پژوهشنامه ریخته گری
سال سوم شماره 1 (پیاپی 8، بهار 1398)

In this investigation Al-4.3% alloy was treated through gas induced semi-solid (GISS) process. The average grain size decreased significantly from several millimeters to less than 100μm through this process. Results showed that GISS changed the structure from fully dendritic to globular morphology. Casting defects such as shrinkage porosities were not detected in the GISS samples. The average grain size decreased when the inert gas flow rate increased. Period time of inert gas injection did not have a significant effect on average grain size. Increasing the staring temperature of gas purging led to an increase in average grain size at constant gas flow rate and gas purging duration. The GISS sample which was prepared through inert gas purging of 20s and having flow rate of 4L.min-1 at 670˚C, had the minimum average grain size (76μm) and maximum sphericity (0.86). According to the results, GISS led to a significant decrease in hot tearing susceptibility from 14 for conventional cast sample to 1 for 680-10S-2L GISS sample. Therefore, high quality castings can be produced using GISS process.

The purpose of this study was evaluation of the directional solidification of GTD111 super alloy bars by Bridgman method, and then, to determine numerically the effect of the heat transfer boundary conditions on the directional solidification rate and the resulting microstructure and grains. For this purpose, in the first step, the heat transfer boundary conditions at the intersection of melt-mold and melt-chill-plate in the Bridgman furnace during solidification of a cluster, consisting of GTD111 rods, were determined, and then the grain structure of the rods after the metallography operation, in both longitudinal and transverse sections, appeared. In the second step, the intended boundary conditions were introduced into the Pro-Cast software and the solidification process was simulated. The grain growth pattern in both longitudinal and transverse sections was achieved and the results of both virtual and real metallography compared. After assuring the accuracy of the simulation results, the effect of the temperature gradient on the pattern of columnar grain growth with the change of the thermal conditions was modeled and its effect on the secondary dendrite arm spacing were obtained. The results show that at a growth rate of about 4mm/min, and a temperature gradient of 80-100°C/mm, the mean secondary dendrite arm spacing will be approximately 100- 120 microns

The low cost stir casting is one of the most applicable methods to fabricate Aluminum metal matrix composites. In this study, stir casting technique is utilized to produce Silicon carbide/Aluminum alloy composite. To achieve a homogeneous and suitable distribution, reinforcing particle surface properties is investigated by applying various thermal and mechanical treatments. SiC Oxidation performed at 950˚C for 1 hour and keeping at 650˚C for 2 hours as heat treatment. Mechanical treatment comprised milling aluminum and SiC powder. 10% vol. fraction Silicon carbide particles with average size of 2, 10 and 40 microns were gradually added to the aluminum matrix stirring at 640˚C. After stirring, the composite was cast at 700˚C in a steel mold. Observing the composite microstructure using OM and SEM reveals that reinforcing particles are dispersed in the matrix. However, heat treated reinforcing particles at 950˚C and keeping at 650˚C leads to fabricate a more homogenous composite of a specified percentage of reinforcing particles in which particles could be easily wetted by the matrix. In samples with SiC particles milled with Al powder, particles were not desirably bounded and disturbed in the matrix. Oxidation at 950˚C and keeping at 650˚C Shows better wettability and distribution of particles. This specimen also has the highest hardness of 139 HV, indicating the proper bonding of particles and their uniform distribution in the matrix.

The purpose of this investigation, is study the effect of melt holding time (3 , 20 min) in vacuum induction melting furnace (VIM) with alumina crucible on the amount of gases, γ' precipitates and stress rupture properties of Rene 80 superalloy. For this purpose, the amount of O, N gasses and chemical composition were analyzed. The microstructure was assessed by scanning electron microscopy (SEM) and stress rupture properties was performed at 980˚C. The results showed that with increasing the melt holding time from 3 to 20 min, γ' volume fraction reduced from 46 to 37% and also, oxygen level of the alloys increased from 79 to 165 ppm because of the alumina reduction. The morphology of γ' changed from semi-cubic to cubic one. The result of mechanical test showed that the stress-rupture life of samples reduced from 22.6 to 19 hours by increasing melt holding time because of reducing γ' volume fraction.

Running system design is one of the most important variables in the casting of ductile iron components by the In-mold process. In this research, a new gating system (based on the critical gate velocity) was used to cast the sample parts. The effect of three magnesium solubility factors (0.03 and 0.04 and 0.05) and three different reaction chamber heights and also the effect of chill on the microstructure and nodule counts in various position of the gate to the casting were investigated. Optical microscopy showed that the optimum nodule count and uniform distribution through the pieces achieved in the 0.03 solubility factor and 30 mm reaction chamber height. By increasing the reaction chamber height, the amount of spheroidization and the number of graphite in a square millimeter (nodule counts) decreased. Also, the higher cooling rate of the pieces results in higher nodule counts.

In this study, IN738LC super alloy samples were solutionized at 1210 °C for 5 hours. After solutionizing, the samples were cooled in both air and liquid nitrogen. One of the air-cooled samples after reaching the room temperature was stored in liquid nitrogen for 2 hours. To investigate the effects of high cooling rate and sub-zero operations on the microstructure and hardness, the samples were characterized by scanning electron microscopy (SEM) and hardness test. Deeper sub-zero operations reduced the size of the sediment to 117 nm. While cooling in liquid nitrogen reduced the size of the sediment to 76 nm. This suggests that the cold weather in the air has provided the opportunity for growth to precipitate. By increasing cooling rate and keep at deep zero, the hardness of the samples decreased, so that the minimum hardness was related to the sample stored in liquid nitrogen. The results of this research can be applied to new blades after casting and also under service and to help improve the heat treatment process.