制造工程与技术Chapter 11 Metal-Casting Processes

1、Chapter 11 Metal-Casting Processes 第11章 金属铸造工艺Content11.1 Introduction 简介11.2 Sand Casting 砂型铸造11.3 Shell-mold Casting 壳型铸造11.4 Expendable-Pattern Casting (Lost Foam) 一次模铸造消失泡沫11.5 Plaster-Mold Casting 石膏型铸造11.6 Ceramic-Mold Casting 陶瓷型铸造11.7 Investment Casting 熔模铸造11.8 Vacuum Casting 真空铸造11.9 Perma

2、nent-Mold Casting 永久型铸造11.10 Slush Casting 凝壳铸造11.11 Pressure Casting 压力铸造11.12 Die Casting 压模铸造11.13 Centrifugal Casting 离心铸造11.14 Squeeze Casting and Semisolid-Metal Casting 积压铸造和半固态金属成型11.15 Casting Techniques for Single-Crystal components 单晶零件的铸造技术11.16 Rapid Solidification (Amorphous Alloys) 快速

3、凝固非晶和金11.17 Inspection of Castings 铸件检验11.18 Melting Practice and Furnaces 熔化操作和熔炉11.19 Foundries and Foundry Automation 铸造厂与铸造自动化11.1 IntroductionThe first castings were made during the period 4000-3000 B.C., using stone and metal molds for casting copper. Various casting processes have been develo

4、ped over a long period of time, each with its own characteristics and application, to meet specific engineering and service requirements (Table 11.1). Many parts and components are made by casting, including cameras (Fig. 11.1), carburetors (汽化器), engine blocks, crankshafts (曲轴), automotive componen

5、ts (Fig. 11.2), agricultural and railroad equipment, pipes and plumbing fixtures (管子附件), power tools, gun barrels (枪炮管), frying pans (煎锅), and very large components for hydraulic turbines (水轮机) (Fig. 11.3).ProcessAdvantagesLimitationsSandAlmost any metal cast; no limit to size, shape or weight; low

6、cooling cost.Some finishing required; somewhat coarse finish; wide tolerances.Shell moldGood dimensional accuracy and surface finish; high production ratePart size limited; expensive patterns and equipment required.Expendable patternMost metals cast with no limit to size; complex shapes.Patterns hav

7、e low strength and can be costly for low quantities.Plaster moldIntricate shapes, good dimensional accuracy and finish; low porosity.Limited to nonferrous metals; limited size and volume of production; mold making time relatively long.Ceramic moldIntricate shapes; close tolerance parts; good surface

8、 finish.Limited size.InvestmentIntricate shapes; excellent surface finish and accuracy; almost any metal cast.Part size limited; expensive patterns mold and labor.Permanent moldGood surface finish and dimensional accuracy; low porosity; high production rateHigh mold cost; limited shape and intricacy

9、; not suitable for high-melting point metals.DieExcellent dimensional accuracy and surface finish; high production rate.Die cost is high; part size limited; usually limited to nonferrous metals; long lead time.CentrifugalLarge cylindrical parts with good quality; high production rate.Equipment is ex

10、pensive, part shape limitedTABLE 11.1 Summary of Casting processes, Their advantages and LimitationIntricate: 复杂的 finish: 光洁度 intricacy: 复杂 nonferrous: 非铁磁的 lead time: 交付周期Transmission valve body: 变速阀体hub rotor: 转子毂disk-brake cylinder: 制动盘柱体 transmission housing: 变速箱体Two trends are currently having

11、a major impact on the casting industry. The first is continuing mechanization and automation of the casting process, which has led to significant changes in the use of equipment and labor. Advanced machinery and automated process-control systems have replaced traditional methods of casting. The seco

