Metal Forming Processes PDF

METAL FORMING PROCESSES 145 14. Surface finish is good. Surface finish is not so good due to oxidation at high tempe...

METAL FORMING PROCESSES 145 14. Surface finish is good. Surface finish is not so good due to oxidation at high temperatures. 15. It is easy to control the dimensions within the It is difficult to control the dimensions because of tolerance limit. contraction occurring during cooling. 16. Ordinary steels can be used for shaping and hence Alloy steels are necessary for shaping and hence the cost of the cold working plant is less. the cost of the hot working plant is high. 17. Handling of materials is easy. Handling of materials is difficult. 3.3. ROLLING Rolling is a forming operation on cylindrical rolls wherein cross-sectional area of a bar or plate is reduced with a corresponding increase in length. The metal is thinned and elongated by compression and shear forces but increased in width only slightly. Because of the high surface finish maintained on the rolls, the surface of stock is burnished by the rolling action and attains a smooth bright finish. This process is one of the most widely used of all the metal working processes, because of its high productivity and low cost. Rolling would be able to produce components having constant cross-section throughout its length. Many shapes such as I, T, L and channel sections are possi- ble, but not very complex shapes. It is also possible to produce special sections such as railway wagon wheels by rolling individual pieces. Rolling is normally a hot working process unless specifically mentioned as cold rolling. 3.3.1. Principle and Mechanism of Rolling — The process is illustrated in Fig. 3.1. The rolls are in contact with the passing metal piece over a sufficient distance, represented by the arc LM. The angle LOM subtended at the centre of the roll by the arc LM is called the ‘angle of contact’ or the ‘maximum angle of bite’. It is the friction between the surfaces of the metal piece and the rolls which provides the required grip of rolls over the metal piece to draw the latter through them. The greater the coefficient of friction more the possible reduction. — The pressure exerted over the metal by the roller is not uniform throughout, it is mini- mum at both the extremities L and M and maximum at a point, known as no-slip point or the point of maximum pressure. At this point the surfaces of the metal and the roll move at the same speed. Before reaching this point, i.e., from L to S the metal moves slower than the roll and the frictional force acts in the direction to draw the metal piece into the rolls. After crossing the neutral point S, i.e., from S to M, the metal moves faster than the roll surface, as if it is being extruded, and the friction opposes the travel tending to hold the metal track. This results in setting up of stresses within the metal to obstruct its reduction. Refer to Fig. 3.1. Let ti, li, bi and tf, lf and bf be the initial and final thickness, lengths and breadths of the metal piece respectively. Then, Absolute draft, δt = (ti – tf) mm Absolute elongation, δl = (lf – li) mm Absolute spread, δb = (bf – bi) mm — Spread is proportional to the draft and depends upon the thickness and width of the job. Spread increases with increase in roll diameter and co-efficient of friction, as well as with a fall in temperature of the metal in course of hot rolling. 146 A TEXTBOOK OF MANUFACTURING TECHNOLOGY Vr Roll (Top) Metal piece O a L S Neutral plane P M ti Vi mP Vf tf P mP M S a Angle of bite O Roll (Bottom) bi bf Fig. 3.1. Metal rolling process. δt F ti − t f I × 100 Relative draft, Rt = ti = GH ti JK lf Elongation co-efficient, γ=. li l At the moment of bite, two forces act on the metal from the side of each roll, radial or normal force P and the tangential forced frictional force µP where µ is the co-efficient of friction between the metal and roll surfaces. The part would be dragged in if the result- ant of horizontal component of the normal force P and tangential force µP is directed in that direction. In the limiting case, P sin α = µP cos α...(3.1) or, µ = tan α...(3.1 (a)) or, α = tan–1 µ...(3.1 (b)) When α > tan–1 µ, the metal would not enter the space between the rolls automatically, that is unaided. The maximum permissible angle of bite (or contact) depends upon the value of ‘µ’ which in turn depends upon : — Materials of rolls ; — Job being rolled ; — Roughness of their surfaces ; — Rolling temperature ; — Speed. METAL FORMING PROCESSES 147 l In hot rolling, the value of α and hence of µ should be greater since the maximum possible reduction is desired. Usually in hot rolling lubrication is not necessary. — In case of primary reduction rolling mills such as blooming or rough rolling mills for structural elements the rolls may sometimes be “ragged” to increase µ (Ragging is the process of making certain fine grooves on the surface of the roll to increase the friction). In cold rolling, since the rolling loads are very high µ should not be much. Rolls for cold rolling are ground and lubricants are employed to reduce µ. The usual values of biting angles are : 2°–10°...... For cold rolling of oiled sheet and strip ; 15°–20°...... For hot rolling of sheet and strip ; 24°–30°...... For hot rolling of heavy billets and blooms. l The volume of metal that enters the rolling stand should be the same as that leaving it except in initial passes when there might be some loss due to filling of voids and cavities in the ingots. Since the area of the cross-section gets decreased, the metal leaving the rolls would be at a higher velocity than when it entered. Initially when the metal enters the rolls, the surface speed of rolls is higher than that of the incoming metal, whereas the metal velocity at the exit is higher than that of surface speed of the rolls. Between the entrance and exit, the velocity of the metal is continuously changing, whereas the roll velocity remains constant. Vf − Vr Now, Forward slip = × 100 Vr Vr − Vi Backward slip = × 100 Vr where, Vi = Initial metal speed, Vf = Final metal speed, and Vr = Surface/peripheral speed of rolls. Ai − Af % age of cold work = × 100 Ai tibi − tf bf = × 100 ti bi ti − t f = × 100 (∵ bi ≅ bf) ti where Ai and Af are the initial and final areas of cross-section. Process variables in rolling process : The main process variables in rolling process are : (i) Diameter of roll. (ii) Angle of bite. (iii) Speed of rolling. (iv) Strength of work material. 148 A TEXTBOOK OF MANUFACTURING TECHNOLOGY (v) Temperature. (vi) Roll gap or draft. (vii) Co-efficient of friction. (viii) Dimensions of sheet. The rolling load (P) can be calculated as : P = l. b. pm where, l = Roll-strip contact length, b = Breadth of sheet, and pm = Mean specific pressure. Since l depends on roll diameter and angle of bite, it is approximately given as : l≅ R. δt...(3.2) where, R = Roll radius and, δt = draft = ti – tf pm depends on R, ti, tf, and yield strength of work material. l Hot rolling is carried out to roll ingots into slabs, blooms or billets. On further hot rolling, plates, bars, rounds, structural shapes and rails are obtained. Because of limita- tions in equipment and workability of metals, rolling is done in progressive steps, that is, a number of passes through the rolls may be required to get the required configura- tion. For example, ten roll passes are required to get a 100 mm × 100 mm billet reduced to a 12 mm rod (Fig. 3.2). The initial few passes are designed to merely reduce the cross- sectional area, while the intermediate passes not only reduce the area but also try to bring the shape close to the final shape. Final or finishing passes bring the material to the required shape and size. 1 2 100 × 100 mm billet 3 4 5 6 7 8 9 10 Fig. 3.2. Roll passes to get a 12 mm rod from 100 × 100 mm billet. l Cold rolling is employed to finish bars, rods, sheets and strips of most common metals because this process provides better dimensional accuracy, good surface finish and improved physical properties. Roll materials commonly used are cast iron, cast steel and/or forged steel because of high strength and high wear resistance requirements. l Fig. 3.3 shows the effect of both cold working and hot working on the microstructure of cast metals. METAL FORMING PROCESSES 149 Elongated Beginning of recrystallization grains Recovery Recrystallization completed Cold formed Low heat High heat Undeformed Beginning of recrystallization metal Recrystallization completed Grains Grain growth elongated (reheated) Hot formed Reheated Fig. 3.3. Effect of both cold working and hot working on the microstructure of cast metals. 3.3.2. Rolling Stand Arrangement The arrangement of rolls in a rolling mill, also called rolling stand, varies depending on the application. The names of the rolling stand arrangements are given by the number of rolls em- ployed. The various possible configurations are presented in Fig. 3.4 to Fig. 3.8. (a) Two-high (b) Two-high reversible Fig. 3.4. Two-high rolls. 