《材料工程基礎(chǔ)》課件：Chapter four（2005）

1、Chapter four: The Working of Metals 第四章 金屬塑性成型1.概述2.鍛造3.板料沖壓4.軋制5.擠壓6.拉拔塑性變形塑性變形：物體在外力作用下產(chǎn)生變形，當(dāng)去除力后仍有殘余的變形，為塑性變形。一、概述 s彈性變形彈性變形塑性變形塑性變形 加工硬化加工硬化：金屬材料發(fā)生塑性變形時，隨著變形的增大，強(qiáng)度增大，塑性和韌性降低的現(xiàn)象稱為加工硬化。 OA AB B B B e pC回復(fù)與再結(jié)晶回復(fù)與再結(jié)晶n回復(fù)回復(fù)：金屬材料產(chǎn)生塑性變形后，加熱時材料的力學(xué)性能和物理性能得到恢復(fù)，此現(xiàn)象稱為回復(fù)。加工硬化基本上保留。減小內(nèi)應(yīng)力n再結(jié)晶再結(jié)晶：金屬材料產(chǎn)生塑性變形后，加熱在

2、原來的組織中重新產(chǎn)生無疇變的新晶粒的現(xiàn)象，稱為再結(jié)晶。內(nèi)應(yīng)力和加工硬化完全消除。再結(jié)晶過程示意圖n冷變形冷變形：再結(jié)晶溫度以下的變形容易產(chǎn)生加工硬化，強(qiáng)度高，表面質(zhì)量好！變形量不大n熱變形熱變形：再結(jié)晶溫度以上的塑性變形，變形拉力小表面氧化，強(qiáng)度低，容易變形，變形量大。n所以往往先后熱變形，最后一道是冷變形。n性能特點(diǎn)：二、鍛造n自由鍛造n模型鍛造n胎模鍛造鍛造自動化生產(chǎn)自由鍛造自由鍛的工序可以分為：基本工序，輔助工序和修整工序n基本工序有：拔長Drawing out 鐓粗Compression 沖孔Punching 擴(kuò)孔 錯移Offset 扭轉(zhuǎn)Twisting等n特點(diǎn)：1）單件或小批量生產(chǎn)2

3、）適應(yīng)性強(qiáng) 空空 氣氣 錘錘（1）確定變形工序（2）設(shè)計鍛模及模膛 特點(diǎn)：生產(chǎn)率高 接近零件形狀，尺寸精確、表面光潔、加工余量小 復(fù)雜的鍛造。適合中小型鍛件的大批量生產(chǎn)。（3）特殊模鍛，精密模鍛，超塑性模鍛 模型鍛造胎模鍛造特點(diǎn)：鍛模加工簡單 3.胎模的種類 多模膛鍛造多模膛鍛造三、板料沖壓 沖壓所用的設(shè)備包括剪床和沖床，板料沖壓可分為分離工序和變形工序兩大類1. 分離工序，包括：剪切，落料，沖孔和修整2變形工序： 彎曲拉伸成形翻邊 3其他成形方法：旋壓成形，爆炸成形 簡單沖模連續(xù)沖模復(fù)合沖模剪剪 床床沖沖 床床四軋制1. 板材軋制2. 型材軋制3. 管材軋制4. 輥鍛軋制（齒輪等） 輥輥 鍛

4、鍛 機(jī)機(jī)五擠壓正擠壓，用于帶孔或?qū)嵭牧慵磾D壓，復(fù)合擠壓，徑向擠壓，等靜壓擠壓六拉拔drawing n制備細(xì)線材，薄壁管，有加工硬化，變形量不能太大，要中間熱處理，再結(jié)晶退火 Main contentsnIntroductionnForgingnPressed and deep drawingnRolling and extrusionHammer froging (自由鍛)Drop forging （模鍛）RollingDieContainerRamThe methods by which metallic materials are mechanically shaped into oth

5、er product forms are called working processes. The very extensive use of metals is due in large measure to their ability to tolerate considerable amounts permanent deformation without fracture. The products resulting from the working of metals are known as wrought （可煅的） products. The processes used

6、to change ingots into wrought forms are called primary working processes. Further working by additional methods is often required and these are known as secondary working processes. Although the main purpose of working a metal is to produce the required shape, the process may also result in an impro

7、vement in the structure and properties of the material. In the production of a finished shape in wrought metal, the size of the starting material and the sequence of the operations must be such that thorough working throughout the cross-section of the material is given. Metal that is worked largely

8、in one direction usually has different properties in different directions relative to generally be required to justify its complete production by mechanical working; otherwise the shape may be machined or fabricated from standard wrought stock（毛坯）. Anisotropy (各向異性)Depending on the temperature range

9、 in which the working is carried out, working processes can be classified as either cold-working or hot-working operations. As the temperature of a metal is raised, its strength decreases and the metal becomes softer and more plastic. Hence hot working needs less power than cold working and the defo

