Strengthing mechanism of in-situ synthesized particles in friction stir processed aluminum alloy

Friction stir processing (FSP) was conducted by using aluminum alloy plate 1100-H14. Prior to stiring, a rectangular groove was machined on the plate along the center of stir pass, in which nickel powder was filled. Via in-situ synthesis during the processing thermal cycle, Ni particles in Al base metal would react with Al to form hard particles of NiAl intermetallics. Thus, a composite layer would be fabricated. The results show that Ni particles were stir-crushed to pieces and NiAl₃ reactive particles, in submicron size, were formed. A portion of NiAl₃ particles were further refined by stir-crushing. The microhardness test shows evident increase of the hardness distributions in the composite layer. Compared with ceramic powders added in FSP, physical models were proposed to describe both particle-strengthing mechanisms. In-situ synthesesed intermetallic particles have the crystal boundaries to base metal with strong metallic bonds (twinned crystal boundary, for example), which makes difficulty for dislocatons to pass around that forms crystal boundary strengthing mechanism, whereas the stir-crush refined particles issue fine particle dispersive strengthing mechanism. However, in the friction stir processing by adding hard ceramics particles, much weak bonds between particles and base metal contributes little material strengthing as dislocations may easily pass around.