Effect of Substrate Thickness on Micro-Hardness of Direct Laser Deposited Ti-6Al-4V Parts

The heat transfer occurring during laser-based additive manufacturing (LBAM) processes can impact part quality and post-manufactured thermo-mechanical properties – including microstructure and hardness. Effectively managing the heat loss of the manufactured part to the substrate (conduction) and environment (convection) is of great importance. This study experimentally investigates the use of Laser Engineered Net Shaping (LENS) for the additive manufacture of cylindrically-shaped Ti-6Al-4V parts. Using an OPTOMEC LENS ® 750 system, the effect of substrate thickness on post-LENS microhardness and porosity was experimentally investigated. The resultant, transient temperature distribution near the substrate/part intersection was measured using an array of substrate-embedded, type-C (tungsten-rhenium) thermocouples. The effect of the substrate on the substrate/part temperature response was investigated by varying the substrate thickness. The density of the LENS-manufactured part was found to be consistent by ± 0.04 g/cc in relation to each substrate thickness; however, these densities were less than the average density for wrought Ti-6Al-4V suggesting that the parts were somewhat porous for the selected LENS operating parameters. The average Vickers hardness along the crosssection of the part increased with substrate thickness. Compared to the average 349HV for Ti-6Al-4V, the hardness values were 343 HV for the 0.635 cm plate, 359 HV for the 1.27 cm plate, and 368 HV for the 2.54 cm plate. The maximum temperature experienced at the part intersection decreased as plate thickness increased. Heating rate calculated at the outer radius of the part appeared to cause the radial hardness of the first deposited layer to follow a similar curve when built upon the 1.27 cm and 2.54 cm plates.