Principles of Manufacturing Engineering PDF

Ministry of Higher Education and ‫وزارة التعلين العالى والبحث العلوي‬ Scientific Research OBOUR HIGH INSTITUTE for Engineering and ‫هعهد العبىر العالى للهندست والتكنىلىجيا‬ Technology Basic Science Departments ‫قسن العلىم األساسيه‬ Principles of...

Ministry of Higher Education and ‫وزارة التعلين العالى والبحث العلوي‬ Scientific Research OBOUR HIGH INSTITUTE for Engineering and ‫هعهد العبىر العالى للهندست والتكنىلىجيا‬ Technology Basic Science Departments ‫قسن العلىم األساسيه‬ Principles of Manufacturing Engineering Prepared By Basic Science Departments Members Contents CHAPTER 1: Introduction of Manufacturing I.1 What Is Manufacturing? 2 1.2 An over view of Manufacturing's History. 2 1. 3 Metal Forming Processes 4 1. 5 Surface Finishing Processes 5 CHAPTER 2: ENGINEERING MATERIALS 2.1. Introduction 7 2.2. CLASSIFICATION OF ENGINEERING MATERIALS 7 2.3.Metals and Alloys 8 2.4 PROPERTIES OF METALS 8 2.4.1 Physical Properties 9 2.4.2 Chemical Properties 10 2.4.3 Thermal Properties 10 2.4.4 Electrical Properties 10 2.4.5 Magnetic Properties 11 2.4.6 Optical Properties 11 2.4.7 Mechanical Properties 12 2.5 TESTING OF METALS 17 2.5.1 Tensile test 17 2.5.2 Compression Test 19 2.5.3 Testing of Hardness 21 2.5.4 Testing of Impact Strength 22 2.5.5 Testing of Fatique 23 2.5.6 Testing of Creep 24 2.6 CHOICE OF MATERIALS FOR THE ENGINEERING APPLICATONS 25 CHAPTER 3: CASTING 3.1 INTRODUNCTION 27 3.2 SIGNIFICANCE OF FLUDITY 27 3.3 PERMANENT MOLD OR GRAVITY DIE CASTING 28 3.4 SLUSH CASTING 30 3.5 PRESSURE DIE CASTING 31 3.6 ADVANTAGES OF DIE CASTING OVER SAND CASTING 36 3.7 SHELL MOLD CASTING 36 3.8 CENTRIFUGAL CASTING 38 3.9 CONTINNOUS CASTING 41 3.10 PLASTICS MOLDING PROCESSES 42 3.11 Injection die Molding 43 CHAPTER 4: FUNDAMENTALS OF METAL FORMING 4.1 INTRODUCTION 45 4.2 OVERVIEW OF METAL FORMING 45 4.3. MATERIAL BEHAVIOR IN METAL FORMING 49 4.5 FRICTION AND LUBRICATION IN METAL FORMING 54 CHAPTER 5: POWDER METALLURGY and FORMING PROCESSES ON PLASTIC 5.1 POWDER METALLURGY 57 5.2 CHARACTERISTICS OF METAL POWDERS 62 5.3 PROCESSING OF POWDERS 62 5.4 FORMING PROCESSES ON PLASTIC 64 5.4.1 Injection moulding. 65 5.4.2.Extrusion moulding. 66 5.4.3.Blow moulding. 66 CHAPTER 6: Material Joining processes 6.1 Material Joining processes 68 6.2 Why do we need joining? 69 6.2.1. Fusion Welding 69 6.2.1.1. Oxy-Acetylene Welding 70 6.2.1.2. Arc Welding. 70 6.2.1.3. Gas Shielded Arc Welding 70 6.2.1.4. Plasma Arc Welding 71 6.2.1.5. Electron Beam Welding 71 6.2.1.6. Laser-beam Welding. 71 6.3. Solid State Welding 72 6.3.1. Cold welding 72 6.3.2. Ultrasonic welding 72 6.3.3. Resistance welding 72 6.4. Brazing 73 6.5. Soldering 74 7 6.6. Gluing (Adhesive bonding) 74 6.7. Mechanical Fastening 75 CHAPTER 7: Welding 7.1 INTRODUCTION 77 7.2 TERMINOLOGICAL ELEMENTS OF WELDING PROCESS 77 7.2.1 Edge preparations 78 7.2.2 Welding joints 78 7.2.2.1 Lap weld joint 79 7.2.2.2 Butt weld joint 79 7.2.3 Welding Positions 80 7.2.3.1 Flat or Downhand Welding Position 80 7.2.3.2 Horizontal Welding Position 81 7.2.3.3 Veritical Welding Position 81 7.2.3.4 Overhead Welding Position 81 7.3 ADVANTAGES AND DISADVANTAGES OF WELDING 81 7.4 CLASSIFICATION OF WELDING AND ALLIED PROCESSES 82 7.5 GAS WELDING PROCESSES 84 7.5.1 Oxy-Acetylent Welding 85 7.5.2. Types of Welding Flames 85 7.5.32 Safety Recommendations for Gas Welding 90 7.6 ARC WELDING PROCESSES 92 7.6.1 Arc Welding Equipment 93 7.6.2 Carbon Arc Welding 7.6.3 Shielded Metal Arc Welding (SMAW) or Manual Metal Arc Welding (MMAW) 97 7.6.3.1 Functions of Electrode Coating Ingredients 99 7.6.4 Submerged Arc Welding 100 7.6.5 Gas Tungusten Arc Welding (GTAW) or Tungusten Inert Gas Welding (TIG) 101 7.6.6 Gas Metal ARC Welding (GMAW) or Metal Inert Gas Welding (MIG) 102 7.6.7 Safety Recommendations for ARC Welding 103 7.7 RESISTANCE WELDING 106 7.7.1 Types of Resistance welding 107 7.7.1.1 Spot Welding 107 7.7.1.2 Resistance Seam Welding 109 7.7.1.3 Resistance Projection Welding 111 7.7.1.4 Resistance Upset Butt and Flash Butt Welding 111 7.8 BRAZING 113 7.9. SOLDERING 114 7.9.1. Application of heat and solder 115 7.9.2 Solders 115  R CHAPTER 8: Material Removal Processes 8.1 Introduction 117 8.2. Principal Conventional Machining Processes 117 CHAPTER 9: METAL CUTTING 9.1 INTRODUCTION 133 9.2 CUTTING TOOL 134 9.2.1 Single Point Cutting Tools 134 9.2.2 Nomenclature Single Point Tool 135 9.2.3 Tool Signature 137 9.3 MECHANICS OF METAL CUTTING 137 9.4 TYPES OF CHIPS 138 9.5 COOLENTS OR CUTTING FLUIDS OR EMULSIONS 139 9.5.1 Functions or Uses of Collents or Cutting Fluids 140 9.6 NON TRADITIONAL OR UNCONVENTIONAL MACHINING PROCESSES 140 9.6.1 CLassification of Unconventional Machining Processes 141 CHAPTER 10: SURFACE FINISHING PROCESSES 10.1 INTRODUCTION 143 10.2 Different surface finishing processes are described below. 143 10.2.1.HONING 143 10.2.2Honing Machines 144 10.2.3.LAPPING 144 10.2.4 Machine Lapping 145 10.2.5.Lapping Applications 145 10.3. POLISHING AND BUFFING 145 10.3.1 POLISHING 146 10.3.2. Polishing Tool 146 10.3.3. Different between Lapping and Polishing 146 10.4.BUFFING 146 10.5 Application of buffing produces 147 10.6. SUPER FINISHING 147 10.7. Major applications of super finishing 10.8. GRINDING 148 10.9. POLISHING 148 Introduction of Manufacturing 1 1.1 What Is Manufacturing? b t Take a minute to examine the mechanical pencil, light fixture, chair, cell phone, u and computer while you start reading this chapter. You will soon see that all these items, D along with their many separate parts, are constructed from various materials and have c been manufactured into the products you are currently viewing. It's also important to p note that certain items, including a door key, paper clip, nail, and spoon, comprise just i one piece. However, the great majority of the things in our environment are made up of p multiple separate parts that are joined and constructed via a series of processes known as t manufacturing. i p s c m a c i t b s a c n e e s a f a a e f Figure I.1: John Deere tractor showing the variety of materials and processes c incorporated. Source: Shutterstock/Nils Versemann. R 1.2 An over view of Manufacturing's History. i I Production began between 5000 and 4000 B.C. It is therefore older than records b of history, which go back to the Sumerians, around 3500 B.C. Primitive cave drawings r and markings on clay tablets and stones needed (a) a brush and pigment of a certain kind, l as seen in the 16,000-year-old prehistoric cave paintings in Lascaux, France; (b) a way to S first scratch the clay tablets and then bake them, as seen in the 3000 B.C. cuneiform r scripts and pictograms; and (c) simple tools for making incisions and carvings on stone t surfaces, as seen in the Egyptian hieroglyphs. o c The first products manufactured for particular purposes were household artifacts, E which were usually constructed of stone, wood, or metal. Silver, lead, tin, bronze, and E 2 textile machinery and machine tools for cutting metal. Mechanization soon moved to the United States, where it continued to be further developed. p p A major advance in manufacturing began in the early 1800s, with the design, t production, and use of interchangeable parts, conceived by the American manufacturer e and inventor E. Whitney (1765–1825). Prior to the introduction of interchangeable parts, 4 much hand fitting was necessary, because no two parts could be made exactly alike. By o contrast, it is now taken for granted that a broken bolt can easily be replaced with an f identical one produced decades after the original was made. Further developments soon followed, resulting in countless consumer and industrial products which we now cannot imagine being without. d c Beginning in the early 1940s, several milestones were reached in all aspects of v manufacturing, as can be observed by a review of Table I.2. Note particularly the f progress that has been made during the 20th century, as compared with those achieved t during the 40-century long period from 4000 B.C. to 1 B.C. c t For example, in the Roman Empire (around 500 B.C. to 476 A.D.), factories were available for mass production of glassware; the methods used were generally very slow, and much manpower was required in handling the parts and operating the machinery. Today, production methods have advanced to such an extent that (a) aluminum beverage a cans are made at rates of more than 500 per minute, with each can costing about four c cents to make; (b) holes in sheet metal can be punched at rates of 800 holes per minute; M and (c) light bulbs are made at rates of more than 2000 bulbs per minute, each costing i less than one dollar. f The period from the 1940s to the 1990s was characterized by mass production and expanding global markets. Initially, the United States had a dominant position, as it s was the only developed nation with an intact infrastructure following World War II; m however, this advantage dissipated by the 1960s. The quality revolution began to change d manufacturing in the 1960s and 1970s, and in the 1980s, programmable computers t became widely used. m a The digital manufacturing era began around 1990. As a fundamental change in manufacturing operations, powerful computers and software are now fully integrated across the design and manufacturing enterprise. Advances in communications, some Internet-based, have led to further improvements in organizations and their capabilities. The effects are most striking when considering the origin and proliferation of additive manufacturing.. Prior to 1990, the prototype of a part could be produced only through intensive effort and costly manufacturing approaches, requiring significant operator skill. Today, a part can first be drafted in a CAD program, then produced generally in a matter of minutes or hours (depending on size and part complexity) without the need for hard tools or skilled labor. Prototyping systems have become more economical, faster, and with improved raw materials. The term digital manufacturing has been applied to reflect the notion that manufacturing parts and components can take place completely through such computer Aided driven CAD and production machinery. 