نشریه ریخته گری
پیاپی 96 (پاییز و زمستان 1389)

Microstructure of well worn gas turbine blade made of IN738LC Nickel-based superalloy was investigated.The microstructural characterization involved use of both optical as well as electron microscopes including application of EDS technique for studying carbide analysis. The blade was remelted under neutral atmosphere as a part of charge for casting new blade and the microstructure and hardness was investigated. In the yield sample the ordered cubical γ' precipitate was observed. Precipitation films of carbides at the interdendritic regions were caused in increasing hardness and thus the creep ductility was decreased.

The taguchi method (with L''32(2×43×8) orthogonal array) was used as the xperimental design to determine the optimum conditions for production of grey cast irons or in other words producing non-cooled cast irons to increase the graphitization, normalize the size of flake graphites and decrease the carbidization and standard deviation of flake graphites size in solidification rates higher than critical limits. The experimental condition were studied in the range of 0.2-1.5 W/m.K for heat conduction of mould (M), 3.45-3.80% for carbon percent in chemical composition (C), 1350-1450°C for pouring temperature (T), 0-0.5% for nucleas percent (N), and 3-20 mm for part’s thickness (P). The obtained optimum conditions for increasing the graphitization from this study were M=0.2W/m.K, C=3.80%, T=1350°C, N=0.5%, and P=20mm. Under these conditions, the optimum graphitization percent was 95%. To normalize the size of graphites and decrease thecarbidization and standard deviation of graphites size while keeping the graphitization, three series of experiments, in alternative conditions, were performed. Total optimum conditions that obtained from this study were M=0.2W/m.K, C=3.68%, T=1350°C, N=%0.3, and P=20mm.

Study of oxidation behaviour of molten magnesium alloy in different atmospheres is necessary due to the best understanding of protection mechanism of surface film. In this examines characterisation of surface oxidation film formed on pure molten magnesium in pure argon (UPH), dry air, SF6 and HFC-R134a are considered by study of cross section of bubbles that formed by injection of gases into melt in permanent die. Morphology, thickness and chemical composition of surface film studied by uses of the SEM, EDX and GDDAS techniques. The results indicated in all of cases except argon, the film was locally uniform with no evidence of special nuclei. A uniform and thin film formed under atmospheres included fluorine and MgF2 phase is detected in film, also the lower O/F ratio complete conversion of available fluorine into the film was indicated. A thick, wrinkled and folded surface film formed in dry air atmosphere and MgO is dominate phase in surface film.

In general, Aluminum –Silicon alloys have dendritic structure and the silicon particles in their structures are blade form. In the present paper, studied efficacy of semisolid casting terms and modification with Sr on the A356 alloy structure, thus together structure be globular and modify silicon particles form, the conclusive structure refine properly.Hence, first, some specimens produced in semisolid casting situation, then again some specimens produced by applying both of Sr modification and semisolid casting terms, and at last specimens produced by conventional casting and in presence of Sr modifier. For surveying cooling velocity effect, in all of cases, casting accomplished in sand and die moulds. Comparing metallographical results revealed that by applying semisolid casting terms and attend modifier element, samples have globular structures with reformed secondary phase.

The present paper is focused on the AZ91/Al2O3 composite obtained via infiltration of alumina preform by molten AZ91 alloy. The microstructure and mechanical properties of AZ91/Al2O3 composite were investigated as a function of applied pressure and volume fraction of reinforcement.For this purpose the applied pressure on the melt was chosen as 0, 50, 75 and 100 MPa and it was kept until solidification was completed. Results revealed that with increasing the applied pressure to 100 MPa, ultimate compressive strength and ductility of composite, improved as compare to the gravity casting specimens due to decrease of matrix grain size and better reinforcement-matrix cohesion. Also the strength and porosity of samples increased by increasing the volume fraction of reinforcement, and the matrix grain size decreased.

In this study, the efficient parameters of the microstructural properties such as casting thickness, partial remelting heat treatment and cooling rate have been investigated. The testes were carried out by the use of a fluidity test mold. An optical microscope equipped with an image analyzing system was used to study the characteristics. The result showed that solution and partial remelting heat treatment has beneficial effects on the microstructure of the alloy. To obtain the optimum conditions for heat treatment, the specimens were hold in semi-solid state at different temperatures and holding times. Furthermore, the effect of cooling rate was investigates.

Oxidation, Scaling, Thermal fatigue and Abrasive wear all contribute towards deterioration of the rolls surface. Submerged arc welding process with tubular flux-cored wires (FIFO process) is more applicable in roll cladding and hardfacing. In this study, 4 type of AISI420 martensitic stainless steel flux-cored wires with various carbon content produced by wire forming process. An one meter length carbon steel roll chosen as base metal for submerged arc cladding. Finally, the cladded roll cooled slowly below thermal insulation. Because of proper preheating temperature selection, Nondestructive test results showed that no cracking occurred on the roll surface. Microstructural investigation of samples cleared an Austenitic-martensitic microstructure in the clad region that has high hardness and good wear resistance.

By adding ternary or quaternary elements to NiTi alloys, transformation temperatures can be increased or decreased in order to qualify the alloy for a broader range of applications. Due to their high transformation temperatures, good shape memory behavior and lower price, NiTiHf high temperature alloys proved to be promising for high temperature technical applications. This research deals with fabrication and characterization of the Ni49Ti36Hf15 alloy. The ingots were fabricated by alloying and casting of pure materials in a vacuum induction furnace under two different vacuum conditions of 10-1 and 10-4 mbar. After homogenization at 1000°C, ingots were hot rolled at 850°C in order to ameliorate the microstructure. Microstructural investigations were carried out using optical and scanning electron microscopy, and compositional and phase examinations were performed by using XRD and EDS analysis. Results demonstrate that the microstructures of the alloys are composed of B19´ martensite, but different second phases. In the sample prepared under the vacuum of 10-1 mbar, second phase was Titanium-Hafnium Oxide Second phases, especially oxide kinds, not only deteriorate ductility but also lessen transformationtemperatures. Rolling process at elevated temperature results in the annihilation of voids and breaking as well as better distribution of the second phase in the alloy prepared at higher vacuum condition.

In this research the effect of vacuum condition in the VIM furnace on the chemical compositon and mechanical properties of Inconel 718 has been investigated. The results show that under argon atmosphere the Al and Ti loss was significant, however under vacuum atmosphere the loss was lower. Also, result of gas analysis indicated that with increasing vacuum degree, the oxygen and nitrogen content decrease; however in higher vacuum degree there is not any diffrence in gas content of samples. Increasing vacuum content decrease tensile and yield strength of alloys as well as increase in elongation due to eleminating inclusion content and vapourizing Al and Ti in high vacuum degree. On the other hand, with increasing vacuum degree, mode of fracture changes from brittle fracture to ductile fracture. The high Si content which cause laves phase formation in microstructure is the main reason for decreasing ductility in comparison to standard condition.