Metal Machining Episode 4 pot

recently, these were not machinable. The data come from compression testing at room temperature and at low strain rates of initially unworked metal. The detail is presented in Appendix 4.1. Although machining generates high strain rates and temperatures, these data are useful as a first attempt to relate the severity of machining to work material plastic flow behaviour. A more detailed approach, taking into account variations of material flow stress with strain rate and temperature, is introduced in Chapter 6. Work heating is also considered in Chapter 2. Temperature rises in the primary shear zone and along the tool rake face both depend on fU work tanf/k work . Figure 3.2(a) summa- rizes the conclusions from equation (2.14) and Figures 2.17(a) and 2.18(b). In the primary shear zone the dimensionless temperature rise DT(rC)/k depends on fU work tanf/k work and the shear strain gï. Next to the rake face, the additional temperature rise depends on fU work tanf/k work and the ratio of tool to work thermal conductivity, K*. Figure 3.2(b) summarizes the typical thermal properties of the same groups of work materials whose mechanical properties are given in Figure 3.1. The values recorded are from room temper- ature to 800˚C. Appendix 4.2 gives more details. Figures 3.1 and 3.2 suggest that the six groups of alloys may be reduced to three as far as the mechanical and thermal severity of machining them is concerned. Copper and aluminium alloys, although showing high work hardening rates, have relatively low shear stresses and high thermal diffusivities. They are likely to create low tool stresses and low temperature rises in machining. At the other extreme, austenitic steels, nickel and titanium alloys have medium to high shear stresses and work hardening rates and low thermal diffu- sivities. They are likely to generate large tool stresses and temperatures. The body centred cubic carbon and alloy steels form an intermediate group. The behaviours of these three different groups of alloys are considered in Sections 3.1.3 to 3.1.5 of this chapter, after sections in which the machining of unalloyed metals is Work material characteristics in machining 83 Fig. 3.1 Shear stress levels and work hardening severities of initially unstrained, commonly machined, aluminium, copper, iron (b.c.c. and f.c.c.), nickel and titanium alloys Childs Part 1 28:3:2000 2:38 pm Page 83 described. It will be seen that these groups do indeed give rise to three different levels of tool stress and temperature severity. This is demonstrated by presenting representative experimentally measured specific cutting forces (forces per unit feed and depth of cut) and shear plane angles for these groups as a function of cutting speed. Then, primary shear zone shear stress k, average normal contact stress on the rake face (s n ) av and average rake face contact temperature (T rake ) av are estimated from the cutting data. A picture is built up of the stress and temperature conditions that a tool must survive in machining these materials. The primary shear plane shear stress is estimated from (F c cos f – F T sin f)sin f k = ——————————— (3.1) fd The average normal contact stress on the tool rake face is estimated from the measured normal component of force on the rake face, the depth of cut and the chip/tool contact length l c : F c cos a – F T sin a (s n ) av = ———————— (3.2) l c d l c is taken, from the mean value data of Figure 2.9(a), to be cos(f – a) l c = 1.75f ————— [m + tan(f – a)] (3.3) sin f Finally, temperatures are estimated after the manner summarized in Figure 3.2. 84 Work and tool materials Fig. 3.2 Thermal aspects of machining: (a) a summary of heating theory and (b) thermal property ranges of Al, Cu, Fe, Ni and Ti alloys Childs Part 1 28:3:2000 2:38 pm Page 84 The machining data come mainly from results in the authors’ possession. The exception are data on the machining of the aluminium alloy Al2024 (Section 3.1.2), which are from results by Kobayashi and Thomsen (1959). The data on machining elemental metals come from the same experiments on those metals considered by Trent in his book (Trent, 1991). 