Papers by Author: Zheng Bin Xu

Abstract: The scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) were used to characterize samples prepared from the surface scum of melt, the middle flake residues on the furnace wall and the “bottom bulk” after the A357 alloy pouring. The morphology and impurities composition of these features were investigated. Some of particles and regions in samples during the analyses were also studied in detail. The results of analyses show that the surface scum of melt include Al2O3, MgO or MgAl2O4; the impurities of the middle flake residues is comprised of Al2O3, MgO or MgAl2O4, as well as SiO2 particle, Al9Fe2Si2 compound and dimple like C-rich areas; the bottom bulk contains C-rich impurities such as Al4C3 or some of unknown phase or elemental carbon.

Abstract: Aluminum alloy A357 can be heat treated in order to obtain optimum mechanical properties. Moreover, SDAS (Second dendrite arm spacing) is an important structure feature in cast aluminum alloys. The effects of SDAS on the homogenization of Al-Si-Mg alloy are investigated through measurement of SDAS and hardness analysis. The results show that the homogenization time increases as the value of SDAS increases and the homogenization temperature decreases in the homogenization process. The homogenization kinetics equation is: . In addition, the hardness after homogenization treatment at 540°C for 6 hours is advanced with the reduction of SDAS.

Abstract: This present work investigate the hydrogen content in Al-12Si alloy at different holding temperatures of 993K, 1023K, and 1053K and under different ambient relative humidity 30%RH, 50%RH, 80%RH. The relationship of the hydrogen content with atmosphere relative humidity and the reaction time was investigated. A HYSCAN II analyzer was used to evaluate the hydrogen content in aluminum melts. The experimental results show that the hydrogen content increased with the holding temperature and the relative humidity. At the temperature 1053K, the hydrogen content has an inverse change. The hydrogen content is more depend on the liquid structure than physical mass transfer and chemical reaction because of the sudden change in liquid microstructure. A group of kinetic regression equations of the hydrogen absorption in Al-12Si melts was obtained.

Abstract: Microporosity in cast aluminum alloys formed during solidification of castings can be due to the evolution of dissolved hydrogen gas from the liquid metal, the inability of liquid metal to feed through interdendritic channels, or a combination of both. The secondary dendrite arm spacing (SDAS) of the casting may affect either the evolution of hydrogen gas or the ability of feeding. In this investigation, a method based on the Nearest-Neighbor-Distance (NND for short) cluster analysis of image analysis data was used for distinguishing the gas and shrinkage pores in A357 alloy, in order to study the effects of SDAS on the microporosity formation. It shows that the shrinkage pores are prone to form in the specimens with small SDAS. The discrete, isolated gas microporosity is prone to form in the specimens with large SDAS.

Abstract: Hydrogen is the only gas that is appreciably soluble in aluminum and its alloy. Its solubility is small but its effects on the mechanical properties and finishing characteristics of aluminum alloys are significant. In this paper, a patented degassing process, which is based on the vacuum metallurgy principle, is proposed. The diffusion equation was used for explaining hydrogen transfer in the liquid aluminum under the function of a porous sucking head. RPT experiment was carried out to evaluate effectiveness of the process. The experiment indicates that the hydrogen content can be dramatically reduced to 0.05ml/100gAl owing to the use of this new process.

Abstract: The influences of Sn addition of 3%-30% on the friction and thermal expansion properties of Al-Sn bearing alloys were investigated in this paper. The experiments were carried out on a specially designed friction testing machine and a dilatometer. It is shown by the experiments that the friction coefficient and hardness of the alloys decrease with the increasing of Sn content. The friction coefficient decreases dramatically when Sn content is higher than 6%. On the other hand, the friction coefficient is also dependent on the load. The friction coefficient increases as the load rises. The linear CTE is also a function of temperature and Sn contents. CTE increases when the temperature rises but has no obvious differences for the alloy system when the temperature is below 80°C. As the temperature rises, the differences become large among the alloy system. The higher the Sn content, the lower the CTE.

Abstract: In this paper, a new low-pressure die-casting with degassing function, casting under adjusted pressure (CAP), is proposed. The working principle of CAP process is introduced in details. This paper focuses on the influences of moisture of compressed air on the hydrogen content within the melt aluminum during the cyclic vacuum-pressurizing programs for CAP process. The experimental results indicate that degassing under vacuum acts positive in refining aluminum melt. The relative moisture of compressed air exerts a significant influence on the hydrogen content of aluminum melt. After the melt aluminum is exposed to the air with a relative moisture of 85% for 90 min, the hydrogen content of the melt aluminum can get to 0.4 ml/100g/Al; while under the circumstance of CAP process, the hydrogen content is maintained between 0.05 and 0.15 ml/100g/Al depending on the moisture capacity of the air, which virtually has no more influence on the casting qualities. Under the condition of dry compressed air, the hydrogen content can be kept at a level as low as 0.05ml/100g/Al, which facilitates to obtain high air-tightness castings.

Abstract: In the present paper, TiB2/Al composite with 5% volume fraction of TiB2 was fabricated by LSM method. The effects of purification and degassing methods on TiB2/Al composite were examined by means of X-ray diffraction (XRD) and image analysis. Hydrogen contents in the molten composites were detected and compared among flux, inert gas and vacuum purification processes. The experimental results indicate that under general cast condition a majority of the TiB2 particles distribute on grain boundary, and only a few particles disperse within grains. The flux and vacuum purifications have no virtual impact on the distribution of TiB2 and the fraction of TiB2 remains the same after purification. However, degassing with inert gas will be detrimental; the TiB2 particles will be separated and removed from the matrix. The hydrogen contents for flux, inert gas and vacuum processes are 0.15ml/100g/Al, 0.12ml/100g/Al and 0.12ml/100g/Al respectively.