Uday A. Dabade 1, Suhas S. Joshi 1* and V. V. Bhanuprasad 2

Transcription

1 Uday A. Dabade 1, Suhas S. Joshi 1* and V. V. Bhanuprasad 2 1 Department of Mechanical Engineering, IIT Bombay, Mumbai , India 2 Defence Metallurgical Research Laboratory, Hyderabad , India ABSTRACT The difficulty in machining of MMCs arises not only from the excessive wear of the cutting tools but also from the fracturing of the reinforcement particles on machined surfaces that leaves behind adhered particle fragment, pits and cavities. This characteristic of MMCs machining tend to adversely affect the integrity of the machined surfaces. Therefore this paper presents an experimental analysis of integrity of machined surfaces on Al/SiC/30p/220 composites assessed in the terms of residual stresses induced in the machined surfaces and the micro-hardness variation below the machined surfaces. A Taguchi method-based experimentation that has been performed on Al/SiC/30p/220 composite shows that an increase in feed rate, cutting speed and depth of cut induces lower compressive to tensile residual stresses in machined surfaces. Similarly, use of wiper insert geometry too helps induce higher compressive stresses in the machined surfaces. In general an altered material zone (AMZ) of about 180 µm below to machined surfaces was evident from the micro-hardness measurement. Keywords: Metal matrix composites, Residual stress, Micro-hardness, Wiper insert geometry, Taguchi method. 1. INTRODUCTION Metal matrix composites (MMCs) have found wide applications in automobile and aerospace industries due to their high strength-to-weight ratio as well as heat and wear-resistant. Particularly, discontinuously reinforced materials such as aluminum matrix composites have become practical engineering materials. The SiC particles cause excessive wear of cutting tools [1-2]. At the same time, the particles get fractured and their fragments remain adhered to the machined surfaces. Sometimes, the dislodged particles from the machined surfaces leave cavities/ craters behind on the machined surfaces. Therefore, these composites though have many good mechanical properties, have a number of difficulties in their machining too. A number of researchers have worked on identifying the extent of difficulties in machining of composites by quantifying the tool wear [3-8], * Corresponding author: ssjoshi@iitb.ac.in assessing the cutting forces and surface roughness [9-10]. But the demand for high quality product necessitates attention towards the quality of machined surfaces, which is usually measured in terms of residual stresses and/or micro-hardness variation in the machined surfaces. It is know that these aspects of machined surfaces govern performance, longevity and reliability of the components made of composites. There are very few papers, which deal with integrity of machined surfaces on composites. While Allison and Davis [11] have found presence of both tensile and compressive type of residual stresses on the machined surfaces, Kannan and Kishawy [12] have shown presence of tensile (thermal) stresses in the machined surfaces on MMCs. At the same time, Dabade and Joshi, [13] have reported use of wiper insert geometry to induce compressive or lower tensile stresses on machined surfaces of Al/SiCp composites. Therefore, it appears that very limited analysis of the integrity of machined surfaces on the composites has been performed. This paper presents an experimental analysis of integrity of machined surfaces on Al/SiCp composites in terms of residual stresses induced and variation in the micro-hardness beneath the machined surfaces. 2. EXPERIMENTAL DESIGN AND PROCEDURE 2.1 Work Material In this study, Al-2124/SiC/ 30p/220 composite material has been used. It may be understood that this material with 30 % volume fraction of reinforcement and 220 mesh size (65 µm) represents one of the most difficult conditions from the machining point of view. It has been fabricated by powder metallurgy route. A micrograph of this composite depicting uniformity in the distribution of reinforcement is shown in Figure 1. Typical composition of Al2124-matrix alloy is presented in Table 1.

