PSI - Issue 17

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

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

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materials without the requirement of welding consumables like electrodes, fluxes, fillers, shielding gases etc. The primary source of heat generation in FSW is due to the friction between the non-consumable rotating tool and workpiece. During this solid state joining process, the rotation and translation of tool pin plunged into the workpiece introduces flow of the plasticized material and completes the weld. However, in FSW, the parameters like rotational speed of tool (rpm), traverse (weld) speed (mm/min), tool pin profiles etc. affects the heat generation, flow/intermixing, and joint strength. Thus, selecting the right process parameters and tool pin profile is important to obtain sound weld. Raturi et al. (2019) studied the effect of traverse speed, tool rotational speed, tool pin profiles and preheating during dissimilar AA6061-AA7075 friction stir welding and observed that pin profiles, rotational and traverse speed have significant effect on quality and joint strength. Ajri and Shin (2017) analyzed different process parameters to study the defect formation during FSW of AA7075-T6. They reported that the tunnel was formed in lower half of advancing side when rotational speed (rpm) was lower and welding speed (mm/min) was close to optimum whereas groove like defect on advancing side (visible from the top) was formed at higher rotational speed and lower welding speed. Ilangovan et al. (2015) studied dissimilar friction stir weld between AA6061-T6 and AA5086-O for strength, failure and material flow using different pin profiles namely straight cylindrical, threaded cylindrical and taper cylindrical. They reported that the threaded pin profile resulted in good flowability and produced sound joint. Reza-E-Rabby et al. (2018) analyzed tool pin thread interruptions (from zero i.e. no to four) on friction stir weld quality and reported that thread interruptions (three flats) produce defect free weld under all the examined conditions. Rajakumar el al. (2011) examined the influence of process parameters (axial force, rotational and welding speed) and tool parameters (pin and shoulder diameter) on tensile strength, yield strength and notch tensile strength during FSW of AA7075-T6. Yan et al. (2006) studied notch strength and notch sensitivity of friction stir welded AA2524-T351 and AA2024-T351 joints providing notch at different regions of welded zone and they reported that all zones were notch insensitive. Moreira et al. (2009) examined mechanical and metallurgical behavior of dissimilar FSW of AA6061-T6 and AA6082-T6 and compared with friction stir weld of individual alloy. They reported that the tensile properties of dissimilar weld lies between the properties of individual metal alloy joints and the joint failed from the AA6082-T6 side where minimum hardness value was noticed. In another work, Moreira et al. (2012) also studied the fracture behavior and fatigue crack growth propagation in AA2195-T8X (aluminum-lithium alloy) FSW joints . Đurđević et al. (2015) analyzed fatigue crack growth propagation in friction stir welded AA2024-T351 using numerical method. Yokoyama et al. (2018) carried out FSW of AA6061-T6 and examined tensile properties of weld in both transverse and longitudinal orientation followed by modeling using Ludwik equation. They reported that the tensile properties of longitudinal specimen were greater than transverse one due to difference in the microstructure in gauge length. The review of literature indicates that different welding parameters and tool pin profiles significantly influence the heat input, material flow etc. and selection of tool pin profiles, process parameters are therefore important to avoid formation of defects like tunnels, voids etc. which act as failure initiation site and affects the mechanical properties of the joints. Thus, in the present study, an in-situ imaging approach using high speed camera was employed to appraise the crack initiation and propagation during tensile loading of longitudinal, transverse samples of dissimilar FSW joints between AA6061-T6 and AA7075-T651 prepared with plain cylindrical and threaded with three intermittent flat faces tool pins. Joint failure behaviors were also studied by finite element (FE) analysis incorporating ductile damage criteria to have an insight to the failure mechanism. To create a realistic FE model both geometrically and mechanically, the FE model was segmented into different regions - stir zone, thermo mechanically affected zone, heat affected zone and base metal. In addition, tunnel/void region was also incorporated in FE model. FE analysis and experiment were also carried out for notch tensile sample (notch at the weld stir zone) to study notch sensitivity. Fractographic analysis was carried out to study failure behavior of the welded joints.

2. Materials and Methods

In the present study, AA6061-T6 and AA7075-T651 plates of thickness 6.1 mm were used for dissimilar FSW. The plates of dimensions 160 mm × 100 mm were placed in butt joint configuration and FSW carried out along plate rolling direction. For all welding, AA6061-T6 was placed on advancing side (AS) and AA7075-T651 on retreating side (RS). The process parameters like rotational speed of 660 rpm, welding speed of 63 mm/min and total plunge depth of 6 mm which were kept same for all the experiments. Tools employed for the present study was