STUDIES ON MACHINABILITY OF Al. SIC PARTICULATE METAL MATRIX COMPOSITE

Abstract

Composite materials have received considerable attention in this era of energy crisis as it fulfills the demand of manufacturing industries for lightweight components with reliable product life cycle and superior properties. Furthermore, conventional monolithic materials have limitations in terms of achievable combinations of strength, stiffness, coefficient of expansion and density. Therefore, material scientists and engineers are always striving to combine materials across the basic categories of metals, ceramics and polymers in order to synergize and obtain superior combination of properties in the resulting composite material. Metal matrix composites (MMCs) are, thus, engineered synergistic combinations of two or more materials, where the geometrically continuous constituent is a metal/alloy, in which one or more selected constituent (s) may be distributed to obtain tailored properties. Generally, the constituents of a composite are insoluble in each other but should form strong interfacial bond to synergize the mechanical properties. Particulate metal matrix composites (PMMCs) consist of reinforcing particles embedded in a metal matrix resulting in combinations of high specific strength and specific modulus. Moreover, systematic design and synthesis procedures can be developed to achieve unique combinations of engineering properties, such as high elevated-temperature strengths, better radiation and corrosion resistance, low coefficient of thermal expansion, electrical and thermal conductivity. Metals such as aluminum, titanium, magnesium, copper, nickel, iron and their alloys are generally used as matrix. The reinforcing phase is usually in form of either continuous or discontinuous fibers, whiskers, short fibers or particles. The reinforcing particles such as boron, carbon, alumina and silicon carbide are available commercially in varying size. The PMMCs can be produced through a number of routes including mechanical alloying, powder metallurgy and various casting methods such as stir casting, squeeze casting, compo casting. The stir casting is one of the promising methods because the conventional infrastructure of the existing foundries could be used and the cost of ii processing is relatively .low as compared to that in other methods. The important factors that control the properties of PMMCs are the properties of the constituents, volume fraction, shape, size and geometric arrangement of reinforcement, microstructure and grain size, properties of interface, residual stresses, secondary processing like extrusion and heat treatment etc. Commercial application of PMMCs will have to overcome a few inherent limitations. The PMMCs may generally suffer from decreased ductility and fracture toughness as compared to that of the matrix metal/alloy, when the reinforcement is too hard and brittle. There are chances for segregation of particles during flow of melt-particle slurry in the mould as well as during solidification, particularly for complex design of components involving small holes, thin and/or multiple partitions etc. Further, the casting method is non viable for rapid mass production of components that have different requirements like, micro size, nano finish, net shape etc. Thus, for several applications there is requirement of different advanced or conventional machining processes to attain dimensional and geometric precision as well as good surface fmish on the components that are made of PMMCs. In this context, studies on machinability of metal matrix composites assume importance. Machinability is defined as the machining response to a number of factors such as machined surface, material removal rate, ease of chip formation, cutting tool life, required cutting forces and power, and cutting temperature. The aluminium based silicon carbide particles reinforced metal matrix composite (Al-SiCp MMCs) not only have good mechanical properties and wear resistance but are also economically viable. Therefore Al-SiCp MMCs have found many applications in the structural, aerospace and automotive industry. But it is very difficult to machine them due to extremely high hardness and abrasive nature of reinforced silicon carbide particles, which leads to excessive wear of conventional cutting tool materials, premature failure of cutting tools and poor surface finish on the machined surface. There is a growing demand for optimum machining of these composites notwithstanding their inherent problem of machining, in order to utilize the superior properties of PMMC materials. The use of advanced machining processes and costly cutting tools are very expensive and so, their use in commercial applications is not feasible. Thus; there 111 is a need to put more efforts to study machining of PMMCs using conventional machining and commonly available tools. The machinability performance evaluation criteria depend significantly on the material and geometries of the cutting tool and on the cutting parameters. The mechanical and physical properties of Al-SiCn MMCs depend on the weight fraction and size of SiC, reinforcement, which play vital role in their machining behaviours. The available literature show that there is a lack of information in the form of recommended guidelines for machining of PMMCs. Therefore, an extensive study involving wider range and more number of process parameters is required to evolve such recommendations for machining of PMMCs. The extent of experimental work is limited by cost and efforts and there is a need for an accurate, understandable and cost effective theoretical model for optimisation to arrive at the cutting conditions satisfying the requirements of manufacture like production rate, performance criteria and economy. The turning process can be optimised either for single-pass or multi-pass operation. Published literature show that some of traditional and advanced methods for optimisation are successful in locating optimal solution but they are usually slow in convergence and require gradient calculations. Few of them are only applicable when types and/or number of constants are limited. Other approaches face the problem of being trapped at local optimum. The novel approach of genetic algorithm (GA) is based on principle of natural biological evolution-survival of the fittest. The GA provides a robust, adaptive search and global optimisation. Recently, it has received considerable and increasing attention in the field of optimisation due to its effectiveness. The present study has been carried out in this context as explained in the Chapter- 1. A critical