2 12 I J M E M S: 5(1) 2013 Table 1 Composition of 2124Al-matrix Alloying element wt. % Cu 5.4 Mg 1.5 Fe 0.4 Si 0.3 Cr 0.2 Ti 0.2 Zn 0.07 Mn 0.02 Al Balance radius on the cutting edge see Fig. 2(d). It is anticipated that these geometries will reduce the residual stresses in the machined surfaces. Other process parametric conditions were chosen based on the inputs from literature and earlier experiments performed by authors group. The response variables considered are micro-hardness variation and the surface residual stresses in the axial cutting direction. Factors/interactions Table 2 Control Factors and their Levels Level Tool nose radius (mm) * Insert geometry W1 W2 NW Feed rate (mm rev -1 ) Cutting speed (m min -1 ) Depth of cut (mm) * Dummy level. It fulfills the requirement of L 27 orthogonal array and provides more precise information about the effect of 0.8 mm tool nose radius on the chosen response variables 18]. Figure 1: Microstructure of Al/SiC/30p/220 Composite 2.2 Design of Experiment A Taguchi method-based design of experiment [14] using L 27 (3 13 ) orthogonal array, has been used in this work. The chosen values of independent variables along with their levels are shown in Table 2. The three insert cutting edge geometries selected are depicted in Figs. 2(a-d). The geometries W1 and W2 vary in the chamfer angle as shown in Figs. 2(b) and 2(c) respectively and have wiper cutting edge geometry. The NW type is conventional type insert with a honed 2.3 Experimental Setup and Procedure The turning experiments on the MMCs were performed on a CNC Turning (EMCO/PCTURN Model 345-II) machine using CBN inserts with conventional and wiper type insert geometry see Figs. 2 (a-d). Tool holder of type DCLNL 2020K12 that provides 6º negative rake and 6º cutting edge inclination angle has been used. Machining was carried out over a length of 7 mm for each machining conditions given by the L27 orthogonal array. For the analysis, the 7 mm length of the turned sample was cut-off from the cylindrical rod by hack-saw at extremely gentle conditions. The specimen were again cut radially into three pieces, of which one was used for residual measurement; the second was used for measuring micro-hardness variation along the cross-section. The third section was used for other analysis. Tool nose radius 0.4 / 0.8mm Rake face A A Chamfer width 100μm 20º 100μm 30º 100μm Honed edge 30º Figure 2: Insert Geometries Indicates (a) Tool Nose Radius; (b-d) Cutting Edge Preparation along Section A-A

3 Characteristics of Machined Surfaces on Al/SiCp Metal Matrix Composites 13 The first part of the sample was then encapsulated in an epoxy mold and then subjected to a series of polishing stages across the machined surface (axial direction). These were then etched using Keller s reagent (2.5ml nitric acid, 1ml Hydrochloric acid, 1ml Hydrofluoric acid, and 95ml distilled water) for 50 seconds. Later the samples were used for microhardness measurement to analyse altered material zone (AMZ) and effect of machining on microhardness variation beneath the machined surfaces. The measurement of micro-hardness on each specimen was done by taking indentations on the cross-sections at 30 to 240 ¼m below the machined surfaces at an interval of 30 µm. A load of Kgf was applied for 10 second duration in this measurement. Care is taken such that the indentations are made in the matrix and away from the SiCp interface. The samples for surface residual stress measurement were electro-polished for 25 seconds with electrolyte A2 (90 ml distilled water, 730 ml Ethanol, 100 ml Butylcellosolve and 78 ml Perchloric acid, at Voltage of 20 V). The samples were then mounted on X-ray diffractometer and uniaxial stresses in axial direction were determined using X-ray diffraction; XRD. The sin 2 y technique was used to calculate the residual stress values on the machined surfaces. 