3  H ( s d C ( C W S a n d Figure I.2: A flexible hybrid electronic wound care system. The device integrates communication ability (in this case a Bluetooth ability), sensors to detect the oxygen 1 levels at the wound, and oxygen generating devices to increase oxygen level to optimize healing. These devices use combinations of printed electronic devices and silicon-based integrated circuits. Source: Courtesy of NextFlex. d b Machine learning algorithms, and the incorporation of physics-based i mathematical models of manufacturing processes and systems, has led to the ability to p apply advanced controls to the manufacturing enterprise. c A Advanced models of manufacturing processes, combined with the detailed f measurement of the manufacturing and service environment of a product (Big Data), 8 lead to the computer-based representation of the product, referred to as a digital twin. v The digital twin represents a virtual model of the part, and accurate performance models a applied to the virtual twin can predict failure or required service of the actual part. c r p These developments are a natural extension of the computer revolution that a started in the 1990s, and developments are certain to continue. p w 1. 3 Metal Forming Processes i p Forming processes encompasses a wide variety of techniques, which make use of j suitable force, pressure or stresses, like compression, tension and shear or their p combination to cause a permanent deformation of the raw material to impart required c shape. These processes are also known as mechanical working processes and are mainly M classified into two major categories i.e., hot working processes and cold working m processes. In these processes, no material is removed; however it is deformed and m displaced using suitable stresses like compression, tension, and shear or combined u stresses to cause plastic deformation of the materials to produce required shapes. Such m processes lead to production of directly usable articles which include kitchen utensils, c rods, wires, rails, cold drink bottle caps, collapsible tubes etc. Some of the important d metals forming processes are: w a o 4 joined together to produce desired shape and size of the product. The joining processes are carried out by fusing, pressing, rubbing, riveting, screwing or any other means of assembling. These processes are used for assembling metal parts and in general fabrication work. Such requirements usually occur when several pieces are to be joined together to fabricate a desired structure of products. These processes are used developing steam or water-tight joints. Temporary, semi-permanent or permanent type of fastening to make a good joint is generally created by these processes. Temporary joining of components can be achieved by use of nuts, screws and bolts. Adhesives are also used to make temporary joints. Some of the important and common joining processes are: (1) Welding (plastic or fusion), (2) Brazing, (3) Soldering, (4) Riveting, (5) Screwing,(6) Press fitting, (7) Sintering, (8) Adhesive bonding, (9) Shrink fitting, (10) Explosive welding, (11)Diffusion welding, (12) Keys and cotters joints, (13) Coupling and (14) Nut and bolt joints. 1. 5 Surface Finishing Processes Surface finishing processes are utilized for imparting intended surface finish on the surface of a job. By imparting a surface finishing process, dimension of part is not changed functional either a very negligible amount of material is removed from the certain material is added to the surface of the job. These processes should not be misunderstood as metal removing processes in any case as they are primarily intended to provide a good surface finish or a decorative or protective coating on to the metal surface. Surface cleaning process also called as a surface finishing process. Some of the commonly used surface finishing processes are: (1) Honing, (2) Lapping, (3) Super finishing, (4) Belt grinding, (5) Polishing, (6) Tumbling, (7)Organic finishes, (8) Sanding, (9) debarring, (10) Electroplating, (11) Buffing, (12) Metal spraying, (13) Painting, (14) Inorganic coating, (15) Anodizing, (16) Sherardizing, (17) Parkerizing, 5 ENGINEERING MATERIALS 6 2.1. Introduction Engineering materials used to manufacture of articles or products, dictates which manufacturing process or processes are to be used to provide it the desired shape. Sometimes, it is possible to use more than one manufacturing processes, then the best possible process must be utilized in manufacture of product. It is therefore important to know what materials are available in the universe with it usual cost. What are the common characteristics of engineering materials such as physical, chemical, mechanical, thermal, optical, electrical, and mechanical? How they can be processed economically to get the desired product. The basic knowledge of engineering materials and their properties is of great significance for a design and manufacturing engineer. The elements of tools, machines and equipment's should be made of such a material which has properties suitable for the conditions of operation. In addition to this, a product designer, tool designer and design engineer should always be familiar with various kinds of engineering materials, their properties and applications to meet the functional requirements of the design product. They must understand all the effects which the manufacturing processes and heat treatment have on the properties of the engineering materials. The general classification 2.2. CLASSIFICATION OF ENGINEERING MATERIALS A large numbers of engineering materials exists in the universe such as metals and nonmetals leather, rubber, asbestos, plastic, ceramics, organic polymers, composites and semiconductor. Some commonly used engineering materials are broadly classified as shown in Fig. 2.1. Leather is generally used for shoes, belt drives, packing, washers etc. It is highly flexible and can easily withstand against considerable wear under suitable conditions. Rubber is commonly employed as packing material, belt drive as an electric insulator. Asbestos is basically utilized for lagging round steam pipes and steam pipe and steam boilers because it is poor conductor of heat, so avoids loss of heat to the surroundings. Engineering materials may also be categorized into metals and alloys, ceramic materials, organic polymers, composites and semiconductors. The metal and alloys have tremendous applications for manufacturing the products required by the customers. 7 2.3. Metals and Alloys Metals are polycrystalline bodies consisting of a great number of fine crystals. Pure metals possess low strength and do not have the required properties. So, alloys are produced by melting or sintering two or more metals or metals and a non-metal, together. Alloys may consist of two more components. Metals and alloys are further classified into two major kind namely ferrous metals and non-ferrous metals. (a) Ferrous metals are those which have the iron as their main constituent, such as pig iron, cast iron, wrought iron and steels. (b) Non-ferrous metals are those which have a metal other than iron as their main constituent, such as copper, aluminum, brass, bronze, tin, silver zinc, invar etc. Fig. 2.1Classification of engineering materials 2.4 PROPERTIES OF METALS The important properties of an engineering material determine the utility of the material which influences quantitatively or qualitatively the response of a given material to imposed stimuli and constraints. The various engineering material properties are given as under. 8  Physical properties Chemical properties Thermal properties Electrical properties Magnetic properties Optical properties, and Mechanical properties These properties of the material are discussed as under. 2.4.1 Physical Properties The important physical properties of the metals are density, color, size and shape (dimensions), specific gravity, porosity, luster etc. Some of them are defined as under. 1. Density Mass per unit volume is called as density. In metric system its unit is kg/mm3. Because of very low density, aluminum and magnesium are preferred in aeronautic and transportation applications. 2. Color It deals the quality of light reflected from the surface of metal. 3. Size and shape Dimensions of any metal reflect the size and shape of the material. Length, width, height, depth, curvature diameter etc. determines the size. Shape specifies the rectangular, square, circular or any other section. 4. Specific Gravity Specific gravity of any metal is the ratio of the mass of a given volume of the metal to the mass of the same volume of water at a specified temperature. 9  5. Porosity A material is called as porous or permeable if it has pores within it. 2.4.2 Chemical Properties The study of chemical properties of materials is necessary because most of the engineering materials, when they come in contact with other substances with which they can react, suffer from chemical deterioration of the surface of the metal. Some of the chemical properties of the metals are corrosion resistance, chemical composition and acidity or alkalinity. Corrosion is the gradual deterioration of material by chemical reaction with its environment. 2.4.3 Thermal Properties The study of thermal properties is essential in order to know the response of metal to thermal changes i.e. lowering or raising of temperature. Different thermal properties are thermal conductivity, thermal expansion, specific heat, melting point, thermal diffusivity. Some important properties are defined as under. Melting Point Melting point is the temperature at which a pure metal or compound changes its shape from solid to liquid. It is called as the temperature at which the liquid and solid are in equilibrium. It can also be said as the transition point between solid and liquid phases. Melting temperature depends on the nature of inter-atomic and intermolecular bonds. Therefore higher melting point is exhibited by those materials possessing stronger bonds. Covalent, ionic, metallic and molecular types of solids have decreasing order of bonding strength and melting point. Melting point of mild steel is 1500°C, of copper is 1080°C and of Aluminum is 650°C. 2.4.4 Electrical Properties The various electrical properties of materials are conductivity, temperature coefficient of resistance, dielectric strength, resistivity, and thermoelectricity. These properties are defined as under. 10  1. Conductivity Conductivity is defined as the ability of the material to pass electric current through it easily i.e. the material which is conductive will provide an easy path for the flow of electricity through it. 2. Temperature Coefficient of Resistance It is generally termed as to specify the variation of resistivity with temperature. 