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英語(yǔ)翻譯
Forming
Forming can be defined as a process in which the desired size and shape are obtained through the plastic deformations of a material. The stresses induced during the process are greater than the yield strength, but less than the fracture strength, of the material. The type of loading may be tensile, compressive, bending, or shearing, or a combination of these. This is a very economical process as the desired shape, size, and finish can be obtained without any significant loss of material. Moreover, a part of the input energy is fruitfully utilized in improving the strength of the product through strain hardening.
The forming processes can be grouped under two broad categories, namely, cold forming, and hot forming. If the working temperature is higher than the recrystallization temperature of the material, then the process is called hot forming. Otherwise the process is termed as cold forming. The flow stress behavior of a material is entirely different above and below its recrystallization temperature. During hot working, a large amount of plastic deformation can be imparted without significant strain hardening. This is important because a large amount of strain hardening renders the material brittle. The frictional characteristics of the two forming processes are also entirely different. For example, the coefficient of friction in cold forming is generally of the order of 0.1, whereas that in hot forming can be as high as 0.6. Further, hot forming lowers down the material strength so that a machine with a reasonable capacity can be used even for a product having large dimensions.
The typical forming processes are rolling, forging, drawing, deep drawing, bending, and extrusion. For a better understanding of the mechanics of various forming operations, we shall briefly discuss each of these processes.
Rolling
In this process, the job is drawn by means of friction through a regulated opening between two power-driven roll. The shape and size of the product are decided by the gap between the rolls and their contours. This is a very useful process for the production of sheet metal and various common sections, e.g., rail, channel, angle, and round.
Forging
In forging, the material is squeezed between two or more dies to alter its shape and size. Depending on the situation, the dies may be open or closed.
Drawing
In this process, the cross-section of a wire or that of a bar or tube is reduced by pulling the workpiece through the conical orifice of a die. When high reduction is required, it may be necessary to perform the operation in several passes.
Deep Drawing
In deep drawing, a cup-shaped product is obtained from a flat sheet metal with the help of a punch and a die. The sheet metal is held over the die by means of a blank holder to avoid defects in the product.
Bending
As the name implies, this is a process of bending a metal sheet plastically to obtain the desired shape. This is achieved by a set of suitably designed punch and die.
Extrusion
This is a process basically similar to the closed die forming. But in this operation, the workpiece is compressed in a closed space, forcing the material to flow out through a suitable opening, called a die. In this process, only the shapes with constant cross-sections (die outlet cross-section) can be produced.
Advantages and Disadvantages of Hot and Cold Forming
Now that we have covered the various types of metal working operations, it would only be appropriate that we provide an overall evaluation of the hot and cold working processes. Such a discussion will help in choosing the proper working conditions for a given situation.
During hot working, a proper control of the grain size is possible since active grain growth takes place in the range of the working temperature. As a result, there is no strain hardening, and therefore there is no need of expensive and time-consuming intermediate annealing. Of course, strain hardening is advisable during some operations (viz., drawing) to achieve an improved strength; in such cases, hot working is less advantageous. Apart from this, strain hardening may be essential for a successful completion of some processes (e.g., in deep drawing, strain hardening prevents the rupture of the material around the bottom circumference where the stress is maximum). Large products and high strength materials can be worked upon under hot conditions since the elevated temperature lowers down the strength and, consequently, the work load. Moreover, for most materials, the ductility increases with temperature and, as a result, brittle can also be worked upon by the hot working operation. It should, however, be remembered that there are certain materials (viz., steels containing sulphur ) which become more brittle at elevated temperatures. When a very accurate dimensional control is required, hot working is not advised because of shrinkage and loss of surface metal due to scaling. Moreover, surface finish is poor due to oxide formation and scaling.
The major advantages of cold working are that it is economical, quicker, and easier to handle because here no extra arrangements for heating and handling are necessary. Further, the mechanical properties normally get improved during the process due to strain hardening. What is more, the control of grain flow directions adds to the strength characteristics of the product. However, apart from other limitations of cold working (viz., difficulty with high strength and brittle materials and large product sizes), the inability of the process to prevent the significant reduction brought about in corrosion resistance is an undesirable feature.
