外文翻译-铸造及其他成形工艺

附录
英文：
Casting and Other Forming Processes DeGarmo, E. Paul et al. Materials and Processes in Manufacturintg. John Wiley
& Song, 1998. P205-211
Casting
Casting is the introduction of molten metal into a cavity or mold where, upon solidification, it becomes an object whose shape is determined by mold configuration. Casting offers several advantages over other method of metal forming: it is adaptable to intricate shapes, to extremely large pieces, and to mass production; it can provide parts with uniform physical and mechanical properties through out and, depending on the particular material being cast, the design of the part, and the quantity being produced, its economic advantages can surpass other processes.
Categories
Two broad categories of metal-casting processes exist: ingot casting (which includes continuous casting) and casting to shape. Ingot castings are produced by pouring molten metal into a permanent or reusable mold. Following solidification these ingots (or bars, slabs, or billets, as the case may be) are then further processed mechanically into many new shapes. Casting to shape involves pouring molten metal into molds in which the cavity provides the final useful shape, followed only by machining or welding for the specific application.
Ingot casting Ingot castings make up the majority of all metal castings and are separated into three categories: static cast ingots, semi-continuous or direct-chill cast ingots, and continuous cast ingots.
Static cast ingots Static ingot casting simply involves pouring molten metal into a permanent mold. After solidification, the ingot is withdrawn from
the mold and the mold can be reused. This method is used to produce millions of tons steel annually.
Semi-continuous cast ingots A semi continuous casting process is employed in the aluminum industry to produce most of the cast alloys from which rod, sheet, strip, and plate configurations are made. In this process molten aluminum is transferred to a water-cooled permanent mold which has a movable base mounted on a long piston. After solidification has progressed from the mold surface so that a solid "skin" is formed, the piston is moved down. Finally the piston will have moved its entire length, and the process is stopped. Conventional practice in the aluminum industry utilizes suitably lubricated metal molds. However, technological advances have allowed major aluminum alloy producers to replace the metal mold (at least in part) by an electromagnetic field so that molten metal touches the metal mold only briefly, thereby making a product with a much smoother finish than that produced conventionally.
Continuous cast ingots Continuous casting provides a major source of cast material. In the steel and copper industry and is growing rapidly in the aluminum industry. In this process molten metal delivered to a permanent mold, and the casting begins much in the same way as in semi continuous casting. However, instead of the process ceasing after a certain length of time, the solidified ingot is continually sheared or cut into lengths and removed during casting. Thus the process is continuous, the solidified bar or strip being removed as rapidly as it is being cast. This method has many economic advantages over the more conventional casting techniques; as a result, all modem steel mills produce continuous cast products.
Casting to shape Casting to shape is generally classified according to the molding process, molding material or method of feeding the mold. There are four basic types of these casting processes: sand, permanent-mold, die, and centrifugal
Sand casting This is the traditional method which still produces the
largest volume of cast-to-shape pieces. It utilizes a mixture of sand grains, water, clay, and other materials to make high-quality molds for use with molten metal. This "green sand" mixture is compacted around a pattern (wood, plaster, or metal), usually by machine, to 20-80% of its bulk density. The two halves of the mold (the cope and drag) are closed over cores necessary to form internal cavities, and the whole assembly is weighted or damped to prevent floating of the cope when the metal is poured.
Other casting processes which utilize sands as a basic component are the shell, carbon dioxide, investment casting, ceramic molding and plaster molding processes. In addition, there are a large number of chemically bonded sands which are becoming increasingly important.
Permanent-mold casting Many high-quality castings are obtained by pouring molten metal into a mold made of cast iron, steel, or bronze. Semi permanent mold materials such as aluminum, silicon carbide, and graphite may also be used. The mold cavity and the gating system are machined to the desired dimensions after the mold is cast: the smooth surface from machining thus gives a good surface finish and dimensional accuracy to the casting. To increase mold life and to make ejection of the casting easier, the surface of the mold cavity is usually coated with carbon soot or a refractory slurry; these also serve as heat barriers and control the rate of cooling of the casting. The process is used for cast iron and nonferrous alloys with advantages over sand casting such as smoother surface finish, closer tolerances, and higher production rates.
