SOME STUDIES ONWEAR,MACHINABILITY AND WELDABILITY OF AL-12%SI-TIC IN-SITU COMPOSITES

Abstract

It is apparent that technological development depends on advances in the field of materials. One may design most durable and highly efficient automobile or aircraft; however, without appropriate materials to fulfil the design requirements, the product may not be realistic. Accordingly, to enhance the performance of engineering materials in line with technological development, engineers and scientists are always striving to improve existing materials or to produce new materials. Metal Matrix Composites (MMCs) are example of such newly emerging engineering materials. In the past few decades, monolithic materials and alloys used in the automotive, aerospace and naval industries have been gradually replaced by particulate reinforced aluminium alloy composites. However, the application of these composite materials has not been wide spread due to higher processing cost, irregular wear characteristics and difficulty in machining and joining. The purposes of the current study is therefore to prepare an aluminium alloy composite having better mechanical and wear properties as well as improved machining and welding characteristics. To achieve the desired objectives, Al-12%Si/TiC composite has been prepared by in-situ route through direct reaction process. In-situ synthesizing techniques have economical advantage over the traditional ex-situ techniques. To prepare the said composite, initially, Al-12%Si alloy was melted in a muffle furnace using graphite crucible. At 6000C commercially pure titanium was added and the processing temperature was increased up to 11000C to facilitate the diffusion of Titanium in the Aluminium melt. Thereafter, activated charcoal was added and the processing temperature was further increased to 12000C. The melt was then hold for 30 minutes in the same processing temperature, so as to obtain sufficient exothermic reactions that yields the required Al-12%Si/TiC composites. To determine the appropriate process parameters that yield the desired in-situ composite, number of trial experiments has been conducted by varying the processing temperature and the reaction time. The initial values were determined based on literature. To analyse the formation of TiC reinforcement, its size and distribution as well as the formation of other inter metallic phases, optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive x-ray (EDX) and x-ray diffraction (XRD) techniques were used. The mechanical properties viz. tensile strength, hardness, ductility and percentage porosity v of the Al-12%Si/TiC in-situ composites have also been examined. The results revealed that the addition of TiC substantially enhances the tensile strength and hardness of the Al-12%Si matrix with a scarification of some ductility. The intended application of the experimental composite is in the production of weight and wear sensitive automotive components like brake discs, cylinder heads, cylinder blocks, pistons, etc. Accordingly, the abrasive wear characteristics, machinability and weldability behaviour of the said composites have been also studied. The abrasive wear characteristics of the experimental composite were examined using a pin-on-disc apparatus designed for such purpose. The experiment was conducted based on full factorial design of experimental technique with three factors and three levels. Applied load, sliding distance and weight percentage of reinforcement were considered as process variables and weight loss and coefficient of friction as the output measures. The abrasive wear process has been modelled based on statistical approach by developing regression equations. The process was also optimized with surface response optimization techniques. The results revealed that TiC reinforcement had significant contribution in enhancing the wear resistance properties of Al-12%Si matrix. The established equations have been found reliable to predict the weight loss and coefficient of friction. The process was also successfully optimized with in an accuracy of 6.06%. To conduct the machinability experiments, a single pass dry turning operations was performed on a lathe machine using uncoated carbide tools. The tests were carried out at various cutting speeds, feed rates and depth of cuts based on 33 full factorial plan of experiment. Tool wear mechanism, cutting force (Ft), surface roughness (Ra) and physical appearance of chips were analysed as the output measures. Multiple linear regression models have been developed to correlate the cutting speed, feed rate and depth of cut with the surface roughness (Ra) and cutting forces (Ft). The machining parameters were also optimized using desirability-based approach response surface methodology. It has been observed that effective machining of Al-12%Si/10wt%TiC composites with uncoated carbide tools is possible at a relative higher cutting speed of about 150 m min-1 and at minimum feed rates of 0.06 mm rev-1. The derived multiple regression equations correlate the measured variables and the cutting parameters with a reasonable degree of accuracy. The optimization result also exhibited a close agreement between the predicted and achieved optimized values of Ft and Ra within 9.71% of error. vi In the actual industrial applications of materials, welding is the vital process to fabricate components, assemblies and complete structures. However, joining of metal matrix composites (MMCs) by conventional fusion welding process is very challenging. The irregular distribution and segregation of reinforcements, deleterious reactions as well as weld defects such as porosity in the fusion zone are the major hindrances. Accordingly, in the current investigation, friction stir welding (FSW), which is one of the solid state joining processes was selected to examine the weldability of the prepared composite. To prepare the welding plates, the as cast composite ingots were hot rolled and machined at a size of 150mm x 50mm x 5mm. The FSW experiment was conducted on a 7.5 HP vertical milling machine with tools having flat shoulders and threaded probes. Normally, the FSW tools used for MMCs are exceptionally harder than those used for monolithic materials to minimize the possibility of excessive tool wear due to the abrasive nature of the reinforcement particles. Accordingly, the tool shoulder used in the current investigation was made from die steel and the tool probe was made from titanium. The tools were further hardened by oxyacetylene gas flame followed by water quench. Twelve experiments were planned based on full factorial design of experiment technique with three shoulder diameters, two rotational speeds, two traverse speeds and a constant vertical load of 6 kN. To predict the influence of the FSW process parameters on the UTS, percentage elongation and micro hardness of the weld joint, mathematical models were develop based on statistical approach. Multi-response optimization was also done to optimize the FSW joint characteristics of the composites. The results revealed that percentage elongation of the FSW joint enhanced by 14.3%, whereas the ultimate tensile strength (UTS) and micro-hardness reduced by 15% and 40% respectively. This indicates that, by applying proper tool materials and welding parameters FSW butt joints of Al-12%Si/10wt%TiC in-situ composites having a joint efficiency of 85% can be successfully produced. The developed multiple regression equations also satisfactorily predicted the influence of the input variables on welds. The observed 0.07 to 2.98% error in the optimality test results assured the adequacy of the adopted optimization technique. In general, the outcomes of the present investigation revealed that Al-12%Si/TiC in-situ composites can be successfully synthesized through direct reaction process at 12000C with 30 minute reaction time and further, the material can be utilized in real industrial applications particularly in the weight and wear sensitive areas such as automotive engine parts.