MICROSTRUCTURE DEVELOPMENT IN MULTI-PASS LASER MELTING OF AISI 8620 STEEL

An existing thermal model for laser melting and additive manufacturing (AM) was expanded to include phase transformation and hardness predictions for an alloy steel and coupled to experimental results. The study was performed on AISI 8620, a popular case-hardening, steel to understand microstructural and property effects for potential repair applications. The experimental samples were polished, etched with nital and picral for comparison, imaged, and Vicker’s microhardness was taken at 0.5 and 0.2 kg loads. The etched images revealed a transformation zone slightly larger than the melt zone in all cases including a gradient in transformation along the outer edges of the transformation zones. The microhardness measurements revealed that the lower energy cases provided a higher hardness in the melted region even after tempering due to multiple passes. But the overall hardness was higher than what is to be expected of a fully martensitic structure in AISI 8620. The phase transformation model qualitatively shows a similar microstructure where molten regions turn completely to martensite. The model also predicts a transformation zone larger than the melt pool size, as well as the transformation of pearlite but not ferrite near but not in melt pool. This observation is experimentally verified showing a heat affected zone where pearlite is clearly transformed but not ferrite outside the transformation zone comprised of complete martensite. The hardness model predicts a lower hardness than the experiments but is similar to what is expected based on published Jominy End Quench tests. The cases in the regime dominated by conductive heat transfer show good agreement with the predictions of melt pool shape and hardness by the thermal model. However, at higher powers and lower speeds, the fluid flow influenced the shape of the melt pool and the hat transfer in its vicinity, and the model was less accurate.