Effect of graphene content on the microstructure and mechanical properties of graphene-reinforced Al-15Si-4Cu-Mg matrix composites

A graphene-nanoflakes (GNFs)-reinforced GNFs/Al-15Si-4Cu-Mg composite was prepared through low-temperature ball-grinder milling and vacuum hot-press sintering. The influences of the GNFs mass content on the microstructural and mechanical properties of the GNFs/Al-15Si-4Cu-Mg composite were investigated via scanning electron microscope, X-ray diffraction, energy disperse spectroscopy, and transmission electron microscope. Meanwhile, tensile strength and micro-hardness tests were conducted. The corresponding result show that for the specimens with 0.4% and 0.8% (mass fraction) GNFs in mass fraction, the nanoflakes are concentrated on the border of the aluminum alloy grain and played a major role in restraining the matrix grain expansion and avoiding crystal particle coarsening. Moreover, the interface bonding between the GNFs and Al-15Si-4Cu-Mg matrix is strong. There are primary β-Si particles, Mg₂Si, and Al₂Cu-phase precipitated dispersedly throughout the aluminum matrix. The strong interface bonding between the GNFs and Al-15Si-4Cu-Mg matrix leads to the effective impeding of the dislocation slippage and the improvement in the properties of the GNFs/Al-15Si-4Cu-Mg composites. With the addition of the 1.0% GNFs, it is difficult for the GNFs to disperse but easy for them to cluster together to form black impurities on the grain border, inducing brittle Al₄Cu₂Mg₈Si₇ phase precipitation along the aluminum alloy grain boundary. As the content of GNFs increases, the composite tensile strength first increases and then decreases. With an addition of 0.8% GNFs, the composite exhibited higher strength and micro- hardness (321 MPa of tensile strength and HV 98 of micro hardness), with the strength and micro-hardness increasing by 19.3%和46.2%, respectively, compared with the pure Al-15Si-4Cu-Mg composite without added GNFs. With the addition of 0.4% GNFs, the yield strength reaches 221 MPa; however, the micro-hardness and ductility (elongation rate) are enhanced. The combined properties of the GNFs/Al-15Si-4Cu-Mg composite obtained are clearly improved.