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英文原文
The machinability of a material
1. 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 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 becoming entangled in the cutting zone.
Because of the complex nature of cutting operations, it is difficult to establish relationships that 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.
2. Machinability 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 Rephosphorized 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 inclusions 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 increased hardness. Harder steels result in 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 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 as a solid lubricant 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 cutting speeds and feeds —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 10xx, 11xx, 12xx, 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 “l(fā)ow carbon,” a condition that improves their corrosion resistance.)
However, because lead is a well-known toxin and a pollutant, 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 possible 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 is 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 a problem, necessitating machine tools with high stiffness. However, ferritic stainless steels (also 300 series) have good machinability. Martensitic (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 materials.
The Effects of Other Elements in Steels on Machinability. The presence of aluminum and silicon in steels is always harmful because these elements combine with oxygen to form aluminum oxide and silicates, which are hard and abrasive. These compounds increase tool wear and reduce machinability. It is essential to produce and use clean steels.
Carbon and manganese have various effects on the machinability of steels, depending on their composition. 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 machinability 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, such 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 strong 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 properties 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 steels, because of the formation of iron sulfide, unless sufficient manganese is present to prevent such formation. At room temperature, the mechanical properties of re-sulfurized steels depend on the orientation of the deformed manganese sulfide inclusions (anisotropy). Re-phosphorized steels are significantly less ductile, and are produced solely to improve machinability.
3. 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 relief angles are recommended. 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 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 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, although cast copper alloys are easy to machine. Brasses are easy to machine, especially with the addition 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 exercised 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 is similar to that of stainless steels.
