Use of Alloying to Effect an Equiaxed Microstructure in Additive Manufacturing and Subsequent Heat Treatment of High-Strength Titanium Alloys

Brian A Welk,Zachary Kloenne,Hamish L Fraser,Stephen Fox,Nevin Taylor,Kevin J Chaput

doi:10.1007/s11661-021-06475-3

Brian A Welk, Zachary Kloenne + Show 4 more

Open Access

https://doi.org/10.1007/s11661-021-06475-3

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Abstract

This paper addresses the use of alloying additions to titanium alloys for additive manufacturing (AM) with the specific objective of producing equiaxed microstructures. The additions are among those that increase freezing ranges such that significant solutal undercooling results when combined with the rapid cooling rates associated with AM, and so be effective in inducing a columnar-to-equiaxed transition (CET). Firstly, computational thermodynamics has been used to provide a simple graphical means of predicting these additions; this method has been used to explore additions of Ni and Fe to the alloy Ti–6Al–4V (Ti64). Secondly, an experimental means of determining the minimum concentration of these alloying elements required to effect the CET has been developed involving gradient builds. Thirdly, it has been found that additions of Fe to Ti64 cause the alloy to change from an α/β Ti alloy to being a metastable β-Ti alloy, whereas additions of Ni do not produce the same result. This change in type of Ti alloy results in a marked difference in the development of microstructures of these compositionally modified alloys using heat treatments. Finally, hardness measurements have been used to provide a preliminary assessment of the mechanical response of these modified alloys.

Highlights

THERE has been considerable recent interest in the literature regarding the application of Additive Manufacturing (AM) to the production of net-shape components of metallic alloys.[1,2]
Among various structural metallic materials, Ti alloys have received much attention, because of the attractive aspects of this processing route, and because the as-printed high-performance Ti alloys processed by AM tend to be free of cracks.[3]
It appears that alloying additions to Ti alloys, which increase the alloys’ freezing ranges, may be an effective method of inducing a columnar-to-equiaxed transition (CET) during AM powder deposition, where it is the significant degree of undercooling prior to nucleation of b grains that can be achieved because of the large freezing range combined with the rapid cooling rates associated with AM

Summary

Introduction

THERE has been considerable recent interest in the literature regarding the application of Additive Manufacturing (AM) to the production of (near) net-shape components of metallic alloys.[1,2] Among various structural metallic materials, Ti alloys have received much attention, because of the attractive aspects of this processing route, and because the as-printed high-performance Ti alloys processed by AM tend to be free of cracks.[3]. The second approach makes use of additions of inoculants to control solidification, such as those used in the optimization of microstructure in high-strength Al alloys.[7] This approach has been demonstrated in Ti alloys with the addition of boron as well as lanthanum, an issue of formation of intermetallic compounds arises which may limit mechanical properties, in fatigue.[8,9] The third approach uses alloying additions to effect a change of solidification mode An example of this method involves peritectic Ti alloys for 3D printing.[10] In this example, Ti–La and Ti–Fe–La alloys were printed, and the resulting microstructures consist of elongated and tortuous a grains with some refined equiaxed grains. The role of the dilute additions is to increase the freezing range of Ti alloys in order to promote

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