Dissimilar Thickness Al Alloy Weld by Single/Double Pass FSW
8.2 Analysis of the Weld
8.2.4 Fatigue Life Analysis
The highest tensile strength specimen of DPBF and SPTF configurations (i.e., specimen of experiment WPDT7) were considered for the fatigue test. The measured
Chapter 8 and fracture surface of the specimen for enhanced assessment of the welded specimens.
The fatigue tests were performed under stress amplitudes of 66.6 MPa to 133.1 MPa in the interval of 16.6 MPa and at a stress ratio (R) of -1. It was found that the number of failure cycle of the welded specimens changes with stress levels for different joint configuration as shown in the Fig. 8.7.
Fig.8.6 Variation of impact energy with tool rotational speed and PTR
Fig.8.7 Fatigue life of SPTF, DPBF and BM.
The measured fatigue life of the welded specimens (experiment WPDT7) and BM at various stress amplitude are shown in Fig. 8.7. The fatigue life of AA1050 FSWed specimens at higher stress amplitude is appreciably lower than the un-welded BM and which is attributed to grain refinement in the weld zone (Uematsu et al., 2013).
It is evidential from the figure that at low stress condition fatigue life of welded specimens are comparable with that of BM. It can also be observed that DPBF specimen has better fatigue life than the SPTF due to proper joint and less stress concentration.
Failure zone of the fatigue specimens at different stress levels are represented in the Fig. 8.8. Most of the DPBF specimens failed at the thinner side of the material rather than the welded region because of slanting weld bead that has more thickness compared to the thinner plate. In case of SPTF specimens failure locations vary with stress levels
Dissimilar Thickness Al Alloy Weld by Single/Double Pass FSW
due to step in the specimens. Fracture occurred at the boundary between NZ and TMAZ with stress amplitude of 133.1 and 116.5 MPa. Whereas, at stress amplitudes of 100 and 83.2 MPa, the fracture location shifted towards the boundary between TMAZ and HAZ and at lower stress levels fracture occurred at the boundary between the HAZ and BM.
Fig.8.8 Some of the fatigue tested specimen showing the failure zone at different stress ratio, (a) DPBF specimen, (b) SPTF specimen.
Fatigue fracture of any microcrystalline ductile metal like AA1050 can be divided into at least three different fracture zones namely, (a) crack initiation area, (b) stage I and stage II of crack propagation, (c) void nucleation, coalescence and growth.
The crack initiation site is illustrated in Fig. 8.9(a) for the specimen tested at 66.6 MPa stress amplitude. Due to a low stress level, this specimen features a single crack initiation site, and the river marks pattern, shown in Fig. 8.9(a), indicates the direction of crack growth. The crack developed area (marked by arrow) is followed by stage I zone of crack propagation with stage II of crack propagation with striation mark Fig. 8.9(b-c).
Finally, the crack exhibits a combination of void nucleation, coalescence and growth of the fractured area, Fig. 8.9(d). Grain size plays an important role on fatigue life in the low stress high cycle regime in which stage I cracks predominate. One of the predominant crack growth mechanisms is inter-granular crack propagation which shows localized regions of combination of ductile and brittle fractured surface and characterized by a pure transgranular cracking propagation (Fig. 8.9e). The ductile fracture region has large amounts of varied dimples spread over the specimen and at the brittle surface the crack initiated and spread towards the ductile zone. Figure 8.9(d) shows crack propagation regime characterized by parallel fatigue patches containing fine striations. In each loading cycle fatigue striation was formed and the crack growth rate can be estimated from the spacing between the striations. However, the exact location of striation is difficult to identify and it needs careful observation throughout the specimen since load variation at slow crack growth affect the growth rate.
Chapter 8
Fig.8.9 SEM fractographs of fatigue crack growth for different regions.
Crack growth line against the specimen for Exp. No. WPDT7 can be seen in the SEM fatigue fractographs represented in Fig. 8.10(a-d). Fatigue life of a specimen is highly sensitive to crack initiation phase and the fatigue crack initiates through easy growth route. During fatigue testing, it was observed that a site for crack initiation is generated from the edge of the specimen and propagated towards the center of the specimen as shown in Fig. 8.10(a-b). On the other hand there were no weld defects at the edges so it is believed that the crack initiated due to repeated loading cycles. FSW specimens have fatigue strength comparable to the BM because of reduction of crystal size, increase in hardness due to dynamic ageing during fatigue test, recrystallization and grain refinement during FSW process (Uematsu et al., 2013).
The striations observed in DPBF specimen-7 is illustrated in Fig. 8.10c and d.
The striation marks of the specimen are shallow and uniform throughout the fracture area. For a ductile FCC material duplex slip is the primary criterion for striation formation. As the specimen of DPBF has finer grain size than the SPTF specimen, dislocation movement becomes difficult and requires high stress to move and increase interfacial volume that leads to small planar slip distance. The grain size increases from NZ to TMAZ, HAZ and BM therefore it is expected that the fracture path should shift towards the coarse grain. The absence of such shift of fracture path may be due to very small variation of the grain size. Microhardness can also have an effect on fatigue
Dissimilar Thickness Al Alloy Weld by Single/Double Pass FSW
fracture path line. A comparison has been made between two specimens before and after the fatigue test at 66.6 MPa stress level. It was observed that the hardness of the NZ increases after the fatigue test but BM hardness decreases after the test. This is due to cyclic loading the hardened BM became soften and soften NZ became work hardened.
Fig.8.10 SEM analysis of fatigue fractographs and crack growth regions of specimen-7 (a) SPTF, (b) magnified view of (a), (c) DPBF and (d) magnified view of (c).