review of exiting literature on synthesis and machining of the.Al-SiCp MMCs is given in Chapter-2. Literature on the modelling and optimisation of turning process has also been covered in the review. The gaps in the existing literature representing the current state of knowledge of machining of PMMCs have been identified and the objectives of present study have been' formulated to fill up some of this gap. 1V Chapter-3 deals with the experimental procedures followed in the present work involving synthesis, characterisation and machinability study of Al-SiCp composites. Three types of composites have been investigated - cast, as received and heat-treated wrought composites. Aluminium based cast composite containing 7 wt % of SiC particles as reinforcement and magnesium as wetting element has been successfully synthesized in the laboratory by stir casting route. In order to compare mechanical properties of cast composite, Al-Mg matrix alloy has also been cast without addition of particles. The 23.2 wt % wrought composite has been procured from M/S Metallic Composite for 21St Century, USA and heat-treated under T6 condition in the laboratory. Powder X-ray diffraction, optical emission spectroscopy and atomic absorption spectroscopy have been carried out for cast composite to determine chemical composition. The weight percentage of SiC particles and volume percentage of porosity have also been estimated. The polished samples have been examined under optical microscope and scanning electron microscope to determine the distributions of SiC. particles. On polished samples Brinell macro and Vickers micro hardness measurements have been carried out. The ultimate tensile strength (UTS) and percent elongation at break have been determined by tensile test. For machinability study of selected composites, longitudinal turning operation without use of cutting fluid has been carried out with selected cutting tools - HSS tool bit, TCGX-16-T3-04-AL (H 10) and TCGX-16-T3-04-AL (CD 1810) inserts, on conventional lathe machine. As current machining process happened to be non-linear, the 33 full factorial design has been selected for the experimentation. The machinability performance has been evaluated in terms of (a) cutting forces, (b) surface roughness of the machined surface, (c) cutting tool wear (d) built-up-edge (BUE) formation and (e) type of chips. The Kistler piezoelectric three components dynamometer, three load charge amplifiers and data acquisition card have been used to measure cutting forces. Surface roughness of the machined surface has been measured using a profilometer. After turning a fixed volume of composite material under selected test conditions, average width of flank wear and average height of BUE on cutting tools have been measured using Large Tool Maker's Microscope. The optical images of flank and rack faces of cutting u tools after turning have been taken using optical microscope and SEM to observe the wear types. The morphology and topography of chips have been observed under SEM. The type of chips has been correlated with changes in cutting process parameters. The constant and exponents of Taylor's tool life equation have been found for different pair of HSS tool bit and H10 insert with cast and as received wrought composites. The formulation of theoretical model for multi-pass orthogonal longitudinal turning and its optimisation using genetic algorithm are given in chapter-4. The model determines the optimised process parameters for turning of Al-SiCP MMCs by minimising unit production time without violating any imposed constraints. The unit production time has been chosen as the index of utility. Using preventive tool replacement strategy, several rough passes with a final finish pass have been considered in this model. The cutting force, surface finish of machined surface, cutting tool life, required power for rough and finish pass(s) and available range of process parameters on lathe machine are considered as machining constraints. A user-friendly genetic algorithm based optimisation approach using MATLAB® software has been developed for optimizing the above-mentioned index of utility under the given constraints to find the optimal machining parameters. Using optimised cutting process parameters, experiments have been carried out to determine the cutting tool wear and the performance evaluation criteria such as cutting forces and surface roughness. The theoretical and experimental results of cutting forces and surface roughness under optimised and non-optimised cutting parameters have been compared for validation purpose. The second order fitting of the experimental results has been carried out by the analysis of variance and it has led to equations for different performance criteria like cutting forces and surface roughness revealing their dependence on the cutting parameters and their interactions for different composite — cutting tool combinations. Results and discussion are presented in two segments in chapter-5 and 6. Chapter-5 contains the results on characterisation and machinability of Al-SiC, MMCs. The microstructures of selected composites show that the distribution of SiC particles into aluminium based matrix is fairly homogeneous. The results of optical emission spectroscopy and atomic vi absorption spectroscopy confirm the presence of magnesium in the matrix of cast composite. X-ray diffraction analysis of cast composite also shows the presence of magnesium oxide. The Brinell hardness and ultimate tensile strength of cast composite have respectively increased and decreased when the Al-Mg alloy has been reinforced by SiC particles, while for as received wrought composite, after heat-treatment both these properties increase. Results of turning on Al-SiCp composites show that the cutting forces increase with increasing cutting speed when HSS tool bit is used, but for H10 and CD 1810 inserts, the variation in cutting forces is negligible. At higher cutting speed, the softening of the matrix is overcome by the hard reinforcing particles and the cutting forces do not reduce as it happens in the . case of monolithic metals/alloys. The high wear of HSS tool bit at higher cutting speed introduces more friction force at the wear land so the cutting force increases. The cutting forces increase with increasing feed during turning of Al-SiCn composites with cutting tools used in the current investigation. The increase in feed results in similar trends of variation in cutting forces as observed during machining of monolithic material. The cutting forces increase with increasing depth of cut during turning of Al-SiC1, composites with HSS