The effects of cooling rate 0.15, 1.5, 15, 150, and 1.5 × 105 °C/s on the microstructures and mechanical properties of Al-13Si-4Cu-1Mg-2Ni cast piston alloy were investigated. The results show that with an increase of solidification cooling rate, the secondary dendrite arm spacing (SDAS) of this model alloy can be calculated using the formula D = 47.126v − 1/3. The phases formed during the solidification with lower cooling rates primarily consist of eutectic silicon, M-Mg₂Si phase, γ-Al₇Cu₄Ni phase, δ-Al₃CuNi phase, ε-Al₃Ni phase, and Q-Al₅Cu₂Mg₈Si₆ phase. With the increase in the solidification cooling rate from 0.15 to 15 °C/s, the hardness increased from 80.9 to 125.7 HB, the room temperature tensile strength enhanced from 189.3 to 282.5 MPa, and the elongation at break increased from 1.6% to 2.8%. The ε -Al₃Ni phase disappears in the alloy and the Q phase emerges. The δ phase and the γ phase change from large-sized meshes and clusters to smaller meshes and Chinese script patterns. Further increase in the cooling rate leads to the micro hardness increasing gradually from 131.2 to 195.6 HV and the alloy solidifying into a uniform structure and forming nanocrystals.

In this paper, Al₂O₃/7075 composites were prepared by mechanical alloying with subsequent hot-pressing sintering, and the effect of Al₂O₃ nanoparticle on the mechanical and tribological behavior of 7075 was studied. The mechanical property results showed that the hardness and compressive strength of Al₂O₃/7075 composites first increased and then decreased with Al₂O₃ amount increasing, and 5 wt.% Al₂O₃ addition made the material exhibit excellent comprehensive mechanical properties. The tribological properties also indicated that 5 wt.% Al₂O₃ nanoparticle significantly improved the high-temperature wear resistance of 7075 alloys. Thus, all the mechanical and tribological results confirmed that the addition of Al₂O₃ nanoparticle was a better strengthening way for 7075 alloys at high temperatures.

The oxidation mechanism of vermicular graphite cast iron was studied. The oxidation reaction starts from graphites and diffused slowly. Graphites in vermicular graphite are interconnected, coral-like clusters, providing the main oxidation core and channel. The worm-like graphites on the surface are mostly oxidized and form oxide affected zones. The oxide films are composed of a loose oxide layer with the phases of Fe₃O₄, Fe₂O₃, and FeO, and a dense passivation layer with FeO and Fe₂SiO₄. After oxidation, pearlites in the vermicular graphite cast iron are decomposed into ferrite and cementite at high temperatures.

Al 7075 alloy, 15 wt.% VN/7075 composites, and 20 wt.% TiB₂-TiC_x/7075 composites were prepared by ball milling with subsequent hot-pressing sintering. The microstructure, hardness, and wear properties at room temperature to 200 °C of Al 7075-based composites with different reinforcement phases were discussed. The grain uniformity degree values of 15 wt.% VN/7075 composites and 20 wt.% TiB₂-TiC_x/7075 composites were 0.25 and 0.13, respectively. The reinforcement phase was uniformly distributed in 15 wt.% VN/7075 composites and 20 wt.% TiB₂-TiC_x/7075 composites, almost no agglomeration occurred. The order of hardness was 20 wt.% TiB₂-TiC_x/7075 composites (270.2 HV) > 15 wt.% VN/7075 composites (119.5 HV) > Al 7075 (81.8 HV). At the same temperature, the friction coefficient of 15 wt.% VN/7075 composites was the lowest, while the volume wear rate of 20 wt.% TiB₂-TiC_x/7075 composites was the lowest. With the increase of temperature, the wear mechanism of Al 7075 changed from spalling wear to oxidation wear and adhesion wear. However, the wear mechanisms of 15 wt.% VN/7075 and 20 wt.% TiB₂-TiC_x/7075 composites changed from abrasive wear at room temperature to wear mechanism (oxidation wear, abrasive wear, and adhesive wear) at medium and low temperature. Comprehensive wear test results indicated that 20 wt.% TiB₂-TiC_x/7075 composites had excellent tribological properties at medium and low temperature.