Chao Li Ma

Microstructural Evolution of Fe-Based Metallic Glass/2024-Al Powder during High Energy Ball Milling Process

Nanostructured Al alloy powder was prepared by ball milling of a mixture of Fe-based metallic glass (FMG) powder and 2024-Al alloy powder. Microstructural evolution and mechanical properties of the milled composite powder were examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and micro-hardness test. It is revealed that after 24h milling, the grain size of the Al alloy powder reduced to about 30nm, and the FMG particles were uniformly distributed throughout the Al matrix. The mechanical test indicated that the micro-hardness of the powder was significantly improved.

Preparation of Multilayered Ti-Al Alloys by Solid Reaction

The multilayered materials with different combinations of Ti, Al and Ti-Al intermetallics were prepared by heat treatment and hot pressing (HP) with elemental foils. The microstructures and phase formation of the obtained samples were detected by scanning electron microscopy (SEM), electron probe microanalysis (EPMA), energy dispersive spectroscopy (EDS) and X-ray diffractometer (XRD). When the HP is applied under the melt point of aluminum, aluminum is the only diffusive element across the oxide films on the surface of the initial foils; however, some unusual TiAl3 particles are found in the multilayered structure due to the broken of oxide films; after hot pressing for 4 hours, all the aluminum was consumed; many voids exist at the centerline of TiAl3 layers, which are mainly caused by Kirkendall effect and the difference of molar volumes between reactants and products; before the aluminum is completely consumed, TiAl3 is the only product in the solid reaction under the melting temperature of aluminum; however, other Ti-Al intermetallics like Ti3Al and TiAl are formed in the updated temperature diffusion after aluminum is consumed.

The Effects of Two-Stage Aging on the Microstructure and Mechanical Properties of the New Al-Si-Cu-Mg Alloy

Two-stage aging treatments were applied to Al-6Si-2Cu-0.5Mg casting alloy, and the influence of aging treatment parameters on the microstructure and mechanical properties was investigated. The experimental results indicated that the microstructure and mechanical properties of Al-6Si-2Cu-0.5Mg alloy were significantly influenced by the aging time and temperature, and the elongation remarkably increased with the aging time increasing before 7 h. In the case of the alloy aged at 200°Cfor 2h, higher hardness and tensile strength were obtained, which may be attributed to precipitation of a large amount of Cu-rich phases. However, the higher elongation was achieved in the alloy under 200°C aging treatment for 5h, while its hardness and tensile strength slightly decreased. It is mainly due to the amount of the Cu-rich phases decrease slightly, but the morphology of the phases evolved from plate-like to bulk-like structure.

Effect of Iron-Rich Intermetallic and Eutectic Si Accumulation on Al-Si-Mg Alloy

Al-Si-Mg alloys, an important aluminum cast alloys, are excessively used inmanufacturing of critical components due to their high strength to weight ratio, flexibility ofmanufacturing designs, economic processing and capital material cost for automotive industry. Thisresearch is aimed to study microstructure evolution including distribution and morphology ofiron-rich intermetallic, as well as eutectic Si accumulation and their effect on mechanical propertiesof Al-Si-Mg (A356) casting alloy after artificial ageing. The results show that formation of iron-richintermetallic and eutectic Si accumulation resulted in surprisingly opposite mechanical properties,especially ductility. The elongations deceased with increase of area of accumulated eutectic Si and theamount of needle-like iron-rich intermetallic. When the area of accumulated eutectic Si reached 31%of the microstructure of the A356 alloy, the strength and elongation were respectively damaged to129.69 MPa and 1.05%. Moreover, the amount of needle-like iron-rich intermetallic increased, thestrength and elongation respectively decreased.

Solidification Behavior and Mechanical Properties of Al-Si-Mg Alloy with Ti Addition

The solidification behavior, microstructural evolution and mechanical properties of Al-Si-Mg foundry alloy with different Ti additions were investigated in the present study. The solidification behavior of those A357 alloys was analyzed through thermal analysis. The microstructures were examined by optical microscopy (OM), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The results showed that the addition of Ti could refine grains of A357 as-cast alloy due to a good restriction on the grain growth, but Ti could not refine secondary dendrite arm spacing (SDAS), thus mechanical properties of the A357 as-cast alloy did not improved significantly. After T6 heat treatment, the microstructure with α-Al dendrites with the Al-Si eutectics at interdendritic space was replaced by a homogeneous α-Al matrix with a nonuniform dispersion of discrete, spheroidizing and coarse silicon particles. Hence, compared with the as-cast alloys, both of the strength and ductility of the T6 treated alloys are significantly improved, and an optimal combination of strength and elongation of the A357 alloy can be achieved by the 0.8 wt.% Ti addition after T6 heat treatment.

