Laser-Directed Energy Deposition Additive Manufacturing of Nickel-Titanium Coatings: Deposition Morphology, Microstructures, and Mechanical Properties

To overcome the disadvantages of comparatively poor surface hardness and deprived resistance against wear of Ti-6Al-4V alloy, the protective NiTi coatings are fabricated at different processing parameters by laser-directed energy deposition (LDED). This work presents a comprehensive study of deposition morphology, phase constituents, microstructural evolution, composition distribution, and mechanical properties of as-fabricated NiTi deposition tracks. The relationship among processing parameters and deposition morphology, phase constituents, and mechanical properties is established. The microstructural evolution mechanism and the composition distribution on the cross section of deposition tracks are revealed. Results show that the deposition tracks are successfully formed under all laser processing parameters, and the dilution ratio increases with the increase in laser power and scanning speed. The deposition tracks consist of NiTi2, NiTi, and alpha-Ti. According to the microstructural analysis, planar crystals are formed between the substrate and the deposition tracks, and columnar and equiaxed dendrites are found in the deposition tracks. The average microhardness increases continuously with the increase in laser power. The deposition track prepared at laser power of 1400 W has the highest average microhardness of 732.45 HV0.2. The deposition track at this laser power exhibits excellent wear properties, with a wear rate of 2.25 x 10(-6) mm(3) Nm(-1).