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Nickel based precipitation hardening superalloys are widely used in various applications such as aircraft, nuclear reactor and space industry due to their excellent high temperature performance and resistance to metallurgical and structural variations. However, it is very difficult to fabricate components with complex shapes from these superalloys by conventional manufacturing technologies. Additive manufacturing (AM) technology offers an alternative way to meet the demanding needs of lightweight, complexity and integrated manufacturing by building layers by layer without the limitations of traditional processes. Selective Laser Melting (SLM) is one of the promising AM technologies that can be used to produce aerospace parts from heat-resistant alloys. Although in738 can be successfully printed by SLM with good properties, metallurgical defect such as liquation cracks are still present.
Liquation cracking of non-weldable superalloys can be reduced by optimizing the main process parameters such as deposition cooling rate and preheat temperature. However, it is not easy to eliminate cracks in in738 since liquation cracks occur mainly along grain boundaries. In order to understand the underlying reason of these cracks, we have studied the microstructure evolution during and after SLM process with different build heights. We found that the variation of g eutectic phase with build heights is responsible for the formation of these cracks.
Further, we have conducted an in-depth characterization of the microstructure of IN 738 by electron microscopy and energy dispersive X-ray spectroscopy. We also performed a comparative study on the material composition of failed turbine blade and in738LC alloy. It was found that the failure of turbine blade was caused by chemical erosion and corrosion of the in738LC alloy at a depth of 4 mm from surface.