Process-Property Relationships and Microstructure Evolution in Friction Stir Welded Thin Sheet 2024-T3 Aluminum Alloy.

Optimum welding parameters, viz, tool rotation, welding speed, plunge depth and tool design are established for friction stir welding (FSW) 1 mm thick 2024-T3 aluminum alloy by using design of experiments and the effects of these parameters on the weld microstructure and mechanical properties are studied. Hardness profiles indicate the presence of two minima in the weld heat-affected zone (HAZ), one directly adjacent to the thermal-mechanically affected zone (TMAZ), and a second farther away from the TMAZ/HAZ boundary, separated by a region of maximum weld hardness. Average hardness in the HAZ minimum adjacent to the TMAZ decreases with an increase in heat input. Various regions of the welds are characterized using transmission-electron microscopy and changes in the type, size and density of precipitates are related to the changes in hardness across the weld. The hardness minima in the HAZ adjacent to the TMAZ, and at the HAZ location further from the TMAZ, are associated with the presence of overaged S precipitates and dissolution of GPB zones, respectively. The hardness peak in the HAZ between these minima results from the presence of GBP II zones.