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英語翻譯 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 藝中引起的應力大于材料的屈服強度 但小于材料的斷裂強度 加載的類型可以是 拉應力 壓應力 彎曲應力或剪應力 或者是這些類型的組合 這是個很經(jīng)濟的方 法 因為可以獲得所需的形狀 尺寸和光潔度而無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變硬化提高產(chǎn)品的強度上得到了卓有成效的利用 成形工藝可以分為以下兩個大類 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 這一個過程就被稱為冷變形 材 料的流動應力在再結(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 塑性變形而無大的冷作硬化 這一點很重要 因為大的冷作硬化會使材料變脆 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 0 6 此外 熱變形降低了材料的強度 結(jié)果甚至可以 使用具有一定容量的機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 種成形操作地機械學原理 我們將簡要討論每一種方法 軋制 在這一工藝中 通過一個調(diào)整過的位于兩個動力驅(qū)動的軋輥之間的孔利用摩擦 來拉伸工件 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 材料在兩個和多個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 當截面需要減小很多時 也許有必要通過幾個階段來完成此操作 拉深 在拉深中 杯形產(chǎn)品是在一個凸模和一個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坯料壓板來避免產(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)計適當?shù)耐鼓：桶寄硗瓿?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 工件被壓進一 個封閉空間迫使材料通過一個被稱為磨具的適當?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)點與缺點 既然我們已經(jīng)談及了各類金屬加工工序 現(xiàn)在我們應該給熱加工和冷加工工藝 一個總體評價了 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 因為活躍晶粒在加工溫度范圍內(nèi)會生長 就有可能適當控制 晶粒尺寸 于是沒有冷作硬化 因此無須昂貴耗時的中間退火 當然 在一些操作 如拉延 中應變硬化是可取的 可以提高強度 在這些情況下 熱加工幾乎沒有 優(yōu)勢 除此之外 要成功地完成一些工藝應變硬化可能是必不可少的 如在拉深中 冷作硬化可防止應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)品和高強度材料 因為提升的溫度降低了強度 進而降低了載荷 此外 對于大多數(shù)材料來說 延展性隨溫度而增大 因此易碎的材料也可采用熱加 工工序來加工 但是應該記住某些材料 如含硫磺的鋼 在提高溫度時會變得更脆 當需要非常精確地控制尺寸時 熱加工就不太適合 因為金屬的表面會生成氧化皮 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)點是經(jīng)濟 操作更為迅速 容易 因為無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 機械特性通常得以提高 此外 晶粒流 動方向的控制增加了產(chǎn)品的強度特性 然而 冷加工除了其他局限性以外 如難以 加工高強度和脆性材料以及大的產(chǎn)品尺寸 該工藝還有一個不好的特性 即無法 防止防腐蝕性能明顯減小