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

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.

β grain refinement during solidification of Ti-6Al-4V in Wire-Arc Additive Manufacturing (WAAM)

“…Strongly partitioning elements are segregated into the liquid, slowing the growth of the columnar front while also promoting nucleation by increasing the constitutional undercooling [15]. Elements which have previously been investigated for this purpose in Ti alloys fabricated using AM technologies include Be [16], B [17], La [18], Fe or Ni [19]. Y additions to Ti-64 have resulted in an order of magnitude refinement of the β grain size, attributed to both the solutal effect of Y on growth restriction as well as the potential heterogeneous nucleation of β on yttria during solidification [20].…”

Insights into Phase Evolution and Mechanical Behavior of the Eutectoid Ti–6.4(wt%)Ni Alloy Modified with Fe and Cr

“…and anisotropy. Zhang et al [16] used Cu additions, but similar effects have been demonstrated using Fe, [17][18][19][20] Ni, [19,21,22] and Co. [23] The transition metals mentioned before belong to the group of β-eutectoid-forming elements when added to Ti. [1,24] Upon heat treatment below the eutectoid temperature, the β-phase decomposes into α phase and various intermetallic phases with different stoichiometry and crystallography.…”

“…Recently, several works explored the introduction of transition metals into titanium alloys. [16][17][18][19][20][21][22][23] Zhang et al [16] suggested that alloying elements with pronounced partitioning behavior into the remaining melt during solidification and a steep liquidus curve provide a high growth restriction factor, resulting in grain refinement and a strong reduction of texture DOI: 10.1002/adem.202300177 Titanium alloys gain increasing importance in industry due to the expansion of advanced manufacturing technologies such as additive manufacturing. Conventional titanium alloys processed by such technologies suffer from formation of large primary grains and anisotropy of mechanical properties.…”

“…[24] As a result of the created microstructure, varying effects on the mechanical properties are observed. Due to the potential for grain refinement upon solidification [19,21] and the option to form intermetallic phases (in reasonable times, that is, by using an active eutectoid-forming element), [26] Ni was chosen as major alloying element in the present study.…”

“…Recently, several works explored the introduction of transition metals into titanium alloys. [ 16–23 ] Zhang et al [ 16 ] suggested that alloying elements with pronounced partitioning behavior into the remaining melt during solidification and a steep liquidus curve provide a high growth restriction factor, resulting in grain refinement and a strong reduction of texture and anisotropy. Zhang et al [ 16 ] used Cu additions, but similar effects have been demonstrated using Fe, [ 17–20 ] Ni, [ 19,21,22 ] and Co. [ 23 ]…”