Reinforcement strategies for producing functionally graded materials by friction stir processing in aluminium alloys

• Research different strategies for reinforcing Al with hard particles to produce FGMs.
• Pre-deposition of particles + FSP leads to homogeneous thin layers.
• Window of processing parameters is narrow and controlled by Ω/Vx ratio.
• Consumable tool drilled and filled with particles + FS produce heterogeneous thick layers.

The present work addresses the production of aluminium based FGM composites by friction based processes, namely FSP and a consumable tool approach. Several strategies were investigated, featuring the use of SiC and alumina reinforcements. These strategies were implemented and surfaces analyzed to evaluate the influence of deposition and processing on particle distribution and homogeneity. Three strategies were studied: a square shaped groove packed with reinforcement particles, the pre-deposition of a uniform layer of particles prior to FSP with a non consumable tool, and the last one consisted of a consumable rod in aluminium drilled with holes placed in different positions along a radial line filled with reinforcing particles. Coatings were examined by optical microscopy, scanning electron microscopy, as well as, hardness testing. The strategies investigated proved to be possible in the production of surface layers reinforced with hard materials by FSP in aluminium based alloys. While the pre-deposition of reinforcements directly on plate surface enables the simplest approach to produce composites by FSP, the packing of reinforcements in grooves can increase the composite layer thickness. The consumable rod approach allows depositing composite layers soundly bonded to the substrate while avoiding FSP tool wear.