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.