Heat treatment of electron beam melted (EBM) Ti-6Al-4V: microstructure to mechanical property correlations

Abstract

Purpose The paper presents a wide range of post processing heat treatment cycles performed to Electron Beam Melted (EBM) Ti6Al4V alloy and establishes correlations of heat treat process to microstructure and mechanical property (microhardness). The research also identifies the optimal heat treatment to obtain the best microstructure and mechanical properties (hardness and tensile). Design/methodology/approach Rectangular bars fabricated using EBM was used to study the different heat treatment cycles. A variety of heat treatments from sub ß-transus, super ß-transus, near ß-transus and solution aircool plus ageing were designed. After the heat treatment process, the samples were analysed for, α lath width, prior ß grain size, microhardness and nanohardness. Tensile tests were done for the heat treated samples showing most refined α lath structure with uniform globular grains. Findings A clear correlation was observed between α lath width and the microhardness values. The solution aircooled plus aged samples exhibited the best refinement in α-ß morphology with uniform equiaxed grains. The tensile properties of the solution aircooled plus aged samples were comparable to that of the EBM printed samples and better than ASTMF1472 specifications. Originality/value There is hardly any prior work related to post processing heat treatment of EBM built Ti6Al4V other than HIP treatments. The variety of heat treatment cycles and its influence in microstructure and properties, studied in this research, gives a clear understanding on how to tailor final microstructures and select the optimal heat treatment process.

On the Effect of Electron Beam Melted Ti6Al4V Part Orientations during Milling

“…The microstructures show differences between the top and side faces due to the variations in thermal gradients that existed along these faces. Overall, the as-printed EBM samples exhibited superior strength compared to those of the wrought [32] and heat-treated samples [54] due to the refined lamellar α and β microstructure.…”

Microstructure and Mechanical Properties of Ti-6Al-4V Fabricated by Electron Beam Melting

“…In general, the microstructure of EBM Ti-6Al-4V alloys is diversified and the mechanical properties vary greatly. Fine needle-like α phase separated by the β phase was found to exist in EBM Ti-6Al-4V parts [5][6][7][8], while fine lamellar α/β dual-phase microstructure was a typical microstructure of EBM parts [4,9,10]. Sinan et al [8] found martensite in the top area of the samples with heights of 2 and 5 mm, but the martensite in the top area of the samples with a height of 18 mm was not obvious.…”

“…Hrabe et al [20] demonstrated that vertically oriented EBM Ti-6Al-4V parts showed a 30% lower elongation compared to horizontally oriented parts. Different heat treatments were performed on Ti-6Al-4V by Galarraga et al [22] and Raghavan et al [5] to study the effects of heat treatments on the unique microstructure formed during the EBM fabrication process. When different heat treatment cycles are applied to EBM Ti-6Al-4V materials, the final microstructure can be tailored, and the mechanical properties can also be improved.…”

Thermomechanical Simulations of Residual Stresses and Distortion in Electron Beam Melting with Experimental Validation for Ti-6Al-4V

“…For instance, in Figure 23c, it can be seen that layer-2 receives several heat cycles due to electron beam scans on subsequent layers as compared to layer-8, which only receives 2 heat cycles (see Figure 23d). Usually, the higher heat accumulation and rapid extraction of heat favor the higher concentration of the beta phase and vice versa for the alpha phase [35,36]. Therefore, in Figure 23a, more β phase and wider α grains could be observed.…”