PSI - Issue 17

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Ashu Garg et al. / Procedia Structural Integrity 17 (2019) 456–463

459

Ashu Garg et al. / Structural Integrity Procedia 00 (2019) 000 – 000

Assigning the individual material properties to TMAZ and HAZ regions on both AS and RS was difficult due to narrow and microstructural gradient, however, hardness was easier to measure. Thus, the ultimate tensile strength ( ) and yield strength ( ) of these regions were calculated from Vickers hardness ( ) value using equation 2. = 1 + ( 1 × ) = 2 + ( 2 × ) (2) Where a i , b i are material dependent constants and were obtained by fitting the experimental data for the present case. The tensile testing of two specimens cut from each of the base materials AA6061-T6 and AA7075-T651 were carried out to measure the values of and for both the base materials. Later, the Vickers hardness ( ) was also measured for both the specimens cut each from the base materials. Finally the values of , and were used in equation 2 to obtained the value of a i , and b i . For FE analysis, the model geometry was meshed by 4 noded tetrahedral elements (C3D4) with finer mesh size for the center welded region (SZ, TMAZ and HAZ regions) where global mesh size was kept as 0.4 mm. The boundary conditions applied to one side of the tensile specimen as fixed (grip region in experiment) and total displacement upto failure was applied to the other end as recorded from the experiment (refer Fig. 1(d)). The fractographic analysis using field emission scanning electron microscope (GeminiSEM 500, Zeiss, Germany) were also performed and reported. Optical microscopic images were captured from the cross-section of FSW joints prepared with CYL and TIF tool pins to observe the different zones and are shown in Fig. 1(c). Due to contact around circumference and face of the CYL tool pin with the base metal, high heat is expected to generate leading to flow of plasticized material around the tool pin surface. However, for CYL tool pin under the process condition used, a tunnel was observed at the bottom of the SZ. For the joint prepared with TIF tool pin no such tunnel was observed. The plasticized material gets fully delivered and flows around the intermittent threads on the tool pin periphery and intermittent flat regions of the TIF tool pin sweeps plasticized material from retreating to advancing side and vice versa. The welding was repeated and two samples for each of the transverse, longitudinal and base metal were tested. The tensile stress strain plots for base metals (AA6061-T6 and AA7075-T651) and samples in longitudinal (entirely through SZ) and transverse direction of the FSW joints prepared with CYL and TIF tool pin are presented in Fig. 2(a) and (b), respectively. From Fig. 2 it may be noted that the UTS for longitudinal tensile specimen is comparatively higher than that of transverse specimen for joints prepared with CYL as well as TIF tool pin since the longitudinal specimen solely contains the stirred metal with recrystallized fine grains whereas, transverse specimen contains all zones e.g. SZ, HAZ, TMAZ and BM with difference in the grain structure and hardness. During tensile loading, the presence/size/location of the tunnel can influence the strength of welded joints. 3. Results and Discussion

600

500

AA7075-T651 (BM)

400

Notch specimen

specimen Longitudinal

AA6061-T6 (BM)

300

200

Tensile stress (MPa)

specimen Transverse

100

specimen Longitudinal

specimen Transverse

(a)

Strain (%)

(b)

Fig. 2. Tensile stress versus strain curves for friction stir welds prepared with (a) CYL; (b) TIF tool pin.

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