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1、1/10 机械工业 2004 年 3 月第 1 版 20.9 MACHINABILITY The machinability of a material usually defined in terms of four factors:1、Surface finish and integrity of the machined part;2、Tool life obtained;3、Force and power requirements;4、Chip control.Thus,good machinability good surface finish and integrity,long
2、tool life,and low force And power requirements.As for chip control,long and thin(stringy)cured chips,if not broken up,can severely interfere with the cutting operation by being entangled in the cutting zone.Because of the plex nature of cutting operations,it is difficult to establish relationships t
3、hat quantitatively define the machinability of a material.In manufacturing plants,tool life and surface roughness are generally considered to be the most important factors in machinability.Although not used much any more,approximate machinability ratings are available in the example below.20.9.1 Mac
4、hinability Of Steels Because steels are among the most important engineering materials(as noted in Chapter 5),their machinability has been studied extensively.The machinability of steels has been mainly improved by adding lead and sulfur to obtain so-called free-machining steels.Resulfurized and Rep
5、hosphorized steels.Sulfur in steels forms manganese sulfide inclusions(second-phase particles),which act as stress raisers in the primary shear zone.As a result,the chips produced break up easily and are small;this improves machinability.The size,shape,distribution,and concentration of these inclusi
6、ons significantly influence machinability.Elements such as tellurium and selenium,which are both chemically similar to sulfur,act as inclusion modifiers in resulfurized steels.Phosphorus in steels has two major effects.It strengthens the ferrite,causing 2/10 increased hardness.Harder steels result i
7、n better chip formation and surface finish.Note that soft steels can be difficult to machine,with built-up edge formation and poor surface finish.The second effect is that increased hardness causes the formation of short chips instead of continuous stringy ones,thereby improving machinability.Leaded
8、 Steels.A high percentage of lead in steels solidifies at the tip of manganese sulfide inclusions.In non-resulfurized grades of steel,lead takes the form of dispersed fine particles.Lead is insoluble in iron,copper,and aluminum and their alloys.Because of its low shear strength,therefore,lead acts a
9、s a solid lubricant(Section 32.11)and is smeared over the tool-chip interface during cutting.This behavior has been verified by the presence of high concentrations of lead on the tool-side face of chips when machining leaded steels.When the temperature is sufficiently high-for instance,at high cutti
10、ng speeds and feeds(Section 20.6)the lead melts directly in front of the tool,acting as a liquid lubricant.In addition to this effect,lead lowers the shear stress in the primary shear zone,reducing cutting forces and power consumption.Lead can be used in every grade of steel,such as 10 xx,11xx,12xx,
