Welding, Joining and Additive Manufacturing International Conference 2015

Selective laser melting (SLM) is establishing itself for manufacturing of functional components from powder material. Titanium and aluminum alloys are in high demand by aerospace industry to be employed for specific applications. For the SLM process to be qualified for aerospace applications, the mechanical performance of the SLM parts needs to be determined with reliability. Quasi-static mechanical properties of many SLM-manufactured titanium and aluminum alloys have been determined and are at par with those of wrought materials. However, for dynamic applications, fatigue performance is very critical and needs to be understood in depth for reliable designs. This study focuses on investigations on the fatigue and fracture behavior of titanium Ti-6Al-4V and aluminum Al-4047 alloys. A novel fatigue testing method - Rapid Fatigue Performance Identification method (RAPID) - has been developed to determine Woehler curves for different process configurations. Additive nature of the process usually results in a higher surface roughness (Ra 10-15 µm) which is detrimental for fatigue performance. Post processing techniques, e.g. polishing, sand blasting and hot isostatic processing, have been applied to improve the fatigue performance. Current aerospace standards require that the fatigue life needs to be determined until 109 load cycles compared to the previously accepted threshold of 107 cycles. Therefore, fatigue investigations have been carried out from high cycle fatigue (HCF) to very high cycle fatigue (VHCF) regions. VHCF was investigated using ultrasonic fatigue testing system. Fatigue crack growth studies have also been carried out to determine the influence of process-induced defects on the fracture behavior, which is an important parameter for damage tolerant designs. Results show that the fatigue performance of SLM-manufactured parts in the as-built condition is extremely impaired due to rough surface. However, it can be improved by appropriate post processing to be not only at par with that of wrought material, but a significant further increase in fatigue limit can be achieved e.g. compared to the fatigue limit of 400 MPa for conventionally manufactured Ti-6Al-4V at 107 cycles, a fatigue limit of about 500 MPa was achieved at 109 cycles for SLM manufactured alloy. The phenomenon is attributed to the process-specific microstructure. Therefore, SLM process has the capability to be qualified for aerospace applications.