Study of fatigue crack growth behaviour of Al-Mg and Al-Mg-Sc-Zr alloys

Abstract

Fatigue crack growth behaviour of Al-Mg and Al-Mg-Sc-Zr alloys is studied under different orientations and stress ratios. The microstructures and the fracture morphologies of each alloy are observed. For both alloys, they have Brass, S and Goss texture; while only Al-Mg alloy has Cube texture which indicates recrystallisation; as fatigue crack propagated, intergranular fracture transformed into transgranular fracture accompanied with gradually coarser crack surfaces. Al-Mg alloy is featured by large recrystallised grain, favourable plasticity, rougher crack surfaces, and with relatively strong crack growth resistance before rapid growth. While Al-Mg-Sc-Zr alloy with small grain size and relatively smoother crack surfaces, shown cyclic stability at higher Δ K level.

Fatigue crack growth behavior in an aluminum alloy Al–Mg–0.3Sc produced by wire based directed energy deposition process

“…A higher crack growth rate was attributed to a straight and smoother crack path (no crack deflection) in samples with smaller grains 24 . Similar findings were also made where Al–Mg alloys with smaller grains showed a higher FCGR than the material with larger grains 25 . A slower crack growth rate in material with larger grains was attributed to micro‐scale crack path deflections that were manifested by rougher crack surfaces.…”

“…Conventional cold‐rolled Al–Mg–Sc alloy, 25 on the other hand, shows a different trend: it initially had a comparable crack growth rate, only marginally lower than that of the current WAAM material near and within the Paris law region. When ∆ K is greater than 15 MPa m 1/2 , the cold‐rolled material continued to keep the stable crack growth while the WAAM material entered the fast crack growth stage and failed.…”

“…Experimental test data in terms of crack growth rate (d a /d N ) versus applied stress intensity factor range (∆ K ) for R = 0.1 is presented in Figure 3A with a zoomed view in Figure 3B. Comparison with literature data on a similar Al–Mg–Sc alloy processed by L‐PBF 39 and a conventional cold rolled alloy 25 . WAAM material shows a higher value in the threshold stress intensity factor range ∆ K th (average ∆ K th ~1.8 MPa m 1/2 for vertical cracks and 2.7 MPa m 1/2 for horizontal crack samples) compared to the L‐PBF material (average ∆ K th ~1.2 MPa m 1/2 ), 38 indicating that the WAAM material exhibited higher resistance to the onset of fatigue crack propagation.…”

“…(A) Crack growth rate versus stress intensity factor range for R = 0.1 and comparison with a similar AM alloy 38 or cold rolling 25 ; (B) zoomed view showing anisotropic behavior in the lower Δ K region. Data discrepancy from horizontal crack “test‐3” (in red colour) will be discussed in the following paragraphs.…”

“…When ∆ K is greater than 15 MPa m 1/2 , the cold‐rolled material continued to keep the stable crack growth while the WAAM material entered the fast crack growth stage and failed. This is due to the altered grain orientation against the crack propagation direction after rolling 25 ; hence, it postponed the fast crack growth stage and increased the resistance to cyclic failure (the maximum stress intensity factor range being 40% greater than that of the WAAM material).…”