Preparation and mechanical properties of closed-cell CNTs-reinforced Al composite foams by friction stir welding

Closed-cell carbon nanotubes (CNTs)-reinforced Al composite foams were successfully fabricated by the combination of friction stir welding and heat treatment. The microstructure and elemental composition of the CNTs/Al composite foams were analyzed by scanning electron microscopy and energy dispersive analysis (SEM/EDS). Simultaneously, the mechanical properties of the CNTs/Al composite foams were investigated by the quasi-static compression test. The results show that when the rotation speed is 1000 rpm matching with the welding speed of 300 mm/min, the surface morphology of the precursor is smooth and dense. TiH₂ and CNTs can be mixed evenly in the aluminum matrix by the strong stirring of the stirring needle. The pore size and pore morphology of the CNTs/Al composite foams are highly sensitive to change in holding time. The precursor of composite foam occurs with sufficient expansion with highly spherical pores at 680 °C for 15 min. Simultaneously, due to the intense stirring of the pin tool, most CNTs are embedded in the Al foam matrix, and some entangled CNTs have become flat while maintaining the structural integrity. In the stress-strain curves, CNTs/Al composite foams have higher yield stress and wider plateau region than pure Al foam. Both compressive property and energy absorption capacity of CNTs/Al composite foams have a tendency to increase with decreasing porosity and increasing the strain rate. The peak stresses of CNTs/Al composite foams are in the range of 8.5–10.3 MPa at the strain rate of 0.01 mm/s, which are 3–4 times higher than that of pure Al foam.