10、rmation can be carried out more rapidly. Because the tensile strength is much reduced at high temperatures, hot working usually involves the use of compressive stresses. On the other hand, cold working achieves a high-quality surface finish and is usually used in the final stages of shaping. 4.2 Col

11、d Working Cold working is usually carried out at room temperature and is often the finishing stage in production. The effect of cold working is to distort and elongate the grains in the direction of working. The metal becomes harder and stronger as internal stress is increased so that the characteri

12、stic ductility is much reduced. The increase in hardness resulting from the plastic deformation caused by cold working is referred to as working hardening. As cold working proceeds, the degree of work hardening is increased, the metal loses ductility, and the metal requires an increasing applied str

13、ess to cause further deformation. A stage is reached when any attempt to cause further deformation will cause fracture of the metal. 4.3 Hot Working Hot working is a shaping process carried out at temperatures above the recrystallization temperature of the material being worked. Deformation and recr

14、ystallization take place at the same time, and no distortion of grains or work hardening occurs. Typical hot-working temperature ranges for a few common metals are given in table 4.1. Table 4.1 Hot working temperaturesMetalM e l t i n gpointApprox.Recrystalli-sation Temp.Hot-workingRange oCIron15354

15、501200-900Copper1083200900-650Aluminum660150500-350Zinc42020170-110Hot working should be completed by the time the temperature of the materials has cooled to just above the recrystillisation temperature, so that the finished product will have a fine grain size with good mechanical properties. If the

16、 working is finished at a temperature far above the recrystallisation temperature, then the final grain size will be large, resulting in a low-strength material. Hot-worked material is usually cooled rapidly after the final working in order to minimise grain growth. Some materials contain brittle co

17、nstituents and/or low melting point impurities at grain boundaries and these can be troublesome during hot working. At hot-working temperatures these impurities may melt, leaving the grain boundaries weakened and the grains separated by liquid, so that, on working, the metal crumbles and is said to

18、be hot-short. 4.4 Rolling Rolling may be carried out as a hot or cold operation. The process involves feeding the metal into the gap between rolls revolving in opposite directions. The compressive forces exerted by the rolls cause the metal to deform and an increase in length is obtained as a result

19、 of the reduction in section, the metal leaving the rolls faster than it enters. In the manufacture of flat products plain rolls are used, while grooved rolls are necessary for producing sections. In the whole process of producing a rolled product, several rolling-passes involving different sets of

20、rolls may be needed. Each set of rolls is held in a housing and is referred to as a mill stand. Several mill stands may be used in conjunction to make up the rolling mill. 4.5 Forging Forging is usually a hot working process and may be carried out either under a hammer in which the blow has a high v

21、elocity, or under a press which exerts a squeezing action. In general, the hammer blows are relatively light, while presses exert a heavy load. Consequently the deformation extends to a much greater depth in press-forged material than in hammer-forged material. Hammer forging is used to shape ingots

22、 weighing less than about 5 tones. Lengths of hot rolled bar may also be forged by hammer. The material is given a number of sharp blows and the work is carried out much faster than under a press, but the effects of the work do not penetrate as deeply as in press forging. Massive foundations are req

23、uired because the shock of the hammer blows must be absorbed. Drop forging is concerned with the production of relatively large numbers of forgings from one die block. One half of the die block is attached to the anvil and the other to the tup. The hot metal is forced into the impression formed by t

24、he two halves of the die. To ensure complete filling of the die, excess stock metal must be used and the dies must have a flash gutter to collect this excess in order to allow the dies to close and give the correct dimensions. 4.6 Extrusion In extrusion the metal is squeezed through a die in similar

25、 fashion to toothpaste being forced out of a tube. Since large forces are required, the process is usually carried out as a hot-working operation, although cold extrusion is sometimes possible. Because the deformation is achieved entirely by compressive forces, it is possible to extrude relatively b

26、rittle alloys. Extrusion is a very important process for the production of bars, rods, sections and tubes in non-ferrous metals and alloys. Large reductions may be achieved by extrusion and this is normally quoted in terms of an extrusion ratio R whereR = Original cross-sectional area : Cross-sectio

27、nal area of product Thus an extrusion ratio of 10:1 is equivalent to 90% reduction.Extrusion processes may be classified as follows:1. Direct extrusion2. Indirect (or Inverted) extrusion3. Hydrostatic extrusion4. Impact extrusionDirect ExtrusionA cast metal billet is heated to the required temperatu

28、re and placed in the container of the extrusion press (fig. 4.12). The hydraulic ram then applies pressure to the billet causing the metal to be forced through the orifice（小孔）. The container and the die are fixed and therefore remain stationary during the process. Consequently the billet moves relat

29、ive to the container so that friction arises, causing a peculiar flow of the metal during extrusion in as much as the center of the billet moves forward faster than the outside. Hollow sections, including tubes, may be produced by forcing a bored（膛） billet through a die using a mandrel（芯棒） (fig. 4.13). The billet moves through the die as pressure is exerted and the section is formed in the space between the die and the mandrel. The extrusion may also be carried out by piercing the hot billet and extruding in one operation. Tubes