150 A TEXTBOOK OF MANUFACTURING TECHNOLOGY Top roll Back-up roll Working roll Centre roll Bottom roll Fig. 3.5. Three-high rolls. Fig. 3.6. Four-high rolls. Back-up rolls Working roll Pinch rolls to feed Fig. 3.7. Cluster roll. Fig. 3.8. Planetary mill. 1. Two-high rolls : l Both the rolls rotate in opposite directions to one another as shown in Fig. [3.4 (a)]. Their direction of rotation is fixed and cannot be reversed. Thus, the work can be rolled by feeding from one direction only. — The space between the rolls can be adjusted by raising or lowering the upper roll. The position of the lower roll is fixed. l There is another type of two-high mill [Fig. 3.4(b)] which incorporates a drive mecha- nism that can reverse the direction of rotation of the rolls. This facilitates rolling of the workpiece continuously through back-and-forth passes between the rolls. This type of rolling mill is known as two-high reversing mill. These stands are more expensive compared to the non-reversible type because of the reversible drive needed. 2. Three-high rolls : Refer to Fig. 3.5. l This stand arrangement is used for rolling of two continuous passes in a rolling sequence without reversing the drives. After all the metal has passed through the bottom roll set, the end of the metal is entered into the other set of the rolls for the next pass. For this purpose a table-tilting arrangement is required to bring the metal to the level with the rolls. l This arrangement may be used for blooming, billet rolling or finish rolling. 3. Four-high rolls : Refer to Fig. 3.6. l This rolling stand is essentially a two-high rolling mill, but with small-sized rolls. The other two rolls are the back-up rolls for providing the necessary rigidity to the small rolls. l These mills are generally employed for subsequent rolling of slabs. The common products of these mills are hot or cold rolled sheets and plates. METAL FORMING PROCESSES 151 4. Cluster rolls : Refer to Fig. 3.7. l It consists of two working rolls of smaller diameter and four or more back-up rolls of larger diameter. The number of back-up rolls may go as high as 20 or more, depending upon the amount of support needed for the working rolls during the operation. l This type of mill is generally used for cold rolling. 5. Planetary mill. For the rolling arrangements requiring large reduction, a number of free rotating wheels instead of a single small roll, are fixed to a large back-up roll in the planetary rolling mill arrangement shown in Fig. 3.8. 3.3.3. Defects in Rolling The various defects in rolling process are enumerated and discussed below : 1. Surface defects. 2. Structional defects. 1. Surface defects : These defects may result from : — Inclusions and impurities in the material ; — Scale, rust, dirt ; — Roll marks ; — Other causes related to the prior treatment and working of the material. In hot rolling blooms, billets, and slabs, the surface is usually preconditioned by various means, such as by torch (scarfing), to remove scale. 2. Structural defects : These defects distort or affect the integrity of the rolled product. Rolling direction (i) Wavy edges (ii) Zipper cracks (iii) Edge cracks (iv) Alligatoring Fig. 3.9. Typical defects in flat rolling. (i) Wavy edges. Refer to Fig. 3.9 (i). These are caused by bending of the rolls ; the edges of the strip are thinner than the centre. Because the edges elongate more than the centre and are restrained from expanding freely, they buckle. (ii), (iii) Zipper cracks and edge cracks : Refer to Fig. 3.9 (ii), (iii). Zipper cracks in the centre of strip and edge cracks are usually caused by low ductility and barreling. (iv) Alligatoring. Refer to Fig. 3.9 (iv). Alligatoring is a complex phenomenon that results from inhomogeneous deformation of the material during rolling or from defects in the original cast ingot, such as piping. Note : “Residual stresses” can be generated in rolled sheets and plates because of inhomogeneous plastic deformation in the roll gap. — Small-diameter rolls or small reductions tend to work the metal plastically at its surfaces. This working generates compressive residual stresses on the surfaces and tensile stresses in the bulk. — Large-diameter rolls and high reductions, however, tend to deform the bulk to a greater extent than the surfaces, because of frictional constraint at the surfaces along the arc of contact. This situation generates residual stresses that are opposite to those of the previous case. 152 A TEXTBOOK OF MANUFACTURING TECHNOLOGY 3.4. FORGING 3.4.1. Introduction Forging is the process by which heated metal is shaped by the application of sudden blows or steady pressure and characteristics of plasticity of material are made use of. l Forging can be carried out at room temperature (cold working), or at elevated tempera- tures, a process called warm or hot forging, depending on the temperature. — Simple forgings can be made with a heavy hammer and an anvil by techniques used by blacksmiths for centuries. Usually, though, a set of dies and a press are required. The three basic categories of forging are open die, impression die, and closed die. l Typical parts made by forging today are : — Crankshafts and connecting rods for engines ; — Turbine discs, gears, wheels, bolt heads, hand tools ; — Many types of structural components for machinery and transformation equipment. 3.4.2. Advantages and Disadvantages of Forging Following are the advantages and disadvantages of forging process : Advantages : 1. Forging improves the structure of metal and hence its mechanical properties. 2. Forging distorts the previously created uni-directional fibre in such a way as to strengthen the component. 3. Owing to intense working, flaws are seldom found and the workpiece has a high reliability. 4. Forgings are easily welded. 5. Rapid duplication of components. 6. Ability of the forging to withstand unpredictable loads. 7. Metal removing in machining is minimum. 8. The forging can withstand unpredictable loads. 9. The surface of the forging is relatively smooth. 10. Superior machining qualities. 11. Minimum weight per unit strength and better resistance to shock. 12. Forgings can be held to within fairly close tolerances. Disadvantages : 1. The initial cost of dies and the cost of their maintenance is high. 2. In hot forging, due to high temperature of metal, there is rapid oxidation or scaling of the surface resulting in poor surface finish. 3. Forging operation is limited to simple shapes and has limitations for parts having under- cuts, re-entrant surfaces, etc. 4. Forgings are usually costlier than castings. 3.4.3. Classification of Forging Forging can be classified in two ways : 1. Hand forging. 2. Machine forging. Hand Forging :
Hand forging or blacksmithing is employed for small quantity production and for special work. METAL FORMING PROCESSES 153 Generally speaking, the accuracy obtained is less than that of drop forging. Chimney In hand forging the metal is heated in a Smith’s forge or hearth (Fig. 3.10). It consists of a hearth for holding the fuel, a cast iron tuyere for supplying air blast to the fire, a centrifugal blower driven by a power Tuyere or preferably electric motor, to produce the blast, a chim- nozzle Water tank ney to carry the smoke and poisonous gases to air, a Fire brick water tank behind the hearth to water cool the tuyere, a cool bunker to stock coal or coke, a water trough in front for quenching cutting tools and an air valve to control the blast. In operation, the work is paced in Air the fire pot and heated to the proper temperature for valve Water forging. Cool bunker trough Fig. 3.11 shows the various tools used in Fig. 3.10. Smith’s forge. smithy. The list of important smithy tools and their uses are given below : Top Straight peen Bottom Cross peen Sledge Swages Flatter Jaws Handles Ball peen Flat tong Set hammer Clip tong Square bit Cold Pick-up tong Work set Punch Hardie Hot set Gouge Hordie Fuller hole Anvil face Pritchel hole Anvil horn Body Body Swage Block Anvil Fig. 3.11. Tools used in smithy. 154 A TEXTBOOK OF MANUFACTURING TECHNOLOGY Tools used in smithy Name of tool Use 1. Sledge hammers, straight, flat and — To forge big jobs (heavy work). cross peen 2. Smith’s ball peen hammer — To forge light and medium work. 3. Tongs, flat or square bit pick up — To hold the hot work. tong. 4. Chisel long cold set — To cut cold metal. 5. Hot set — To cut hot metal. 6. Fullers, top and bottom — To shape inside curves. To form corrugations for elongating metal. 7. Swages, top and bottom — To shape convex surfaces and to give finish to round, square, hexagonal or octagonal shaped sections. 8. Flatter or flattener — To give smooth finish to flat surfaces. 9. Set hammer — To form square shoulders and to clean the rounding in corners. 10. Punches — To make recesses of any shape in hot metal. 