,QWHUQDWLRQDO-RXUQDORI)DWLJXH ² Contents lists available at ScienceDirect International Journal of Fatigue journal homepage: www.elsevier.com/locate/ijfatigue Overloading effect on the fatigue strength in resistance spot welding joints of a DP980 steel ⁎ J.H. Ordoñeza, R.R. Ambriza, , C. Garcíab,G.Plascenciaa, D. Jaramilloa a Instituto Politécnico Nacional CIITEC-IPN, Cerrada de Cecati S/N, Col. Sta. Catarina, C.P. 02250 Azcapotzalco, Ciudad de México, Mexico b Facultad de Ingeniería, Universidad Autónoma de San Luis Potosí, Av. Dr. Manuel Nava No. 8, Zona Universitaria, C.P. 78290 San Luis Potosí, S.L.P., Mexico ARTICLE INFO ABSTRACT Keywords: Dual phase steel DP980 sheets were joined by resistance spot welding (RSW) process. Mechanical resistance of DP980 steel the welds was characterized by microhardness, tensile shear and fatigue tests. A significant hardness decreases Resistance spot welding was observed in the RSW lap joints with respect to the base material, which was attributed to phase transfor- Fatigue behavior mations during the heating and cooling of the steel. Fatigue Wöhler curves using a fixed load ratio of 0.1 were Compressive residual stresses obtained. It was found that the spot weld at the nugget interface close to the fusion zone induced a critical stress concentration factor, which decreased the fatigue life of the joints in the as-welded condition. Failure of the welds was initiated at the interface between welded sheets. Two predominant fatigue fracture modes were observed associated with mode I/III cyclic loading, which were correlated with the fatigue crack initiation and propagation stages. Compressive residual stresses were induced by a loading-unloading cycle on the spot welds, which tends to increase the fatigue life of the joints when compared to the as-welded condition. 1. Introduction attributed to the stress concentration effect produced by the nugget zone reducing the fatigue life of the welded joints. Shariati and Nejad Dual-phase steels are composed of a ferrite matrix with martensite [7] studied resistance spot welded U-shape specimens of low carbon as a second phase. This dual microstructure allows to obtain a balance steel subjected to tensile cyclic loading. They observed a stress con- between strength and ductility, which is very attractive to reduce the centration effect at the nugget interface that resulted in either a pull-out weight in automobiles. The manufacture of vehicles requires the use of or a fatigue crack growth failure as a function of the applied load, welding processes, such as the resistance spot welding RSW (a vehicle specimen thickness and nugget diameter. Bandyopadhyay et al. [8], has around three thousand welding spots) [1–3]. However, due to the performed a failure analysis on DP980 joints that were fractured under heat generated during the RSW process, microstructural changes are tensile tests; by SEM images they found void initiation at the interface induced, which affects the mechanical behavior of the welded joints. of ferrite and martensite grains on the necked region. The micro- Some studies have been carried out on this regard [4–8]. The soft zone structural and mechanical properties changes induced by the conven- produced in the heat affected zone (HAZ) by the RSW process on dual- tional RSW process represents a challenge in the use of high strength phase steels (DP600, DP780 and DP980) has been studied by Nayak steels for the automotive industry, because the fatigue strength can be et al. [4]. They observed an increase in the volume of martensite in the seriously affected. Despite the work performed on the RSW joints, re- fusion zone, as well as tempering of the martensite in the HAZ (soft covery mechanisms for the fatigue life had been limited reported in the zone formation). Additionally, these authors reported a reduction of the literature. Fujimoto et al. [9] reported the use of shot blasting proces- mechanical properties in the welded joints because of the micro- sing on RSW