Erjun Guo

Tensile properties of ZnO- and ZnAl2O4-coated aluminum borate whiskers reinforced aluminum composites at elevated temperatures

ZnO- and ZnAl2O4-coated aluminum borate whiskers reinforced pure aluminum composites were fabricated by squeeze casting. The tensile properties of the composites at elevated temperatures were tested. The results indicate that the maximum tensile stress of the composite decreases with the increase of tensile temperature; however, the tensile strain of the composite initially increases and later declines. ZnO, especially, ZnAl2O4 coating of the whiskers can effectively improve the ultimate tensile strength of the composite at elevated temperatures. On the basis of fractographic analysis, the fracture mechanisms of the composites at elevated temperatures were also investigated.

Modelling of dendritic growth under forced convection in solidification of Al–7Si alloy

Abstract An integrated microscale model is used to describe the dendritic growth morphology of Al–7Si alloy under forced convection, which combines the two-dimensional finite difference method (FDM) with the cellular automaton (CA) model. The FDM is used to simulate the induced fluid flow and solute transfer. The CA model is used to depict the dendritic morphology. Simulations are performed to investigate the influences of processing variables on morphological evolution under stirring. Calculated results reveal that at the intermediate undercooling, the growth morphology changes from dendritic to rosette as the rotation speed increases. The dendritic growth is promoted by increasing the undercooling. The rotation speed has a minor influence on microstructure formation for the cases of lower and higher undercoolings. A globulitic structure is formed as the nucleation density is increased. Changing the rotation speed is found to have a negligible influence on morphological evolution for the grain refined alloy.

Effect of annealing temperature on microstructure and tensile properties of Al–Mg alloy 5A06 sheet

Abstract In order to optimise heat treatment condition for improving the stability of microstructure and properties of 5A06 alloy sheet, annealing treatment of the sheet was conducted in the temperature range of 175–380°C for 1 h. The microstructure and tensile properties of the sheets were evaluated. The results show that the deformed microstructure of 5A06 alloy sheets in cold rolled state undergoes recovery, recrystallisation and the precipitation of Mg contained phases during annealing process from 175 to 380°C. The initial recrystallisation temperature is ∼250°C. By tensile tests and microstructure observation, the microstructure and properties of the sheets are relatively stable when annealing temperature reaches to 310°C.

Preparation and tribological properties of homogeneously dispersed graphene-reinforced aluminium matrix composites

ABSTRACT Aluminium matrix composites reinforced with homogeneously dispersed graphene (Gr/Al) were fabricated by electrostatic self-assembly and powder metallurgy. The microstructure of the composites was observed. The effects of graphene content, applied load and sliding speed on the tribological properties of the composites were also investigated. The results show that graphene is homogeneously dispersed in the aluminium matrix. The coefficient of friction for the composites initially increases and later decreases with increasing graphene content. It is lowest in the 0.3 wt-% Gr/Al composite, which is a decrease of approximately 20% when compared to that of pure aluminium. The coefficient of friction of the composites appears to increase significantly when increasing the applied load. However, it decreases with increases in the sliding speed.

Design and tensile properties of aluminum borate whiskers reinforced aluminum composite with low whisker volume fraction

Abstract ZnO was firstly coated on the surface of aluminum borate whiskers (ABOw) by sol–gel. Then, ZnO-coated ABOw was added into soybean slurry to make a preform with low volume fraction and the preform was sintered at high temperatures to obtain a sufficient strength. ABOw reinforced aluminum composite (ABOw/Al) was fabricated by squeeze casting. Interfacial microstructures, tensile properties, and fracture mechanisms of the ABOw/Al composite were investigated. The results show that the coating of ABOw and the addition of soybean sacrificial filler can effectively decrease the volume fraction of whiskers in the composite. The ultimate tensile strength of composite has not changed much with the decrease of whiskers volume fraction in the composite. However, the elongation to fracture of the composite evidently increases and it can reach 11.1% at room temperature.