11. Hardie — To nick the bar and to shape the cold work. 12. Anvil — To forge art, bend and shape the work. 13. Swage block — To shape or bend the work to any form and to knock heads of bolts, etc. 14. Gouge — To cut plates to curves. Forging on anvil is usually done with : (i) one man or (ii) two men-two handed working—the smith and his striker. The former uses a small hammer, the latter the sledge. To indicate, where he requires his mate to strike a blow the smith lightly taps the work with the small hammer ; the striker’s job is to hit the spot with the sledge. If working three handed the same procedure is followed, a ligh tap from the smith preceding a heavy one from the striker. To indicate when to finish, the smith taps the anvil with his hammer. Upsetting. It (Fig. 3.12) is the process of increasing cross- sectional dimensions when forging. The process implies that cross- section is increased and the length decreases. It may be done in a number of ways, each varying according to the details of the article Fig. 3.12. Upsetting. required and the equipment in the shop. The simplest is to place the heated article on the anvil and hammer directly on the upper end. This increases the cross-section and reduces the length of the metal being worked. Drawing down. It is the process of increasing the length of a bar at the expense of its cross-sectional area. It is illustrated in Fig. 3.13. Setting down is a localised drawing down or swaging Fig. 3.13. Drawing down. operation. Punching is the process of removing a slug of metal, METAL FORMING PROCESSES 155 generally cylindrical, by using a hot punch over the pritchel hole of the anvil, over a hole of correct size in the swage block. Cutting out. It is the process of cutting large holes of various shapes by using a hot chisel over a hole in the sewage block. Forging Machines : A forging machine is one which is designed to shape a metal article while the material is in hot plastic state. The term forging machine in its widest sense includes : 1. Drop stamp (whether of rope, belt or board type). 2. Steam hammer. 3. Pneumatic hammer. 4. Hydraulic hammer. 1. Drop stamp. The drop stamp of board type (Fig. 3.14) is widely employed when shaping hot bars, and finally to bring the work to size and shape between a set of drop stamping dies. In board hammer the tup is attached to a board which passes between two rollers. The latter run in an over head attachment, are belt-driven and run in opposite directions. The tup is lifted by means of eccentric (foot, or hand operated or self acting) and they (eccentrics) cause the rollers to grip or release the board, when the board is gripped by the rollers their direction of rotation is such as to lift it (board) and the attached tup, when the board is released the tup falls with it. The height of lift depends upon the timing of release, which is instantaneous. When producing small drop forgings or hot pressings the drop stamps in its various forms is a very effective method Fig. 3.14. Drop stamp of board type. of obtaining the desired results. For shallow sheet metal Steam in work drop stamp is first class production machine as it per- to forge mits a solid blow to be struck without any fear of bending a Piston crank or breaking a press frame. 2. Steam hammer. A steam hammer (Fig. 3.15) Steam in operates on the principle of the steam engine. The main parts to lift ram are frame, a steam chest or cylinder, piston, piston rod and the anvil. The hammer head is attached to the piston rod Steel rod and is raised by admitting steam in the cylinder through the valve beneath the piston. The downward stroke of the Ram hammer is obtained by exhausting the steam from beneath Die the piston and admitting from above the piston. The ham- mer descends by gravity and steam pressure is 5.5 to 8.5 Work bar. For varying the intensity of the hammer blows, light to Die heavy, steam is admitted below the piston while the ham- Stationary mer is descending to create cushioning to the falling ham- anvil mer. The steam inlet and outlet are controlled by a special slide valve. For generating steam a boiler is required. A wide Fig. 3.15. Steam hammer. range of work is done on this class of forging machine. 156 A TEXTBOOK OF MANUFACTURING TECHNOLOGY 3. Pneumatic hammer. In pneumatic Air pump hammer (Fig. 3.16) air is compressed on both up- Ram cylinder ward and downward strokes of the piston which is worked by the electric motor. This compressed air cylinder is supplied to the ram cylinder by the long valve Control lever kept between the two cylinders which is moved by the control lever. By lowering and raising of con- Driving Ram pulley guard trol lever, the strokes and the speeds of the blows per minute can be varied from 50 to 200. The steam and air hammers are designed Anvil Motor to give sharp and fast blows, reproducing to a block marked extent the action of the smith and his ham- mer. They may be used with a standard pair of anvils or with a set of dies, the latter often being so designed that the metal can be drawn out to the approximate length and width, and then placed in Fig. 3.16. Pneumatic hammer. the dies for the final shaping stage. The flash, which is formed, is clipped off as the last operation. 4. Hydraulic hammer. For the large castings and in cases where a heavy pressure is required the use of hydraulic hammer is restored to. The hydraulic forging machine being sluggish in action cannot usually compare with the steam or air hammer which operate more quickly, for small and medium sized forgings. The main advantage of the hydraulic forging press is that it gives a definite squeeze and the time element permits the material to flow. 3.4.4. Basic Categories of Forging Following are the three basic categories of forging : 1. Open-die forging. 2. Impression-die forging. 3. Closed-die forging. 3.4.4.1. Open-die forging This type of forging is distinguished by the fact that the metal is never completely confined as it is shaped by various dies. Most open-die forgings are produced on flat-V, or swaging dies (Fig. 3.17). Round swaging dies and V dies are used in pairs or with a flat die. The top die is attached to the ram of the press, and the bottom die is attached to the hammer anvil or, in the case of press open-die forging, to the press bed. As the workpiece is hammered or pressed, it is repeatedly manipulated between the dies until hot working forces the metal to the final dimensions. Flat dies V-dies Flat die and V-die Swage dies Fig. 3.17. Types of dies used in open-die forging. l Open-die forging, in its simplest form generally involves placing a solid cylindrical workpiece between two flat dies (platens) and reducing its height by compressing it. This METAL FORMING PROCESSES 157 process is known as upsetting. Under ideal conditions, a solid cylinder deforms as shown in Fig. 3.18 (a) ; this is known as homogeneous deformation. Fig. 3.18 (b) shows defor- mation in upsetting with friction as the die-workpiece interfaces ; the specimen develops a barrel shape. Die Workpiece hi di df hf Die (a) Friction force hf hf di (b) Fig. 3.18 — Barreling caused by friction can be minimised by an effective lubricant or ultrasonic vibration of the platens. The use of heated platens or thermal barrier at interfaces will also reduce barrel is hot working. Advantages of open-die forging : (i) Simple to operate. (ii) Simple for low production volume. (iii) Inexpensive tooling and equipment. (iv) Wide range of workpiece sizes can be used. Limitations : (i) Suitable for simple shapes only. (ii) Can be employed for short run production only. (iii) It is difficult to maintain moderately close tolerances. (iv) Material utilisation is poor. (v) Less control in determining grain flow, mechanical properties and dimensions. (vi) Fairly skilled workers are required. (vii) Since machining is often required, final cost of production may be higher than other forging methods. 3.4.4.2. Impression-die forging l In impression-die forging, the workpiece acquires the shape of the die cavities (impres- sions) while it is being upset between the closing dies. l In the simplest example of this type of forging, two dies are brought together, and the workpiece undergoes plastic deformation until its enlarged sides touch the side walls of the die (Fig. 3.19). A small amount of material is forced outside the die impression, 158 A TEXTBOOK OF MANUFACTURING TECHNOLOGY forming flash that is gradually thinned. The flash cools rapidly and presents increased resistance to deformation, effectively becoming a part of the tool, and helps build up pressure inside the bulk of the workpiece that aids material flow into unfilled impressions. Die Blank Flash Die Fig. 3.19. Impression-die forging. 