Effect of Laser Energy Density and Solid Solution on Microstructure and Properties of Selective Laser Melted NiTi Alloy

NiTi alloy was prepared by laser selective melting process with NiTi alloy powder as raw materials under different laser powers and scanning speeds. The effects of laser energy density (26.67–111.11 J · mm⁻³) and 950 ℃×2 h solid solution on the microstructure, relative density and mechanical properties of NiTi alloy were investigated. The results show that with the increase of laser energy density, the relative density of the deposited NiTi alloy firstly increased, then decreased and then slightly increased, and reached the highest value (99.89%) under the laser energy density of 50.00 J · mm⁻³. With the increase of laser energy density, the nickel content in the alloy decreased, the phase transition temperature increased, the B2 austenite phase decreased, and the B19´ martensite phase increased; the microstructure of the alloy did not change much, and it was equiaxed crystal perpendicular to the forming direction and columnar crystal parallel to the forming direction; the grain shape changed from irregular shape to regular shape. After solid solution, the grain morphology did not change significantly, but the structure was more uniform. With the increase of laser energy density, the tensile strength and percentage elongation after fracture of the deposited and solid-soluted alloys both basically increased first, then decreased and then increased, and the hardness decreased. When the laser energy density was 50.00 J · mm⁻³, the deposited alloy had the best tensile property, with the tensile strength of 769 MPa and the percentage elongation after fracture of 6.73%, and after solid solution the tensile strength and percentage elongation after fracture were 762 MPa, 7.27%, respectively. The optimal SLM process parameters of NiTi alloy were laser power of 100 W, scanning speed of 800 mm · s⁻¹, scanning spacing of 100 mm and powder thickness of 30 mm.