Thermal and microstructural aspects of the laser direct metal deposition of waspaloy

A potential problem in applying the laser direct metal deposition (LDMD) technique to the fabrication of superalloys is the possibility of an inconsistent microstructure and gamma-prime constituency throughout a component. Understanding the causes for these inconsistencies is a crucial step towards eliminating it and facilitating widespread application of the technique. This article uses thermocouple and pyrometric thermal monitoring of the LDMD process and optical metallographic, scanning electron microscope, and electron backscattered diffraction analyses of components built from Waspaloy to correlate LDMD process parameters and final part microstructural characteristics. Temperatures in thin wall structures show a good match to classical Rosenthal heat flow models. The Waspaloy grain morphology and orientation are found to be sensitive to LDMD power and powder mass flow rate parameters, with columnar grains forming preferentially at lower powder mass flow rates. Results cannot be explained purely in terms of established maps that relate microstructure to temperature gradient at the solidification front and its velocity. This leads to the conclusion that intra melt pool factors such as local fluctuations in temperature gradients and changes in nucleation density are significant.