Tailoring microstructure and strengthening mechanism of anisotropic elastocaloric effect in NiTi shape memory alloys by laser directed energy deposition scanning strategy

Abstract

The NiTi shape memory alloys (SMAs) with elastocaloric effect are applied to new solid-state refrigeration technologies with green and zero-global warming potential, and additive manufacturing technologies provide new opportunities to advance their development. In this study, four scanning strategies for laser directed energy deposition (LDED) additive manufacturing involving the direction change of adjacent hatch lines and the rotation of adjacent deposited layers were designed, and the NiTi SMAs were synthesized in situ from two metal powders, Ni and Ti. The microstructure evolutionary behaviors and reinforcement mechanisms of anisotropic elastocaloric effects were analyzed. The rotation of two adjacent deposited layers demonstrates the potential to inhibit the growth of Ti₂Ni precipitation phase dendrite arms and to modulate their morphology and content, and to disperse the direction of the temperature gradient and reduce the stresses and deformations resulting from the thermal cycling process. The differences in microstructure evolution behaviors of different scanning strategies make NiTi SMAs exhibit anisotropy in stress-strain response behaviors and elastocaloric effects. The results of the analysis of the elastocaloric effect show that the temperature drop generated at the surface of the alloy is a linearly increasing function of the maximum compressive stress. The customized microstructure of “semicircular arc” shaped grains within the double cross-section was obtained by the simultaneous rotation of adjacent hatch lines and deposited layers, exhibiting a synergistic enhancement of phase strain and elastocaloric effects. The cooling effect up to −9.4 °C (unloaded at 1100 MPa) was increased by 224 % compared to the conventional strategy. This study provides a way to advance the rapid development of solid-state refrigeration technology by customizing the microstructure of NiTi SMAs and obtaining the desired elastocaloric effect via LDED additive manufacturing.