制造工程與技術(shù)Chapter 11 Metal-Casting Processes

1、Chapter 11 Metal-Casting Processes 第11章 金屬鑄造工藝Content11.1 Introduction 簡介11.2 Sand Casting 砂型鑄造11.3 Shell-mold Casting 殼型鑄造11.4 Expendable-Pattern Casting (Lost Foam) 一次模鑄造消失泡沫11.5 Plaster-Mold Casting 石膏型鑄造11.6 Ceramic-Mold Casting 陶瓷型鑄造11.7 Investment Casting 熔模鑄造11.8 Vacuum Casting 真空鑄造11.9 Perma

2、nent-Mold Casting 永久型鑄造11.10 Slush Casting 凝殼鑄造11.11 Pressure Casting 壓力鑄造11.12 Die Casting 壓模鑄造11.13 Centrifugal Casting 離心鑄造11.14 Squeeze Casting and Semisolid-Metal Casting 積壓鑄造和半固態(tài)金屬成型11.15 Casting Techniques for Single-Crystal components 單晶零件的鑄造技術(shù)11.16 Rapid Solidification (Amorphous Alloys) 快速

3、凝固非晶和金11.17 Inspection of Castings 鑄件檢驗(yàn)11.18 Melting Practice and Furnaces 熔化操作和熔爐11.19 Foundries and Foundry Automation 鑄造廠與鑄造自動(dòng)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 (曲軸), automotive componen

5、ts (Fig. 11.2), agricultural and railroad equipment, pipes and plumbing fixtures (管子附件), power tools, gun barrels (槍炮管), frying pans (煎鍋), and very large components for hydraulic turbines (水輪機(jī)) (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: 復(fù)雜的 finish: 光潔度 intricacy: 復(fù)雜 nonferrous: 非鐵磁的 lead time: 交付周期Transmission valve body: 變速閥體hub rotor: 轉(zhuǎn)子轂disk-brake cylinder: 制動(dòng)盤柱體 transmission housing: 變速箱體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 容許尺寸公差 and no defects . 當(dāng)前有兩個(gè)趨勢對鑄造工業(yè)起重要沖擊作用。第一個(gè)是正在持續(xù)進(jìn)行的鑄造過程機(jī)械化和自動(dòng)化，它導(dǎo)致鑄造設(shè)備和勞動(dòng)力方面發(fā)生重大變化。先進(jìn)的機(jī)械和自動(dòng)的過程控制系統(tǒng)已經(jīng)取代了傳統(tǒng)的鑄造方法。第二個(gè)是對具有嚴(yán)格尺寸公差和五缺陷的高質(zhì)量鑄件的需求正在增長 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 復(fù)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 (印記), (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 (草圖) 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 (滲透率), good collapsibility (退讓性).Mulling: 混沙, clay (bentonite): 粘土 (皂土), as cohesive agent (粘性劑) to bond sand particles, giving sand strength; Zircon (ZrSiO4) (鋯英石), olivine (Mg2SiO4) (橄欖石), and iron silicate (Fe2SiO4) (硅酸鐵) sands ar

19、e often used in steel foundries for their low thermal expansion; chromite (FeCr2O4) (亞鉻酸鐵，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 (濕型砂), 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 (變形), lower collapsibility resulting in more susceptible to hot tearing, low production rate. Cold-boxed mold process, various organic (有機(jī)的) and inorganic (無機(jī)的) binders (粘結(jié)劑) 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(樹脂), Harding by self, called cold-setting processes. The major component of sand mold is shown in fig. 11.4. Fig. 11.4 Schematic illustration 示意圖 o

23、f a sand mold, showing various features 性質(zhì)、特點(diǎn).Flask: 型盒, cheek: 中砂箱, Vent: 通氣孔, parting line: 分型線, cope: 上砂箱， drag: 下砂箱, core sand: 型芯砂, riser: 冒口, pouring basin: 轉(zhuǎn)包，澆注槽, sprue, runners and gates, 注流口, 流道, 內(nèi)澆道,choke: 節(jié)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 (脫模劑，分型劑) to facilitate their removed from the molds.O

