Dissimilar Thickness Al Alloy Weld by Single/Double Pass FSW
8.2 Analysis of the Weld
8.2.1 Tensile Strength Analysis
The stress-strain curves for BM and SPBF, DPBF and SPTF welded specimens of Exp No. WPDT7 are shown in Fig. 8.1(a). Figure 8.1(b-f) represents the summary of the result found in the tensile test for theckness ratios of 1.33, 1.67 and 2.0 respectively.
The tensile properties of SPBF, DPBF and SPTF speciems are non-identical for same process parameters setting (Fig. 8.1d-f). Out of these three joint configurations, DPBF specimens yield highest UTS (Fig. 8.1e), as well as YS and percentage of elongation compared to other two configurations. The highest average UTS is 166.39 MPa (specimen WPDT7 of DPBF joint configuration), which is 99% of the BM. The minimum strength of the entire experimental data set is more than 72% of the BM. The YS and percentage of elongation of the best welded specimen (specimen WPDT7 of DPBF joint configuration) are 99% and 99.5% of the BM, respectively. In case of DPBF joint configuration, all the welded specimens are having more than 90% UTS (Fig. 8.1e) to that of the BM. From the tensile test results, it is observed that double pass FSW with bottom flat surface gives better weld quality compared to the single pass FSW. In DPBF configuration, the tool has maximum contact on the thicker plate in the first pass, which plough and deposit extra plasticized material on the thinner plate to compensate thickness difference of the plates. In the second pass, proper contact between tool and workpiece is established that leads to proper material mixing and good weld quality.
All the welded specimens of DPBF configuration broke outside the weld region (shown in Fig. 8.2a), that indicates UTS of the welded segment is stronger than the BM.
This is attributed to finer grains in the NZ due to double stirring action that leads to higher UTS. In case of SPBF tensile tested specimens, shown in Fig. 8.2(b), all the specimens were broken at the stir zone. It is due to improper mixing of plasticized material as inadequate heat was generated due to mismatching of tool shoulder and plate surfaces. But in case of SPTF configuration, shown in Fig. 8.2(c), tensile specimens were broken at the interference of TMAZ and HAZ at the thinner material sidedue to
Chapter 8 step at the bottom of the joint resulting heterogeneous strain changes because of stress concentration.
Fig.8.1. (a) Stress Vs. strain of Exp. WPDT7 with BM and, (b-f) variation of tensile properties with PTR.
Fig.8.2 Photographs of some of the tensile tested specimens, (a) DPBF, (b) SPBF and (c) SPTF specimen.
To understand the effect of PTR on the weld quality, tensile properties of the joints are compared, shown in Fig 8.1(b-f). As the thickness ratio increases, it is observed that the joint efficiency decreases irrespective of the welding conditions and configurations.
This may be due to asymmetry in the thermal profile of the joint and increase of stress- concentration factor at the joint of the tested specimens. As the thickness ratio increases, high volume of plasticized material need to be ploughed and deposited on the thinner plate at higher rate to get proper weld quality, which requires higher heat generation and material flow during the process. However the heat generation rate decreases as PTR increases due to improper surface contact between the tool shoulder and workpieces.
However, this problem can be eliminated using double pass welding. It has been found
Dissimilar Thickness Al Alloy Weld by Single/Double Pass FSW
that the weld strength in case of DPBF configuration is more than 90% of BM for all considered PTR.
Some of the fractured surface images of the tensile tested specimens are presented in Fig. 8.3. It was found that most of the specimens against SPBF joint configuration failed with less tensile load compared to DPBF. Due to difference in plate thickness, material flowed in step, shown in Fig. 8.3(a-b), that leads to poor weld quality. In SPBF configuration, the type of fracture was not fully ductile rather it was mixture of ductile and brittle fracture due to featureless fracture surface that include river like patterns, and feather markings. With the creation of the step the specimen experience shearing and overloading. The mixture of ductile and brittle fractures initiates dislocation movement at the NZ. Al alloys are generally considered to fail as ductile fracture due to FCC (face centered cubic) structure as the presence of numerous active slip systems. In some cases, where fracture occurred within the FSW joint, the fracture surfaces resembled that of a brittle fracture. Most of the cases the fracture consist both ductile and brittle fracture which is termed as a quasi-cleavage fracture (Zadpoor et al., 2010 and ASM Handbook, Fractography, 1987).
Fig.8.3 (a) Tensile tested fractured surface of SPBF specimen, (b) enlarged view of (a), (c) DPBF specimen with cup shape on first part of the fractured surface, (d) cone shape on the counter surfaces, (e) tensile fractured surface of SPTF specimen and (f) enlarged
view of (e).
Chapter 8 The Fig. 8.3(c-d) indicates ductile fracture as it includes dimple, cup shape on first part of the fractured surface and cone shape on the counter surface. Presence of cup and cone features on the fracture surface gives higher bonding strength and ductility.
Also SPTF tensile fractured surface, Fig. 8.3(e-f), indicates ductile fracture including similar feature of DPBF specimen but less deep dimples indicating less ductility due to stirring effect of the tool pin.