Fatigue Crack Initiation Behaviour of Additive Manufactured Ti-6Al-4V
T.P.A. Cheylus (TU Delft - Aerospace Engineering)
Calvin D. Rans – Mentor
Santiago Garcia Espallargas – Graduation committee member
S. Van der ZWAAG – Coach
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Abstract
This master thesis investigates the effect of surface morphology of Selectively Laser Melted Ti-6Al-4V on fatigue performance. The literature study performed here iden- tifies roughness as one of three major causes for the decreased performance of the additive manufactured alloys (along side porosity and residual stresses). In order to eliminate the two other causes for poor fatigue, and isolate the surface roughness, a hot isostatic pressing (HIP) treatment was performed on every sample. Luckily, a first set of tests proved that chemical treatments, specifically electropolishing and plasma- polishing, greatly improve the surface quality. A second series tests (rotating bending fatigue tests) concluded that, although specific roughness parameter such as Ra reach lows of 0.2μ there was no statistically sound relation between fatigue performance and an improved surface roughness. Upon further investigation of the specimens, it was discovered through CT scans, that their porosity was in fact much closer to that of non HIP treated specimens. This master thesis illustrates once more that the effect of surface roughness on SLM Ti-6AL-4V alloys cannot be properly identified if the bulk material shows internal defects.