Search

7A85-T74 forged aluminum alloy was chosen as the experimental material, and the microstructure, tensile properties and impact energy of the alloy were investigated after 5 h of thermal exposure at room temperature to 240℃. The mechanism of the influence of the microstructure on the mechanical properties of 7A85-T74 aluminum alloy was also analyzed by transmission electron microscopy. The results show that the grain size of 7A85-T74 aluminum alloy does not change much in the temperature range of 80-240℃, but the precipitation phase changes significantly with the increase of temperature. Below 120℃, the precipitate size, tensile properties and impact absorption energy do not change significantly with increasing thermal exposure temperature, and the precipitation strengthening mechanism is a mixture of dislocation cutting precipitates and dislocation bypassing precipitates. With the increase of the thermal exposure temperature from 120℃ to 240℃, the precipitate average radius increases from 3.8 nm at room temperature to 12.3 nm, and the precipitate changes from η' phase to η phase. The yield strength and tensile strength of the alloy decrease significantly by 45.7% and 33.5% respectively compared with that of room temperature and the elongation, reduction of area and impact energy of the alloy increase significantly. The precipitation strengthening mechanism changes to dislocation bypassing precipitates, and the fracture mode changes from mixed fracture consisting of intergranular fracture and dimple transgranular fracture to dimple transgranular fracture. The effect of precipitate size on the strength and impact energy of the alloy discusses based on the precipitation strengthening theory, and the results of the theoretical analysis are consistent with the experimental results.

This study examined the effect of pouring temperature on the microstructure, tensile properties and hot-tearing susceptibility (HTS) of a high-pressure die-cast (HPDC) Al–Zn–Mg–Cu alloy. The microstructure and pore defects of the HPDC samples were analyzed, and a semi-quantitative assessment method was proposed to evaluate HTS for HPDC. Results showed that the pouring temperature has pronounced influence on the tensile properties and HTS of the HPDC Al–Zn–Mg–Cu alloy. The tensile strength and elongation reach the highest point at the near-liquidus pouring temperature of 650 °C and decrease with higher or lower pouring temperature, whereas the HTS value presents the opposite trend. Microstructural investigation showed that all tear surfaces had pores and Fe-rich compounds were present at grain boundaries in samples poured at semi-solid temperature. Higher porosity levels occurred with higher or lower pouring temperatures than 650 °C. With consideration of the combined action of porosity and Fe-rich compounds, the effect of pouring temperature on the tensile properties and HTS of the HPDC Al–Zn–Mg–Cu alloy could be well explained. [ABSTRACT FROM AUTHOR]

Semisolid slurry of A357 aluminum alloy was prepared using a temperature-controllable electromagnetic stirrer and rheo-diecast at different temperatures. The effects of pouring temperature and electromagnetic stirring (EMS) on the porosity in rheo-diecast samples, as well as the relation between porosity and mechanical properties, were investigated. The results show that pouring temperature and EMS had minor influences on rheo-diecast microstructure but marked influence on the porosity. With decreasing slurry pouring temperature, the porosity decreased first and then increased, whereas the maximum pore ratio (ratio of shape factor to diameter of the largest pore) increased first and then decreased. The maximum pore ratio determines the level of tensile strength and elongation, and higher mechanical properties can be obtained with smaller and rounder pores in samples. The mechanical properties of the rheo-diecast samples increased linearly with increasing maximum pore ratio. The maximum pore ratio was 1.43 µm−1, and the minimum porosity level was 0.37% under EMS condition for the rheo-diecast samples obtained at a pouring temperature of 608 °C. With this porosity condition, the maximum tensile strength and elongation were achieved at 274 MPa and 4.9%, respectively. It was also revealed that EMS improves mechanical properties by reduction in porosity and an increase in maximum pore ratio. [ABSTRACT FROM AUTHOR]

