A microstructure analysis is carried out to optimize the process parameters of a randomly oriented discrete length hybrid carbon fiber reinforced carbon matrix composite. The composite is fabricated by moulding of a slurry into a preform, followed by hot-pressing and carbonization. Heating rates of 0.1, 0.2, 0.3, 0.5, 1, and 3.3°C/min and pressures of 5, 10, 15, and 20 MPa are applied during hot-pressing. Matrix precursor to reinforcement weight ratios of 70:30, 50:50, and 30:70 are also considered. A microstructure analysis of the carbon/ carbon compacts is performed for each variant. Higher heating rates give bloated compacts whereas low heating rates give bloating-free, fine microstructure compacts. The compacts fabricated at higher pressure have displayed side oozing of molten pitch and discrete length carbon fibers. The microstructure of the compacts fabricated at low pressure shows a lack of densification. The compacts with low matrix precursor to reinforcement weight ratios have insufficient bonding agent to bind the reinforcement whereas the higher matrix precursor to reinforcement weight ratio results in a plaster-like structure. Based on the microstructure analysis, a heating rate of 0.2°C/min, pressure of 15 MPa, and a matrix precursor to reinforcement ratio of 50:50 are found to be optimum w.r.t attaining bloating-free densification and processing time.
Inconel 718 is widely used superalloy in the Indian space program for high temperature application. Some of the newer applications envisage use of this alloy in very critical high pressure oxygen carrying vessels. The alloy is frequently used in welded condition which requires extensive characterization of various types of welds viz Electron beam welding (EBW) and Gas tungsten arc welding (GTAW). In many cases the weldability of Inconel 718 is found to be limited by microfissuring phenomenon in the weld heat affected zone. Microfissures are fine intergranular cracks and their severity strongly depends on pre-weld solution treatment temperature (grain size), weld heat input and concentration of impurities (B, P and S) in the base metal. In the present work, a study was undertaken to compare the microfissuring tendency in EBW and GTAW processes using two pre-weld solution treatment temperatures. The samples were solution treated at 970°C and 1050°C to generate different grain sizes. Amount of heat input and cooling rate were calculated since they are known to affect the microfissuring and an effort was made to understand their role on the microfissuring. It was observed that microfissuring susceptibility is more at coarser grain size. Severity is more in EBW. The reasons for this phenomenon have been discussed in this paper correlating microfissuring with microstructures and other factors. Procedures to achieve minimal microfissuring during welding have also been brought out.
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