Friction Stir Welding of Metal Matrix Composites to Dissimilar Aluminum Alloys: Optimization of Weld Quality and Tool Life

The emergence of metal matrix composites (MMC) offer many advantages for the aerospace industry including high specific stiffness and strength. Joining of MMCs can be very challenging as riveting can add significant weight, complexity and cost. Friction Stir Welding (FSW) can join MMCs, but often experiences severe wear of the tool due to the SiC particles limiting its commercial viability. This work will summarize a development effort to design long life FSW tools and optimize the welding parameters in several MMCs for use as stiffener for aerospace panels. The MMCs under consideration are Materion’s 217XG (2124A/SiC/17p), 225XF (2124A/SiC/25p), and 620XF (6061A/SiC/20p) SupremEX plate material while the skin material will be bare 0.190” 2024-T3. One key attribute of the MMC’s under consideration are that they contain sub-micron particulate (0.3µm to 0.7µm average) which is expected to reduce the chance of tool chipping during machining.

FSW tools were designed using a next generation TiC based MMC for high wear resistance at elevated temperatures. The welding parameters were optimized using a series of design of experiments (DOE) focusing maximizing weld strength and travel speed for each alloy in lap joint configuration. The DOEs focused upon the effects of rotational velocity, travel speed, pin length and plunge depth on weld strength and interface disruption. Through experimental work, it was determined that deformation of the AA2024 into the MMC top sheet was detrimental to weld properties. Weld quality and the effects of FSW on the MMC were examined using lap shear tensile testing, mini-tensile testing, metallographic examination, and scanning electron microscopy. Tool life was examined using repeated bead of plates welding trials in the small particle MMCs and an additional large particle (5 µm) MMC for a variety of FSW tool materials to determine the best tool material and effects of SiC particle size.