Effect of processing parameters and composition on microstructure and phase transformation in clad layers by additive manufacturing of Ni-Ti alloy

Abstract

Nickel Titanium alloy (Nitinol) is well known for its distinctive shape-memory effect and superelasticy effect beside its great resistance of corrosion and bioxmechanical compatibility. Thesexproperties have empowered its applications particularly within the bio medical and aerospace field. Despite these exceptional properties, Manufacturing of nitinol by conventional procedures are exceptionally troublesome and costly and consequently, must be inspected. Therefore, additive manufacturing specifically the laser-based ones were used recently. In this thesis, Laser metal deposition (LMD) process of Nitinol was investigated in addition to the effect of process parameters on the properties of Nitinol. Preparation of the Specimens and the tests needed to be done were carried out in The National Laser Centre in Pretoria- South Africa. The effect of Heat treatment on Nitinol’s Superelasticity, Hardness, Microstructure, and phases transformation was also evaluated. Systematic characterization of Nitinol Specimens was done utilizing Optical Microscopy, Energy dispersive X-ray analysis (EDX) and Nano-indentation test. Specimens of nitinol was synthesized with different processing parameters using laser cladding and its properties were investigated and compared to one another to get the optimum processing parameters to synthesize a near net shape, fully dense Nitinol component with reliable properties. At first, eight preliminary Specimens were prepared with different processing parameters in terms of laser power, scan speed and rotating speed by which the powder is deposited while keeping the spot diameter constant. The objective of preparing the first batch of Specimens was to investigate its microstructure and find which one had the martensite phase in its microstructure. After the Specimens were mounted, polished, and then etched, it was clearly observed that all of the Specimens have dendrite arms concentrated at the diffusion zone between the substrate and the clad layer, Specimen A (Laser power = 1.5 kW, Scan speed = 1.5 m/min, Ti/Ni 2/1.4 rpm) has cracks and pores. However, the rest of the Specimens were crack free except for Specimen (Laser power = 1.25 kW, Scan speed = 1.5 m/min, Ti/Ni 1.5/1.5 rpm) which has a crack, nevertheless, it was the most promising Specimen among the first batch that possess the martensite phase in its microstructure and the least number of pores. Depending on this finding, a second batch of Specimens with processing parameter near that of Specimen H was prepared to investigate its microstructure, Elemental chemical