Effect of a constant laser energy density on the evolution of microstructure and mechanical properties of NiTi shape memory alloy fabricated by laser powder bed fusion

Abstract

As the key parameter in the laser powder bed fusion (L-PBF), the laser energy density can directly determine the comprehensive quality of NiTi shape memory alloys (SMAs). However, the specific role of laser parameters under the energy density value is rarely discussed. In this work, the effect of specific laser power and scanning speed variation on the properties of NiTi alloy under the same energy density (62.5 J/mm3) was studied. Results show that the phase transformation temperatures decrease as the Ni content was increasing with the simultaneous increases in laser power and scanning speed. Under the influence of Ni content, the SMA samples with low phase transformation temperatures were fully austenite at room temperature. In the cyclic compression test at Af + 15 °C, the sample built under the lowest laser power and scanning speed showed the best superelasticity effect. Higher scanning speed resulted in a stronger <100>//BD texture, and the increase of laser power produced a deeper molten pool, which remelted the solidified part at the bottom and further enhanced the <100>//BD texture. The TEM characterization shows that more Ni4Ti3 precipitates with larger sizes are obtained in the sample with the lowest laser power and scanning speed. The finite element analysis shows the lowest peak temperature in the molten pool under the lowest laser power and scanning speed, which can promote the precipitation of Ni4Ti3 particles. The promoted formation of Ni4Ti3 precipitates enhanced the austenite stability and further enhanced the superelasticity of NiTi alloy fabricated at the lowest laser power. The present work shows that the specific laser parameter under the constant energy density value can further improve the comprehensive performance of L-PBF NiTi SMAs.