DIRECTED ENERGY DEPOSITION PROCESSING OF ALPHA-BETA, NEAR-ALPHA, AND BETA TITANIUM ALLOYS

Abstract

Directed energy deposition (DED), or laser powder fed additive manufacturing (LPF-AM) is a process whereby a conical stream of metal powder is concentrated at the focal point of a laser, providing sufficient energy to create a stable melt pool. This melt pool can then be strategically deposited onto a substrate, forming layers. By slicing 3D parts into a series of 2D shapes, these layers can be stacked onto one another, creating the desired geometry. This additive manufacturing (AM) technique offers significant advantages over subtractive machining with regards to limiting material waste and the ability to produce complex geometries not achievable by traditional machining or forming processes. Compared to other fusion-based AM processes, DED offers faster material deposition rates, resulting in faster build times. Additionally, DED allows for functionally graded materials, near net-shape part repair, laser cladding and hybrid additive/subtractive manufacturing.

In the context of titanium alloys, DED has been mainly utilized for processing the industrial workhorse, Ti-6Al-4V. As such, the initial research aimed to compare the manufacturing capabilities of several vendors of DED equipment to determine the resultant variability in mechanical and microstructural properties and establish a baseline for future works. A single lot of AncorTi-64 was sent to several OEMs of DED equipment and their builds were examined on the basis of dimensional accuracy, hardness, density, tensile properties (pulling perpendicular as well as parallel to the build direction), optical micrography, scanning electron microscopy, and differential scanning calorimetry. Ti-64 builds showed considerable variation in dimensional accuracy, density, and mechanical properties between vendors. Tensile samples built horizontally showed near-wrought properties from all vendors, indicating the importance of part-specific build orientation.

Two other industrially relevant titanium alloys were chosen to further understand the feasibility of processing titanium using DED. These were the near-alpha Ti-6242 and the beta alloy Beta-21s. In both instances, process parameters were varied in a statistically driven manner to achieve fully dense samples. The first design of experiments (DOE) varied laser power and traverse rate, measuring density as the dependent variable. Based on the results of this smaller scale experiment, a larger experiment was prepared varying laser power, traverse rate, hatch spacing, layer height and powder feed rate. The optimal process parameters were then used to create tensile bars according to ASTM E-8 and heat treated using industrial heat treatments to obtain mechanical properties. Both Ti-6242 and Beta-21s were successfully processed, achieving >99.9% of theoretical density with appropriate processing parameters. DED specimen of each alloy also exhibited near-wrought mechanical properties as well as heat treatment responses consistent with their underlying fundamental metallurgy.