3. EXPERIMENTAL RESULTS AND DISCUSSIONS 3.1 Absolute Magnitude of Micro-hardness The micro-hardness variation along the cross-section of the machined surfaces at an interval of 30 µm and up to a depth of 240 µm is shown in Figs. 3 (a-b). It appears that the micro-hardness is high near the machined surface (at 30 µm depth) but gradually decreases at a distance of 180 µm, refer Fig. 3(b). Thus, it appears that a depth of about 180 µm is affected due to machining and is called as altered material zone (AMZ) for the present Al/ SiC/30p/220 composite material. (a) (b) Figure 3: Micro-hardness Measurement (a) Typical Specimen of Vickers micro-hardness Indentations, (b) Micro-hardness at Different Locations Beneath the Machined Surface 3.2 Statistical Analysis of Experimental Results The statistical analysis of residual stresses in axial direction and micro-hardness (at 30 µm depth) on machined surfaces were performed using MINITAB version-14 software. The ANOVA results for residual stress and micro-hardness are presented in Tables 3 and 4 respectively. The AOM plots for the surface residual stresses and micro-hardness (at 30 µm depth) are sh own in Figs. 4 (a-e) and 5 (a- e) respectively. In the following sections, the effect of process parameters on residual stress and micro-hardness has been discussed Effect of Feed Rate The P-values of ANOVA in Tables 3 and 4 indicate that the feed rate is the most significant parameter, which influences the residual stresses and the microhardness during machining of Al/SiC/30p/220 composites. The AOM plot in Fig. 4(c), indicate that

4 14 I J M E M S: 5(1) 2013 Table 3 ANOVA for Residual Stress Parameter DOF Seq SS Adj SS Adj MS F P Tool nose radius Insert geometry Feed rate (mm/rev.) Cutting speed (m/min.) Depth of cut (mm) Tool nose radius * Insert geometry Tool nose radius * Feed rate Insert geometry * Feed rate Error Total S = R-Sq = 85.35% R-Sq (adj) = 57.68% Bold value indicates the significance at 90 % confidence level Table 4 ANOVA for Micro-hardness Parameter DOF Seq SS Adj SS Adj MS F P Tool nose radius Insert geometry Feed rate (mm/rev.) Cutting speed (m/min.) Depth of cut (mm) Tool nose radius * Insert geometry Tool nose radius * Feed rate Insert geometry * Feed rate Error Total S = R-Sq = 80.01% R-Sq (adj) = 42.25% Bold value indicates the significance at 90 % confidence level Tool nose radius (mm) Insert geometry Feed rate (mm/rev) Cutting speed (m/min) Depth of cut (mm) Tool nose radius (mm) Insert geometry Feed rate (mm/rev) Cutting speed (m/min) Depth of cut (mm) Mean of Residual stress (MPa) Mean of Microhardness at 30µm (VHN) W1 W2 NW (a) (b) (c) (d) (e) Figure 4: (a-e) AOM plot for residual stress Figure 5: W1 W2 NW (a) (b) (c) (d) (e) (a-e) AOM Plot for Micro-hardness at 30µm Beneath the Machined Surface an increase in the feed rate from 0.05 to 0.2 mm rev -1 changes the residual stresses from higher compressive to lower compressive or tensile. At the same time, micro-hardness reduces as the feed rate increases; see Fig. 5(c). Now considering the variation in residual stresses, the magnitude of residual stresses change from highly compressive at the lowest feed rate (0.05 mm rev -1 ) to the lower tensile at highest feed rate (0.2 mm rev -1 ). The compressive residual stresses at the lowest feed rate (0.5 mm rev -1 ) could be due to extremely low volume of material being removed primarily at room temperature. But as the feed rate increases, the rate of deformation as well as the volume of material being removed increases. This leads to a gradual increase in the deformation temperature, thereby increasing the contribution of thermal dependent deformation in the whole. This aspect has been more evident in the composites, where the abrasive reinforcements

