制造工程與技術(shù)(熱加工)11[1].2

1、Metal Casting ProcessesCHAPTER 11PART IIMetalCastinaProcess Investment Casting Vacuum Casting Permanent-Mold Casting Slush Casting Pressure Casting Die Casting Centrifugal CastingInvestment Casting Also called lost-wax process First used 4000 一 3000 BC The pattern is made of wax or of a plastic by m

2、olding or rapid prototyping techniques Term investment derives from the fact that the pattern is in vested with the refractory material Need careful handling because they are not strong eno ugh to withsta nd the forces in volved in mold maki ng Wax can be recovered and reusedInvestment Casting Proce

3、ssbInitial pattern, traditionally wax, is produced slightly larger than final metal part Wax assembly is thensuccessively dipped inceramic slurry and stuccoedwith sand to produce shell.Metal delivery system is thenassembled on the pattern.The gating system is usuallymade from standard waxshapes.Wax

4、assembly is then successiuelv dipped in ceramic slurry and stuccoed with sand to produce shell.Metal delivery system is then assembled on the pattern. The gating system is usually made from standard wax shapes.Investment Casting One-piece mold一 Dried in the air一 Heated to 90 - 175 C一 Held in verted

5、for 12 hrs to melt out wax一 The mold is then heated to 650 - 1150 C for about 4 hrs depending on the metal to be cast to drive off the water of crystallization一 After the metal has been poured the mold is broken up and the cast is removed一 A number of patter ns can be joined to make one mold called

6、a tree which increases production rateOn e-piece mold - co nt一 Small parts The tree can be inserted on to a flask and filled with slurry in vestment The investment is then placed into a chamber and evacuated to remove air bubbles Next it is placed in a vacuum drawing machine to produce fine detail 一

7、 Not a cheap process一 Produces fine details一 Good surface finish一 Few or no finishing operations一 Can produce intricate parts from parts weighing 1g 一 35Kg Ex : Investment die casting examples3. SHELL BUILDING : The shell is built by immersing the assembly in a liquid ceramic slurry and then into a

8、bed of extremely fine sand. Up to eightSchematic illustration of investment casting1. WAX INJECTION : Wax replicas of the desired castings are produced by injection molding. These replicas are called patter ns.2. ASSEMBLY : The patterns are attached to a central wax stick, called a sprue, to form a

9、casting cluster or assembly.layers may be applied in this manner.4. DEWAX : Once the ceramic is dry, the wax is melted out, creating a negative impression of the assembly within the shellII7. CUT OFFThe parts are cut away from the central sprue using a high speed friction saw6. KNOCKOUTWhen the meta

10、l has cooled and solidified, the ceramic shell is broken off by vibration or water blasting.5. CONVENTIONAL CASTINGIn the conventional process, the shell is filled with molten metal by gravity pouring. As the metal cools, the parts and gates, sprue and pouring cup become one solid casting.8. FINISHE

11、D CASTINGSAfter minor finishing operations, the metal castings-identical to the original wax patterns- are ready for shipment to the customer.CeramioShell Casting一 Variation of the investment-casting process一 Uses same type of wax or plastic pattern as investment casting一 Patten is then dipped into

12、fluidized bed of Fine grained fused silica Zircon flour一 Pattern is then dipped into coarser grained silica to build up additional coatings and proper thickness to withstand the thermal shock of pouring一 The rest of the procedure follows the investment casting processFig : Investment casting of an i

13、ntegrally cast rotor for a gas turbine. (a) Wax pattern assembly, (b) Ceramic shell around wax pattern. (c) Wax melted out and the mold is filled under a vacuum, with molten super alloy.(d) The cast rotor produced to net or near-net shape..Mixture of fine sand and urethane is molded over meta

14、l dies a cured with amine vaporThe mold is partially immersed into molten metal held in an induction furnaceThe metal is melted in air or in a vacuum(b)huhiction funiaceCastingVacuum /jJ/ Mold/ MoldMolten metal5. Altem ative to in vestment, shell- mold, and greervsand casting6. Relatively low costTh

15、e molten metal is usually 55 C above the liquidus temperature 一 begins to solidify within a fraction of a secondPermanent-Molcl Casting Called hard-mold casting Two halves of a mold are made from materials such as iron, steel, bronze, or other alloys The mold cavity and gating system are machined in

