Development of healable aluminium-alloys reinforced with magnesium-silicon particles by friction stir processing

The objective of this master thesis is to develop new aluminium-based metal matrix composites (MMCs) by means of friction stir processing (FSP), which inherit the ability to heal damaged regions with the help of fast-diffusing atoms triggered by a brief heating treatment. To this aim, Al6063-T4 was selected as base material, which got enriched with different powder mixtures consisting of magnesium and silicon. The main focus laid in analysing the influence of those reinforcements on the healing kinetics and the mechanical properties when varying their amount and Mg:Si-ratio. The powder was placed inside a groove on the Al6063-T4 plates and was then embedded into the base metal by FSP. After eight passes, small and finely-dispersed precipitates were obtained, whereas performing more passes caused growth and elongation of the particles. Additional silicon amplified this expansion, generated higher volume fractions of particles and led to a gradient of their size beginning at the bottom of the stirred zone and increasing towards the top layer. Microhardness measurements revealed the same trend, where the top of the nugget zone exhibited superior hardness compared to the bottom region. Homogeneous distributions were achieved after eight passes of FSP and the enrichment of the base material by Mg-Sipowder enhanced the mean hardness by 10-15% depending on the proportions. Finally, the healing abilities were investigated by tensile testing of heat-treated specimens. The pure magnesium reinforced composition presented considerable recovery capabilities, whereas supplementary silicon provoked a noticeable diminishing of the damage mitigation effect. Nonetheless, both powder-enriched compositions exhibited a significant increase in yield and ultimate tensile strength.