1、Chapter four: The Working of Metals 第四章 金属塑性成型1.概述2.锻造3.板料冲压4.轧制5.挤压6.拉拔塑性变形塑性变形：物体在外力作用下产生变形，当去除力后仍有残余的变形，为塑性变形。一、概述 s弹性变形弹性变形塑性变形塑性变形 加工硬化加工硬化：金属材料发生塑性变形时，随着变形的增大，强度增大，塑性和韧性降低的现象称为加工硬化。 OA AB B B B e pC回复与再结晶回复与再结晶n回复回复：金属材料产生塑性变形后，加热时材料的力学性能和物理性能得到恢复，此现象称为回复。加工硬化基本上保留。减小内应力n再结晶再结晶：金属材料产生塑性变形后，加热在
2、原来的组织中重新产生无畴变的新晶粒的现象，称为再结晶。内应力和加工硬化完全消除。再结晶过程示意图n冷变形冷变形：再结晶温度以下的变形容易产生加工硬化，强度高，表面质量好！变形量不大n热变形热变形：再结晶温度以上的塑性变形，变形拉力小表面氧化，强度低，容易变形，变形量大。n所以往往先后热变形，最后一道是冷变形。n性能特点：二、锻造n自由锻造n模型锻造n胎模锻造锻造自动化生产自由锻造自由锻的工序可以分为：基本工序，辅助工序和修整工序n基本工序有：拔长Drawing out 镦粗Compression 冲孔Punching 扩孔 错移Offset 扭转Twisting等n特点：1）单件或小批量生产2
3、）适应性强 空空 气气 锤锤（1）确定变形工序（2）设计锻模及模膛 特点：生产率高 接近零件形状，尺寸精确、表面光洁、加工余量小 复杂的锻造。适合中小型锻件的大批量生产。（3）特殊模锻，精密模锻，超塑性模锻 模型锻造胎模锻造特点：锻模加工简单 3.胎模的种类 多模膛锻造多模膛锻造三、板料冲压 冲压所用的设备包括剪床和冲床，板料冲压可分为分离工序和变形工序两大类1. 分离工序，包括：剪切，落料，冲孔和修整2变形工序： 弯曲拉伸成形翻边 3其他成形方法：旋压成形，爆炸成形 简单冲模连续冲模复合冲模剪剪 床床冲冲 床床四轧制1. 板材轧制2. 型材轧制3. 管材轧制4. 辊锻轧制（齿轮等） 辊辊 锻
4、锻 机机五挤压正挤压，用于带孔或实心零件反挤压，复合挤压，径向挤压，等静压挤压六拉拔drawing n制备细线材，薄壁管，有加工硬化，变形量不能太大，要中间热处理，再结晶退火 Main contentsnIntroductionnForgingnPressed and deep drawingnRolling and extrusionHammer froging (自由锻)Drop forging （模锻）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 (各向异性)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. 4.1
30、3). 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 can be extruded cold as well as hot. 4.7 Cold Drawing Cold drawin
31、g involves reducing the cross-section and increasing the length by pulling the metal through a die at room temperature. The material being drawn should possess high ductility and a reasonably good tensile strength. Common products of cold drawing include bright bars, rods, tubes and wire. The starti
32、ng material is either hot-rolled stock (ferrous) or extruded material (non-ferrous). Wire DrawingA very important application of cold drawing is in the production of wire. The starting material is hot worked rod about 5 to 20 mm in diameter. In general, the smallest diameter which will give the requ
33、ired mechanical properties in the finished product is used as starting material. After a certain amount of drawing through the die the material will become so brittle that further working without cracking is impossible. The brittleness is removed by annealing which is carried out in a controlled atmosphere to avoid oxidation.
