A novel method, powder thixoforming, for net-shape forming of the particle-reinforced Aluminum matrix composites in semi-solid state has been proposed based on powder metallurgy combining with thixoforming technology. The microstructural evolution and phase transformations have been investigated during partial remelting of the 2024 bulk alloy, prepared by cold pressing of atomized alloy powders to clarify the mechanisms of how the consolidated powders evolve into small and spheroidal primary particles available for thixoforming. The effect of heating temperature on the resulting semisolid microstructure has also been discussed. The results indicate that the microstructural evolution includes three stages—the initial rapid coarsening of the fine grains within the powders, the formation of continuous liquid layer on the primary particle surface (the original powder), and the final coarsening—that result from the phase transformations of θ→α, α→L, and α→L and L→α, respectively. The coarsening rate of the primary particles is low, and one original powder always evolves into one spheroidal particle with a continuous liquid layer surface. Properly raising the heating temperature is beneficial for obtaining an ideal semisolid microstructure.

The microstructure and tensile properties of the thixoforged in situ Mg₂Si_p/AM60B composite were characterized in comparison with the as-cast composite and thixoforged AM60B. The results indicate that the morphology of α-Mg phases, the distribution and amount of β phases and the distribution and morphology of Mg₂Si particles in thixoforged composite are completely different from those in as-cast composite. The Mg₂Si particles block heat transfer and prevent the α-Mg particles from rotation or migration during reheating. Both the thixoforged composite and thixoforged AM60B alloy exhibit virtually no porosity in the microstructure. The thixoforged composite has the highest comprehensive tensile properties (ultimate tensile strength (UTS)) of 209 MPa and an elongation of 10.2%. The strengthening mechanism of the Mg₂Si particle is the additive or synergetic effect of combining the load transfer mechanism, the Orowan looping mechanism and the dislocation strengthening mechanism. Among them, the load transfer mechanism is the main mechanism, and the latter two are minor. The particle splitting and interfacial debonding are the main damage patterns of the composite.

To impart electrical conductivity on magnesium alloy micro-arc oxidation coatings, a graphite/epoxy conductive layer was prepared on the surface of a ceramic layer in this work, focusing on wear behavior and corrosion resistance of the coating. At a graphite weight of 80 wt%, the square resistance of the coating decreased to 217.6 kΩ/□, and it exhibited good resistance. Combined with the distribution of graphite particles in the coating and the change in surface resistance, we determined that the conductive mechanism of the coating occurred through quantum tunneling when the graphite content was 60 wt%. When the graphite content increased from 60 to 80 and 100 wt%, the formation of conductive paths on the surface of the coating further improved the conductivity. The hardness of the organic coatings was positively related to the graphite content. Analysis of the wear scars and wear debris after dry friction and wear testing showed that the wear forms of the coating consisted of abrasive wear when the graphite content was in the range of 20–40 wt%. When the graphite content was in the range of 60–100 wt%, the wear forms of the coating consisted of abrasive wear and peeling wear.

The influences of reheating duration on microstructures and tensile properties of thixoforged in situ Mg₂Si_p/AM60B composites have been investigated. The results indicate that the reheating time affects the volume fraction, composition and distribution of the liquid phase, the evolution of the liquid pools within the primary particles, and the morphology and distribution of the Mg₂Si particles of the semisolid feedstock. These changes then affect the subsequent solidification behavior and deformation mechanism during thixoforging, which are responsible for the resulting microstructure and tensile properties. Mg₂Si particles play a strengthening role for the matrix through the load transfer mechanism. The appropriate reheating time is 60 min at 600 °C when the mold preheating temperature is at 300 °C. The ultimate tensile strength and elongation of the resulting composite are up to 209 MPa and 11.9%, respectively.

The plate-like iron-rich intermetallic phases in recycled aluminum alloys significantly deteriorate the mechanical properties. In this paper, the effects of mechanical vibration on the microstructure and properties of the Al-7Si-3Fe alloy were systematically investigated. Simultaneously, the modification mechanism of the iron-rich phase was also discussed. The results indicated that the mechanical vibration was effective in refining the α-Al phase and modifying the iron-rich phase during solidification. The forcing convection and a high heat transfer inside the melt to the mold interface caused by mechanical vibration inhibited the quasi-peritectic reaction: L + α-Al₈Fe₂Si → (Al) + β-Al₅FeSi and eutectic reaction: L → (Al) + β-Al₅FeSi + Si. Thus, the plate-like β-Al₅FeSi phases in traditional gravity-casting were replaced by the polygonal bulk-like α-Al₈Fe₂Si. As a result, the ultimate tensile strength and elongation were increased to 220 MPa and 2.6%, respectively.