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1、金属加工性能 1. 简介 加工条件和材料的物理性能对材料的切削加工有直接影响。被描述为”加工的物质条件”的各种条件和特性，它们单独或累加地直接影响和决定加工材料的加工性能。运转条件、刀具材料、几何尺寸和加工工件的需求作业间接影响加工性能，并且经常用于克服加工材料所呈现的复杂情况。另一方面如果忽视它们这些因素，这些因素可以导致出现增加加工难度的情况。对影响加工性能和加工工艺所有因素的完全理解有助于选择材料和工件设计，已达到最佳加工方案和最大生产效率的目的。2 加工材料的条件 以下八个因素决定加工材料的的条件，显微结构、晶粒尺寸、热处理、化学成分、制造、硬度、屈服强度、拉伸强度。显微结构：一种金属
2、的显微结构是指通过蚀刻和抛光表面，在显微镜下检测金属的晶粒或晶体结构。具有相似晶粒显微结构的金属由相似的加工性能特性，但是在同一个加工工件上有多种影响加工性能的晶粒显微结构。 粒度：金属的晶粒尺寸和机构是金属加工性能的一般衡量指标。晶粒规则和细小的金属更加容易切削和抛光。这种金属不但延展性好，而且具有“粘性”晶粒尺寸中等大小的金属呈现折中的金属切削和抛光加工特性。金属硬度一定与晶粒尺寸相关，并且它是一般作为机械加工的指标。 热处理：提供所需的金属特性，在固态状态下，金属经过间或的一系列加热和冷却处理。金属可以改善加工性能，如减少脆性、消除应力、增强硬度或其他变化。 化学成分：金属的化学成分是决
3、定金属加工性能的主要因素。化学成分的影响不总是很明确。因为多种元素构成的金属合金，每种元素都对金属加工性能有影响，而他们影响的叠加则不确定。关于钢铁的化学成分与加工性能联系可以这样归纳，但非铁合金过于复杂多样而不能这样归纳。 制造：无论金属是被热轧、冷轧、冷拉、熔炼、锻造都会影响它的晶粒尺寸、塑性、硬度、晶体结构和其加工性能。“加工”是指用传统的加工工艺，捶打或材料成型为可以随时改变的组件或构件。加工金属分为棒材、钢坯、卷板、条料、板材或管材。铸造要将熔融金属浇铸到一个模具中以得到要求低的、相似的形状，有些情况是不需要加工的。浇铸需要的模具可以用沙子、石膏、金属或其他材料制作而成。硬度：书本上
4、硬度的定义材料抵抗变形的趋势。硬度的衡量经常用布氏硬度或洛氏硬度方法。测量硬度的方法是采用预订的负载或重力，用一个特定尺寸和形状的压头压紧测试材料的表面。布氏硬度或洛氏硬度读数与压痕到材料表面的距离有关。越大的压痕，布氏硬度或洛氏硬度读数越小；大的布氏硬度或洛氏硬度读数，则压痕到材料表面的距离就很小。根据定义，是一种硬度极大的金属。图1展示了如何测量硬度。 布氏硬度测试要嵌入一个特定直径的钢球，用一公斤的负载在材料表面测试。布氏硬度值（BHN）是在测试工件表面留下的球形压痕的每单位面积负载（平方毫米）。这种标准化的测量方法提供了统一的方法来测量比较不同加工材料的硬度或某种材料硬化过程。 洛氏硬
5、度测试可以采用多种尺寸的压头和多种负载测试。硬度测试或洛氏硬度有几种不同的硬度等级。就实际应用来说有三种最受流行的等级。这个测试的设计如下：洛氏硬度 测试等级 应用A 钨合金、其他薄的、硬的、条状硬质材料。B 中等硬度、退火条件下的低碳钢。C 材料硬度大于洛氏硬度100HBS 在一般实际加工中，低硬度材料可以使生产效率提高，因为切削速度依据硬度选择（硬度越小，切削速度越高）。加工工件硬度的增加对刀具的使用寿命产生不利影响。对于既定的切削速度和硬度，切削应力和温度的只能更加使刀具的使用寿命减少。对于钻孔和车削，切削温度的增加是对刀具寿命不利的，因为它产生的额外热量使刀具边缘磨损加速。在切削过程中
6、，材料硬度的增加使负荷增加，导致刀具边缘过早的磨钝。 屈服应力：拉伸测试是比较金属材料物理性质的方法。拉伸测试可以得出屈服强度、拉伸强度和其他热处理金属材料多的物理性质。还有，这种测试经常用于比较不同的金属材料的物理性质。拉伸测试要采用一个圆柱棒或轴，在液压机上用一个逐渐增大的力从另一端拉伸。 在测试开始前，在圆柱棒上二英寸和八英寸处各做一记号，当圆柱棒受到逐渐增大的力作用时，两个记号的距离变大。如果负载可以使圆柱棒和记号回到原来的位置，则材料在弹性变形区域。测试有这样一个过程，如果负载使圆柱棒的记号回不到原来位置，则发生永久变形。屈服强度的测量就在永久变形前那个点。图2展示屈服强度如何测试。
7、 屈服强度的测试在永久变形发生点之前。屈服强度用每平方英寸多少磅（PSI）来描述。永久变形之前过渡区横截面积除以负载来决定。这种材料特性视具体情况而定，因为它的屈服强度在热处理后可以改变。硬度增大导致屈服强度增大。因为硬度增加，它需要更大的力来产生永久变形。屈服强度不能与弯曲强度和拉伸强度混淆，因为这些特性与屈服强度不同，与当前状态无关。通过定义可以知道，具有高屈服强度（产生永久变形的力）的材料在加工作业中切削开始时需要一个更大的力。这表明一个材料的屈服强度增加，产生更强切削应力的刀头形状和切削更小的切削尺寸来抵御加工区域增加的负荷。因此具有相对较高屈服强度的金属比中行等屈服强度的金属更难加工
8、，刀具的使用寿命更短。 拉伸强度：一个材料的拉伸强度随着热处理使其屈服强度的增加而增加。这个物理材料特性也是有一个拉伸测试确定。拉伸强度（或极限强度）由拉伸测试产生的最大负荷除以测试样品过渡区域横截面积来定义。因此，拉伸强度像屈服强度一样用PSI来表示。这样定义的好处是由于一种材料的当前特性不止与物理特性有关，就像屈服强度、硬度一样，经过热处理，值可以改变。因此，在材料确定的情况下，每一种硬度读数对应着不同的拉伸强度和屈服强度。 在加工作业中正如屈服强度的增加，则切削力增加一样，拉伸强度增加，切削力增加。同样的，加工工件拉伸硬度的增加，为了生产效率和刀具使用寿命则需要产生更强切削力的切削刀头几
9、何形状。3.加工材料的物理特性 某个材料的物理特性包括弹性模量、热膨胀、加工硬化等物理特性。 弹性模量：在前面提到的条件下，以同样的方式进行可拉伸试验来定义弹性模量。然而与硬度、屈服强度和拉伸强度不同，弹性模量是某种材料的特定属性，因此不会受到热处理的影响。这种特定的属性是衡量材料受到外力发生绕度变形的指标。这种特性用PSI来描述。主要用处是为金属提供了几百万的PSI。2、4或8尺6长的木棒支撑两端，在中间挂两百磅的负荷，下降的尺寸高度是同样长度尺寸、挂同样负荷钢材的17倍。不是因为钢铁更硬和结实，而是钢铁的弹性模量是木材的17倍。 生产实践表明，具有相对较低弹性模量的加工工件一般需要前角度数
10、为正或很大的刀具切割几何形状。前角为正的刀具产生较小的切削力，用这种刀具弹性材料的切屑变多了。前角度数很大的刀具容易发生咬刀，产生材料的剪切。前角度数为负的刀具当刀具开始切削时，会产生很大的切削力，使加工材料的尺寸发生局部变形。 热导率：材料通常被标记为导热材料或绝缘材料。导热材料容易以较快的速度从更热或更冷的物体上传递热量，而绝缘体阻止热量的传递。导热率是描述某种材料传递热量效率的衡量方法。因此，具有相对较高导热率的材料视为导体，具有相对较低导热率的材料视为绝缘体。具有相对较低导热率的的金属材料不能较快够传递热量。因此，加工这些材料时，切削刀具和加工工件变得很热。这个过程加速了刀具的磨损和减
