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Improving Keyhole Stability during Laser Welding of AA5xxx Alloys
Abstract:
Laser welding of the magnesium-bearing AA5xxx aluminum alloys is often beset by keyhole instability, especially in the lap through joint configuration. This phenomenon is characterized by periodic collapse of the keyhole leaving large voids in the weld zone. In addition, the top surface can exhibit undercut and roughness. In full penetration welds, keyhole instability can also produce a spikey root and severe top surface concavity. These discontinuities could prevent a weld from achieving engineering specification compliance, pose a craftsmanship concern, or reduce the strength and fatigue performance of the weld. In the case of a full penetration weld, a spikey root could compromise part fit-up and corrosion protection, or damage adjacent sheet metal, wiring, interior components, or trim.
Previous work at Ford Research and Advanced Engineering evaluated the potential of a number of factors and techniques to improve keyhole stability in laser welding AA5xxx alloys without filler wire, including penetration, travel speed, laser beam inclination angle in the direction of travel, shield gas composition and delivery method, beam diameter, and beam oscillation frequency.[1] It was found that higher travel speeds improved keyhole stability and thereby weld strength.
The objective of the present investigation was to evaluate the effect of travel speed on keyhole stability during laser welding of AA5xxx alloys with filler wire in lap through joint configuration. The investigation found that higher travel speeds reduce voids and other concomitant keyhole instability discontinuities, thereby increasing weld strength.