3.1.1 Machining elemental metals Although the elemental metals copper, aluminium, iron, nickel and titanium have little commercial importance as far as machining is concerned (with the exception of aluminium used for mirrors and disk substrates in information technology applications), it is interest- ing to describe how they form chips: what specific forces and shear plane angles are observed as a function of cutting speed. The behaviour of alloys of these materials can then be contrasted with these results. Figure 3.3 shows results from machining at a feed of 0.15 mm with high speed steel (for copper and aluminium) and cemented carbide (for iron, nickel and titanium) tools of 6˚ rake angle. At the lowest cutting speeds (around 30 m/min), except for titanium, the metals machine with very large specific forces, up to 8 GPa for iron and nickel and around 4 GPa for copper and aluminium. These forces are some ten times larger than the expected shear flow stresses of these metals (Figure 3.1) and arise from the very low shear plane angles, between 5˚ and 8˚, that occur. These shear plane angles give shear strains in the primary shear zone of from 7 to 12. As cutting speed increases to 200 m/min, the shear plane angles increase and the specific forces are roughly halved. Further increases in speed cause much less variation in chip flow and forces. The titanium material is an exception. Over the whole speed range, although decreases of specific force and increases of shear plane angle with cutting speed do occur, its shear plane angle is larger and its specific forces are Work material characteristics in machining 85 Fig. 3.3 Cutting speed dependence of specific forces and shear plane angles for some commercially pure metals ( f = 0.15 mm, α = 6º) Childs Part 1 28:3:2000 2:38 pm Page 85 smaller than for the other, more ductile, metals. A reduction in forces and an increase in shear angle with increasing speed, up to a limit beyond which further changes do not occur, is a common observation that will also be seen in many of the following sections. Although the forces fall with increasing speed, the process stresses remain almost constant. Figure 3.4 shows aluminium to have the smallest primary shear stress, k, followed by copper, iron, nickel and titanium. The estimated average normal stresses (s n ) av lie between 0.5k and 1.0k. This would place the maximum normal contact stresses (which are between two and three times the average stress) in the range k to 3k. This is in line with the estimates in Chapter 2, Figure 2.15. The different thermal diffusivities of the five metals result in different temperature vari- ations with cutting speed (Figure 3.5). For copper and aluminium, with k taken to be 110 and 90 mm 2 /s respectively (Appendix 4.2), fU work tanf/k work hardly rises to 1, even at the cutting speed of 300 m/min. Figure 3.2 suggests that then the primary shear temperature rise dominates the secondary (rake) heating. The actual increase in temperature shown in 86 Work and tool materials Fig. 3.4 Process stresses, derived from the observations of Figure 3.3 Fig. 3.5 Temperatures estimated from the observations of Figure 3.3 Childs Part 1 28:3:2000 2:38 pm Page 86 Figure 3.5 results from the combined effect of increasing fraction of heat flowing into the chip and reducing shear strain as cutting speed rises. Iron and nickel, with k taken to be 15 and 20 mm 2 /s respectively, machine with fU work tanf/k work in the range 1 to 10 in the conditions considered. In Figure 3.5, the primary shear and average rake face temperatures are distinctly separated. Over much of the speed range, the temperature actually falls with increasing cutting speed. This unusual behaviour results from the reduction of strain in the chip as speed increases. Finally, titanium, with k taken to be 7.5 mm 2 /s, machines with fU work tanf/k work from 7 to 70. The rake face heating is dominant and a temperature in excess of 800˚C is estimated at the cutting speed of 150 m/min. 3.1.2 Effects of pre-strain and rake angle in machining copper In the previous section, the machining of annealed metals by a 6˚ rake angle tool was considered. Both pre-strain and an increased rake angle result in reduced specific cutting forces and reduced cutting temperatures, but have little effect on the stressses on the tool. These generalizations may be illustrated by the cutting of copper, a metal sufficiently soft (as also is aluminium) to allow machining by tools of rake angle up to around 40˚. Figure 3.6 shows examples of specific forces and shear plane angles measured in turning annealed and heavily cold-worked copper at feeds in the range 0.15 to 0.2 mm, with high speed steel tools of rake angle from 6˚ to 35˚. Specific forces vary over a sixfold range at the lowest cutting speed, with shear plane angles from 