機(jī)械類英文文獻(xiàn)+翻譯(共10頁)

1、精選優(yōu)質(zhì)文檔-傾情為你奉上機(jī)械工業(yè)出版社 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

2、 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 diffic

3、ult 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 avail

4、able in the example below. 20.9.1 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 fr

5、ee-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, shap

6、e, 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 ferrit

7、e, 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 contin

8、uous 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

9、 alloys. Because of its low shear strength, therefore, lead acts as 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

10、. When the temperature is sufficiently high-for instance, at high cutting 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 c

11、onsumption. 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 improve

12、s 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 eliminati

13、ng 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 tur

14、n, 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 mac

15、hine. Chatter can be s 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

16、 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 t

17、o 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 s

18、teels (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

19、 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 negli

20、gible. 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

21、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 em

22、brittlement, 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 temper

23、ature, the mechanical properties of resulfurized steels depend on the orientation of the deformed manganese sulfide inclusions (anisotropy). Rephosphorized steels are significantly less ductile, and are produced solely to improve machinability. 20.9.2 Machinability of Various Other Metals Aluminum i

24、s 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

25、 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

26、 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

27、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 additio

28、n 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 exercised because of its high rate of oxidation and the danger of fire (the element is pyrophoric). M

29、olybdenum 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

30、. 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 machinab

31、ility 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 Materials Graphite is abrasive; it requires hard, abrasion-resistant, sharp too

32、ls. 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 supp

33、ort of the workpiece. 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.

34、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 simi

35、lar 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, mac

36、hining 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 processing parameters, such as ducti

37、le-regime cutting (Section 22.4.2). 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. 20.9.4 Thermally Assisted Machining Metals and alloys that are difficul

38、t 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 inexpensiv

39、e 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

40、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

41、chip 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 可機(jī)加工性 一種材料的可機(jī)加工性通常以四種因素的方式定義： 1、分的表面光潔性和表面完整性。 2、刀具的壽命。 3、切削力和功率的需求。 4、切屑控制。 以這種方式，好的可機(jī)加工性指的是好的表面光潔性和完整性，長的刀具壽命，低的切削力和功率需求。關(guān)

42、于切屑控制，細(xì)長的卷曲切屑，如果沒有被切割成小片，以在切屑區(qū)變的混亂，纏在一起的方式能夠嚴(yán)重的介入剪切工序。 因為剪切工序的復(fù)雜屬性，所以很難建立定量地釋義材料的可機(jī)加工性的關(guān)系。在制造廠里，刀具壽命和表面粗糙度通常被認(rèn)為是可機(jī)加工性中最重要的因素。盡管已不再大量的被使用，近乎準(zhǔn)確的機(jī)加工率在以下的例子中能夠被看到。 20.9.1 鋼的可機(jī)加工性 因為鋼是最重要的工程材料之一（正如第5章所示），所以他們的可機(jī)加工性已經(jīng)被廣泛地研究過。通過宗教鉛和硫磺，鋼的可機(jī)加工性已經(jīng)大大地提高了。從而得到了所謂的易切削鋼。 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

43、區(qū)引起應(yīng)力。其結(jié)果是使切屑容易斷開而變小，從而改善了可加工性。這些夾雜物的大小、形狀、分布和集中程度顯著的影響可加工性?；瘜W(xué)元素如碲和硒，其化學(xué)性質(zhì)與硫類似，在二次硫化鋼中起夾雜物改性作用。 鋼中的磷有兩個主要的影響。它加強(qiáng)鐵素體，增加硬度。越硬的鋼，形成更好的切屑形成和表面光潔性。需要注意的是軟鋼不適合用于有積屑瘤形成和很差的表面光潔性的機(jī)器。第二個影響是增加的硬度引起短切屑而不是不斷的細(xì)長的切屑的形成，因此提高可加工性。 含鉛的鋼 鋼中高含量的鉛在硫化錳夾雜物尖端析出。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。因為它的低抗剪強(qiáng)度。因此，鉛充當(dāng)固體潤滑

44、劑并且在切削時，被涂在刀具和切屑的接口處。這一特性已經(jīng)被在機(jī)加工鉛鋼時，在切屑的刀具面表面有高濃度的鉛的存在所證實。 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，鉛降低第一剪切區(qū)中的剪應(yīng)力，減小切削力和功率消耗。鉛能用于各種鋼號，例如10XX，11XX，12XX，41XX等等。鉛鋼被第二和第三數(shù)碼中的字母L所識別（例如，10L45）。（需要注意的是在不銹鋼中，字母L的相同用法指的是低碳，提高它們的耐蝕性的條件）。 然而，因為鉛是有名的毒素和污染物，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）。結(jié)果，對于估

45、算鋼中含鉛量的使用存在一個持續(xù)的趨勢。鉍和錫現(xiàn)正作為鋼中的鉛最可能的替代物而被人們所研究。脫氧鈣鋼 一個重要的發(fā)展是脫氧鈣鋼，在脫氧鈣鋼中矽酸鈣鹽中的氧化物片的形成。這些片狀，依次減小第二剪切區(qū)中的力量，降低刀具和切屑接口處的摩擦和磨損。溫度也相應(yīng)地降低。結(jié)果，這些鋼產(chǎn)生更小的月牙洼磨損，特別是在高切削速度時更是如此。 不銹鋼 奧氏體鋼通常很難機(jī)加工。振動能成為一個問題，需要有高硬度的機(jī)床。然而，鐵素體不銹鋼有很好的可機(jī)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度和耐月牙洼磨損性。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而耐磨。 鋼中其它元素在可機(jī)加工性方

46、面的影響 鋼中鋁和矽的存在總是有害的，因為這些元素結(jié)合氧會生成氧化鋁和矽酸鹽，而氧化鋁和矽酸鹽硬且具有磨蝕性。這些化合物增加刀具磨損，降低可機(jī)加工性。因此生產(chǎn)和使用凈化鋼非常必要。 根據(jù)它們的構(gòu)成，碳和錳鋼在鋼的可機(jī)加工性方面有不同的影響。低碳素鋼（少于0.15%的碳）通過形成一個積屑瘤能生成很差的表面光潔性。盡管鑄鋼的可機(jī)加工性和鍛鋼的大致相同，但鑄鋼具有更大的磨蝕性。刀具和模具鋼很難用于機(jī)加工，他們通常再煅燒后再機(jī)加工。大多數(shù)鋼的可機(jī)加工性在冷加工后都有所提高，冷加工能使材料變硬并且減少積屑瘤的形成。 其它合金元素，例如鎳、鉻、鉗和釩，能提高鋼的特性，減小可機(jī)加工性。硼的影響可以忽視。氣態(tài)

47、元素比如氫和氮在鋼的特性方面能有特別的有害影響。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。越高的含氧量，就產(chǎn)生越低的縱橫比和越高的可機(jī)加工性。 選擇各種元素以改善可加工性，我們應(yīng)該考慮到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，鋁會使鋼變脆（液體金屬脆化，熱脆化，見1.4.3節(jié)），盡管其在室溫下對力學(xué)性能沒有影響。 因為硫化鐵的構(gòu)成，硫能嚴(yán)重的減少鋼的熱加工性，除非有足夠的錳來防止這種結(jié)構(gòu)的形成。在室溫下，二次磷化鋼的機(jī)械性能依賴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)生成來提高機(jī)加工性。 20.9.2 其它不同金屬的機(jī)加工性 盡管越軟的品種易于生成積屑瘤，但鋁通常很容易被機(jī)加工，導(dǎo)致了很差的