5 Characteristics of Machined Surfaces on Al/SiCp Metal Matrix Composites 15 tend to increase the friction and consequently the heat generation with increase in the feed rate. Therefore, as the highest feed rate level, 0.2 mm rev -1 is reached, the residual stress trend change to tensile from compressive, see Fig. 4(c). While the residual stresses increase with an increase in the feed rate, the micro-hardness of the composites tend to reduce with increase in the feed rate. This trend is significantly evident at 30µm depth below the machined surface, see Fig. 5(c). Since, the micro-hardness measurement at 30µm depth is closer to the machined surface; it is felt that the trend could be related to the variation in the residual stresses as shown in Figs. 6 (a-b), the compressive residual stresses would offer more resistance to penetration of the indenter during the micro-hardness testing. Therefore, at lower feed rate when the residual stresses are highly compressive, the micro-hardness is higher. An increase in the feed rate from 0.05 to 0.2 mm rev -1 deteriorates the quality of the machined surface and more boundary defects are generated by fracturing/breaking of SiC particles. This forms pit holes, cavities and cracks on the machined surfaces, which release the stresses. They also contribute towards the introduction of tensile or lower compressive stresses and reduce the micro-hardness along the machined surface cross-sections; see Figs. 6 (a-b). Figure 6: Conceptual Model Showing the Effect of Surface Residual Stresses on Micro-hardness Indentation (a) Compressive Residual Stress and (b) Tensile Residual Stress Effect of Depth of Cut The residual stresses and the micro-hardness are significantly influenced by the depth of cut during machining of Al/SiC/30p/220 composites; refer P- values of ANOVA in Table 3 and Table 4. The AOM plot in Fig. 4(e) indicates that an increase in depth of cut changes the residual stresses from compressive to the tensile side on the machined surface. This is related to the rise in machining temperature due to an increase in contact length between the cutting edge and the reinforcement particles present in work material as well as reduction in chip side flow angle with an increase in depth of cut. The contact length between the cutting edge and the work material l, while machining with cutting tools of nose radius r is given by equation (1) [15]- a r 1 cos k 1 f r l r kr sin 2r sin kr (1) Where, l = length of contact between the cutting edge and the work material (mm), r = tool nose radius (mm), f = feed rate (mm/rev), a = depth of cut (mm) and k r = approach angle = 95º, for present experimental work. It is observed that at 0.4 mm tool nose radius and 0.05 mm rev -1 feed rate, the change in depth of cut from 0.2 to 0.6 mm causes an increase in contact length by 88.76% and further increase to 1 mm depth of cut increases the contact length by 47.02%. Thus, the contact between the abrasive SiC particles present in the composites increases with an increase in depth of cut and results an increase in machining temperature. Hence, an increase in depth of cut causes an increase in l which might increase machining temperature and induces higher thermal stresses on machined surfaces. Thus, lower compressive or tensile stresses are observed on the machined surfaces at higher depth of cut, refer AOM plot in Fig. 4(e). While the residual stresses increase with an increase in the depth of cut, the micro-hardness of

6 16 I J M E M S: 5(1) 2013 the composites tend to reduce with an increase in the depth of cut. This trend is significantly evident at 30 µm depth below the machined surface; refer Fig. 5(e). As depicted by schematic in Figs. 6(a-b), at lower depth of cut (0.2 mm), when the residual stresses are highly compressive, the micro-hardness is much higher and at higher level of depth of cut (1.0 mm) the micro-hardness is lower since the lower compressive stresses are observed on the machined surface, refer Fig. 4(e) Effect of Cutting Speed The effect of change in cutting speed is evident on the residual stresses and the micro-hardness is nonsignificant to the change in cutting speed during machining of Al/SiC/30p/220 composites, refer P- values of ANOVA in Tables 3 and 4. An increase in cutting speed from 40 to 80 m min -1 increases the deformation rate and work material temperature. This causes thermal softening of work material