Texture and Grain Boundary Network Evolution of Laser Powder Bed Fusion Processed Pure Ni During Post-printing Annealing

Abstract

Commercially pure Ni was additively manufactured using laser powder bed fusion. The printed specimens were then subjected to annealing treatments in a range of 700 °C to 1200 °C to investigate the evolution of the microstructure using electron backscatter diffraction mapping and a five-parameter analysis of the grain boundary character distribution. The post-printing annealing treatment resulted in microstructural changes in respect to grain shape and size, the grain boundary character distribution and the overall texture. However, the extent of these changes strongly depended on the annealing temperature. The grain structure and overall texture changed only modestly when annealed below 900 °C. The grains grew by about 10 pct and this led to a small (≈ 15 pct) increase in the areas of boundaries with low energy (111) planes at the expense of higher energy grain boundary planes with the (001) orientation. Static recrystallization was the dominant microstructure evolution mechanism at annealing temperatures greater than 1000 °C, where new equiaxed grains replaced the initial printed microstructure, enhancing the relative areas of low energy grain boundaries (i.e., ∑ 3 and ∑ 9) by a factor of more than five. This change increased the population of boundaries with the low energy (111) plane by a factor of five and decreased the strength of the texture by more than 50 pct through twinning. The resultant microstructure is expected to ultimately improve the material properties, where the increase in the relative areas of ∑ 3 boundaries would enhance the corrosion resistance and fracture toughness of material, and the texture weakening diminishes the anisotropy in mechanical behaviour.