3.4.4.3. Closed-die forging l Though not quite correct, the example shown in Fig. 3.19 is also referred to as closed-die forging. In true closed-die forging, no flash is formed, and the workpiece is completely surrounded by dies, while in impression-die forging, any excess metal in the die cavity is formed into a flash. Since no flash can be formed in closed-die forging, proper control of the volume of material is essential to obtain a forging of desired dimensions (and to avoid generating extreme pressures in the dies from overfilling). One approach to getting the right amount of metal for the die cavity and reduce forging time is the use of roll-formed shapes or extruded preform shapes. Advantages : (i) Can be used for production of complex shapes. (ii) Good dimensional accuracy and reproducibility. (iii) Suitable for high production rate. (iv) Less time consuming than open-die forging. (v) Workpiece materials are utilised effectively. (vi) The grain flow of the metal can be controlled ensuring high mechanical properties. (vii) Since the forgings are made with smaller machining allowances, therefore, there is a considerable reduction in the machining time and consumption of metal required for the forging. Limitations : (i) More than one step required for each forging. (ii) Finishing required for achieving final shape. (iii) High equipment and tooling cost. (iv) Appropriate die set for production of each component. l Both open-die and closed-die forgings can be carried out in hot or cold state. l Cold forging obviously requires higher deformation energy and is usually carried out for only those materials which are sufficiently ductile at room-temperature. Cold forged parts have better dimensional accuracy and have good surface finish. ‘Hot forged parts’ although require lower forces but give inferior finish and dimensional accuracy. 3.4.5. Methods of Forging 3.4.5.1. Drop forging This method of forging uses a closed impression die to obtain the desired shape of the com- ponent. The shaping is done by the repeated hammering given to the material in the die cavity. The equipment employed for delivering the blows are called drop hammers. METAL FORMING PROCESSES 159 The die used in drop forging consists of two halves. The lower half of the die is fixed to the anvil of the machine, while the upper half is fixed to the ram. The heated stock is kept in the lower die while the ram delivers four to five blows on the metal in quick succession so that the metal spreads and completely fills the die cavity. When the two die halves close, the complete cavity is formed. Too complex shapes with internal cavities, deep pockets, re-entrant shapes, etc. cannot be obtained in drop forging due to the limitation of the withdrawal of the finished forging from the die. In drop forging, the final desired shape cannot be obtained directly from the stock in a single pass. Depending on the shape of the component, and the desired grain flow direction, the mate- rial should be manipulated in a member of passes ; the various passes used are : l Fullering impression (Reducing the stock to the desired size). l Edging impression (Preform). l Bending impression (Required for those parts which have a bent shape). l Blocking impression (Semi-finishing impression). l Finishing impression (Final impression). l Trimming (Removal of extra flash present around the forging). Typical products obtained in drop forging are : — Wrench — Crane hook — Crank — Crankshaft — Connecting rod, etc. (For forging sequence of connecting rod refer to Q. 3.17). Note : The difference between drop forging and smith forging is that in drop forging closed-impres- sion dies are used and there is drastic flow of metal in the dies due to repeated blows the impact of which compels the plastic metal to conform to the shape of the dies ; whereas in the smith forging open face dies are used and the hammering of the heated metal is done by hand tools to get the desired shape by judgement. 3.4.5.2 Press forging l In press forging, the metal is shaped not by means of a series of blows as in drop forging, but by means of a single continuous squeezing action. This squeezing is obtained by means of presses. Owing to the continuous action of the press, the material gets uni- formly deformed throughout its entire depth. The impressions obtained in press forging are clear compared to that of the likely jarred impressions which are likely in the drop forged components. l The press is generally of vertical type and the squeezing action is carried completely to the centre of the part being pressed. Forging presses are of two types : Mechanical and hydraulic. Mechanical presses may be either of screw type used for brass forging only ; or crank type. Presses can be readily automated. — “Hydraulic presses” are used for heavy work and “mechanical press” for light work. Mechanical presses operate faster than the hydraulic presses, but hydraulic presses are designed to provide greater squeezing force. l For press forging operation, the drive should be capable of giving huge force which is needed at the end of the stroke when the metal is forced into desired shape. For this purpose, copper alloys are well suited as these flow easily in the die and are readily extruded. Advantages of press forging : Following are the advantages of press forging over drop forging : 1. Presses provide a faster rate of production because the die in press forging is filled in a single stroke. 160 A TEXTBOOK OF MANUFACTURING TECHNOLOGY 2. Superior structural quality of the product. 3. Quicker operation comparatively. 4. High output even with unskilled operators. 5. Low susceptibility to failure and simple maintenance. 6. Uniform forgings with exacting tolerances and low machining allowances. 7. Alignment of the two die halves can be move easily maintained than with hammering. 3.4.5.3 Machine forging The machine forging, as it involves the upsetting operation, sometimes it is simply called upset forging. Like press forging, in machine forging also, the material is plastically deformed by squeeze pressure into the shape provided by the dies in the forging machine ; but unlike press forging, it operates in horizontal direction. It is the forging method which is often selected when certain parts are required with an increased volume of metal at the centre or only at one end. l Upsetting machines called up setters are generally horizontal acting. The die set con- sists of a die and a corresponding punch or a heading tool. The die consists of two parts, one called stationary gripper die which is fixed to the machine frame and the other, movable gripper die, which moves along with the die slide of the upsetter. The stock is held between these two gripper dies by friction. The upset forging cycle starts with the movable die sliding against the stationary die to grip the stock. The two dies when in closed position, form the necessary die cavity. Then the heading tool advances against the stock and upsets it to completely fill the die cavity. Having completed the upsetting, the heading tool moves back to its back position. Then the movable gripper die releases the stock by sliding backwards. In machine forging, similar to drop forging, the operation is carried out in a number of stages. The die cavities required for the various operations are all arranged vertically on the gripper dies. The stock is then moved from one stage to the other in a proper sequence till the final forging is ready. Fig. 3.20 shows the operation of upsetting the end of a bar. Fixed die Heading tool Stock Movable die Finished part Fig. 3.20. Upset forging. METAL FORMING PROCESSES 161 l The automatic forging machines are employed for the mass production of : — Nuts — Bolts — Rivets — Wood screws — Roller and ball bearing balls — Gear blanks — Valve stems — Axles — Couplings, etc. Advantages : 1. Better quality of forging. 2. Since there is no or little draft is needed on forging made by upsetters, therefore, there is saving in material and also machining expenses. 3. The upsetting process can be automated. 4. As compared to drop forging hammers, forging machines have a higher productivity and their maintenance is much cheaper. 5. In forging machine the forging is accompanied by little or no flash (whereas in drop forging the flash is quite large). Disadvantages : 1. High tooling cost. 2. It is difficult to forge intricate, non-symmetric and heavy jobs on a forging machine. 3. Owing to the material handling difficulties it is not convenient to forge heavier jobs. 4. The maximum diameter of the stock which can be upset is limited (about 250 mm maximum). 3.4.6 Other Forging Processes Under this heading the following processes will be discussed : 1. Roll forging 2. Rotary forging or swaging 3. High velocity forging (HVF) 4. Orbital forging 5. Incremental forging 6. Liquid metal forging 7. Gatorizing. 3.4.6.1 Roll forging The primary function of the forging rolls is Guide to reduce the cross-section of a bar over a designed flange length or to produce taper on its surface over a cer- tain length. Roll The machine used is called a roll forging machine, and consists of two horizontal rolls Stock (Fig. 3.21) arranged directly over each other. These rolls are not completely circular ; about half or more portion of these rolls is cut away to allow the stock to enter through them. One or more sets of grooves, according to the shape required on the job surface, are arranged on the circular portion of the rolls. The total reduction is accomplished in several stages. When the rolls open, the bar is fed into the groove and rolled. During the next opening it is Fig. 3.21. Roll forging. shifted to the next groove and rolled. This continues 162 A TEXTBOOK OF MANUFACTURING TECHNOLOGY till the total reduction is achieved. After each rolling the bar is turned through 90°, before feeding into the next groove to prevent the flash formation. Pressure on rolls may be as high as 1 MN. l Roll forging is employed for producing long slender forged components, such as axles and leaf springs. 