Bimetallic gradient alloys have attracted research attention recently due to their potential applications in the aerospace and automobile industries. In this study, Al-20Si/7075 bimetallic gradient alloys were successfully manufactured by co-spray forming and the roll process. We investigated the thermal expansion behavior of the gradient alloy. It was found that the coefficients of thermal expansion increased with silicon content and increased temperature, reaching the highest point at 573 K, after which they decreased on account of the relaxation of residual thermal stress and the silicon desolvation from the supersaturated aluminum phase. The measured thermal expansion coefficient can be roughly predicted through the traditional theoretical models. Our results revealed the thermal expansion behavior of Al-20Si/7075 bimetallic gradient alloys and would improve the development of new type aluminum–silicon alloy for electronic packaging.

Spray forming technology has been adopted to prepare essential castings in various industry fields. Understanding what happen during the impaction of droplet on deposited layer is critical to optimize spray forming processes and so improve the final casting quality. The present work investigated the morphology evolution of droplet and deposited layer during impaction, with particularly emphasis on the nonrigid feature of deposited layer in practical spray forming. The results demonstrate that the deposited layer would suffer obvious deformation under the impaction of flighting droplet, which produce puddle-like structure on the deposited layer. The droplet spreading process would be suppressed by this puddle-like structure when the droplet temperature is not high enough, which might introduce gap between the droplet and the deposited layer, so might cause tiny holes in the final casting. Moreover, the simulation results suggest that multiple droplet impaction might cause column structure on the deposited layer, this would seriously increase the surface roughness of the final casting. The investigations hint that the mechanical properties of deposited layer should be considered when studying the microstructure evolution and mechanism of spray forming casting.

In this paper, we report the results obtained from the hot compression behaviour of co-spray-formed Al-Si/7075 bimetallic gradient alloys by investigating the five selected layers divided evenly from top to bottom. It was found that with a decrease in the strain rate and an increase in temperature, the hot compression flow stress of deposited alloys decreases gradually, as expected. The relationship between flow stress, temperature and strain rate follows the Arrhenius relationship. The deformation activation energy of the co-spray formed gradient alloy gradually increases from bottom to top in the height direction, which are 100.15, 167.35, 185.54, 208.78 and 220.83 KJ/mol, indicating that the degree of difficulty in the deformation of the alloy is affected by changes in the silicon content of the alloy. Finally, the flow stress constitutive equation is calculated for each layer of alloy material, which is instructive for the subsequent study of the densification process of co-spray-formed gradient alloys.

To find the suitable conditions under which nano-SiO2 can exhibit a significant impermeability enhancement effect and the mechanism underlying this effect, comparisons between the permeability-related properties of a nano-SiO2-filled cement paste and those of a reference cement paste composed of different water/cement (W/C) ratios were carried out in this research. Permeability-related properties of cement paste, such as the chloride-ion penetration coefficient (D nssm), water permeability coefficient (K p), and initial water sorptivity coefficient (S i), were tested. Furthermore, Power’s model, mercury intrusion porosimetry data, and the general effective media theory were also applied to analyse the evolution mechanism. The results indicate that the effect of nano-SiO2 on the enhancement of the impermeability becomes more remarkable at a lower W/C ratio. The decreasing rates of D nssm, K p, and S i increase as the W/C ratio decreases. Furthermore, it can be concluded that the effects of nano-SiO2 on promoting the hydration, refining the pore structure, narrowing the width of microcrack and thus enhancing the impermeability of cement paste become much clearer as the W/C ratio decreases.

To determine the spray forming process parameters of 7075/Al–Si bimetallic gradient composite plate with two gas atomizers, a calculation model of the plate has been established by using the finite element software ANSYS. The effects of different motion trajectory, advance speed, swing cycle and spray center distance on shape, and silicon distribution of deposited plate have been simulated by the APDL programming language. The results show that a smooth and uniform surface is obtained when motion trajectory is in a regular jaggies mode. The deposited plate varies from platform to stepped shape with a center distance increasing from 20 mm to 50 mm; meanwhile, the width of the transition zone decreases gradually. As the period increases to 8 s, the silicon distribution of each layer presents a jagged fluctuation. Both the thickness of the deposited plate and the width of the transition zone decrease as the advance speed increases, except the silicon distribution. Finally, the modeling and simulation of the co-spray formed 7075/Al–Si bimetallic gradient composite plate are validated by experimental investigations and the simulation results are in good agreement with the actual results.

The distribution model of temperature for the billets was established based on the porosity formation mechanism. By calculating the numerical relationship between the temperature of the deposition layer and the formation ability of the porosity, the distribution of porosity for spray formed 7075 aluminum billets was predicted and the influence of superheat on the formation probability of porosity was analyzed. The results show that a denser billet can be obtained with the average temperature of 813 K of the droplets and a liquid fraction of 50%. The porosity would increase whether the temperature further increased or decreased. Finally, the 7075 aluminum alloy was prepared by spray deposition technique with the optimum process parameters. It can be found that the distribution and types of the porosity were different at different locations. These results are in good agreement with the porosity distribution law.

Theγ-aminopropyltriethoxysilane film was deposited on 6061aluminium alloy surface.The corrosion resistance of the silane film was investigated in 3.5% NaCl solution,using Tafel polarization curves.The results showed that silane solution was stable when the silane concentration was 3%,pH value was 12and time of hydrolysis was 72hat room temperature.When aluminum alloy specimen in silane solution was treated by dip-coating 3min, curing temperature 180℃and curing time 120min.the obtained silane films on 6061aluminium alloy surface had the best corrosion resistance,the corrosion current density reduced and corrosion potential rose.

The evolution of microstructure and coefficient of thermal expansion (CTE) of the Al-50Si (wt.%) alloy manufactured by spray deposition followed by hot isostatic pressing (HIP) are systematically investigated. The results indicate that the microstructure of the deposited alloy is composed of primary Si with average size of 12.5 +/- 0.1 mu m and alpha-Al. The CTE of the deposited alloy is higher than the corresponding alloy produced by casting due to the high solid solubility of Al in Si. After HIP, the CTE is lower than the parent as-deposited alloy owing to the high solid solubility of Si in Al. The residual thermal stress results in a higher CTE during the second heating as a result of the CTE mismatch between the Al matrix and the primary Si particles. Furthermore, the measured CTE value is in good agreement with the Turner model after complete densification by HIP at 843 K. (C) 2014 Elsevier Ltd. All rights reserved.

The advantage of concrete containing nano-TiO2 in resisting the coupled effects of chloride diffusion and scouring with respect to pure concrete was studied in this paper. Because of the movement in exposed concrete surface induced by scouring and the deterioration in concrete microstructure caused by chloride salt accumulation, an increasing mutual accelerative effect between the chloride diffusion and the scouring abrasion was experimental observed, which agreed with the theoretical simulation results. Benefited from the improvement in microstructure and porosity compared with the pure concrete, concrete containing 1% nano-TiO2 in the weight of cement showed a better impermeability as well as the abradability. Correspondingly, a better performance in resisting the coupled effects of chloride diffusion and scouring was founded for the concrete containing nano-TiO2 compared to the pure concrete, and this advantage increased upon the time.