Presentation on theme: "Metal Casting Processes Part 2"— Presentation transcript:
1Metal Casting Processes Part 2 Manufacturing Processes, 1311Dr Simin NasseriSouthern Polytechnic State University
2Permanent Mold Casting Processes Students’ presentations Economic disadvantage of expendable mold casting: a new mold is required for every castingIn permanent mold casting, the mold is reused many timesThe processes include:Basic permanent mold castingDie castingCentrifugal casting
3The Basic Permanent Mold Process Uses a metal mold constructed of two sections designed for easy, precise opening and closingMolds used for casting lower melting point alloys are commonly made of steel or cast ironMolds used for casting steel must be made of refractory material, due to the very high pouring temperatures
4Permanent Mold Casting Figure Steps in permanent mold casting: (1) mold is preheated and coatedFigure Steps in permanent mold casting: (2) cores (if used) are inserted and mold is closed, (3) molten metal is poured into the mold, where it solidifies.
5Advantages and Limitations Advantages of permanent mold casting:Good dimensional control and surface finishMore rapid solidification caused by the cold metal mold results in a finer grain structure, so castings are strongerLimitations:Generally limited to metals of lower melting pointSimpler part geometries compared to sand casting because of need to open the moldHigh cost of mold
6Applications of Permanent Mold Casting Due to high mold cost, process is best suited to high volume production and can be automated accordinglyTypical parts: automotive pistons, pump bodies, and certain castings for aircraft and missilesMetals commonly cast: aluminum, magnesium, copper‑base alloys, and cast iron
7Slush CastingSlush Casting is a special type of permanent mold casting, where the molten metal is not allowed to completely solidify. After the desired wall thickness is obtained, the not yet solidified molten metal is poured out. This is useful for making hollow ornamental objects such as candlesticks, lamps, statues etc.
8Slush CastingLow-melting-point metals such as lead, zinc, and tin are used.The exterior appearance is important, but the strength and interior geometry of the casting are minor considerations.
9Thin wall castings can be made. Low Pressure CastingInstead of using gravity to assist in the metal pour and flow in the mold, a low pressure of up to 0.1 MPa (15 psi) gas is applied to the molten metal.This maintenance of pressure on the melt causes complete fill of the mold and compensates for any shrinkage on cooling.Thin wall castings can be made.
10Low Pressure Casting Mechanical properties are superior. Since no riser is used (unlike a regular casting), the yield is generally higher since the metal in the pressurized feed tube is still molten and the mold is ready for the next shot right away.Molten metal is always cleanerGas porosity and oxidation defects are minimized.
11Vacuum Permanent Mold Casting Similar to the low-pressure permanent mold casting, where a vacuum is used instead of a pressure.Reduced air pressure from the vacuum in the mold is used to draw the liquid metal into the cavity (rather than forcing it by pressure)Thin wall castings can be made as in the low-pressure permanent mold casting. In addition, the yields are high since no risers are used.Advantages: Reduced air porosity, greater strength.
12Die CastingA permanent mold casting process in which molten metal is injected into mold cavity under high pressurePressure is maintained during solidification, then mold is opened and part is removedMolds in this casting operation are called dies; hence the name die castingUse of high pressure to force metal into die cavity is what distinguishes this from other permanent mold processes
14Die Casting MachinesDesigned to hold and accurately close two mold halves and keep them closed while liquid metal is forced into cavityTwo main types:Hot‑chamber machineCold‑chamber machine
15Hot-Chamber Die Casting Metal is melted in a container, and a piston injects liquid metal under high pressure into the dieHigh production rates parts per hour not uncommonApplications limited to low melting‑point metals that do not chemically attack plunger and other mechanical componentsCasting metals: zinc, tin, lead, and magnesium
16Hot-Chamber Die Casting Figure Cycle in hot‑chamber casting: (1) with die closed and plunger withdrawn, molten metal flows into the chamber
17Hot-Chamber Die Casting Figure Cycle in hot‑chamber casting: (2) plunger forces metal in chamber to flow into die, maintaining pressure during cooling and solidification.
