Microstructure and mechanical properties of a novel β titanium metallic composite by selective laser melting

Abstract

Selective laser melting (SLM) is an additive manufacturing process in which functional, complex parts are produced by selectively melting consecutive layers of powder with a laser beam. This flexibility enables the exploration of a wide spectrum of possibilities in creating novel alloys or even metal–metal composites with unique microstructures. In this research, Ti6Al4V-ELI powder was mixed with 10 wt.% Mo powder. In contrast to the fully α′ microstructure of Ti6Al4V after SLM, the novel microstructure consists of a β titanium matrix with randomly dispersed pure Mo particles, as observed by light optical microscopy, scanning electron microscopy and X-ray diffraction. Most importantly, the solidification mechanism changes from planar to cellular mode. Microstructures after heat treatment indicate that the β phase is metastable and locate the β transus at ∼900 °C, and tensile properties are equal to or better than conventional β titanium alloys.

Introduction

In selective laser melting (SLM) a high-power laser locally melts successive layers of powder to produce complex-shaped three-dimensional metal parts. The highly localized heat input leads to fast melting and solidification, resulting in a unique microstructure. SLM possesses several advantages over other production techniques, such as a high material use efficiency, a high level of flexibility and near net shape production of geometrically complex structural parts. The process details and applications have been widely reviewed elsewhere (e.g. [1]).

Titanium alloys are among the most widely used alloys in SLM due to their high specific strength and excellent biocompatibility. Much research effort has thus been focused on the influence of process parameters on the microstructure and related mechanical properties of titanium parts produced by SLM. For example, literature reports on SLM of pure Ti [2], Ti6Al7Nb [3], Nitinol [4], [5], other β titanium alloys [6] and, most importantly, both Ti6Al4V [7], [8], [9], [10] and Ti6Al4V-ELI [11], [12]. Concerning Ti6Al4V and TI6AL4V-ELI, all authors report a microstructure consisting of martensitic α′ within columnar prior β grains, the orientation of which depends on the scanning strategy.

Combining different powders and processing the mixture via SLM opens up a whole new and exciting research field. Hua et al. [13] combined elemental powder of Ti, Al and V to produce a bulk Ti6Al4V part. All three elements were mixed homogeneously in the melt and mechanical properties were found to be at least equal to those of parts produced with prealloyed powder. Several authors [14], [15], [16], [17], [18] have reported on the production of several types of in situ metal matrix composites (MMCs), including different titanium matrix composites. More specifically, Collins et al. [19] were the first to mix Ti and Mo powder via additive manufacturing, using laser cladding. By creating compositionally graded structures, the microstructure and hardness for varying Mo content were examined, and it was concluded that the highest hardness of 450 HV is reached by adding 15 wt.% Mo. Recently, Almeida et al. [20] also used laser cladding and performed tensile tests on parts with different Mo content. The optimal combination of a low Young’s modulus (75 GPa) and adequate hardness (240 HV) was obtained by adding 13 wt.% Mo.

In this paper, the solidification, microstructure, mechanical properties and response to heat treatment of SLM parts produced using a mixture of Ti6Al4V-ELI powder with 10 wt.% Mo powder are described. Throughout the text, the novel material will be compared with regular Ti6Al4V-ELI processed by SLM under the exact same conditions. All of the following results or relevant quoted sources refer to Grade 23 Ti6Al4V-ELI and not Grade 5 Ti6AL4V. Consequently, the “-ELI” suffix will be dropped for clarity of the text.