Fundamentals of Metal Casting (Fall 2023) Cairo University PDF

Faculty of Engineering Cairo University Fall 2023. ENG001- Fundamentals of Manufacturing Engineering FUNDAMENTALS OF METAL CASTING ENG-G001  FUNDAMENTALS OF METAL CASTING 1. Overview of Casting Technology 2. Heating and Pouring 3. Soli...

Faculty of Engineering Cairo University Fall 2023. ENG001- Fundamentals of Manufacturing Engineering FUNDAMENTALS OF METAL CASTING ENG-G001  FUNDAMENTALS OF METAL CASTING 1. Overview of Casting Technology 2. Heating and Pouring 3. Solidification and Cooling ENG-G001  Metal Casting Metal casting definition Metal casting can be defined as a shaping process, where solid metal is transferred to liquid phase then poured in a cavity having the shape of the object to be cast and allowed to solidify ENG-G001  Casting Process in which molten metal flows by gravity or other force into a mold where it solidifies in the shape of the mold cavity The term casting also applies to the part made in the process Steps in casting seem simple: 1. Melt the metal 2. Prepare mold 3. Pour it into a mold 4. Let it freeze 5. Finishing. ENG-G001  Capabilities and Advantages of Casting Can create complex part geometries Can create both external and internal shapes Some casting processes are net shape; others are near net shape Can produce very large parts Some casting methods are suited to mass production ENG-G001  Disadvantages of Casting Different disadvantages for different casting processes: – Limitations on mechanical properties – Poor dimensional accuracy and surface finish for some processes; e.g., sand casting – Safety hazards to workers due to hot molten metals – Environmental problems ENG-G001  Parts Made by Casting Big parts – Engine blocks and heads for automotive vehicles, wood burning stoves, machine frames, railway wheels, pipes, church bells, big statues, pump housings Small parts – Dental crowns, jewelry, small statues, frying pans All varieties of metals can be cast, ferrous and nonferrous ENG-G001 Melting furnaces ENG-G001  electric arc furnace it can be used to produce stainless steel and alloyed steel. ENG-G001  Induction melting furnace These types of furnaces are used for special alloy steels, and for melting and refining of brasses and aluminum alloys. ENG-G001  The Mold in Casting Contains cavity whose geometry determines part shape – Actual size and shape of cavity must be slightly oversized to allow for shrinkage of metal during solidification and cooling – Molds are made of a variety of materials, including sand, plaster, ceramic, and metal. ENG-G001  Two Categories of Casting Processes 1. Expendable mold processes – uses an expendable mold which must be destroyed to remove casting – Mold materials: sand, plaster, and similar materials, plus binders 2. Permanent mold processes – uses a permanent mold which can be used over and over to produce many castings – Made of metal (or, less commonly, a ceramic refractory material ENG-G001  Mold: cope (upper half) & drag (bottom Features of a Sand Mold half) The gating system – pouring cup, (down)sprue, and runner Pouring cup: this serves to accept the molten metal and provide for a large cross-sectional area control metal flow into the sprue. Sprue: this provide a pressure head to assure proper filling of the mold. Runner: transport molten metal from the sprue to the casting. Risers: provide molten metal to account for shrinkage and control the cooling across the mold. Core: features the internal surfaces of the casting. It may require supports Schematic illustration of a sand mold various (called chaplets ) to hold it in position in features. the mold cavity during pouring. ENG-G001  Open Molds and Closed Molds Two forms of mold: (a) open mold, simply a container in the shape of the desired part; and (b) closed mold, in which the mold geometry is more complex and requires a gating system (passageway) leading into the cavity. ENG-G001  Solidification of pure metals Due to chilling action of mold wall, a thin skin of solid metal is formed at the interface after pouring. Skin thickness increases to form a shell around the molten metal as solidification progresses Rate of freezing depends on heat transfer into mold, as well as thermal properties of the metal. ENG-G001  Solidification of Metals Transformation of molten metal back into solid state Solidification differs depending on whether the metal is – A pure element or – An alloy ENG-G001  Cooling Curve for a Pure Metal A pure metal solidifies at a constant temperature equal to its freezing point (same as melting point) Cooling curve for a ENG-G001 pure metal during casting.  Solidification of Alloys Most alloys freeze over a temperature range rather than at a single temperature Phase diagram for a copper-nickel alloy system and (b) associated cooling curve for a 50%Ni-50%Cu composition during casting. ENG-G001  Solidification Time TST=Cm(V/A)n where TST = total solidification time; V = volume of the casting; A = surface area of casting; n = exponent usually taken to have a value = 2; and Cm is mold constant [Chvorinov's constant] To feed molten metal to main cavity, TST for riser must greater than TST for main casting ENG-G001  Gating system A gating system refers to all passageways through which the molten metal passes to enter the mold cavity. The basic components of a simple gating system are pouring basin, sprue, runner, gates ENG-G001  Mold Filling (Pouring) Time: Velocity of the molten metal leaving the bottom of the sprue of height h is given by: v  2 gh The volumetric rate of flow Q through the bottom of the sprue is Q=Av A = cross-sectional area at the bottom of the sprue We can estimate the time required to fill a mould cavity of the volume V as ENG-G001  Types of casting processes Sand Casting: Sand casting uses natural or synthetic sand (lake sand) which is mostly refractory material called silica (SiO2). The sand grains must be small enough so that it can be packed densely; however, the grains must be large enough to allow gasses formed during the metal pouring to escape ENG-G001  Types of Mold sand The mold sand is composed chiefly from two substances silica and clay. Their types are: 1-Green sand: 18-30% clay, 6-8% water--soft-light and porous Weak sand used for all ferrous and nonferrous metals in small and medium size 2-Dry sand: dried or baked green sand, for large casting 3- Facing sand: Possess high strength and refractoriness made of silica sand and clay and different forms of carbon are used, It forms the face of the mould 4-Core sand: Silica sand mixed with core oil and other binding materials, Used for making the cores ENG-G001  Parts of sand molds The mold is made of two parts, the top half is called the cope, and bottom part is the drag. Mold cavity: The gap between the two parts. The geometry of the Pattern: The shape of the patterns is identical to the shape of the product. Pouring cup: The top of the funnel. Sprue: The pipe-shaped neck of the funnel.