پژوهشنامه ریخته گری
سال دوم شماره 2 (پیاپی 5، تابستان 1397)

The effect of beryllium addition (200 ppm and 2000 ppm) on dynamically formed double oxide film of aluminum A356 alloy was ‎investigated. Dynamically double oxide films were obtained by gravity casting in a cup of thin-walled steel at different free ‎fall of melt and solidified under a vacuum of 80 mbar. The melt quality according to the bifilm index for melts containing ‎‎2000 ppm of beryllium for molten free fall height of 200 and 300 mm is 5.6 and 5.20 mm, respectively. Scanning electron ‎microscopy (SEM) images show a change in the morphology and thickness of the young double oxides containing the ‎beryllium element in dynamic conditions. By adding 2000 ppm of beryllium, the thickness of the film is reduced. For ‎amount of 200 ppm, the edges of the oxide screed and its greater thickness are oxide film properties. Reducing the thickness ‎can mean increased flexibility of oxide films composed of melt containing beryllium. Also, the radiographic results of RPT ‎samples showed that the cavities were reduced by adding beryllium.‎

In this study, centrifugal casting method was used to produce a hyper-eutectic composite Al-Mg2Si ‎with hybrid microstructure and graded hardness. For this purpose, two cylinders with a chemical ‎composition of Al-20Si and Al-20Si-9Mg (weight percent) were cast using a vertical centrifugal ‎casting machine. Then the microstructure and hardness of the cast cylinders were studied along the ‎radial direction by optical and scanning electron microscopes and brinell method, respectively. The ‎results show that in Al-20Si cast cylinder; in contrary to the outer layer which shows only a binary ‎eutectic Al-Si microstructure, the inner layer contains a high volume fraction of in situ segregated ‎primary silicon particles inside the eutectic matrix. By addition of 9% Mg to Al-20Si alloy, the inner ‎layer of the cast cylinder form a hybrid microstructure containing of both Si and Mg2Si particles ‎inside the eutectic matrix, while the outer layer shows only a ternary eutectic Al-Si-Mg2Si ‎microstructure. As a result of these microstructure changes along the radial direction of the ‎cylinders, not only the hardness of both cylinders are smoothly increased from the outer towards the ‎inner layers, but also all radial sections of the Al-20Si-9Mg cylinder in compared with the other one, ‎indicate higher hardness (about 18 brinell).

The goal of this research is an investigation on the effect of 2 parameters of casting process including cooling rate of mold and pouring temperature on the 2 important characterization including Vickers hardness and wear resistance via pin on disc of bearing bronze SAE660. The melting had done by transistor induction furnace and then, pouring had done in sand mold in four different casting conditions, including pouring temperatures of 950C and 1200C, and cooling with water and air and finally, the microstructure via scanning electron microscopy, Vickers hardness and wear resistance via pin on disc of the SAE660 bronze had compared. The results indicated that the effect of pouring temperature can be ignored for wear resistance but the minimum weight loss due to wear via pin on disc (or maximum wear resistance) obtained when the samples cooled with air compared to water cooling, because the contents of lead, intermetallic compounds and the phase of solid solution is more in the samples cooled with air compared to water cooling. In addition, maximum hardness obtained for low pouring temperature.

The pile-up of insoluble build-ups in induction pressure pouring machines has been one of the important problems in casting of ductile iron. The main constituent of these build-ups is magnesium oxide (MgO). The formation of MgO is resulted by oxidation of the magnesium which was used for graphite spheroidisation. In this research, the effectiveness of two solutions has been studied to overcome this problem in pouring machines of a casting plant. The solutions were the addition of a flux into the melt and applying nitrogen atmosphere instead of air. The chemical analysis of the melt, slag and build-ups were done using X-ray diffraction (XRD), wet chemistry method and quantometery technique. For the first approach, the best efficiency was obtained with addition of 900 g/ton flux into the ladle before inoculation. Without flux addition, the electrical current drop was drastic (~350 A in 16 days). However, the flux addition resulted in a slight drop in the electrical current (~50 A in 28 days). Moreover, the flux addition has changed the main constituent of build-ups from MgO to iron oxide (Fe2O3). The using of nitrogen atmosphere resulted in a significant decrement in the fading rate of magnesium from 0.017% in 36 min to 0.0146 % in 50 min.

A380 Aluminum alloy is suited for producing of different parts with proper strength and low weight because of having 8.5% silica and a suitable temperature range. Also, due to the wide usage of Al-A380 alloy in various industries such as automobile and aerospace industries, the use of some methods to improve the mechanical properties of this alloy can help to develop its applications. In the process of semisolid forming, by applying shear stress on alloy, dendritic structures converted to the globular ones that have better properties than the structure of common casting. The main objective of this study is to improve the microstructure and mechanical properties and determine the desired levels of propagation temperature and slope length in a semi-solid casting of Al-A380 alloy with the aid of a cooling slope method. The grain diameter, hardness and porosity in the samples were considered as important output characteristics in this study. Investigation of the microstructure of Al-A380 alloy in this study showed that the temperature of 605°C on the 600mm length and using the coolant system is the most appropriate casting that can be the best condition for this alloy. It is because of in this temperature the least grain diameter (32µm) is made. The hardness in this condition was obtained around 98HB that can be related to the effect of the shear stress rate.

In the current study, the effects of bismuth and T6-heat treatment on the microstructural and tensile properties of SiC-reinforced Al-Si alloy (LM13) have been investigated. The alloys were subjected to the solution and ageing treatment to investigate its effect on the characteristics of eutectic silicon particles and tensile properties. The microstructures of specimens were examined by using optical and scanning electron microscopy. According to microstructural results, the most improvement in the modification of eutectic silicon phase was achieved by the addition of 1 wt. % bismuth. On the other hand, applying T6-heat treatment spherodize the silicon particles and thus modify the microstructure of the alloys. Adding bismuth and applying heat treatment simultaneously intensify the modification effect of each of them. The tensile testing was employed to measure the quality index (QI) of the alloys for evaluating the modification efficiency of the alloy with different bismuth contents. The reduction in both tensile strength and ductility after addition of higher contents of bismuth was found to be due to the formation of bismuth–rich phases in the microstructure. The results of fractography showed that bismuth in its optimum level changed the fracture mode from brittle to ductile. This is due to the reduction in dimples size.