Microstructural variation due to heat gradient of a thick friction stir welded aluminum 7449 alloy

Abstract

Welding of precipitation strengthened (PS) aluminum alloys leads to a knockdown of mechanical properties in the weld zone. Reduced mechanical properties in the weld region, impact the design of load-bearing structures resulting in heavier engineering components. So far even friction stir welding (FSW), an advanced solid-state welding technique, has not solved the issue of a reduction in mechanical properties of PS aluminum alloys, especially in thick plates. This issue remains unresolved due to limited choice of processing conditions and the lack of clear insight into the FSW of precipitation strengthened Al alloys. The present study combines experimental and computational tools to maximize the strength of the welded structure of a PS Al 7449 alloy. A series of welds were made using various parameters, which ranged from cold runs (low rotational rate and high traverse speed) to hot runs (high rotational rate and low traverse speed). Even though the heat gradient is observed for all welds, and a difference is observed at the top and bottom of the nugget for all welds, a set of parameters (150 RPM/ 8 IPM) was chosen for microstructural evaluation. This particular combination showed significant higher hardness values at the bottom of the nugget compared to the base metal. The selection of FSW processing parameters were guided by a calibrated finite element thermal model of FSW. Because of the plate thickness, a significant heat gradient exists through the thickness of the weld, resulting in microstructural variations in the welded zone. Differential scanning calorimetry and transmission electron microscopy captured the microstructural evolution in the welded structure. Mechanical properties were measured using Vickers microhardness on the welded plates. The heat affected zone (HAZ) and the nugget were of primary interest. An improvement in HAZ has been observed after natural aging, while exceptionally high strength was observed at the bottom of the nugget region independent of the aged condition.