Investigation of the high strain rate behaviour and impact toughness of Ti6Al4V (ELI) parts built by the EOS M280 DMLS System with standard process parameters : as-built and stress relieved

Thesis (Master of Engineering in Mechanical Engineering) -- Central University of Technology, Free State, 2018The high strain rate behaviour and the impact toughness of Direct Metal Laser Sintering (DMLS) produced Ti6Al4V (ELI) parts were investigated. The as-built (AB) specimens were also taken through stress-relieving heat treatment and are hereinafter referred to as SR samples or specimens. The high strain rate deformation of the two forms of the alloy (AB and SR) was studied using the Split Hopkinson Pressure Bar (SHPB) test, in compression and tension. Two high strain rates of 400 s-1 and 700 s-1 in compression and 250 s-1 and 360 s-1 in tension were used in these tests. The test results were used to investigate the relative strain rate dependence of flow stresses and fracture strain. Microstructural analysis was carried out to study the dominant fracture mechanisms on the deformed surfaces of the compression and tensile test samples using optical and scanning electron microscopy. Measurements of Vickers microhardness for the test samples were taken before and after the compression and tensile tests. The results of the high rate tensile and compression tests showed significant strain rate sensitivity in both compression and tension. Moreover, the rate sensitivity was seen to be relatively higher in the SR samples than in the AB samples. Microstructural analysis of the tested specimens showed that adiabatic shear bands dominated the deformed surfaces in the compression test samples. Multiple cracks that were more pronounced on the surfaces of the tested specimens were seen to initiate randomly on the deformed surfaces of the tensile test samples. The Vickers microhardness of the tested samples were observed to be higher for loaded specimens than for the unloaded specimens. The impact properties of the AB and SR DMLS Ti6Al4V (ELI) specimens were studied using an instrumented Charpy impact tester. The transition curves for the absorbed energy and lateral expansion were obtained by performing the experiments in the temperature range 130 ºC to 250 ºC. The effect of the orientation of the v-notch on the standard test specimen with relation to the base plate of the DMLS machine was investigated. Furthermore, the effects of stress-relieving heat treatment on the notch toughness of DMLS Ti6Al4V (ELI) specimens was also studied. The analysis of the fracture surfaces resulting from the tests at various temperatures was done using a scanning electron microscope (SEM). The values of absorbed energy and the lateral expansion of the specimens that were determined from this series of tests indicated that specimens built with the v-notch facing the base plate of the DMLS machine had better impact toughness and notch ductility in comparison to those built with the v-notch facing away from the base plate over most of the temperature range of testing. Besides improving toughness, stress-relieving heat treatment gave rise to a shift of the ductile-to-brittle transition temperatures (DBTT) to lower values. The study further established that the DMLS Ti6Al4V (ELI) retains appreciable notch toughness even at sub-zero temperatures