18Cold‑Chamber Die Casting Machine Molten metal is poured into unheated chamber from external melting container, and a piston injects metal under high pressure into die cavityHigh production but not usually as fast as hot‑chamber machines because of pouring stepCasting metals: aluminum, brass, and magnesium alloysAdvantages of hot‑chamber process favor its use on low melting‑point alloys (zinc, tin, lead)
19Cold‑Chamber Die Casting Figure Cycle in cold‑chamber casting: (1) with die closed and ram withdrawn, molten metal is poured into the chamber
20Cold‑Chamber Die Casting Figure Cycle in cold‑chamber casting: (2) ram forces metal to flow into die, maintaining pressure during cooling and solidification.
21Molds for Die CastingUsually made of tool steel, mold steel, or maraging steelTungsten and molybdenum (good refractory qualities) used to die cast steel and cast ironEjector pins required to remove part from die when it opensLubricants must be sprayed into cavities to prevent stickingMaraging Steel: a strong tough low-carbon martensitic steel which contains up to 25 percent nickel and in which hardening precipitates are formed by aging
22Advantages and Limitations Advantages of die casting:Economical for large production quantitiesGood accuracy and surface finishThin sections are possibleRapid cooling provides small grain size and good strength to castingDisadvantages:Generally limited to metals with low metal pointsPart geometry must allow removal from die
23Centrifugal CastingA family of casting processes in which the mold is rotated at high speed so centrifugal force distributes molten metal to outer regions of die cavityThe group includes:True centrifugal castingSemicentrifugal castingCentrifuge casting
24True Centrifugal Casting Molten metal is poured into rotating mold to produce a tubular part (we have radial symmetry)In some operations, mold rotation commences after pouring rather than beforeParts: pipes, tubes, bushings, and ringsOutside shape of casting can be round, octagonal, hexagonal, etc , but inside shape is (theoretically) perfectly round, due to radially symmetric forces
25True Centrifugal Casting Figure Setup for true centrifugal casting.
26Semicentrifugal Casting Centrifugal force is used to produce solid castings rather than tubular partsMolds are designed with risers at center to supply feed metalDensity of metal in final casting is greater in outer sections than at center of rotation (because of condensation)Examples: wheels and pulleys
27Centrifuge CastingMold is designed with part cavities located away from axis of rotation, so that molten metal poured into mold is distributed to these cavities by centrifugal forceUsed for smaller partsRadial symmetry of part is not required as in other centrifugal casting methods
28Furnaces for Casting Processes Furnaces most commonly used in foundries:CupolasDirect fuel‑fired furnacesCrucible furnacesElectric‑arc furnacesInduction furnaces
29CupolasVertical cylindrical furnace equipped with tapping spout near baseUsed only for cast ironsAlthough other furnaces are also used, the largest tonnage of cast iron is melted in cupolasThe "charge," consisting of iron, coke, flux, and possible alloying elements, is loaded through a charging door located less than halfway up height of cupola
30Direct Fuel‑Fired Furnaces Small open‑hearth in which charge is heated by natural gas fuel burners located on side of furnaceFurnace roof assists heating action by reflecting flame down against chargeAt bottom of hearth is a tap hole to release molten metalGenerally used for nonferrous metals such as copper‑base alloys and aluminum
31Crucible FurnacesMetal is melted without direct contact with burning fuel mixtureSometimes called indirect fuel‑fired furnacesContainer (crucible) is made of refractory material or high‑temperature steel alloyUsed for nonferrous metals such as bronze, brass, and alloys of zinc and aluminumThree types used in foundries: (a) lift‑out type, (b) stationary, (c) tilting
32Crucible FurnacesFigure Three types of crucible furnaces: (a) lift‑out crucible, (b) stationary pot, from which molten metal must be ladled, and (c) tilting-pot furnace.