lap joints of high strength steel sheets (1.2 mm-thickness). structural changes. Mediratta et al. [5], determined that the decreased The fatigue strength was improved because of high compressive re- hardening occurs when there is a uniform distribution of dislocations sidual stresses induced by the shot blasting to the RSW lap joint. On the due to the dispersion of martensite. Farabi et al. [6] studied the mi- contrary, tensile residual stresses decreased the fatigue strength, but crostructure and mechanical properties in dissimilar welds (DP600- they can be diminished through careful control and optimization of the DP980), they observed that fine grains and continuous martensite heat input in welding processes [10]. In view of the massive usage of around the ferrite grains allowed to obtain the best performance in the the RSW processes by the automobiles manufacturers industry results fatigue life. Also, it has been found that the fatigue failure on RSW was relevant to propose additional operations routes to improve the ⁎ Corresponding author. E-mail address: [email protected] (R.R. Ambriz). https://doi.org/10.1016/j.ijfatigue.2018.12.026 Received 13 November 2018; Received in revised form 20 December 2018; Accepted 27 December 2018 $YDLODEOHRQOLQH'HFHPEHU (OVHYLHU/WG$OOULJKWVUHVHUYHG 7UDQV1RQIHUURXV0HW6RF&KLQD í $VVHVVPHQWRIJDVWXQJVWHQDUFZHOGLQJWKHUPDOF\FOHVRQ,QFRQHODOOR\ 0+(51È1'(=55$0%5,=5&257e6 &0*Ï025$*3/$6&(1&,$'-$5$0,//2 ,QVWLWXWR3ROLWpFQLFR1DFLRQDO&,,7(&,31&HUUDGDGH&HFDWL61&RO6WD&DWDULQD $]FDSRW]DOFR&LXGDGGH0p[LFR&30p[LFR ,QVWLWXWRGH,QYHVWLJDFLyQHQ0HWDOXUJLD\0DWHULDOHV 8QLYHUVLGDG0LFKRDFDQDGH6DQ1LFROiVGH+LGDOJR$3&30RUHOLD0LFKRDFiQ0p[LFR 5HFHLYHG$SULODFFHSWHG2FWREHU $EVWUDFW+HDWPRYLQJVRXUFHPRGHOVDORQJZLWKWUDQVLHQWKHDWDQDO\VLVE\ILQLWHHOHPHQWPHWKRGZHUHXVHGWRGHWHUPLQHZHOGWKHUPDO F\FOHVDQGLVRWKHUPDOVHFWLRQVREWDLQHGIURPWKHDSSOLFDWLRQRIDJDVWXQJVWHQDUFZHOGLQJEHDGVRQ,QFRQHOSODWHV$QDO\WLFDO 5RVHQWKDO¶VWKLFNSODWHPRGHO DQGILQLWHHOHPHQWUHVXOWVVKRZDQDFFHSWDEOHDSSUR[LPDWLRQZLWKWKHH[SHULPHQWDOZHOGWKHUPDOF\FOHV 7KHLVRWKHUPDOVHFWLRQVGHWHUPLQHGE\QXPHULFDOVLPXODWLRQVKRZDEHWWHUDSSUR[LPDWLRQZLWKWKHH[SHULPHQWDOZHOGLQJSURILOHIRU GRXEOHHOOLSVHPRGHOKHDWGLVWULEXWLRQWKDQ*DXVVPRGHO7RDQDO\]HWKHPLFURVWUXFWXUDOWUDQVIRUPDWLRQSURGXFHGE\GLIIHUHQWFRROLQJ UDWHVLQWKHIXVLRQDQGKHDWDIIHFWHG]RQHV9LFNHUVPLFURKDUGQHVVPHDVXUHPHQWV SURILOHDQGPDSSLQJUHSUHVHQWDWLRQ ZHUHFRQGXFWHG $ KDUGQHVV GHFUHPHQW IRU WKH KHDW DIIHFWHG ]RQH a +9 DQG IXVLRQ ]RQH a +9 LQ FRPSDULVRQ ZLWK EDVH PDWHULDO a+9 ZDVREVHUYHG7KLVEHKDYLRUKDVEHHQDWWULEXWHGWRWKHKHWHURJHQHRXVVROXELOL]DWLRQSURFHVVRIWKHȖƎSKDVH QLFNHO PDWUL[ ZKLFKDFFRUGLQJWRWKHFRQWLQXRXVFRROLQJíWUDQVIRUPDWLRQ FXUYH SURGXFHGWKH/DYHVSKDVHįDQG0&WUDQVLWLRQSKDVHV JHQHUDWLQJDORVVLQKDUGQHVVFORVHWRWKHIXVLRQ]RQH .H\ZRUGV,QFRQHOJDVWXQJVWHQDUFZHOGLQJ *7$: ZHOGWKHUPDOF\FOHILQLWHHOHPHQWPHWKRGKHDWPRYLQJVRXUFH HVVHQWLDO WR DQDO\]H WKH ZHOGLQJ WKHUPDO F\FOHV ,QWURGXFWLRQ GHWHUPLQHG H[SHULPHQWDOO\ DV ZHOO DV E\ QXPHULFDO DQG DQDO\WLFDOPRGHOVWRFRUUHODWHWKHPZLWKWKHFRQWLQXRXV ,QFRQHOLVDSUHFLSLWDWLRQKDUGHQHGQLFNHOEDVH FRROLQJíWUDQVIRUPDWLRQGLDJUDP &&7 VXSHUDOOR\ 7KLV PDWHULDO LV XVHG LQ D ZLGH YDULHW\ RI 7KHEDVLFWKHRU\RIKHDWIORZGHYHORSHGE\)RXULHU DSSOLFDWLRQVGXHWRLWVH[FHOOHQWPHFKDQLFDOSURSHUWLHVDW DQGDSSOLHGWRKHDWPRYLQJVRXUFHVE\526(17+$/>@ FU\RJHQLF DQG KLJK WHPSHUDWXUHV DV ZHOO DV FRUURVLRQ LVVWLOOWKHPRVWSRSXODUDQDO\WLFDOPHWKRGIRUFDOFXODWLQJ UHVLVWDQFHLQDJJUHVVLYHHQYLURQPHQWV WKHWHPSHUDWXUHGLVWULEXWLRQLQZHOGVDVKDVEHHQVKRZQ 7KHKLJKUHVLVWDQFHRI,QFRQHO XOWLPDWH WHQVLOH LQ5HIV>í@ VWUHQJWK RI *3D>@ LVREWDLQHGE\WKHSUHVHQFHRI 526(17+$/¶V PRGHO GRHV QRW SURYLGH DQ ILQH KDUG DQG GLVSHUVHG SUHFLSLWDWHV Ȗƍ DQG ȖƎ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í@,QVRPHVWXGLHVWKHKHDWIOX[LQWKH RIWKHUPRG\QDPLFDOO\DQGVWUXFWXUDOO\VWDEOHSKDVHVOLNH ZHOGLQJGLUHFWLRQLVLJQRUHG>í@,QVXFKFDVHVWKH 1E&7L1LDQG/DYHVSKDVHV>@2QWKLVFRQWH[WLWLV VXUIDFH*DXVVLDQKHDWVRXUFHPRGHOLVJHQHUDOO\XVHGIRU &RUUHVSRQGLQJDXWKRU55$0%5,=(PDLOUUDPEUL]#LSQP[ '2,6 Int. 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