Die casting A further development of the permanent molding process is die casting. Molten metal is forced into a die cavity under pressures of 100 -100, 000 psi. Two basic types of die-casting machines are hot-chamber and cold-chamber. In the hot-chamber machine, a portion of the molten metal is forced into the cavity at pressures up to about 2, 000 psi. The process is used for casting low-melting-point alloys such as lead, zinc, and tin.
In the cold-chamber process the molten metal is ladled into the injection
cylinder and forced into the cavity under pressures which are about 10 times those in the hot-chamber process. High-melting-point alloys such as aluminum-, magnesium-, and copper-base alloys are used in this process. Die casting has the advantages of high production rates, high quality and strength, surface finish on the order of 40—100-microinch rms (root mean square), and close tolerances, with thin sections.
Rheocasting is the casting of a mixture of solid and liquid. In this process the alloy to be cast is melted and then allowed to cool until it is about 50% solid and 50% liquid. Vigorous stirring promotes liquid like properties of this mixture so that it can be injected in a die-casting operation. A major advantage of this type of casting process is expected to be much reduced die erosion due to the lower casting temperatures.
Centrifugal casting Inertial forces of rotation distribute molten metal into the mold cavities during centrifugal casting, of which there are three categories: true centrifugal casting, semi centrifugal casting, and centrifuging. The first two processes produce hollow cylindrical shapes and parts with rotational symmetry respectively. In the third process, the mold cavities are spun at a certain radius from the axis of rotation; the centrifugal force thus increases the pressure in the mold cavity.
The rotational speed in centrifugal casting is chosen to give between 40 and 60g acceleration. Dies may be made of forged steel or cast iron. Colloidal graphite is used on the dies to facilitate removal of the casting.
Successful operation of any metal-casting process requires careful consideration of mold design and metallurgical factors.
Sheet and Plate Bending
Bending is a method of producing shapes by stressing metal beyond its yield strength, but not past its ultimate tensile strength. The forces applied during bending are in opposite directions, just as in the cutting of sheet metal. Bending forces, however, are spread farther apart, resulting in plastic distortion
of metal without failure.
The bending process appears to be simple; yet, in reality, it is a rather complex process involving a number of technical factors. Included are characteristics of the work piece material flow and reactions during various stages of deformation, the effect of tooling design on force required to form the bend, and the type of equipment used.
In the large, varied field of sheet metal and plate fabricating, several types of bending machines are used. Press brakes predominate in shops that process heavy-gage materials, because they are well suited to such applications and also because they are adaptable to other metalworking operations, such as punching, piercing, blanking, notching, perforating, embossing, shearing, and drawing.
Light-gage metal typically is formed with specialized bending machines, which are also described as leaf, pan, or box brakes; as wing folders; and as swivel benders. Equipment of this type is often manually operated.
The principal kinds of equipment used to bend sheet metal and plate can be grouped into the following categories:
Mechanical press brakes—elongated presses with numerous tooling options. Work is performed by means of energy released from a motor-driven flywheel. These machines normally have a 3" or 4" stroke length.
Hydraulic press brakes—stretched C-frame presses that are likewise compatible with a wide range and diversity of tooling. High-pressure oil in hydraulic cylinders supplies the force, which is directed downward in most models. The stroking length usually exceeds 6".
Hydraulic-mechanical press brakes—presses with drives that combine hydraulic and mechanical principles. In operation, oil forces a piston to move arms that push the ram toward the bed.
Pneumatic press brakes—low- tonnage bending machines that are available with suitable tooling options.
Bending brakes—powered or manual brakes commonly used for bending
light-gage sheet metal.
Special equipment—custom-built benders and panel formers designed for specific forming applications.
Terms used to describe various aspects of sheet metal bending are illustrated Bend Allowance
Bend allowance is the dimensional amount added to a part through elongation during the bending process. It is used as a key factor in determining the initial blank size.
The length of the neutral axis or bend allowance is the length of the blank. Since the length of the neutral axis depends upon its position within the bend area, and this position is dictated by the material type and thickness and the radius and degree of bend, it is impossible to use one formula for all conditions. However, for simplicity a reasonable approximation with sufficient accuracy for practical usage when air bending is given by the following equation: L=A/360*2π(R+kt)
or
L=0.017453A(R+kt)
where:
L = bend allowance (arc length of the neutral axis) in. or mm
