Understanding the Microstructure Formation of Ti-6Al-4V during Direct Laser Deposition via In-Situ Thermal Monitoring

Marshall, G., Young, W. J., II, Thompson, S.M., Shamsaei, N., Daniewicz, S., & Shao, S. (2016). Understanding the Microstructure Formation of Ti-6Al-4V during Direct Laser Deposition via In-Situ Thermal Monitoring. JOM. TMS. 68(3), 778-790. DOI:10.1007/s11837-015-1767-z.

Abstract

Understanding the thermal phenomena associated with Direct Laser Deposition (DLD) is an important step toward obtaining ‘process-property-performance’ relationships for various designed parts and materials, as well as achieving increased process control for meeting application constraints. In this study, a thermally monitored Laser Engineered Net Shaping (LENSTM) system was used with time-invariant (uncontrolled) build parameters to construct Ti-6Al-4V cylinders. During fabrication, the part’s thermal history and melt pool temperature were recorded via an in-chamber infrared (IR) camera and a dual-wavelength (DW) pyrometer, respectively. These tools demonstrate the use of non-destructive thermographic inspection (NTI) for ensuring target quality and/or microstructure. For the chosen part geometry, the melt pool was found to be approximately 40-50% superheated during DLD, reaching temperatures as high as 2500 ºC. Temperature gradients varied and peaked around 1000 ºC/mm along the diameter of the relatively small cylinders. Cooling rates within the melt pool were found to increase as maximum melt pool temperature increased, for instance, from 12,000 ºC/s – 25,000 ºC/s. The post-DLD Ti-6Al-4V microstructure was found to vary from columnar near the substrate, or substrate affected zone, to equiaxed approximately 2-3 mm from the substrate. Bulk heating of the part due to successive layer deposits was shown to promote α’’ to an α+β decomposition, while prior-β grains were observed near and far from the substrate.