Effect of laser rescanning on the grain microstructure of a selective laser melted Al-Mg-Zr alloy

Abstract

The microstructures of alloys created via Additive Manufacturing (AM) can vary substantially from those present in cast or wrought products, due to the very rapid solidification associated with AM.While numerous studies have investigated the process-microstructure relationship of alloys created by Selective Laser Melting (SLM), few have investigated the effects of laser rescanning to alter the microstructure or take advantage of the rapid solidification conditions the process provides. This study investigates the effect of single-or multiple pass laser scanning upon the grain structure of Addal-loy™, a new Al-Mg-Zr alloy strengthened via L1 2 Al 3 Zr precipitates. The bottom of the melt pools consisted of fine equiaxed grains (1.3 µm) that nucleated from primary Al 3 Zr (100-400 nm) precipitates. The top of the melt pool consists of columnar grains (up to 40 µm long), consistent with lack of Al 3 Zr nucleants due to Zr solute trapping from increased solidification velocities. Additional laser scanning (a second or third scan) reduces the amounts of columnar grains and increased the number equiaxed grains. The change is attributed to a shallower melt pool remelting the columnar grain region upon rescanning, due to reduced laser energy absorption and increased heat losses in the solid.

New Aluminum Alloys Specifically Designed for Laser Powder Bed Fusion: A Review

“…A similar bimodal microstructure was observed by Griffiths et al in Sc-free Al-Mg-Zr (Addalloy TM ) LPBF samples [77]. The authors focused the attention on the effect of laser rescanning on the microstructure of Addalloy TM observing a refinement of the microstructure thanks to the remelting of columnar grains and the precipitation of Al 3 Zr seeds.…”

Selective Laser Melting and Remelting of Pure Tungsten

“…The high‐temperature fluids with a low density inevitably rose; on the contrary, the low‐temperature fluids with a high density sank, which leads to convection of fluids in the molten pool . The heat flow remelts the previously formed columnar crystals and triggers them to peel off . The detached grains are engulfed into the molten pool and formed free grains, which may become the source of crystal nuclei in fine‐grain zone (FGZ).…”

“…The detached grains are engulfed into the molten pool and formed free grains, which may become the source of crystal nuclei in fine‐grain zone (FGZ). Additional heating from laser rementing can also influence the grain microstructure . The remelted microstructures, which are taken from horizontal surfaces of tungsten, are shown in Figure .…”

“…The remelted microstructures, which are taken from horizontal surfaces of tungsten, are shown in Figure . In addition, the fine‐grain structure could be caused by the higher cooling rate for the remelting step due to the difference in laser–material interaction and higher thermal conductivity in solid than powder . With the laser beam moving, the chilled grains are less likely to grow and form the FGZ near the centerline of the melt pool .…”

Effect of Laser Rescanning on the Characteristics and Residual Stress of Selective Laser Melted Titanium Ti6Al4V Alloy

“…The results show that the remelting strategy in SLM improves the surface quality and relative density. Griffiths et al [ 21 ] found that because of the reduced laser energy absorption and increased heat losses in the solid, the laser rescanning reduced the amount of columnar grains and increased the number of equiaxed grains. Xiao et al [ 22 ] studied the effect of the rescanning cycle on the relative density, microstructure, residual stress, and mechanical properties of SLMed Ti6Al4V.…”