3. Dielectric Strength It means insulating capacity of material at high voltage. A material having high dielectric strength can withstand for longer time for high voltage across it before it conducts the current through it 4. Resistivity It is the property of a material by which it resists the flow of electricity through it. 5. Thermoelectricity If two dissimilar metals are joined and then this junction is heated, a small voltage (in the millivolt range) is produced, and this is known as thermoelectric effect. It is the base of the thermocouple. Thermo -couples are prepared using the properties of metals. 2.4.5 Magnetic Properties Magnetic properties of materials arise from the spin of the electrons and the orbital motion of electrons around the atomic nuclei. In certain atoms, the opposite spins neutralize one another, but when there is an excess of electrons spinning in one direction, magnetic field is produced. Many materials except ferromagnetic material which can form permanent magnet, exhibit magnetic affects only when subjected to an external electro-magnetic field. Magnetic properties of materials specify many aspects of the structure and behavior of the matter. Various magnetic properties of the materials are magnetic hysteresis, coercive force and absolute permeability which are defined as under. 11  1. Magnetic Hysteresis Hysteresis is defined as the lagging of magnetization or induction flux density behind the magnetizing force or it is that quality of a magnetic substance due to energy is dissipated in it on reversal of its magnetism. Below Curie temperature, magnetic hysteresis is the rising temperature at which the given material ceases to be ferromagnetic, or the falling temperature at which it becomes magnetic. Almost all magnetic materials exhibit the phenomenon called hysteresis. 2. Coercive Force It is defined as the magnetizing force which is essential to neutralize completely the magnetism in an electromagnet after the value of magnetizing force becomes zero. 3. Absolute Permeability It is defined as the ratio of the flux density in a material to the magnetizing force producing that flux density. Paramagnetic materials possess permeability greater than one whereas di-magnetic materials have permeability less than one. 2.4.6 Optical Properties The main optical properties of engineering materials are refractive index, absorptivity, absorption co-efficient, reflectivity and transmissivity. Refractive index is an important optical property of metal which is defined as under. Refractive Index It is defined as the ratio of velocity of light in vacuum to the velocity of a material. It can also be termed as the ratio of sine of angle of incidence to the sine of refraction. 2.4.7 Mechanical Properties Under the action of various kinds of forces, the behavior of the material is studied that measures the strength and lasting characteristic of a material in service. The mechanical properties of materials are of great industrial importance in the design of tools, machines and structures. These properties are 12 structure sensitive in the sense that they depend upon the crystal structure and its bonding forces, and especially upon the nature and behavior of the imperfections which- exist within the crystal itself or at the grain boundaries. The mechanical properties of the metals are those which are associated with the ability of the material to resist mechanical forces and load. The main mechanical properties of the metal are strength, stiffness, elasticity, plasticity, ductility, malleability, toughness, brittleness, hardness, formability, cast ability and weld ability. These properties can be well understood with help of tensile test and stress strain diagram. The few important and useful mechanical properties are explained below. 1. Elasticity It is defined as the property of a material to regain its original shape after deformation when the external forces are removed. It can also be referred as the power of material to come back to its original position after deformation when the stress or load is removed. It is also called as the tensile property of the material. 2. Proportional limit It is defined as the maximum stress under which a material will maintain a perfectly uniform rate of strain to stress. Though its value is difficult to measure, yet it can be used as the important applications for building precision instruments, springs, etc. 3. Elastic limit Many metals can be put under stress slightly above the proportional limit without taking a permanent set. The greatest stress that a material can endure without taking up some permanent set is called elastic limit. Beyond this limit, the metal does not regain its original form and permanent set will occurs. 4. Yield point At a specific stress, ductile metals particularly ceases, offering resistance to tensile forces. This means, the metals flow and a relatively large permanent set takes place without a noticeable increase in load. This point is called yield point. Certain metals such as mild steel exhibit a definite yield point, in which case the yield stress is simply the stress at this point. 13  5. Strength Strength is defined as the ability of a material to resist the externally applied forces with breakdown or yielding. The internal resistance offered by a material to an externally applied force is called stress. The capacity of bearing load by metal and to withstand destruction under the action of external loads is known as strength. The stronger the material the greater the load it can withstand. This property of material therefore determines the ability to withstand stress without failure. Strength varies according to the type of loading. It is always possible to assess tensile, compressive, shearing and torsional strengths. The maximum stress that any material can withstand before destruction is called its ultimate strength. The tenacity of the material is its ultimate strength in tension. 6. Stiffness It is defined as the ability of a material to resist deformation under stress. The resistance of a material to elastic deformation or deflection is called stiffness or rigidity. A material that suffers slight or very less deformation under load has a high degree of stiffness or rigidity. For instance suspended beams of steel and aluminum may both be strong enough to carry the required load but the aluminum beam will “sag” or deflect further. That means, the steel beam is stiffer or more rigid than aluminum beam. If the material behaves elastically with linear stress-strain relationship under Hooks law, its stiffness is measured by the Young’s modulus of elasticity (E). The higher is the value of the Young’s modulus, the stiffer is the material. In tensile and compressive stress, it is called modulus of stiffness or “modulus of elasticity”; in shear, the modulus of rigidity and this is usually 40% of the value of Young’s modulus for commonly used materials; in volumetric distortion, the bulk modulus. 7. Plasticity Plasticity is defined the mechanical property of a material which retains the deformation produced under load permanently. This property of the material is required in forging, in stamping images on coins and in ornamental work. It is the ability or tendency of material to undergo some degree of permanent deformation without its rupture or its failure. Plastic deformation takes place only after the elastic range of material has been exceeded. Such property of material is important in forming, shaping, 14 extruding and many other hot or cold working processes. Materials such as clay, lead, etc. are plastic at room temperature and steel is plastic at forging temperature. This property generally increases with increase in temperature of materials. 8. Ductility Ductility is termed as the property of a material enabling it to be drawn into wire with the application of tensile load. A ductile material must be strong and plastic. The ductility is usually measured by the terms, percentage elongation and percent reduction in area which is often used as empirical measures of ductility. The materials those possess more than 5% elongation are called as ductile materials. The ductile material commonly used in engineering practice in order of diminishing ductility are mild steel, copper, aluminum, nickel, zinc, tin and lead. 9. Malleability Malleability is the ability of the material to be flattened into thin sheets under applications of heavy compressive forces without cracking by hot or cold working means. It is a special case of ductility which permits materials to be rolled or hammered into thin sheets. A malleable material should be plastic but it is not essential to be so strong. The malleable materials commonly used in engineering practice in order of diminishing malleability are lead, soft steel, wrought iron, copper and aluminum. Aluminum, copper, tin, lead, steel, etc. are recognized as highly malleable metals. 10. Hardness Hardness is defined as the ability of a metal to cut another metal. A harder metal can always cut or put impression to the softer metals by virtue of its hardness. It is a very important property of the metals and has a wide variety of meanings. It embraces many different properties such as resistance to wear, scratching, deformation and machinability etc. 15  11. Brittleness Brittleness is the property of a material opposite to ductility. It is the property of breaking of a material with little permanent distortion. The materials having less than 5% elongation under loading behavior are said to be brittle materials. Brittle materials when subjected to tensile loads, snap off without giving any sensible elongation. Glass, cast iron, brass and ceramics are considered as brittle material. 12. Creep When a metal part when is subjected to a high constant stress at high temperature for a longer period of time, it will undergo a slow and permanent deformation (in form of a crack which may further propagate further towards creep failure) called creep. 13. Formability It is the property of metals which denotes the ease in its forming in to various shapes and sizes. The different factors that affect the formability are crystal structure of metal, grain size of metal hot and cold working, alloying element present in the parent metal. Metals with smal1 grain size are suitable for shallow forming while metal with size are suitable for heavy forming. Hot working increases formability. Low carbon steel possesses good formability. 14. Cast ability Cast ability is defined as the property of metal, which indicates the ease with it can be casted into different shapes and sizes. Cast iron, aluminum and brass are possessing good cast ability. 