33Electric‑Arc Furnaces Charge is melted by heat generated from an electric arcHigh power consumption, but electric‑arc furnaces can be designed for high melting capacityUsed primarily for melting steel
35Induction FurnacesUses alternating current passing through a coil to develop magnetic field in metalInduced current causes rapid heating and meltingElectromagnetic force field also causes mixing action in liquid metalSince metal does not contact heating elements, environment can be closely controlled to produce molten metals of high quality and purityMelting steel, cast iron, and aluminum alloys are common applications in foundry work
37Ladles Last slide of students’ presentations Moving molten metal from melting furnace to mold is sometimes done using cruciblesMore often, transfer is accomplished by ladlesFigure Two common types of ladles: (a) crane ladle, and (b) two‑man ladle.
39Additional Steps After Solidification TrimmingRemoving the coreSurface cleaningInspectionRepair, if requiredHeat treatment
40TrimmingRemoval of sprues, runners, risers, parting‑line flash, fins, chaplets, and any other excess metal from the cast partOtherwise, hammering, shearing, hack‑sawing, band‑sawing, abrasive wheel cutting, or various torch cutting methods are used
41Removing the Core If cores have been used, they must be removed Most cores are bonded, and they often fall out of casting as the binder deterioratesIn some cases, they are removed by shaking casting, either manually or mechanicallyIn rare cases, cores are removed by chemically dissolving bonding agentSolid cores must be hammered or pressed out
42Surface CleaningRemoval of sand from casting surface and otherwise enhancing appearance of surfaceCleaning methods: tumbling, air‑blasting with coarse sand grit or metal shot, wire brushing, polishing and buffing, and chemical pickling (to give a light finish to by bleaching or painting and wiping)Surface cleaning is most important for sand castingIn many permanent mold processes, this step can be avoided
43Heat Treatment Castings are often heat treated to enhance properties Reasons for heat treating a casting:For subsequent processing operations such as machiningTo bring out the desired properties for the application of the part in service
45Casting QualityThere are numerous opportunities for things to go wrong in a casting operation, resulting in quality defects in the productThe defects can be classified as follows:General defects common to all casting processesDefects related to sand casting process
46General Defects: Misrun A casting that has solidified before completely filling mold cavityFigure Some common defects in castings: (a) misrun
47General Defects: Cold Shut Two portions of metal flow together but there is a lack of fusion due to premature freezingFigure Some common defects in castings: (b) cold shut
48General Defects: Cold Shot Metal splatters during pouring and solid globules form and become entrapped in castingFigure Some common defects in castings: (c) cold shot
49General Defects: Shrinkage Cavity Depression in surface or internal void caused by solidification shrinkage that restricts amount of molten metal available in last region to freezeFigure Some common defects in castings: (d) shrinkage cavity
50Sand Casting Defects: Sand Blow Balloon‑shaped gas cavity caused by release of mold gases during pouringFigure Common defects in sand castings: (a) sand blow
51Figure 11.23 Common defects in sand castings: (b) pin holes Sand Casting Defects: Pin HolesFormation of many small gas cavities at or slightly below surface of castingFigure Common defects in sand castings: (b) pin holes
52Sand Casting Defects: Penetration When fluidity of liquid metal is high, it may penetrate into sand mold or core, causing casting surface to consist of a mixture of sand grains and metalFigure Common defects in sand castings: (e) penetration
53Sand Casting Defects: Mold Shift A step in cast product at parting line caused by sidewise relative displacement of cope and dragFigure Common defects in sand castings: (f) mold shift
55Foundry Inspection Methods Visual inspection to detect obvious defects such as misruns, cold shuts, and severe surface flawsDimensional measurements to insure that tolerances have been metMetallurgical, chemical, physical, and other tests concerned with quality of cast metal
57Metals for CastingMost commercial castings are made of alloys rather than pure metalsAlloys are generally easier to cast, and properties of product are betterCasting alloys can be classified as:FerrousNonferrous
58Molten Facts Metal Tin, Lead, Zinc (786F) Aluminum 1220 F Howto?Barely HotHotVery HotToo HotTempBelow 1000F1000F-1500F1500F-2250F2250F or HigherMetalTin, Lead,Zinc (786F)Aluminum 1220 FBrass, Bronze, Gold, Silver or CopperIron, SteelToolsThese alloys can be melted on the stove in a soup can. Caution: Most low-melting alloys are TOXIC, vent well and use a respirator.Aluminum can be melted in a coffee can on the BBQ, use propane, wood or charcoal for fuel.A gas or electric crucible furnace is typical.Electric Induction furnaces are used for large commercial foundries.Cupola furnaces use coke (refined coal) for smaller batches.SafetyneedsSafety GlassesGloves and GlassesThick shirt and pants.Glasses and gloves."Going into a volcano" suit !