11、少了刀具的使用寿命。具有底导热率的金属材料在切削区域（在加工工件和切削刀具之间）加足量的冷却液，来增加刀具的使用寿命。 热膨胀：许多材料，尤其是金属材料当它们的温度上升时，尺寸有变大的趋势。这种物理特性描述为热膨胀。金属的膨胀率的多样化取决于某种金属或合金的具体情况。金属的热膨胀率由热膨胀系数决定。具有越大的热膨胀系数，金属温度上升时或发生导热时，产生的热膨胀变形就越大。例如，在100华氏温度下100英寸的钢材棒料，当温度上升时，尺寸变为100.065英寸。相同条件的100英寸铝材棒料，在相同状况下为100.125英寸。在在这个实验中，铝材棒料的改变的尺寸几乎是钢材棒料改变尺寸的二倍。这明确的
12、不同表明不同的材料有不同的热膨胀系数。 在一般加工方法中，具有较大热膨胀系数的材料的加工很难符合尺寸公差要求。因为一个加工工件温度的微弱增加都会导致尺寸的变化。加工这种类型的金属需要足够的冷却液以保证尺寸的稳定性。另外，用前角度数为正的切削刀具可以减少加工温度。 加工硬化：许多金属在冷加工时会有硬度参数上升的物理特性。冷加工涉及改变金属物体的形状，如弯曲、成型、滚压和造型。当金属被改变形状，内部应力的增加使金属局部硬化。一种金属与另一种金属的内部硬度改变速率和程度是多种多样的。热在金属加工硬化中也起着重要作用。当金属产生加工硬化时，金属温度上升。它的作用像催化剂一样产出更硬的加工工件。 具有加
13、工硬化特性的加工工件在加工中不应用大量冷却液。另外，切削速度因与之有关联，为了达到生产效率，不能不计后果的改变材料的切削速度。加工工件在加工过程中，加工硬化决定高速切削产生的额外热量。薄的切屑应避免在这些材料上加工使用，因为加工实践中由于摩擦产生的热量会造成前面提到的影响。刀具前角度数为正、低切削应力、中等的切削速度和进刀量一般在这种材料上加工很有效率。4.金属加工 术语“加工性能”是金属与布氏硬度160、美国钢铁协会B1112高速切削低碳钢，一种材料的加工难易程度。美国钢铁协会做了这种材料车削180英尺的测试，并与其他几种材料比较了测试结果。如果B1112代表100%等级，具有相对较低等级的
14、材料则较难加工，超过100%则较容易加工。某种材料的加工性能等级评定需要切削速度、表面粗糙度和刀具寿命等综合考虑到加工性能等级排列。下面的表格多种金属的加工性能等级排列。材料材料硬度加工性能等级6061铝材190%7075 铝材120%B1112 钢材160BHN100%416 不锈钢200BHN90%1120钢160BHN80%1020 钢材148BHN5.加工性能判定影响加工性能的因素已经解释过，下面讨论四种判断加工性能的方法：刀具使用寿命：金属切削以及低的速度磨损刀具则认为加工性能好，反之亦然。含有小的硬质杂物的加工工件材料与耐研磨金属具有相似的机械加工特性，在切削过程中不需要消耗跟多的
15、能量。还有，这种材料的加工性能比较低，因为它的耐磨特性是加速刀具磨损的主要原因，增加加工成本。刀具应力和能量消耗作为影响工件材料加工性能的因素因为以下两个原因：1. 金属较容易切削即刀具进给容易说明加工工件材料具有较好的加工性能等级。2. 更实用的加工性能概念是与应力和能量消耗有关的每部分加工的最低成本和合适的加工成本。表面处理：在工件切削加工过程中，表面处理的质量有时对判定加工性能等级有很大用处。有些工件像其它工件一样不用高的精度表面处理。影响表面粗糙度的基本原因是切屑和刀具的切削边缘增厚。柔软、韧性材料较容易形成厚的刀具切削边缘。不锈钢合金的气体涡轮和其它有较高硬度的金属也趋于用较厚的刀具
16、边缘加工。具有很大剪切取得的材料趋于减少厚度增加的影响。这些材料包括铝合金、冷加工钢材、高速切削钢、黄铜、和钛合金。如果表面质量作为加工性能的唯一指标。这些金属的加工性能等级要高于前面一组。切屑形状：有种加工性能等级基于加工过程中产生的切屑形状类型。这种加工性能可能是由切屑的处置和来判定。产生长线型切屑的材料获得低等级，就像产生细粉装切屑的材料。本来具有细段切屑、半个或整个螺旋切屑的材料获得最高等级。切屑的处置十分昂贵。线型切屑对机床和已加工表面质量有很大威胁。然而，切屑的形成是与机床和材料有函数变化关系。这种方法的等级排定由于断裂切屑的提供而改变。Machinability Of Metal
17、s1.Introduction The condition and physical properties of the work materials have a direct influence on the machinability of a work material.The various conditions and characteristics described ascondition of work material, individu ally And in combinations,directly influence and determine the machin
18、ability. Ope rating conditions,toolmaterial and geo-metry ,and workpiece requirements exercise indirect effects on machinability ang can offen be used to overcome difficult conditions presented by the work material.On the other hand ,they can create situations that increase machining difficulty if t
19、hey are ignored .A thorough understanding of all of the factors affecting machinability and machining will help in selecting material ang woekpiece designs to achieve the optimum machining combinations critical to maximum productivity.2.Condition of Work Material The following eight factors determin