15. Weld ability Weld ability is defined as the property of a metal which indicates the two similar or dissimilar metals are joined by fusion with or without the application of pressure and with or without the use of filler metal (welding) efficiently. Metals having weld ability in the descending order are iron, steel, cast steels and stainless steels. 16 2.5 TESTING OF METALS Metal testing is accomplished for the purpose of for estimating the behavior of metal under loading (tensile, compressive, shear, torsion and impact, cyclic loading etc.) of metal and for providing necessary data for the product designers, equipment designers, tool and die designers and system designers. The material behavior data under loading is used by designers for design calculations and determining weather a metal can meet the desired functional requirements of the designed product or part. Also, it is very important that the material shall be tested so that their mechanical properties especially their strength can be assessed and compared. Therefore the test procedure for developing standard specification of materials has to be evolved. This necessitates both destructive and non- destructive testing of materials. Destructive tests of metal include various mechanical tests such as tensile, compressive, hardness, impact, fatigue and creep testing. A standard test specimen for tensile test is shown in Fig. 2.2. Non-destructive testing includes visual examination, radiographic tests, ultrasound test, liquid penetrating test and magnetic particle testing. Fig. 2.2 Tensile test specimen 2.5.1 Tensile test A tensile test is carried out on standard tensile test specimen in universal testing machine. Fig. 2.3 shows a schematic set up of universal testing machine reflecting the test specimen griped between two cross heads. Fig. 2.4 shows the stress strain curve for ductile material. Fig. 2.5 shows the properties of a ductile material. 17 Fig. 2.3 Schematic universal testing machine Fig.2.4 Stress strain curve for ductile material Fig. 2.5 Properties of a ductile material 18 2.5.2 Compression Test Compression test is reverse of tensile test. This test can also be performed on a universal testing machine. In case of compression test, the specimen is placed bottom crossheads. After that, compressive load is applied on to the test specimen. This test is generally performed for testing brittle material such as cast iron and ceramics etc. Fig. 2.6 shows the schematic compression test set up on a universal testing machine. The following terms have been deduced using figures pertaining to tensile and compressive tests of standard test specimen. Hook’s Law Hook’s law states that when a material is loaded within elastic limit (up to proportional limit), stress is proportional to strain. Strain Strain is the ratio of change in dimension to the original dimension. Fig. 2.6 Schematic compression test set up on a universal testing machine 19  Tensile Strain The ratio of increase in length to the original length is known as tensile strain. Compressive Strain The ratio of decrease in length to the original length is known as compressive strain. Modulus of Elasticity The ratio of tensile stress to tensile strain or compressive stress to compressive strain is called modulus of elasticity. It is denoted by E. It is also called as Young’s modulus of elasticity. E = Tensile Stress/Tensile Strain Modulus of Rigidity The ratio of sheer stress to shear strain is called modulus of rigidity. It is denoted by G. G = Shear Stress/Shear Strain Bulk Modulus The ratio of direct stress to the volumetric strain (ratio of change in volume to the original volume is known as volumetric strain) is called Bulk modulus (denoted by K). K = Direct stress/volumetric strain Linear and Lateral Strain When a body is subjected to tensile force its length increases and the diameter decreases. So when a test specimen of metal is stressed, one deformation is in the direction of force which is called linear strain and other deformation is perpendicular to the force called lateral strain. Poisson’s Ratio The ratio of lateral strain to linear strain in metal is called poisson’s ratio. Its value is constant for a particular material but varies for different materials. 20  Proof Resilience The maximum amount of energy which can be stored in an elastic limit is known as proof resilience. Modulus of Resilience The proof resilience per unit volume of a material is modulus of resilience or elastic toughness. 2.5.3 Testing of Hardness It is a very important property of the metals and has a wide variety of meanings. It embraces many different properties such as resistance to wear, scratching, deformation and machinability etc. It also means the ability of a metal to cut another metal. The hardness of a metal may be determined by the following tests. (a) Brinell hardness test (b) Rockwell hardness test (c) Vickers hardness (also called Diamond Pyramid) test (d) Shore scleroscope Fig. 2.7 shows Rockwell hardness testing machine. 21 2.5.4 Testing of Impact Strength When metal is subjected to suddenly applied load or stress, it may fail. In order to assess the capacity of metal to stand sudden impacts, the impact test is employed. The impact test measures the energy necessary to fracture a standard notched bar by an impulse load and as such is an indication of the notch toughness of the material under shock loading. Izod test and the Charpy test are commonly performed for determining impact strength of materials. These methods employ same machine and yield a quantitative value of the energy required to fracture a special V notch shape metal. The most common kinds of impact test use notched specimens loaded as beams. V notch is generally used and it is get machined to standard specifications with a special milling cutter on milling machine in machine shop. The beams may be simply loaded (Charpy test) or loaded as cantilevers (Izod test). The function of the V notch in metal is to ensure that the specimen will break as a result of the impact load to which it is subjected. Without the notch, many alloys would simply bend without breaking, and it would therefore be impossible to determine their ability to absorb energy. It is therefore important to observe that the blow in Charpy test is delivered at a point directly behind the notch and in the Izod test the blow is struck on the same side of the notch towards the end of the cantilever. Fig. 2.8 shows the impact testing set up arrangement for charpy test. The specimen is held in a rigid vice or support and is struck a blow by a traveling pendulum that fractures or severely deforms the notched specimen. The energy input in this case is a function of the height of fall and the weight of the pendulum used in the test setup. The energy remaining after fracture is determined from the height of rise of the pendulum due to inertia and its weight. The difference between the energy input and the energy remaining represents the energy absorbed by the standard metal specimen. Advance testing setups of carrying out such experiments are generally equipped with scales and pendulum-actuated pointers, which provide direct readings of energy absorption. 22  Fig. 2.8 Schematic impact testing machine setup 2.5.5 Testing of Fatique Material subjected to static and cyclic loading, yield strength is the main criterion for product design. However for dynamic loading conditions, the fatigue strength or endurance limit of a material is used in main criteria used for designing of parts subjected to repeated alternating stresses over an extended period of time. Fig 2.9 shows a fatigue test set up determining the fatigue strength of material. The fatigue test determines the stresses which a sample of material of standard dimensions can safely endure for a given number of cycles. It is performed on a test specimen of standard metal having a round cross-section, loaded at two points as a rotating simple beam, and supported at its ends. The upper surface of such a standard test specimen is always in compression and the lower surface is always in tension. The maximum stress in metal always occurs at the surface, halfway along the length of the standard test specimen, where the cross section is minimum. For every full rotation of the specimen, a point in the surface originally at the top Centre goes alternately from a maximum in compression to a maximum in tension and then back to the same maximum in compression. Standard test specimens are tested to failure using different loads, and the number of cycles before failure is noted for each load. The results of such tests are recorded on graphs of applied stress against the logarithm of the number of cycles to failure. 23  Fig. 2.9 Schematic fatigue test setup 2.5.6 Testing of Creep Metal part when is subjected to a high constant stress at high temperature for a longer period of time, it will undergo a slow and permanent deformation (in form of a crack which may further propagate further towards creep failure) called creep. Creep is time dependent phenomena of metal failure at high constant stress and at high temperature such subjecting of at steam turbine blade. A schematic creep testing setup is shown in Fig. 2.10. Test is carried out up to the failure of the test specimen. Fig. 2.10 Schematic creep testing setup 24 2.6 CHOICE OF MATERIALS FOR THE ENGINEERING APPLICATONS The choice of materials for the engineering purposes depends upon the following factors: 1- Availability of the materials, 2-Properties needed for meeting the functional requirements, 3- Suitability of the materials for the working conditions in service, and 4- The cost of the materials. 25 CASTING 26 3.1 INTRODUNCTION Casting process is one of the earliest metal shaping techniques known to human being. It means pouring molten metal into a refractory mold cavity and allows it to solidify. The solidified object is taken out from the mold either by breaking or taking the mold apart. The solidified object is called casting and the technique followed in method is known as casting process. The casting process was discovered probably around 3500 BC in Mesopotamia. In many parts of world during that period, copper axes (wood cutting tools) and other flat objects were made in open molds using baked clay. These molds were essentially made in single piece. The Bronze Age 2000 BC brought forward more refinement into casting process. For the first time, the core for making hollow sockets in the cast objects was invented. The core was made of baked sand. Also the lost wax process was extensively used for making ornaments using the casting process. Casting technology was greatly improved by Chinese from around 1500 BC. For this there is evidence of the casting activity found in China. For making highly intricate jobs, a lot of time in making the perfect mold to the last detail so hardly any finishing work was required on the casting made from the molds. Indus valley civilization was also known for their extensive use of casting of copper and bronze for ornaments, weapons, tools and utensils. But there was not much of improvement in the casting technology. From various objects that were excavated from the Indus valley sites, they appear to have been familiar with all the known casting methods such as open mold and piece mold. This chapter describes the fluidity of molten metal, different casting techniques and various casting defects occurring in casting processes. 