Role of bead shape and dispersed intermetallic phases in determining the strength of CMT brazed DP780 lap joints

Abstract

In this study, cold metal transfer (CMT) brazing technique was used for lap joining of 1.2 mm thick galvanized dual phase (DP780) steel using CuAl10Fe filler wire. The aim of this study was to evaluate the role of bead shape and dispersion of intermetallic phases in the deposited bead and their effect on load bearing capacity of brazed lap joints. Results showed that Fe-Al-Cu intermetallic phases were formed and distributed in the deposited bead due to melting and dissolution of base metal. Bead shape and amount of dispersed intermetallic phases in the bead significantly affected joint strength. The modes of failures were categorised as horizontal interface failure (mode 1), bead failure (mode 2) and vertical interface failure (mode 3) depending on effective bonding area (LH-LV) and LH/θ ratio. Failure load also increased with phase fraction of dispersed intermetallic. In addition, intermetallic phase fraction in bead increased by inserting 0.1 and 0.2 mm thick strips between the overlapping sheets. This successfully resulted in the failure of joint from the base metal i.e. 100% joint efficiency at a heat input of 184 J/mm. An increase of about 20% in the joint strength was achieved on altering the work angle to both 0° and 40° at a heat input of 133 J/mm. Excessive spreadability and severe loss of base metal may lead to the degradation of joint performance under dynamic loading and corrosive environment, hence it is suggested to keep the work angle between 0° and 40°.