A =bend angle, de
R = inside radius of part, in. or mm
t = metal thickness, in. or mm
k = constant, neutral-axis location
Theoretically, the neutral axis follows a parabolic arc in the bend region; therefore, the k factor is an average value that is sufficiently accurate for practical applications. A value of 0.5 for k places the neutral axis exactly in the center of the metal. This figure is often used for some thickness. One manufacturer specifies k according to sheet thickness and inside radius of the bend; when R is less than 2t, k = 0.33; when R is 2t or more, t=0. 50.
Types of Bending
The basic types of bending applicable to sheet metal forming are straight bending, flange bending and contour bending.
Straight Sending During the forming of a straight bend the inner grains are compressed and the outer grains are elongated in the bend zone. Tensile strain builds up in the outer grains and increases with the decreasing bend radius. Therefore, the minimum bend radius is an important quantity in straight bending since it determines the limit of bending beyond which splitting occurs.
Flange Bending Range bend forming consists of forming shrink and stretch flanges. This type of bending is normally produced on a hydrostatic or rubber-pad press at room temperature for materials such as aluminum and light-gage steel.
Parts requiring very little handwork are produced if the flange height and free-form-radius requirements are not severe. However, forming metals with low modulus of elasticity to yield strength ratios, such as magnesium and titanium, may result in undesirable buckling, a and b. Also, splitting may result during stretch-flange forming as a function of material elongation. Elevated temperatures utilized during the bending operation enhance part formability and definition by increasing the material ductility and lowering the yield strength, providing less spring back and buckling.
Contour Bending Single-contour bending is performed on a three-roll bender or by using special feeding devices with a conventional press brake. Higher production rates are attained using a three-roll bending machine. Contour radii are generally quite large; forming limits are not a factor. However, spring back is a factor because of the residual-stress buildup in the part; therefore, over forming is necessary to produce a part within tolerance.
Stretch Bending Stretch bending is probably the most sophisticated bending method and requires expensive tooling and machines. Furthermore,
stretch bending requires lengths of material beyond the desired shape to permit gripping and pulling. The material is stretched longitudinally, past its elastic limit by pulling both ends and then wrapping around the bending form. This method is used primarily for bending irregular shapes; it is generally not used for high production.
Drawing
Drawing is an operation wherein the workpiece is pulled through a die, resulting in a reduction in outside dimensions.
Wire and Bar Drawing
Among the variables involved in the drawing of wires and bars are properties of the original material, percent reduction of cross-sectional area, die angle and geometry, speed of drawing, and lubrication. The operation usually consists of swaging the end of a round rod to reduce the cross-sectional area so that it can be fed into the die; the material is then pulled through the die at speeds as high as 8, 000 feet per minute. Short lengths are drawn on a draw bench, while long lengths (coils) are drawn on bull blocks. Most wire drawing involves several dies in tandem to reduce the diameter to the desired dimension.
Die angles usually range from 6 to 15°, the actual angle depending on die and workpiece materials. Reductions in cross-sectional area vary 10—45% in one pass, although theoretically the maximum reduction per pass for a perfectly plastic material is 63%.
Die materials are usually alloy steels, carbides, and diamond. Diamond dies are used for drawing fine wires. The purpose of the die land is to maintain dimensional accuracy. The force required to pull the workpiece through the die is a function of the material, die angle, coefficient of friction, and reduction in cross-sectional area. The work applied to the process comprises three components: ideal work of deformation, friction work, and redundant work due to no uniform deformation within the material. Depending on a number of factors, there is an optimum die angle for which tile drawing force is a minimum.
In cold drawing, the strength of the material increases due to work hardening.
Temperature rise in drawing is important because of its effect on die life, lubrication, and residual stresses. Also, a defect in drawn rods is the rupturing of the core, called cuppy core. The tendency for such internal rupturing increases with increasing die angle, friction, and inclusions in the original material, and with decreasing reduction per pass.
The magnitude of residual stresses in a drawn material depends on the die geometry and reduction. The surface residual stresses are generally compressive for light reductions and tensile for intermediate or heavy reductions.
