Residual stress of Ti6Al4V (ELI) parts manufactured by direct metal laser sintering

Abstract

This study focuses on the residual stress in Ti6Al4V (ELI) parts manufactured by direct metal laser sintering (DMLS). During DMLS a laser beam scans over the powder layer thus melting the powder and previous layer. High-concentrated energy input leads to high thermal gradients. The track-by-track, layer-by-layer nature of DMLS technology and the thermocycling of the process leads to complex stress distribution and deformation of parts during manufacturing. A literature review of the DMLS process is given; the capabilities of DMLS are overviewed, and a practical medical solution is demonstrated as a prominent market component of DMLS and its capabilities. Types of residual stress, its origin and effect on the performance of materials are described. Focus is given to the methods for quantitative and qualitative determination of residual stress in DMLS objects. The thermal gradient and shrinkage play a vital role in the development of residual stresses. Methodology for the investigations of the microstructure, roughness, deformation and microhardness of the specimens are outlined. Experiments which were conducted with their respective results are included, the results being scrutinized as well as the relevant interpretation to reach adequate scientific conclusions. Relative comparisons are also done in terms of received data and literature. Some numerical simulations are done for predictive and comparative purposes. The results show high values of residual stress in Ti6Al4V (ELI) specimens produced by DMLS. The residual stresses in samples with successively increasing thickness and in primitive 3D components were investigated. It was found that volumetric and shape relations in terms of residual stress and DMLS objects exist. Applied post process heat treatment is adequate in relieving the majority of residual stress. The sensitivity of Ti6Al4V (ELI) alloy to interstitial elements is also indicated in the data during heat treatment. The consequences of residual stress experienced during this study demand creative solutions for the purpose of a wider application of this technology. Some promising directions for further investigation of residual stress in DMLS objects based on the study’s findings are done.