Fabrication and Heat Treatment of Selective Laser Melting Produced NiTi Shape Memory Alloy

Although additive manufacturing provides more possibilities for NiTi shape memory alloy with flexible structural design,there are still challenges in fabricating fully dense NiTi components with complex structure by laser additive manufacturing.Studying the influence of laser scanning speed on the density and surface quality is very significant.In the selected process window of 400～1400 mm/s,the relative density of samples was greater than 99%.But when the scanning speed was greater than 600 mm/s,cracks appeared at the bottom.The results of tensile tests at an optimized printing speed of 400 mm/s show that the average tensile strength of the deposited and heat treated specimens were 675 MPa and 782 MPa,respectively.And the maximum elongation of deposited specimens and heat treated specimens were 19.7% and 17.95% respectively.That means,the tensile strength of the specimen increases after the 500 ℃ annealing heat treatment,but the elongation decreases.The fracture mechanism of the deposited and heat treated specimens is the joint action of cleavage and plastic fracture.The martensitic phase transformation and inverse phase transformation starting temperatures（Ms and As） for the heat-treated specimens were 35.8 ℃ and 10.0 ℃,respectively,which is obtained by DSC experiments.For temperature between Ms and As,the alloy consists of austenite and martensite phases.The EBSD results revealed that the specimens were mainly composed of B19' martensite at 20 ℃ room temperature.