A semisolid slurry of Al-Zn-Mg-Cu alloy was prepared by adding 0.1 wt.% Al-5Ti–1 B master alloy and rheo-diecast with different injection velocities. The average size of the primary α-Al particles in the rheo-diecast parts decreased and its shape factor increased with the increase of the injection velocity. The microstructure was obtained with an average particle diameter of 52 μm and a shape factor of 0.69 for the primary α-Al particles at 1.0 m s −1 . With increasing injection velocity, porosity decreased to a minimum of 0.9%, at an injection velocity of 0.7 m s −1 , and then increased with the increase in injection velocity. At this velocity, the maximum tensile strength and elongation of the rheo-diecast samples were achieved, which were 252 MPa and 2.8%, respectively. After T6 heat treatment, the tensile strength and elongation of the samples were 403 MPa and 2.1%, an increase and decrease of 60% and 25%, respectively, compared to the as-cast samples. [ABSTRACT FROM AUTHOR]

In this paper semisolid slurry of Al-6.3Zn-1.9Mg-1.8Cu-0.15Zr alloy with different levels of Sc was prepared, and the effect of Sc content on the morphology of microstructure and mechanical properties of the rheo-diecasting samples was investigated. The results indicate that the excellent semisolid slurry of the wrought Al alloy could be obtained with the addition of 0.22%Sc. Sc additions not only influenced the morphology of the primary α-Al particles in the slurry, but also promoted the formation of fine α-Al globules in the solidification process of the remaining liquid, which contributed to the considerable improvement in tensile strength and elongation. The tensile strength and elongation of the rheo-diecasting samples with 0.45%Sc after T6 heat treatment increased by 82% and 147% respectively compared to the die-casting samples of the base alloy. [ABSTRACT FROM AUTHOR]

• The effects of laminar plasma quenching on the microstructure of AISI 52100 bearing steel were studied. • Laminar plasma quenching can refine grains and increase carbon content in martensite to enhance mechanical properties. • Laminar plasma quenching can reduce the carbides in the matrix and reduce the self-corrosion current density, and improve the corrosion resistance. In this work, the effect of laminar plasma quenching on microstructure and corrosion resistance of AISI 52100 bearing steel was studied and its mechanism was discussed. The results indicate that the microstructure of bearing steel material treated by laminar plasma quenching changed from tempered martensite to fine cryptocrystalline martensite, and the quantity of carbide obviously decreased. The average grain size is 746 nm, the carbide content is 3.5 %, and the microhardness is 900 HV 0.5. The self-corrosion current density of the treated material is 1.8456 × 10−5 A/cm2 in 3.5 wt% NaCl solution, and it has excellent corrosion resistance. The carbon atoms more easily dissolved into α-Fe to form a supersaturated solid solution due to rapid high-temperature heating and cooling during laminar plasma treatment. The decrease in the amount of carbides after laminar plasma quenching is the cause for the improvement in the corrosion resistance of the bearing steel. [ABSTRACT FROM AUTHOR]

A series of characterization tests were performed to elucidate the high-cycle fatigue behavior in a type of steel often used in high-speed railway wheel rims. The specimens were cyclically-loaded with a constant stress amplitude of 370 MPa at the stress ratios of −1 at a test temperature range from −60 °C to 60 °C. It was found that the steel showed differences in the evolution of microstructures (slip system, dislocation structure, austenite transformation, sub-grains), which led to significant changes in the mechanism of dissipation of strain and fatigue behavior. However, all microstructure orientations on fracture surfaces within grains or sub-grains were approximately uniform and were inclined to primary slip planes {110} and {112} in BCC crystal structure. In addition, the rough set theory model was introduced for the attribute reduction of characteristic parameters. Four sets of attribute reductions based on decisions were obtained, and each one had only three characteristic parameters. Developing a new parameter attribute reduction model was of great importance in building a comprehensive understanding of the characteristic parameters as well as the development of new methods for reliable fatigue lifetime calculations. [ABSTRACT FROM AUTHOR]