پژوهشنامه ریخته گری
سال چهارم شماره 2 (پیاپی 13، تابستان 1399)

The effect of chemical modification of a hyper eutectic Al-Si alloy using phosphorus ‎on the efficiency of solutionizing heat treatment in modifying the primary Si has ‎been studied in the current paper. In order to enhance the effect of heat treatment in ‎the modification of the primary Si, the cast specimens modified by phosphorous were ‎solutionized and then microstructurally and mechanically compared with the heat treated ‎unmodified ones. To characterize the microstructure and mechanical properties, ‏optical microscopy, scanning electron microscopy (SEM) and tensile test were ‎performed. The results revealed that chemical modification reduces the primary ‎silicon particles size without any change in Si particles shapes‏. Nevertheless, due to ‎increasing the surface to volume ratio of the particles, the influence of the post heat ‎treatment on the shape of the primary silicon particles was enhanced. Based on the ‎results, the primary silicon particles’ nodularity in the phosphorus modified ‎specimens increased from 32 to 47 percent upon heat treatment process. In addition, ‎‎32 and 33 percent increments in the ultimate tensile strength and ductility were ‎obtained, respectively, after applying the post heat treatment to the P-modified ‎specimens compared to the as cast ones.

Cast Al-15Mg2Si composites due to appropriate specific strength and tribological properties are ‎widely used in automotive and aerospace industries. The improved properties of these composites ‎are governed by the correct controlling of morphology, size, and distribution of primary Mg2Si ‎particles. In this study the effect of solidification cooling rate (chill modification) was investigated ‎on structure and quality index of Al-15Mg2Si composite. The casting operation was performed in ‎different molds with varying cooling rates of 2.7, 5.5, 17.1, and 57.5 °C/s. According to the ‎microstructural observation and image analysis results, increasing the solidification rate resulted in ‎refinement of primary Mg2Si particles and improvement of their distribution in the matrix. The ‎increased solidification cooling rate, moreover, refined the grains so that increasing cooling rate ‎from 2.7 to 57.5 °C/s resulted in 93% reduction in grain size. These microstructural variations ‎improved the composite tensile strength, fracture strain, and quality index. The quality index of ‎composite solidified at 2.7 °C/s was found to be lower than that of composite solidified at cooling ‎rate of 57.5 °C/s by 240%.

The aim of this study was to investigate the effect of temperature and time of aging on the microstructure and hardness of the novel Co-based superalloy with the composition of Co-7Al-7W-4Ti-2Ta. For this purpose, the new generation cobalt-based superalloy ingot cast by the VIM and VAR furnaces. Then aging treatment was performed at 700, 800 and 900°C for 8, 16 and 24 hours. Then, the specimens were cooled in air. Subsequently, the specimens were subjected to microstructural examinations by optical and electron microscopy and hardness test. The results showed that by increasing aging temperature from 700°C to 800°C, the γʹ precipitation and its growth occurred and its volume fraction and size increased. Therefore, the maximum hardness obtained for the specimen aged at 800°C. By increasing the temperature of aging to 900oC, the hardness decreased which was attributed to the precipitation of harmful β phase containing Co-Al-Ti and consumption of γʹ alloying elements results in reduction of the γʹ volume fraction.

In the present study, IN738LC superalloy ingot was re-melted and cast, then heat treated under ‎standard conditions. Microstructural studies with field emission scanning electron microscopy ‎‎(FESEM) equipped energy dispersive spectroscopy (EDS) showed that after re-melting and ‎controlled casting, the amount of porosity and size of γ' precipitates decreases. Also, with ‎applying the standard heat treatment cycle, the primary γ' precipitates less than 1µm in size and ‎cubic morphology and nano‏spherical‏ ‏secondary γ' precipitates appeared and eutectic γ-γˈ ‎regions were completely eliminated. Vickers hardness test results under a load of 10 Kg showed ‎that the lowest hardness is related to the solutionized sample due to remove the coarsened γ' ‎precipitates and the highest hardness (833 H.V.) is obtained after aging. According to the results ‎of the tensile test at ambient and 750◦C temperatures, by re-melting and applying the standard ‎heat treatment due to structural modification, strength and elongation increased at both ambient ‎temperature and 750◦C. Also, with applying the standard heat treatment, failure mechanism of ‎IN738LC superalloy was transferred from brittle trans-dendritic failure to brittle inter-dendritic ‎failure with little footprints of ductile fracture.‎

In this study, Ag-9Pd alloy is introduced as a suitable alloy with a high luminosity for the ring production process. Initially, due to the high price of it, the amount of palladium required to achieve 100 grams of Ag-9Pd (Silvadium) alloy was calculated by the HSC Chemistry 5 software. In order to verify the calculations results, the alloy developed by coagulation casting and rapid solidification was assessed by ICP. As a result, the evaluation indicated great calculations precision. Also, the microstructure, hardness, and luminosity of the produced alloy were compared with both pure silver and Ag-4Pd alloy. The results of microscopic images depicted that Ag-9Pd alloy had smaller dendritic spaces, more homogeneous structures, as well as no defects. Raising the volume fraction of zinc-enriched PdCu and nickel-based precipitates increased the hardness based on the Orvan mechanism. Therefore, as the amount of Pd rises, the luminosity and luminosity life span of the alloy increased due to the improvement in oxidation resistance.

The optimal design of sprue well in of aluminum sand casting process is considered ‎in the present study. The lack of is appropriate design for this component of gating ‎system could cause filling defects such as sand erosion, oxide films and gaseous ‎bubbles trap within the melt. In this study, formerly suggested designs in the ‎literature are firstly examined and then a new optimal design, based on logical try-‎and-error using computer simulation, is introduced to avoid their limitations. The ‎performance of new design is compared to former ones by means of computer ‎simulation. Finally, to validate simulation results, experimental studies are conducted ‎using water and molten metal fluid flow in side transparent sand mold. A high speed ‎camera is employed to capture the fluid flow pattern in the transparent mold. The ‎reasonable agreement between experimental results and computational ones support ‎the validity of numerical simulation and feasibility of presented new design.‎‎