Microstructure and mechanical behaviour of Ti―6Al―7Nb alloy produced by selective laser melting

Selective laser melting (SLM) is an advanced manufacturing technology providing alternative method of producing complex components directly from 3D computer models. The purpose of this work is to determine the influence of the SLM manufacturing strategy on mechanical properties and microstructure of the as-built Ti―Al―Nb alloy. Specimens of Ti―6Al―7Nb were produced in three versions of the specimen axis orientation with respect to its build direction. Mechanical characteristics of the alloy were determined by tensile and compression testing, as well as hardness measurements. Microstructures were characterised utilising optical microscopy, scanning electron microscopy and X-ray diffraction analysis. It was found that the as-built Ti―6Al―7Nb alloy has microstructure of α′ martensite hardened by dispersive precipitates of the second phase, which results in higher tensile and compressive strengths, but lower ductility in comparison to those of an alloy manufactured by conventional methods. The layered microstructure of the material gives it a significant anisotropy of Young's modulus, moderate anisotropy of mechanical properties, but strong anisotropy of sensitivity to the build porosity. The paper develops understanding of the relationships between the strategy of layered manufacturing of the Ti―6Al―7Nb alloy and its microstructural and mechanical characteristics. This is important for future applications of the SLM technology for producing Ti―6Al―7Nb parts, e.g. the custom medical implants.

Research Highlights
► The as-built SLM Ti―6Al―7Nb alloy has a layered microstructure of α′ martensite.
► Size and orientation of the α′ plates are determined by the SLM building strategy.
► The layered microstructure gives the alloy an anisotropy of Young's modulus.
► The building strategy influences anisotropy of material sensitivity to the built porosity.