12、nd is the increasing demand for high-quality castings with close dimensional tolerances 容许尺寸公差 and no defects . 当前有两个趋势对铸造工业起重要冲击作用。第一个是正在持续进行的铸造过程机械化和自动化，它导致铸造设备和劳动力方面发生重大变化。先进的机械和自动的过程控制系统已经取代了传统的铸造方法。第二个是对具有严格尺寸公差和五缺陷的高质量铸件的需求正在增长 表。ProcessTypical materials castWeight (kg)Typical surfacefinish (m

13、, Ra)Porosity*Shape complexity*Dimensional accuracy*Section thickness (mm)MinMaxMinMaxSandAll0.05No limit5-2541-233No limitShell moldAll0.05100+1-342-322Expendable patternAll0.05No limit5-204122No limitPlaster moldNonferrous (Al, Mg, Zn, Cu)0.0550+1-233-421InvestmentAll (high melting pt.)0.005100+1-

14、3311175Permanent moldAll0.53002-32-33-41250DieNonferrous (Al, Mg, Zn, Cu)0.05501-21-23-410.512Centrifugalall5000+2-101-23-432100TABLE 11.2 General Characteristics of Casting Processes* Relative rating: 1 best, 5 worst. Expendable molds 一次性模, made of sand, plaster, ceramic and similar materials, used

15、 just one time, shown in 11.2-11.8.Permanent mold 永久性模, used many time, shown in 11.9-11.14;Composite mold 复合模, made by two different materials in order to get desired properties.This chapter is organized as 3 major categories based on different mold materials as follow: 11.2 SAND CASTINGSand castin

16、g, the oldest casting process, consists of (a) placing a pattern having the shape of the desired casting in sand to make an imprint (印记), (b) incorporating a gating system, (c) filling the resulting cavity with molten metal, (d) allowing the metal to cool until it solidifies, (e) breaking away the s

17、and mold, and (f) removing the casting.Till now, the sand casting is still the most prevalent (普遍的，流行的) form of casting.Fig. 11.5 Outline (草图) of production steps in a typical sand-casting operation 11.2.1 SandsSilica sand (SiO2), two types: naturally banded (bank sand: 岸沙), synthetic (lake sand: 湖沙

18、)Requirement: fine-grained sand results in lower mold permeability (渗透率), good collapsibility (退让性).Mulling: 混沙, clay (bentonite): 粘土 (皂土), as cohesive agent (粘性剂) to bond sand particles, giving sand strength; Zircon (ZrSiO4) (锆英石), olivine (Mg2SiO4) (橄榄石), and iron silicate (Fe2SiO4) (硅酸铁) sands ar

19、e often used in steel foundries for their low thermal expansion; chromite (FeCr2O4) (亚铬酸铁，尖晶石) is used for it high heat-transfer characteristics. 11.2.2 Types of Sand MoldsThree basic types of sand molds: green-sand (湿型砂), cold-box (冷芯盒), and no-bake (自硬的) molds.Green molding sand: mixture of sand,

20、clay and water, moist or damp while metal poured, least expensive method of making mold.Skin-dried method, dried in air of torches (吹焰器), used for large castings because of their high strength.Oven (烤箱) dried method: dried in oven before metal poured, stronger than green-sand molds and better dimens

21、ional accuracy and surface finish; drawbacks: distortion (变形), lower collapsibility resulting in more susceptible to hot tearing, low production rate. Cold-boxed mold process, various organic (有机的) and inorganic (无机的) binders (粘结剂) are blended (混合) into the sand to bond the grains chemically for gre

22、ater strength, more dimensional accuracy, but more expensive compared with green-sand mold.No-bake mold process, mixed with synthetic (合成的) liquid resin(树脂), Harding by self, called cold-setting processes. The major component of sand mold is shown in fig. 11.4. Fig. 11.4 Schematic illustration 示意图 o

23、f a sand mold, showing various features 性质、特点.Flask: 型盒, cheek: 中砂箱, Vent: 通气孔, parting line: 分型线, cope: 上砂箱， drag: 下砂箱, core sand: 型芯砂, riser: 冒口, pouring basin: 转包，浇注槽, sprue, runners and gates, 注流口, 流道, 内浇道,choke: 节流口11.2.3 Patterns (模型)Patterns are used to mold the sand mixture into the shape of