Extensive study has been made of lubrication in rod and wire drawing. The most common lubricants are various oils containing fatty or chlorinated additives, chemical compounds, soap solutions, and sulfate and oxalate coatings. The original rod to be drawn is usually surface-treated by pickling to remove scale, which can be abrasive and thus considerably reduce die life. For drawing of steel, chemically deposited copper coatings are also used. If the lubricant is applied to the wire surface it is called dry drawing; if the dies and blocks are completely immersed in the lubricant, the process is called wet drawing.
Tube Drawing
Tubes are also drawn through dies to reduce the outside diameter and to control the wall thickness. The thickness can be reduced and the inside surface finish can be controlled by using an internal mandrel (plug). Various arrangements and techniques have been developed in drawing tubes of many materials and a variety of cross sections. Dies for tube drawing are made of essentially the same materials as those used in rod drawing.
Deep Drawing
A great variety of parts are formed by this process, the successful operation of which requires a careful control of factors such as blank-holder pressure, lubrication, clearance, material properties, and die geometry. Depending on many factors, the maximum ratio of blank diameter to punch diameter ranges
from about 1.6 to 2.3.
This process has been extensively studied, and the results show that two important material properties for deep draw ability are the strain-hardening exponent and the strain ratio (anisotropy ratio) of the metal. The former property becomes dominant when the material undergoes stretching, while the latter is more pertinent for pure radial drawing. The strain ratio is defined as the ratio of the true strain in the width direction to the true strain in the thickness direction of a strip of the sheet metal. The greater this ratio, the greater is the ability of the metal to undergo change in its width direction while resisting
Anisotropy in the sheet plane results in earring, the appearance of wavy edges on drawn cups. Clearance between the punch and the die is another factor in this process: this is normally set at a value of not more than 1.4 times the thickness of the sheet. Too large a clearance produces a cup whose thickness increases toward the top, whereas correct clearance produces a cup of uniform thickness by ironing. Also, if the blank-holder pressure is too low, the flange wrinkles; if it is too high, the bottom of the cup will be punched out because of the increased frictional resistance of the flange. For relatively thick sheets it is possible to draw parts without a blank holder by special die designs.
Punching and blanking
Though punching and blanking are the most common sheet metal operations involving H shearing of the metal strips, (2) lancing, (3) slitting, (4) nibbling, and (5) trimming.
In the notching operation, material is removed from the side of a sheet metal, whereas lancing makes cuts partway through the metal without producing any scrap. Lancing is frequently combined with bending to form tabs. Slitting is an operation to cut a coiled sheet metal lengthwise to produce narrower strips. In the nibbling operation, complicated shapes are cut out from a sheet metal by producing overlapping notches, starting either from the outer boundary or from a punched hole. Without using any special tool, a simple, round or triangular
punch of small dimensions is reciprocated at a fixed location. The sheet metal is guided to obtain the desired shape of the cut. Trimming refers to the removal of the excess material in a flange or flash.
In reducing the operation time and cost, the design of the die and punch for blanking plays an extremely important role. An accurate relative location of the punch and the die is maintained with the help of a set of guide posts. The stripper helps in removing the sheet metal workpiece from the punch during the return stroke, whereas the spring loaded push-off pins help in removing the blank from the punch face. The stripper also acts as a blank holder to prevent drawing.
To optimize space and time, more than one operation can be performed in a stroke. Such an assembly is commonly known as a compound die. It is obvious that piercing of the inner hole has to be performed before blanking. Sometimes, a combination of drawing (or bending) and blanking is also used for economy.
In the foregoing situation, more than one operation is performed in only one location. However, it is also possible to use a series of die-punch elements at different) locations. Here, one operation is performed at each station and the metal stock is advanced to the next station. Thus, a continuous operation is possible.
Another important aspect of the blanking operation is to minimize the scrap by an optimum layout design (also known as nesting). The minimum gap between the edge of the blank and the side of the strip is given as g =t + 0. 015h, where t is the thickness of the strip and h is the width of the blank. The gap between the edges of two successive blanks b depends on the strip thickness values of b. Sometimes, the relative direction of grain flow (when a rolled strip is used as stock) with respect to the blank is specified. In such a case, the freedom of nesting is nearly lost. In a circular blank, some saving in the scrap may be achieved only through a choice of multiple rows.
铸造及其他成形工艺
铸造
铸造是将熔化的金属导入型腔或铸模中，在那里金属一旦凝固就会变成一个形状由铸模轮廓确定的物件。

