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• Elemental distribution and correlation analysis of welded joints was investigated. • Defect-free welding joint with improved performance was obtained with MnN < 20%. • The nitrogen conversion process from powder to molten pool was discussed. • MnN powder started decomposing at 800℃ and produced Mn 4 N and Mn 2 N. The lack of Mn content and the importance of Mn to nitrogen content had been confirmed in this study and the welding of high-nitrogen steel with different proportions of MnN/CrN mixed powders was investigated. The distribution of pore defects in the welds and welding stability were analyzed by the synchronous acquisition of photographs using a high-speed camera, current–voltage curves, and plasma spectra. The nitrogen conversion process from powder to molten pool was studied, and the optimized MnN ratio was obtained by observing the microstructure changes in the weld. CrN powder did not produce intermediate products after melting, and it mixed well with the molten pool after melting/decomposition. Cr of the powder significantly improved nitrogen solubility in the austenite, thereby inhibiting micro pores; no CrN/Cr 2 N precipitates were found in the welds. The addition of Mn increased the austenite ratio; however, the MnN powder started decomposing at 800 ℃ and produced intermediate products Mn 4 N, Mn 2 N and N 2 , forming an unstable arc and keyhole and resulting in more pore defects. Thus, a defect-free welding joint with high tensile strength and toughness was obtained by controlling the proportion of MnN to < 20 wt%. When the proportion of MnN was > 30 wt%, pores were formed, which deteriorated the mechanical properties of the joint. [ABSTRACT FROM AUTHOR]

In this study, a ZK60 Mg alloy fabricated by laser powder bed fusion (LPBF) is subjected to in-situ aging treatment by preheating the substrate. The effects of preheating temperature on the microstructure, mechanical properties, and corrosion resistance of the LPBF-processed ZK60 alloy are systematically analyzed. The results show that at the preheating temperatures of 35, 90, and 180 °C, the average grain size of the samples does not change much, ranging between 7.0 and 8.2 μm. As the preheating temperature increases, the mechanical properties of the alloy increase. The specimens treated at a preheating temperature of 180 °C exhibit a yield strength (YS) of 201 ± 5 MPa, ultimate tensile strength (UTS) of 291 ± 7 MPa, and elongation (EL) of 14.7 ± 0.8%. This is attributed to the precipitation of a large amount of β' 1 phase in the alloy during the aging state in the LPBF process. However, the corrosion resistance first improves and then deteriorates with the increase in the preheating temperature. Under a preheating temperature of 90 °C, the corrosion resistance is improved due to the reduction in the galvanic corrosion area by the precipitated phases. When the preheating temperature is increased to 180 °C, the precipitated phases and α-Mg form a large number of microgalvanic cells, which increases the active center for corrosion and deteriorates the corrosion resistance. These findings indicate that the desired mechanical properties and corrosion resistance of the LPBF-processed ZK60 alloy can be obtained by selecting an appropriate preheating temperature. • The cracks significantly reduce in LPBF-processed ZK60 alloy by substrate preheating. • The mechanical properties increase with increasing preheating temperature. • The corrosion resistance first improves and then deteriorates. • β' 1 phase is beneficial to mechanical properties but deteriorates corrosion resistance. [ABSTRACT FROM AUTHOR]