Effect of post-weld heat treatment on the mechanical properties of 2219-O friction stir welded joints

Abstract

As a solid-state process, friction stir welding (FSW) can avoid the formation of solidification cracking and porosity associated with fusion welding processes and significantly improve the weld properties of aluminum alloys [1]. However, many studies on the mechanical properties of FSW joints of heat-treatable aluminum alloys such as 2017-T351 [2], 2024-T6 [3], 2195-T8 [4], 2519-T87 [5], 6061-T6 [6, 7], 6063-T5 [8] and 7075-T651 [9] have indicated that FSW gives rise to softening of the joints and results in the significant degradation of the mechanical properties. In order to restore the mechanical properties of the FSW joints, a post-weld ageing heat treatment to 6082 aluminum alloy joints has been performed [10]. When the base material was in T6 condition, the microhardness in the weld zone increased, but the one in the heat-affected zone decreased; while the base material is in T4 condition, the micro-hardness in the entire joints increased. Another post-weld solution and ageing heat treatment to 6061-O aluminum alloy joints indicated that the micro-hardness values across the joint significantly increased, while the toughness of the joints deteriorated [11]. These results imply that the effects of the post-weld heat treatment (PWHT) on the mechanical properties of the joints are related not only to the base material conditions, but also to the PWHT processes. The present letter demonstrates the effect of the post-weld solution-ageing heat treatment on the mechanical properties of 2219O aluminum alloy FSW joints. The emphasis is placed on the tensile properties and fracture locations of the joints. The base material used in this study was a 5-mm-thick 2219-O aluminum alloy plate, with the chemical compositions and mechanical properties listed in Table I. The welding samples, 260 mm by 50 mm, were longitudinally butt-welded using an FSW machine. The welding tool size and welding parameters are listed in Table II. After welding, the samples were cut into two parts, one part for the PWHT and the other part for the as-welded examinations. The PWHT process includes solid solution at 535 ◦C for 32 min, quenching in water at 25 ◦C, and ageing at 165 ◦C ∗Author to whom all correspondence should be addressed. for 18 hr. After the heat treatment, all the joints, including as-welded joints, were cross-sectioned perpendicular to the welding direction for metallographic analyses and tensile tests. The cross-sections of the metallographic specimens were polished with a diamond paste, etched with Tucker’s reagent and observed by optical microscopy. Vickers hardness distributions in the joints were measured along the centerlines of the cross sections under a load of 4.9 N for 10 s, and the distance between the adjacent measured points was 1 mm. The room-temperature tensile test was carried out at a crosshead speed of 1 mm/min using an Instron-1186 testing machine. The marked length of each specimen was 50 mm, and the tensile properties of each joint were evaluated using three tensile specimens cut from the same joint. Fig. 1 shows the tensile properties of the joints welded at different welding speeds before and after the heat treatment. The tensile properties of the as-welded joints almost do not change with the welding speed (see Fig. 1a). The tensile strength is equivalent to that of the base material, and the elongation is changed between 10 and 12%. However, the tensile properties of the heat-treated joints increase with increasing welding speed (see Fig. 1b). The tensile strength is up to 385 MPa, equivalent to 2.4 times that of the base material, and the maximum elongation is 3.2%. These results indicate that the heat-treated joints possess a higher tensile strength and a lower elongation than the as-welded joints. Fig. 2 shows the fracture locations of the joints before and after the heat treatment. As seen from this figure, the as-welded joints fracture in the base material zone (BMZ) (see Fig. 2a), while the heat-treated joints fracture in the weld zone (WZ) (see Fig. 2b). This implies that the PWHT process has a significant effect on the fracture locations of the joints. That is to say, the BMZ is a weak part of the joints before the heat treatment, while the WZ becomes the weak part of the joints after the heat treatment is performed. In most cases, the tensile properties and fracture locations of the joints are related to the hardness