铸造技术基础培训的英文课件

1、Fundamental of Materials Forming-Casting basic conceptMain contents1、铸造的概念及特点、铸造技术的发展； Fundamentals of Metal-Casting2、金属铸造工艺 Metal-Casting Processes 砂型铸造及其工艺设计； 特种铸造及其工艺；3、铸件的结构设计、材料及经济性 Metal Casting: Design, Materials, and Economics5、先进铸造技术。 Advanced CastingSand Casting Sand Casting processThe con

2、cept for Casting Schematic illustration of a typical riser-gated casting. Risers serve as reservoirs, supplying molten metal to the casting as it shrinks during solidification. See also Fig. 11.4 Source: American Foundrymens Society.Why Sand Casting? In metal casting work, as with almost every kind

3、of manufacture, high precision means high cost. In a case we are making a small number of objects so we cant really afford to make a sophisticated metal mold (i.e. we cant do die casting). The usual approach in this case is to make disposable molds out of sand or plaster. The kind of disposable mold

4、, and the techniques used to make the mold, are again determined by questions of cost and quality. For a very intricate, precise result, lost wax or lost polystyrene foam casting is used. For less intricate workpieces, or lower precision requirements, direct sand casting can be adequate.What is Sand

5、 Casting?All casting, not just sand casting, is about pouring or squirting a molten substance into a cavity. All casting methods require two basic things:A cavity which is the shape of the desired final object, and A way to put molten substance into the cavity. In the sand casting process, the cavit

6、y is made of sand which is held together by a binder (a substance that holds the sand grains together). Sand casting is used to produce a wide variety of metal components with complex geometries. These parts can vary greatly in size and weight, ranging from a couple ounces to several tons. Some smal

7、ler sand cast parts include components as gears, pulleys, crankshafts, connecting rods, and propellers. Larger applications include housings for large equipment and heavy machine bases. Sand casting is also common in producing automobile components, such as engine blocks, engine manifolds, cylinder

8、heads, and transmission cases.Die-Casting Examples(a)(b)Figure 11.1 (a) The Polaroid PDC-2000 digital camera with a AZ91D die-cast, high purity magnesium case. (b) Two-piece Polaroid camera case made by the hot-chamber die casting process. Source: Courtesy of Polaroid Corporation and Chicago White M

9、etal Casting, Inc.Casting ExamplesFigure 11.2 Typical gray-iron castings used in automobiles, including transmission valve body (left) and hub rotor with disk-brake cylinder (front). Source: Courtesy of Central Foundry Division of General Motors Corporation.Figure 11.3 A cast transmission housing.So

10、lidification ProcessesStarting work material is either a liquid or is in a highly plastic condition, and a part is created through solidification of the materialSolidification processes can be classified according to engineering material processed: MetalsCeramics, specifically glasses Polymers and p

11、olymer matrix composites (PMCs)Classification of solidification processesCastingProcess in which molten metal flows by gravity or other force into a mold where it solidifies in the shape of the mold cavityThe term casting also applies to the part made in the processSteps in casting seem simple: Melt

12、 the metalPour it into a moldLet it freezeCapabilities and Advantages of CastingCan create complex part geometriesCan create both external and internal shapesSome casting processes are net shape; others are near net shapeCan produce very large parts Some casting methods are suited to mass production

13、Disadvantages of CastingDifferent disadvantages for different casting processes: Limitations on mechanical propertiesPoor dimensional accuracy and surface finish for some processes; e.g., sand castingSafety hazards to workers due to hot molten metalsEnvironmental problemsParts Made by CastingBig par

14、ts: engine blocks and heads for automotive vehicles, wood burning stoves, machine frames, railway wheels, pipes, church bells, big statues, and pump housings Small parts: dental crowns, jewelry, small statues, and frying pans All varieties of metals can be cast, ferrous and nonferrous Overview of Ca

15、sting TechnologyCasting is usually performed in a foundry Foundry = factory equipped for making molds, melting and handling molten metal, performing the casting process, and cleaning the finished castingWorkers who perform casting are called foundrymenThe Mold in Casting Contains cavity whose geomet

16、ry 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 Two forms of mold: (a) open mold, simply a container in the shap

17、e 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 Two Categories of Casting ProcessExpendable mold processes uses an expendable mold which must be destroyed to remove castingMold materials: sand, p

18、laster, and similar materials, plus bindersPermanent mold processes uses a permanent mold which can be used many times to produce many castingsMade of metal (or, less commonly, a ceramic refractory materialAdvantages and DisadvantagesMore intricate geometries are possible with expendable mold proces

