پژوهشنامه ریخته گری
سال ششم شماره 2 (پیاپی 20، تابستان 1402)

In this study, the effect of slope casting parameters including pouring temperature, slope angle, mold material, and vibration on the microstructure and mechanical properties of AXE622 alloy (Mg-Al-RE) was investigated. The mentioned parameters were changed at two levels. It is clearly observed that in this process, the shape of the α-Mg dendrites changes to the globular and rosseta shape morphology in the matrix. Increasing the slope from 30 to 60°, caused the development of non-dendritic (spherical) structure, increased the ultimate strength (62%), and improved the elongation by 17%. Selecting the melting temperature of 680 and 700° C caused the short run and decreased the ultimate strength, respectively. In this regard, by pouring the melt at 680 °C, the semi-solid casting was successfully completed and the mechanical properties of the alloy were improved. Comparing to the pouring temperature of 700 ° C, at the pouring temperature of 680 ° C, ultimate strength and elongation increase by 33% and 5%, respectively. The results indicated that the mold material change from sand to metal, led to decrease in the average of grain size (13%), less dendritic structures, increasing ultimate strength and elongation by 81% and 15% respectively. Applying vibration to the slope surface, led to reduce average grain size, increase hardness and ultimate strength, so that the ultimate strength increased by 3% compared to the sample without vibration.

In this study, the mechanical properties of gas turbine blades are investigated by investment casting method. These Nickel-based alloys blades, with standard name, CM88Y, are made in Ukraine. Currently, these blades are used in Zorya brand gas turbines in most power plants in Iran. Therefore the technical information about this alloy are very limited. In this investigation, the CM88Y Nickel base alloy as a Ni-Co heat resistant super-alloy was assessed by a lot of destructive tests. Mechanical and creep properties as well as the phase microstructure for a turbine blade were measured after 6000 and 18,000 hours. The creep results by adding Larson-Miller parameter, revealed that the 18,000-hour blade with the hot-spot zone defect has a creep life less than 3,800 hours due to the TBC erosion and hot-spot defect. Also, the gamma prime phase has changed from a cubic morphology to a spherical shape associated to the coarsening phenomenon.

The purpose of this research is to investigate the effect of different heat treatment cycles on structural changes and mechanical properties of Ti-48Al-2Cr-2Nb intermetallic compound. Heat treatment at 1175°C for 24 hours resulted in the formation of a duplex structure including γ grains along with colonies of γ and α2 layers with an average grain size of 1160 micrometers. Heat treatment at 1400°C for 30 minutes created a near lamellar structure of γ and α2 phases with a grain size of 1300 micrometers. By increasing the time to 60 minutes, a fully lamellar structure with an average grain size of 1120 micrometers was created. The decrease in grain size with increasing the heat treatment time is mostly attributed to the completion of the recrystallization process with the creation of new colonies and the destruction of the remaining gamma grains. Heat treatment at a temperature of 1380C for 45 minutes resulted in the formation of a fully lamellar structure with a grain size of 950 micrometers, but the grain size distribution is more uniform than the lamellar structure formed at a temperature of 1400 degrees. By increasing the volume fraction of colonies, the hardness increased. The hardness of duplex structure was 282 Vickers and the near lamellar structure was 320 Vickers. The results of the stress rupture test showed that the creep rupture life of the lamellar structure was 115 hours, while the life of duplex structure was 22 hours. This difference is attributed to the small distance between the layers.

In this research, the effect of the hot-pressing process on the microstructure and hardness of A390 cast aluminum-silicon alloy was investigated. The results were prepared using an optical microscope and hardness tester. The microstructure of the casting sample included very coarse primary silicones, acicular eutectic silicones, intermetallic compounds, and large alpha-phase dendrites. By increasing the amount of strain in the hot-pressing process, primary and eutectic silicones as well as intermetallic compounds were broken and finer, and their distribution in the alpha matrix became more uniform. The obtained results showed that the microstructure of the casting alloy has been improved in terms of the modification of primary silicon particles, eutectic silicon, and intermetallic compounds, as well as the uniform distribution of these particles and the removal of porosity. The initial silicon size is dramatically reduced from over 100 µm (for the cast sample) to less than 5 µm (after the seventh pass). By increasing the strain up to the fourth pass, the hardness of the alloy decreased from 87 to 65 HB. With the further increase of the strain from the fourth pass to the seventh (final) pass, the hardness value increased to 81 HB.

The purpose of this research is to investigate the possibility of reducing the solutionizing heat treatment duration of CMSX-4 single-crystal superalloy, which normally takes about 18 to 20 hours according to the standard. In this research, three solution heat treatment cycles with reduced time on single crystal samples of CMSX-4 superalloy made by the vertical Bridgman method, has been done and its results have been analyzed. The separation of alloying elements, the percentage of eutectic pools and the amount of porosities have been accurately evaluated based on microstructural measurements with optical microscope (OM), scanning electron microscope (SEM) and energy diffraction spectroscopy (EDS). The results showed that the solutionizing treatment performed with a reduced duration compared to the standard, caused the complete removal of inter-dendritic eutectic pools and the segregation of alloy elements has been reduced by about 30% on average compared to the as-cast sample. But the results have shown that even the longest suggested heat treatment duration was not enough to remove the porosity formed during the solutionizing process and the percentage of porosity has been reached from 0.83% in the as-cast sample to 1.01, 1.75 and 1.41% in heat treated samples for 8.5, 9.5 and 10.5 hours respectively.

In the current study the effect of mold mechanical vibration during the solidification was studied on microstructure and corrosion behavior of Zn-4Si composite. According to the microstructural observation results, mechanical vibration substantially improved the SiP particle distribution and refined them. The image analysis results showed that mechanical vibration at 20, 40, and 60 Hz reduced the average size of SiP particles by 34, 55, and 75%, and increased their number density by 6, 16, and 36 times, respectively. Mechanical vibration at the 20, 40, and 60 Hz also decreased the average grain size by 50, 68, and 76%, respectively and increased the equiaxed zone in castings. The results of Tafel and impedance corrosion tests at 3.5 wt. % NaCl solution implied on increasing the corrosion current and shifting the corrosion potential to the more negative values in mechanically vibrated samples. The corrosion current of as-cast and 60 Hz samples were determined as -1.3610-5 and -2.3310-5 A, respectively. Mechanical vibration also reduces the corrosion resistance of samples where the resistance of 60 Hz sample (about 76 ohm) is lower than that of as-cast sample by about 45%. The increased density of grain boundaries and fine distribution of primary Si particles (as cathodic points) in the composite matrix are characterized as the most important factors decreasing the corrosion resistance of the composite. This is because they increased the number and interspacing of the galvanic cells within the matrix and exhibited appropriate locations for pitting.