Compared to conventional material processing techniques, selective laser melting (SLM) technology possesses several advantages, including rapid processing capabilities and the ability to fabricate complex parts. The increasing demand for lightweight aluminum matrix composites has imposed higher requirements on their preparation methods. Graphene has excellent mechanical properties and is an ideal reinforcement material for metal matrix composite. However, the uniform dispersion of graphene into metals remains a challenge, and the strengthening mechanism of graphene nano-platelets (GNPs) in SLM-fabricated GNPs/AlSi10Mg composite materials needs to be explored. This study aims to investigate the effect of varying GNPs mass fraction (0, 0.1%, 0.3%, and 0.5%) on the microstructure and mechanical properties of SLM-fabricated AlSi10Mg composites, to reveal the strengthening mechanism of GNPs-reinforced AlSi10Mg composites.

Different types of GNPs/AlSi10Mg composite powders were prepared using a QM-3SP4 ball mill. The ball-milling parameters were set as follows: a ball material ratio of 8∶1, speed of 230 r/min, and ball-milling time of 2 h. Different types of GNPs/AlSi10Mg composite materials were prepared using a laser melting forming equipment (AM400, Renishaw, UK). Tensile testing was conducted using a universal tensile-testing machine (SHIMADZU AG-X plus). Sample hardness was measured using a precision automatic turret digital microhardness tester (JMHVS-1000AT type). The crystallographic structure was analyzed using an electron backscatter diffusion (EBSD) equipment, integrated into a JEOL JSM-7800F field-emission scanning electron microscope. The microstructure and phase composition of the composite materials were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS).

EBSD inverse pole figure (IPF) maps of the longitudinal section of the AlSi10Mg alloy and 0.5%GNPs/AlSi10Mg composite show that the addition of graphene does not change the grain orientation but refine the grains (Fig.4). The EBSD grain boundary misorientation distribution in Fig.5 indicates that the addition of GNPs increases the number of nucleation sites and hinders grain growth, resulting in an increase in LAGBs inside the melt pool and a decrease in the toughness. The SEM and XRD results show that the microstructures of the matrix and composite are composed of fine grains, coarse grains, and heat-affected zones. The dark grey color represents the aluminum solid solution phase, while the light grey network structure represents the eutectic silicon phase. Tensile tests show that the ultimate tensile strength of the 0.1%GNPs/AlSi10Mg composite material is (383±4) MPa, with an elongation at break of 8.4%±0.14%, indicating a good ductility strength. As the amount of GNPs increases, the ultimate tensile strength (UTS) and yield strength (YS) of the GNPs/AlSi10Mg composite materials show a decreasing trend. With an increase in the GNPs content, the fracture morphology of the composite material (Fig.9) shows a series of small cleavage steps, a river-like pattern, small tearing edge undulations, and plastic fracture characteristics formed by ductile dimples, transitioning towards brittle straight fractures. The strengthening mechanism of the 0.1%GNPs/AlSi10Mg composite material formed by selective laser melting is mainly controlled by the synergistic effect of thermal mismatch and load transfer strengthening. The pinning effect of the GNPs on the dislocations provides additional interfaces that can hinder their movement. In the 0.1%GNPs/AlSi10Mg composite material, the interface bonding between the GNPs and matrix is strong, and the dislocation resistance is high, resulting in the best plasticity and toughness among the composite materials.

The preferred orientation of the SLM-fabricated AlSi10Mg alloy is〈100〉. The addition of GNPs does not change the preferred orientation of the GNPs/AlSi10Mg composite material, although it reduces the proportion of the large-angle grain boundaries in the composite material. The phase composition of all GNPs/AlSi10Mg composite materials investigated are α-Al and eutectic silicon phases. As the GNPs content in the composite material increases, the hardness also increased, reaching a maximum of 168 HV. However, an increase in the number of GNPs led to an increase in the number of defects in the composite material. The ultimate tensile strength, yield strength, and elongation of the 0.1%GNPs/AlSi10Mg composite materials are (417±4) MPa, (254±5) MPa, and 8.4%±0.14%, respectively. These values gradually decrease to (224±6) MPa, (150±3) MPa, and 4.0%±0.45%, respectively, for 0.5%GNPs/AlSi10Mg composite material. The strengthening of the 0.1%GNPs/AlSi10Mg composite material is mainly controlled by the synergistic effect of the thermal mismatch and load transfer strengthening.