Numerical simulation of macrosegregation during solidification of WE43 (Mg–4.3 wt-% Y–2.0 wt-% Nd–0.6 wt-% Gd) magnesium alloy

The formation of macrosegregation driven by multicomponent thermosolutal convection during solidification of WE43 magnesium alloy is numerically studied. A two-dimensional solidification model that describes the conservation of mass, momentum, energy and solute is presented. The experiments are performed in the resin sand mould with a graphite chill placed on one side of the cavity. The validation of computer codes is carried out by comparisons with the experimentally measured cooling curves and concentrations. The influences of mould material and characteristic grain size on macrosegregation are numerically investigated. For a total sand mould casting, the magnitude of macrosegregation is reduced, although the solidification rate is slower than that cooled with a graphite chill. Increasing the characteristic grain size not only causes a serious positive segregation but also leads to the formation of channel segregates. The channel segregation occurs thanks to the weak instabilities of the growing front triggered by the fluid flow.

Effect of ZnAl2O4 coating on the compressive behaviors of aluminum borate whiskers-reinforced aluminum composites at elevated temperatures

Abstract Aluminum borate whiskers (ABOw) with or without ZnAl2O4 coating reinforced pure aluminum composites (ABOw/Al, ABOw/ZnAl2O4/Al) were fabricated by squeeze casting. The effects of ZnAl2O4 coating on the compressive behaviors, microstructures, and matrix textures of the composites were investigated at different temperatures and strain rates. The results indicate that the maximum compressive flow stress of the composites almost linearly decreases with the increase in temperature. The maximum compressive flow stress of ABOw/ZnAl2O4/Al composite is higher than that of ABOw/Al composite at the same temperature when the strain rate is larger, however, it is reversed when the strain rate is smaller. It is more serious for the fracture of whiskers in ABOw/ZnAl2O4/Al composite than that in ABOw/Al composite at the high compressive strain rate. However, the average length of whiskers in ABOw/ZnAl2O4/Al composite is larger than that in ABOw/Al composite at the low compressive strain rate. The strong matrix texture in ABOw/ZnAl2O4/Al composite appears at the high compressive strain rate, however, it is observed in ABOw/Al composite at the low compressive strain rate.

Preparation of NiO coated aluminum borate whiskers by a solgel process and an ultrasonic dispersion technique

NiO coated aluminum borate whiskers were prepared by a sol-gel process and an ultrasonic dispersion technique. The preparation process and characterizations of NiO coated aluminum borate whiskers were investigated. The results show that during the heating process of the xerogel coated aluminum borate whiskers, one endothermic peak appears at 100 °C and three exothermic peaks appear in the range of 150-350 °C according to the differential thermal analysis. And no weight loss occurs after 350 °C according to the thermogravimetry analysis. After the xerogel coated aluminum borate whiskers are calcined at 500 °C, the surface of each whisker is uniformly coated by a layer of NiO film and the NiO coated whiskers can be easily dispersed in water through an ultrasonic apparatus. Moreover, No phase transformation of NiO and interfacial reaction between NiO and aluminum borate whiskers occur when the xerogel coated aluminum borate whiskers are calcined in the range of 500 and 900 °C.

Preparation of Fe 2 O 3 coated aluminum borate whiskers by a sol-gel method

Fe<inf>2</inf>O<inf>3</inf> coated aluminum borate whiskers were prepared by a sol-gel process. The preparation process and characterizations of Fe<inf>2</inf>O<inf>3</inf> coated aluminum borate whiskers were investigated. The results show that after Fe<inf>2</inf>O<inf>3</inf> xerogel coated aluminum borate whiskers are calcined at 400°C, Fe<inf>2</inf>O<inf>3</inf> can be uniformly coated on the surface of the whiskers. Moreover, Fe<inf>2</inf>O<inf>3</inf> does not take place phase transformation and no interfacial reaction takes place between Fe<inf>2</inf>O<inf>3</inf> and aluminum borate whiskers in the range of 400 and 800°C.