11、41xx,etc.Leaded steels are identified by the letter L between the second and third numerals(for example,10L45).(Note that in stainless steels,similar use of the letter L means“low carbon,”a condition that improves their corrosion resistance.)However,because lead is a well-known toxin and a pollutant
12、,there are serious environmental concerns about its use in steels(estimated at 4500 tons of lead consumption every year in the production of steels).Consequently,there is a continuing trend toward eliminating the use of lead in steels(lead-free steels).Bismuth and tin are now being investigated as p
13、ossible substitutes for lead in steels.Calcium-Deoxidized Steels.An important development is calcium-deoxidized steels,in which oxide flakes of calcium silicates(CaSo)are formed.These flakes,in turn,reduce the strength of the secondary shear zone,decreasing tool-chip interface and wear.Temperature i
14、s correspondingly reduced.Consequently,these steels produce less crater wear,especially at high cutting speeds.Stainless Steels.Austenitic(300 series)steels are generally difficult to machine.Chatter can be s problem,necessitating machine tools with high stiffness.However,ferritic stainless steels(a
15、lso 300 series)have good machinability.M3/10 artensitic(400 series)steels are abrasive,tend to form a built-up edge,and require tool materials with high hot hardness and crater-wear resistance.Precipitation-hardening stainless steels are strong and abrasive,requiring hard and abrasion-resistant tool
16、 materials.The Effects of Other Elements in Steels on Machinability.The presence of aluminum and silicon in steels is always harmful because these elements bine with oxygen to form aluminum oxide and silicates,which are hard and abrasive.These pounds increase tool wear and reduce machinability.It is
17、 essential to produce and use cleansteels.Carbon and manganese have various effects on the machinability of steels,depending on their position.Plain low-carbon steels(less than 0.15%C)can produce poor surface finish by forming a built-up edge.Cast steels are more abrasive,although their machinabilit
18、y is similar to that of wrought steels.Tool and die steels are very difficult to machine and usually require annealing prior to machining.Machinability of most steels is improved by cold working,which hardens the material and reduces the tendency for built-up edge formation.Other alloying elements,s
19、uch as nickel,chromium,molybdenum,and vanadium,which improve the properties of steels,generally reduce machinability.The effect of boron is negligible.Gaseous elements such as hydrogen and nitrogen can have particularly detrimental effects on the properties of steel.Oxygen has been shown to have a s
20、trong effect on the aspect ratio of the manganese sulfide inclusions;the higher the oxygen content,the lower the aspect ratio and the higher the machinability.In selecting various elements to improve machinability,we should consider the possible detrimental effects of these elements on the propertie