and introduces higher thermal stresses on the machined surfaces; see Fig. 4(d). Hence, at 80 m min -1 cutting speed lower compressive stresses are observed due to thermal softening of Al-matrix. Simultaneously at 80 m min -1 cutting speed a portion of the heat generated gets dissipated into the matrix and softens the Al-matrix. This causes a reduction in the micro-hardness beneath the machined surface; refer Fig. 5(d). Similar results were reported by Kishawy and Kannan [7] while machining of 20% alumina MMCs in the range of m min -1 cutting speed. However, further increase in the cutting speed from 80 to 120 m min -1 increases the strain rate and the material becomes difficult to deform and work hardened. A slight increase in micro-hardness at 120 m min -1 cutting speed is the evidence of work hardening; refer Fig. 5(d). At the same time, an increase in the cutting speed from 80 to 120m min -1 reduces the friction between the abrasive SiC particles and the cutting edge, and at higher cutting speed more amount of heat will be carried away by the chip. Thus the mechanical stresses are more prominent than the thermal stresses at higher cutting speed (120 m min -1 ) and more compressive stresses are introduced on the machined surfaces. Thus, as depicted by a schematic model in Figs. 6 (a-b), at lower cutting speed (40 m min -1 ), when the residual stresses are highly compressive, the microhardness is much higher and at middle level of cutting speed (80 m min -1 ) the micro-hardness is lower since the lower compressive stresses are observed on the machined surface Effect of Tool Nose Radius The P-values of ANOVA in Tables 3 and 4 indicate that the residual stresses and the micro-hardness beneath the machined surfaces have not been influenced by the tool nose radius. However, the AOM plot in Fig. 4(a) indicates that an increase in tool nose radius from 0.4 to 0.8mm introduces more compressive stresses on the machined surfaces. Since, at 0.8 mm tool nose radius the interface contact between the tool and the work piece is more than 0.4 mm tool nose radius. Thus, an increase in tool nose radius increases the passive (ploughing) force, which is further resolved into radial as well as feed force and causes an increase in the tensile stresses behind the end cutting edge resulting higher compressive stresses on the machined surfaces. Similar results were observed in literature in metal cutting indicating that machining using higher tool nose radius introduce higher compressive stresses on the machined surfaces [16-18]. The AOM plots in Fig. 5(a) indicate that the change in tool nose radius does not have any influence on the micro-hardness. This result deviates from the conceptual relation between the residual stress and micro-hardness depicted by a schematic in Figs. 6(a-b) Effect of Insert Geometry The influence of change in insert geometry has no significance on the residual stresses and microhardness; refer P-values of ANOVA in Tables 3 & 4. Though the influence of change in insert geometry is not evident on the residual stresses and microhardness the variation in their magnitude focuses attention towards the optimum selection of insert geometry that introduces more compressive stresses and micro-hardness on the machined surfaces. The AOM plots in Figs. 4(b) and 5(b) indicate that wiper type insert geometries W1 and W2 introduces higher compressive stresses and micro-hardness than conventional NW type insert geometry, refer Figs. 4(b) and 5(b). While the residual stress reduces, the microhardness of the composites tends to increase with W2 type inserts; this follows the relation between the residual stresses and the micro-hardness, see Figs. 6 (a-b). 4. CONCLUDING REMARKS Based on the experimental results the following conclusions can be drawn:

