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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Fig. 2(b) shows the tensile stress-strain behavior of the notch specimen (notch at SZ) of the FSW joint made with TIF tool pin. From the experiments, the notch tensile strength was obtained around 323 MPa. Thereby, the notch strength ratio (NSR), the ratio of ultimate tensile strength of notch specimen to smooth specimens, is found to be more than unity (i.e. NSR > 1). The value of notch strength ratio greater than unity indicates that the friction stir weld between AA6061 and AA7075 obtained under the tested condition using TIF tool pin was notch insensitive or in other terms notch strengthened i.e. the presence of notch in the weld nugget does not decrease the local strength. The NSR > 1 or higher notch tensile strength than UTS of regular specimen indicates that the large plastic deformation around the notch root also results in some stress relaxation around the notch tip. Thus, the ability of material to accommodate plastic flow under high level of stress concentration may also increases which enhances the fracture stress and thus avoided the notch weakening or notch sensitivity.

(a)

(b)

AA6061 (AS)

AA6061 (AS)

TMAZ HAZ

TMAZ HAZ

TMAZ HAZ

TMAZ HAZ

SZ

SZ

SZ

SZ

TMAZ HAZ AA7075 (RS)

AA7075 (RS)

Fig. 3. Progressive failure captured using (a) in-situ imaging; (b) FE analysis for friction stir weld prepared with CYL tool pin.

The progressive failure of FSW joint prepared with CYL tool pin under tensile loading captured through high speed imaging as well as FE analysis (using ductile damage model) are shown in Fig. 3(a). It has been observed that the failure started from the tunnel surrounded regions wherein first the thin connected region at weld bottom failed followed by crack initiation and propagation through the boundary of SZ and TMAZ and finally separated through the TMAZ on the AS (AA6061) of the welded joints. Due to stress concentration in and around the sharp edge/corner of tunnel region, crack initiated from such sites and propagated beyond SZ since the recrystallized, finer grains are present in the SZ. A similar trend of crack propagation and failure has been predicted by the FE analysis (refer Fig. 3(b)) which indicated higher stresses and strain confined near the tunnel regions where from the crack initiated and propagated through boundary of SZ and TMAZ as observed from the experiments.

(a)

(b)

AA6061 (AS)

AA6061 (AS)

~ 50°

~ 46°

TMAZ HAZ

TMAZ HAZ

TMAZ HAZ

TMAZ HAZ

SZ

SZ

SZ

SZ

AA7075 (RS)

AA7075 (RS)

Fig. 4. Progressive failure captured using (a) in-situ imaging; (b) FE analysis for friction stir weld prepared with TIF tool pin.

Fig. 4(a) shows the progressive failure of FSW joint obtained with TIF tool pin. Due to the absence of tunnel in the weld nugget, the formation of necking was observed on softer material side (AA6061-T6 on the advancing side) and subsequently, crack initiation and propagation was noticed through the region adjoining of the BM and HAZ on