Precipitation phenomena in Al2O3/Al-Mg-Si metal matrix composites manufactured via friction stir processing
Marta Lipińska, Florian Pixner, Andreas Hütter, Milena Mras, Kacper Jaworski, Norbert Enzinger, M. Lewandowska
Abstract
In this study, metal matrix composites (MMC) were fabricated from a precipitation-hardened Al-Mg-Si alloy and reinforced with Al 2 O 3 nanoparticles using a friction stir processing (FSP) technique. Post-process heat treatment (PPHT) was carried out to initiate the precipitation processes. The results were compared with those of FSP-treated samples without reinforcement, for which the aging parameters had been optimized. The results showed that the presence of reinforcement effectively inhibited grain growth. Regardless of the PPHT conditions, the average grain size of the MMC was ∼ 4 µm. On the other hand, the presence of Al 2 O 3 reinforcement strongly influenced precipitation processes. After FSP and subsequent aging, the formation of precipitate-free zones was observed next to the Al 2 O 3 particles, while precipitation processes occurred in the inter-particle regions. The resulting β” precipitates were larger than those observed in the non-reinforced sample. Full heat treatment (consisting of solution annealing, water quenching and aging) applied to the MMC after FSP resulted in homogeneous but accelerated precipitation processes, leading to a precipitate state typical of the overaged condition. The loss of precipitation strengthening cannot be compensated by a reduction in grain size alongside Al 2 O 3 particle strengthening. Therefore, the highest microhardness was measured for a non-reinforced sample that underwent FSP followed by solution annealing, water quenching and aging. This sample’s microstructure consisted of coarse grains (∼ 400 µm) with small, homogeneously distributed β’’ precipitates. The combination of FSP and heat treatment offers great potential for producing high-strength composite materials, however, PPHT conditions must be optimized. • Al 2 O 3 /Al-Mg-Si composites were manufactured via friction stir processing. • Reinforcement had a detrimental effect on the precipitation phenomena. • Aging resulted in the presence of precipitate-free zones next to the reinforcement. • Full heat treatment resulted in overaging compared to unreinforced samples. • The strengthening from Al 2 O 3 and grain size is smaller than that from precipitates.