3.4.6.2. Rotary forging or swaging l This process can be used to reduce diameters of round bars or tubings. The other applications include fabrication of stepped and tapered shafts, pipes with forged out ends, etc. l The process can be done both in hot and cold state. However, cold working is preferable because of the greater use of handling and better surface finish obtained. l In swaging, also known as rotary swaging or radial forging, a solid rod or tube is reduced in diameter by the reciprocating radial movement of two or four dies (Fig. 3.22). Die tube Outer ring Rollers Mandrel + Die Hammers (four in all) giving radial motion (a) Side view (b) Front view (c) to the dies shown in (b). Fig. 3.22. Swaging process. The die movements are generally obtained by means of a set of rollers in cage. The internal diameter and thickness of the tube can be controlled with or without mandrels. Mandrels can also be made with longitudinal groove (similar in appearance to a splined shaft) ; thus, internally shaped tubes can be swaged. l The swaging process is usually limited to workpiece diameters of about 50 mm, al- though special machinery has been built-up to swage gun barrels of large diameter. — Die angles are usually only a few degrees and may be compound, that is, the die may have more than one angle, for more favourable material flow during swaging. — Lubricants are used for improved surface finish and longer die life. l The process is generally carried out at room temperature. l Parts produced by swaging have improved mechanical properties and good dimensional accuracy. Advantages : (i) Tooling cost is low. (ii) Low initial investment. (iii) Consistency of the product. (iv) Labour cost is low. (v) Rapid production. (vi) Maintenance is easy. Limitation : The process is limited to parts of symmetrical cross-section only. METAL FORMING PROCESSES 163 3.4.6.3. High velocity forging (HVF) High velocity forging is also known as High Energy Rate Forming (HERF) or Pneumatic- Mechanical High Velocity Forging (PMHVF). HVF machines provide a very high impact rate at a very low operating cost. These ma- chines are based on essentially the same principles as are drop hammers and impact forging machines. They provide greater energies for a given ram weight by using ram velocities of the order of 2 to 10 times those of hammers. These are based upon the following methods of releasing energy (i) Chemicals—High explosives, propellants, gas mixtures ; (ii) Electrical—Exploding wires, spark discharge, magnetic field. These machines can be effectively used for forging, extruding, compacting and many other metal forging operations. The most efficiently performed operation, however, is the closed die forging of metals, although even ceramics can be worked on these machines. l A wide variety of materials can be formed or forged including exotic and refractory metals, stainless steels, non-ferrous alloys, and high-strength materials, some of which are not usually forgeable. Advantages : 1. Complex parts can be made is one blow. 2. Draft allowances are reduced and is some cases eliminated. 3. Scaling and decarbonisation of metal surface during the operation are almost elimi- nated. 4. Extrusion of even such materials, which either cannot be extruded on the conventional machines or will need conventional machines of giant sizes for this purpose, is also possible on these machines. 5. Tolerance and surface finish are improved over those obtained with conventional forging techniques. 6. Because strength and fatigue resistance are improved, parts can be made smaller. 7. Repeatability is excellent. 8. Overall production cost is very low. Limitations : 1. Part configuration is usually limited to one-piece dies. If two-piece or split dies are neces- sary, the economics of process become debatable. 2. There is a limitation of size and weight of the product these machines can handle. Extremely large and very heavy forgings (say above 25 kg) cannot be easily produced on these machines. Applications : A few examples of the type of products made on these machines are : — Valve bodies ; — Gears ; — Engine housings ; — Rocket components ; — Missile components, etc. 3.4.6.4. Orbital forging (or Rota forming) It is a cold forming process. In this process, the pressure of the upper die of the workpiece is concentrated on a small area at any time, and not on the total workpiece area as in conventional forging. The upper die is slightly inclined to the vertical axis of the machine and it imparts a high frequency circular 164 A TEXTBOOK OF MANUFACTURING TECHNOLOGY rocking motion across the top surface of the workpiece. At the same time, the workpiece is slowly moved hydraulically upward and pressed against the orbiting upper die. The forging operation is completed when the hydraulic ram touches a preset stop. The ram is then lowered and the hydrau- lically operated ejector ejects the forging from the lower die. Advantages : 1. Low equipment cost. 2. Press capacity requirements are only 5 to 10 per cent of that needed for conventional forging (since only a small portion of die actually contacts the workpiece). 3. Better surface finish. Limitation : The limitation of this process is that the forging is obtained by filling the lower die and so the bottom surface of the upper die should be flat and smooth. l Examples of parts produced by this process are : Parts flanged with indented or crown shapes and discs, etc. 3.4.6.5 Incremental forging l In this forging method, very big forgings are made by working different areas of the forging into shape, one at a time. l As only a limited area is worked at any time, the forging equipment can be much smaller in capacity as compared to conventional forging. This makes it possible to forge huge parts on presses of modest capacity. Limitation : The workpiece tends to cool below the forging temperature as it moves from incremental step to step. Reheating the part to its original forging temperature may destroy the thermomechanical work already done on it in the case of many alloys. It may be prevented as follows : (i) By reheating the workpiece progressively to a lower temperature after its temperature falls below the forging temperature. (ii) By covering the billet using insulating blankets and other coatings so as to reduce the heat loss. 3.4.6.6. Liquid metal forging l Liquid metal forging process also called squeeze casting is hybrid between conventional casting and forging methods and produces complex shaped components from molten metal in a single step. l In this process of forging, the molten metal is poured into the bottom forging die and allowed to solidify partially. Then the upper and bottom dies are closed and pressure is applied and maintained for a fixed time until solidification is completed. The dies are then opened and the forged component is ejected from the bottom die. Advantages : This forging process claims the following advantages over conventional casting and forging processes : 1. Low capacity presses are required. 2. Economical from material point of view. 3. Yield is higher in comparison to sand castings, since gates and risers are not needed. 4. The tooling and equipment needed are basically simple, low cost and readily available. 5. No gas and shrinkage porosities. 6. Thinner and more complex components can be produced. 7. The cast as well wrought alloys can be used. 8. The mechanical properties of the product are comparable to those by conventional forging ; they are considerably better than sand castings. METAL FORMING PROCESSES 165 Limitations : 1. This process cannot be employed to produce all the component shapes. 2. Liquid forged parts necessitate different heat treatments (cooling rates being faster, the effect on microstructure of the component will be different as compared to forging or casting). 3.4.6.7 Gatorizing In this forging technique the forging stock is preconditioned in inert atmosphere to obtain a temporary condition of fine grained microstructure resulting in low strength and high ductility (This is accomplished by mechanically working the workpiece at temperature slightly below the recrystallisation temperature). After preconditioning, the isothermal forging operation is carried out at slightly below the recrystallisation temperature. Finally, certain heat treatment operations are done on the forgings to get normal high strength and hardness. l This forging method is employed for making aircraft components of nickel and titanium based alloys. 3.4.7. Defects in Forging Although the forging process generally gives superior quality products compared to other manufacturing processes, there are some defects that are likely to come if proper care is not taken in forging process design. A brief description of such defects is given below : 1. Cold shut. This usually occurs at the corners and at right angles to the surface. — This is caused mainly by the improper design of the die wherein the corner and fillet radii are small as a result of which the metal does not flow properly into the corner and ends up as a cold shut. 2. Unfilled section. It is similar to misrun in casting and occurs when metal does not completely fill the die cavity. — It is usually caused by using insufficient metal or insufficient heating of the metal. 