Fatigue Performance of Laser Additive Manufactured Ti–6Al–4V in Very High Cycle Fatigue Regime up to 109 Cycles

Abstract

Additive manufacturing technologies are in the process of establishing themselves as an alternative production technology to conventional manufacturing, such as casting or milling. Especially laser additive manufacturing (LAM) enables the production of metallic parts with mechanical properties comparable to conventionally manufactured components. Due to the high geometrical freedom in LAM, the technology enables the production of ultra-light weight designs, and therefore gains increasing importance in aircraft and space industry. The high quality standards of these industries demand predictability of material properties for static and dynamic load cases. However, fatigue properties especially in the very high cycle fatigue (VHCF) regime until 10 9 cycles have not been sufficiently determined yet. Therefore, this paper presents an analysis of fatigue properties of laser additive manufactured Ti-6Al-4V under cyclic tension-tension until 10 7 cycles and tension-compression load until 10 9 cycles. For the analysis of laser additive manufactured titanium alloy Ti-6Al-4V, Woehler fatigue tests under tension-tension and tension-compression were carried out in the high cycle and VHCF regime. Specimens in stress-relieved as well as hot-isostatic-pressed conditions were analyzed regarding crack initiation site, mean stress sensitivity, and overall fatigue performance. The determined fatigue properties show values in the range of conventionally manufactured Ti-6Al-4V with particularly good performance for hot-isostatic-pressed additive-manufactured material. For all conditions, the results show no conventional fatigue limit but a constant increase in fatigue life with decreasing loads. No effects of test frequency on life span could be determined. However, independently of testing principle, a shift of crack initiation from surface to internal initiation could be observed with increasing cycles to failure.

Effects of build orientation and thickness of allowance on the fatigue behaviour of 15–5 PH stainless steel manufactured by DMLS

“…Most cracks started from porosities just beneath the surface (80 to 100 μm): an example (the initiation point is indicated by an arrow) is shown in Figure 9 (C). Similar cracks, initiating from different size porosities, located just beneath the surface, have also been observed in, 31 following fatigue cycling of Ti-6Al-4 V.…”

“…However, in these papers, the size effect of the block, from which the part is machined, is studied from the point of view of fracture mechanics, whereas similar investigations dealing with high cycle fatigue properties are still missing. Moreover, the fatigue performance of machined Ti-6Al-4V samples with suitable selection of allowance has also been the topic of the study, 31 which confirms the general interest in this topic. Issues of novelty also arise from considering three orientations with respect to the vertical stacking direction: horizontal, vertical and slanted.…”

Sensitivity of direct metal laser sintering Maraging steel fatigue strength to build orientation and allowance for machining

“…The parts have all been produced by DMLS using a commercial machine by EOS. The fatigue performance of machined Ti-6Al-4V samples with suitable selection of allowance has also been the topic of the study, 26 which confirms the general interest in this point. The here reported results indicate a remarkably slower crack growth rate, following machining from oversized blocks.…”

Very high cycle fatigue and fatigue crack propagation behavior of selective laser melted AlSi12 alloy

“…Ultrasonic fatigue testing systems which can achieve a frequency of 20 kHz can now test a specimen until 10 9 cycles in less than one day. Several studies have suggested a decrease of fatigue strength after 10 7 cycles [20][21][22] it does not exhibit a fatigue limit [22]. A real fatigue limit can only be achieved for bcc metals with ideal structure containing no inhomogenities and when there is no influence of environmental factors like temperature and corrosive environment.…”