19、sesPart shapes in permanent mold processes are limited by the need to open moldPermanent mold processes are more economic in high production operationsWhy Do We Prefer Casting?Near net shapeLow scrapRelatively quick processLarge hollow shapesNo limit to sizeReasonable to good surface finishMetal Cas

20、ting ProcessesSand CentrifugalShell Die Ceramic Investment Permanent mold Plaster moldEvoparative PatternMetals processed by casting Sand casting 60% Investment casting 7% Die casting 9% Permanent mold casting 11% Centrifugal casting 7% Shell mold casting 6%(b) Sand casting moldSand Casting Mold Ter

21、msMold consists of two halves: Cope = upper half of moldDrag = bottom halfMold halves are contained in a box, called a flaskThe two halves separate at the parting lineForming the Mold CavityMold cavity is formed by packing sand around a pattern, which has the shape of the partWhen the pattern is rem

22、oved, the remaining cavity has desired shape of cast partThe pattern is usually oversized to allow for shrinkage of metal as it solidifies and cools Sand for the mold is moist and contains a binder to maintain shape Cores in the Mold CavityThe mold cavity provides the external surfaces of the cast p

23、artIn addition, a casting may have internal surfaces, determined by a core, placed inside the mold cavity to define the interior geometry of partIn sand casting, cores are generally made of sandGating System Channel through which molten metal flows into cavity from outside of mold Consists of a down

24、sprue, through which metal enters a runner leading to the main cavity At top of downsprue, a pouring cup is often used to minimize splash and turbulence as the metal flows into downsprueRiser Reservoir in the mold which is a source of liquid metal to compensate for shrinkage during solidification Th

25、e riser must be designed to freeze after the main casting in order to satisfy its function Heating the MetalHeating furnaces are used to heat the metal to molten temperature sufficient for casting The heat required is the sum of: Heat to raise temperature to melting point Heat of fusion to convert f

26、rom solid to liquid Heat to raise molten metal to desired temperature for pouring Pouring the Molten MetalFor this step to be successful, metal must flow into all regions of the mold, most importantly the main cavity, before solidifying Factors that determine success:Pouring temperaturePouring rateT

27、urbulence Transformation of molten metal back into solid stateSolidification differs depending on whether the metal is a pure element or an alloyFigure 10.4 Cooling curve for a pure metal during castingA pure metal solidifies at a constant temperature equal to its freezing point (same as melting poi

28、nt)Solidification of Pure Metals Due to chilling action of mold wall, a thin skin of solid metal is formed at the interface immediately after pouringSkin thickness increases to form a shell around the molten metal as solidification progressesRate of freezing depends on heat transfer into mold, as we

29、ll as thermal properties of the metal Characteristic grain structure in a casting of a pure metal, showing randomly oriented grains of small size near the mold wall, and large columnar grains oriented toward the center of the casting (a) Phase diagram for a coppernickel alloy system and (b) associat

30、ed cooling curve for a 50%Ni50%Cu composition during castingMost alloys freeze over a temperature range rather than at a single temperature Characteristic grain structure in an alloy casting, showing segregation of alloying components in center of castingSolidification TimeSolidification takes timeT

31、otal solidification time TST = time required for casting to solidify after pouringTST depends on size and shape of casting by relationship known as Chvorinovs RuleChvorinovs Rule where TST = total solidification time; V = volume of the casting; A = surface area of casting; n = exponent usually taken

32、 to have a value = 2; and Cm is mold constantMold Constant in Chvorinovs RuleCm depends on mold material, thermal properties of casting metal, and pouring temperature relative to melting pointValue of Cm for a given casting operation can be based on experimental data from previous operations carried

33、 out using same mold material, metal, and pouring temperature, even though the shape of the part may be quite different What Chvorinovs Rule Tells UsA casting with a higher volumetosurface area ratio cools and solidifies more slowly than one with a lower ratioTo feed molten metal to main cavity, TST

34、 for riser must greater than TST for main castingSince riser and casting mold constants will be equal, design the riser to have a larger volumetoarea ratio so that the main casting solidifies first This minimizes the effects of shrinkage Shrinkage of a cylindrical casting during solidification and c

35、ooling: (0) starting level of molten metal immediately after pouring; (1) reduction in level caused by liquid contraction during cooling (dimensional reductions are exaggerated for clarity in sketches)(2)reduction in height and formation of shrinkage cavity caused by solidification shrinkage; (3) fu