– the liquid metal is poured into the pouring cup and flows down the sprue. Runners: Are the horizontal hollow channels that connect the bottom of the sprue to the mould cavity. Gate: The region where any runner joins with the cavity is Cores: represents the interior holes in the product ENG-G001  Example In sand casting of a cylindrical disk with outer diameter 30 cm and thickness of 5 cm, the part has a central cylindrical cavity with diameter of 10 cm and thickness of 5 cm. a) Draw a longitudinal section in mold showing its main parts. b) Calculate the solidification time for the casting if Chvorinov’s constant = 4 min/cm2. c) If a Spherical riser is considered to compensate shrinkage during solidification, find the diameter of riser, where the solidification time of it is greater by 20% than the solidification time of the casting. d) Assuming that the gating system volume equal riser volume, if the down-sprue height is 10 cm long, its cross-sectional area at the top = 5 cm2 and at the base = 2 cm2. Calculate mold filling time to fill the entire mold (including cavity, riser, and gating system). ENG-G001 Shell-mold Casting ENG-G001  Procedures of shell mold casting The 2-piece pattern is made of metal (e.g. aluminum or steel), it is heated to between 175°C-370°C, and coated with a lubricant, e.g. silicone spray. - Each heated half-pattern is covered with a mixture of sand and a thermoset resin/epoxy binder. The binder glues a layer of sand to the pattern, forming a shell. - The assembly is baked to cure it. - The patterns are removed, and the two half-shells joined together to form the mold; metal is poured into the mold. - When the metal solidifies, the shell is broken to get the part ENG-G001  Expendable-pattern casting (lost foam process) The process is as follows:- - The pattern is dipped in slurry of water and clay. It is dried to get a hard shell around the pattern. - The shell-covered pattern is placed in a container with sand for support, and liquid metal is poured from a hole on top. - The foam evaporates as the metal fills the shell; upon cooling and solidification, the part is removed by breaking the shell.. There are no runners, risers, gating or parting lines. The process is used to manufacture crank-shafts for engines, aluminum engine blocks, ENG-G001 Expendable mold casting Details ENG-G001  Plaster-mold casting The mold is made by white powder mixing from (CaSO4) and silica flour. The powder is relatively not strong enough at temperature above 1200°C, so this method is mainly used to make castings from non-ferrous metals, e.g. zinc, copper, aluminum, and magnesium. ENG-G001  Ceramic mold casting Like plaster-mold casting, except that ceramic material is used (e.g. silica or powdered Zircon ZrSiO4). The process is as follows:- - The ceramic slurry forms a shell over the pattern; - It is dried in a low temperature oven, and the pattern is removed - Then it is backed by clay for strength and baked in a high temperature oven to burn off any volatile substances. - The metal is cast same as in plaster casting. This process can be used for parts such as impellor blades (for turbines, pumps, or rotors). ENG-G001 Ceramic mold castings. ENG-G001  Investment casting (lost wax process) The basic steps of the investment casting process are : 1. Preparing the heat-disposable wax patterns in a die. 2. Assembly of the prepared patterns onto a gating system 3. (covering) the pattern assembly with a refractory slurry which builds the shell. 4. Melting the pattern assembly (burning out the wax) by firing 5. The liquid metal is poured into the formed mold 6. Once the metal solidifies, the shell is removed 7. (cutting off) of the pouring basin and gates ENG-G001 The Basic Steps of the Investment Casting Process. ENG-G001  Permanent mold casting the two halves of the mold are made of metal, cast iron, steel, or refractory alloys. The cavity, including the runners and gating system are machined into the mold halves Before molding, the surface is covered with a spray of graphite or silica, which acts as a lubricant. This improves the flow of the liquid metal, and it allows the cast part to be withdrawn from the mold more easily. It is used for producing pistons gear blanks, cylinder heads, and other parts made of low melting point metals, e.g. copper, bronze, aluminum, magnesium. ENG-G001 principle of permanent mold casting. ENG-G001  Pressure die casting process Cold chamber die casting In this process the molten metal is forced under pressure to flow into the mold cavity. The mold cavity is made of special alloy heat resisting steel to insure serves life of the mold principle of cold chamber die casting process ENG-G001 hot chamber die casting ENG-G001  Centrifugal casting Centrifugal casting uses a permanent mold that is rotated about its axis at a speed between 300 to 3000 rpm as the molten metal is poured. Centrifugal forces cause the metal to be pushed out towards the mold walls, where it solidifies after cooling. Centrifugal casting for pipe production ENG-G001  Semi-Centrifugal Casting Such items as wheels and pulleys are cast in a semi- centrifugal setup. This type of mold rotates very slowly to cause the metal to first flow to the outer rim. where the mold cavity is filled from rim to hub not from bottom to top as is the case of common gravity pouring. ENG-G001