7 Characteristics of Machined Surfaces on Al/SiCp Metal Matrix Composites 17 Feed rate, cutting speed and depth of cut significantly influence the residual stresses and micro-hardness beneath the machined surfaces. An increase in feed rate, cutting speed and depth of cut introduces tensile stresses in machined surfaces and reduces the micro-hardness beneath the machined surfaces. The depth of altered material zone appears to be around 180 µm depth based on this analysis. The wiper cutting edge geometry with 0.8 mm tool nose radius helps in introducing higher compressive stresses in the machined surfaces. A correlation between the residual stresses and micro-hardness exists. Micro-hardness increases with compressive stress and reduces with tensile stresses on the machined surfaces of composites. Within the scope of this experimentation, use of W2 type insert geometry (with 30º chamfer), 0.8 mm tool nose radius, 0.1 mm rev -1 feed rate, 0.6 mm depth of cut and 40 mm min -1 cutting speed has been found to introduce the highest compressive stresses in the machined surfaces on Al/SiC/30p/220 composites. ACKNOWLEDGEMENT The authors would like to express their thanks to DST- FIST sponsored Precision Prototyping Facility to carry out the emperimental work presented here. References [1] Kunz, J. M., and Bampton, C. C.; Challenges to Developing and Producing MMCs for Space Applications, Journal of Materials, 53(4), 22 25, [2] Joshi, S. S., Ramakrishnan, N. and Ramakrishnan, P.; Micro-structural Analysis of Chip Formation During Orthogonal Machining of Al/SiCp Composites, Journal of Engineering Materials and Technology, 123, , [3] Quan, Y. M., Zhou, Z. H. and Ye, B. Y.; Tool Wear and its Mechanism for Cutting SiC Particle-reinforced Aluminium Matrix Composites, Journal of Materials Processing Technology, 100, , [4] Brun, M. K., Lee, M. and Gorsler, F.; Wear Characteristics of Various Hard Materials for Machining SiCp Reinforced Aluminium Alloy, Wear, 104, 21 29, [5] Tomac, N., and Tonnessen, K.; Machinability of Particulate Aluminium Matrix Composites, Annals of CIRP, 41(1), 55 58, [6] Ping Chen, High-Performance of SiC Whisker- Reinforced Aluminium Composite by Self-propelled Rotary Tools, Annals of the CIRP, 41(1), 59 62, [7] Cronjager, L. and Meister, D.; Machining of Fiber and Particle-reinforced Aluminium, Annals of CIRP, 41(1), 63 66, [8] Joshi, S. S., Ramakrishnan, N., Nagarwalla H. E. and Ramakrishnan P.; Wear of Rotary Tools in Machining of Al/SiCp Composites, Wear, 230, , [9] Davim J. Paulo. and Baptista A. Monteiro, Relationship between Cutting Force and PCD Cutting Tool Wear in Machining Silicon Carbide Reinforced Aluminium, Journal of Materials Processing Technology, 103, , [10] Pedersen, W. and Ramulu, M.; Facing SiCp/Mg Metal Matrix Composites with Carbide Tools Journal of Materials Processing Technology, 172, , [11] Davis, L. C. and Allison, J. E.; Residual Stresses and other Effects on Deformation in Particle Reinforced Metal Matrix Composites, Metallurgical Transactions A, 24A, , [12] Kannan, S. and Kishawy, H. A.; Surface Characteristics of Machined Aluminium MMCs, International Journal of Machine Tools and Manufacture, 46, , [13] Uday A. Dabade and Suhas S. Joshi; Surface Integrity and Surface Finish of Machined Surface of Al/SiCp Composites, Proceeding of 4 th National Conference on Precision Engineering (COPEN 2005) held at Jadavpur University, Kolkata, India, 75 80, [14] Phadke M. L.; Quality Engineering using Robust Design, Prentice Hall Publication, New Jersey, [15] Pramanik, A, Zhang, L. C. and Arsecularatne, J. A.; Prediction of Cutting Forces in Machining of Metal Matrix Composites, International Journal of Machine Tools and Manufacture, 46, , [16] Henriksen, E. K. and Ithaca, N. Y.; Residual Stresses in Machined Surfaces, Transactions of ASME, Journal of Applied. Mechanics, 73, 69 76, [17] Shreyes N. Melkote and Jeffrey D. Thiele; Effect of Tool Edge Geometry and Work Piece Hardness on Surface Generation in the Finish hard Turning of AISI Steel Journal of Materials Processing Technology, 94, , [18] Liu, C.R. and Barash, M. M.; Variables Governing Patterns of Mechanical Residual Stress in a Machined Surface, Journal of Engineering for Industry, ASME, 104, , 1982.