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譯文題目： Casting
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Casting
Casting is a manufacturing process in which molten metal is poured or injected and allowed to solidify in a suitably shaped mold cavity. During or after cooling, the cast part is removed from the mold and then processed for delivery.
Casting processes and cast-material technologies vary from simple to highly complex. Material and process selection depends on the part’s complexity and function, the product’s quality specifications,and the projected cost level.
Castings are parts that are made close to their final dimensions by a casting process. With a history dating back 6,000 years, the various casting processes are in a state of continuous refinement and evolution as technological advances are being made.
? Sand Casting
Sand casting is used to make large parts (typically iron, but also bronze, brass, aluminum). Molten metal is poured into a mold cavity formed out of sand (natural or synthetic). The processes of sand casting are discussed in this section, including patterns, sprues and runners, design considerations, and casting allowance.The cavity in the sand is formed by using a pattern (an approximate duplicate of the real part), which are typically made out of wood, sometimes metal. The cavity is contained in an aggregate housed in a box called the flask. Core is a sand shape inserted into the mold to produce the internal features of the part such as holes or internal passages. Cores are placed in the cavity to form holes of the desired shapes. Core print is the region added to the pattern, core, or mold that is used to locate and support the core within the mold. A riser is an extra void created in the mold to contain excessive molten material. The purpose of this is to feed the molten metal to the mold cavity as the molten metal solidifies and shrinks, and thereby prevents voids in the main casting.
In a two-part mold, which is typical of sand castings, the upper half, including the top half of the pattern, flask, and core is called cope and the lower half is called drag, as shown in Fig.3.1. The parting line or the parting surface is line or surface that separates the cope and drag. The drag is first filled partially with sand, and the core print, the cores, and the gating system are placed near the parting line. The cope is then assembled to the drag, and the sand is poured on the cope half, covering the pattern, core and the gating system. The sand is compacted by vibration and mechanical means. Next, the cope is removed from the drag, and the pattern is carefully removed. The object is to remove the pattern without breaking the mold cavity. This is facilitated by designing a draft, a slight angular offset from the vertical to the vertical surfaces of the pattern. This is usually a minimum of 1.5mm(0.060in.), whichever is greater. The rougher the surface of the pattern, the more the draft to be provided.
The molten material is poured into the pouring cup, which is part of the gating system that supplies the molten material to the mold cavity. The vertical part of the gating system connected to the pouring cup is the sprue, and the horizontal portion is called the runners and finally to the multiple points where it is introduced to the mold cavity called the gates. Additionally there are extensions to the gating system called vents that provide the path for the built-up gases and the displaced air to vent to the atmosphere. The cavity is usually made oversize to allow for the metal contraction as it cools down to room temperature. This is achieved by making the pattern oversize. To account for shrinking, the pattern must be made oversize by these factors on the average. These are linear factors and apply in each direction.
These shrinkage allowances are only approximate, because the exact allowance is determined by the shape and size of the casting. In addition, different parts of the casting might require different shrinkage allowances. Sand castings generally have a rough surface sometimes with surface impurities, and surface variations. A machining (finish) allowance is made for this type of defect.
In general, typical stages of sand casting operation include (as shown in Fig.3.2):
1. Patterns are made. These will be the shape used to form the cavity in the sand.
2. Cores may also be made at this time. These cores are made of bonded sand that will be broken out of the cast part after it is complete.
3. Sand is mulled (mixed) thoroughly with additives such as bentonite to increase bonding and overall strength.
4. Sand is formed about the patterns, and gates, runners, risers, vents and pouring cups are added as needed. A compaction stage is typically used to ensure good coverage and solid molds. Cores may also be added to make concave or internal features for the cast part. Alignment pins may also be used for mating the molds later. Chills may be added to cool large masses faster.
5. The patterns are removed, and the molds may be put through a baking stage to increase strength.
6. Mold halves are mated and prepared for pouring metal.
7. Metal is preheated in a furnace or crucible until is above the liquidus temperature in a suitable range (we don’t want the metal solidifying before the pour is complete). The exact temperature may be closely controlled depending upon the application. Degassing, and other treatment processes may be done at this time, such as removal of impurities (i.e. slag). Some portion of this metal may be remelted scrap from previously cast parts—10% is reasonable.
8. The metal is poured slowly, but continuously into the mold until the mold is full.
9. As the molten metal cools (minutes to days), the metal will shrink and the volume will decrease. During this time molten metal may backflow from the molten risers to feed the part and maintain the same shape.
10. Once the part starts to solidify small dendrites of solid material form in the part. During this time metal properties are being determined, and internal stresses are being generated. If a part is allowed to cool slowly enough at a constant rate then the final part will be relatively homogenous and stress free.
