Elevated temperature- and temperature dependence of the quasi-static, uniaxial tensile behaviour of additively manufactured ti6al4v(eli)

Abstract

The use of additive manufacturing (AM) techniques has increased in the past years. These technologies have a significant potential to replace traditional manufacturing methods such as casting, forging, moulding, machining, and joining. Laser powder bed fusion (LPBF) technologies are widely used in AM to produce complicated structures consisting of interconnected parts with minimal waste of material. Additively manufactured metallic parts exhibit features such as surface roughness, porosity, and residual stress that affect the mechanical properties of the parts and determine the sites where fatigue failure is likely to initiate. Post-heat treatment processes such as stress relief and high-temperature annealing are employed to improve the microstructure and, therefore, mechanical properties of additively manufactured metallic parts. The Ti6Al4V alloy is the most popular titanium alloy due to its appealing combination of characteristics such as high strength and lightweight. The alloy can be used in a wide range of applications in the aerospace, automotive, and biomedical sectors at both room temperature and high temperatures. The Ti6Al4V alloy is used in aircraft for various parts, including window frames, compressor blades, and fan disks. In the biomedical industry, the alloy is used for bone implants and dental prostheses. It finds application in the automotive industry in the manufacture of parts such as outlet valves, suspension springs, and wheels. Additively manufactured Ti6Al4V occurs in various microstructures, including martensite, lamellar, equiaxed, bimodal, and the Widmanstätten microstructures. It has been almost two centuries since fatigue was first recognised in metals subjected to repetitive or fluctuating stress due to applied load. Until today, fatigue has remained a big problem in the engineering world, occurring with little to no warning. Failure due to fatigue in engineering structures has been thoroughly studied, and engineers have developed techniques to mitigate it, but its prevention is never assured. However, some measures have been developed to study and manage fatigue. Metals and their alloys can display different types of fracture surfaces depending on the prevailing temperature, microstructure, stress state, and methods of manufacture. Consequently, further research on the fatigue behaviour of metallics is still required. Testing at high temperatures can affect the fatigue properties of metallics. Hence, metallics that are used at high temperatures need a combination of microstructural stability and mechanical strength. As temperature rises, microstructural changes take place in metals, which affect their mechanical properties. These include strength, ductility, and stiffness. The fatigue behaviour of metallics is dependent on the microstructure and its mechanical properties. Metallics undergo a decrease in fatigue strength and fatigue resistance with increased temperature, which leads to a reduction in their fatigue life. Therefore, studies on the effect of temperature on the fatigue behaviour of the alloy are necessary to guide their use in industry. In this study, uniaxial tensile test and fatigue test specimens were manufactured using a DMLS EOSINT M290. The specimens printed were exposed to stress relief and high-temperature annealing heat treatment. Uniaxial tensile tests and fatigue tension-tension tests were performed at various temperatures. After both uniaxial tensile testing and fatigue testing, scanning electron microscopy (SEM) was used to study the characteristics of failure on the fracture surfaces and investigate the causes of failure. An optical microscope was used to establish the changes from the edge of the fracture surface away along the gauge length. Vickers microhardness tests were performed to measure the hardness of the alloy. The Ti6Al4V alloys used in the aircraft industry undergo temperature variations while in operation, which might have an impact on their quasi-static behaviour. Therefore, this study aimed to investigate the effect of elevated temperatures on the quasi-static behaviour of additively manufactured titanium Ti6Al4V(ELI) alloy. Considerations of this effect on the fatigue behaviour of the alloy led to the extension of the material in the literature review to fatigue. Moreover, preliminary fatigue testing at room temperature and two elevated temperatures was conducted, and the results were analysed to lay the groundwork for future fatigue testing at elevated temperatures. The results showed that a test temperature of 475 °C significantly influenced the mechanical properties, and a temperature below 325 °C had little effect on the fatigue behaviour of the alloy. Temperature changes showed little influence on the fracture characteristics and the microstructures of the alloy.