21、s and strength of the machined part in service.At elevated temperatures,for example,lead causes embrittlement of steels(liquid-metal embrittlement,hot shortness;see Section 1.4.3),although at room temperature it has no effect on mechanical properties.Sulfur can severely reduce the hot workability of
22、 steels,because of the formation of iron sulfide,unless sufficient manganese is present to prevent such formation.At room temperature,the mechanical properties of resulfurized steels depend on the orientation of the deformed manganese sulfide inclusions(aniso4/10 tropy).Rephosphorized steels are sig
23、nificantly less ductile,and are produced solely to improve machinability.20.9.2 Machinability of Various Other Metals Aluminum is generally very easy to machine,although the softer grades tend to form a built-up edge,resulting in poor surface finish.High cutting speeds,high rake angles,and high reli
24、ef angles are remended.Wrought aluminum alloys with high silicon content and cast aluminum alloys may be abrasive;they require harder tool materials.Dimensional tolerance control may be a problem in machining aluminum,since it has a high thermal coefficient of expansion and a relatively low elastic
25、modulus.Beryllium is similar to cast irons.Because it is more abrasive and toxic,though,it requires machining in a controlled environment.Cast gray irons are generally machinable but are.Free carbides in castings reduce their machinability and cause tool chipping or fracture,necessitating tools with
26、 high toughness.Nodular and malleable irons are machinable with hard tool materials.Cobalt-based alloys are abrasive and highly work-hardening.They require sharp,abrasion-resistant tool materials and low feeds and speeds.Wrought copper can be difficult to machine because of built-up edge formation,a
27、lthough cast copper alloys are easy to machine.Brasses are easy to machine,especially with the addition pf lead(leaded free-machining brass).Bronzes are more difficult to machine than brass.Magnesium is very easy to machine,with good surface finish and prolonged tool life.However care should be exer
28、cised because of its high rate of oxidation and the danger of fire(the element is pyrophoric).Molybdenum is ductile and work-hardening,so it can produce poor surface finish.Sharp tools are necessary.Nickel-based alloys are work-hardening,abrasive,and strong at high temperatures.Their machinability i
29、s similar to that of stainless steels.Tantalum is very work-hardening,ductile,and soft.It produces a poor surface finish;tool wear is high.5/10 Titanium and its alloys have poor thermal conductivity(indeed,the lowest of all metals),causing significant temperature rise and built-up edge;they can be d
30、ifficult to machine.Tungsten is brittle,strong,and very abrasive,so its machinability is low,although it greatly improves at elevated temperatures.Zirconium has good machinability.It requires a coolant-type cutting fluid,however,because of the explosion and fire.20.9.3 Machinability of Various Mater
