پژوهشنامه ریخته گری
سال چهارم شماره 3 (پیاپی 14، پاییز 1399)

The aim of the present study is to produce advanced metal matrix composites in an innovative route. In fact, reinforcement phase in the advanced composite is a 3D-copper open cell foam as a continuous precursor and liquid of Zamak5 alloy is penetrated into porous space of the 3D-copper foam by casting method. Then effect of supper-heat was investigated on the liquid permeability of the molten Zamak5 and inter-diffusion at Cu/Zamak5 interface during casting in a metallic mold. Results show that by preparing open-cell copper foam with 800 micrometers thickness, 8-20 PPI, and 65 Celsius degree super-heat for Zamak5 casting alloy, production of an advanced 3D-Cu foam-Zamak5 without misrun and cold shut defects during the filling of the metallic mold can be obtained. These condition led to a complete link between 3D-copper foam and Zamak5 matrix, by interfusion mechanism at interface of Cu (3D)/Zamak5 at 450 casting temperature. Also, the 3D copper foam as an internal chill led to formation of fain dendritic structure.

The effects of 0.05, 0.1 and 0.15 wt% C on the microstructure, hardness and tensile properties of annealed Haynes 25 were investigated. The alloys were melted in a vacuum induction melting furnace and purified through electro slag remelting process then solution annealed at 1200˚C for 30 min. The results indicated that W-riched M6C carbides developed in the microstructure and its amount increase with increase of carbon. By increase of Carbon from 0.05 to 0.15wt%, grain size decrease from 44 to 35 µm. The carbides act as nucleator and also retard grain growth that cause to reduce grain size. Mechanical properties investigation shows that with increasing carbon, as a result of grain refinement and the proper size and morphology of carbides, hardness increases from 255 to 290 Hv, yield strength from 450 to 510 MPa and ultimate tensile strength from 946 to 1088 MPa but ductility decreases slightly.

X-750 Superalloy is a precipitation hardened nickel-chromium alloy that maintains its corrosion and oxidation resistance up to 704 °C. Although with increasing temperatures up to 704 °C most of the effects of hard deposition are eliminated, the heat treatment components have a good strength up to 982 °C. For this reason, it is important to apply heat treatment to stabilize the precipitation phases in the X-750 superalloy. The purpose of this study was to investigate the effect of cooling rate on the microstructure and changes in the morphology and size of the particles size of γ' in the X-750 superalloy as well as the relationship between microstructure and hardness. For this purpose, during two-step heat treatment of solution and aging treatment to obtain optimum γ' particle size, different cooling rates after solution treatment were taken and visual analyzes were performed on the obtained microstructures. The results showed that nucleation of secondary γ' particles occurred after cooling rate after solution but their growth was dependent on the aging temperature. The formation of secondary γ' is also dependent on the cooling environment after dissolution and with increasing cooling rate, the amount of secondary γ' depositional particles decreases. Also, with increasing cooling rate after solution treatment, the average size of initial γ' particles decreased from 126.5 to 30 nm and their percentage increased from 59 to 66.91. The highest hardness belongs to the 2-AC heat treatment cycle, which has the highest amount of γ' particles.

In this study, open-cell magnesium foams with irregular and spherical morphology in different sizes and amounts of porosity were produced using the melt infiltration method into a NaCl space holder material. After determining the amount of closed porosity, the effects of morphology and porosity on the mechanical properties of the produced foams were investigated. Structure evaluation of the foams showed that more than 93% of the cells of the produced foams have open porosity and the foams with the irregular cells have a higher amount of the closed cells. Also, the amount of porosity of the foams varied from 54 to 62%. The results of the compression test showed that with increasing the porosity percentage, mechanical properties such as Young's modulus of the foams (E*) and stress of the plateau zone (s*PL) decreased. Also, the irregular cell foams have a higher energy absorption than the spherical cell foams due to the wider plateau zone in the stress-strain curve. The in vitro corrosion rate of the foams was also studied by immersion test in the simulated body fluid (SBF) for two foams specimens with spherical cell morphology and the lowest closed cell amount. The results of the corrosion test have also shown that the size of the porosity of the foams can affect the corrosion rate, so that the smaller cell size foams had higher corrosion rate than the larger cell size foams.

High entropy alloys (HEAs) are a new class of multicomponent alloys in which the solid solution phase is formed as the main phase. Due to the variety of elements in their composition, these alloys have a good capacity to design alloys with desired properties such as lower weight and lower cost. In recent years, several studies have been conducted on refractory high- entropy alloys (RHEAs) for high temperature applications. However, refractory metals are generally very high density and somewhat expensive, so it is important to design the composition of these alloys so that the weight and cost of raw materials can be reduced. In this study, using calculations of thermodynamic and physical parameters, the composition of elements in W-Mo-Ta-Nb-V and W-Mo-Cr-Ti-Al RHEAs were optimized so that while reaching the solid solution phase, weight and cost are minimized. Finally, W10Mo27Cr21Ti22Al20 RHEA was obtained as the alloy with optimal composition in W-Mo-Cr-Ti-Al system and then melted and casted by VAM method. This RHEA contains a solid solution phase with BCC structure as the main phase and a small amount of Laves phase as a minor phase, which has a good agreement with the calculation results. SEM-BSE image of the as-cast RHEA exhibits a typical dendritic microstructure that the dendrites are enriched with elements with higher melting points such as W and Mo and interdendrites are enriched with Cr, Ti and Al.

In the present study, cast IN718 superalloys solutionized at 1050◦C for 150 min. Afterwards, the samples were quenched with three various cooling rates including 240◦C/s, 5◦C/s and 0.1◦C/s. After solutionizing, the samples were aged at 750◦C for 30, 60, and 90 hrs. Microstructural investigation was carried out by optical microscopy (OM), Field Emission Scanning Electron Microscopy (FESEM), and X-ray diffraction (XRD). The results indicated that as the cooling rate increased, the conditions for complete dissolution were provided and only a small amount of MC carbide (TiC) phase remains in the matrix. By reducing the cooling rate and increasing aging time, due to the increase in volume fraction and size of secondary precipitates, the shear transformation of the γ'' to δ increases, and consequently, the δ precipitates increased. After solutionizing, as the cooling rate increased, the hardness due to the decrease in the amount of cooling precipitates decreased. Besides, as the aging time increases, due to the abnormal growth of precipitates, hardness first increased and then experienced a decrease.