پژوهشنامه ریخته گری
پیاپی 3 (زمستان 1396)

Simulation softwares are not able to design and can only show various aspects of the suggested design by the running system designer. In this paper, the design of running system with the variation of pouring time (in the range of 5 -20 seconds) for a hollow cylinder component weighing approximately 56 kg was carried out based on the choke calculation method. The number of gates used in the design were 2, 4 and 6 and different types and locations of the ingates were considered. The optimal design and pouring time achieved in this work improves the filling and feeding conditions, allowing the control of the linear velocity based on critical gate velocity criteria and a nonturbulent filling pattern of the mold cavity. The maximum average velocity in the gate was decreased from 2 m. s -1 to 0,17 m. s -1 followed by the control of the cooling rate, improved filling conditions, lowering the shrinkage and the likelihood of its formation has decreased by 41%.

In this paper, the nondestructive eddy current method was used for evaluation of microstructure and hardness of the heat treated cast steel. For this purpose, eight reference samples in same sizes were prepared from the cast block of hypereutectoid low alloy steel called FMU-226 and then the samples were heat treated under different cycles such as full annealing, quenching and quench-tempering. The microstructural studies were performed using scanning electron microscope (SEM), optical microscope and Vickers hardness (HV) instrument as destructive tests. Then, the microstructure and hardness of the samples were evaluated nondestructively by an eddy current test device at different frequencies. The capacities of different eddy current characters such as resistivity, inductive reactance, impedance and resistivity to impedance ratio were examined for developing a mathematical model to hardness measuring and microstructural recognitions. The results showed that the eddy current method can be nondestructively recognized and separated the steel parts according to their microstructure and hardness, rapidly. It was recognized that the eddy current characters were related to the current frequency, variation of retained austenite content in matrix, type of matrix microstructure and carbides morphologies. By using of impedance plane in the eddy current test at 20 kHz, it can be visually detected the microstructure of the heat treated steel. In addition, the impedance (Z) at this frequency was recognized as proper eddy current character for hardness measuring and the special equation for quality control of the heat treated steel was presented.

The impact of the viscosity of the coating, sand grade, and vibration time was investigated on the exhaust of the gaseous products in the lost-foam casting of an A356 aluminum alloy. In this regard, a foam stepwise model with five different thicknesses was used. Three coatings of different viscosities (15, 20, and 25 Pas.sec), three grades of sand (20, 50, 80 AFS), and different vibration times (60, 90, 120, and 180 sec) were considered in the study. To analyze these parameters, their impact on the defects, porosities, and the surface roughness was evaluated. The results showed that the most important defects to be mentioned include being unable to completely fill the mold cavity, the increased porosity in the casting as a result of the excessively thick coating, and the compression of the sand (which blocks the porosities in the mold) due to the wrong choice of refractory coating, sand grade, and vibration time. A coating with a viscosity of 20 Pas.sec, along with 50 AFS grade sand, and a vibration time of 60 sec yielded the best result in terms of defects, porosity (3.7 %) and surface roughness (18 µm).

The aim of the present study is to produce Sn base lead-free nanocomposite solders reinforced by nanoparticles with rapid solidification technique and compare their mechanical, electrical and thermal properties with conventional SAC (Sn-3.8Ag-0.7Cu) solder. Therefore, four lead-free soldering alloys Sn-3.8Ag-0.7Cu-XAl (X = 0, 0.25, 0.5, 1) were alloyed using a vacuum arc remelting (VAR) furnace. Then with melt spinning technique ribbons of nanocomposite solders reinforced with Cu6Sn5 and Ag3Sn intermetallic compounds nanoparticle were manufactured. The microstructural, mechanical, electrical and thermal properties of these nanocomposite solders were investigated using scanning electron microscopy, X-ray diffraction, Vickers hardness method, four-point resistance measurement method and differential scanning calorimetry (DSC). The results showed the uniform distribution of nanoparticles intermetallic compounds Cu6Sn5 and Ag3Sn in the solder matrix and a 30% significant increase of micro-hardness, negligible variation in the specific electrical resistance, and a 3 degrees increase in the melting temperature of the new nanocomposite solder compared to conventional SAC solder.

In this research, cooling curve thermal analysis and interrupted quenching technique were employed to examine interactive effect of Sr-Bi on porosity characteristics of the Al-7Si-0.4Mg alloy commonly used in industry. Based on obtained cooling curve profile and plotting the first and second derivative curves, solidification process of the alloy was studied. Solid fraction curves were calculated and corresponding exact temperatures for precipitation of 20, 45 and 80% solid for alloys with different Sr/Bi ratios, namely 0.1, 0.34 and 0.46 were determined. Subsequently, alloys were quenched rapidly at determined temperatures in the same solid fraction. Porosity characteristics consist of size, area and density were measured using an optical microscope equipped with image analyzer software. Results show that the size, area and density of porosity were affected by Sr/Bi ratio and solid fraction percentage. Area percentage and size of porosity increased by 75% and 141% respectively with increase of Sr/Bi ratio from 0.1 to 0.46. Critical solid fraction for the nucleation and growth of porosity in the alloys were determined at 70% and 80%respectively while Sr and Bi co-existed.

The common defect in Al-5Cu-1Ag alloy is hot tear; so it is necessary to solve it. Thixocasting and rheocasting are semi-solid casting methods that could be useful for hot tear decreasing in the alloys with wide solidification range. In this research after alloying process, casted in direct casting (DC) and semi solid casting (SC). Samples were aged for 18 hours in 150 °C and microstructure studies by optical and scanning electron microscope were performed. EDX was used for analysis and image analyzing done by CLEMEX software. Brinell hardness and tensile strength measurement were used to investigate mechanical properties. Semi-solid behavior and optimum solid fraction during the process were studied by Thermo Calc. software. Results shows that the dendritic structure in DC mode samples could change to a spherical structure with 48 μm diameter and 0.78 sphericity grains in SC method. Thus, hardness increased up to %50; tensile strength and elongation also increased 20 % and 77 %, respectively. According to thermodynamic simulation results the optimum temperature range for Al-5Cu-1Ag alloy rheocasting process is 545-612 °C.