Hot cracking in laser braze joint of high strength stainless steel using gold brazing filler metal - prevention of hot cracking by controlling thermal cycle

The hot cracking behaviour in the diode laser braze joint of 13Cr-4Ni stainless steel using Au-18Ni, Ag-10Pd and Ag-21Cu-25Pd filler metals has been investigated. The types of joint investigated were the T fillet joint and the L fillet joint which simulated the second braze bead in the T fillet joint. Tandem beam brazing was also carried out in order to prevent the hot cracking by post-heating treatment with a trailing beam. A centreline crack, characterised as a ductility dip crack, occurred in the second braze bead of the T fillet joint using Au-Ni filler metal. On the other hand, no cracks occurred in either the first or the second braze bead in T fillet braze joints using Ag-Pd and Ag-Cu-Pd filler metals. The hot cracking susceptibility of the Au-Ni braze metal was evaluated by the spot Varestraint test. Most of the cracks observed in the spot Varestraint test specimen were also characterised as ductility dip cracks, and the susceptibility to such cracking increased with increasing the augmented strain. The ductility dip temperature range (DTR) was estimated from the crack position and length to be 1000-1250 K at strains over 0-4%. Numerical analyses of the thermal stress and strain revealed that the plastic strain-temperature curve intersected the DTR in the single beam brazing, but did not intersect the DTR with post-heating during the laser brazing. The effect of post-heating on the crack prevention was verified by tandem beam brazing of L fillet joint with a trailing beam. No cracks occurred in the braze bead made with a trailing beam at laser powers of 200-300 W. The authors concluded that hot cracking in the Au-Ni braze metal could be successfully prevented by controlling the thermal cycle during the laser brazing process.