3.2 SIGNIFICANCE OF FLUDITY Fluidity of molten metal helps in producing sound casting with fewer defects. It fills not only the mold cavity completely and rapidly but does not allow also any casting defect like “misrun” to occur in the cast object. Pouring of molten metal properly at correct temperature plays a significant role in producing sound castings. The gating system performs the function to introduce clean metal into mold cavity in a manner as free of turbulence as possible. To produce sound casting gate must also be designed to completely fill the mold cavity for preventing casting defect such as misruns and to promote feeding for establishing proper temperature gradients. Prevent casting defect such as misruns without use of excessively high pouring temperatures is still largely a matter of experience. To fill the 27 complicate castings sections completely, flow rates must be high but not so high as to cause turbulence. It is noted that metal temperature may affect the ability of molten alloy to fill the mold, this effect is metal fluidity. 1t include alloy analysis and gas content, and heat-extracting power of the molding material. Often, it is desirable to check metal fluidity before pouring using fluidity test. Fig. 3.1 illustrates a standard fluidity spiral test widely used for cast steel. “Fluidity” of an alloy is rated as a distance, in inches, that the metal runs in the spiral channel. Fig. 3.1Fluidity spiral test Fluidity tests, in which metal from the furnace is poured by controlled vacuum into a flow channel of suitable size, are very useful, since temperature (super-heat) is the most Significant single variable influencing the ability of molten metal to fill mold. This test is an accurate indicator of temperature. The use of simple, spiral test, made in green sand on a core poured by ladle from electric furnace steel melting where temperature measurement is costly and inconvenient. The fluidity test is same times less needed except as a research tool, for the lower melting point metals, where pyrometry is a problem. In small casting work, pouring is done by means of ladles and crucibles. There are some special casting methods which are discussed as under. 3.3 PERMANENT MOLD OR GRAVITY DIE CASTING This process is commonly known as permanent mold casting in U.S.A and gravity die casting in England. A permanent mold casting makes use of a mold or metallic die which is permanent. A typical 28 permanent mold is shown in Fig. 3.1. Molten metal is poured into the mold under gravity only and no external pressure is applied to force the liquid metal into the mold cavity. Fig. 3.1 A typical permanent mold However, the liquid metal solidifies under pressure of metal in the risers, etc. The metallic mold can be reused many times before it is discarded or rebuilt. These molds are made of dense, fine grained, heat resistant cast iron, steel, bronze, anodized aluminum, graphite or other suitable refractoriness. The mold is made in two halves in order to facilitate the removal of casting from the mold. It may be designed with a vertical parting line or with a horizontal parting line as in conventional sand molds. The mold walls of a permanent mold have thickness from 15 mm to 50 mm. The thicker mold walls can remove greater amount of heat from the casting. For faster cooling, fins or projections may be provided on the outside of the permanent mold. This provides the desirable chilling effect. There are some advantages, disadvantages and application of this process which are given as under the use of core Advantages (i) Fine and dense grained structure is achieved in the casting. (ii) No blow holes exist in castings produced by this method. (iii) The process is economical for mass production. (iv) Because of rapid rate of cooling, the castings possess fine grain structure. (v) Close dimensional tolerance or job accuracy is possible to achieve on the cast product. 29  (vi) Good surface finish and surface details are obtained. (vii) Casting defects observed in sand castings are eliminated. (viii) Fast rate of production can be attained. (ix) The process requires less labor. Disadvantages (i) The cost of metallic mold is higher than the sand mold. The process is impractical for large castings. (ii) The surface of casting becomes hard due to chilling effect. (iii) Refractoriness of the high melting point alloys. Applications (i) This method is suitable for small and medium sized casting such as carburetor bodies, oil pump bodies, connecting rods, pistons etc. (ii) It is widely suitable for non-ferrous casting. 3.4 SLUSH CASTING Slush casting is an extension of permanent mold casting or metallic mold casting. It is used widely for production of hollow casting without the use of core. The process is similar to metallic mold casting only with the difference that mold is allowed to open at an early stage (only when a predetermined amount of molten metal has solidified up to some thickness) and some un-solidified molten metal fall down leaving hollowness in the cast object. The process finds wide applications in production of articles namely toys, novelties, statutes, ornaments, lighting fixtures and other articles having hollowness inside the cast product. 30 3.5 PRESSURE DIE CASTING Unlike permanent mold or gravity die casting, molten metal is forced into metallic mold or die under pressure in pressure die casting. The pressure is generally created by compressed air or hydraulically means. The pressure varies from 70 to 5000 kg/cm2 and is maintained while the casting solidifies. The application of high pressure is associated with the high velocity with which the liquid metal is injected into the die to provide a unique capacity for the production of intricate components at a relatively low cost. This process is called simply die casting in USA. The die casting machine should be properly designed to hold and operate a die under pressure smoothly. There are two general types of molten metal ejection mechanisms adopted in die casting set ups which are: (i) Hot chamber type (a) Gooseneck or air injection management (b) Submerged plunger management (ii) Cold chamber type Die casting is widely used for mass production and is most suitable for non-ferrous metals and al1oys of low fusion temperature. The casting process is economic and rapid. The surface achieved in casting is so smooth that it does not require any finishing operation. The material is dense and homogeneous and has no possibility of sand inclusions or other cast impurities. Uniform thickness on castings can also be maintained. The principal base metals most commonly employed in the casting are zinc, aluminum, and copper, magnesium, lead and tin. Depending upon the melting point temperature of alloys and their suitability for the die casting, they are classified as high melting point (above 540°C) and low melting point (below 500°C) alloys. Under low category involves zinc, tin and lead base alloys. Under high temperature category aluminum and copper base alloys are involved. There are four main types of die-casting machine which are given as under. Hot chamber die casting machine Cold chamber die casting machine. Air blown or goose neck type machine Vacuum die-casting machine 31  Some commonly used die casting processes are discussed as under. Hot chamber die-casting Hot chamber die-casting machine is the oldest of die-casting machines which is simplest to operate. It can produce about 60 or more castings of up to 20 kg each per hour and several hundred castings per hour for single impression castings weighing a few grams. The melting unit of setup comprises of an integral part of the process. The molten metal possesses nominal amount of superheat and, therefore, less pressure is needed to force the liquid metal into the die. This process may be of gooseneck or air-injection type or submerged plunger type-air blown or goose neck type machine is shown as in Fig. 3.2. It is capable of performing the following functions: (i) Holding two die halves finally together. (ii) Closing the die. (iii) Injecting molten metal into die (iv) Opening the die. (v) Ejecting the casting out of the die. Fig. 3.2 Air blown or goose neck type die casting setup A die casting machine consists of four basic elements namely frame, source of molten metal and molten metal transfer mechanism, die-casting dies, and metal injection mechanism. It is a simple 32 machine as regards its construction and operation. A cast iron gooseneck is so pivoted in the setup that it can be dipped beneath the surface of the molten metal to receive the same when needed. The molten metal fills the cylindrical portion and the curved passageways of the gooseneck. Gooseneck is then raised and connected to an airline which supplies pressure to force the molten metal into the closed die. Air pressure is required for injecting metal into the die is of the order of 30 to 45 kg./cm2. The two mold halves are securely clamped together before pouring. Simple mechanical clamps of latches and toggle kinds are adequate for small molds. On solidification of the die cast part, the gooseneck is again dipped beneath the molten metal to receive the molten metal again for the next cycle. The die halves are opened out and the die cast part is ejected and die closes in order to receive a molten metal for producing the next casting. The cycle repeats again and again. Generally large permanent molds need pneumatic or other power clamping devices. A permanent mold casting may range in weight from a few grams to 150 kg. for aluminum. Cores for permanent molds are made up of alloy steel or dry sand. Metal cores are used when they can be easily extracted from the casting. A dry sand core or a shell core is preferred when the cavity to be cored is such that a metal core cannot possibly be withdrawn from the casting. The sprees, risers, runners, gates and vents are machined into the parting surface for one or both mold halves. The runner channels are inclined, to minimize turbulence of the incoming metal. Whenever possible, the runner should be at the thinnest area of the casting, with the risers at the top of the die above the heavy sections. On heating the mold surfaces to the required temperature, a refractory coating in the form of slurry is sprayed or brushed on to the mold cavity, riser, and gate and runner surfaces. French chalk or calcium carbonate suspended in sodium silicate binder is commonly used as a coating for aluminum and magnesium permanent mold castings. Chills are pieces of copper, brass or aluminum and are inserted into the mold’s inner surface. Water passages in the mold or cooling fins made on outside the mold surface are blown by air otherwise water mist will create chilling effect. A chill is commonly used to promote directional solidification. Cold chamber die casting Cold chamber die casting process differs from hot chamber die casting in following respects. 