11. Once the part has completely solidified below the eutectic point it may be removed with no concern for final metal properties. At this point the sand is simply broken up, and the part removed. At this point the surface will have a quantity of sand adhering to the surface, and solid cores inside.
12. A bulk of the remaining sand and cores can be removed by mechanically striking the part. Other options are to use a vibrating table, sand/shot blaster, hand labor, etc.
13. The final part is cut off the runner gate system, and is near final shape using cutters, torches, etc. Grinding operations are used to remove any remaining bulk. 14. The part is taken down to final shape using machining operations. And cleaning operations may be used to remove oxides, etc.
? Investment casting
Investment casting is also known as the lost wax process. This process is one of the oldest manufacturing processes. The Egyptians used it in the time of the Pharaohs to make gold jewelry (hence the name Investment) some 5,000 years ago.
Intricate shapes can be made with high accuracy. In addition, metals that are hard to machine or fabricate are good candidates for this process. It can be used to make parts that cannot be produced by normal manufacturing techniques, such as turbine blades that have complex shapes, or airplane parts that have to withstand high temperatures.
The mold is made by making a pattern using wax or some other material that can be melted away. This wax pattern is dipped in refractory slurry, which coats the wax pattern and forms a skin. This is dried and the process of dipping in the slurry and drying is repeated until a robust thickness is achieved.
After this, the entire pattern is placed in an oven and the wax is melted away. This leads to a mold that can be filled with the molten metal. Because the mold is formed around a one-piece pattern (which does not have to be pulled out from the mold as in a traditional sand casting process), very intricate parts and undercuts can be made. The wax pattern itself is made by duplicating using a stereo lithography or similar model—which has been fabricated using a computer solid model master.
The materials used for the slurry are a mixture of plaster, a binder and powdered silica, a refractory, for low temperature melts. For higher temperature melts, sillimanite or alumina-silicate is used as a refractory, and silica is used as a binder.
Depending on the fineness of the finish desired additional coatings of sillimanite and ethyl silicate may be applied. The mold thus produced can be used directly for light castings, or be reinforced by placing it in a larger container and reinforcing it more slurry.
Just before the pour, the mold is pre-heated to about 1,000℃(1,832℉) to remove any residues of wax, harden the binder. The pour in the pre-heated mold also ensures that the mold will fill completely. Pouring can be done using gravity, pressure or vacuum conditions. Attention must be paid to mold permeability when using pressure, to allow the air to escape as the pour is done.
Tolerances of 0.5% of length are routinely possible, and as low as 0.15% is possible for small dimensions. Castings can weigh from a few grams to 35kg (0.1oz to 80lb), although the normal size ranges from 200g to about 8kg(7oz to 15 lb). Normal minimum wall thicknesses are about 1mm to about 0.5mm(0.040~ 0.020 in.) for alloys that can be cast easily. The types of materials that can be cast are aluminum alloys, bronzes, tool steels, stainless steels, stellite, hastelloys, and precious metals. Parts made with investment castings often do not require any further machining, because of the close tolerances that can be achieved.
? Centrifugal Castin
Centrifugal casting (Fig.3.3) as a category includes centrifugal casting, semi-centrifugal casting and centrifuging. In centrifugal casting, a permanent mold is rotated about its axis at high speeds (300 to 3,000rpm) as the molten metal is poured.
The molten metal is centrifugally thrown towards the inside mold wall, where it solidifies after cooling. The casting is usually a fine grain casting with a very fine-grained outer diameter, which is resistant to atmospheric corrosion, a typical situation with pipes. The inside diameter has more impurities and inclusions, which can be machined away. Only cylindrical shapes can be produced with this process. Size limits are up to 3m(10feet) diameter and 15m(50 feet) length. Wall thickness can be 2.5mm to 125mm(0.1~5.0in.). The tolerances that can be held on the OD can be as good as 2.5mm (0.1in.) and on the ID can be 3.8mm(0.15in.). The surface finish ranges from 2.5mm to 12.5mm(0.1~0.5in.) rms(root-mean-square).
Typical materials that can be cast with this process are iron, steel, stainless steels, and alloys of aluminum, copper and nickel. Two materials can be cast by introducing a second material during the process. Typical parts made by this process are pipes, boilers, pressure vessels, flywheels, cylinder liners and other parts that are axis-symmetric.
Semi-centrifugal casting. The molds used can be permanent or expendable, can be stacked as necessary. The rotational speeds are lower than those used in centrifugal casting.
The center axis of the part has inclusion defects as well as porosity and thus is suitable only for parts where this can be machined away. This process is used for making wheels, nozzles and similar parts where the axis of the part is removed by subsequent machining.
