پژوهشنامه ریخته گری
سال ششم شماره 1 (پیاپی 19، بهار 1401)

Rene 80 is a cast alloy which widely used in turbine blade industry. The purpose of this investigation is to study the effect of crucible type (Zirconia and Alumina) in vacuum induction melting furnace (VIM) with alumina rucible 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 Crucible type has no considerable effect on the γ' size and morphology but γ' volume fraction increased about 3% in the alloy melts in Zirconia crucible in comparison to Alumina one. The result of mechanical test showed that the stress-rupture life of sample melts in Alumina and Zirconia crucibles are 22.6 and 24.7 respectively because of reducing γ' volume fraction in the alloy melt in Alumina crucible.

This study investigated the effect of adding barium elements with various weight percentages of 0.1, 0.2, 0.4, 0.6, 0.8, and 1% on different phases, mechanical properties, and wear resistance of the Al-20Mg2Si composite. The results showed that adding 0.2% barium modified the primary Mg2Si particles. However, with further increasing Ba content, this effect disappeared. The primary Mg2Si particles changed from coarse dendritic to small polygon. The average size decreased from 1178.5 to 289.1 μm, and the normalized area decreased from 1 to 0.7. The aspect ratio of the particles decreased from 1.2 to 1.1, and the number of particles increased from 9 to 41 mm2. The mechanism of barium modification can be heterogeneous nucleation on Al2Si2Ba particles and growth restriction. The eutectic Mg2Si structure also became fine flakes. The α-Al15Si2 (Fe, Mn) 3 emerged in addition to β-Al5FeSi after adding Ba. However, adding Ba did not cause any significant changes in the Al5Mg8Si6Cu2 and Al2Cu phases. Elastic Modulus, yield strength, ultimate tensile strength, elongation percentage, impact energy, and hardness increased by 22%, 24%, 30%, 20%, 33%, and 19% after adding 0.2% Ba, respectively. A large part of the fracture surface of the specimens was composed of cleavage plates, which indicated a brittle fracture mode consistent with a small elongation percentage. Modification decreased the wear rate from 1.9 to 1.2 mm3 / km, reduced the coefficient of friction from 0.58 to 0.54, and the weight loss from 7.1 to 4.8 mg for a sliding distance of 2000 m.

In casting, one way to reduce hot tearing is by modifying the microstructure. In this study, the microstructure of A206 alloy was modified using a melt shearing process at various times, and the effect of melt shearing time on hot tearing behaviour was analyzed. A steel chamber with two baffles and a spiral stirrer was used to apply shear stress to the melt. The sheared melt was then cast in a graphite mold in the form of a constrained rod, and optical and SEM microscopes were utilized to investigate grain size. To evaluate the susceptibility to hot tearing, the quantity of crack length density was used as a criterion. The results revealed that melt shearing process reduces the size of shrinkage pores, making it harder to initiate hot tearing, and increases the alloy's resistance against hot tearing. Also, due to decreasing the size of shrinkage pores, the ultimate strength is increased up to 57%. As the melt shearing time increased up to 3 minutes, the as-cast microstructure became less uniform; however, after 3 minutes, the microstructure became significantly more uniform. Additionally, the grain size decreased up to 37%, and the rate of this decrease accelerated after 3 minutes, leading to an increase in resistance to hot tearing. The decrease in grain size due to melt shearing caused an increase in the number of grain boundaries and a reduction in the thickness of the grain boundary precipitation layer, resulting in a softer behaviour observed on the fracture surface.

The medical cast CoCrMo ASTM F75 alloy is used in orthopedic implants such as artificial toggle and knee. In this alloy, due to the existence of defects such as chemical inhomogeneities and large grain size, heat treatment is performed. In this research the effect of solution annealing heat treatment on the microstructural evolution and mechanical properties of two ASTM F75 alloy (the free of carbon specimen and the specimen containing 0.21 wt% carbon) was investigated. The solution treatment done at 1175, 1225 and 1275oC for 0.5, 1, 2 and 4 hours. Subsequently the specimens quenched in water. The results show that in the specimen without carbon, the σ precipitates were formed. However, in the specimen containing carbon, the M23C6 carbides and σ were formed. Besides, by adding carbon the volume fraction of carbides increased and reached to 6.55 % leads to increase in strength. By increasing annealing temperature from 1175 to 1275 oC and the time from 0.5 to 4 hours, the volume fraction and the size of carbides have been decreased. In the specimen containing carbon, by annealing at 1225oC for 1 hours the distribution of precipitates became homogenous in the microstructure. Besides, most of carbides at the grain boundaries in the cast condition were disappeared by heat treatment. Finally, in the specimen containing carbon in the heat-treated condition, the mechanical properties such as the ultimate tensile strength raised up to 19% and the ductility increased twice in comparison to the one in the cast condition

In this article, the effects of stabilized zirconia oxide by Y2O3 and silicon carbide on physical ‎stability of magnesite based refractories was evaluated. For this purpose, the simultaneous deposition was used to produce MgO-ZrO2 and Y2O3-MgO-ZrO2 precursor ceramic powders, and then these compounds were analyzed using various analytical methods, including QELS laser diffraction (quasi-elastic light scattering), XRD, BET and SEM.  Evaluate. Nanoscale powders with a specific surface area greater than 60 m2/g were created. Using the Vickers indentation method, which was done to determine the hardness of the porous ceramic that was composed of synthetic powders, the hardness of the ceramics is increased by the incorporation of Y2O3, and this process also helps to stabilize the entire cubic crystal phase.

In this study the effect of mechanical vibration (20 Hz, 40 Hz, and 60 Hz) during solidification on microstructure, hardness, and dry sliding wear resistance of Zn-4Si composite was studied. According to the results, Si addition led to the formation of primary Si (SiP) and eutectic Si (SiE) in the microstructure. This increased the hardness of Zn matrix from 30 BHN to more than about 60 BHN. However, uneven distribution (agglomeration) of SiP particles resulted in uneven hardness within the composite matrix. Applying the mechanical vibration during the solidification of composite improved the SiP particle distribution and refines them. According to the pin-on-disk sliding wear results, at the applied pressures of 0.25, 0.5, and 0.75 MPa, the sliding wear resistance of the modified composite (at the 60 Hz frequency) is higher than that of the as-cast composite by 50, 60, and 65%, respectively. Moreover, at the applied pressure of 0.75 MPa the average friction coefficient of mechanically-vibrated composite (60 Hz) is lower than that of as-cast composite by about 23%. Based on worn surface and wear debris analyses the substantial improvement of wear resistance in modified composites can be attributed to the improved tribolayer stability due to the strengthening effect of hard Si particles on the substrate material.