1- Melting unit is generally not an integral part of the cold chamber die casting machine. Molten metal is brought and poured into die casting machine with help of ladles. 2- Molten metal poured into the cold chamber casting machine is generally at lower temperature as compared to that poured in hot chamber die casting machine. 33  3- For this reasoning, a cold chamber die casting process has to be made use of pressure much higher (of the order of 200 to 2000 kgf/cm2) than those applied in hot chamber process. 4- High pressure tends to increase the fluidity of molten metal possessing relatively lower temperature. 5. Lower temperature of molten metal accompanied with higher injection pressure with produce castings of dense structure sustained dimensional accuracy and free from blow-holes. 6. Die components experience less thermal stresses due to lower temperature of molten metal. However, the dies are often required to be made stronger in order to bear higher pressures. There are some advantages, disadvantages and application of this process which are given as under. Advantages 1. It is very quick process 2. It is used for mass production 3. castings produced by this process are greatly improved surface finish 4. Thin section (0.5 mm Zn, 0.8 mm Al and 0.7 mm Mg) can be easily casted 5. Good tolerances 6. Well defined and distinct surface 5- Lower temperature of molten metal accompanied with higher injection pressure with produce castings of dense structure sustained dimensional accuracy and free from blow-holes. 6- Die components experience less thermal stresses 5. Lower temperature of molten metal accompanied with higher injection pressure with produce castings of dense structure sustained dimensional accuracy and free from blow holes. due to lower temperature of molten metal. However, the dies are often required to be made stronger in order to bear higher pressures. 6. Die components experience less thermal stresses due to lower temperature of molten metal. However, the dies are often required to be made stronger in order to bear higher pressures. There are some advantages, disadvantages and application of this process which are given as under. Advantages 1- It is very quick process 2- It is used for mass production 3- Castings produced by this process are greatly improved surface finish 4- Thin section (0.5 mm Zn, 0.8 mm Al and 0.7 mm Mg) can be easily casted 5- Good tolerances 34  6- Well defined and distinct surface 7- Less nos. of rejections 8- Cost of production is less 9- Process require less space 10-Very economic process 11-Life of die is long 12-All casting has same size and shape. Disadvantages 1-Cost of die is high. 2- Only thin casting can be produced. 3-Special skill is required. 4- Unless special precautions are adopted for evaluation of air from die-cavity some air is always entrapped in castings causing porosity. 5-It is not suitable for low production. Applications 1- Carburetor bodies 2- Hydraulic brake cylinders 3- Refrigeration castings 4- Washing machine 5- Connecting rods and automotive pistons 6- Oil pump bodies 7- Gears and gear covers 8- Aircraft and missile castings, and 9- Typewriter segments 35  3.6 ADVANTAGES OF DIE CASTING OVER SAND CASTING 1- Die casting requires less floor space in comparison to sand casting. 2- It helps in providing precision dimensional control with a subsequent reduction in machining cost. 3- It provides greater improved surface finish. 4- Thin section of complex shape can be produced in die casting. 5- More true shape can be produced with close tolerance in die casting. 6- Castings produced by die casting are usually less defective. 7- It produces more sound casting than sand casting. 8- It is very quick process. 3.7 SHELL MOLD CASTING Shell mold casting process is recent invention in casting techniques for mass production and smooth surface finish. It was originated in Germany during Second World War. It is also called as Carning or C process. It consists of making a mold that possesses two or more thin shells (shell line parts, which are moderately hard and smooth with a texture consisting of thermosetting resin bonded sands. The shells are 0.3 to 0.6 mm thick and can be handled and stored. Shell molds are made so that machining parts fit together-easily. They are held using clamps or adhesive and metal is poured either in a vertical or horizontal position. They are supported using rocks or mass of bulky permeable material. Thermosetting resin, dry powder and sand are mixed thoroughly in a Muller. Complete shell molding casting processes is carried in four stages as shown in Fig. 3.3. In this process a pattern is placed on a metal plate and it is then coated with a mixture of fine sand and Phenol-resin (20:1). The pattern is heated first and silicon grease is then sprayed on the heated metal pattern for easy separation. The pattern is heated to 205 to 230°C and covered with resin bounded sand. After 30 seconds, a hard layer of sand is formed over pattern. Pattern and shell are heated and treated in an oven at 315°C for 60 secs., carried in four stages as shown in Fig. 3.4. In this process a pattern is placed on a metal plate and it is then coated with a mixture of fine sand and Phenol-resin (20:1). The pattern is heated first and silicon grease is then sprayed on the heated metal pattern for easy separation. The pattern is heated to 205 to 36 230°C and covered with resin bounded sand. After 30 seconds, a hard layer of sand is formed over pattern. Pattern and shell are heated and treated in an oven at 315°C for 60 secs. Fig. 3.3 Shell mold casting process Phenol resin is allowed to set to a specific thickness. So the layer of about 4 to 10 mm in thickness is stuck on the pattern and the loose material is then removed from the pattern. Then shell is ready to strip from the pattern. A plate pattern is made in two or more pieces and similarly core is made by same technique. The shells are clamped and usually embedded in gravel, coarse sand or metal shot. Then mold is ready for pouring. The shell so formed has the shape of pattern formed of cavity or projection in the shell. In case of unsymmetrical shapes, two patterns are prepared so that two shell are produced which are joined to form proper cavity. Internal cavity can be formed by placing a core. Hot pattern and box is containing a mixture of sand and resin. Pattern and box inverted and kept in this position for some time. Now box and pattern are brought to original position. A shell of resin-bonded sand sticks to the pattern and the rest falls. Shell separates from the pattern with the help of ejector pins. It is a suitable process for casting thin walled articles. The cast shapes are uniform and their dimensions are within close limit of tolerance ± 0.002 mm and it is suitable for precise duplication of exact parts. It has various advantages which are as follows. There are some advantages and disadvantages of this process which are given as under. 37  Advantages The main advantages of shell molding are: Applications (i) Suitable for production of casting made up of alloys of Al, Cu and ferrous metals (ii) Bushing (iii) Valves bodies (iv) Rocker arms (v) Bearing caps (vi) Brackets (vii) Gears 3.8 CENTRIFUGAL CASTING In centrifugal casting process, molten metal is poured into a revolving mold and allowed to solidify molten metal by pressure of centrifugal force. It is employed for mass production of circular casting as the castings produced by this process are free from impurities. Due to centrifugal force, the castings produced will be of high density type and of good strength. The castings produced promote directional solidification as the colder metal (less temperature molten metal) is thrown to outside of casting and molten metal near the axis or rotation. The cylindrical parts and pipes for handling gases are most adoptable to this process. Centrifugal casting processes are mainly of three types which are discussed as under 1- True centrifugal casting 2- Semi-centrifugal casting and 38  3- Centrifuged casting True Centrifugal Casting In true centrifugal casting process, the axis of rotation of mold can be horizontal, vertical or inclined. Usually it is horizontal. The most commonly articles which are produced by this process are cast iron pipes, liners, bushes and cylinder barrels. This process does not require any core. Also no gates and risers are used. Generally pipes are made by the method of the centrifugal casting. The two processes namely De Lavaud casting process and Moore casting process are commonly used in true centrifugal casting. The same are discussed as under: De Levaud Casting Process Fig 3.4 shows the essential components of De Levaud type true centrifugal casting process. The article produced by this process is shown in Fig 13.6. In this process, metal molds prove to be economical when large numbers of castings are produced. This process makes use of metal mold. The process setup contains an accurately machined metal mold or die surrounded by cooling water. The machine is mounted on wheels and it can be move lengthwise on a straight on a slightly inclined track. At one end of the track there is a ladle containing proper quantities of molten metal which flows a long pouring spout initially inserted to the extremity of the mold. As pouring proceeds the rotating mold, in the casting machine is moved slowly down the track so that the metal is laid progressively along the length of the mold wall flowing a helical path. The control is being achieved by synchronizing the rate of pouring, mold travel and speed of mold rotation. After completion of pouring the machine will be at the lower end of its track with the mold that rotating continuously till the molten metal has solidified in form of a pipe. The solidified casting in form of pipe is extracted from the metal mold by inserting a pipe puller which expands as it is pulled. 