120 CHAPTER 7 Alpha-Beta Brass Alpha-Beta Brass also known as duplex brass and Muntz metal is the traditional material which represents commonly the soft engineering alloys. This alloy consists of two phases Alpha and Beta. The Beta phase is harder and more brittle at room temperature than Alpha; therefore these alloys are more difficult to cold work than the Alpha Brass. At elevated temperatures, the Beta phase becomes more plastic and since most of these alloys may be heated into the single phase Beta region, with excellent hot working properties. 7.1. Experimental Details In order to establish the clear picture of burnishing process, a series of experiments were conducted on brass which find wide range of industrial applications. In these experiments, the work pieces were burnished after turning on lathe, keeping the roller burnishing tool fixed in the lathe tool dynamometer. The dynamometer is employed to measure three force components, along x, y and z directions (force in z direction is taken as burnishing force).
121 7.1.1. Material The work piece material is brass, and its nominal composition is given in Table 7.1. The alloy is obtained in half-hardened condition. Table 7.1: Chemical composition of the experimental material alphabeta Brass Composition, in Wt. % Material Fe Cu Zn Pb Alpha beta Brass 57.2 Bal. 3.2 0.156 7.2. Results and Discussion 7.2.1. Surface roughness The values of surface finish, a direct measurement of surface roughness before and after burnishing as a function of burnishing speed and burnishing feed are given in Table 7.2 and 7.3, respectively. From these data (Tables 7.2 and 7.3 and Figs. 7.1 and 7.2) the optimal value of surface finish is found to be 0.19 µm. The corresponding optimum burnishing conditions are: 140N burnishing force, 22.6 burnishing speed and 2 nd burnishing pass.
122 Table 7.2: Comparison of surface finish values before and after burnishing for a 30 mm diameter work piece of Brass as a function of force and burnishing speed. Material force (N) Burnishing speed () Surface finish before burnishing Ra (µm) Surface finish after burnishing Ra (µm) First pass Second pass Third pass % increase in surface finish First pass Second pass Third pass Brass 110 120 140 6.03 1.87 1.19 1.21 1.43 36.36 35.29 23.52 9.55 1.18 1.70 1.77 1.48 6.07 2.20 18.23 14.57 1.85 1.07 1.31 1.55 42.16 29.18 16.21 22.60 1.85 0.98 0.95 0.59 47.02 48.64 68.10 35.60 2.44 0.71 0.78 0.45 70.90 68.03 81.55 6.03 2.33 0.29 0.29 0.69 87.55 87.55 70.38 9.55 2.12 0.38 0.24 0.25 82.07 88.67 88.20 14.57 2.37 0.29 0.25 0.38 87.76 89.45 83.96 22.60 2.89 0.48 0.30 0.49 83.39 89.61 83.04 35.60 2.18 1.09 0.48 0.57 50.0 77.98 73.85 6.03 1.71 1.01 1.20 1.45 40.93 29.82 15.20 9.55 2.22 1.00 1.53 0.82 54.95 31.08 63.06 14.57 2.54 1.59 0.87 1.04 37.40 65.70 59.05 22.60 2.28 0.19 0.19 0.68 91.66 91.66 70.17 35.60 2.59 0.59 0.32 0.94 77.22 87.64 63.70
123 Table 7.3: Comparison of surface finish values before and after burnishing for a 30 mm diameter work piece of Brass as a function of burnishing feed. Material Burnishing feed (mm/rev) Surface finish before burnishing Ra (µm) Surface finish after burnishing Ra (µm) 9.55 14.57 22.6 35.6 9.55 % increase in surface finish 14.57 22.6 35.6 0.111 2.22 1.00 1.53 1.27 0.82 54.95 31.08 42.79 63.06 Brass 0.032 2.54 1.59 0.87 1.20 1.04 37.40 65.74 52.70 59.05 0.063 2.28 1.60 1.23 0.19 0.68 29.82 46.05 91.66 70.17 0.095 2.59 0.59 0.32 0.75 0.94 77.22 87.64 71.04 63.70 80 % increase in surface finish 70 60 50 40 30 20 1 st pass 2 nd pass 3 rd pass 10 (a) Depth of cut 0.1 mm 0 0 10 20 30 40 Speed,
124 % increase in surface finish 100 90 80 70 60 50 40 30 20 1 st pass 2 nd pass 3 rd pass 10 (b) Depth of cut 0.2 mm 0 0 10 20 30 40 Speed, 100 90 % increase in surface finish 80 70 60 50 40 30 20 1 st pass 2 nd pass 3 rd pass 10 (c) Depth of cut 0.3 mm 0 0 10 20 30 40 Speed, Fig. 7.1: Variation of burnishing speed with % increase in surface finish for different passes with different optimal forces for Brass
125 % increase in surface finish 100 90 80 70 60 50 40 30 20 9.55 14.57 22.6 35.6 10 0 0.02 0.04 0.06 0.08 0.10 0.12 Feed, mm/rev Fig. 7.2: Variation of burnishing feed with % increase in surface finish at different speeds in alpha-beta Brass 7.2.2. Microstructure The optical micrographs in Fig. 7.3 clearly show microstructures consisting of equally axed grains with large amount of intra-granular particles. Mechanically modified layer of varied thickness was found to be present at the surface as a consequence of burnishing values of burnishing depth as a function of extent of burnishing (unburnished, 1 st, 2 nd and 3 rd passes) are marked in the micrographs of Fig. 7.3 and the data are included in Table 7.4 clearly shows that maximum burnishing depth happens to occur in 1 st pass. One should be careful in determining
126 the burnishing depth as both burnishing affected and chemically affected surface layers appear almost similar and their visual contrast is almost the same. The variation in depth of these zones is measured from micrographs and the same are given in Fig. 7.4. (a) (b) (c) (d) Fig. 7.3: Optical micrograph of brass showing the depth of burnishing in (a) Unburnished, (b) Burnished 1 st pass, (c) Burnished 2 nd pass and (d) burnished 3 rd pass conditions
