Material interactions in laser polishing powder bed additive manufactured Ti6Al4V components

Abstract

Laser polishing (LP) is an emerging technique with the potential to be used for post-build, or in-situ, precision smoothing of rough fatigue-initiation prone surfaces of additive manufactured (AM) components. LP uses a laser to re-melt a thin surface layer and smooths the surface by exploiting surface tension effects in the melt pool. However, rapid resolidification of the melted surface layer and the associated substrate thermal exposure can significantly modify the subsurface material. This study has used an electron beam melted (EBM) Ti6Al4V component, representing the worst case scenario in terms of roughness for a powder bed process, as an example to investigate these issues and evaluate the capability of the LP technique for improving the surface quality of AM parts. Experiments have shown that the surface roughness can be reduced to below Sa=0.51 µm, which is comparable to a CNC machined surface, and high stress concentrating defects inherited from the AM process were removed by LP. However, the re-melted layer underwent a change in texture, grain structure, and a martensitic transformation, which was subsequently tempered in-situ by repeated beam rastering and resulted in a small increase in sub-surface hardness. In addition, a high level of near-surface tensile residual stresses was generated by the process, although they could be relaxed to near zero by a standard stress relief heat treatment.

Material Characterization, Thermal Analysis, and Mechanical Performance of a Laser-Polished Ti Alloy Prepared by Selective Laser Melting

“…This resulted in a coarse lamellae structure in the microstructure, as shown in Figure 2d. The dimension of the microstructure with an average size of 9.8 µm in the laser-polished layer is smaller than that of the substrate with an average size of 48.6 µm due to the rapid re-melting and solidification process [21].…”

Laser polishing of additive manufactured Ti6Al4V alloy: a review

“…1. 42 There are currently two forms of laser polishing. One is to fix the printed parts on a numerically controlled X-Y-Z stage and then perform polishing.…”

“…In addition to the selection of process parameters, the influence of other factors on the surface roughness of laser-polished Ti6Al4V alloy should also be taken into account. Tian et al 42 measured the surface roughness of laser-polished Ti6Al4V using a Nanofocus μScan laser profilometer and a white light interferometer with different resolutions and found that the Sa parameter was reduced from as-received 21.46 to 5.5 μm and 0.51 μm, respectively. The difference in the measurement results was mainly due to the size of the sampling area, namely, 2 mm × 3 mm and 0.13 mm × 0.17 mm.…”

Additive manufacturing titanium components with isotropic or graded properties by hybrid electron beam melting/hot isostatic pressing powder processing

“…By varying the level of preheat the bed temperature is controlled by the software. For Ti-6Al-4V the intention is to maintain the bed at 650 °C, which is high enough to reduce the residual stresses in built parts to near zero 15 . This removes the need for a stress relief heat treatment following manufacture, typically required for selective laser melted (SLM) parts.…”