Atomic layer deposition of Al2O3 to Ti–6Al–4V feedstock improves powder oxidation resistance and tensile strength in additively manufactured parts

Abstract

With the ever-increasing interest into additive manufacturing (AM) via laser powder bed fusion (PBF-L) of Ti–6Al–4V , there have been numerous efforts to introduce different coatings onto the metal powder feedstock in order to achieve benefits in the PBF-L process. Whether for the purpose of dispersion strengthening, enhanced flowability, or grain refinement, there are several dry coating techniques that can be used to introduce a coating onto the metal powder. In the present investigation, we leverage atomic layer deposition (ALD), a thin-film coating technique, to uniformly coat a continuous layer of Al${}_2$O${}_3$ onto a batch of Grade 23 Ti–6Al–4V powder. This ALD-coated feedstock was then used in PBF-L to build specimens for analysis of the microstructure and mechanical properties in the as-built and hot isostatic pressing (HIP) treated state. The ALD-coated feedstock displayed greater Hall flowability and oxidation resistance when measured via thermogravimetric analysis in air, with no significant changes to microstructure or bulk morphological features. The microstructure of the as-built and HIPed material stemming from the ALD-coated powder batch was indistinguishable from the material made from virgin powder feedstock, indicating that the ALD Al${}_2$O${}_3$ layer dissolved into the melt pool and went into solution. The ALD condition had an approximate 80 MPa increase in tensile strength in the as-built condition, and an approximate 70 MPa increase in strength in the HIPed condition due to solid solution strengthening from the increased oxygen content of the ALD coating. The ALD HIPed condition, while somewhat lower in elongation and reduction of area relative to the virgin material after HIP, remained within common material specification requirements for PBF Ti–6Al–4V . While there was an increase in measured oxygen content after the ALD process, this work serves as a proof-of-concept for pre-treating Ti–6Al–4V feedstocks with ALD methodology to achieve benefits in flowability, oxidation resistance, and solid solution strengthening.