20、e the condition of the work material microstructure, grain size,heat treatmen,chemical, composition ,fabri cation hardness ,yield ,and tensile . Microstructure: The microstructure of a metal refers to its crystal or grain structure as shown through examination of etched and polished surface under a
21、microscope .Metal whose microstructures are similar have like machining properties .But there can be variations in the mincrostructure of the same workpiece ,that will affect machinability. Grain Size :Grain size and structure of a metal serve as general indicators of its machinability .A metal with
22、 small undistorted grains tends to cut easily an finish easily .Such a metal is ductile ,but it is also gummy.Metal of an intermediate grain size represent a compromise that permits both cutting and finishing machinability . Hardness of a metal must be correlated with grain size and it is generally
23、used as an indicator of machinability. Heat Treatment:To provide desired properties in metals ,they are sometimes put through a series of heating and cooling opertions when in the solid state .A material may be treated to reduce brittleness ,remove stress ,to obtain hardness ,or to make other change
24、s that affect machinability. Chemical Composition :Chemical composition of a metal is a major factor in determing its machinability .The effects of composition though ,are not always clear ,because the elements that make up an alloy metal ,work both singly and collectively .Certain generalizations a
25、bout chemical composition of steels in relation to machinability can be made ,but non-ferrous alloy are too numerous and varied to permit such generalizations. Fabrication: Whether a metal has been rolled ,cold rolled ,cold drawn,cast,or forged will affect its grain size ,ductility , hardness ,struc
26、ture-and therefore-its machinability . The termwroughtrefers to the hammering or forming of materials into permanfactured shapes which are readily altered into components or products using traditional manufacturing techniques.Wrought metals are defined as that group of materials which are mechanical
27、ly shaped into bars,billets, rolls, sheets,plates or tubing. Casting involves pouring molten metal into a mold to arrive at a near component shape which requires minimal,or in some cases no machining. Molds for these operations are made from sand ,plaster,metals and a variety of other materials. Har
28、dness:The textbook definition of hardness is the tendency for a material to resist deformation.Hardness is offten measured using either the Brinell or Rockwell scale. The method used to measure hardness involves embedding a specific size and shaped indentor into the surface of the test material,usin
29、g a predetermined load or weight.The distance the indentor penetrates the materials surfacewill correspond to a specific Brinell or Rockwell hardness reading.The greater the indentor surface penetration,the lower the ultimate Brinell or Rockwell number,and thus the lower the corresponding hardness l
