Meaning
Reduction in hardness and yield strength occurring in the metal immediately adjacent to a weld bead results from the intense thermal cycle of the joining process. This heat-affected zone softening is particularly critical in work-hardened or precipitation-hardened aluminum alloys used for battery casings and busbars. The localized high temperature causes grain growth, over-aging, or recrystallization in the region that did not melt.
This weaker zone becomes the most likely point of failure under mechanical load.
Microstructural Transformation
Rapid heating followed by slower cooling alters the delicate phase distribution within the cold-worked or heat-treated alloy. In strain-hardened metals, the elevated temperature triggers recrystallization, which replaces the high-strength deformed grains with soft, stress-free grains. For precipitation-hardened alloys, the thermal pulse causes the strengthening precipitates to coarsen or dissolve into the matrix.
These microstructural changes permanently reduce the local hardness of the metal.
Mechanical Strength
Tensile testing of welded joints typically results in failure within this softened region rather than the weld metal itself. The localized reduction in yield strength means that any structural strain concentrates almost entirely within the heat-affected zone softening zone. This concentration can cause premature ductile failure of the joint at loads far below the nominal rating of the base metal.
Mitigation Strategy
Minimizing the total heat input through the use of high-speed laser welding reduces the width of the affected zone.
