Objective Selective laser melting(SLM) is an important method to realize functional optimization design and manufacture lightweight metal parts. The parts fabricated by SLM possess have a fine microstructure and excellent mechanical properties due to the rapid cooling rate. Some typical metals, such as aluminum alloys, Ni alloys, and Ti alloys, have been manufactured successfully by SLM and are used widely in the aerospace, automobile, and marine industries. In recent years, aluminum matrix composites have attracted considerable attention because of the advantageous properties of the matrix and reinforcement materials. Compared to other reinforced particles, SiC particles are the most common ceramic reinforcement because of their easy availability, low cost, and high hardness. However, few studies have examined the relative density, microstructure, and properties of SLMed SiC/Al composites, particularly the tensile properties. In this study, 5%SiC/AlSi7Mg composite specimens were prepared by SLM at different process parameters, and an almost entirely dense specimen was obtained. SiC particles and Al4SiC4 phases formed during the in situ reaction were distributed uniformly throughout the aluminum matrix, and strong metallurgical bond existed at the interface. Such aluminum matrix composites posse high tensile strength and yield strength but low ductility compared with the SLMed aluminum alloy. The fracture mode of the SLMed composites was mainly brittle fracture.
Methods The original powders used in this study were SiC powders and gas atomized AlSi7Mg powders. The mixed powders with 5% SiC powders were prepared using a V-type mixer. The SiC/AlSi7Mg composite specimens were then fabricated with SLM 125 equipment using different SLM process parameters in an argon atmosphere. Subsequently, the Archimedes method was used to measure the relative densities of the composite specimens. The microstructure of the SLMed composites was observed by optical microscopy and scanning electron microscopy after grinding, polishing, and etching in Keller reagent. The phase identification of the specimen was analyzed by X-ray diffraction. The tensile properties were examined using an electronic universal testing machine at room temperature. In addition, the fracture morphology of the composite was also characterized by scanning electron microscopy.
Results and Discussions With increasing scanning speed and hatch spacing, the relative densities of the SLMed SiC/AlSi7Mg composites increased initially and then decreased (Fig.4). The relative density of the composite reached up to 99.2 % under the optimized process parameters (laser power of 300 W, scanning speed of 1400 mm/s, hatch spacing of 0.12 mm, and layer thickness of 30 μm). The typical fine zone, coarse zone, and heat-affected zone also can be found in the microstructure of the SLMed composite. New needle-like Al4SiC4 phases formed during the SLM process because of the in situ reaction of SiC particles and molten aluminum matrix. The SiC particles and Al4SiC4 phases were distributed uniformly in the matrix due to the Marangoni flow. The in situ reactions occurring on the surface of SiC particles promoted the wettability of the SiC particles and molten aluminum matrix. No pores or cracks were observed in the interface, indicating a strong metallurgical bonding. The SiC and Al4SiC4 reinforced phases in the matrix enhanced matrix strength that could bear the stresses transferred from the matrix. However, they also hindered the dislocation movement and interface migration, and the deformation resistance of the composite was improved. The tensile strength and yield strength of the SLMed composite increased to 452 MPa and 280 MPa, respectively, but the elongation decreased to 4.5%. The cleavage plane observed in the fracture morphology also showed brittle fracture.
Conclusions The SiC/AlSi7Mg composite specimens were manufactured successfully by the SLM process. The relative density of the SLMed composite increased initially and then decreased with increasing scanning speed and hatch spacing. The SLMed composite exhibited a relative density of 99.2% under the optimized parameters. The microstructure of the composite was similar to the SLMed aluminum alloy, where the typical fine zone, coarse zone, and heat-affected zone exist. The new Al4SiC4-reinforced phases were formed in the aluminum matrix and at the interface of the SiC particles and matrix caused by the in situ reactions between the SiC particles and molten aluminum alloy. Good metallurgical bonding in the interface was formed. The SiC and Al4SiC4-reinforced phases were distributed uniformly throughout the aluminum matrix. The strength of the SLMed composite was improved by the addition of SiC particles and the formation of an Al4SiC4 phase, but the ductility decreased compared to SLMed AlSi7Mg. The tensile strength, yield strength, and elongation of the SLMed composite were 452 MPa, 280 MPa, and 4.5%, respectively. The fracture mode of the SLMed composites was mainly brittle fracture.