8˚ to 32˚. The left panel of Figure 3.7 shows that the estimated tool contact stresses change little with rake angle, although they are clearly larger for the annealed than the pre-strained material. The right-hand panel shows that the temperature rises are halved on changing from a 6˚ to 35˚ rake angle tool. These observations, that tool stresses are determined by Work material characteristics in machining 87 Fig. 3.6 Specific force and shear plane angle variations for annealed (•) and pre-strained (o) commercially pure copper ( f = 0.15 to 0.2 mm, α = 6º to 35º) Childs Part 1 28:3:2000 2:38 pm Page 87 the material being cut and do not vary much with the cutting conditions, while tempera- tures depend strongly on both the material being cut and the cutting conditions, is a contin- uing theme that will be developed for metal alloys in the following sections. 3.1.3 Machining copper and aluminium alloys It is often found that alloys of metals machine with larger shear plane angles and hence lower specific forces than the elemental metals themselves. Sometimes a strong reason is a lower value of the strain hardening parameter Dk/k max , at other times the chip/tool fric- tion (as indicated by the friction coefficient) is less; and at others again it is not at all obvi- ous why this should be so. But even when the specific forces are lower, the tool contact stress can be higher. In this section, examples of machining two copper and one aluminium alloy are taken to illustrate this. Figure 3.8 records the behaviours of a CuNi and a CuZn alloy. The CuNi alloy, with 80%Ni, might better be considered as a Ni alloy. However, it machines at a higher shear plane angle at a given cutting speed than either copper or nickel, despite its strain-harden- ing characteristic being similar to or more severe than either of these (Appendix 4.1). The CuZn alloy (an a-brass) is a well-known very easy material to machine. Its shear plane angle is twice as large as that of Cu, despite having a similar strain-hardening characteris- tic (Appendix 4.1 again) and an apparently higher friction interaction with the tool (as judged by the relative sizes of its specific thrust and cutting forces). (Figure 3.8 describes the machining of an annealed brass. After cold-working, even higher shear plane angles, and lower specific forces are obtained.) These two examples are ones where the reason for the easier machining of the alloys compared with the elemental metals is not obvious from their room temperature, low strain rate mechanical behaviours. Figure 3.9 shows machining data for an aluminium alloy. In this case the variation of behaviour with rake angle is shown. At a rake angle and speed comparable to that shown in Figure 3.3, the shear plane angle is five times as large and the specific cutting force is half as large for the alloy as for pure Al. In this case both the strain-hardening and friction factors are less for the alloy than for pure Al. For both the copper and aluminium alloy examples, the primary shear plane shear stress and the average rake contact stresses are similar to, or slightly larger than, those for the 88 Work and tool materials Fig. 3.7 Average rake face contact stresses and temperatures, from the results of Figure 3.6 Childs Part 1 28:3:2000 2:38 pm Page 88 elemental metals. Figure 3.8 shows only the values of k, but (s n ) av may be calculated to be ≈ 0.6k. Figure 3.9 shows both k and (s n ) av . It also shows that, in this case, the estimated rake face temperature does not change as the rake angle is reduced. This is different from the observations recorded in Figure 3.7: perhaps the maximum temperature is limited by melting of the aluminium alloy? Work material characteristics in machining 89 Fig. 3.8 Observed and calculated machining parameters for two copper alloys ( f = 0.15 mm, α = 6º) Fig. 3.9 Machining parameter variation with rake angle for Al22024-T4 alloy, at a cutting speed of 175 m/min and f = 0.25 mm Childs Part 1 28:3:2000 2:39 pm Page 89 The choice in Figure 3.9 of showing how machining parameters vary with rake angle has been made to introduce the observation that, in this case, at a rake angle of around 35˚ the thrust force passes through zero. Consequently, such a high rake angle is appropriate for machining thin walled structures, for which thrust forces might cause distortions in the finished part. However, the main point of this section, to be carried forward to Section 3.2 on tool materials, is that the range of values