Processing and Characterization of Inconel 625–Sic Metal Matrix Composites by Direct Metal Laser Sintering

Abstract

Metal Matrix Composites (MMCs) are gaining wide spread popularity because of their superior mechanical properties such as high mechanical strength; wear resistance, excellent thermal conductivity and ability to retain strength at high temperature. Most of the work on MMCs till date is focused on development of aluminium, copper, magnesium, and titanium based MMCs. Lot of interest is shown now-a-days on aerospace and high temperature materials such as Inconel 625 superalloy which finds an application in important sectors like gas turbines, air craft engine components and electronic parts like cathode ray tube spiders, and springs. Also, excellent resistance to corrosion in purity water environment led to its use in the construction of control rods in nuclear reactors. Most of MMCs are developed by various methods like powder metallurgy, conventional casting, and very few researchers have reported about the processing of MMCs by different laser based additive manufacturing processes. In the present thesis work, Nickel based Inconel-625 (IN625) metal matrix composites were processed using “Direct Metal Laser Sintering” (DMLS) additive manufacturing process. Silicon carbide (SiC) particles, coated with NiP and IN625 particles are premixed using hexagonal double cone blender for homogeneity of particulates. MMCs are developed in DMLS under nitrogen atmosphere using CO2 laser and deposition of IN625 with addition of 1, 3, 5 weight percentage of NiP coated SiC particles was done using laser additive manufacturing process. The physical and mechanical behaviour of the MMCs were thoroughly examined. The micro-structure, density, micro-hardness, tensile properties, corrosion properties and machinability of the developed composites were studied. The distribution of SiC particles and microstructures were characterized by using optical and scanning electron micrographs. The results of the experiments were clearly reveal that the interface integrity between the SiC particles and the IN625 matrix, the mixed particulates flowability, the SiC ceramic particles and laser beam interaction, and the mechanical and physicaliv properties of the developed MMCs were improved effectively by the use of NiP coated SiC particles. The results reveal that the NiP coated SiC particles can be used to reinforce IN625 using laser additive DMLS process. The micro-structure of IN625 matrix become more refined with the addition of more SiC particles, and the shape of the grains switched over from columnar dendrite to cellular equi-axed form. There is an increment in hardness by 33 % above the base IN625 material because of rarefaction in micro-structure with more addition of NiP coated SiC particulates. Lower density and hardness, at higher scan speed, due to increased porosity and higher density and hardness, due to high dislocation density and also because of excellent bonding between matrix and reinforcement was observed. Significant improvements in the tensile properties are observed and are due to micro-structure refinement and strengthening effect by addition of NiP coated SiC particles. With the addition of 3 weight percentage of NiP coated SiC, the UTS and YS increases by 12 % and 10 % respectively, compared to laser deposited IN625. The addition of NiP coated SiC particles into IN625 matrix beyond 3 weight percentage resulted in drastic reduction in UTS, YS and elongation because of increase in thermal cracking and due to large stress concentration around the reinforcement. This will lead to premature failure during straining of composites. Further the corrosion studies indicate that the rate of corrosion increases with increase in laser scan speed because of increased porosity. The corrosion rate decreases with the increase in weight percentage of reinforcement because of effective laser absorption by NiP coated SiC particles. The machinability studies was carried out using Wire Electrical Discharge Machine (WEDM), and the results shows that the discharge current and weight percentage of NiP coated SiC have profound effect on machining time and surface roughness of developed composites.