36、rther reduction in height and diameter due to thermal contraction during cooling of the solid metal (dimensional reductions are exaggerated for clarity in our sketches)Solidification ShrinkageOccurs in nearly all metals because the solid phase has a higher density than the liquid phase Thus, solidif

37、ication causes a reduction in volume per unit weight of metalException: cast iron with high C content Graphitization during final stages of freezing causes expansion that counteracts volumetric decrease associated with phase change Shrinkage AllowancePatternmakers account for solidification shrinkag

38、e and thermal contraction by making mold cavity oversized Amount by which mold is made larger relative to final casting size is called pattern shrinkage allowance Casting dimensions are expressed linearly, so allowances are applied accordingly Directional SolidificationTo minimize damaging effects o

39、f shrinkage, it is desirable for regions of the casting most distant from the liquid metal supply to freeze first and for solidification to progress from these remote regions toward the riser(s)Thus, molten metal is continually available from risers to prevent shrinkage voids The term directional so

40、lidification describes this aspect of freezing and methods by which it is controlledAchieving Directional SolidificationDesired directional solidification is achieved using Chvorinovs Rule to design the casting itself, its orientation in the mold, and the riser system that feeds itLocate sections of

41、 the casting with lower V/A ratios away from riser, so freezing occurs first in these regions, and the liquid metal supply for the rest of the casting remains open Chills internal or external heat sinks that cause rapid freezing in certain regions of the casting (a) External chill to encourage rapid

42、 freezing of the molten metal in a thin section of the casting; and (b) the likely result if the external chill were not usedCast Structures of Metals 金属的铸态组织 Figure 10.1 Schematic illustration of three cast structures of metals solidified in a square mold: pure metals(纯金属); (b) solid-solution alloy

43、s （固溶合金）; and (c) structure obtained by using nucleating agents（孕育剂）. Source: G. W. Form, J. F. Wallace, J. L. Walker, and A. Cibula.Chill zone 激冷区Columnar zone 柱状区Equiaxed zone 等轴晶区Equiaxed Structure 等轴结构Riser DesignRiser is waste metal that is separated from the casting and remelted to make more c

44、astings To minimize waste in the unit operation, it is desirable for the volume of metal in the riser to be a minimumSince the geometry of the riser is normally selected to maximize the V/A ratio, this allows reduction of riser volume as much as possible 冒口 RiserTRANSLATION Gating system 浇注系统Trap co

45、ntaminant 阻挡杂质Oxides 氧化物 inclusions 夹杂Premature cooling turbulence 紊流Gas entrapment 气泡 impurity 杂质 Figure 10.8 A test method for fluidity using a spiral mold. The fluidity index is the length of the solidified metal in the spiral passage. The greater the length of the solidified metal, the greater i

46、s its fluidity.fluidity index 流动性指标 spiral 螺旋螺旋形流动性试样 spiral passageFluidity Test 流动性测试What influence “the fluldity of molten metal”?1.Characteristics of molten metal:Viscosity(粘性); Surface tension(表面张力); Inclusions(夹杂);Solidification pattern of the alloy(合金的凝固方式).2.Casting parametersMold design(模具设

47、计);Mold material and its surface characteristics(模具材料及其表面特性);Degree of superheat(过热度);Rate of pouring(浇注速度);Heat transfer(热传导); 液态收缩 Liquid shrinkage 凝固收缩 Solidification shrinkage 固态收缩 Solid shrinkage 熔融金属的收缩shrinkage（Contraction） Liquid shrinkage refers to the reduction in volume when the metal cha

48、nges from liquid to solid state at the solidus temperature, Solid shrinkage is the reduction in volume caused, when metal loses temperature in solid state. caused, when metal loses temperature in solid state. Solidification Contraction for Various Cast Metals 收缩是液态合金凝固冷却过程中的必然现象，它产生缩孔与缩松，内应力，变形和裂纹，对

49、铸造工艺影响很大。 缩孔与缩松shrinkage cavity 铸造应力 casting stress铸件变形 casting deformation铸件裂纹 casting crackDefects p253 Metallic projections 突起类缺陷 Fins 披缝 Flash飞边 Swells 隆起 Rough surface粗糙表面 Cavities 孔穴类缺陷 blowholes 气孔 pinholes 针孔 shrinkage cavity(porsity)缩孔 Discontinuities 不连续缺陷 Crack 开裂 Cold (Hot)tearing 冷(热)裂 Cold shut 冷隔 Coarse grain 粗大晶粒 segregates偏析 intergranular晶间 Defective surface 表面缺陷 surface folds 表面折叠 laps表面结疤( Scars