铸造有着若干其他金属成型方法没有的优点：它适合于复杂形状，适合于超大工件，而且适合于批量生产；它能始终给部件提供均匀的物理和机械特性；而且视所铸造的特殊材料、部件的设计和所生产的数量而定，它的经济优势可能超过其他工艺。

分类
金属铸造工艺有两大类：铸锭（包括连续铸造）和铸型。

铸锭是将熔化的金属倒入永久的或可以重复使用的铸模中制造出来的。

凝固之后，这些锭（或棒料、板坯或方坯，根据容器而定）被进一步机械加工成多种新的形状。

铸型包括将熔化的金属倒人铸模，铸模的型腔提供了最终有用的形状，之后仅需根据具体应用进行加工和焊接。

铸锭铸锭占整个金属铸件中的一大部分，分为3类：静态铸锭、半连续或直冷式铸锭和连续铸锭。

静态铸锭静态铸锭仅是单纯将熔化的金属倒入永久的铸模中凝固后，将铸锭从铸模中抽出，铸模可以再次使用。

每年用这种方法生产出数以百万吨的钢。

半连续铸锭半连续铸锭工艺在铝工业中用于制造大多数的铸造合金，由这些合金加工出棒料、薄板、板条和板材的形状。

在这一工艺中，熔化的铝被传送到一个由水冷却的永久铸模中，在铸模的长活塞上装有活动底座。

在铸模表面进一步凝固而形成
一层坚硬的“皮”之后，活塞向下运动，更多的金属连续填入容器中。

最后，活塞运动至全长，过程停止。

铝工业中常规的方法是利用适当润滑的金属铸模。

然而，工艺的改进已经允许主要的铝合金生产者用一种电磁场取代（至少部分取代）金属铸模以使熔化的金属仅仅短暂地接触到金属铸模，因此与传统方法相比可以生产出光洁度更高的产品。