30、evel.Therefore,high Brinell or Rockwell numbers or readings represent a minimal amount of indentor penetration into the workpiece and thus,by definition,are an indication of an extremety hard part.Figure 1 shows how hardness is measured.Figure 1 Hardness is measured by depth of indentations made. Th
31、e Brinellhardness test involves embedding a steel ball of a specific diameter,using a kilogram load ,in the surface of a test piece. The Brinell Hardness Number (BHN) is determined by dividing the kilogram load by the area(in squre millimeters) of the circel created at the dimple or impression left
32、in the workpiece surface.This standardized approach provides a consistent method to make comparative tests between a variety of workpiece materials or a single material which has undergone various hardening processes. The Rockwell test can be performed with various indentor sizes and loads.Several d
33、ifferent scales exist for the Rockwell methord or hardness testing. The three most popular are outlined below in terms of the actual application the test is designed to address:Rockwell TestingScale ApplicatioA For tungsten carbide and other extremely hard material & thin,hard,sheets.B For medium ha
34、rdness low and medium carbon steels in the annealed condition.C For material than RockwellB100 In terms of general machining pratice,low material hardness enhances productivity,since cutting speed is offten selected based on material hardness(the lower the hardness, the higher the speed).Tool life i
35、s adversely affected by an increase in workpiece hardness,since the ctting loads and temperatures rise for a specific cutting speed with part hardness, thereby reducing with toll life .In drilling and turning ,the added cutting temperature is detrimental to tool life ,since it produces excess heat c
36、ausing accelerated dege wear.In milling,increased material hardness produces higher impact loads as inserts enter the cut,which often leads to a premature breakdown of the cutting edge. Yield Strengh:Tensile test work is used as a means of comparison of metal material conditions.These tests can esta
37、blish the yield strengh ,tensile strengh and many other conditions of a material based on its heat treatment.In addition,these tests are used to compare different workpiece material.The tensile test involves taking a cylindrical rod or shaft,and pulling it from opposite ends with a progressively lar
38、ger force in a hydraulic machine .Prior to the start of the test ,two marks either two or eight inches apart are made on the rod or shaft.As the rod is systematically subjected to increased loads,the marks begin to move farther apart.A material is in the so-called elastic zonewhen the load can be re
39、moved form the rod and the marks return to their initial distance apart of either two or eight inches.If the test is allowed to proresss,a point is reached where ,whenthe load is removed the marks will not return to their initial distance apart.At this point,permanent set or deformation of the test
