Ultrafine-Grained Al/Al2Cu Composite Formation via Friction Stir Processing of Cold-Sprayed Coatings

Abstract We demonstrate a shear-deformation-driven, solid-state phase transformation pathway for the formation of an ultrafine-grained Al/Al $$_2$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mn>2</mml:mn> <mml:mrow/> </mml:mmultiscripts> </mml:math> Cu composite via friction stir processing of a Cu cold-sprayed coating on an AA6061 aluminum substrate. This approach leverages the severe plastic deformation and high strain-rate environment inherent to friction stir processing to drive localized interdiffusion and solid-state reactions between the Cu coating and the Al alloy substrate. The processed surface exhibits a significant increase in hardness ( $$\approx $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>≈</mml:mo> </mml:math> 250 HV), compared to both the AA6061 substrate ( $$\approx $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>≈</mml:mo> </mml:math> 100 HV) and the as-deposited Cu coating ( $$\approx $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>≈</mml:mo> </mml:math> 132 HV); these measured hardness values represent an increase of 1.8-times and 2.4-times relative to the Cu CS coating and AA6061 substrate, respectively. This hardness enhancement is attributed to the uniform distribution of fine-grained Al $$_2$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mn>2</mml:mn> <mml:mrow/> </mml:mmultiscripts> </mml:math> Cu reinforcement within an Al(Cu) matrix, as confirmed by transmission electron microscopy and atom probe tomography. Unlike conventional precipitation hardening, here, discrete Al $$_2$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mn>2</mml:mn> <mml:mrow/> </mml:mmultiscripts> </mml:math> Cu grains are directly formed and dispersed among Al grains, resulting in a hetero-grained microstructure that transitions into a single-phase matrix below the processed zone. Our results demonstrate the potential of integrating solid-state deposition with high-speed mechanical mixing to generate unique, non-equilibrium microstructures that bypass equilibrium melting constraints and exceed the performance of conventional thermomechanical processing routes.