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

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 (LENS™) 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 camera and a dual-wavelength pyrometer, respectively. These tools demonstrate the use of non-destructive thermographic inspection for ensuring target part 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°C. Temperature gradients varied and peaked around 1000°C/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°C/s to 25,000°C/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.