Exploiting Metastability and Lattice Defects for Microstructural Engineering of Selective Laser Melted Ti-6Al-4V

Abstract

Introduction Additive manufacturing (AM) is a technology of enormous potential, yet also of high complexity. Understanding and exploiting AM requires very interdisciplinary research efforts since material, processing, AM-design and engineering, build strategies, post-treatments and surface finishing are all interdependent. Particularly for class-one components in aerospace applications, achieving high material quality and performance in a robust AM process such as Selective Laser Melting (SLM) is a major hurdle. For the development of a manufacturing chain for a Ti-6Al-4V rocket engine turbo-pump impeller, different SLM processing and post-heat-treatment strategies for Ti-6Al-4V including intensified intrinsic heat treatment and in situ high-temperature build space heating to obtain stabilized -microstructures with fine -particles and films have been studied. Moreover, the transfer from coupon level to a complex part will be discussed. Methods In this study a SLM Solutions 280HL machine with a custom high-temperature build space heating was used. Results Heat treatments act to stabilize the initial as-built microstructures by decomposition of the acicular martensite in conjunction with pronounced element partitioning. New -phase nucleates at prior lattice defects in the as-built microstructures, leading to favorable materials properties. The interplay between SLM process parameters and the part geometry was studied using High Energy X-ray Diffraction in a section of an impeller manufactured with the optimized SLM bulk parameters. A mapping of the phase distribution shows that the local thermal history differs strongly in filigree and more massive sections and leads to substantial differences in the obtained microstructures and porosities compared to coupon specimen. Conclusions The process parameters and heat treatments were successfully optimized with coupon specimen to achieve stabilized -microstructures allowing for high strength and ductility. When used for manufacturing actual parts, however, the local geometry plays a key role: Results from studies carried out with cubes or cylinders can differ substantially from those in complex parts despite identical SLM parameters.