Fatigue Performance Of Friction Stir Butt Welds In A6000 Series Aluminum Alloy

Abstract

Friction stir welding is a novel solid state joining process for making low cost, energy efficient butt welds in aluminum alloy extrusions. The plate edges are clamped against a backing plate and the material is plastically deformed and stirred by a rotating tool moving along the joint line. The resulting weld bead is flush with the surface and exhibits little distortion. The material in the weld and heat affected zone (HAZ) has a fine-grained microstructure and a high tensile strength compared with welds produced by conventional arc welding methods. The present investigation was undertaken to determine the fatigue properties of friction stir welds in 5 mm thick plates in an AA6082 alloy. Extruded plates in the T4 condition were used in the test program. S-N tests in pulsating tension at R = 0.5 were performed on specimens with the weld transverse to the stress direction. Reference tests were made on the base material. Crack growth data were obtained for material in the weld metal, in the HAZ and base material. S-N tests were also made on conventional MIG butt welds from the same batch material to enable a comparison of the two welding methods. The results indicate that the fatigue strength of transverse friction stir welds is approximately 50 percent higher than the fatigue strength of MIG butt welds. The crack growth rates obtained for the weld material were lower than in the base material, probably due to a more fine grained microstructure in the weld region. INTRODUCTION The friction stir welding process has recently been developed as a cost effective alternative to conventional metal inert gas (MIG) and tungsten inert gas (TIG) Transactions on Engineering Sciences vol 8, © 1995 WIT Press, www.witpress.com, ISSN 1743-3533 226 Surface Treatment Effects II welding in aluminum alloys [1]. A major advantage of friction stir welding is that it is a solid state process involving a much lower heat input than that required in conventional arc welding methods. The weld itself and its adjacent narrow heat affected zone both have a very fine-grained microstructure with high mechanical strength. The high tensile strength of the weld material and the favorable geometry would also indicate that friction stir welds could have high levels of fatigue strength. A testing program was implemented to determine the fatigue properties of transverse butt welds of two alloys in the AA6000 series. The data presented in this paper are results from introductory tests on specimens fabricated from extruded plates in AA6082 material in the T4 temper condition. THE FRICTION STIR WELDING PROCESS In friction stir welding the plates to be joined are clamped on a backing plate to prevent movement A cylindrical shouldered tool with a specially profiled pin is rotated at a high speed, see Fig. la. The pin is slowly brought into contact with the joint line, and the material is heated by friction and plasticised in an annular volume around the pin. As the pin is lowered into the plates, soft material is extruded at the surface. Upon further lowering of the pin and movement along the joint line the shoulder face contacts the plate surface and the plasticised material is compressed against the face of the shoulder. The soft material is mashed by the leading face of the pin profile and transported to the trailing face of the pin where it consolidates and cools to form a solid-phase weld. The generation of a friction stir weld has many similarities with extrusion seam welds that form when material is joined in the weld chamber of an extrusion die [2]. The material flow, however, is somewhat different due to the more extensive mechanical mixing of the material from the two plates in the friction stir process. The properties of the weld are closely related to the tool technology. The tool bit shape and material determines the heating, plastic flow and forging pattern. Development of the friction stir welding process has up to now been concentrated mainly on butt and lap joints, however, introductory tests have shown that friction stir welding is suitable for a wide range of joint configurations [4], as shown in Fig. 2. EXPERIMENTAL PROGRAM The specimens were fabricated from AA6082 alloy plate material, in the T4 (as-extruded) condition. The plate thickness was 5 mm. The mechanical properties are listed in Table 1. Transactions on Engineering Sciences vol 8, © 1995 WIT Press, www.witpress.com, ISSN 1743-3533 Surface Treatment Effects II 227 Table 1. Mechanical properties of the AA6082 alloy in T4 temper.