Grain refinement and mechanical properties for AISI304 stainless steel single-tracks by laser melting deposition: Mathematical modelling versus experimental results

Many of the significant mechanical properties of stainless steel, including ultimate tensile strength, yield strength and hardness, the ductile-brittle transition temperature, and susceptibility to environmental embrittlement, can be improved by grain size refinement. Hall-Petch relation identifies that the improvement can be quantified in a constitutive relation. In this study, a new mathematical model to calculate the number of grains and their average size inside a single printed layer via the laser melting deposition (LMD) process is proposed. The printed layer's thermal history concerning the moving laser beam and co-axial addition of powder debits was analyzed and used to calculate the thermal stress and strain rate. The average grain size within the printed layer was calculated using the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model. The mechanical properties, including ultimate tensile strength, yield strength, and hardness, were estimated using the average grain size. For single depositions of AISI 304 stainless steel powder debits on a steel substrate, dedicated experiments were performed to verify the model's trustworthiness. Scanning electron microscopy was used to quantify the number and size of grains. Vickers hardness tests were conducted to confirm the mechanical performances predicted by the developed model. It was found that the primary operating parameters strongly influence the grain type, resulting in three varieties of grains: (a) quasi-continuous circular, (b) long lath-shaped, and (c) a combination of the two mentioned above. The deposited layer's thermal history influences the thermal stresses and controls the growing grains' average size. A strong correlation between experimental and computational results, within the range of 10-15 %, and 8-10%, was obtained for the average grain size and Vickers hardness test. The laser scanning speed and laser power were in an inverse relation with the average grain size, while a direct relationship was noticed between the powder feed rate and average grain size. The mechanical computational and experimental results show that the highest yield strength (=208 MPa), ultimate tensile strength (=722 MPa), and hardness (=278 HV) were obtained for the finest grain structure.

Automated summary

This study introduces the improvement of stainless steel mechanical properties through grain size refinement, proposing a new mathematical model to calculate the number and average size of grains within a single printed layer via the laser melting deposition process. The experimental results show a strong correlation between computational and experimental results for grain size and Vickers hardness tests. Additionally, the study found that the operating parameters impact the grain type, resulting in different varieties of grains, and identified relationships between parameters such as laser scanning speed, laser power, and powder feed rate with the average grain size. The highest mechanical properties were achieved for the finest grain structure.