Effect of alloy composition on the microstructure of titanium alloys produced by wire & Arc Additive Manufacturing

Abstract

Additive Manufacturing (AM) technologies seek to minimise material waste in the manufacturing industry. Such technologies are able to be used with various plastics, metals and composites and can impact a multitude of industries. Users of titanium and its alloys can benefit from additive manufacturing due to the challenges present in the subtractive fabrication of titanium components. While AM is a solution to the issue of high material waste observed in titanium manufacturing, most AM technologies suffer from columnar, anisotropic microstructures that hinder their widespread use. A review of recent literature has highlighted the need for the development of an effective grain refining solution to additively manufactured titanium components. Solute addition was chosen as the method of grain refinement due to its success in cast titanium.Silicon and carbon additions were found to be effective in refining the microstructure of AM titanium components. The addition of up to 0.75wt% silicon to commercially pure titanium manufactured by wire and arc AM resulted in a significant reduction of the prior-b grain size. Additions of up to 0.41wt% carbon were also made to Ti-6Al-4V components built using wire and arc AM. Microstructural analysis revealed that the prior-b grain size and a-lath length reduced by a factor of 5-6. The addition of 0.1wt% carbon proved to be the most effective in increasing yield and tensile strengths, with an improvement of 9% over unalloyed Ti-6Al-4V. For higher levels of carbon, strength and ductility both decreased. The addition of 0.1% carbon also resulted in a 7% increase in maximum compressive strength when compared to unalloyed Ti-6Al-4V.The effectiveness of both solutes showed that grain refiners utilised for cast titanium can also be applied in AM processes. However, while both solutes refined microstructure to a degree, columnar grains remained. It was determined that potent nucleant particles must be added in conjunction with solutes to achieve a homogenous equiaxed microstructure in AM titanium components. This research has also shown that the interdependency theory and the growth restriction factor are both applicable to the wire and arc AM process. These theories can be utilised by future researchers to further investigate the grain refinement of AM titanium. The experimental methodologies developed during the course of this research can also aid future researchers. The findings from this research can be utilised in future research to develop commercially viable grain refiners for not only titanium, but other materials as well. The effectiveness of silicon and carbon can also be tested using other AM technologies to further understand the mechanisms of grain refinement during AM.