连续铸锭连续铸锭为钢和铜工业提供了主要的铸材资源，而且在铝工业中增长迅速。

在这一工艺中，熔化的金属被送到一个永久铸模中，铸造开始时与半连续铸造极为相似。

然而，该过程不是在一定时间后停止，凝固的铸锭被连续剪成或切成一定长度并且在铸造过程中被运走。

因此，该过程是连续的，凝固的棒料或板条像铸造一样被迅速运走。

与传统铸造工艺相比这一方法有许多经济优势；因此，所有的现代钢厂都生产连续铸造的产品。

铸型铸型通常按照造模的方法、造模的材料或进铸模的方法进行分类，铸造工艺有四种基本类型：砂型铸造、永久型铸造、压模和离心式铸造。

砂型铸造这是一种传统的、仍在生产大量铸型工件的铸造方法。

它利用沙粒、水、粘土和其他材料来制作供熔化的金属使用的高质量的铸模。

这种“绿沙”混合物通常由机器压实在（木制的、石膏的或金属的）模型周围，压实到松装密度的20-80%。

沙模及其他铸模的基本部分铸模的两半（上下型箱）盖在型芯上构成必要的内腔，整个组件被加以重物或被夹紧以防止浇注金属时上箱移动。

其他利用沙子作为铸模基本成分的铸造方法有壳型法、二氧化碳法、熔模造型法、陶瓷造型法和石膏造型法。

此外，有大量的化学粘合的沙子，它们正变得越来越重要。

永久型铸造很多高质量的铸件是通过将熔化的金属倒人一个由铸铁、钢或者铜制作的铸模中获得的，也可使用半永久铸模材料如铝、金刚砂和石墨。

铸模铸造好后，铸模的型腔和浇注系统被加工成所需尺寸，加工得到的光滑表面就给了铸件高表面光洁度和尺寸精度。

为了提高铸模寿命并使铸件推出更加容易。

铸模的型腔表面通常被覆以碳黑或是难熔的泥浆；这此材料也被用作热的屏障以控制铸件的冷却速度。

该法用于铸铁和非金属合金的铸造，与砂型铸造相比其优势是表面光洁度更光滑、公差更小、生产率更高。

压铸永久型铸造的进一步发展即为压铸熔化的金属在100—100,000磅／平方英寸的压力下被强行压入铸模型腔中。

两种基本压铸机型是热室压铸机和冷室压铸机。

在热室压铸机中，一部分熔化的金属在高达2000磅／平方英寸压力下被强制压入型腔中。

该法用于铸造低熔点金属如铅、锌和锡。

在冷室法中，熔化的金属被舀进压注缸中并在10倍于热室法的压力下被强行压人型腔中。

高熔点的合金如铝基镁基和铜基合金用该法。

压铸的优点有高生产率、高质量和高强度表面，光活度等级为40-100徽英寸有效值。

以及公差小、壁薄等。

流变铸造是固体与液体混合物的铸造。

在这一方法中，先熔化要铸的合金再使其冷却至约50%固体，50%液体。

强烈搅拌促进了这种混合物的类液特性，使其能被注入压铸工艺中。

这类铸造方法的主要优点是由于铸造温度低可以大大降低铸模的侵蚀。

离心式铸造在离心式铸造中，旋转的惯性力将熔化的金属散布到铸模的型腔中，离心式铸造有3类：真正的离心式铸造、半离心式铸造和离心法。

前两种方法分别用于制造空心筒形部件和轴对称的部件。

在第三种方法中，铸模的型腔被绕着旋转轴以某一半径旋转，离心力增加了铸模型腔中压力。

在离心式铸造中的旋转速度选为加速度为40-60g.铸模可以由锻钢或铸铁制造。

铸模上用石墨乳以便更容易地取出铸件。

任何金属铸造工艺的成功实施都需要仔细地考虑模具设计和冶金因
薄板与板材的弯曲
弯曲是一种通过给金属施加超出其屈服强度但不超过其极限抗拉强度的压力来引起变形的方法。

在弯曲过程中施加的力与金属薄板的切割一样，方向相反。

但是，弯曲力向远处展开，引起金属的塑性扭曲而不会破坏。

弯曲过程似乎简单，但事实上，它是一种包含很多技术因素的相当复杂的过程。

包含的因素有工件材料的特性、各变形阶段材料的流动和反应、工具设计对于成形弯曲件所需力的影响以及使用设备的类型。

金属薄板与板材的加工领域范围大、变化大，使用了几类弯板机。

压弯机在加工大厚度板材的车间占优势，不仅因为它们很适合这样用．还因为它们适合于其他金属加工工序，如冲孔、打孔、落料、开缺口，穿孔、压花、剪切和拉延。

小厚度板材典型的成型方式是使用专用弯板机，也被称为薄板、盘子或盒子压弯机；称为弯边机以及转盘弯折机。

这种类型的设备常常由手工操作。

用于薄板与板材弯曲的机器主要类型可分为以下几类：
．机械压弯机—能选择多种工艺装置的延长了的压力机。

由马达驱动的飞轮释放出的能量来作功。

这些机器通常具有3-4英寸的行程长度。

．液压式压弯机—拉伸的C形架弯折机，也可兼容广泛的、多种多样的工艺装置。

液压油缸里的高压油提供力，在大多数模型中力是向下的。

行程长度通常超过6",
．液压-机械式弯板机-将液压与机械原理组合起来驱动的压力机。

运行时，油液
．迫使活塞移动工作臂．工作特推动推杆移向床身。

．气动压弯机－小吨位的弯板机，有适合的工艺装置选项。

．压弯机—动力或人力压弯机，通常用于弯曲小厚度金属薄板。

．专用设备—定制的折弯机以及为特殊成形用所设计的面板成形机。

弯曲公差
弯曲公差是在弯曲过程中通过延长使部件尺寸增加的量。

在确定毛坯的初始尺寸时，它被作为一个关键因素。

中心轴的长度或者弯曲公差的长度即为毛坯的长度。

既然中心轴的长度取决于其所在弯曲区域内的位置．这一位置由材料的类型和厚度以及弯曲的半径和程度来确定，就不可能把一个公式用于所有情况。

但是，为了简化，在气动弯曲时实际使用的具有足够精度的合理近似值由下面的方程给出：
L=A/360 x2π(R+kt)
L=0.017453A(R+kt）
其中：
L=弯曲公差（中性轴的弧长）英寸或毫米
A=弯曲角，度数
R=部件内径，英寸或毫米
t=金属厚度．英寸或毫米
k=常数．中心轴位置
理论上讲，中心轴在弯曲区呈抛物线状的弧形；因此．,k因子是对于实际应用来讲足够精确的一个平均值。

