Abstract
AbstractTitanium as a high-performance material offers great potential for a wide range of applications with different aspects like lightweight-constructions, biocompatibility as well as design and is used in various industrial areas like the transportation sector or in medical and sports engineering. To exploit the full potential of titanium, there is still a need for innovative and efficient joining techniques compared to conventional methods. The process of friction stir welding as a solid-state joining method is able to produce high-quality similar joints of titanium. Nevertheless, there are still some challenging aspects when friction stir welding titanium like the formation of oxide layers that affect the mechanical properties especially in the field of fatigue negatively. The superposition of power ultrasound on the friction stir welding process can address this issue. Therefore, the hybrid process of ultrasound enhanced friction stir welding was applied on similar Ti6Al4V/Ti6Al4V joints in a butt joint configuration to reduce the amount of oxide layers and improve the fatigue lifetime of the joints. For the joint configuration, a positive impact of the power ultrasound could be identified by the breakup of oxide layers and an increase in the tensile strength of 16%. Also stepwise load increase tests revealed a higher estimated fatigue strength for ultrasound enhanced friction stir welded specimen in this butt joint configuration.
Highlights
Global society's demand for increasing mobility is countered by continuously rising energy costs
The temperature curve of the Friction stir welding (FSW) process reaches higher temperature values with an average temperature of about 950° C and runs more homogenously than the ultrasound enhanced friction stir welding (USE-FSW) process, which shows an average temperature of about 915° C. This lower process temperature development for the USE-FSW could be attributed to the acousto-plastic effect, which could have caused a softening of the titanium and therewith a change in the friction conditions and is in accordance with the literature [24]
Both curves are located in the diffusion-controlled transformation zone for Ti6Al4V extending from 800° C–995° C, what leads to the assumption that lamellar α- and β-titan will be present in the resulting microstructure for both joints with varying volume content
Summary
Global society's demand for increasing mobility is countered by continuously rising energy costs. Additional investigations on friction stir welding of similar Ti6Al4V/Ti6Al4V joints often proved the presence of oxide layers in the joining zone [7, 9]. With respect to an improved stirring as well as a reduction of oxide layers for similar joints, hybrid joining processes such as ultrasound enhanced friction stir welding (USE-FSW) have been developed [13, 14]. The occurrence of the acousto-plastic effect can reduce the yield stress of the joining partners [16] The combination of these two mechanisms of action results in numerous advantages, including the plasticization of a larger volume of material, more homogeneous stirring of the joining zone, better through-welding and the reduction or avoidance of intermetallic phases (dissimilar composites) and oxide layers (similar composites) [17–21]. There is no literature on the application of power ultrasound enhanced friction stir welding on similar titanium/titanium joints and subsequent investigations on the process behavior as well as microstructural and mechanical properties
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