PSI - Issue 56

S Anand Kumar et al. / Procedia Structural Integrity 56 (2024) 65–70 / Structural Integrity Procedia 00 (2019) 000–000

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is best realized when applied to metals which are difficult to process through other techniques [3]. Any complex shaped metallic components can be produced by the LPBF process on the LPBF equipment through 3D CAD data. It provides a better alternative manufacturing route than traditional manufacturing techniques in aspects like high flexibility, high material use efficiency and near-net-shape geometries [4]. LPBF has been widely used in medical, military, aerospace, and automobile manufacturing. LPBF represents the advantageous process. However, some problems with large thermal gradients exist due to considerable heat input and short interaction. The influence of various parameters is still a matter of interest for research for its biological and biomechanical compatibility. Titanium alloys comprising high fracture toughness, high strength-to-weight ratio and superior mechanical properties have been widely used in the biomedical and aerospace industries [5]. Ti6Al4V using conventional processing methods limits its extensive application due to poor machinability and high cost. Moreover, high energy consumption and material wastage occur during the production of titanium alloys through conventional methods. Consequently, researchers have been studying the non-conventional processing of titanium alloys through AM route. AM process through LPBF techniques offers more excellent benefits when compared to the conventional process. Thijs et al. studied the microstructural evolution of Ti6Al4V during the LPBF process. Due to the higher temperature gradient during the LPBF process epitaxial growth, elongated grains emerge and acicular martensitic phase formed in Ti6Al4V alloy [6]. Losertová et. al. [7] focused on the properties of porous Ti6Al4V specimens processed by selective laser melting. The material was tested in tension and compression with and without heat treatment. The as-built stage consisted of prior- β grains filled with acicular α ΄martensite and displayed high yield strength but limited ductility. In similar work by Naeem Eshawish et al. [8], the microstructure, even after stress relieving (SR) at 704º C for two h, shows fine α’ martensitic phase. Most research and development on Ti-6Al-4V fabricated by LPBF has focused on the microstructure of the as built samples and the adjustment of the process parameters to increase the quality of products [9-10]. This research studies the influence of heat treatments i.e. SR, on the microstructure and hardness on electron beam (EB) welded joint of Ti6Al4V alloy processed by selective laser melting (LPBF). It is reported that the acicular martensite α’ phase is formed during the LPBF process which changes to a lamellar mixture of α+β after heat treatment below the critical temperature (T0 at approximately 893ºC) [11]. In the present study, the welding process was optimized and welds were analyzed to study the microstructure and its effects on the mechanical properties.

Nomenclature T 0

critical temperature martensitic phase

α’

α β

hcp phase bcc phase

1. 2. Material and methods The welding was carried out on two state-of-condition samples, i.e. (i) as-printed (AP) Ti6Al4V parts were welded (AP+AP), and (ii) stress relieving (SR) Ti6Al4V LPBF parts were welded with as-printed LPBF processed parts (SR+AP). The LPBF process parameters included a laser power range of 170 to 200W, a scanning speed of 500 to 1250mm/s, a controlled layer thickness of 0.03 mm, a hatch distance of 0.1mm, a laser beam diameter of 0.1mm, a beam offset of 0.015mm, a stripe width of 5mm and a 67° rotation scanning strategy. The thickness of the as printed sample is 10 x 30 x 5 mm 3 . The stress relieving heat treatment was performed by heating to 650ºC and holding for three h, followed by furnace cooling in an inert atmosphere by maintaining Argon gas supply. The preliminary edge preparations and cleaning with acetone were performed before welding. The welding was performed using a 6 kW, 60- kV electron beam welding (EBW) setup. A circular beam oscillation at 200 Hz having a diameter of 100 μm was used and surface focus condition was maintained during welding. The welding speed of 1500 mm/min with a beam current of 20 mA was employed. The power density was in the range of 1.525 × 10 5 W/mm². The weld samples shown in Fig.1 were analyzed and compared regarding weld microstructures, bead profile, microhardness