26、ne-piece patterns (整體模), also called loose or solid patterns, are generally used for simpler shapes and low-quantity production, wood, inexpensive.Split patterns (分體模): Two-piece patterns made such that each part form a portion (局部) of the cavity, complicated shapes. Match-plate patterns (雙面模板模), 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: 快速原型機(jī)制造Pattern design is a crucial(決定性的) aspect of the total casting operation. The design should provide for metal shrinkage (收縮), ease of removal from the sand mold by means of a taper (坡度) or draft (減面率) (Fig. 11.7), and proper metal flow in the mold cavity.脫模角度

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 (退讓性), made by sand aggregates (聚合體). The core is anchored (固定) by core point (核點(diǎn) ), or metal support (chaplets:撐子，型芯撐 ), 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 (吹砂機(jī)，型芯吹砂機(jī)). 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 (型芯撐 ) 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: 錘子、榔頭， tamp: 夯實(shí), ram: 搗實(shí) Molding machine: 模塑造型機(jī), jolt: 震動(dòng), anvil: 鐵砧. 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 (平衡壓實(shí)活塞); and (d) flexible diaphragm (彈性膜片.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 (水平分型線), wher

34、eas in squeezing, compaction is highest at the squeezing head. Thus, more uniform compaction can be obtained by combining squeezing (擠壓) and jolting (震動(dòng)).Conventional flat head: 常規(guī)平封頭, Profile head: 齒形封頭, Equalizing squeeze piston: 均衡壓實(shí)活塞, Flexible diaphragm: 柔軟膜片Vertical flakless molding (垂直無箱模): e

35、asy and very high production rate;Sandslinger: (投砂器), impeller: 葉輪，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 (沖壓制模): the sand is compacted by controlled explosion (

36、爆炸) or instantaneous (瞬時(shí)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: 震動(dòng), sand blasting: 吹砂磨蝕, shot blasting: 噴丸清理, grit: 吹砂, sawing: 鋸斷， shearing：剪斷，abrasive wheel： 研磨輪 11.3 SHELL-MOLD CASTING (殼型鑄造)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 (脫模劑，分型劑) such as silicone, and (c) clam

39、ped to a box or chamber, as shown in Fig. 11.13.Ejector pin: 起模桿，頂桿, clamp: 夾住，夾緊, permeability: 滲透性, 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 (葉片) for turbines. Molding materials commonly used are shells, plaster (灰漿), and sand with binder (粘結(jié)劑), 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: 肋條 Fig. 11.14 (a) Schematic illustration of a semipermanent composite mold. (b) A composite mold used in casting aluminum-alloy torque convert (扭矩變換器). 11.3.2 Sodium Silicate Process 水玻璃化工藝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 沖壓石墨制模In this process, rammed (沖實(shí)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 (濕度) and temperature. The casting procedures are similar to those of sand molds.11.4 EXPENDABLE-PATTERN CASTING (LOST FOAM):一次模鑄造消失泡沫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: 揮發(fā)性碳?xì)浠衔? 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: 耐火水性漿料, 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: 氣缸體蓋, crankshaft: 曲軸, brake components: 制動(dòng)元件 Saturn: 土星, Recent developments include the use of polymethyl methacrylate (PMMA: 有機(jī)玻璃), and polyalkylene carbonate (聚烷碳酸鹽) 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 石膏模鑄造In the plaster-mold casting process, the mold is made of plaster of pairs

52、(gypsum, or calcium sulfate: 石膏, 硫酸鈣), 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 (脫水) in an autoclave (pressurized oven: 高壓爐) 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: 熱固性塑料, 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 (精密鑄造) because of the high dimensional accuracy and good surface finish obtained. Typical parts made are lock components, gears, valves, fittings (配件), tooling (封面壓花件), and ornamen

57、ts (飾物). Weight range: 10kg, usually 125-250g, also 1g.11.6 CERAMIC-MOLD CASTING 陶瓷模鑄造Clamp: 夾緊, back: 支撐, fireclay: 耐火土, firebrick: 耐火磚, torch: 吹焰器The ceramic-mold casting process, also called cope-and-drag investment casting (熔模鑄造), 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 (粘結(jié)劑) 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: 葉輪，cutters (刀具) for machining operations, dies for metalworking (金屬加工), 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: 漿料, stripping:脫模, torch: 吹焰器 Fig11.17 A typical ceramic mold (Shaw process) for casting steel dies (沖壓模) used in hot forging (鍛造). Fireclay: 火泥Steel band:鋼筘帶11.7 INVESTMENT CASTING (熔