Effect of gradient layer thickness within a single SLM-built IN738 alloy on grain morphology and post-treatment grain boundary evolution

Layer thickness strongly affects microstructural evolution during SLM. In this work, a single IN738 specimen containing four layer-thickness regions (30, 60, 90 and 120 μm) was fabricated to clarify the effect of layer thickness on the as-built grain structure and post-treatment response. As layer thickness increases, the as-built microstructure changes from relatively fine grains with higher local orientation gradients to coarser grains with reduced columnar continuity and a stronger tendency toward equiaxed growth, while the average KAM decreases from 2.02° to 1.10°. After post-treatment, KAM values in all regions decrease to 0.24°–0.27°, but the 60 μm region exhibits the shortest transverse HAGB length. The results indicate that an intermediate layer thickness provides the most favorable post-treatment grain-boundary topology in SLM-built IN738.