estimated for k follows the range expected from Figure 3.1 and the estimated values of (s n ) av range from 0.5 to 1.0k. This is summarized in Table 3.4 which also contains data for the other alloy systems to be considered next. 3.1.4 Machining austenitic steels and temperature resistant nickel and titanium alloys The austenitic steels, NiCr, and Ti alloys are at the opposite extreme of severity to the aluminium and copper alloys. Although their specific forces are in the same range and their shear plane angles are higher, the tool stresses and temperatures (for a given speed and 90 Work and tool materials Table 3.4 Approximate ranges of k and ( σ n ) av estimated from machining tests Alloy system Stress (MPa) Al Cu Fe(bcc) Fe(fcc) Ni Ti k 200–400 300–550 350–750 500–800 550–850 550–700 ( σ n ) av 120–370 150–400 200–550 400–700 300–800 600–700 Fig. 3.10 Specific force and shear plane angle variations for some austenitic steels, nickel-chromium and titanium alloys ( f = 0.1 to 0.2 mm, α = 0º to 6º) Childs Part 1 28:3:2000 2:39 pm Page 90 feed) that they generate are significantly higher. Figure 3.10 presents observations for two austenitic steels, a NiCr and a Ti alloy. One of the austenitic steels (the 18Cr8Ni material) is a common stainless steel. The 18Mn5Cr material, which also contains 0.47C, is an extremly difficult to machine creep and abrasion resistant material. The NiCr alloy is a commercial Inconel alloy, X750. In all cases the feed was 0.2 mm except for the Ti alloy, for which it was 0.1 mm. The rake angle was 6˚ except for the NiCr alloy, for which it was 0˚. Specific cutting forces are in the range 2 to 4 GPa. Thrust forces are mainly between 1 and 2 GPa. Shear plane angles are mainly greater than 25˚. In most cases, the chip forma- tion is not steady but serrated. The values shown in Figure 3.10 are average values. Figure 3.11 shows stresses and temperatures estimated from these. The larger stresses and temper- atures are clear. 3.1.5 Machining carbon and low alloy steels Carbon and alloy steels span the range of machinability between aluminium and copper alloys on the one hand and austentic steels and temperature resistant alloys on the other. There are two aspects to this. The wide range of materials’ yield stresses that can be achieved by alloying iron with carbon and small amounts of other metals, results in their spanning the range as far as tool stressing is concerned. Their intermediate thermal conductivities and diffusivities result in their spanning the range with respect to tempera- ture rise per unit feed and also cutting speed. Work material characteristics in machining 91 Fig. 3.11 Process stresses and temperatures derived from (and symbols as) Figure 3.10 Childs Part 1 28:3:2000 2:39 pm Page 91 Figure 3.12 shows typical specific force and shear plane angle variations with cutting speed measured in turning steel bars that have received no particular heat treatment other than the hot rolling process used to manufacture them. At cutting speeds around 100 m/min the specific forces of 2 to 3 GPa are smaller than those for pure iron (Figure 3.3), but as speed increases, the differences between the steels and pure iron reduce. In the same way as for many other alloy systems, the shear plane angles of the ferrous alloys are larger than for the machining of pure iron. In the hot rolled condition, steels (other than the austenitic steels considered in the previous section) have a structure of ferrite and pearlite (or, at high carbon levels, pearlite and cementite). For equal coarsenesses of pearlite, the steels’ hardness increases with carbon content. The left panel of Figure 3.13 shows how the estimated k and (s n ) av values from the data of Figure 3.12 increase with carbon content. Additional results have been included, for the machining of a 0.13C and a 0.4C steel. An increase of both k and (s n ) av with %C is clear. The right panel of the figure likewise shows that the increasing carbon 92 Work and tool materials Fig. 3.12 Representative specific force and shear plane angle variations for hot rolled carbon and alloy steels ( f = 0.15 mm, α = 6º) Fig. 3.13 Process stresses and temperatures derived from Figure 3.12 Childs Part 1 28:3:2000 2:39 pm Page 92 [...]... Al2O3/SiC(wh.) Si3N4/Sialon HSS 600–2000 100–550 ≈100* 75–120 25–55 11–35 10–35 10–22 10–35 15–30 19– 24 2.0 2.0** 1.9–2.1 3.0–3 .4 4.0 4. 1 2 .4 2.7 3.2–3.6 3.8 4. 0 ≈3 .4* 2.1–2.3 3.6–3.8 3.1 3.8 4. 2 4. 7 4. 9 4. 