Electron Backscattered Diffraction to Estimate Residual Stress Levels of a Superalloy Produced by Laser Powder Bed Fusion and Subsequent Heat Treatments.

Mathieu Terner,Hyun-Uk Hong,Sara Biamino,Jiwon Lee,Giulio Marchese

doi:10.3390/ma13204643

Abstract

Metal Additive Manufacturing and Laser Powder Bed Fusion (LPBF), in particular, have come forth in recent years as an outstanding innovative manufacturing approach. The LPBF process is notably characterized by very high solidification and cooling rates, as well as repeated abrupt heating and cooling cycles, which generate the build-up of anisotropic microstructure and residual stresses. Post-processing stress-relieving heat treatments at elevated temperatures are often required in order to release some of these stresses. The effects of 1 h–hold heat treatments at different specific temperatures (solutionizing, annealing, stress-relieve and low-temperature stress-relieve) on residual stress levels together with microstructure characterization were therefore investigated for the popular Alloy 625 produced by LPBF. The build-up of residual stress is accommodated by the formation of dislocations that produce local crystallographic misorientation within grains. Electron backscattered diffraction (EBSD) was used to investigate local misorientation by means of orientation imaging, thereby assessing misorientation or strain levels, in turn representing residual stress levels within the material. The heavily constrained as-built material was found to experience full recrystallization of equiaxed grains after solutionizing at 1150 °C, accompanied by significant drop of residual stress levels due to this grains reconfiguration. Heat treatments at lower temperatures however, even as high as the annealing temperature of 980 °C, were found to be insufficient to promote recrystallization though effective to some extent to release residual stress through apparently dislocations recovery. Average misorientation data obtained by EBSD were found valuable to evaluate qualitatively residual stress levels. The effects of the different heat treatments are discussed and suggest that the peculiar microstructure of alloys produced by LPBF can possibly be transformed to suit specific applications.

Highlights

For several years, Metal Additive Manufacturing has become an undisputable alternative to conventional manufacturing
The microstructure of the Alloy 625 produced by Laser Powder Bed Fusion (LPBF) as-built and after the four isothermal heat treatments was observed at different length scale
The popular Alloy 625 was produced by LPBF and subjected to 1 h–hold heat treatments at different temperatures

Summary

Introduction

Metal Additive Manufacturing has become an undisputable alternative to conventional manufacturing. It is notably characterized by very high solidification and cooling rates, as high as 105 –106 ◦ C/s, as well as repeated and sharp heating-and-cooling cycles as the laser beam travels over the powder bed and the consolidated parts according to the selected scanning strategy [1,2,3,4,5,6] These unique characteristics, as opposed to conventional manufacturing, are responsible for the peculiar microstructure of LPBFed parts. This microstructure can be effectively controlled, to some extent, by varying the numerous process parameters This severe thermal cycling leads to stresses sometimes approaching or even exceeding the elastic limit of the materials associated with the build-up of anisotropic residual stresses. While processing parameters can be optimized with regards to their significant effect on residual stress buildup, post-processing stress relieving heat treatments at elevated temperatures to be applied shortly after manufacturing are often advised in order to release these stresses to some extent

Methods

Discussion

Conclusion