24、 the casting. They may be made of wood, plastic, or metal. The selection of a pattern material depends on the size and shape of the casting, the dimensional accuracy, the quantity of castings required, and the molding process.Because patterns are used repeatedly to make molds, the strength and durab

25、ility (持久性) of the material selected for patterns must reflect the number of castings that the mold will produce. They may be made of a combination of materials to reduce wear in critical regions. Patterns are usually coated with a parting agent (脱模剂，分型剂) to facilitate their removed from the molds.O

26、ne-piece patterns (整体模), also called loose or solid patterns, are generally used for simpler shapes and low-quantity production, wood, inexpensive.Split patterns (分体模): Two-piece patterns made such that each part form a portion (局部) of the cavity, complicated shapes. Match-plate patterns (双面模板模), po

27、pular type of mounted pattern in which two-piece patterns are constructed by securing (定位) each half of one or more split patterns to the opposite sides of a single plate. Gating system is in the drag side, to produce smaller casting in large number.Fig. 11.6 A typical metal match-plate pattern used

28、 in sand castingRapid prototyping: 快速原型机制造Pattern design is a crucial(决定性的) aspect of the total casting operation. The design should provide for metal shrinkage (收缩), ease of removal from the sand mold by means of a taper (坡度) or draft (减面率) (Fig. 11.7), and proper metal flow in the mold cavity.脱模角度

29、Fig. 11.7 Taper on patterns for ease of removal from the sand mold11.2.4 Cores (型芯)Cores are placed in the mold cavity before casting to form the interior surface of the casting and are removed from the finished part during shakeout and further processing. Strength, permeability, ability to withstan

30、d head, and collapsibility (退让性), made by sand aggregates (聚合体). The core is anchored (固定) by core point (核点 ), or metal support (chaplets:撑子，型芯撑 ), and made in similar manner for mold made. Cores are formed in core-boxes (砂芯盒). The sand can be packed into the boxes with sweeps, or blown into the bo

31、x by compressed air from core blowers (吹砂机，型芯吹砂机). The latter has the advantages of producing uniform cores and operating at very high production rate.Fig. 11.8 Examples of sand cores showing core prints (型芯座) and chaplets (型芯撑 ) to support cores.11.2.5 Sand-Molding MachineThe oldest known method of

32、 molding is to compact the sand by hand hammering (tamping) or ramming it around the pattern.Hammer: 锤子、榔头， tamp: 夯实, ram: 捣实 Molding machine: 模塑造型机, jolt: 震动, anvil: 铁砧. Fig11.9 Various designs of squeeze heads for mold making: (a) conventional flat head; (b) profile (仿形) head; (c) equalizing squee

33、ze pistons (平衡压实活塞); and (d) flexible diaphragm (弹性膜片.Process: the flask, molding sand, and pattern are first placed on a pattern plate mounted on an anvil, and then jolted upward by air pressure at rapid intervals. Jolting produces the highest compaction at the horizontal parting line (水平分型线), wher

34、eas in squeezing, compaction is highest at the squeezing head. Thus, more uniform compaction can be obtained by combining squeezing (挤压) and jolting (震动).Conventional flat head: 常规平封头, Profile head: 齿形封头, Equalizing squeeze piston: 均衡压实活塞, Flexible diaphragm: 柔软膜片Vertical flakless molding (垂直无箱模): e

35、asy and very high production rate;Sandslinger: (投砂器), impeller: 叶轮，ram: 填塞 Fig. 11.10 vertical flaskless molding. (a) Sand is squeezed between two halves of the pattern. (b) Assemlbed molds pass along an assembly line for pouring.Impact molding (冲压制模): the sand is compacted by controlled explosion (

