PSI - Issue 17

Ashu Garg et al. / Procedia Structural Integrity 17 (2019) 456–463 Ashu Garg et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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The high speed imaging of progressive failure of notch tensile specimen prepared with TIF tool pin is also carried out as shown in Fig. 6(a) and the FE prediction of failure pattern is shown in Fig. 6(b). Fig. 6(a) shows that when uniaxial tensile load is applied the crack initiated from the notch and the crack path progressed through the dynamically recrystallized SZ. Due to the stress concentration around the sharp notch during loading leads to the crack initiation and failure. It has also been noticed that crack initiation and opening was from the bottom side of the weld stir nugget region (refer Fig. 6(a)) and a small cup-cone type of failure was observed indicating uniform ductile fracture. The notch opening from the bottom side of the weld nugget is seemingly due to the formation of narrow SZ where flow of material is mostly driven by the tool pin, whereas, at the top side of the weld nugget the plastic flow of material is driven by both tool pin as well as tool shoulder that led to the formation of wider SZ. The FE analysis also indicates that the higher stresses are confined near the notch root where from crack initiation and propagation during loading was observed.

(a)

(b)

AA6061 (AS)

AA6061 (AS)

SZ

AA7075 (RS)

AA7075 (RS)

Fig. 6. Progressive failure captured using (a) in-situ technique; (b) FE method for notch tensile specimen prepared with TIF tool pin.

The fractographic analysis of welded joints was further carried using SEM and are shown in Fig. 7. The fracture morphology of joint obtained with CYL tool pin is shown in Fig. 7(a). Fracture surface examined near the tunnel region indicates that the material separates from this region under tearing with some presence of shallow dimples. However, fractograph observed from the center of fracture surface shows numerous fine dimples and micro-voids indicating the sound bonding and ductile fracture of the joint. Fractographs of joint obtained with TIF tool also represent numerous micro-voids and fine equiaxed dimples all throughout the fracture surface signifying a sound bonding between the two plates and ductile failure of the joint (refer Fig, 7(b)). Sound welding with no tunnel for joint prepared with TIF tool pin exhibits highest joint strength and ductile type of failure from the interface of HAZ region on the AS.

Shear fracture marks

Fine equiaxed dimples and micro-voids

Fine dimples

Shallow dimples

Crack initiation site

Dimples

Fine dimples

Dimples and micro-voids

Fine dimples

(b)

(a)

(c)

Fig. 7. Fractographic analysis of (a) tensile sample obtained with CYL tool pin; (b) tensile sample obtained with TIF tool pin; (c) notch tensile sample obtained with TIF tool pin