tool bit. But during turning with H 10 and CD 1810 inserts, only the main cutting force component increases with increasing depth of cut while the other two cutting force components remain more or less in the same range. Thus, overall cutting force increases with increasing depth of cut and this trend is nearly similar to those observed in the case of monolithic metals/alloys. The variation of average and maximum surface roughness with increasing cutting speed as observed on turned Al-SiC1, composites show different trends from those observed in monolithic materials. For monolithic material the surface roughness decreases with increasing cutting speed, while during turning of Al-SiCp composites surface roughness may either increases or remains steady as the combination of cutting tool-composite changes. The surface roughness of turned sample of Al-SiCP composites increases with increasing feed, as similar to that observed in monolithic metals/alloys. The effect of change in the depth of cut on average and maximum surface roughness is inconsistent as the combination of cutting tool-composite vii changes, while in case of monolithic material; the depth of cut does not apparently influence the surface finish. The SEM images of the finish side of the chips show that a number of pits, debonded particles, feed marks and cracks are significantly reduced during turning with CD 1810 insert as compared to that with either HSS tool bit or H10 inserts. The formation of built-up-edge and/or de-bonding of SiC particles are mainly responsible for poor surface finish during machining of PMMC materials. The wear on selected cutting tools increases with increasing feed or depth of cut when the remaining cutting parameters are kept constant during turning of Al-SiCp composites. These trends are similar to those observed with the monolithic materials. The wear on HSS tool bit is the highest followed by H 10 and then CD 1810 inserts during turning of the same Al-SiCp composites under the same cutting conditions. The SEM images of worn cutting tools confirm that the major type of cutting tool wear during turning of PMMCs is the flank wear. However, high notch wear has been observed during turning of as received wrought composite with HSS tool bit presumably due to higher wt % of SiC particles in this material compared to cast composite. For all cutting tool - composite combinations investigated in the present study., the shape of chips are loose are and/or short washer type under various cutting conditions. It reveals z . that continuous turning of Al-SiCp composites does not result in the formation of continuous chip and so, it could be carried out without any chip breaker. The width of chip and curling radius of chip increases with increasing depth of cut and feed respectively. The present study indicates that HSS tool bit is unsuitable because of quick failure in respect of all the performance evaluation criteria and only CD1810 insert is hard enough to turn these composites satisfactorily. Since CD 1810 is very expensive, the H10 and CD1810 inserts are recommended for rough and finish turning of Al-SiCu composites on the basis of the results obtained in the present study. Chapter-6 deals with results and discussion of modelling and optimisation for multi-pass turning of PMMCs. The comparison of theoretical and experimental results indicates that the viii theoretical estimate of main cutting force provides a fair estimate of the experimental results apart from random errors in the experiments but the theoretical resultant cutting force provides an overestimate of experimental results. For cast composite machined with HSS tool bit and H 10 insert, and for as received wrought composite machined with HSS tool bit, the theoretical average surface roughness provides an underestimate of experimental results. But for as received wrought composite machined with H 10 and CD 1810 inserts, the theoretical average surface roughness provides an overestimate of experimental results. After machining of heat treated wrought composite with H 10 and CD 1810 inserts, the theoretical estimates of average surface roughness provides a fair estimate of the experimental results. For most of the test conditions, the observed maximum surface roughness is higher than the theoretical estimate indicating that the theoretical maximum surface roughness provides an underestimate of experimental results. The de-bonding/fracture of hard SiC particles during turning could be responsible for this enhanced maximum surface roughness observed experimentally. Proposed hybrid optimisation genetic algorithm based approach for multi-pass longitudinal turning operation gives more realistic and better results of optimum utility index with optimised process parameters for PMMC materials using the experimentally found cutting tool life constant and exponents. For rough and finish turning of cast and as received composite with HSS tool bit and H10 insert under optimised cutting conditions, the calculated and experimental results of the main and the resultant cutting forces match fairly well particularly at lower forces, but at higher cutting forces, the calculated forces are relatively .higher than the experimental forces. Similarly, the calculated and experimental average and maximum surface roughness match fairly, particularly at lower surface roughness, but at higher surface roughness, there is slight variation. However, the theoretical estimate of cutting forces as well as surface roughness provides a reasonable estimate. Thus, using optimised cutting process parameters through proposed optimisation model, one may get better results of performance criteria during turning of PMMC materials. ix The second order fitting equations of cutting force components obtained from experimental results show their dependence on cutting speed directly and also through interaction terms involving feed and depth of cut. But the empirical equation for the main cutting force used in the theoretical model does not involve cutting speed at all. The cutting force components observed, indirectly depend on the cutting tool material as evident from the different fitting equations obtained for turning the same composite material with different cutting tools of similar geometry. The second order fitting equations obtained through analysis of variance of both the average and the maximum surface roughness show their dependence on the depth of cut and the cutting speed. But the empirical equations used in the theoretical model do not involve these two cutting parameters. Finally, chapter 7 summarizes the conclusion of the present investigation. x