پژوهشنامه ریخته گری
سال دوم شماره 1 (پیاپی 4، بهار 1397)

The effects of squeeze casting process (SQC) on the as cast microstructure, casting defects and hardness of A390 aluminum alloy were investigated and compared by sand and permanent mold casting processes. The squeeze casting was performed using a hydraulic press and 200°C preheated steel die at 120MPa. The microstructural evaluations and hardness variations were measured. The thermal analyzer technique was used to determine the solidification cooling curve and solidified phases. Result exhibited that the microstructure of the alloy was refined and modified by SQC compered by the sand mold and the permanent mold casting. In addition, the shrinkage defects were severely decreased. The intermetallic phases, the primary silicon and eutectic silicon particles were refined. The quantitative analyses of microstructures showed that the squeeze casting decreased the average area of primary silicon particles and its aspect ratio in amount of 74% and 17% respectively compared with sand casting. Also by using SQC, the hardness of the alloy increased to 30 and 18% compared to sand and permanent casting methods respectively.

In this study, the effect of Ca, Sr, Mn, and Be on modification and quality index ‎improvement of A356 aluminum alloy containing 1.2 wt% Fe impurity was studied in as-‎cast and heat treated conditions. According to the results, Sr and Ca modified the eutectic Si ‎particles and decreased the size of detrimental β-Al5FeSi compounds so that their average ‎size reduced by 23 and 18%, respectively. The addition of Be and Mn did not affect the ‎eutectic Si, but converted the brittle b-platelets to the α-Fe Chinese-scripts. Due to this ‎microstructural variation, the quality index of the alloys modified by Ca, Sr, Be, and Mn has ‎increased by 58, 32, 31, and 17%, respectively. T6 heat treatment led to the precipitation ‎hardening of the alloy, thermally modified the eutectic Si particles, and fractured the b-‎particles. Although, it did not exert significant effect on the size and volume fraction of α-‎compounds. The maximum improvement of quality index in heat-treated samples (36 %) ‎was observed in non-modified base alloy whilst the amplitudes of the quality index ‎improvement in the Ca, Sr, Be and Mn modified samples were 6, 13, 21, and 19 ‎respectively. Increasing of porosity content and the remaining of rather large α-compounds ‎were found to be the most important factors responsible for the relative reduction of the ‎quality index in modified and heat treated samples.‎

Demand for improvements of efficiency in the gas turbines is a driving force for development of ‎new materials generations. Recently, novel Co-Al-W cobalt-base superalloys containing ‎strengthening‎ γ' phase which are capable of using at high temperature have been subject of ‎intense research. The main purpose of this study is to investigate and evaluate present phases in ‎a multi-components Co-Al-W superalloy by both experimental methods and JMatPro software. ‎An alloy belong to this category was cast in VIM furnace. The samples were characterized by ‎SEM-EDS, FE-SEM, XRD and DSC instruments. In the as cast condition, it was found that the ‎matrix phase (‎γ‎) only contained gamma prime phase (‎γ') and MC carbides (riched in Ti and W ‎elements). The JMatPro software simulated precisely the present phases and their formation ‎temperature. Moreover, the software data demonstrated the ‎‏µ‏‎ phase must have presented in the ‎microstructure. According to experimental and simulation results, heat treatment at 900°C for ‎‎48 hours was carried out on the alloy which revealed the tungsten-riched ‎‏µ‏‎ phase in the ‎microstructure. It was found that the formation of the thermodynamically stable ‎‏µ‏‎ phase is ‎restricted depending on the kinetics of the reaction.‎

In the current research, the effect pouring temperature, mechanical vibration of mold, mold temperature and partial remelting on microstructure and mechanical properties of hypereutectic Al-Si alloy was investigated. For designing of experiment, Taguchi method is used. Signal to noise ratio and analysis of variance were used for finding optimized parameters and contribution percent of parameters on amount of output character. After finding the optimized parameters for producing primary billet, partial remelting process is carried out in three temperatures of 540, 555 and 565°C and five holding times of 5, 30, 60, 90 and 120 min. Scanning electron microscope and optical microscope have been used to evaluate the microstructure. The average particle diameter, distribution factor and sphericity of primary Si and Eutectic Si were analyzed by image analysis software. Result of analysis showed that sample with pouring temperature of 750°C, mold temperature of 350°C and frequency of vibration of 60Hz, had maximum hardness and tensile strength. On the other hand, the result of microstructure showed partial remelting process led to increase the sphericity and decrease of average particle diameter of Eutectic Si, but no slight effect on the size of primary Si.

To reach an acceptable bimetallic composite product from two metals, interface characteristics needs to be investigated. In this work joining of commercial aluminium and a plain carbon steel and the formation and growth of intermetallic compounds at the interface was investigated. Steel rods were immersed into the pure aluminium melt at temperatures of 680, 720, 760 and 800 Celsius and kept for different times of 5, 10, 15 and 20 minutes, respectively and then were taken out and cooled in the air. After cross sectioning and sampling, the microstructure of interface between steel substrate and aluminium was studied by optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and Vickers hardness tests. The results showed that two intermetallic compounds, FeAl3 and Fe2Al5 are formed in the interface. Increasing the holding time and temperature changed the interface thickness and had no clear effects on the intermetallics types. Vickers hardness tests showed that the hardness of reaction interface was higher than aluminum and steel substrate due to presence of the above mentioned intermetallics. Kinetic of intermetallics formation are discussed.

In this paper, foam defects of A356 closed cell aluminum foams produced in different process were evaluated. For this purpose, the different foam samples were separately produced using two foaming agents, Titanium Hydride and Calcium Carbonate, without addition of stabilized ceramic particles into the molten metal. Foaming process using 1-2 wt.% TiH2 was performed in the temperature ranges of 650 to 750 °C, for 0.5-2 minutes holding times. Another foaming process was also performed with 2.5-3.5 wt% CaCO3 at 700 °C and for 10-20 minutes holding time. Casting defects included the drainage, the coalescence and the cell size distribution were evaluated using the optical and scanning electron microscopic images and density measurements. The results showed that the homogeneous cellular foam of Al-Si-Mg alloys can be produced using TiH2 and CaCO3 foaming agents and without using the stabilized ceramic particles. The optimal foam are produced using 3 wt.% CaCO3 or using 1.5 wt.% TiH2 additions, separately. In each of two foam productions processes, the drainage and the coalescence foam defects are increased with increasing the holding time. While the drainage and the coalescence foam defects increase with increasing the foaming temperature in TiH2 foamed specimens and with decreasing of the mixing time in CaCO3 foamed specimens.