Abstract
Titanium alloys such as Ti-6Al-4V built by most of the additive manufacturing processes are known to contain process induced defects, non-conventional microstructure and strong crystallographic texture; all of which can affect the fatigue strength. In this study we evaluated the effect of crystallographic orientation of α and α lath width around gas pore defects on the high cycle fatigue life of Wire + Arc Additive Manufactured Ti-6Al-4V by means of Electron Back Scattered Diffraction. Here we show that variations in crystallographic orientation of α lath and its width in the vicinity of the crack initiating defect were the main reasons for the considerable scatter in fatigue life. Pyramidal slip systems with high Schmid factor active around the defects resulted in longer fatigue life compared to pyramidal slip with lower Schmid factor. In the absence of pyramidal slip, cracks initiated from active prismatic slip systems. When considering the influence of the microstructure, a higher number of smaller α laths around the defect resulted in longer fatigue life, and vice versa. Overall, the fatigue crack initiation stage was controlled collectively by the complex interaction of porosity characteristics, α lath width and its crystallographic orientation at the crack initiation location.
Highlights
Fatigue performance of titanium alloy Ti-6Al-4V (Ti64) produced by additive manufacturing (AM) processes suffers from defects, processdependent microstructures and strong crystallographic texture of grains [1]
Previous study on Wire + Arc Additive Manufacturing (WAAM) built Ti64 has showed that these columnar grains grew across the entire build height and had an average grain width of 1 to 2 mm [28]
The higher values of the critical resolved shear stress (CRSS) associated with the pyramidal slip system might have delayed the onset deformation and the crack initiation under cyclic loading, higher fatigue life was found in S1 and S4 compared to S2
Summary
Fatigue performance of titanium alloy Ti-6Al-4V (Ti64) produced by additive manufacturing (AM) processes suffers from defects, processdependent microstructures and strong crystallographic texture of grains [1]. Easier prismatic slip activation was observed compared to basal due to the lower strength of prismatic slip systems They have concluded that combination of high Schmid factor and high tensile stress is required for crack formation on basal planes, whereas crack initiation on prismatic planes involves surface roughening mechanism due to high Schmid factor and single slip. Large areas of local crystallographic misorientation were observed in regions around defects indicating inhomogeneous stress distribution [20] From this it is evident that the presence of strong crystallographic orientation within the material and at and around the defects influence the fatigue life of AM built materials. The present study is focused on investigating the reasons of scatter in fatigue life presented in [22] with the aim to identify the competing factors from defects, the microstructure and crystallographic orientation of α in an as-built WAAM Ti64 material
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