39  Fig. 3.4 De Levaud type true centrifugal casting process. Moore Casting System Moore casting system for small production of large cast iron pipes employs a ram and dried sand lining in conjunction with end pouring. As the mold rotates, it does not move lengthwise rather its one end can be raised up or lowered to facilitate progressive liquid metal. Initially one end of the mold is raised as that mold axis gets inclined. As the pouring starts and continues, the end is gradually lowered till the mold is horizontal and when the pouring stops. At this stage, the speed of mold rotation is increased and maintained till the casting is solidified. Finally, the mold rotation is stopped and the casting is extracted from the mold. Semi-Centrifugal Casting It is similar to true centrifugal casting but only with a difference that a central core is used to form the inner surface. Semi- centrifugal casting setup is shown in Fig. 3.5. This casting process is generally used for articles which are more complicated than those possible in true centrifugal casting, but are axi- symmetric in nature. A particular shape of the casting is produced by mold and core and not by centrifugal force. The centrifugal force aids proper feeding and helps in producing the castings free from porosity. The article produced by this process is shown in Fig. 3.6. Symmetrical objects namely wheel having arms like flywheel, gears and back wheels are produced by this process. 40  Fig3.5 Semi-centrifugal casting setup Fig. 3.6 Article produced by semi centrifugal casting process Centrifuging Casting Centrifuging casting setup is shown in Fig. 3.5. This casting process is generally used for producing non-symmetrical small castings having intricate details. A number of such small jobs are joined together by means of a common radial runner with a central sprue on a table which is possible in a vertical direction of mold rotation. The sample article produced by this process is depicted in Fig. 3.6. 3.9 CONTINNOUS CASTING In this process the molten metal is continuously poured in to a mold cavity around which a facility for quick cooling the molten metal to the point of solidification. The solidified metal is then continuously extracted from the mold at predetermined rate. This process is classified into two categories namely Asarco and Reciprocating. In reciprocating process, molten metal is poured into a holding furnace. At the bottom of this furnace, there is a valve by which the quantity of flow can be changed. The molten metal is poured into the mold at a uniform speed. The water cooled mold is reciprocated up and down. The solidified portion of the casting is withdrawn by the rolls at a constant speed. The movement of the rolls and the reciprocating motion of the rolls are fully mechanized and properly controlled by means of cams and follower arrangements. 41  Advantages of Continuous Casting (i) The process is cheaper than rolling (ii) 100% casting yield. (iii) The process can be easily mechanized and thus unit labor cost is less. (iv) Casting surfaces are better. (v) Grain size and structure of the casting can be easily controlled. Applications of Continuous Casting (i) It is used for casting materials such as brass, bronzes, zinc, copper, aluminum and its alloys, magnesium, carbon and alloys etc. (ii) Production of blooms, billets, slabs, sheets, copper bar etc. (iii) It can produce any shape of uniform cross-section such as round, rectangular, square, hexagonal, fluted or gear toothed etc. 3.10 PLASTICS MOLDING PROCESSES There are various methods of producing components from the plastics materials which are supplied in the granular, powder and other forms. Various plastics molding processes are: 1- Compression Molding. 2- Transfer Molding 3- Injection Molding. 4- Blow Molding. 5- Extrusion Molding 6- Calendaring. 7- Thermoforming. 8- Casting Two major processes from the above are discussed as under. 42 3.11 Injection die Molding In this process, thermoplastic materials soften when heated and re-harden when cooled. No chemical change takes place during heating and cooling. As illustrates the injection molding process. The process involves granular molding material is loaded into a hopper from where it is metered out in a heating cylinder by a feeding device. The exact amount of material is delivered to the cylinder which is required to fill the mold completely. The injection ram pushes the material into a heating cylinder and doing so pushing bushes a small amount of heated material out of other end of cylinder through the nozzle and screw bushing and into cavities of the closed mold. The metal cooled in rigid state in the mold. Then mold is opened and piece is ejected out material heating temperature is usually between 180°-280°C. Mold is cooled in order to cool the mold articles. Automatic devices are commercially available to maintain mold temperature at required level. Injection molding is generally limited to forming thermoplastic materials, but equipment is available for converting the machines for molding thermosetting plastics and compounds of rubber 43 FUNDAMENTALS OF METAL FORMING 44 4.1 INTRODUCTION Metal forming includes a large group of manufacturing processes in which plastic deformation is used to change the shape of metal workpieces. Deformation results from the use of a tool, usually called a die in metal forming, which applies stresses that exceed the yield strength of the metal. The metal therefore deforms to take a shape determined by the geometry of the die. Metal forming dominates the class of shaping operations identified in Chapter 2 as the deformation processes (Fig. 2.1). Stresses applied to plastically deform the metal are usually compressive. However, some forming processes stretch the metal, while others bend the metal, and still others apply shear stresses to the metal. To be successfully formed, a metal must possess certain properties. Desirable properties include low yield strength and high ductility. These properties are affected by temperature. Ductility is increased and yield strength is reduced when work temperature is raised. The effect of temperature gives rise to distinctions between cold working, warm working, and hot working. Strain rate and friction are additional factors that affect performance in metal forming. We examine all of the reissues in this chapter, but first let us provide an overview of the metal forming processes. 4.2 OVERVIEW OF METAL FORMING Metal forming processes can be classified into two basic categories: bulk deformation processes and sheet metalworking processes. Each category includes several major classes of shaping operations, as indicated in Fig. 4.1 44  Fig. 4.1. Classification of metal forming operations. Bulk Deformation Processes Bulk deformation processes are generally characterized by significant deformations and massive shape changes, and the surface area-to-volume of the work is relatively small. The term bulk describes the workparts that have this low are at volume ratio. Starting work shapes for these processes include cylindrical billets and rectangular bars. Fig. 4.2 illustrates the following basic operations in bulk deformation: Fig 4.2. Deformation processes: (a) rolling, (b) forging, (c) extrusion, and (d) drawing. Relative motion in the operations is indicated by v; forces are indicated by F 44 Rolling. This is a compressive deformation process in which the thickness of a slab or plate is reduced by two opposing cylindrical tools called rolls. The rolls rotate so as to draw the work into the gap between them and squeeze it. Forging. In forging, a workpiece is compressed between two opposing dies, so that the die shapes are imparted to the work. Forging is traditionally a hot working process, but many types of forging are performed cold. Extrusion. This is a compression process in which the work metal is forced to flow through a die opening, thereby taking the shape of the opening as its own cross section. Drawing. In this forming process, the diameter of a round wire or bar is reduced by pulling it through a die opening. Sheet Metalworking Sheet metalworking processes are forming and cutting operations performed on metal sheets, strips, and coils. The surface area-to-volume ratio of the starting metal is high; thus, this ratio is a useful means to distinguish bulk deformation from sheet metal processes. Press working is the term often applied to sheet metal operations because the machines used to perform these operations are presses (presses of various types are also used in other manufacturing processes). A part produced in a sheet metal operation is often called a stamping. Sheet metal operations are always performed as cold working processes and are usually accomplished using a set of tools called a punch and die. The punch is the positive portion and the die is the negative portion of the tool set. The basic sheet metal operations are sketched in Fig. 4.3 and are defined as follows: Bending. Bending involves straining of a metal sheet or plate to take an angle along a (usually) straight axis. 44 Drawing. In sheet metalworking, drawing refers to the forming of a flat metal sheet into a hollow or concave shape, such as a cup, by stretching the metal. A blank holder is used to hold down the blank while the punch pushes into the sheet metal, as shown in Fig. 4.3(b). To distinguish this operation from bar and wire drawing, the terms cup drawing or deep drawing are often used. Shearing. This process seems somewhat out-of-place in a list of deformation processes, because it involves cutting rather than forming. A shearing operation cuts the work using a punch and die, as in Fig. 4.3(c). Although it is not a forming process, it is included here because it is a necessary and very common operation in sheet metalworking. Fig 4.3. Basic sheet metalworking operations: (a) bending, (b) drawing, and (c) shearing: (1) as punch first contacts sheet, and (2) after cutting. Force and relative motion in these operations are indicated by F and v. The miscellaneous processes within the sheet metalworking classification in Fig. 4.1 include a variety of related shaping processes that do not use punch and die tooling. Examples of these processes are stretch forming, roll bending, spinning, and bending of tube stock. 