Tailoring microstructure evolution and mechanical properties of a high-performance alloy steel through controlled thermal cycles of a direct laser depositing process.

Direct laser deposition (DLD), as a popular metal additive manufacturing process, shows advantages of technical flexibility and high efficiency to gain a high-performance alloy steel component. However, during the processing of DLD, the deposited steel layer is affected by the subsequent layer depositing. The DLD block shows different microstructure and mechanical properties at the bottom, middle and top of the deposited parts. To date, there are few research works about the effects of inter-layer interval time and laser power on the microstructure evolution and mechanical properties of the deposited layers. In this study, the idle time and laser power layer by layer during DLD of 12CrNi2 steel were controlled to cause the deposited layers to maintain a high cooling rate, while the bottom deposited layer was subjected to a weak tempering effect. Results show that a high proportion of martensite is produced, which improves the strength of the deposited layer. Under the laser scanning strategy of laser power 2,500 W, scanning velocity 5 mm·s−1, powder feeding rate 11 g·min−1, overlap rate 50%, and a laser power difference of 50 W and a 2 min interval, the tensile strength of the deposited layer of 12CrNi2 steel is in the range of 873–1,022 MPa, and the elongation is in the range of 16.2%–18.9%. This study provides a method to reduce the tempering effect of the subsequent deposition layers on the bottom layers, which can increase the proportion of martensite in the low-alloy high-strength steel, so as to improve the yield strength of the alloy steel. [ABSTRACT FROM AUTHOR]