پژوهشنامه ریخته گری
سال سوم شماره 2 (پیاپی 9، تابستان 1398)

To evaluate the effect of magnesium content on the microstructure and hardness of the Al-Si-Mg composites in the centrifugal casting method, three cylinders with the chemical composition of Al-20Si-XMg (X = 6, 9, 12) (as weight percent) were cast. Then the microstructure and hardness of the different radial sections were studied by optical microscope, SEM equipped with a micro-analysis system (EDS), and standard brinell hardness testing method, respectively. The phase diagram of Al-20Si-XMg system was plotted as a function of Mg% using Thermo-Calc software. Also JMat Pro software was employed to plot the variation of the mass fraction and density of the in situ formed phases during the solidification of the alloys. The results show clearly that while the coarse Mg2Si particles are formed in high Mg content alloys; however, these particles along with the primary Si particles, both, due to the low density, based on Stokes' law in fluid mechanics, are centripetally segregated towards the inner layers of the cylinders. In addition, by increasing the Mg content of the alloys from 6% to 9% then 12% the volume fraction and average size of the Mg2Si particles in inner layer of the cylinders, both, increase respectively from less than 7% to about 28% and from less than 54 microns to about 166 microns. But, since Mg2Si particles are softer than Si particles, by increasing the volume fraction of the Mg2Si particles, the hardness of the inner layers of the cylinders reduces from 86 to 81 and then 78 brinell.

In recent years, manganese-nickel alloys have been studied by many researchers due to their unique magnetic properties. The purpose of this study was to investigate the effect of pressure of the compartment on the manganese evaporation process and to determine critical and inhibit evaporation pressures and temperature for this element in manganese-nickel alloys (70Mn-25Ni-5Cr and 40Mn-55Ni-5Cr). In this regard, manganese-nickel alloys in a vacuum induced melting furnace, under the atmosphere of argon by positive pressure (0.4, 0.6, 0.8, 1, 2, 2.5, 3, 4, and 4.5) bar melted at temperatures of 1450, 1550 and 1650°C. For the above conditions, the rate of evaporation was calculated and illustrated by plotting evaporation rate graphs with pressure, the critical pressure and impeding pressure for manganese evaporation determined. The results showed that pressures of 1 bar and 3 bar were sequencly critical pressure and impeding pressure for manganese evaporation in manganese-nickel alloys. It was also found that the increase in melting temperature caused an increase in the evaporation rate, but the critical pressure and inhibit pressure remained unchanged for manganese evaporation in the alloys.

The Austempering process that leads to formation of a bainitic ‎microstructure in cast iron is significantly affected by any variation in ‎austempering time and temperature as well as chemical composition of ‎samples. The present work is aimed to reveal the effects introduced by ‎aluminum substitution for silicon in the alloy and austempering ‎treatment on bainite transformation kinetics. Samples of the same size ‎were initially austenitizing at 900°C for 2h in order to achieve a uniform ‎austenitic microstructure, followed by austempering at four upper ‎bainitic temperatures of (375, 400, 425 and 450°C) at various ‎times (1 to 512 min.). Microstructural analyses were carried out by ‎means of optical and electron microscopes and observed that by ‎increasing austempering temperature the thickness of ferritic bainitic ‎plates increases from 0.31 μm in 375°C to 0.63 μm in 450°C. It was ‎concluded that austempering transformation in aluminum ductile cast ‎iron is somehow similar Si cast iron. The difference is that the rate of ‎the first stage of austempering was higher while other stages went ‎through the same rates as the other types of cast iron.‎

The application of Ni-hard 4 cast irons are similar to high chromium cast irons but the characteristic which differs Ni-hard 4 ‎cast iron from high  chromium cast irons is its great hardenability. In the present research the effect of tungsten on Ni-hard 4 cast ‎irons and its effect on the microstructure and formation of carbides are investigated. Also the aim of this research is to ‎simultaneously increase the impact resistance and toughness. For this reason samples according to ASTM A532 standard were ‎prepared without the addition of tungsten while the other samples were prepared by adding different percentages of tungsten to ‎the melt by casting method. After the casting procedure; heat treatment, impact, macro-hardness and micro-hardness tests were ‎done. Studies showed that the addition of tungsten leads to the increase in the hardness of the matrix phase and the formation of ‎M7C3 carbides and it also increases the volume fraction of the carbides and decreases the average diameter of carbides. Due to ‎these reasons the hardness and impact resistance simultaneously increase.‎

In the present study, solidification process of Al-Fe eutectic alloys at a metallic mold was done by finite difference method; and the hot tearing sensitivity was investigated. The mold was designed for directional solidification toward the mold center. The results of simulation revealed that the solidification was done in two stages. In the first stage, the mushy zone thickness was increased and in the second stage it was decreased. As the Fe content was increased, the time of first stage was decreased while the time of second one was independent of Fe amounts. In this work, it was supposed that the hot tear is formed only in a pero-eutectic continuous solid network, so, a criterion for hot tearing susceptibility was introduced based on the mushy zone thickness and local solidification time. In this criterion, the hot tears filled with eutectic melt were account as healed hot tears. The results depicted that all alloys could be sensitive to hot tearing during first stage. Because of low cooling rate during the second stage, the melt experiences near equilibrium solidification; therefore the pero-eutectic solid network could not be formed in Al-1.5wt% alloy and consequently this alloy was not sensitive to hot tear. The most sensitivity was related to Al-1wt%Fe and Al-1.5wt%Fe alloys. At the second stage of solidification, the most sensitivity was related to Al-1wt%Fe alloy.

Al–Si alloys are widely used in automotive components, especially with eutectic composition, for cylinder heads, pistons and valve lifters. In the current study, the effect of different concentration of nickel has been investigated on microstructural evolution and tensile strength properties of Al-12%Si-1%Mg-1%Cu-X%Ni alloys. The microstructural assessment of alloys was carried out by an optical (OM) and scanning electron microscope (SEM) equipped with energy dispersive spectrometer (EDS) analysis. The tensile testing method was performed for both modified and unmodified specimens. The obtained result indicated that the Ni can effectively refine eutectic silicon particles. Depending on the chemical composition of the alloy specimens, different containing phases such as Τ-Al9FeNi, δ-Al3CuNi, γ-Al7Cu4Ni, Q-Al5Cu2Mg8Si6, α-Al15(MnFe)3Si2 and ε-Al3Ni intermetallic in interdendritic regions have been detected. Furthermore, with the increasing of nickel concentration, Ni-containing intermetallics translate from Al3Ni to Al7Cu4Ni or Al3CuNi and their morphologies change from short and long strip-like to chines-script or reticular-shape. Moreover, the results showed that the mechanical performance has been related to the microstructural features. The ultimate tensile strength (UTS) at room temperature increases from 157 MPa to 220 MPa and then decrease to 195 MPa.