Tantalum is very work-hardening, ductile, and soft. It produces a poor surface finish; tool wear is high.
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 difficult 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.
4. Machinability of Various Materials
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 depths of cut and feed, relatively high speeds, and proper support of the work-piece. Tools should be sharp.
External cooling of the cutting zone may be necessary to keep the chips from becoming “gummy” and sticking to the tools. Cooling can usually be achieved with a jet of air, vapor mist, or water-soluble oils. 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 cutting. 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 capacity of the component. 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 Nano ceramics and with the selection of appropriate processing parameters, such as ductile-regime cutting .
Metal-matrix and ceramic-matrix composites can be difficult to machine, depending on the properties of the individual components, i.e., reinforcing or whiskers, as well as the matrix material.
5. Thermally Assisted Machining
Metals and alloys 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 heat—a torch, induction coil, high-energy beam (such as laser or electron beam), or plasma arc—is forces, (b) increased tool life, (c) use of inexpensive 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 work-piece. Also, the original microstructure of the work-piece may be adversely affected by elevated 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.
6. SUMMARY
Machinability is usually defined in terms of surface finish, tool life, force and power requirements, and chip control. Machinability of materials depends not only on their intrinsic properties and microstructure, but also on proper selection and control of process variables.
中文翻譯
切削加工性
1、一種材料的切削加工性通常從四個方面來定義：
(1)、已切削部分的表面光潔度和表面完整性。
(2)、刀具的壽命。
(3)、切削力和切削的功率需求。
(4)、切屑控制。
由上述可知，好的切削加工性指的是好的表面光潔度和完整性，長的刀具壽命，低切削力和功率需求。至于切屑控制，細(xì)長而卷曲的切屑，如果沒有及時清理，就會在切削區(qū)纏繞，嚴(yán)重影響切削工序。
由于切削工序的復(fù)雜性，因此很難建立一個定量確定一種材料切削加工性的關(guān)系式。在制造廠里，刀具壽命和表面粗糙度通常被認(rèn)為是切削加工性中最重要的影響因素。盡管切削性能指數(shù)使用的并不多，但基本的切削性能指數(shù)在下面的材料中仍然被使用。
2.鋼的切削加工性
因為鋼是最重要的工程材料之一（如第5章所示），所以它的切削加工性已經(jīng)被廣泛地研究過。通過加入鉛和硫磺，可以使鋼的切削加工性得到大幅度地提高。從而得到了所謂的高速切削鋼。
二次硫化鋼和二次磷化鋼 硫在鋼中形成硫化錳夾雜物（第二相粒子），這些夾雜物在第一剪切區(qū)形成應(yīng)力集中元。其結(jié)果是使切屑容易斷開而變小，從而改善了切削加工性。這些夾雜物的大小、形狀、分布和集中程度顯著的影響切削加工性。化學(xué)元素如碲和硒，其化學(xué)性質(zhì)與硫類似，在二次硫化鋼中起雜質(zhì)改性作用。
鋼中的磷有兩個主要的作用。第一它加強(qiáng)鐵素體，增加硬度。越硬的鋼，就會對切屑的形成和表面光潔度越有利。需要注意的是軟鋼是很難加工的，因為軟鋼加工容易產(chǎn)生積削瘤而且表面光潔度差。第二個作用是硬度增加會引起短切屑的形成而不是連續(xù)細(xì)長的切屑的形成，因此提高切削加工性。
鉛鋼 鋼中高含量的鉛在硫化錳雜質(zhì)尖端析出。在非二次硫化鋼中，鉛呈細(xì)小而分散的顆粒。鉛在鐵、銅、鋁和它們的合金中是不能溶解的。由于它的低抗剪強(qiáng)度，鉛在切削時充當(dāng)固體潤滑劑，被涂在刀具和切屑的分界處。這一特性已經(jīng)被證實--在切削加工鉛鋼時，在刀具橫向表面的切屑上有高濃度的鉛存在。
當(dāng)溫度足夠高時——例如，在高的切削速度和進(jìn)刀速度下——鉛在刀具前直接熔化，并且充當(dāng)液體潤滑劑。除了這個作用外，鉛還可以降低第一剪切區(qū)中的剪應(yīng)力，減小切削力和降低功率消耗。鉛能用于各種型號的鋼，例如10XX，11XX，12XX，41XX等等。鉛鋼由型號中第二和第三數(shù)碼中的字母L識別（例如，10L45）。（需要注意的是在不銹鋼中，字母L指的是低碳，這是提高不銹鋼耐腐蝕性的先決條件）。
然而，因為鉛是眾所周知的毒素和污染物，因此在鋼的使用中存在著嚴(yán)重的環(huán)境隱患（在鋼產(chǎn)品中每年大約有4500噸的鉛消耗）。于是，消除鉛在鋼中使用是一個必然的趨勢（無鉛鋼）。鉍和錫現(xiàn)正作為最可能替代鋼中鉛的物質(zhì)而被人們所研究。
