Investigation On The Hot Extrusion Microstructure And Properties Of Stir-casting (SiCw+SiCp)/Al Composite

The preparation of aluminum matrix composites by stirring casting method has the advantages of low cost,casting complex shape parts and low cost,but it is difficult to control the distribution state of micro-nano reinforcements and casting defects during stirring casting.In this paer,the preparation method and properties of micron silicon carbide particles(SiCp)and submicron silicon carbide whiskers(SiCw)hybrid enhanced aluminum-based composites by stirring casting were studied.The microstructure and properties of(13vol.%SiCp+2vol.%SiCw)/2024 Al composites were investigated by mechanical ball milling pre-dispersion SiCw and semi-solid stirring casting.The effects of hot extrusion and aging on the properties of the composites were investigated.The whisker was uniformly mixed with aluminum powders by low energy mechanical ball milling at the ratio of aluminum powder to SiCw volume fraction 7:3,spherical ratio 10:1,filling coefficient 0.3,ball milling speed 100 rpm,ball milling time 1 h.No whisker aggregations were observed and the whisker had high aspect ratio.The(SiCp+SiCw)/2024 Al composite was successfully prepared by semi-solid stirring casting using mixed powders of aluminum and SiCw.The semi-solid stirring temperature was 630?,the stirring time was 30 min,and casting temperature was 660? after the stirring process was finished.The homogeneous distribution of particles and whisker reinforcements was achieved in the composites,and the density of the ingot was over 97%.As-cast composite had a tensile strength and elongation of 174.2 MPa and 0.37%,respectively,with obvious brittle fracture characteristics.The fracture surface exhibited a small amount of interfacial debonding between the SiCp and the aluminum matrix,as well as the SiCw fracture phenomenon,indicating that the particles and whiskers played a load bearing role.After aging heat treatment of as-cast composite,the tensile strength increased but plasticity decreased.Under the conditions of extrusion temperature 500? and extrusion ratio 16:1,the composite bar with smooth surface can be obtained at 10 mm/min extrusion speed.A reticular Al2 Cu eutectic phase presented in as-cast composites was broken into fine granular form after hot extrusion.The density of the composites increased to more than 99%,and the SiCp distribution uniformity was improved,while SiCw aligned along the extrusion axial orientation.The mechanical properties of the hot extruded composite was improved significantly after 500? of solid solution for 4 h?190? aging for 8 h.The tensile strength reached 450.7 MPa,elongation 1.5% and elastic modulus around 110 GPa.The strength of the composites decreases with the transition from ductile fracture to brittle fracture under over aging condition.Under the same load conditions,the dry friction coefficient of the composites in the as-cast,extruded and extruded state increased in turn,while the wear rate decreased in turn.The wear mechanism of as-cast composite materials changed from abrasive wear under low load to adhesive wear under high load,and the wear rate of extruded composite materials decreased significantly due to the bearing action of Si C whisker.