59Ferrous Casting Alloys: Cast Iron Most important of all casting alloysTonnage of cast iron castings is several times that of all other metals combinedSeveral types: (1) gray cast iron, (2) nodular iron, (3) white cast iron, (4) malleable iron, and (5) alloy cast ironsTypical pouring temperatures 1400C (2500F), depending on compositionNodular or ductile cast iron: Tiny amounts of magnesium or cerium added to these alloys slow down the growth of graphite precipitates by bonding to the edges of the graphite planes.
60Ferrous Casting Alloys: Steel The mechanical properties of steel make it an attractive engineering materialThe capability to create complex geometries makes casting an attractive shaping processDifficulties when casting steel:Pouring temperature of steel is higher than for most other casting metals 1650C (3000F)At such temperatures, steel readily oxidizes, so molten metal must be isolated from airMolten steel has relatively poor fluidity
61Nonferrous Casting Alloys: Aluminum Generally considered to be very castablePouring temperatures low due to low melting temperature of aluminumTm = 660C (1220F)Properties:Light weightRange of strength properties by heat treatmentEasy to machine
62Nonferrous Casting Alloys: Copper Alloys Includes bronze, brass, and aluminum bronzeProperties:Corrosion resistanceAttractive appearanceGood bearing qualitiesLimitation: high cost of copperApplications: pipe fittings, marine propeller blades, pump components, ornamental jewelryCopper + Zinc = BrassCopper + Tin + other elements= BronzeBrass,Bronze?
63Nonferrous Casting Alloys: Zinc Alloys Highly castable, commonly used in die castingLow melting point – melting point of zinc Tm = 419C (786F)Good fluidity for ease of castingProperties:Low creep strength, so castings cannot be subjected to prolonged high stresses
65Product Design Considerations Geometric simplicity:Although casting can be used to produce complex part geometries, simplifying the part design usually improves castabilityAvoiding unnecessary complexities:Simplifies mold‑makingReduces the need for coresImproves the strength of the casting
66Product Design Considerations Corners on the casting:Sharp corners and angles should be avoided, since they are sources of stress concentrations and may cause hot tearing and cracksGenerous fillets should be designed on inside corners and sharp edges should be blendedFILLET. Concave corner piece usually used at the intersection of right-angle surfaces (that would otherwise meet at an angle) on patterns and core boxes. Fillets in cast shapes lessen the danger of cracks and avoid "fillet shrinkages."
67Product Design Considerations Draft Guidelines:In expendable mold casting, draft facilitates removal of pattern from moldDraft = 1 for sand castingIn permanent mold casting, purpose is to aid in removal of the part from the moldDraft = 2 to 3 for permanent mold processesSimilar tapers should be allowed if solid cores are used
68Draft Minor changes in part design can reduce need for coring Figure Design change to eliminate the need for using a core: (a) original design, and (b) redesign.
69Product Design Considerations Dimensional Tolerances and Surface Finish:Significant differences in dimensional accuracies and finishes can be achieved in castings, depending on process:Poor dimensional accuracies and finish for sand castingGood dimensional accuracies and finish for die casting and investment casting
70Product Design Considerations Machining Allowances:Almost all sand castings must be machined to achieve the required dimensions and part featuresAdditional material, called the machining allowance, is left on the casting in those surfaces where machining is necessaryTypical machining allowances for sand castings are around 1.5 and 3 mm (1/16 and 1/4 in)