成形
成形可以定義為一種通過材料的塑性變形獲得所需尺寸和形狀的工藝。在此工藝中引起的應(yīng)力大于材料的屈服強(qiáng)度，但小于材料的斷裂強(qiáng)度。加載的類型可以是拉應(yīng)力、壓應(yīng)力、彎曲應(yīng)力或剪應(yīng)力，或者是這些類型的組合。這是個(gè)很經(jīng)濟(jì)的方法，因?yàn)榭梢垣@得所需的形狀、尺寸和光潔度而無(wú)需使材料有任何大的損失。此外，一部分輸入的能量在通過應(yīng)變硬化提高產(chǎn)品的強(qiáng)度上得到了卓有成效的利用。
成形工藝可以分為以下兩個(gè)大類，即冷成形和熱成形。如果加工溫度高于材料的再結(jié)晶溫度，那么這一過程就叫熱變形，否則，這一個(gè)過程就被稱為冷變形。材料的流動(dòng)應(yīng)力在再結(jié)晶溫度之上或之下全然不同。在熱加工過程中，可以產(chǎn)生大的塑性變形而無(wú)大的冷作硬化。這一點(diǎn)很重要，因?yàn)榇蟮睦渥饔不瘯?huì)使材料變脆。兩種成形方法的摩擦特性也完全不同。例如，冷成形的摩擦系數(shù)一般為0.1左右，而熱變形的摩擦系數(shù)可以高達(dá)0.6。此外，熱變形降低了材料的強(qiáng)度，結(jié)果甚至可以使用具有一定容量的機(jī)器加工很大尺寸的產(chǎn)品。
典型的成形方法有軋制、鍛造、拉延、拉深、彎曲和擠壓。為了更好地理解各種成形操作地機(jī)械學(xué)原理，我們將簡(jiǎn)要討論每一種方法。
軋制
在這一工藝中，通過一個(gè)調(diào)整過的位于兩個(gè)動(dòng)力驅(qū)動(dòng)的軋輥之間的孔利用摩擦來(lái)拉伸工件。產(chǎn)品的形狀和尺寸由軋輥及其輪廓之間的間隙來(lái)決定。這是一種很有用途的工藝，用于生產(chǎn)金屬薄板和各種常用端面，如鐵軌、槽鋼、角鋼和圓鋼。
鍛造
鍛造時(shí)，材料在兩個(gè)和多個(gè)磨具間受到擠壓以改變其形狀和尺寸。根據(jù)情況不同，磨具可以是開式或閉式。
拉延
在這一工藝中，金屬絲的截面或者是條鋼或鋼管的 截面由于工件被拉過磨具的錐形孔而減小。當(dāng)截面需要減小很多時(shí)，也許有必要通過幾個(gè)階段來(lái)完成此操作。
拉深
在拉深中，杯形產(chǎn)品是在一個(gè)凸模和一個(gè)凹模的幫助下由一塊金屬板獲得的。金屬薄板被放在磨具上利用一個(gè)坯料壓板來(lái)避免產(chǎn)品缺陷。
彎曲
如其名所示，這是一道塑性彎曲一塊金屬薄板以獲得所需形狀的工藝。這道工藝由一套設(shè)計(jì)適當(dāng)?shù)耐鼓：桶寄?lái)完成。。
擠壓
這是一道基本上類似于閉式磨具鍛造的工藝。但是在該工序中，工件被壓進(jìn)一個(gè)封閉空間迫使材料通過一個(gè)被稱為磨具的適當(dāng)?shù)拈_口處流出。在這一工藝中，只有具有固定截面（磨具出口截面）的形狀可以制造。
熱、冷成形的優(yōu)點(diǎn)與缺點(diǎn)
既然我們已經(jīng)談及了各類金屬加工工序，現(xiàn)在我們應(yīng)該給熱加工和冷加工工藝一個(gè)總體評(píng)價(jià)了。這一討論將有助于為給定的情況選擇適合的加工條件。
在熱加工過程中，因?yàn)榛钴S晶粒在加工溫度范圍內(nèi)會(huì)生長(zhǎng)，就有可能適當(dāng)控制晶粒尺寸。于是沒有冷作硬化，因此無(wú)須昂貴耗時(shí)的中間退火。當(dāng)然，在一些操作（如拉延）中應(yīng)變硬化是可取的，可以提高強(qiáng)度；在這些情況下，熱加工幾乎沒有優(yōu)勢(shì)。除此之外，要成功地完成一些工藝應(yīng)變硬化可能是必不可少的（如在拉深中，冷作硬化可防止應(yīng)力最大處的深處圓周周圍的材料斷裂）。只要在熱的狀態(tài)下，就可以加工大件產(chǎn)品和高強(qiáng)度材料，因?yàn)樘嵘臏囟冉档土藦?qiáng)度，進(jìn)而降低了載荷。此外，對(duì)于大多數(shù)材料來(lái)說(shuō)，延展性隨溫度而增大，因此易碎的材料也可采用熱加工工序來(lái)加工。但是應(yīng)該記住某些材料（如含硫磺的鋼）在提高溫度時(shí)會(huì)變得更脆。當(dāng)需要非常精確地控制尺寸時(shí)，熱加工就不太適合，因?yàn)榻饘俚谋砻鏁?huì)生成氧化皮而收縮或損失；再者，由于氧化物的形成和起氧化皮表面光潔度很差。
冷加工的主要優(yōu)點(diǎn)是經(jīng)濟(jì)、操作更為迅速、容易，因?yàn)闊o(wú)須安排額外的加熱和處理。另外，在加工過程中由于冷作硬化，機(jī)械特性通常得以提高。此外，晶粒流動(dòng)方向的控制增加了產(chǎn)品的強(qiáng)度特性。然而，冷加工除了其他局限性以外（如難以加工高強(qiáng)度和脆性材料以及大的產(chǎn)品尺寸），該工藝還有一個(gè)不好的特性，即無(wú)法防止防腐蝕性能明顯減小。