31、ials Graphite is abrasive;it requires hard,abrasion-resistant,sharp tools.Thermoplastics generally have low thermal conductivity,low elastic modulus,and low softening temperature.Consequently,machining them requires tools with positive rake angles(to reduce cutting forces),large relief angles,small
32、depths of cut and feed,relatively high speeds,and proper support of the workpiece.Tools should be sharp.External cooling of the cutting zone may be necessary to keep the chips from being“gummy”and sticking to the tools.Cooling can usually be achieved with a jet of air,vapor mist,or water-soluble oil
33、s.Residual stresses may develop during machining.To relieve these stresses,machined parts can be annealed for a period of time at temperatures ranging from to (to),and then cooled slowly and uniformly to room temperature.Thermosetting plastics are brittle and sensitive to thermal gradients during cu
34、tting.Their machinability is generally similar to that of thermoplastics.Because of the fibers present,reinforced plastics are very abrasive and are difficult to machine.Fiber tearing,pulling,and edge delamination are significant problems;they can lead to severe reduction in the load-carrying capaci
35、ty of the ponent.Furthermore,machining of these materials requires careful removal of machining debris to avoid contact with and inhaling of the fibers.The machinability of ceramics has improved steadily with the development of nanoceramics(Section 8.2.5)and with the selection of appropriate process
36、ing parameters,such as ductile-regime cutting(Section 22.4.2).6/10 Metal-matrix and ceramic-matrix posites can be difficult to machine,depending on the properties of the individual ponents,i.e.,reinforcing or whiskers,as well as the matrix material.20.9.4 Thermally Assisted Machining Metals and allo
37、ys that are difficult to machine at room temperature can be machined more easily at elevated temperatures.In thermally assisted machining(hot machining),the source of heata torch,induction coil,high-energy beam(such as laser or electron beam),or plasma arcis forces,(b)increased tool life,(c)use of i
38、nexpensive cutting-tool materials,(d)higher material-removal rates,and(e)reduced tendency for vibration and chatter.It may be difficult to heat and maintain a uniform temperature distribution within the workpiece.Also,the original microstructure of the workpiece may be adversely affected by elevated
39、 temperatures.Most applications of hot machining are in the turning of high-strength metals and alloys,although experiments are in progress to machine ceramics such as silicon nitride.SUMMARY Machinability is usually defined in terms of surface finish,tool life,force and power requirements,and chip
40、control.Machinability of materials depends not only on their intrinsic properties and microstructure,but also on proper selection and control of process variables.20.9 可机加工性 一种材料的可机加工性通常以四种因素的方式定义：1、分的表面光洁性和表面完整性。7/10 2、刀具的寿命。3、切削力和功率的需求。4、切屑控制。以这种方式，好的可机加工性指的是好的表面光洁性和完整性，长的刀具寿命，低的切削力和功率需求。关于切屑控制，细长