3. Flakes. Basically, these are internal ruptures. — These are caused by the improper cooling of large forging and can be remedied by following proper cooling practice. 4. Scale pits. These are irregular depressions on the surface of the forging. — These are primarily caused because of the improper cleaning of the stock used for forging. 5. Improper grain flow : — This is caused by the improper design of the die which makes the flow of metal not following the final intended directions. 6. Internal cracks : — These can result from too drastic a change in the shape of the raw stock at too fast a rate. 7. Die shift : — This defect is caused by the misalignment of the two die halves, making the two halves of the forging to be of improper shape. 8. Burnt and overheated metal : — This defect is caused by improper heating conditions and soaking the metal too long. 166 A TEXTBOOK OF MANUFACTURING TECHNOLOGY 3.4.8. Cleaning and Finishing of Forgings Cleaning and finishing of forgings include the following : 1. Removal of oxide scale : l Due to the contact of heated steel with air a thin layer of scale (iron oxide) is formed on the surface of steel forging. The amount of scale depends upon the forging tem- perature and length of time of the operation. l The scale can be removed by employing steam or compressed air. 2. Cleaning by pickling : l The hard scale from the surface of the forgings can be removed by pickling process. l The pickling process consists of immersing the forgings in a tank filled with an acid solution, which is 12 to 15 percent concentrate of H2SO4 in water. The solution acts to loosen the hard scale from the forging surface and remove it. 3. Tumbling process : l This process is employed to remove scale and for general cleaning of the forgings. l In this process, the forgings along with some abrasive materials such as coarse sand or small metallic particles are placed in barrel ; the tilted barrel is rotated at low speeds. This action loosens the scale from the surface of the forgings and results in general cleaning of the forgings. 4. Blast cleaning : l The process consists of directing a jet of sand, grit or metallic shots against the forgings. l By this process the scale is removed and a smooth surface finish is imparted to the forging. 3.4.9. Heat Treatment of Forgings The forged parts are generally heat treated for the following reasons : 1. To relieve internal stresses set up during working and cooling. 2. To normalise the internal structure of the metal. 3. To improve machinability. 4. To improve hardness, strength and other mechanical properties. l The common heat treatments given to forged components are annealing, normalising and tempering. 3.4.10. Design Considerations While designing a forging, the following considerations need be given : 1. The parting line of a forging, as far as possible, should lie in one plane. 2. The forged component should ultimately be able to achieve a radial flow of grains or fibres. 3. In order to facilitate easy removal of forgings from the dies, sufficient draft on surfaces should be provided. Generally, a 1° to 5° draft is provided on press forgings and 3° to 10° on drop forgings. 4. As far as possible, sharp corners should always be avoided, to prevent concentration of stresses leading to fatigue failures and to facilitate ease in forging. 5. In order to facilitate an easy flow of metal too thin sections should be avoided. 6. The presence of pockets and recesses in forgings should be avoided. 7. While deciding the forging and finishing temperatures metal shrinkage and forging method should be duly taken into account. 306 A TEXTBOOK OF MANUFACTURING TECHNOLOGY 5. Hose and hose fittings : l Hoses are the rubber and fabric pipes used to connect gas cylinder to blow pipe and are painted black or green for oxygen and red or maroon for acetylene. It should be strong, durable, non-porous and light. l Special fittings are used for connecting hoses to equipment. 6. Safety devices : l Goggles fitted with coloured glasses should be used to protect the eyes from harmful heat ultraviolet rays. l Gloves made of leather, canvas and asbestos should be worn to protect hands from any injury. Gloves should be light so that the manipulation of the torch may be done easily. Other requirements include spark-lighter, apron, trolley, wire brush, spindle key, spanner set, filler rods and fluxes and welding tips. Welding rods (Filler materials) for gas welding : The welding wire or rod used as filler material in gas welding should have a chemical compo- t sition similar to that of the base metal. The welding rod diameter, d = + 1 mm (app.), where t is the 2 thickness of the base metal, mm. l Gas welding ‘‘fluxes’’ (composing of borates or boric acid, soda ash and small amount of other compounds e.g., sodium chloride, ammonium sulphate and iron oxide) must melt at a lower temperature than the metals being welded so that surface oxides will be dissolved before the metal melts. 7.7. ELECTRIC ARC WELDING 7.7.1. Introduction Arc welding is the system in which the metal is melted by the heat of an electric arc. It can be done with the following methods : (i) Metallic arc welding. (ii) Carbon arc welding. (iii) Atomic hydrogen welding. (iv) Shielded arc welding. 7.7.2. Advantages and Limitations Following are the advantages and limitations of electric arc welding : Advantages : 1. Portable and relatively inexpensive equipment. 2. Very versatile process. Limitations : 1. Large heat affected zone. 2. Weld quality depends upon operator’s skill in normal operations. 3. Not suitable for thin sections. 7.7.3. Metallic Arc Welding Refer to Fig. 7.14. In metallic arc welding an arc is established between work and the filler metal electrode. The intense heat of the arc forms a molten pool in the metal being welded, and at the same time melts the tip of the electrode. As the arc is maintained, molten filler metal from the electrode tip is transferred across the arc, where it fuses with the molten base metal. Arc may be formed with direct or alternating current. Petrol or diesel driven generators are widely used for welding in open, where a normal electricity supply may not be available. D.C. may also be obtained from electricity mains through the instrumentality of a transformer and rectifier. A simple transformer WELDING AND ALLIED PROCESSES 307 Flux coated electrode Electrode holder Deposited Flame weld metal Lead clamped to the work Crater Leads of generator or transformer Fig. 7.14. Metallic arc welding. is, however widely employed for A.C. arc welding. The transformer sets are cheaper and simple having no maintenance cost as there are no moving parts. l With Arc system, the covered or coated electrodes are used, whereas with D.C. system for cast iron and non-ferrous metals, bare electrodes can be used. l In order to strike the arc an open circuit voltage of between 60 to 70 volts is required. For maintaining the short arc 17 to 25 volts are necessary ; the current required for welding, however, varies from 10 amp. to 500 amp. depending upon the class of work to be welded. l The great disadvantage entailed by D.C. welding is the presence of arc blow (distortion of arc stream from the intended path owing to magnetic forces of a non-uniform magnetic field). With A.C. arc blow is considerably reduced and use of higher currents and large electrodes may be restored to enhance the rate of weld production. Applications : l The field of application of metallic arc welding includes mainly low carbon steel and the high-alloy austenitic stainless steel. l Other steels like low and medium-alloy steels can however be welded by this system but many precautions need be taken to produce ductile joints. 7.7.4. Carbon Arc Welding Refer to Fig. 7.15. Here the work is connected to Carbon negative and the carbon rod or electrode connected to the Filler electrode positive of the electric circuit. Arc is formed in the gap, rod Arc flame filling metal is supplied by fusing a rod or wire into the Pool of arc by allowing the current to jump over it and it produces molten metal a porous and brittle weld because of inclusion of carbon particles in the molten metal. It is therefore used for filling blow holes in the castings which are not subjected to any Fig. 7.15. Carbon arc welding. of the stresses. l The voltage required for striking an arc with carbon electrodes is about 30 volts (A.C.) and 40 volts (D.C.). l A disadvantage of carbon arc welding is that approximately twice the current is required to raise the work to welding temperature as compared with a metal electrode, while a carbon electrode can only be used economically on D.C. supply. 