127 800 Burnishing layer thickness, m 700 600 500 400 B B1 B2 B3 No of Passes Fig. 7.4: Correlation of burnishing layer thickness with burnishing parameters Table 7.4: Variation of burnishing depth and average micro hardness values in the burnishing zone for Brass. Material Brass Characteristic Burnishing Process BB B1 B2 B3 Micro Hardness 126.76 172.30 180.8 166.7 Burnishing layer thickness 410 650 700 450 [BB Before burnishing, B1 Burnished-1 st pass, B2 Burnished-2 nd pass and B3 Burnished-3 rd pass]
128 7.2.3. Micro hardness The specimens polished to obtain microstructure were further used to determine the variation in micro hardness as a function of distance from the surface. The micro hardness values are found to be almost similar with no systematic variation with the burnishing distance. Hence, an average value of micro hardness is taken as a representative value for each of the experimental condition such as unburnished, burnished-1 st pass, burnished-2 nd pass and burnished-3 rd pass. These data are summarized and included in Table 7.4 and are shown in Fig. 7.5. It is interesting to note that maximum burnished depth (as obtained from optical micrographs) also results in highest values of average micro hardness. 200 B1 B2 B3 150 Micro hardness 100 50 B 0 Brass Fig. 7.5: Correlation of surface microhardness with burnishing parameters
129 7.2.4. Residual stresses The residual stresses that are determined by XRD are given in table 7.5 and shown in Fig. 7.6 as a function of number of passes for the Brass. The data in Fig. 7.6 show that the residual stresses gradually build up with burnishing and exhibit a peak in residual stresses at 1 st burnishing pass. Following this peak in compressive residual stress, further burnishing results in slight decrease of the order of 15-20% in compressive residual stress. Further the peak in residual stress is found to be of the order of 20% of the yield strength of Brass (based on commonly reported representative yield strength value of 300 MPa). Parameters chosen for XRD analysis are wave length: 1.542 A and Bragg angle: 145. Table 7.5: Compressive residual stresses for alpha-beta brass Material Alpha beta Brass Burnishing condition Principal Stress (max) (MPa) Principal Stress (min) (MPa) Direction of Principal Stress * Max shear stress (MPa) Equivalent stress (MPa) BB -8.8-61.2-32.9 26.2 15 B1-68.6-79.7 22.5 5.5 17.1 B2-54.3-112.8 30.1 30.7 14.8 B3-42.1-54.2 31.3 9.1 13.8 [BB Before burnishing, B1 Burnished-1 st pass, B2 Burnished-2 nd pass and B3 Burnished-3 rd pass] * Angle in degrees from the axial direction of the cylindrical sample
130 100 Compressive Residual Stress, MPa 80 60 40 20 0 B B1 B2 B3 Number of Passes Fig. 7.6: Variation of magnitude of residual compressive residual stresses with burnishing pass in case of Brass. (B corresponds to unburnished and B1, B2 and B3, corresponding to 1 st, 2 nd and 3 rd passes of burnishing) 7.3. Implication Surface and machinability are the two key issues among many for a wide range of application areas, where different grades of brasses are used. Few of these applications involve fatigue loading where the compressive residual stresses may significantly enhance fatigue life that
131 is controlled by tensile mean stresses. Hence, in these conditions burnishing is an effective surface modification treatment which can significantly enhance fatigue resistance of brass made components. However, one should note that the beneficial effects progressively diminish as the magnitude of compressive residual stress gradually decrease with service or even completely vanish at medium to high temperatures by effective stress relaxation and more importantly burnishing process has a major limitation that the compressive residual stress could prove fatally harmful and adversely affect the life in compressive-mean-stress-controlled fatigue and creep 7.4. Summary and Conclusions 1. The present study reveals that the burnishing effectively improves surface finish, depth of burnishing, micro hardness and compressive residual stresses. 2. The studies conducted on burnishing till date limit number of passes to a maximum of 4. With the present data where the number of passes is restricted to 3, the brass shows best surface finish in the second pass (though the third pass d oesn t show much degradation in surface finish).
132 3. Mechanically modified layer of varied thickness was found to be present at the surface as a consequence of burnishing values of burnishing depth as a function of extent of burnishing (unburnished, 1 st, 2 nd and 3 rd passes). The present study shows that maximum burnishing depth happens to occur in 1 st pass. 4. The present study revealed one-to-one correlations between burnishing depth, increase in micro hardness and magnitude of compressive residual stresses.