36、爆炸) or instantaneous (瞬时的) release of compressed gases. This method produces molds with uniform strength and good permeability.Vacuum molding (真空模塑法) produces castings with high-quality detail and dimensional accuracy. It is especially well suited for large, relatively flat casting. 11.2.6 The Sand

37、Casting OperationA complete sequence of operations in sand casting is shown in Fig. 11.11.Shake: 震动, sand blasting: 吹砂磨蚀, shot blasting: 喷丸清理, grit: 吹砂, sawing: 锯断， shearing：剪断，abrasive wheel： 研磨轮 11.3 SHELL-MOLD CASTING (壳型铸造)Shell-mold casting was first developed in the 1940s and has grown signifi

38、cantly because it can produce many types of castings with close dimensional tolerances and good surface finish at low cost. In this process, (a) a mounted pattern (模板) made of a ferrous metal of aluminum is heated to 175-370OC, (b) coated with a parting agent (脱模剂，分型剂) such as silicone, and (c) clam

39、ped to a box or chamber, as shown in Fig. 11.13.Ejector pin: 起模杆，顶杆, clamp: 夹住，夹紧, permeability: 渗透性, 11.3.1 Composite MoldsComposite molds are made of two or more different materials and are used in shell molding and other casting processes. They are generally employed in casting complex shapes suc

40、h as impeller (叶片) for turbines. Molding materials commonly used are shells, plaster (灰浆), and sand with binder (粘结剂), metal and graphite. Composite molds increase the strength of the mold, improve the dimensional accuracy and surface finish of castings, and may help reduce overall cost and processi

41、ng time.Rib: 肋条 Fig. 11.14 (a) Schematic illustration of a semipermanent composite mold. (b) A composite mold used in casting aluminum-alloy torque convert (扭矩变换器). 11.3.2 Sodium Silicate Process 水玻璃化工艺The mold material in the sodium silicate process is a mixture of sand and 1.5% to 6% sodium silica

42、te (waterglass) as a binder. The mixture is packed around the pattern and hardened by blowing carbon dioxide gas through it. This process, also known as silicate-bonded sand or the carbon-dioxide (Co2) process, was first used in the 1950s and has been developed further, for example, by using various

43、 other chemicals for binders. 11.3.3 Rammed Graphite Molding 冲压石墨制模In this process, rammed (冲实的) graphite is used to make molds for casting reactive metals such as titanium and zirconium. The mold are packed rather like sand molds, air dried, baked (烘烤) at 175oC, fired at 870oC, then stored under co

44、ntrolled humidity (湿度) and temperature. The casting procedures are similar to those of sand molds.11.4 EXPENDABLE-PATTERN CASTING (LOST FOAM):一次模铸造消失泡沫The expendable-pattern casting process uses a polystyrene (聚苯乙烯) pattern, which evaporates upon contact with molten metal to form a cavity for the ca

45、sting, evaporative-pattern or lost-pattern casting, trade name: Full-Mold process, for ferrous and nonferrous metals, especial for the automotive industry.Pattern made sequence: expendable polystyrene beads (珠子) + 5-8%pentane (正戊烷) (a volatile hydrocarbon: 挥发性碳氢化合物) placed under preheated die usuall

46、y made by aluminum, then expand and take the shape of the die cavity, fuse and bond the heads together, cooled and removed pattern.Complex patterns may also be made by bonding various individual pattern sections using hot-melt adhesive.Molding: water-based refractory slurry: 耐火水性浆料, Pouring molten m

47、etal immediately vaporizes the pattern (ablation: 消融) and fills the mold cavity, completely replacing the space previously occupied by the polystyrene pattern. The heat degrades (depolymerizes: 去聚合) the polystyrene and the degradation products are vented into the surrounding sands. Characteristics:

48、laminar flow, Reynolds number: 400-3000, velocity of flow: 0.1-1m/s, increased velocity by forming patterns with cavity or hollow section. Cooling rate higher than sand casting leads to less fluidity and directional solidification of the metal.Fig. 11.15 Schematic illustration of the expendable patt