脫氧鈣鋼 一個重要的發(fā)展是脫氧鈣鋼，在脫氧鈣鋼中可以形成硅酸鈣的氧化物片。這些片狀物，可以減小第二剪切區(qū)中的應(yīng)力，降低刀具和切屑分界處的摩擦和磨損。溫度也相應(yīng)地降低。于是，這種鋼產(chǎn)生更小的月牙洼磨損，特別是在高速切削時更是如此。
不銹鋼 通常奧氏體鋼很難進(jìn)行切削加工。振動可能是一個問題，這必需要求機(jī)床有足夠的剛度。然而，鐵素體不銹鋼有很好的切削加工性。馬氏體鋼易磨蝕，易于形成積屑瘤，并且要求刀具材料有高的熱硬性和耐月牙洼磨損性。經(jīng)沉淀硬化的不銹鋼強(qiáng)度高、磨蝕性強(qiáng)，因此要求刀具材料硬度高而耐磨。
鋼中其它元素對切削加工性能的影響 鋼中鋁和硅元素的存在總是有害的，因為這些元素結(jié)合氧會生成氧化鋁和硅酸鹽，而氧化鋁和硅酸鹽硬度高且具有磨蝕性。這些化合物會加快刀具磨損，降低切削加工性。因此生產(chǎn)和使用凈化鋼是非常必要的。
根據(jù)它們的構(gòu)成，碳和錳在鋼的切削加工性方面有各種不同的影響。低碳鋼（少于0.15%的碳）容易形成積屑瘤而使毛坯的表面光潔度很低。鑄鋼的切削加工性和鍛鋼的大致相同，但鑄鋼更容易磨蝕。工具鋼和模具鋼很難用于切削加工，通常是在切削加工之前進(jìn)行退火處理。大多數(shù)鋼的切削加工性在冷加工后都有所提高，冷加工能使材料變硬而減少積屑瘤的形成。
其它合金元素，例如鎳、鉻、鉬和釩，能改善鋼的特性，而通常會鋼減小切削加工性。硼的影響可以忽視。氣態(tài)元素比如氫和氮在鋼的特性方面有特別有害的影響。氧已經(jīng)被證明了在硫化錳夾雜物的縱橫比方面有很強(qiáng)的影響。含氧量越高，縱橫比越低且切削加工性越好。
在選擇各種元素以改善切削加工性時，我們應(yīng)該考慮這些元素對已加工零件在使用中的性能和強(qiáng)度的不利影響。例如，當(dāng)溫度升高時，鉛會使鋼變脆，盡管其在室溫下對機(jī)械性能沒有影響。
由于硫化鐵的構(gòu)成，硫元素能嚴(yán)重的降低鋼的熱加工性，除非有足夠的錳元素來防止這種結(jié)構(gòu)的形成。在室溫下，二次硫化鋼的機(jī)械性能取決于變形的硫化錳夾雜物的定位（各向異性）。二次磷化鋼具有更小的延展性，被單獨(dú)生成來提高切削加工性。
3. 其它不同金屬的切削加工性
盡管越軟的材料更易于生成積屑瘤而導(dǎo)致很差的表面光潔度，但鋁通常很容易進(jìn)行切削加工。這需要高的切削速度，高的前角和后角。鑄鋁合金和高含量硅的鍛鋁合金可能具有磨蝕性，它們要求刀具材料硬度更高。在加工鋁材料的工件時尺寸公差控制可能會是一個難題，這是因為它具有高熱膨脹系數(shù)和相對較低的彈性模數(shù)。
鈹和鑄鐵相似。由于它更具磨蝕性和毒性，于是它需要在可控環(huán)境下進(jìn)行加工。
灰鑄鐵通常是可進(jìn)行切削加工的，但也有磨蝕性。鑄件中的游離碳化物降低它們的切削加工性，容易導(dǎo)致刀具破裂或裂口，因此它需要具有強(qiáng)韌性的刀具。在刀具具有足夠硬度的情況下球墨鑄鐵和可鍛鑄鐵是可加工的。
鈷基合金有磨蝕性和高度的加工硬化性。這要求刀具必須鋒利而且具有耐蝕性，并且在加工時進(jìn)給速度要低。
鑄銅合金是很容易進(jìn)行切削加工的，與此相反的是鍛銅因為容易產(chǎn)生積屑瘤而很難進(jìn)行切削加工。黃銅易進(jìn)行切削加工，特別是在添加了一定量鉛的情況下更容易。而青銅比黃銅更難進(jìn)行切削加工。
鎂是很容易加工的，加工后的鎂件具有很好的表面光潔性而且使加工零件的刀具壽命更長。然而，因為鎂極易氧化而燃燒（這種元素易燃），因此我們應(yīng)該要特別小心的使用它。
鉬有很好的延展性和加工硬化性，因此加工后它的表面光潔性很差。所以鋒利的刀具是很很有必要的。
鎳基合金具有加工硬化性和磨蝕性，且在高溫下非常堅硬。它的切削加工性和不銹鋼相似。
鉭具有非常好的加工硬化性，延展性和柔性。加工后零件的表面光潔性很差且刀具磨損非常大。
鈦和鈦的合金導(dǎo)熱系數(shù)很低(的確，是所有金屬中最低的)，因此在加工時會引起明顯的溫度升高和還會產(chǎn)生積屑瘤。它們是很難進(jìn)行切削加工的。
鎢易脆，堅硬，且具有磨蝕性，因此盡管它的性能在高溫下能大幅提高，但它的切削加工性仍很低。
鋯切削加工性很好。然而，因為有爆炸和起火的危險，鋯在加工時要求切削液冷卻性能非常好。
4. 各種材料的機(jī)加工性
石墨具有磨蝕性。它要求刀具硬度高、鋒利和具有一定的耐蝕性。
熱塑性塑料通常熱導(dǎo)性都很差且彈性模數(shù)小、軟化溫度低。因此，加工熱塑性塑料時要求刀具具有正前角（以此降低切削力），較大的后角，較小的切削和進(jìn)給深度，相對較高的切削速度和適當(dāng)?shù)墓ぜС?。另外還要求刀具應(yīng)當(dāng)足夠鋒利。
切削區(qū)外部的冷卻也是很必要的，這可以防止切屑變的有黏性而粘在刀具上。實現(xiàn)冷卻通常是利用空氣流，汽霧或水溶性油。在切削加工時，可能會產(chǎn)生殘余應(yīng)力。為了減小這些應(yīng)力，已加工的部分需要在 的溫度范圍內(nèi)進(jìn)行一段時間的退火，然后緩慢而均一地冷卻到室溫。
熱固性塑料在切削時易脆，并且對熱梯度很敏感。它的切削加工性能和熱塑性塑料的基本相同。
由于有纖維狀切屑的存在，加固塑料具有很強(qiáng)的磨蝕性而且很難進(jìn)行切削加工。切屑的撕裂、拉長和邊界分層是非常嚴(yán)重的幾個問題。它能導(dǎo)致組件的承載能力大大下降。而且，這些材料的切削加工需要對加工碎屑進(jìn)行仔細(xì)清理，從而避免接觸而吸入纖維。
隨著納米陶瓷的發(fā)展和適當(dāng)?shù)膮⒘刻幚淼倪x擇，例如塑性切削，陶瓷器的切削加工性能已大大地提高了。
金屬基復(fù)合材料和陶瓷基復(fù)合材料很難進(jìn)行切削加工，它們依賴于獨(dú)立部件的特性，也就是包括基質(zhì)材料在內(nèi)纖維或金屬須的增強(qiáng)。
5. 熱輔助加工
有些在室溫下很難進(jìn)行切削加工的金屬和其合金在高溫下卻能更容易地進(jìn)行加工。在熱輔助加工時（高溫切削），熱源——一個火源，感應(yīng)線圈，高能束流（例如雷射或電子束），或等離子弧——被集中在切削刀具前的一塊區(qū)域內(nèi)。作用是：（a）降低切削力。（b）增加刀具壽命。（c）減少切削刀具材料使用成本。（d）提高材料切除率。（e）減少振動。
也許很難在工件內(nèi)加熱和保持均一的溫度分布。而且高溫可能會對工件的最初微觀結(jié)構(gòu)產(chǎn)生不利的影響。盡管加工陶瓷（如氮化硅）實驗在進(jìn)行中，，但高溫切削仍大多數(shù)應(yīng)用在高強(qiáng)度金屬及其合金的加工中。
6. 小結(jié)
切削加工性通常從以下幾個方面來定義：已切削部分的表面粗糙度，刀具的壽命，切削力和切削功率的需求以及切屑的控制。材料的切削加工性能不僅取決于其固有特性和微觀結(jié)構(gòu)，也取決于工藝參數(shù)的適當(dāng)選擇與控制
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