Effect of the Spinning Deformation Processing on Microstructure and Mechanical Properties of A356 Wheels

The influences of spinning deformation and heat treatment on microstructure and tensile properties of A356 alloy at different cooling rates were investigated in this study by optical and scanning electron microscopes. The results indicated that spinning deformation enhanced the tensile properties of the alloy due to the reduction of Si size and porosity percentage, especially in the samples with coarse microstructure. Heat treatment increased the strength while decreased the ductility of the alloy because of the precipitation of brittle Mg2Si in Al matrix. It is suggested that the spinning deformation processing is an effective technique to produce A356 alloy wheels with high mechanical properties.

Effects of Cooling Rate on Morphology of Eutectic Si in RE Modified Al-10wt.%Si Alloy

Morphology evolution of eutectic Si in Ce-rich mischmetal (RE) modified Al-10wt.%Si alloy at different cooling rates was investigated. The morphology of eutectic Si and modification mechanism of RE was investigated by scanning electron microscopy (SEM) and Transmission electron microscopy (TEM). The results showed that the RE modified eutectic Si exhibited a plate-like morphology under the low cooling rate (~100K/s). When cooling rate increased to ~600K/s, some branches were observed on the eutectic Si. In the RE modified alloy with a higher cooling rate (~1000K/s), the number of the branches on RE modified eutectic Si increased, and the morphology of eutectic S was modified to coral-like structure. The addition of Sr caused a flake-to-fiber modification of eutectic Si at low cooling rate, and the fiber size decreased with cooling rate increasing. The morphological observations indicated that the morphology of eutectic Si in RE modified alloy was significantly influenced by the cooling rate, while the modification efficiency of RE was lower than that of Sr.

Enhanced Tensile Properties of Graphene-Al5083 Composite Prepared by Hot Pressing and Hot Extrusion

Graphene, with outstanding mechanical properties of 1TPa of Young’s modulus and 130GPa of tensile strength, has been considered as an excellent reinforcement in composites. Graphene nanoplates (GNP) reinforced Al5083 alloy composites were successfully fabricated by powder metallurgy method. Flake-like Al5083 powder was firstly added into a GNP-alcohol solution and then dried in vacuum oven at 343 K for 5 h. The composite powders were sintered at 723K under pressure of 300 MPa for 1 h, followed by extrusion at 623 K. X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed for microstructure characterization. Al4C3 cannot be observed in the composite, suggesting that there was no reaction occurred between GNP and Al matrix during the processing. Tensile test results revealed that the ultimate tensile strength and tensile elongation of 0.3wt.%GNP/Al5083 composites were 580MPa and 15%, respectively, showing optimal combination of strength and ductility. The relevant strengthening mechanisms of the composites were discussed.

Influence of Processing Procedure on Mechanical Properties of A319 by Comparison between Casting and Powder Metallurgy

Al-6wt.%Si-3.5wt.%Cu alloy (A319) were prepared by casting and powder metallurgy (PM) methods to investigate the influence of processing procedure on properties. The casting samples were cast into a horizontal mold after degassing by argon. The PM samples were extruded after hot-press sintering before which Al-11wt.%Si master alloy powder, pure Al powder and 48 h ball-milled Al-40wt.%Cu powder were blended for 12 h at rotation speed of 250rpm. The samples underwent a T6 temper, i.e. solution, water quenching and artificial aging. Microstructures were observed by optical microscope (OM) and scanning electron microscope (SEM), and phases were determined by X-ray diffraction (XRD). The tensile testing was carried out on universal testing machine. The results showed that heat treatment could greatly improve the tensile strength of the samples, especially for the casting counterpart. Discussion was made to analyze the reason that the properties differed.

Meso-Structure Design of B4C-Al/Al Composites by Hot Pressing

According to the theory of meso-structure design, milling powders were blended with un-milled Al particulate to increase ductility. Two kinds of Al particulate-toughened composites were fabricated by using powder metallurgy method, where the mass fraction of B4C in the B4C-Al agglomerate particles was 40%, but 32% and 16% in the whole composite. The microstructure of composites was examined by scanning electron microscope (SEM), and its mechanical properties were studied. The results indicate that Al particulate-toughened sample has a slight plasticity with bulks of aluminum alloy in the composite. But meso-structure design has no effect on improvement on the plasticity and toughness of the sample B4C-Al/Al (16%)(3#), where the mass fraction of B4C in the whole composite is 16%. In the present study, the strengthening and deformation mechanism of the composites were also discussed.