Metal additive manufacturing of Ti6Al4V from blended elemental powders

Abstract

Metal Additive Manufacturing (MAM) processes, such as the Direct Metal Laser Sintering (DMLS) process, have conventionally employed pre-alloyed (PA) metal powder to produce parts with excellent mechanical properties. In a PA powder feedstock, each individual powder particle has the target alloy composition that is required in the final part, therefore producing a part with a good elemental homogeneity, which in turn results in a part that possesses the best mechanical properties that the target alloy has to offer. The production of these PA powders involves a chain of processes, including conventional and modern atomisation processes, which often result in expensive PA powders. Since the nature of the DMLS process entails re-melting these PA powders to solidify them again into the bulk of the part being built, an alternative approach is the use of blended elemental (BE) powders. The term BE can be used in the context of any application wherein a powder blend consisting of purely elemental powders or of elemental and pre-alloyed (master alloy) powders is employed as feedstock. In this approach, in situ alloying of BE powders during DMLS occurs with the consequent elimination of the pre-alloying process, thus resulting in a more cost-effective production route. The Ti6Al4V alloy is widely known as the workhorse of the titanium industry due to its unique combination of properties, which are high strength, lightweight and corrosion resistance [1]. This alloy is mostly used in medical and aerospace applications where its high cost can be justified by the benefits that the alloy offers. The production of parts in the DMLS process using PA Ti6Al4V powder as feedstock results in expensive parts, which makes them unjustifiable for use in common engineering applications. In comparison, the production of parts in the DMLS process through BE Ti6Al4V entails obtaining elemental powders of titanium (Ti), aluminium (Al) and vanadium (V), blending them into the correct ratio and then using them as feedstock for DMLS. This route has the potential to lower the cost of Ti6Al4V parts (and those of other Ti alloys) produced through the DMLS process. Therefore, the aim of this study was to determine the feasibility of using the BE approach to produce the Ti6Al4V alloy through the DMLS process. To achieve this aim, a gradual approach, which entails first producing the Ti6Al4V alloy from the BE approach using a powder blend consisting of elemental Ti and Al-V master-alloy (MA) powders, was employed. This gradual approach was used in an attempt to reduce the complexity of the interaction between the DMLS system and the elemental powder blend to allow for insight into further optimisation and customisation of the DMLS process parameters and BE powders, thus paving the way for the production of the Ti6Al4V from the purely elemental powder blend. A hierarchal approach was employed to determine the feasibility of using these powder blends to produce high-quality Ti6Al4V parts [2]. Firstly, single tracks were produced to determine the process parameters that could be used to produce optimum single tracks (laser power, scanning speed and layer thickness). Secondly, these process parameters were then used to produce single and double layers to determine the optimum hatch distance. Lastly, once a full set of process parameters had been obtained from the range of process parameters investigated, 3D parts were then produced. These 3D parts were analysed to determine the type of microstructure produced and their mechanical properties (tensile tests). The results obtained from the two powder blends used in this study were compared with each other and with the known test values obtained from the wrought and DMLS PA production routes. It was found that the as-built (AB) microstructures of the produced Ti6Al4V parts were similar to those of Ti6Al4V parts produced from the PA route. The mechanical properties of the parts produced from the blended powders were generally superior to the mechanical properties specified in the ASTM F2924-14 standard, which is intended to ensure the production of additively manufactured Ti6Al4V parts with mechanical properties that are comparable to the forged and wrought Ti6Al4V parts. The use of the Al-V MA powder in the powder blend generally offered a greater compositional control during in-situ alloying, thus yielding parts with a chemical composition that was closer to the intended chemical composition in comparison to that of the parts produced from the purely elemental powder blend. The results obtained suggest that in-situ alloying in the DMLS process can be employed to produce high-quality Ti6Al4V parts from powder blends consisting of a combination of elemental and MA powders or purely elemental powders.