The development and processing of nickel titanium shape memory alloys containing palladium using selective laser melting

Baker, Hollie L. (2019). The development and processing of nickel titanium shape memory alloys containing palladium using selective laser melting. University of Birmingham. Ph.D.

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Abstract

Nitinol (NiTi) is a commonly used biomaterial, due to its unique superelasticity and shape memory effect. However, NiTi has a low radiopacity, so it is difficult to observe using electromagnetic radiation once inside the body. The radiopacity of NiTi can be improved by alloying with a denser, ternary element. Platinum (Pt) is a popular choice as a result of its biocompatibility and corrosion resistance. However, alloying with palladium (Pd) is fast becoming a viable alternative due to its similar properties and much lower cost. The overall purpose of this thesis is to investigate the suitability of selective laser melting (SLM) for manufacturing palladium (Pd) modified nickel-titanium (NiTi) shape memory alloys (SMAs), that can be used in biomedical devices within the human body.
This study presents a methodology for in-situ alloying Pd into NiTi using selective laser melting (SLM) to manufacture dense, defect free components. The influence of melt pool dimensions on chemical homogeneity was investigated. The laser power, laser scanning velocity and laser scan spacing were independently varied to alter the energy density of the laser and in turn, modify the dimensions of the melt pool. It has been found that the SLM process parameters can be manipulated to fully diffuse Pd into the NiTi matrix, providing a homogenous microstructure. Out of the three process parameters investigated, the scan velocity had the greatest influence on the melt pool. Thus, increasing the dissolution of Pd as a result of the Marangoni effect and increased melt pool liquid life span.
The suitability of the dense and chemically homogeneous SLM fabricated TiNiPd alloys were investigated for their use in biomedical applications. Furthermore, the effects of Pd on the properties of the binary NiTi alloy were also studied. This was performed by testing the alloy’s phase transformation behaviour, cell biocompatibility, corrosion resistance, superelastic response and X-ray visibility. Overall, it can be concluded that the addition of Pd had advantageous benefits towards improving the properties of binary NiTi for use in biomedical applications. Alloying with 7 at % Pd in substitution for Ni, was found to improve the alloy’s corrosion resistance, X-ray visibility and biocompatibility, thus making the ternary TiNiPd alloy a promising biomaterial.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):