308 A TEXTBOOK OF MANUFACTURING TECHNOLOGY 7.7.5. Atomic Hydrogen Welding Refer to Fig. 7.16. In this system heat is obtained Hydrogen gas from an alternating current arc drawn between two Tungsten tungsten electrodes in an atmosphere of hydrogen. As the electrodes hydrogen gas passes through the arc, the hydrogen molecules are broken up into atoms and they recombine Filler Arc Welding on contact with the cooler base metal generating intense rod flame heat sufficient to melt the surfaces to be welded, together with the filler rod, if used. The envelop of hydrogen gas also shields the molten metal from oxygen and nitrogen and thus prevents weld metal from deterioration. Fig. 7.16. Atomic hydrogen welding. l The welds obtained are homogeneous and smooth in appearance because the hydrogen keeps the molten pool. Advantages : 1. No flux or separate shielding gas is used ; hydrogen itself acts as a shielding gas and avoids weld metal oxidation. 2. Due to high concentration of heat, welding can be carried out at fast rates (specially when filler metal is not needed) and with less distortion of the workpiece. 3. Welding of thin materials is also possible which otherwise may not be successfully carried out by metallic arc welding. 4. The job does not form a part of the electrical circuit. The arc remains between two tung- sten electrodes and can be moved to other places easily without getting extinguished. Limitations : 1. For certain applications, the process becomes uneconomical because of higher operating cost as compared to that of other welding processes. 2. The process cannot be used for depositing large quantities of metals. 3. Welding speed is less as compared to that of metallic arc or MIG welding. Applications : l Atomic hydrogen welding being expensive is used mainly for high grade work on stainless steel and most non-ferrous metals. 7.7.6. Shielded Arc Welding Flux coating In this system molten weld metal is Slag Electrode protected from the action of atmosphere by an coating envelope of chemically reducing or inert gas. Weld Gaseous shield metal As molten steel has an affinity for oxygen Arc stream and nitrogen, it will, if exposed to the Base atmosphere, enter into combination with these metal gases forming oxides and nitrides. Due to this Pool of injurious chemical combination metal becomes molten metal weak, brittle and corrosion resistant. Thus, Fig. 7.17. Shielded arc welding. several methods of shielding have been developed. The simplest (Fig. 7.17) is the use of a flux coating on the electrode which in addition to producing a slag which floats on the top of the molten metal and protects it from atmosphere, has organic constituents which turn away and produce an envelope of inert gas around the arc and the weld. l Welds made with a completely shielded arc are more superior to those deposited by an ordinary arc. WELDING AND ALLIED PROCESSES 309 7.7.7. Arc Blow l Arc blow is the phenomenon of wandering of arc and it occurs in D.C. welding. l When a current flows in any conductor, a magnetic field is formed around the conductor at right angles to the current. Since in the case of D.C. arc welding, there is current through the electrode, workpiece and ground clamp, magnetic field exists around each of these components. The arc thus lacks control as though it were being blown to and by the influence of these complex magnetic fields. This is more common in welding with very high or very low currents, and especially in welding in corners or other confined spaces. Usually arc blow results from the interaction of magnetic fields of the electrode workpiece with that of the arc. The movement of arc blow causes atmospheric gases to be pulled into the arc, resulting in porosity or other defects. The severity of arc blow problem can be reduced by taking the following corrective measures : 1. Change to A.C. welding, if possible (since due to change in the polarity, the effect of magnetic field is nullified). 2. Reduce the current used so that the strength of magnetic field is reduced. 3. Use a short arc length so that filler metal would not be deflected but carried easily to the arc crater. 4. Place more than one ground lead from the base metal (preferably on each from the ends of the base metal plate). 5. The ground cable may be wrapped around the workpieces such that the current flowing in it sets up a magnetic field in a direction which will counteract the arc blow. 7.7.8. Comparison between A.C. and D.C. Arc Welding The Comparison between A.C. and D.C. arc Welding is given below : S. No. Aspects A.C. Welding D.C. Welding 1. Power consumption Low High 2. Arc stability Arc unstable Arc stable 3. Cost Less More 4. Weight Light Heavy 5. Efficiency High Low 6. Operation Noiseless Noisy 7. Suitability Non-ferrous metals cannot Suitable for both ferrous and be joined non-ferrous metals 8. Electrode used Only coated Bare electrodes are also used 9. Welding of thin sections Not preferred Preferred 10. Miscellaneous Work can act as cathode Electrode is always negative while electrode acts as anode and the work is positive. and vice versa. Specifications of A.C. Transformer/D.C. generator : A.C. transformer : Step down, oil cooled = 3 phase, 50 Hz ; Current range = 50 to 400 A ; Open circuit voltage = 50 to 90 V ; Energy consumption = 4 kWh per kg of metal deposit ; Power factor = 0.4 ; Efficiency = 85%. D.C. generator : Motor generator—3 phase, 50 Hz ; Current range = 125 to 600 A ; Open circuit voltage = 30 to 80 V ; Arc voltage = 20 to 40 V ; Energy consumption = 6 to 10 kWh/kg of deposit ; Power factor = 0.4 ; Efficiency = 60%. 310 A TEXTBOOK OF MANUFACTURING TECHNOLOGY Electrodes : The electrodes may be of the following two types : 1. Consumable electrode : (i) Base electrode (ii) Flux coated electrode. 2. Non-consumable electrode : 1. Consumable electrode : (i) Bare electrode : l These electrodes do not prevent oxidation of the weld and hence the joint is weak. They are used for minor repairs where strength of the joint is weak. l Employed in automatic and semi-automatic welding. (ii) Flux-coated electrode : l The flux is provided to serve the following purposes : — To prevent oxidation of the weld bead by creating a gaseous shield around the arc. — To make the formation of the slag easy. — To facilitate the stability of the arc. 2. Non-consumable electrode : l These electrodes are 12 mm in diameter and 450 mm long. l These are not consumed during the welding process. Examples of these electrodes are : Carbon, graphite and tungsten. 7.7.9. Types of Welded Joints The type of joint is determined by the relative positions of the two pieces being joined. The following are the five basic types of commonly used joints : 1. Lap joint 2. Butt joint 3. Corner joint 4. Edge joint 5. T-joint. 1. Lap joint. Refer to Fig. 7.18. Plates Plates Fig. 7.18. Lap joint. Fig. 7.19. Butt joint. l The lap joint is obtained by overlapping the plates and then welding the edges of the plates. l The lap joints may be single traverse, double traverse and parallel lap joints. l These joints are employed on plates having thickness less than 3 mm. 2. Butt joint : l The butt joint is obtained by placing the plates edge to edge as shown in Fig. 7.19. l In this type of joints, if the plate thickness is less than 5 mm, bevelling is not required. When the thickness of the plates ranges between 5 mm to 12.5 mm, the edge is required to WELDING AND ALLIED PROCESSES 311 be bevelled to V or U-groove, while the plates having thickness above 12.5 mm should have a V or U-groove on both sides. l The various types of butt joints are shown in Fig. 7.20. 60° Square butt Single-V—butt Double-V—butt Single-U—butt Double-U—butt Single bevel butt Double bevel butt Single-J—butt Double-J—butt Fig. 7.20. Various types of butt joints. 3. Corner joint. Refer to Fig. 7.21. l A corner joint is obtained by joining the edges of two plates whose surfaces are at an angle of 90° to each other. l In some cases corner joint can be welded, without any filler metal, by melting off the edges of the parent metal. l This joint is used for both light and heavy gauge sheet metal. Fig. 7.21. Corner joint. Fig. 7.22. Edge joint. 4. Edge joint. Refer to Fig. 7.22. l This joint is obtained by joining two parallel plates. l It is economical for plates having thickness less than 6 mm. l It is unsuitable for members subjected to direct tension or bending. 5. T-joint. Refer to Fig. 7.23. l It is obtained by joining two plates whose surfaces are approxi- mately at right angles to each other. l These joints are suitable up to 3 mm thickness. l T-joint is widely used to weld siffeners in aircraft and other thin walled structures. Fig. 7.23. T-joint. 312 A TEXTBOOK OF MANUFACTURING TECHNOLOGY Note : The lap joints, corner joints and T-joints are known as fillet weld joints. The fillet cross- section is approximately triangular. Fig. 7.24 shows the three types of fillet welds. (a) Flush fillet (b) Convex fillet (c) Concave fillet Fig. 7.24 Welding positions : It is easiest to make welds in flat positions, i.e., both the parent metal pieces lying in horizon- tal plane over a flat surface. But, several times it becomes unavoidable to weld the workpieces in some other positions also. The common welding positions are : 1. Flat position 2. Horizontal position 3. Vertical position 4. Overhead position. 