k值为0.5时，中性轴精确地位于金属的中心。

该数常用于一定厚度的金属。

一个制造厂按照薄板的厚度和弯曲内径来规定k值。

当R小于2t时, k=0.33；当R等于或大于2t时，k =0.50。

弯曲的种类
适用于金属薄板成形的基本的弯曲类型有直线弯曲、凸缘弯曲和成形弯曲。

直线弯曲在直线弯曲件的成形过程中．在弯曲区的
内侧晶粒受到压缩而外侧晶粒受到拉伸。

拉伸应变在外侧晶粒产生并随弯曲半径的减小而增大。

因此．最小弯曲半径是直线弯曲中很重要的量，因为它确定了弯曲极限，超过就会反生撕裂。

凸缘弯曲凸缘弯曲成形由收缩凸缘成形和拉伸凸缘成形组成，这种类型的弯曲通常在室温下在液压或胶垫压力机上加工，如铝和小厚度钢等材料。

如果凸缘的高演和自由成形半径要求不高，用它来制造部件储要很少的手工工作。

但是，对于具有较低弹性模量屈服强度比的成形金属，如镁和铁，可能产生不良的翘曲和回弹，而且，由于材料的延长作用，在拉伸凸缘成形过程中可能引起撕裂。

在弯曲工序中，利用提高温度，通过增加材料的延展性及降低屈服强度来增强部件的可成形性和边界成形，减少回弹和翘。

成形弯曲
单向成形弯曲是在一个三辊式压力机或使用专用进给设备与传统的压弯机。

使用三辊式压力机可获得较高的生产率。

弯曲半径一般较大：成形限制不是一个要素。

然而，回弹是一个要素因为在部件内积聚了残余应力；因此．有必要过量成形以制造一个在公差范围内的部件。

拉伸弯曲拉伸弯曲可能是最复杂的弯曲方法，而且需要昂贵的工艺装置和机器。

而且，位伸弯曲需要材料的长度超出所需形状，好用来夹紧和拉拽。

通过拉两端以及缠绕弯曲成形模，材料被纵向拉伸超过其弹性极限。

这种方法主要用于不规则形状的弯曲；一般不用于大量生产。

拉延是将工件拉过一个模子使其外部尺寸减小的一种工艺。

线材与棒料的拉延
线材与棒料拉延的变量有原材料的特性、截面积减少百分比、模子的角度和几何形状、拉延的速度和润滑。

工艺通常包括锻压圆棒的端头来减小截面积以使其能被送进模子；之后，材料被以每分钟高达8000英尺的速度拉过模子。

长度短的用拉床拉延，长的（线材卷）用拉丝机拉延。

大多数线材拉延包括若干个串联模子以将其直径减小至所需尺寸。

模子角度的范围通常是6-15度.实际的角度取决于模子与工件的材料。

通过一次模子，截面积减少的变化量为10-45%，尽管理论上.讲完美的塑性材料每次通过的减少量是63％。

模子的材料通常为合金钢、硬质合金和金刚石。

金刚石模子用于拉延细丝。

模成型段的目的是保持尺寸精度。

将工件拉过模子所需的力是关于材料强度、模子角度、摩擦系数和截面积减少量的函数。

施加于此法的功由3部分组成：变形的理想功、摩擦功和由于材料内部变形不均匀引起的多余功。

根据几个因素，有一个优化的模子角度，此角度拉延力最小。

在冷拉延中，材料的强度因加工硬化而增大。

拉延中温度的升高很重要因为它影响模子寿命、润滑和残余应力。

而被拉延的棒料中的个缺陷是中心部分破裂，叫做凹芯。

这种内部破裂的趋势随着模子角度、摩擦和原材料中的内含物的增加以及每次通过时减小量的下降而增加。

被拉延的材料中的残余应力取决于模子的几何形状和减小量。

减小量小时表面残余应力是压应力；而减小量为中等或大时，是张力。

棒料与线材拉延的润滑已被广泛研究。

最常见的润滑剂是含有油脂或氮化的添加剂、化学合成物、皂碱液以及硫酸盐和草酸盐涂料。

待拉延的原始棒料通常用酸洗进行表面处理以去除锈迹，锈迹可成为磨料从而将模子寿命减小很多。

对于钢的拉延，也会用到以化学方法沉积的铜涂料。

如果将润滑剂用于线材表面，就称之为干式拉延；如果模子和料完全地浸入润滑剂，该工艺就被称为湿式拉延。

管料的拉延
管料也是被拉过模子来减小外径以及控制管壁厚度。

厚度可以减小而且通过使用内芯（芯棒）可以控制内表的表面光洁度。

已经开发出各种设备和技巧来拉延多种材料和截面的管料。

用于管料拉延的模子材料与棒料拉延的模子的材料基本相同。

拉深
许多部件由这种方法制造，成功地操作该法需要仔细地控制诸如毛坯。