Al/Cu Dissimilar FSW Using Third Material
7.2 Analysis of Experimental Results
7.2.1 Variation of Tensile Strength
The tensile strength data for welding cases with interlayer and without interlayer are analyzed in this section. It was observed that all the welded specimens with interlayer (Al/Cu-T 1-12) were failed at the interface of NZ/TMAZ as shown in Fig. 7.1(a) due to interlayer boundary which initiates the failure point on the AS. Specimens obtained from welding without interlayer were also failed in the interface of NZ/TMAZ except Al/Cu- EWF2 which failed at interface of TMAZ/HAZ, shown in Fig. 7.1(b). The failure occurs in Al/Cu-EWF2 on the RS of TMAZ/HAZ interface for the microstructural difference between TMAZ and HAZ, which is a dominating factor than local thinning action by the plunged depth which induced stress concentration as experienced in other welding cases without interlayer. The nominal stress versus strain curves for Exps. Al/Cu-EWF2, Al/Cu-T2, Al/Cu-T6, Al/Cu-T10, which gives highest tensile strength for each set and compared with Al BM, is shown in Fig. 7.1(c). The maximum UTS of the welded joints are around 95%, 105% and 107% respectively of the Al BM in the case of Exps. Al/Cu- EWF2, Al/Cu-T6 and Al/Cu-T10. Al/Cu-T6 and Al/Cu-T10 gives more than 100% of Al BM tensile strength because of strengthening effect of interlayer with proper diffusion
Chapter 7 (Xue et al., 2010), solid solution strengthening (Al-Roubaiy et al., 2014) thin, uniform and controlled formation of IMCs (Kandasamy et al., 2012). Similar strengthening effect was also observed by other researchers (Xue et al., 2010, Kandasamy et al., 2012).
According to Hsu et al. (2006), the major contribution to the high strength of the composite structure are the fine grain size of the Al matrix and the Orowan strengthening due to the dispersion of the fine IMC particles.
Table 7.1 Experimentally measured output responses corresponding to the parameters setting mentioned in Table 3.10
Exp.
No.
UTS (MPa)
YS (MPa)
% Elongation
Flexural stress (MPa)
BA (°)
Avg. H at NZ
(HV)
Maximum H at NZ
(HV)
Al/Cu-EWF1 60.3 59.3 1.7 154.4 30 51.8 141.3
Al/Cu-EWF2 126.0 119.2 8.5 286.5 65 60.4 176.2
Al/Cu-EWF3 101.2 98.2 4.3 213.3 45 57.9 115.2
Al/Cu-EWF4 77.7 55.8 3.2 193.0 35 55.6 191.1
Al/Cu-T1 58.0 55.0 1.2 155.7 15 84.7 257.1
Al/Cu-T2 97.1 84.0 2.8 217.6 30 81.4 242.0
Al/Cu-T3 90.7 79.6 2.7 211.6 25 83.8 254.0
Al/Cu-T4 79.6 55.3 1.5 192.5 20 94.8 255.8
Al/Cu-T5 43.8 36.0 1.5 181.2 10 108.2 312.4
Al/Cu-T6 139.0 115.7 3.2 291.1 80 107.2 309.0
Al/Cu-T7 68.9 56.1 2.8 189.7 25 112.0 325.7
Al/Cu-T8 64.6 55.0 1.3 162.2 15 154.9 509.7
Al/Cu-T9 50.7 44.7 2.2 142.5 15 83.8 215.7
Al/Cu-T10 142.3 128.0 5.7 337.2 85 80.3 206.8
Al/Cu-T11 87.5 73.4 2.6 225.3 45 89.5 220.1
Al/Cu-T12 63.9 60.2 1.3 202.7 20 71.7 235.4
The difference in the tensile properties for the experimental cases (without interlayer and with Ni, Ti, and Zn interlayer) are discussed in this section. The melting point of Ni, Ti and Zn are 1455 °C, 1668 °C and 419.5 °C, respectively. Out of these interlayers Ni and Ti are difficult to mix with Al/Cu by FSW process due to difference in melting point and hence these two materials will remain as boundary so called diffusion layer/interlayer. However Zn interlayer distributed uniformly throughout the NZ due to low melting point as compared to Al and stirring effect of the tool which results in thin and uniform IMCs in the weld zone that improves the weld strength. Out of all cases it was observed that the joint produced from Exp. Al/Cu-T10 with Zn interlayer shows highest tensile strength because of uniform distribution of alloying element Zn at the NZ.
Despite the presence of brittle CuZn5 IMCs (Kuang et al., 2015), eutectic like structure
Al/Cu Dissimilar FSW using Third Material
of Al.71Zn.29 and Al4.2Cu3.2Zn.7 IMCs are formed at NZ and gives highest tensile strength with Zn interlayer. It is also observed sound tensile properties in case of Ti interlayer due to proper flow control and minimum formation of IMCs. From the above study it is observed that different interlayer on the FSW process have different tensile strength. The characteristics and causes of the above IMC layer will be discussed further in the EDX and XRD analyses.
Fig.7.1 (a) Tensile tested specimen with interlayer (b) tensile tested specimen of Al/Cu- EWF2, (c) stress vs. strain curves of Al/Cu-EWF2, Al/Cu-T2, Al/Cu-T6, Al/Cu-T10 and
Al BM.
The variations of UTS, YS and percentage of elongation with tool rotational speed without and with Ni, Ti and Zn interlayers are shown in Fig. 7.2(a-c), respectively.
In all cases it was observed that with the increase in the rotational speed (upto 1200 rev/min), weld strength and percentage of elongation found to follow an increasing trend.
However, as the speed increases further (> 1200 rev/min), weld strength and percentage of elongation values follow a decreasing trend. During experiments, it was observed that at higher tool rotational speed, more flash of plasticized weld material was generated due to high frictional heat which leads to thinning of welded area. However, a constant plunging depth of 0.1 mm was maintained in all the experiments at constant welding speed and tool offset. Due to local thinning weld strength decreased at high rotational speed. Moreover, at higher tool rotational speed, overlapped layered structure may develop at the Al/Cu interface which leads to easy crack initiation and poor tensile properties. Comparing the UTS and YS with interlayer and without interlayer it was observed that Al/Cu-EWF2 (at 1200 rev/min) (Fig. 7.2a, b) gives good strength but highest strength is observed in case of Al/Cu-T10 (at 1200 rev/min) specimen because of fine and uniform distributed Zn interlayer. Despite the presence of minor IMCs, the highest strength observed in case of Zn interlayer because of Al.71Zn.29 and Al4.2Cu3.2Zn.7
IMCs present around the edges of the weld consolidate the joint. When using a Ni
Chapter 7 interlayer (Al/Cu-T2, at 1200 rev/min Fig. 7.2a, b) the tensile strength is less compared to without interlayer because of improper diffusion with defective joint interfaces (Fig.7.16) which lowers the strength. However, strength in case of Ti interlayer (Al/Cu- T6, at 1200 rev/min Fig. 7.2a, b) is more than Ni interlayer but less than Zn interlayer because of proper diffusion at the joint interface. When considering the percentage of elongation (Fig. 7.2c), it is observed that Al/Cu-EWF cases give higher percentage of elongation compared to joints with interlayer cases. This is due to lower hardness resulting in higher ductility of Al/Cu-EWF cases.
Fig.7.2 Effect of without (Al/Cu-EWF) and with (Ni, Ti and Zn) interlayer on (a) UTS, (b) YS, (c) percentage of elongation.