1. Flat position. Refer to Fig. 7.25. l In this welding position, the welding is done from the upper side of the joint and the welding material is normally applied in the downward direction. l On account of the downward direction of application of welding material this position is also sometimes called as downward position. Weld beads Electrode arc Fig. 7.25. Flat position. Fig. 7.26. Horizontal position. 2. Horizontal position. Refer to Fig. 7.26. In this case, the weld is deposited upon the side of a horizontal and against a vertical surface. 3. Vertical position. Refer to Fig. 7.27. l In this position, the axis of the weld remains either vertical or at an inclination of less than 45° with the vertical plane. l The welding commences at the bottom and proceeds upwards. l The tip of the torch is kept pointing upwards so that the pressure of the outcoming gas mixture forces the molten metal towards the base metal and prevents it from falling down. Fig. 7.27. Vertical position. WELDING AND ALLIED PROCESSES 313 4. Overhead position. Refer to Fig. 7.28. l In this case, the welding is performed Workpiece from the underside of the joint. The Weld workpieces remain over the head of the welder. Axis of weld l The workpieces as well as axis of the weld all remain in approximately hori- zontal plane. Fig. 7.28. Overhead position. l It is reverse of flat welding. 7.8. THERMIT WELDING Refer to Fig. 7.29. It is the method of uniting iron or steel parts by surrounding the joint with steel at a sufficient high temperature to fuse the adjacent surfaces of the Crucible parts together. Pouring — Here a wax pattern of gate Steel metal box desired size and shape is Slag basin prepared around the joint Thermit Weld or region where the weld Riser is to be affected. Iron — The wax pattern is then plug Heating surrounded by sheet iron gate box and the space be- Mould Thermit Parts being welded tween box and pattern is material collar filled and rammed with Fig. 7.29. Thermit welding. sand. — After cutting, pouring and heating gates and risers a flame is directed into the heating oven due to which the wax pattern melts and drains out, the heating is continued to raise the temperature of the parts to be welded. — The thermit mixture (finely divided aluminium iron oxide) is packed in the crucible of conical shape formed from a sheet-iron casting lined with heat resisting cement and is ignited with magnesium or torch yielding a highly superheated (nearly 3000°C) molten-iron and a slag of aluminium oxide (the reaction is : 8Al + 3 Fe3O4 = 4 Al2O3 + 9Fe + heat). — The molten iron is then run into the mould which fuses with the parts to be welded and forms a thermit collar at the joint. The welds thus obtained are metallurgically very sound and strong. Advantages : 1. Can be used anywhere. 2. Low set-up cost. 3. Not a highly skilled operation. 4. Most suitable for welding of thick sections. Limitations : 1. Only thick sections can be welded. 2. High set-up and cycle time. 314 A TEXTBOOK OF MANUFACTURING TECHNOLOGY Applications : l The process is widely employed in the shipping, steel and railroad industries. l It can also be used for welding non-ferrous parts by selection of a mixture of oxides which on reduction with aluminium will provide an alloy approximating the material to be welded. 7.9. TUNGSTEN INERT-GAS (TIG) WELDING This welding process is also called Gas Tungsten Arc Welding (GTAW) Refer to Fig. 7.30. In this process the heat necessary to Tungsten melt the metal is provided by a very intense electric arc which is electrode struck between a virtually non-consumable tungsten electrode and Power metal workpiece. The electrode does not melt and become a part of source the weld. On joints where filler metal is required, a welding rod Arc Shelding is fed into the weld zone and melted with base metal in the same column gas manner as that used with oxyacetylene welding. The weld zone is shielded from the atmosphere by an inert-gas (a gas which does Puddle not combine chemically with the metal being welded) which is ducted directly to the weld zone where it surrounds the tungsten. Base metal The major inert gases that are used are argon and helium. Fig. 7.30. Tungsten inert-gas TIG process offers the following advantages : (TIG) welding. 1. TIG welds are stronger, more ductile and more corro- sion resistant than welds made with ordinary shield arc welding. 2. Since no granular flux is required, it is possible to use a wide variety of joint designs than in conventional shield arc welding or stick electrode welding. 3. There is little weld metal splatter or weld sparks that damage the surface of the base metal as in traditional shield arc welding. Applications : (i) The TIG process lends itself ably to the fusion welding of aluminium and its alloys, stainless steel, magnesium alloys, nickel base alloys, copper base alloys, carbon steel and low alloy steels. (ii) TIG welding can also be used for the combining of dissimilar metals, hard facing, and the surfacing of metals. 7.10. METAL INERT-GAS (MIG) WELDING This welding process is also called Gas Metal Arc Welding (GMAW). Driving wheels Consumable Refer to Fig. 7.31. The inert-gas consumable electrode electrodes process, or the MIG process is a refinement of the TIG process, however, in this process, the tungsten electrode has been Power replaced with a consumable electrode. The electrode is driven source through the same type of collet that holds a tungsten electrode Arc Shielding by a set of drive wheels. The consumable electrode in MIG process column gas acts as a source for the arc column as well as the supply for the filler material. Puddle MIG welding employs the following three basic processes. Base metal 1. Bare-wire electrode process 2. Magnetic flux process Fig. 7.31. Metal inert-gas welding (MIG). 3. Flux-cored electrode process. WELDING AND ALLIED PROCESSES 315 Advantages : 1. It provides higher deposition rate. 2. It is faster than shielded metal-arc welding due to continuous feeding of filler metal. 3. Welds produced arc of better quality. 4. There is no slag formation. 5. Deeper penetration is possible. 6. The weld metal carries low hydrogen content. 7. More suitable for welding of thin sheets. Limitations : 1. Less adaptable for welding in difficult to reach portions. 2. Equipment used is costlier and less portable. 3. Less suitable for outdoor work because strong wind may blow away the gas shield. Applications : l Practically all commercially available metals can be welded by this method. l It can be used for deep groove welding of plates and castings, just as the submerged arc process can, but it is more advantageous on light gauge metals where high speeds are possible. 7.10.1. Difference between TIG and MIG Welding Processes The difference between TIG and MIG welding processes is given in tabular form below : S. No. Aspects TIG welding MIG welding 1. Name of the process Tungsten inert-gas welding. Metal inert-gas welding. 2. Type of electrode used Non-consumable tungsten electrode. Consumable metallic electrode. 3. Electrode feed Electrode feed not required. Electrode need to be fed at a constant speed from a wire reel. 4. Electrode holder It is called welding torch and has got It is called welding gun or torch. It a cap filled on the back to cover the has facility to continuously feed wire tungsten electrode. It has also got electrodes ; shielding inert-gas, cool- connections for shielding gas, cooling ing water and control table. water and control cable. It may be air- cooled also. 5. Welding current Both A.C. and D.C. can be used. D.C. with reverse polarity is used. 6. Feed metal Filler metal may or may not be used. Filler metal in the form of fire wire is used. 7. Bases metal thickness Metal thickness which can be welded Thickness limited to about 40 mm. is limited to about 5 mm. 8. Welding speed Slow. Fast. 7.11. SUBMERGED ARC WELDING The submerged arc process (which may be done manually or automatically) creates an arc column between a base metallic electrode and the workpiece. — The arc, the end of the electrode, and the molten weld pool are submerged in a finely divided granulated powder that contains appropriate deoxidizers, cleansers and any other fluxing elements. 316 A TEXTBOOK OF MANUFACTURING TECHNOLOGY — The fluxing powder is fed from a hopper that is carried on the welding head. The tube from the hopper spreads the powder in continuous mount in front of the electrode along the line of the weld. — This flux mound is of sufficient depth to submerge completely the arc column so that there is no splatter or smoke, and the weld is shielded from all effects at atmospheric gases. As a result of this unique protection, the weld beads are exceptionally smooth. — The flux adjacent to the arc column melts and floats to the surface of the molten pool ; then it Flux solidifies to form a slag on the top of the welded hopper To power metal. The rest of the flux is simply an insulator supply that can be rec