Centrifuging. Centrifuging is used for forcing metal from a central axis of the equipment into individual mold cavities that are placed on the circumference. This provides a means of increasing the filling pressure within each mold and allows for reproduction of intricate details. This method is often used for the pouring of investment casting pattern. Full-mold casting is a technique similar to investment casting, but instead of wax as the expendable material, polystyrene foam is used as the pattern. The foam pattern is coated with a refractory material. The pattern is encased in a one-piece sand mold. As the metal is poured, the foam vaporizes, and the metal takes its place.
This can make complex shaped castings without any draft or flash. However, the pattern cost can be high due to the expendable nature of the pattern. Minimum wall thicknesses are 2.5mm, tolerances can be held to 0.3% on dimensions. Surface finish can be held from 2.5μm to 25μm(0.1μin. to 1.0μin.) rms(root-mean-square).
Size limits are from 400g(1lb) to several tons. No draft allowance is required. Typical materials that can be cast with this process are aluminum, iron, steel, nickel alloys, copper alloys. Types of parts that can be made using these processes are pump housings, manifolds, and auto brake components.
鑄造
鑄造是一種將熔化的金屬倒入或注入合適的鑄模腔并且在其中固化的制造工藝。在冷卻期間或冷卻后，把鑄件從鑄模中取出，然后進(jìn)行交付。
鑄造工藝和鑄造材料技術(shù)從簡單到高度復(fù)雜變化很大。材料和工藝的選擇取決于零件的復(fù)雜性和功能、產(chǎn)品的質(zhì)量要求以及成本預(yù)算水平。
通過鑄造加工，鑄件可以做成很接近它們的最終尺寸?；厮?,000年歷史，各種各樣的鑄造工藝就如同科技進(jìn)步一樣處于一個不斷改進(jìn)和發(fā)展的狀態(tài)。
? 砂型鑄造
砂型鑄造用于制造大型零件(具有代表性是鐵，除此之外還有青銅、黃銅和鋁)。將熔化的金屬倒入由型砂(天然的或人造的)做成鑄模腔。本節(jié)討論砂型鑄造工藝，包括型模、澆注口、澆道、設(shè)計考慮因素及鑄造余量。 砂型里的型腔是采用型模(真實零件的近似復(fù)制品)構(gòu)成的，型模一般為木制，有時也用金屬制造。型腔整個包含在一個被放入稱為砂箱的箱子里的組合體內(nèi)。砂芯是插入鑄模的砂型，用于生成諸如孔或內(nèi)通道之類的內(nèi)部特征。砂芯安放在型腔里形成所需形狀的孔洞。砂芯座是加在型模、砂芯或鑄模上的特定區(qū)域，用來在鑄模內(nèi)部定位和支撐砂芯。 冒口是在鑄模內(nèi)部增加的額外空間，用于容納過多的熔化金屬。其目的是當(dāng)熔化金屬凝固和收縮時往型腔里補(bǔ)充熔化金屬，從而防止在主鑄件中產(chǎn)生孔隙。
