پژوهشنامه ریخته گری
پیاپی 2 (پاییز 1396)

The effect of Mn addition (2 and 4 wt%) on the microstructure and castability of hypoeutectic Al-2Ni alloy was investigated. According to the results, Mn promotes the formation of Mn(Ni)-rich intermetallics in the microstructure. In the case of Al-2Ni-2Mn alloy the intermetallic compounds are interdendritic type whilst in Al-2Ni-4Mn alloy in addition to interdendritic intermetallics, the large and primary Mn-rich compounds with platelets, polyhedral and dendritic morphologies are present. The fluidity results show that the addition of 2 and 4 wt% Mn enhances the mushy solidification of Al-2Ni alloy leading to a fluidity reduction of 7 and 30%, respectively. Based on the microstructural observations and thermal analysis results, this reduction can be attributed to the formation of primary Mn-rich compounds in the molten alloy. Manganese addition, also, exerts negative impact on the hot tearing resistance of Al-2Ni alloy. The hot tearing susceptibility index (HTS) of Al-2Ni-4Mn alloy is 5 and 12 times higher when compared to those of Al-2Ni-2Mn and Al-2Ni alloys, respectively. SEM investigation of hot teared microcracks and the presence of free dendrites and primary Mn-rich compounds on the fractured surfaces imply on the critical role of primary compounds on the formation of hot tear microcracks.

In this research, the effect of austempering heat treatment time and temperature on the mechanical properties and microstructure of high silicon steel were investigated. The melting process of this alloy was carried out in an induction furnace with the capacity of 20 kg followed by casting in a Y-block mold cavity. After cutting, the samples were austenitized for 2 hours at 1150ºC and austempered at 250 and 300ºC temperatures for 2 to 72 hours. The achieved microstructure showed nanometer layers of bainitic-ferrite, and retained-austenite between them. Increasing the austempering time led to the increment of the amount of bainitic-ferrite. The amount of bainitic-ferrite in samples, austempered at 250ºC, was 84% and in the maximum state was about 91%. A maximum hardness value of 611 HV was obtained in samples austempered at 250ºC for 72 hours.

In this study, the effect of silicon content on the graded microstructure of Al-Si alloys fabricated by centrifugal casting method was investigated. For this purpose, two cylinders with a chemical composition of Al-11.9wt.% Si (hypoeutectic alloy) and Al-20wt.% Si (hypereutectic alloy) were cast through vertical centrifugal casting method. Then the microstructure and hardness of radial sections of the cast cylinders were studied using optical/scanning electron microscopes and standard Brinell test method, respectively. According to the results, two hypo and hypereutectic Al-Si cylinders illustrate two different patterns of functionally graded microstructure thus hardness along the radial direction. In hypoeutectic cylinder, due to the segregation of the heavy and soft α-Al phase in centrifugal force direction, the outer and inner layers show hypoeutectic and eutectic microstructures, respectively. While in the hypereutectic cylinder due to the segregation of the light and hard primary Si particles in the centripetal direction, the outer and inner layers contain eutectic and hypereutectic microstructures, respectively. As a result of these continuous and gradual microstructural changes in radial direction of the cylinders; firstly: the hardness of all radial sections in hypereutectic cylinder is about 10 Brinell more than the hypoeutectic one; secondly: the inner layer of hypoeutectic and outer layer of hypereutectic cylinders both with full eutectic microstructure show similar hardness; thirdly: the hardness of both cylinders moderately increase from outer periphery towards the inner periphery of the cylinders.

Nickel base superalloys are used for their unique properties in the construction of various components of ground and air gas turbines. In the present study, the IN738LC superalloy was solubilized at a temperature of 1090 ° C to 1200 ° C and at a time of 30 to 120 minutes. The samples were aged for 24 hours after solution heat treatment at 800 °C. Before and after aging, samples were examined by optical microscopes (OM), scanning electron microscopy (SEM) and field emission scanning electron microscopy (FESEM). The results showed that increasing the temperature and time of solution heat treatment reduces the volume percentage and the size of the g¢ precipitates, while the volume percentage and size of the g¢ precipitates are increased and decreased, respectively, after aging. Micro hardness tests showed that hardness of the superalloy was strongly influenced by the characteristics of sediments. By increasing the solubility time and reducing the volume percentage of g¢ precipitates, the hardness decreased and increased after aging.

The Ni-based superalloy GTD-111 possesses excellent hot corrosion resistance, oxidation resistance and high strength at elevated temperature; accordingly, the alloy is used in manufacturing of the first stage blades of powerful gas turbines. In this study, the effect of homogenization treatment, partial dissolution with two different cooling rates (air and water) along with aging on the microstructure and micro hardness of directionally solidified GTD-111 were studied. To this end, cylindrical specimen (diameter of 10 mm and height of 40 mm) with columnar-grained structure were produced under growth rate of 6mm/min using Bridgman technique. The macro-structural studies of the DS GTD-111 specimen show that the columnar grains along the longitudinal direction of specimen were provided by temperature gradient in liquid metal. Microstructural analyses illustrate that homogenization treatment led to dissolution of a portion of primary γʹ precipitates. Furthermore, the primary γʹ precipitates in the specimen that was cooled in water after partial solution treatment, were spherical and cubic with curved corner. However, the primary γʹ precipitates were angular cubic in the specimen which was cooled in air after partial solution treatment. Since the γ matrix became supersaturated during the homogenization and partial solution treatment, aging process was effective on nucleation and growth of secondary γʹ precipitates. This was especially more substantial in the specimen which was cooled in water after partial solution. Hardness of the DS specimen was reduced after homogenization. While, aging resulted in increasing the hardness of the DS specimen due to the improvement of morphology, size and distribution of γʹ.

In this research, the effect of shot-peening treatment on the wear behavior of two types of high manganese austenitic steel (Fe-7Mn-0.6Si- 1.2C and Fe-17Mn-0.6Si-1.2C) was investigated. In this regard, two samples with various manganese contents of 7 and 17 weight percent are cast, respectively. In order to achieve a uniform structure of austenite into all samples, solution-annealing process is performed at temperature of 1100°C during 2h. Micro hardness measurement and tensile testing as well as wear test according to the rubber wheel-dry sand method are conducted on all heat-treated samples. Surfacing hardening treatment by using of shot- peening equipment is done during 15 and 30 mines on all samples. The results showed that when the percentage of manganese increased from 7 to 17% in the heat-treated samples, hardness and tensile strength values are increased and improved about 27% and 10%, respectively. The surface hardness of high manganese austenite steels is increased due to the applying surface work hardening with shot-pinning treatment. Thus, the highest hardness is observed in the shot-peened sample at time of 30 min. however, the highest hardness and wear %resistance are determined in the sample containing 7%Mn. The investigation of worn surfaces showed that the wear mechanism was abrasive mechanism. Moreover, the amount of and the depth of grooves are reduced with increasing of hardness values.