5–6.0 5.8–6.8 6.7–7.8 7.9–8.0 7.6–8.0 7.0–7.5 3.2–3.6 12–13 960–990 620– 840 680–710 550–650 49 0–560 390 42 0 380–390 370–395 345 42 5 280–320 220– 240 – 140 – 540 150– 340 390–925 390– 840 48 0– 740 145 –330 180–330... Childs Part 1 28:3:2000 2 :40 pm Page 111 Tool materials 111 Table 3.9 Thermal and elastic properties of cemented carbides and their coatings Thermal conductivity [W/m K] Material αe [10–6K–1] Young’s modulus [GPa] 100°C 1000°C TiC TiN Al2O3 WC-Co* WC-TiC-TaC-Co* Co 7 .4 7.7 9 .4 8 .4 9.0 4. 5–6.0 5.8–6.8 ≈ 12 ≈ 45 0 ≈ 250 ≈ 40 0 550–650 49 0–560 ≈ 180 24 33 19–21 20–28 75–120 25–55 70 38 41 25–26 6–7.5 50–75 20–50... Ni-Cr creep resistant alloy (Nimonic 80) One definition of high speed machining is machining at speeds above those of built-up-edge formation These are the conditions mostly focused on in this book Fig 3. 14 Characteristics of built-up edge (BUE) formation (0.15C steel, f = 0.15 mm, α = 6º) Childs Part 1 28:3:2000 2:39 pm Page 94 94 Work and tool materials Fig 3.15 Speed and feed dependence of built-up... TiC, TiN and Al2O3 on the surfaces of cemented carbides are around 1000˚C Some of the furnace atmospheres and the reactions that lead to the coatings are: TiCl4(gas) + CH4(gas) + H2(gas) ⇒ TiC(solid) + 4HCl(gas) + H2(gas) 2TiCl4(gas) + N2(gas) + 4H2(gas) ⇒ 2TiN(solid) + 8HCl(gas) 2AlCl3(gas) + 3CO2(gas) + 3H2(gas) ⇒ Al2O3(solid) + 6HCl(gas) + 3CO(gas) Coating rates are around 1 mm/hr (for good performance... maintained, to damage the substrate microstructure during manufacture An alternative to the reaction of TiCl4 with CH4 to form TiC is TiCl4(gas) + C(from substrate) + 2H2(gas) ⇒ TiC(solid) + 4HCl(gas) If this occurs, the loss of carbon from the substrate can result in the brittle h-phase (Section 3.2 .4) forming Some manufacturers have avoided this problem by developing coated tools on P-type substrates... 0.3%) The topics of tool wear and life are developed more fully in Chapter 4 Here, Figure 3.17 shows differences in the machining of a low alloy steel (nominally 0.4C1Cr0.2Mo), produced without and with small additions of Ca and S as just described Childs Part 1 28:3:2000 2:39 pm Page 96 96 Work and tool materials Fig 3.17 Machining characterisitcs of a low alloy (•) and a semi-free-cutting low alloy... steels in Figure 3.16, k values are estimated between 40 0 MPa and 45 0 MPa (in line with Figure 3.13) (sn)av values around 300 MPa are estimated for the non-free-cutting steel (also in line with Figure 3.13), but values from 350 MPa to 40 0 MPa are estimated for the free-cutting steels Childs Part 1 28:3:2000 2:39 pm Page 95 Work material characteristics in machining 95 Fig 3.16 Representative specific force... 28:3:2000 2 :40 pm Page 106 106 Work and tool materials Fig 3. 24 The dependence on temperature of (a) thermal conductivity and (b) relative heat capacity, Young’s modulus and thermal expansion coefficient, for HSS(•), carbide/cermet (x), Al2O3(o) and Si3N4(+) based tool materials Fig 3.25 Representative bending fatigue behaviour at room temperature of three tool materials: HSS(•), Al2O3(o) and Si3N4(+) based... 28:3:2000 2:39 pm Page 93 Work material characteristics in machining 93 content gives rise to increasing temperatures for a given cutting speed This comes from the increasing shear stress levels This completes this brief survey of the stresses and temperatures generated by different alloy groups in machining Tool stresses are mainly controlled by the metal being machined and vary little with cutting conditions... Al2O3 (WC-Co) Al2O3 TiC TiN (WC-Co) Al2O3 TiN TiC (WC-Co) TiN TiC Al2O3 (WC-Co) (WC-Co) TiC TiN Al2O3 * turning 0 .45 %C steel at a feed of 0 .4 mm/rev Fig 3.28 The temperature dependence of hardness and standard free energy of formation of some coating materials, from Santhanam and Quinto (19 94) hardest material is in fact the best for flank wear resisitance at the lower cutting speed and the most inert . 2.0** 3.8 4. 2 620– 840 140 – 540 PcBN ≈100* 1.9–2.1 4. 7 4. 9 680–710 150– 340 K-carbide 75–120 3.0–3 .4 4.5–6.0 550–650 390–925 P-carbide 25–55 4. 0 4. 1 5.8–6.8 49 0–560 390– 840 Cermet 11–35 2 .4 2.7 6.7–7.8. 6.7–7.8 390 42 0 48 0– 740 Al 2 O 3 10–35 3.2–3.6 7.9–8.0 380–390 145 –330 Al 2 O 3 /TiC 10–22 3.8 4. 0 7.6–8.0 370–395 180–330 Al 2 O 3 /SiC(wh.) 10–35 ≈3 .4* 7.0–7.5 345 42 5 300–500 Si 3 N 4 /Sialon. data on machining elemental metals come from the same experiments on those metals considered by Trent in his book (Trent, 1991). 3.1.1 Machining elemental metals Although the elemental metals