Abstract
The influence of three deposition strategies on the fatigue crack growth behaviour of Wire + Arc Additive Manufactured (WAAM) Ti–6Al–4V has been investigated in the as-built condition. Test samples were prepared using single pass, parallel pass, and oscillation deposition strategies and tested with cracks propagating parallel and normal to the plane of deposition. Due to the higher local heat input, the oscillation build exhibited a significantly coarser columnar β grain structure as well as a coarser transformation microstructure, compared to the single pass and parallel pass builds, which were very similar. Among the three build methods, the lowest crack growth rates were found with the oscillation build. The crack growth data was found to broadly fall between that of a recrystallized α (mill-annealed) and β annealed wrought material, with the oscillation strategy build behaving more similarly to a β annealed microstructure. The fatigue crack growth rate was lower when cracks were propagated perpendicular to the build layers. For each build strategy, a greater microstructural influence on crack growth rate was found at lower levels of stress intensity factor range (<25 MPa m 1/2 ). However, the anisotropy and scatter in the data was much more significant in the case of the oscillation build. These differences have been attributed to the stronger α microtexture heterogeneity present in the oscillation build, which led to a greater crack deflection and bifurcation, giving rise to lower crack growth rates and a higher sensitivity to the anisotropy caused by the directional β grain structure.
Highlights
The Titanium alloy Ti–6Al–4V (Ti64) is known for its high static and fatigue strength, fracture toughness, and outstanding corrosion resis tance
The curved Heat Affect Zone (HAZ) bands are formed as the result of viewing end-on the four single parallel melt tracks deposited consecutively across the wall width in each added layer, which were deposited with a 50% overlap between passes
This paper has presented a systematic study of the influence of three different deposition strategies in the Wire + Arc Additive Manufactured (WAAM) process, i.e. single pass, parallel pass, and oscillation, on the fatigue crack growth behaviour of the alloy Ti64 in the as-deposited condition
Summary
The Titanium alloy Ti–6Al–4V (Ti64) is known for its high static and fatigue strength, fracture toughness, and outstanding corrosion resis tance. The growth rate of long fatigue cracks has been investigated by Zhang et al [27,28,29,30,31], again in simple single pass deposition WAAM Ti64 samples, which were tested at a cyclic load ratio of R = 0.1, with the crack oriented either parallel or perpendicular to the build layers. The few studies performed on fatigue performance and anisotropy in long crack growth rates in WAAM Ti64 parts seem to be inconclusive [27,29] This prior work only used simple test samples consisting of single pass deposition walls, which limited the maximum sample thickness to 9 mm. The differences in crack growth rates observed are discussed in the context of the different build strategies and their microstructure
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