Effects of temporal pulse shaping on cracking susceptibility of 6061-T6 aluminum Nd:YAG laser welds

Pulsed laser beam welds of 6061-T6 aluminum alloy hermetic packages for encapsulation of sensitive electronic and optoelectronic components typically exhibit severe solidification cracking when a conventional rectangular or on/off laser pulse is employed. In this study, the effects of temporal pulse shaping on the solidification cracking susceptibility of Nd: YAG pulsed laser welds made in 6061-T6 aluminum were investigated. The results showed that it was possible to eliminate solidification cracking in 6061-T6 pulsed Nd: YAG laser seam welds by decreasing the ramp-down gradient of the laser pulse power after the main welding pulse sector. Crack-free welds were produced over a limited range of trailing ramp-down gradients; however, intermittent solidification cracking recurred when the gradient was further decreased. The solidification cracking susceptibility was also found to increase with increasing peak power density of the main welding sector. Use of a trailing ramp-down pulse shape was found to affect the solidification morphology of the welds. The width of the initial planar grain growth layer at the fusion boundaries and the dendrite and cell spacing increased with decreasing ramp-down gradient. EDS measurements of the overall chemical composition of the solidification crack surfaces of welds showed that microsegregation of Mg, Si, Fe, and Cu to the grain boundaries increased with decreasing ramp-down gradients. This was thought to be the reason for the return of intermittent cracking when low ramp-down gradients were used.