16、 to the mold Sand aggregate are placed in to the mold prior to casting for producing cavities Typical core materials are一 Oil-bonded or resin-boned sand一 Plaster一 Graphite一 Gray iron一 Low-carbon steel一 Hot-worked die steel Mold cavity surfaces are coated with refractory slurry to in crease the life

17、of the mold every few castings Mechanical ejectors are used to remove complex parts Can produce high production rates Good surface finishShush Casting Molten metal is poured into the metal mold A desired thickness of the solidified skin is obtained The remaining metal is poured out The mold halves a

18、re then opened and the casting is removedPressure Castirg Used a graphite or metal mold Molten metal is forced into the mold by gas pressure The pressure is maintained until the metal solidifies in the moldUsed for high-quality castingsPressure Castoia(b)Air |irc?SAiirsRNrwdory mbcAHflK衛(wèi) lumlmi盹dhoi

19、d訕卿I Grapliilv mvNpol,r,r, basln G他Chill11Whed Hub Plolc(a) The bottom-pressure casting process utilizes graphite molds for the productin of steel railroad wheels(b) Gravity pouring method of casting a railroad wheel. Note that the pouring basin also serves as a riser.Die Casting Further example of

20、permanent-mold casting Molten metal is forced into the die cavity at pressures ranging from .7MPa 一 700MPa Parts made from here range from:一 Hand tools一 Toys一 Applianee components There are two basic types of die casting machines一 Hot-chamber involves the use of a piston to push molten metal in to t

21、he die cavity一 Cold-chamber 一 molten metal is poured in to the injection chamber & the shot chamber is not heatedHot chamber Diecasting DrocessMolten Meta) 1 The die is closed and the piston rises, opening the port and allowing molten metal to fill the cyli nder. 2. The plunger moves down and seals

22、the port pushing the molten metal through the gooseneck and nozzle into the die cavity, where it is held under pressure until it solidifies 3 The die ope ns and the cores, if any, retract. The casting remains in only one die, the ejector side. The plunger returns, allowing residual molten metal to f

23、low back through the nozzle and goose neck. 4. Ejector pins push the casting out of the ejector die. As the plunger uncovers the filling hole, molten metal flows through the inlet to refill the goose neck, as in step (1).ColdDie casting process 2. The plunger pushes the molten metal into the die cav

24、ity where it is held under pressure until solidification.1. The die is closed and the molten metal is ladled into the cold-chamber shot sleeve 4 Ejector pins push the casting out of the ejector die and the plunger returns to its original position. 3 The die ope ns and the plunger advances, to ensure

25、 that the casting remains in the ejector die. Cores, if any, retract.Process Capabilities and Machine Selection一 Dies are rated according to their clamping force that is needed一 Factors involved in selection of die cast machines are Die size Piston stroke Shot pressure Cost一 Die-casting dies Single

26、cavity Multiple-cavity Combination-cavity Unit dies一 Ratio of Die weight to part weight is 1000 to 1一 Surface cracking is a problem with dies due to the hot metal that is poured in to them一 Has ability to produce strong high- quality parts with complex shapes一 Good dimensional accuracy and surface d

27、etailsVarious types of cavities in a die casting die.a)b)d) Unit die800 ton hot chamber die casting machine, DAM 8005. This is the largesthot chamber machine in the world and costs about $1.25 million.Centrifuqal Casting Utilizes the inertial forces caused by rotation to distribute the molten metal

28、in to the mold cavities First used in the 1800;s Three types of centrifugal casting一 True centrifugal casting一 Semi centrifugal casting一 CentrifugingMoklDrive sliah RqIHts- 1Schematic illustration of the centrifugal casting process. Pipes, cylinder liners, and similarly shaped parts can be cast with

29、 this process.Semi centrifuqal Casting Process(a) Schematic illustration of the semi centrifugal casting process. Wheels with spokes can be cast by this process(b) Schematic illustration oi casting by centrifuging. The molds are placed at the periphery of the machine, and the molten metal is forced into the molds by centrifugal force(a) Melt MetalSqueeze Castinq(c) Close die and apply pressure(d) Eject squeeze casting and charge melt stock and repeat cycle(b) Pour molten metal into dieSequence of operatio