44 4.3. MATERIAL BEHAVIOR IN METAL FORMING Considerable insight about the behavior of metals during forming can be obtained from the stress–strain curve. The typical stress–strain curve for most metals is divided into an elastic region and a plastic region. In metal forming, the plastic region is of primary interest because the material is plastically and permanently deformed in these processes. The typical stress strain relationship for a metal exhibits elasticity below the yield point and strain hardening above it. logarithmic axes. In the plastic region, the metal’s behavior is expressed by the flow curve: Equ.4.1 where K = the strength coefficient, MPa (lb/in2 ); and n is the strain-hardening exponent. The stress s and strain e in the flow curve are true stress and true strain. The flow curve is generally valid as a relationship that defines a metal’s plastic behavior in cold working. Typical values of K and n for different metals at room temperature are listed in Table 4.1. Table 4.1 Typical values of strength coefficient K and strain hardening exponent n for selected metals. Flow Stress The flow curve describes the stress–strain relationship in the region in which metal forming takes place. It indicates the flow stress of the metal the strength property that determines forces and power required to accomplish a particular forming operation. For most metals at room temperature, the stress–strain plot of Fig. 4.4 indicates that as the metal is deformed, its strength increases due to strain hardening. The stress required to continue deformation must be increased to match this increase in strength. Flow stress is defined as the instantaneous value of stress required to continue 44 deforming the material to keep the metal ‘‘flowing.’’ It is the yield strength of the metal as a function of strain, which can be expressed: Fig. 4.4 True stress–strain curve plotted on log–log scale. Equ.4.2 where Yf =flow stress, MPa (lb/in2 ). In the individual forming operations discussed in the following two chapters, the instantaneous flow stress can be used to analyze the process as it is occurring. For example, in certain forging operations, the instantaneous force during compression can be determined from the flow stress value. Maximum force can be calculated based on the flow stress that results from the final strain at the end of the forging stroke. In other cases, the analysis is based on the average stresses and strains that occur during deformation rather than instantaneous values. Extrusion represents this case, Fig.4.2(c). As the billet is reduced in cross section to pass through the extrusion die opening, the metal gradually strain hardens to reach a maximum value. Rather than determine a sequence of instantaneous stress–strain values during the reduction, which would be not only difficult but also of limited interest, it is more useful to analyze the process based on the average flow stress during deformation. 45  Fig. 4.5 Stress–strain curve indicating location of average flow stress Yf in relation to yield strength Y and final flow stress Yf. Average Flow Stress The average flow stress (also called the mean flow stress) is the average value of stress over the stress–strain curve from the beginning of strain to the final (maximum) value that occurs during deformation. The value is illustrated in the stress– strain plot of Fig. 4.4. The average flow stress is determined by integrating the flow curve equation, Equ. 4.2, between zero and the final strain value defining the range of interest. This yields the equation: Equ.4.3 where Yf = average flow stress, MPa (lb/in2 ); and ϵ = maximum strain value during the deformation process. We make extensive use of the average flow stress in our study of the bulk deformation processes in the following chapter. Given values of K and n for the work material, a method of computing final strain will be developed for each process. Based on this strain, Equ. 4.3 can be used to determine the average flow stress to which the metal is subjected during the operation. 45 TEMPERATURE IN METAL FORMING The flow curve is a valid representation of stress–strain behavior of a metal during plastic deformation, particularly for cold working operations. For any metal, the values of K and n depend on temperature. Strength and strain hardening are both reduced at higher temperatures. These property changes are important because they result in lower forces and power during forming. In addition, ductility is increased at higher temperatures, which allows greater plastic deformation of the work metal. We can distinguish three temperature ranges that are used in metal forming: cold, warm, and hot working. Cold Working Cold working (also known as cold forming) is metal forming performed at room temperature or slightly above. Significant advantages of cold forming compared to hot working are (1) greater accuracy, meaning closer tolerances can be achieved; (2) better surface finish; (3) higher strength and hardness of the part due to strain hardening; (4) grain flow during deformation provides the opportunity for desirable directional properties to be obtained in the resulting product; and (5) no heating of the work is required, which saves on furnace and fuel costs and permits higher production rates. Owing to this combination of advantages, many cold forming processes have become important mass-production operations. They provide close tolerances and good surfaces, minimizing the amount of machining required so that these operations can be classified as net shape or near net shape processes There are certain disadvantages or limitations associated with cold forming operations: (1) higher forces and power are required to perform the operation; (2) care must be taken to ensure that the surfaces of the starting workpiece are free of scale and dirt; and (3) ductility and strain hardening of the work metal limit the amount of forming that can be done to the part. In some operations, the metal must be annealed in order to allow further deformation to be accomplished. In other cases, the metal is simply not ductile enough to be cold worked. To overcome the strain-hardening problem and reduce force and power requirements, many forming operations are performed at elevated temperatures. There are two elevated temperature ranges involved, giving rise to the terms warm working and hot working Warm Working Because plastic deformation properties are normally enhanced by increasing workpiece temperature, forming operations are sometimes performed at temperatures somewhat above room temperature but below the recrystallization temperature. The term warm working is applied to this second temperature range. The dividing line between cold working and warm working is often expressed in terms of the melting point for the metal. The dividing line is usually taken to be 0.3 T m, where Tm is the melting point (absolute temperature) for the particular metal. 45  The lower strength and strain hardening at the intermediate temperatures, as well as higher ductility, provide warm working with the following advantages over cold working: (1) lower forces and power, (2) more intricate work geometries possible, and (3) need for annealing may be reduced or eliminated. Hot Working Hot working (also called hot forming) involves deformation at temperatures above the recrystallization temperature. The recrystallization temperature for a given metal is about one-half of its melting point on the absolute scale. In practice, hot working is usually carried out at temperatures somewhat above 0.5Tm. The work metal continues to soften as temperature is increased beyond 0.5Tm, thus enhancing the advantage of hot working above this level. However, the deformation process itself generates heat, which increases work temperatures in localized regions of the part. This can cause melting in these regions, which is highly undesirable. Also, scale on the work surface is accelerated at higher temperatures. Accordingly, hot working temperatures are usually maintained within the range 0.5Tm to 0.75Tm. The most significant advantage of hot working is the capability to produce substantial plastic deformation of the metal far more than is possible with cold working or warm working. The principal reason for this is that the flow curve of the hot-worked metal has a strength coefficient that is substantially less than at room temperature, the strain-hardening exponent is zero (at least theoretically), and the ductility of the metal is significantly increased. All of this results in the following advantages relative to cold working: (1) the shape of the work part can be significantly altered, (2) lower forces and power are required to deform the metal, (3) metals that usually fracture in cold working can be hot formed, (4) strength properties are generally isotropic because of the absence of the oriented grain structure typically created in cold working, and (5) no strengthening of the part occurs from work hardening. This last advantage may seem inconsistent, since strengthening of the metal is often considered an advantage for cold working. However, there are applications in which it is undesirable for the metal to be work hardened because it reduces ductility, for example, if the part is to be subsequently processed by cold forming. Disadvantages of hot working include (1) lower dimensional accuracy, (2) higher total energy required (due to the thermal energy to heat the workpiece), (3) work surface oxidation (scale), (4) poorer surface finish, and (5) shorter tool life. Recrystallization of the metal in hot working involves atomic diffusion, which is a time- dependent process. Metal forming operations are often performed at high speeds that do not allow sufficient time for complete recrystallization of the grain structure during the deformation cycle itself. However, because of the high temperatures, recrystallization eventually does occur. It may occur immediately following the forming process or later, as the workpiece cools. Even though recrystallization may occur after the actual deformation, its eventual occurrence, and the substantial softening of the metal at high temperatures, is the features that distinguish hot workin