40、specimen has taken place.Figure 2 shows how yield strength is measured.Figure 2 Yield strength is measured by pulling a test specimen as shown. Yield strengh is measured just prior to the point before permanent deformation takes place.Yield strengh is stated in pounds per square inch(PSI) and is det
41、ernined by dividing the load just prior to permanent deformation by the cross sectional area of the test specimen.This material property has been referred to as a condition ,since it can be altered during heat treatment.Increased part hardness produces an increase in yield strengh and therefore,as a
42、 part becomes harder,it takes a larger force to produce permanent deformation of the part.Yield should not be confused with fracture strengh,cracking or the actual breaking of the material into pieces,since these properties are quite different and underlated to the current subject. By definitin,a ma
43、terial with high yield strengh (force required per unit of area to create permanent deformation) requires a high level of force to initiate chip formation in a machining operation.This implies that as a materials strength yield increases,stronger insert shapes as well as less positive cutting geomet
44、ries are necessary to combat the additional load encountered in the cutting zone.Mareial hardness and yield strength increase simultaneously during heat treatment.Therefore,materials with relatively high yield strengths will be more difficult to machine and will reduce toll life when compared to mat
45、erials with more moderate strengths.Tensile strength:The tensile strength of a material increase along with yield strength as it is heat treated to greater hardness levels.This material condition is also establishde using a tensile test.Tensile strength (or ultimate strength) is defined as the maxim
46、um load that results during the tensile test,divided by the cross-sectional area of the test specimen.Therefore,tensile strength,like yield strength ,is expressed in PSI.This value is referred to as a material condition rather than a property,since its level just like yield strength and hardness,can
47、 be altered by heat treatment.Therefore,based on the material selected,distinct tensile and yield strength levels exist for each hrdness reading. Just as increased yield strength implied higher cutting forces during machining operations,the same could be said for increased tensile strength. Again,as
48、 the workpiece tensile strength is elevated,stronger cutting edge geometries are required for productive machining and acceptable tool life.3.Physical Properties of Work MaterialPhysical proterties will include those characteristics included in the individual material groups ,such as the modulus of elasticity ,thermal expansion and work hardening. Modulus of Elasticity :the modulus of elasticity can be dete