49、ern casting process, also know as lost foam or evaporative casting.Advantages:Relatively simple, design flexibility;Inexpensive flaskInexpensive polystyrene (聚苯乙烯) , easily pattern made with complex shape, various sizes and fine surface detail.Minimum finishing and cleaning operation requiredAutomat

50、e and economical for long production runs.Typical applications: cylinder heads: 气缸体盖, crankshaft: 曲轴, brake components: 制动元件 Saturn: 土星, Recent developments include the use of polymethyl methacrylate (PMMA: 有机玻璃), and polyalkylene carbonate (聚烷碳酸盐) as pattern materials for ferrous castings.Polystyre

51、ne pattern surrounded by a ceramic shell (Replicast C-S process), which burned prior to pouring molten metal into the mold. Principal advantage: carbon pickup into the metal is entirely avoided.11.5. PLASTER-MOLD CASTING 石膏模铸造In the plaster-mold casting process, the mold is made of plaster of pairs

52、(gypsum, or calcium sulfate: 石膏, 硫酸钙), with the addition of talc (滑石粉) and silica flour (石英粉)to improve strength and to control the time required for the plaster to set. These components are mixed with water, and the resulting slurry is poured over the pattern.After the plaster sets, usually within

53、15min, the pattern is removed and the mold is dried at 120oC to remove the moisture. Higher drying temperature may be used depending on the type of plaster. The mold halves are assembled to form the mold cavity and are preheated to about 120oC. The molten metal is the poured into the mold.Low permea

54、bility, vacuum or under pressure casting used. Antioch process: the molds are dehydrated (脱水) in an autoclave (pressurized oven: 高压炉) for 6-12 hours, then dehydrated in air for 14 hours. Another method of increasing permeability is to use foamed plaster containing trapped air bubbles.Pattern materia

55、ls: aluminum alloys, thermosetting plastic: 热固性塑料, brass, of zinc alloys. Temperature withstand 1200oC. Application: aluminum, magnesium, zinc and some copper alloys. Fine details and good surface finish; more uniform grain structure with less warpage (折曲)obtained due to low cooling rate. Wall thick

56、ness of parts can be 1mm-2.5mm. Plaster-mold, ceramic-mold and investment casting process are know as precision casting (精密铸造) because of the high dimensional accuracy and good surface finish obtained. Typical parts made are lock components, gears, valves, fittings (配件), tooling (封面压花件), and ornamen

57、ts (饰物). Weight range: 10kg, usually 125-250g, also 1g.11.6 CERAMIC-MOLD CASTING 陶瓷模铸造Clamp: 夹紧, back: 支撑, fireclay: 耐火土, firebrick: 耐火砖, torch: 吹焰器The ceramic-mold casting process, also called cope-and-drag investment casting (熔模铸造), is similar to the plaster-mold process, with exception that it us

58、es refractory mold materials suitable for high-temperature applications. The slurry is a mixture of fine-grained zircon (ZrSiO4), aluminum alloys, and fused silica (SiO2), which are mixed with bonding agents (粘结剂) and poured over the pattern.Application: ferrous and other high-temperature alloys, st

59、ainless steels, and tool steels.Good dimensional accuracy and surface finish over a wide range of sizes and intricate shapes, expensiveTypical parts: impellers: 叶轮，cutters (刀具) for machining operations, dies for metalworking (金属加工), and molds for making plastic or rubber components. Parts weighting

60、as much as 700kg have been cast by this process. Fig. 11.16 Sequence of operations in making a ceramic mold. Slurry: 浆料, stripping:脱模, torch: 吹焰器 Fig11.17 A typical ceramic mold (Shaw process) for casting steel dies (冲压模) used in hot forging (锻造). Fireclay: 火泥Steel band:钢筘带11.7 INVESTMENT CASTING (熔