在典型砂型鑄造的兩箱鑄模中，上半部分(包括型模頂半部、砂箱和砂芯)稱為上型箱，下半部分稱為下型箱，見圖3.1所示。分型線或分型面是分離上下型箱的線或面。首先往下型箱里部分地填入型砂和砂芯座、砂芯，并在靠近分型線處放置澆注系統(tǒng)。然后將上型箱與下型箱裝配在一起，再把型砂倒入上型箱蓋住型模、砂芯和澆注系統(tǒng)。，型砂通過振動和機(jī)械方法壓實。然后從下型箱上撤掉上型箱，小心翼翼地取出型模。其目的是取出型模而不破壞型腔。通過設(shè)計拔模斜度—型模垂直相交表面的微小角度偏移量—來使取出型模變得容易。拔模斜度最小一般為1.5mm(0.060in.)，只能比此大。
型模表面越粗糙，則拔模斜度應(yīng)越大。 熔化的金屬從澆注杯注入型腔，澆注杯是澆注系統(tǒng)向型腔提供熔化金屬的部分。將澆注系統(tǒng)的垂直部分與澆注杯連接的是澆注口，澆注系統(tǒng)的水平部分稱為澆道，最后到多點把熔化金屬導(dǎo)入型腔的稱為閘道。 除此之外，還有稱為排放口的澆注系統(tǒng)延長段，它為合成氣體和置換空氣排放到大氣提供通道。 型腔通常大于所需尺寸以允許在金屬冷卻到室溫時收縮。這通過把型模做得大于所需尺寸來達(dá)到。為解決收縮效應(yīng)，一般而言型模做得比所需尺寸大，必須考慮線性因素并作用于各個方向。
收縮余量僅僅是近似的，因為準(zhǔn)確的余量是由鑄件的形狀和尺寸決定的。另外，鑄件的不同部分也可能需要不同的收縮余量。砂型鑄件一般表面粗糙，有時還帶有表面雜質(zhì)和表面變異。對這類缺陷采用機(jī)加工(最后一道工序)的余量。
一般而言，砂型鑄造作業(yè)的典型階段包括(如圖3.2所示)
1. 制作型模。做成用于在型砂中形成型腔的形狀。
2. 同時還要制作砂芯。這些砂芯用粘結(jié)砂做成，等鑄件完成后將被打碎取出。
3. 型砂與膨潤土之類的添加劑充分地混合以增強(qiáng)連接及整體強(qiáng)度。
4. 型砂在型模周圍成形，并根據(jù)需要安放閘道、澆道、冒口、排放口和澆注杯等。通常要采取壓緊步驟來保證良好的覆蓋和堅固的鑄型。安放砂芯來制成鑄件的凹形結(jié)構(gòu)或內(nèi)部特征。為了以后鑄模匹配還要用到定位銷。對大質(zhì)量鑄件可能需要加入冷卻物來使其較快冷卻。
5. 取走型模，將鑄模烘焙以增加強(qiáng)度。
6. 匹配上下鑄模，做好澆鑄金屬的準(zhǔn)備。
7. 金屬在熔爐或坩堝中預(yù)熱到高于液化溫度的一個合適范圍內(nèi)(不希望金屬在澆鑄完成前凝固)。確切的溫度要根據(jù)應(yīng)用場合嚴(yán)格控制。在此期間還要進(jìn)行排氣和其它處理步驟，例如去除雜質(zhì)(即熔渣)?？梢约尤胍欢吭仁沁@種金屬鑄件的廢料再融化—10%是適當(dāng)?shù)摹?
8. 將金屬緩慢而連續(xù)地注滿型模。
9. 隨著熔化金屬的冷卻(幾分鐘到幾天)，金屬收縮體積減小。在此期間熔化金屬可能從冒口回流供給零件以保持其形狀不變。
10. 在零件開始凝固其內(nèi)部形成固態(tài)金屬的小型樹枝狀結(jié)晶期間金屬性能被確定，同時也產(chǎn)生了內(nèi)應(yīng)力。如果零件以恒定速率冷卻得足夠緩慢，最終零件將相對均質(zhì)并釋放內(nèi)應(yīng)力。
11. 一旦零件在共析點以下完全凝固，可以不考慮金屬的最后性能而將其取出。這時可以簡單地打碎砂型并取出零件，但零件表面會有大量型砂粘附著，內(nèi)部還有實心的砂芯。
12.大量的剩余型砂和砂芯要通過機(jī)械敲擊零件來去除。其它的選擇還有采用振動臺、噴砂/噴丸機(jī)、手工作業(yè)等等。
13. 最后零件要用刀具、噴槍等切掉澆道閘道系統(tǒng)，這樣就接近最終形狀了。再用磨削作業(yè)去除多余的部分。
14. 通過機(jī)加工將零件切削到最終形狀?？赡苓€要用清洗作業(yè)去除氧化物等。
?熔模鑄造
熔模鑄造也稱為失蠟加工。這是最古老的制造工藝之一。大約在5,000年前的法老王時代，埃及人就用它制造黃金飾品(因此而得名投資)。
復(fù)雜的形狀能被高精度地制造。另外較難機(jī)加工或制作的金屬都能用此工藝。它還能用于生產(chǎn)一般制造技術(shù)無法生產(chǎn)的零件，例如有復(fù)雜形狀的渦輪葉片或必須耐得住高溫的飛機(jī)零件。
