A low-volume, low-cost method for Ti-Nb alloys in the selective laser melting process

Alloying titanium (Ti) with niobium (Nb) offers the possibility to develop a low-stiffness, high- strength Ti alloy whose properties can be tuned via its composition. The changes in mechanical properties are the result of the presence of additional phases induced by the Nb addition. These alloys are yet to see widespread use in the Selective Laser Melting (SLM) process, a technique that allows the simplified fabrication of complex geometries but requires a tight control of the processing parameters and the use of powder with specific characteristics. The established methodologies for the fabrication of powders suitable for the SLM process are based on powder atomisation. These processes require high volume powder production to be financially viable and, consequently are unsuited for small batches used for R&D or in bespoke alloys, (e.g., the Ti-Nb alloys used in this study). To develop a low volume methodology, this work focused on the use of in-situ alloying, heat treatment and ball milled powders, to develop homogenous alloys of the desired composition. These methods were assessed using SLM fabricated samples for microstructural (Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD)), Differential Scanning Calorimetry (DSC) and mechanical (compression and hardness) analysis.

In the first instance it was found that in-situ alloying of elemental powders was able to fabricate alloys with mechanical properties similar to those from more established processes. The addition of Nb content, up to 30 wt.%, led to variations in the Young’s modulus and an increase in the yield strength, linked to the microstructure induced. The use of 20 wt.% Nb (Ti-20Nb) with an SLM volumetric energy density of 230 J/mm3 provided a lower stiffness of 65 GPa, compared to 100 GPa in a pure Ti alloy, and a yield strength of 769 MPa. In addition, this alloy measured a hardness of 276 HV1. However, this process on its own proved unable to produce fully homogenised alloys and as a result was found to be sensitive to variations in processing parameters.

Post fabrication heat treatment followed by furnace cooling was found to aid the homogenisation process and led to an increase in the stiffness from 60-65 GPa to 83-84 GPa, with the diffusion of non-equilibrium phases formed in the SLM fabrication. The use of quenching after heat treatment decreased the stiffness, with a reduction of 31- 40 GPa compared to the furnace cooled alloys, corresponding to changes in the phase compositions present. However, the heat treatment led to the removal of the fine grain structure that resulted from the SLM process. This caused a decrease in the yield strength of up to 345 MPa and a decrease in hardness up to 63 HV1.

Ball milling was introduced in the pre-treatment stage of the powders and proved to be effective at developing a powder that was suitable for the SLM process and consequently capable of fabricating homogenised Ti-Nb alloys. Furthermore, this work addressed the use of Process Control Agents (PCA) that are recommended for the ball milling of powders. The established practice of using an organic PCA, such as stearic acid, led to the introduction of interstitial phases during SLM fabrication with undesirable effects on the mechanical properties. The use of stearic acid led to an increase of hardness by 63 HV1 compared to the alloys milled with the inorganic PCAs (calcium (Ca) and tin (Sn)) that were also investigated. This work found that the use of Ca, was effective as a PCA, with the resultant powder physical properties appropriate for the SLM process, and capable of producing homogeneous Ti-Nb alloys measuring a hardness between 333 - 327 HV1. However, the porosity (2.1%) of the Ti-20Nb alloys fabricated with 110 J/mm3 energy density from the powder milled with Ca was higher than alloys fabricated from powder milled with stearic acid (0.2%). The powder milled with Sn as a PCA performed less well in terms of the flowability, apparent density and homogeneity, compared to powder milled with stearic acid or Ca. As a result, Sn was found not to be effective as a PCA with chemical inhomogeneity and a porosity of 2.0% present after fabrication with 110 J/mm3 energy density. This work shows that homogenised Ti-Nb alloys can be fabricated using ball milling to pre-treat elemental powders, circumventing powder atomisation, with a combination of adjustments to SLM processing parameters and Nb content, followed by heat treatment as post-processing when required.

The results of this work provide a guideline to the development of a method for the production of low-volume bespoke Ti-Nb alloys using the SLM process. It can be anticipated that other alloy compositions could also be produced in the same fashion, opening horizons to mass-customisation of alloys with compositional and geometrical properties matched to the requirements of the end user or application.