Manufacturing high strength aluminum matrix composites by friction stir processing: An innovative approach

Shape Memory Alloys (SMA), e.g., NiTi (nitinol), are good candidates of reinforcement agents for the improvement of mechanical properties of aluminum alloys. Friction Stir Processing (FSP) has been shown to be an appropriate manufacturing process for particulate Aluminum Matrix Composites (AMCs), due to the mitigation of critical intermetallic formation between particles and matrix. However, AMCs processed by FSP with the matrix material made of high strength alloys, in particular 7xxx series, have systematically shown severe defects, such as clusters and cavities that result from poor material flow. Herein, we present an innovative strategy to manufacture Al7075/NiTi composites via FSP avoiding this unacceptable particle clustering. The strategy involves the addition of 7xxx series Al powder in a groove or multi-holes to be stirred together with the NiTi particles, leading to dissociation of the latter. The X-ray computed tomography acquisitions allow for 3D quantitative assessment of the size and spatial distribution of the embedded reinforcements. It is revealed that the groove filling method results in a more homogenous distribution compared to the multi-holes filling method. Moreover, the distribution is found to be dependent on the particle size, as a higher volume fraction of large particles is observed at the upper part of the FSP stir zone. These new AMCs, with a good Al-NiTi interface, present a potential to tailor the composite mechanical properties by exploiting the shape memory effect of the NiTi reinforcements.