The demand for both high-strength and lightweight metals for structural applications to increase vehicle mobility has driven an increase in the research and development of lightweight alloys, including those based on aluminum (Al), to replace commonly-used steel. When alloyed copper (Cu), Al alloys can exhibit high tensile strengths. However, Al-Cu alloys are difficult to join using fusion welding processes due to solidification cracking. To circumvent the issues that arise with fusion welding, friction stir welding (FSW) can be employed. FSW is a solid state joining method that utilizes friction generated heat between a rotating tool and two metal plates to create a joint without the use of additional material. FSW temperatures are lower than the melting point and are historically achieved by controlling the welding speed. The welding temperature has largely been both an uncontrolled and unmonitored variable, which presents an issue when trying to predict or explain the weld microstructures. In addition, most FSW investigations are performed on parts that are less than 12 mm thick. This limits the understanding of processing-microstructure-property relationships of thicker FSW plates. This dissertation studied the effect of FSW on the microstructure and mechanical behavior of 25 mm thick AA2139-T8, a precipitation strengthened Al-Cu-Mg-Ag alloy. Processing-microstructure-property relationships were studied using both constant-speed (150 RPM and 50 mm/min) and constant-temperature FSW (performed at 490 ℗ʻC, 500 ℗ʻC, and 510 ℗ʻC). The microstructural evolution throughout the thickness of the weld was analyzed via optical microscopy, SEM, TEM, and XRD. The mechanical behavior was analyzed using tensile and Vickers hardness experiments. The distribution of the different precipitates was plotted throughout the welded area for the constant-temperature FSW materials. The average matrix grain size decreased from the weld top to bottom, while the precipitate volume percent increased from the weld top to bottom. The stir zone (SZ) exhibited lower strengths and hardness than the base metal. In addition, the larger grains in the upper weld nugget (UWN) of the SZ had a higher hardness than the smaller grains in the lower weld nugget (LWN) of the SZ and this was explained by the dissolution of the precipitates during welding and the associated solid solution strengthening. This study was the first to perform constant-temperature FSW on both an Al-Cu-Mg-Ag alloy and a 25 mm thick plate. Compared with the 500 ℗ʻ and 510 ℗ʻC constant-temperature FSW, the 490 ℗ʻC constant-temperature FSW yielded the smallest average grain size and the highest precipitate volume percent through the SZ. The average grain size increased with welding temperature. The 510 ℗ʻC FSW exhibited the lowest room-temperature (RT) tensile YS, UTS, Îæf, and joint efficiency due to the void formation during the welding. The 490 ℗ʻC and 500 ℗ʻC FSWs failed at the interface between the SZ and the thermomechanically affected zone (TMAZ) and in the HAZ, respectively, which were the weakest links of the weld and dictated the tensile properties. The work in this dissertation provided new insights into the effects of constant temperature FSW on the microstructural evolution and the mechanical properties through the weld thickness. The knowledge gained from this work will not only assist in determining optimal welding parameters for this alloy for targeted applications, but will also serve as a framework for future research targeted at understanding processing-microstructure-property relationships of a variety of metallic systems undergoing not only FSW under controlled conditions but also undergoing different controlled thermomechanical processing treatments.
Read