41、的卷曲切屑，如果没有被切割成小片，以在切屑区变的混乱，缠在一起的方式能够严重的介入剪切工序。因为剪切工序的复杂属性，所以很难建立定量地释义材料的可机加工性的关系。在制造厂里，刀具寿命和表面粗糙度通常被认为是可机加工性中最重要的因素。尽管已不再大量的被使用，近乎准确的机加工率在以下的例子中能够被看到。20.9.1 钢的可机加工性 因为钢是最重要的工程材料之一（正如第 5 章所示），所以他们的可机加工性已经被广泛地研究过。通过 XX 铅和硫磺，钢的可机加工性已经大大地提高了。从而得到了所谓的易切削钢。二次硫化钢和二次磷化钢硫在钢中形成硫化锰夹杂物（第二相粒子），这些夹杂物在第一剪切区引起应力。其结
42、果是使切屑容易断开而变小，从而改善了可加工性。这些夹杂物的大小、形状、分布和集中程度显著的影响可加工性。化学元素如碲和硒，其化学性质与硫类似，在二次硫化钢中起夹杂物改性作用。钢中的磷有两个主要的影响。它加强铁素体，增加硬度。越硬的钢，形成更好的切屑形成和表面光洁性。需要注意的是软钢不适合用于有积屑瘤形成和很差的表面光洁性的机器。第二个影响是增加的硬度引起短切屑而不是不断的细长的切屑的形成，因此提高可加工性。含铅的钢钢中高含量的铅在硫化锰夹杂物尖端析出。在非二次硫化钢中，铅呈细小而分散的颗粒。铅在铁、铜、铝和它们的合金中是不能溶解的。因为它的低抗剪强度。因此，铅充当固体润滑剂并且在切削时，被涂在
43、刀具和切屑的接口处。这一特性已经被在机加工铅钢时，在切屑的刀具面表面有高浓度的铅的存在所证实。当温度足够高时例如，在高的切削速度和进刀速度下铅在刀具前直接熔化，并且充当液体润滑剂。除了这个作用，铅降低第一剪切区中的剪应力，减小切削力和功率消耗。铅能用于各种钢号，例如 10XX，11XX，12XX，41XX 等等。铅钢被第二和第三数码中的字母 L 所识别（例如，10L45）。（需要注意的是在不锈钢中，字母 L 的相同用法指的是低碳，提高它们的耐蚀性的条件）。8/10 然而，因为铅是有名的毒素和污染物，因此在钢的使用中存在着严重的环境隐患（在钢产品中每年大约有 4500 吨的铅消耗）。结果，对于估
44、算钢中含铅量的使用存在一个持续的趋势。铋和锡现正作为钢中的铅最可能的替代物而被人们所研究。脱氧钙钢一个重要的发展是脱氧钙钢，在脱氧钙钢中矽酸钙盐中的氧化物片的形成。这些片状，依次减小第二剪切区中的力量，降低刀具和切屑接口处的摩擦和磨损。温度也相应地降低。结果，这些钢产生更小的月牙洼磨损，特别是在高切削速度时更是如此。不锈钢奥氏体钢通常很难机加工。振动能成为一个问题，需要有高硬度的机床。然而，铁素体不锈钢有很好的可机加工性。马氏体钢易磨蚀，易于形成积屑瘤，并且要求刀具材料有高的热硬度和耐月牙洼磨损性。经沉淀硬化的不锈钢强度高、磨蚀性强，因此要求刀具材料硬而耐磨。钢中其它元素在可机加工性方面的影响
45、钢中铝和矽的存在总是有害的，因为这些元素结合氧会生成氧化铝和矽酸盐，而氧化铝和矽酸盐硬且具有磨蚀性。这些化合物增加刀具磨损，降低可机加工性。因此生产和使用净化钢非常必要。根据它们的构成，碳和锰钢在钢的可机加工性方面有不同的影响。低碳素钢（少于 0.15%的碳）通过形成一个积屑瘤能生成很差的表面光洁性。尽管铸钢的可机加工性和锻钢的大致相同，但铸钢具有更大的磨蚀性。刀具和模具钢很难用于机加工，他们通常再煅烧后再机加工。大多数钢的可机加工性在冷加工后都有所提高，冷加工能使材料变硬并且减少积屑瘤的形成。其它合金元素，例如镍、铬、钳和钒，能提高钢的特性，减小可机加工性。硼的影响可以忽视。气态元素比如氢和
46、氮在钢的特性方面能有特别的有害影响。氧已经被证明了在硫化锰夹杂物的纵横比方面有很强的影响。越高的含氧量，就产生越低的纵横比和越高的可机加工性。选择各种元素以改善可加工性，我们应该考虑到这些元素对已加工零件在使用中的性能和强度的不利影响。例如，当温度升高时，铝会使钢变脆（液体金属脆化，热脆化，见 1.4.3 节），尽管其在室温下对力学性能没有影响。因为硫化铁的构成，硫能严重的减少钢的热加工性，除非有足够的锰来防止这种结构的形成。在室温下，二次磷化钢的机械性能依赖于变形的硫化锰夹杂物的定位（各向异性）。二次磷化钢具有更小的延展性，被单独生成来提高机加工性。20.9.2 其它不同金属的机加工性 9/
47、10 尽管越软的品种易于生成积屑瘤，但铝通常很容易被机加工，导致了很差的表面光洁性。高的切削速度，高的前角和高的后角都被推荐了。有高含量的矽的锻铝合金铸铝合金也许具有磨蚀性，它们要求更硬的刀具材料。尺寸公差控制也许在机加工铝时会成为一个问题，因为它有膨胀的高导热系数和相对低的弹性模数。铍和铸铁相同。因为它更具磨蚀性和毒性，尽管它要求在可控人工环境下进行机加工。灰铸铁普遍地可加工，但也有磨蚀性。铸造无中的游离碳化物降低它们的可机加工性，引起刀具切屑或裂口。它需要具有强韧性的工具。具有坚硬的刀具材料的球墨铸铁和韧性铁是可加工的。钴基合金有磨蚀性且高度加工硬化的。它们要求尖的且具有耐蚀性的刀具材料并
48、且有低的走刀和速度。尽管铸铜合金很容易机加工，但因为锻铜的积屑瘤形成因而锻铜很难机加工。黄铜很容易机加工，特别是有添加的铅更容易。青铜比黄铜更难机加工。镁很容易机加工，镁既有很好的表面光洁性和长久的刀具寿命。然而，因为高的氧化速度和火种的危险（这种元素易燃），因此我们应该特别小心使用它。钳易拉长且加工硬化，因此它生成很差的表面光洁性。尖的刀具是很必要的。镍基合金加工硬化，具有磨蚀性，且在高温下非常坚硬。它的可机加工性和不锈钢相同。钽非常的加工硬化，具有可延性且柔软。它生成很差的表面光洁性且刀具磨损非常大。钛和它的合金导热性(的确，是所有金属中最低的)，因此引起明显的温度升高和积屑瘤。它们是难机
49、加工的。钨易脆，坚硬，且具有磨蚀性，因此尽管它的性能在高温下能大大提高，但它的机加工性仍很低。锆有很好的机加工性。然而，因为有爆炸和火种的危险性，它要求有一个冷却性质好的切削液。20.9.3 各种材料的机加工性 石墨具有磨蚀性。它要求硬的、尖的，具有耐蚀性的刀具。塑性塑料通常有低的导热性，低的弹性模数和低的软化温度。因此，机加工热塑10/10 性塑料要求有正前角的刀具（以此降低切削力），还要求有大的后角，小的切削和走刀深的，相对高的速度和工件的正确支承。刀具应该很尖。切削区的外部冷却也许很必要，以此来防止切屑变的有黏性且粘在刀具上。有了空气流，汽雾或水溶性油，通常就能实现冷却。在机加工时，残余
50、应力也许能生成并发展。为了解除这些力，已机加工的部分要在一定的温度 X 围内冷却一段时间，然而慢慢地无变化地冷却到室温。热固性塑料易脆，并且在切削时对热梯度很敏感。它的机加工性和热塑性塑料的相同。因为纤维的存在，加强塑料具有磨蚀性，且很难机加工。纤维的撕裂、拉出和边界分层是非常严重的问题。它们能导致构成要素的承载能力大大下降。而且，这些材料的机加工要求对加工残片仔细切除，以此来避免接触和吸进纤维。随着纳米陶瓷（见 8.2.5 节）的发展和适当的参数处理的选择，例如塑性切削（见 22.4.2 节），陶瓷器的可机加工性已大大地提高了。金属基复合材料和陶瓷基复合材料很能机加工，它们依赖于单独的成分的