制作鑄型的型模采用石蠟或其它一些能被融化掉的材料做成。石蠟型模浸泡在耐熱漿里，讓它覆蓋型模并形成外殼，然后使其變干。重復(fù)這個浸泡、變干的過程直至獲得足夠的厚度。
完成后把整個型模放在烤箱里融化石蠟。這樣就做成了能填充熔化金屬的鑄型。由于這種鑄型是環(huán)繞整塊型模形成的(無需像傳統(tǒng)的砂型鑄造工藝那樣拔模)，能制作十分復(fù)雜的零件和浮雕。石蠟型模本身能用立體制版或類似的模型復(fù)制—這可以采用計算機(jī)立體模型原版制作。
對較低熔化溫度而言，用于耐熱漿的材料是石膏作粘合劑和用粉末狀硅石作耐溫材料的混合物。對較高熔化溫度而言，則采用硅線石或氧化鋁硅酸鹽作耐溫材料、無水硅酸作粘合劑。
根據(jù)最后所需光潔度也可采用硅線石和乙烷基硅酸鹽。這樣生成的鑄?？芍苯佑糜诒”阼T件或通過將其放在較大容器內(nèi)用更多耐熱漿加強(qiáng)。
在正要澆鑄之前，將型模預(yù)熱到約1,000℃(1,832℉)以去除剩余石蠟、硬化粘合劑。在預(yù)熱的型模中澆鑄也能保證型模完全充滿。澆鑄可采用重力、壓力或真空條件來實現(xiàn)。當(dāng)使用壓力時必須注意滲透性，以便在澆鑄的同時讓空氣逸出。
一般公差可能為長度的0.5%，小尺寸可能低到0.15%。雖然通常尺寸的鑄件重量范圍為200g到約8kg(7oz到15lb)，但實際可從幾克到35kg (0.1oz to 80lb)。對容易鑄造的合金而言，通常壁厚約為1mm到0.5mm(0.040~ 0.020 in.)。 可以用于鑄造的材料類型有，鋁合金、青銅、工具鋼、不銹鋼、鎢鉻鈷合金、鎳基合金和貴金屬。采用熔模鑄造的零件常常不需要進(jìn)一步加工，因為熔模鑄造能達(dá)到精密的公差。
?離心鑄造
離心鑄造(圖3.3)作為一個種類包括了離心鑄造、半離心鑄造和離心法鑄造。離心鑄造中，永久性的型模在熔化金屬澆鑄時以較高速度(300到3,000rpm)繞其軸線旋轉(zhuǎn)。
受離心力作用熔化金屬被拋向型模的內(nèi)壁，在那里冷卻后固化。這種鑄件通常為外徑處晶粒非常細(xì)小的細(xì)晶粒鑄件，能耐大氣腐蝕，典型的情況是管子。內(nèi)徑處則有較多的雜質(zhì)和內(nèi)含物，但可用機(jī)加工去除。只有圓柱形才能用此工藝生產(chǎn)。尺寸限制為直徑大到3m(10feet)、長度大到15m(50feet)。壁厚為2.5mm到125mm(0.1~5.0in.)。外徑公差保持在2.5mm(0.1in.)以內(nèi)，內(nèi)徑公差保持在3.8mm(0.15in.)以內(nèi)。表面粗糙度的有效值(均方根)范圍為2.5mm到12.5mm(0.1~0.5in.)。
可用此工藝鑄造的典型材料有，鐵、鋼、不銹鋼以及鋁、銅和鎳的合金。通過在生產(chǎn)過程中加入第二種材料能進(jìn)行兩種材料鑄造。采用這種工藝制造的典型零件有管子、鍋爐、壓力容器、飛輪、汽缸襯墊和其它軸對稱零件。
半離心鑄造，型?？梢允怯谰眯缘幕蚴窍男缘?，可根據(jù)需要疊加。它的旋轉(zhuǎn)速度比離心鑄造低。
零件的中心軸附近存在缺陷和孔隙，因此僅適用于能將這些機(jī)加工去除的零件。這種工藝被用于制造車輪、管嘴及類似的隨后可用機(jī)加工去除中心軸部分的零件。
離心法鑄造，離心法鑄造用于迫使金屬從設(shè)備的中心軸進(jìn)入分布在圓周上的單獨型腔。它為每個型腔提供了一種增加填充壓力方法并允許再現(xiàn)復(fù)雜細(xì)節(jié)。這種方法常用于澆鑄熔模鑄型。實型鑄造是與熔模鑄造類似的技術(shù)，但它用做型模的消耗材料是聚苯乙烯泡沫而不是石蠟。泡沫型模用難熔材料覆蓋。型模裝入整體砂模中。當(dāng)金屬澆入時，泡沫材料蒸發(fā)，金屬取代其位置。
它能制造沒有拔模斜度和縫脊的復(fù)雜形狀鑄件。然而由于型模的消耗特性，型模成本可能較高。最小壁厚為2.5mm，公差能保持在尺寸的0.3% 之內(nèi)。表面粗糙度的有效值(均方根)能保持在2.5μm至25μm(0.1μin.至1.0μin.)之間。
重量限制從400g(1lb)到數(shù)噸。無需留拔模余量。這種工藝所用的典型材料有，鋁、鐵、鋼、鎳合金、銅合金?？梢圆捎眠@些工藝制造的零件類型有泵殼、復(fù)式接頭和自動剎車部件。