The In situ Laser Powder Bed Fusion of Fe-30Mn-6Si Alloy for Use as a Biodegradable Shape Memory Implant

Abstract

Biodegradable implant materials are more appropriate for temporary support applications compared with their inert counterparts since the former requires no removal surgery because they naturally degrade and eventually dissolve completely during healing. Iron and its alloys are a possible substitute for the commercial magnesium biodegradable implants because of their superior mechanical properties and slower corrosion rates. The addition of manganese and silicon in iron imparts another interesting property to the material–the shape memory effect. There is copious research on the structure and properties of the biodegradable face centred cubic (FCC) Fe-30Mn-6Si shape memory alloy (SMA) that exhibits the reversible FCC austenite to hexagonal close packed (HCP) ε-martensite transformation. However, recent advances in additive manufacturing of metals, brought by the development of the laser powder bed fusion (LPBF) technique, warrant the need for an investigation on the adaptability of the technique in fabricating this alloy composition. The LPBF technique is limited by the need for specialty raw material powder, and this thesis extends the application of the technique in fabricating the Fe-30Mn-6Si shape memory alloy (SMA) from homogenised powder precursors. More so, LPBF processing of Fe-30Mn-6Si alloy from either pre-alloyed powder or blended powder has not been reported.

To successfully fabricate a Fe-30Mn-6Si LPBF product, the influence of key LPBF processing parameters on product quality was identified as a major challenge. This was addressed by investigating the influence of laser power, laser scan speed, laser re-scanning, and their equivalent input energy on the relative density and defect formation. A relative density of over 99% with few processing defects was achieved using the optimised parameters of 175 W laser power, 400 mm/s scan speed, and no re-scanning. The influence of these parameters on the solidification microstructure was also investigated using key techniques, such as X-ray diffraction (XRD) and scanning electron microscopy (SEM) in conjunction with energy dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD). Further, the simulated thermal profile of the melt pool region as a function of process parameters via single scan track experiments was calculated using the finite element method (FEM). These data were used to explain the key microstructural features observed in the as-solidified microstructure of the LPBF alloy as a function of the processing parameters.

The mechanical properties of the LPBF alloy were then assessed by hardness and tensile testing and then compared with a reference alloy produced by arc melting. The hardness of the LPBF as-built alloy was ∼20% higher than the reference alloy. To identify the factors affecting the increased hardness of the former, the influence of grain size and morphology, crystallographic texture, phase constituents (mainly austenite and martensite), and residual strain were investigated. The hardness of the reference alloy was affected mainly by the grain size and residual strain, but for the LPBF-built alloy, the relative volume fractions of austenite and martensite strongly influenced the hardness. Meanwhile, the tensile properties of the LPBF alloy, such as the yield stress, ultimate tensile stress, and ductility, were adversely affected by the internal defects present, such that high temperature homogenisation and hot isostatic pressing (HIP) post-process treatments were investigated to improve these properties.

The homogenisation and HIP treatments increased both the tensile strength and ductility of the LPBF-built alloy. Homogenisation altered the grain morphology by promoting recrystallisation and grain growth, and this increased the tensile strength by ∼80%. The hardness, however, decreased due to a reduction in the volume fraction of HCP martensite in the FCC austenitic microstructure. HIP retained some of the columnar microstructure generated by the LPBF process, marginally increased the density, and increased the tensile strength by ∼65%. The improvement in tensile properties through these post-process treatments allowed for the measurement of LPBF alloy’s shape memory behaviour, whereby a tensile recovery strain of 2% was achieved for the HIP-treated alloy. Finally, the biocorrosion behaviour of the LPBF-processed and HIP-treated alloy was investigated, whereby the in vitro corrosion potential and current density of the alloy were determined to be -769 mV and 5.6 μA/cm2, respectively, indicating a reasonable corrosion rate for this material. Overall, this thesis enabled the first demonstration of the shape memory effect in an LPBF-built Fe-based alloy fabricated from homogenised powder, an alloy which also exhibits biodegradable properties.