Dissimilar Thickness Al Alloy Weld by Single/Double Pass FSW
8.3 Metallographic Analysis
8.3.1 Material Flow and Feature of Microstructure at Various Zones
From the tensile test analysis it was observed that WPDT7 specimen gives highest tensile strength compared to other experiment. However, for metallographic analysis, specimen WPDT9 is considered due to higher PTR that can indicate detail variation of macrostructure and microstructure. Metallographic studies of FSWed specimens WPDT9 are represented for all three cases namely, SPBF, DPBF and SPTF specimens as follows:
8.3.1.1 SPBF Specimen
The microstructures of the base plates with four different thicknesses used in this study did not show any difference in grain size. Figure 8.13(a) represents the weld bead of a SPBF specimen at tool rotational speed of 1100 rev/min with PTR of 2.0 (specimen WPDT9). The microscopic view shows that plasticized material is ploughed from thicker side and deposited in the thinner side. The thickness of weld nugget uniformly decreases from thicker to thinner plates and there is no surface mismatch at the bottom of the weld SPBF cases. The material flow, Fig. 8.13(b), also indicates uniform mixing in the weld nugget with some unfilled cavity defect (Fig. 8.13c). Due to high mismatch of plate thickness plasticized material could not fill the cavity formed by the rotating tool in single pass. The grains in the NZ (Fig. 8.13d) became 4 times finer (average size of
Chapter 8 32 μm) and equiaxed (Hattingh et al., 2016) compared to the respective BM (average size of 120 μm, Fig. 8.13i) due to stirring action of the tool and dynamic recrystallization. The grains in the bottom of NZ (Fig. 8.13e) became elongated compared to the top and middle of NZ due to higher plastic deformation at the tool pin end and forming effect due to insufficient heat generation. Distinguished grains are observed between the NZ and TMAZ, represented in Fig. 8.13(f). The advancing side grains (Fig. 8.13g) are slightly finer compared to the retreating side (Fig. 8.13h) because of non-homogeneous heat generation.
Fig.8.13 (a) Weld bead of SPBF specimen of WPDT9, with macro view at different zones (b) flow of the material in the NZ, (c) defect at NZ in single pass, (d) top of NZ,
(e) bottom of NZ, (f) interface of NZ and TMAZ, (g) advancing side of TMAZ, (h) retreating side of TMAZ and (i) base material.
8.3.1.2 DPBF Specimen
The macro and microstructural images of DPBF specimen of WPDT9 show full penetration with defect free weld as represented in Fig. 8.14(a-g). Weld bead geometry
Dissimilar Thickness Al Alloy Weld by Single/Double Pass FSW
and material flow in the NZ are shown in Fig. 8.14(a) and Fig. 8.14(b-c), respectively. It is observed that material flow in the NZ is non-uniform with a different pattern due to double pass FSW of WPDT. The NZ has been replaced by fine equiaxed recrystallized grain (Fig. 8.14d) compared to BM. Uniform grains (average size is 20 μm) throughout the NZ are observed because of dual stirring of NZ material and higher heat generation.
The cavity defect which was observed in single pass FSW is eliminated in the second pass because of uniform material flow and proper shoulder-workpiece contact. Adjacent to the NZ, it is possible to distinguish both advancing and retreating sides of TMAZ, shown in Fig. 8.14(e-f), respectively. The average grain size of advancing side of TMAZ is 28 μm whereas the same for retreating side it is 32 μm. There is no effect of the tool pin on the HAZ (Fig. 8.14g), this zone is only affected by heat generated during the process. The grains in the HAZ of DPBF are finer (55 μm) than the SPBF specimen (82 μm) because of higher welding temperature in the second pass.
The difference in the onion ring pattern (Fig. 8.14b-c) arises in the advancing and retreating side normally based on difference in the material flow caused by the tool pin rotation. It was observed that onion ring (Zadpoor et al., 2010) rotate according to the direction of the tool rotation progressing from one side to other side. The same effect was observed for all the cases of DPBF specimens. The upper surface onion ring depends on the shoulder contact to the weld top surface and the impact of tool shoulder is different for different PTR. The onion ring of the other part of the NZ depends on material flow caused by pin rotation. It was observed that the difference in material flow in dissimilar thickness joint on both sides of the NZ changes of the onion-ring pattern (Fig. 8.14b-c). The macrograph in the FSW joint cross-section revealed as “classic onion ring structure” (asymmetric vortex-shaped) at the joint interface and has a different morphology for the WPDT joint. The difference is due to generation of more than one onion ring (Fig. 8.14a-c) in the same joint with non-uniform structure. The onion rings are related to the forward motion of the rotating tool making FSW simply as an extrusion process in which a number of semi-cylinders (Zadpoor et al., 2008) are extruded by the rotating tool. Biallas et al. (1999) explained those onion rings are generated due to the reflection of the material flow creating the necessity for through mixing of the two sides of the weld. The above two cases are applicable for dissimilar thickness weld. In the first pass material is extruded by the tool shoulder due to mismatch of plate thickness and
Chapter 8 rearrange in the weld zone and in the second pass reflection of material flow occurs due to proper tool and workpiece contact and heat generation. The flow of the material is not exact bowl shape in SPBF and DPBF configurations because of tilting of either tool or bed.
Fig.8.14 (a) Weld bead of DPBF specimen of WPDT9, (b, c) flow of material in the NZ, (d) NZ, (e) advancing side of TMAZ, (f) retreating side of TMAZ and (g) HAZ.
8.3.1.3 SPTF Specimen
SPTF configuration is like normal FSW process but due to mismatch of plate thickness metal padding was provided at the back side of the thinner plate to retain flat top surface. The main disadvantage in this case is a step crated at the bottom side of the weld, shown in Fig. 8.15(a), which creates stress concentration and thus failure point.
The overall weld bead and flow of material in the NZ is represented in Fig. 8.15(a). The differences in heat absorption and dissipation of the plates due to thickness variation, causes non-uniform weld bead color with onion ring pattern. An enlarged view of the NZ is represented in Fig. 8.15(e) which indicates the classic onion ring pattern. Minor cracks are observed at the bottom of the NZ, shown in Fig. 8.15(f), this may be due to uneven heat dissipation which decreases mechanical strength of the SPTF specimens.
Dissimilar Thickness Al Alloy Weld by Single/Double Pass FSW
The grain size of WPDT9 specimen at the NZ and TMAZ are around 25 μm (Fig. 8.15b) and 40 μm (Fig. 8.15c), which are more than DBPF specimen. The grains at the HAZ (Fig. 8.15d) are similar to DPBF configuration.
Fig.8.15 (a) Weld bead of SPTF specimen of WPDT9, (b) NZ microstructure, (c) TMAZ microstructure on advancing side, (d) HAZ microstructure on advancing side, (e) flow of
material at the NZ and (f) enlarged view of the bottom side of the joint.
From all the welded specimens it seems that there are no metallographic defect in DPBF specimens but in SPBF specimens defect arises due to insufficient heat generation and material flow. SPTF specimens do not show internal defect except minor cracks at the weld root that affect minor decreases of weld strength.