Understanding the role of laser processing parameters and position-dependent heterogeneous elastocaloric effect in laser powder bed fused NiTi thin-walled structures

Abstract

Abstract To reduce the driving load and enhance the heat exchange capacity and elastocaloric refrigeration efficiency, increasing interests in porous structure design and laser-based additive manufacturing (LAM) of NiTi materials with a large specific surface area have been emerging. As a type of characteristic unit of porous components, we mainly focused on the LAM process optimization and elastocaloric effect of NiTi-based thin-walled structures (TWSs) in this work. Firstly, we systemically studied the influence of laser processing parameter on the forming quality and phase transformation behavior of NiTi-based TWS samples. Results showed that high relative density (>99.0%) was inclined to be obtained in a range of 67–133 J mm−3 (laser energy density). Besides, the transformation temperatures (TTs) and enthalpy change roughly showed a positive linear relationship with the applied laser energy density. At an optimized parameter (P = 100 W and v = 1000 mm s−1), the sample exhibited a high relative density (99.88%), good dimensional accuracy, and the lowest TTs. Then, this work emphatically unveiled the position-dependence of phase transformation behavior and elastocaloric effect (eCE) in a NiTi-based TWS sample. It was found that both the TTs and enthalpy change monotonously decreased along the building direction, while the transformation strain kept an increase trend. As a result, the middle portion of the sample exhibited the largest adiabatic temperature change which reached 6.5 K at the applied stain of 4%. The variation in TTs and eCE could be attributed to the heterogeneous solidification microstructure induced by the thermal cycle nature of LAM process.