پژوهشنامه ریخته گری
سال دوم شماره 3 (پیاپی 6، پاییز 1397)

The purpose of this study was to investigate the effect of the semi-solid rotating container process (RCP) on the characteristics of solidification of AZ91 magnesium alloy and its relationship with microstructure morphology. Thermal analysis (CA-CTA) performed under controlled conditions along with semi solid casting process. Rotation of the container, caused decrease in solidification temperature up to maximum 14°C. However, this effect has been much more limited on nucleation temperature of primary α-Mg phase. The temperature of solidification range increased to a maximum of 16 °C, which led to a better control of the semi-solid process. Comparison of the results between fraction of solid and microstructure showed that rotation with 150 RPM is unable to create an effective fluid flow, in spite of low cooling rate. Rotating with 210 RPM, caused sharp increase in the cooling rate and reduced the effect of effective fluid flow in the slurry. It has been observed that the balance between the effect of the cooling rate and the effective fluid flow has been created at the rotational speed of 180 RPM. Thus, the least solid fraction and the most desirable quality of the morphology have been achieved in the microstructure. Rotation of the container decreased the temperature to a maximum of 15°C and increased the solid fraction to a maximum of 6 % at dendrite coherency point (DCP). This proved that RCP casting method postponed the DCP and therefore, the defects caused by casting can be reduced.

In this paper, synergy effect of various bismuth contents and cooling rates on performance of ‎Al-8Si-0.3Mg cast alloy after applying T6 heat treatment was evaluated. Cooling rate was ‎measured based on analyzing cooling curve and its second derivative curve. A step mould with ‎different casting thicknesses was designed to obtain four different cooling rates (0.55 to 6 ‎‎°C/s). Microstructures of sample were evaluated by optical microscopy and scanning electron ‎microscopy (SEM). Results showed that sample modified with Bi and solidified at higher ‎cooling rate required less time for annealing and spherodising of eutectic silicon. Moreover, ‎tensile test results indicated that the highest yield strength (142.2 MPa), ultimate tensile ‎strength (235 MPa) and elongation percentage (6.1%) obtained for the sample containing 0.5 ‎wt% Bi and solidified at the highest cooling rate (6 °C/s). Fracture surface of samples cooled at ‎low cooling rate comprised of cleavage indicating brittle fracture mode resulted in low ‎ductility. Likewise, fracture surface of samples solidified at highest cooling rate exhibits ‎dimples and ductile fracture mode. The counter graph of quality index used to predict the ‎quality of heat-treated samples with different levels of Bi and solidified at various cooling rates ‎showed that the highest index (352.8 MPa) obtained for specimen containing 0.5wt% Bi ‎solidified at 6 °C/s cooling rate.‎

In this paper, effect of solution heat treatment media of IN738LC superalloy on the microstructure and hardness was investigated. Two solution heat treatment medias were selected, solubilizing in the normal or conventional furnace, and solubilizing in the salt base. For this purpose, the samples were solubilized in a temperature range of 1090-1200°C for 20-30 minutes in normal condition and 10-12 minutes in the salt bath, and then aged at 248°C in 850°C. The results of the scanning electron microscope (SEM) and field emission scanning electron microscope (FESEM) showed that increasing the heating rate improves the solution process, and with increasing the temperature and time, the volume fraction of 'γ precipitates decreased by 26% in normal condition and 29% in salt bath. The size of the residual 'γ precipitates after solution in the salt bath is smaller than normal state, and at high temperatures, due to the activation of the aggregation mechanisms, the size and volume fraction of residual precipitates in the salt bath increased compared to the normal condition. Solution in the salt bath reduced the size of the primary'γ compared to the normal condition and increased the volume fraction of the secondary 'γ after the aging. After aging, the total volume fraction of precipitates increased by about 30% and 40%, respectively, in the normal condition and in salt bath. The results showed that the secondary 'γ precipitates are about 16nm smaller than usual and the hardness of the alloy is strongly influenced by the characteristics of 'γ precipitates and the variables of the solution treatment. Increasing the heating rate at the solution resulted in a further increase in hardness after aging and a further reduction in hardness after solution, and in both conditions increasing temperature and time of solution reduced the hardness of the samples after solution and increased after aging.

Many studies have been performed to improve the quality and cleanliness of steel ‎ingots produced by continuous casting process. One of the important defects is entrapping of mold lubricant ‎powder in the ingot. In the paper, the mathematical modeling methods and ‎computational fluid dynamic (CFD) were used to investigate the powder fluid behavior during ‎continuous casting of steel. For this purpose, the factors of powder entrapping and the behavior of the ‎powder in the melt were investigated and some solutions were proposed to prevent these ‎defects. Therefore, the boundary and initial conditions were studied in developed model and the ‎effect of each one were investigated. The results showed ‎that it can be possible to prevent the ‎powder entrapping in the steel ingot by decreasing the casting speed from 0.9 to 0.76 m/min, increasing the nozzle dip depth from ‎‎130 to 140 mm, decreasing the nuzzle diameter from 42 to 36mm, decreasing the powder ‎particles size from 500 to 63 micrometers and by pouring of clean steel melt without the large inclusions.‎

Mg alloy is easily oxidized during melting in the unprotected atmosphere. This affects on the quality and cleanness of the alloy. In addition, the presence of oxide layer on the surface of melt decreases its fluidity so use of magnesium alloys in thin-film casting is faced with Serious limitation. In this paper, effect of adding calcium on the oxidation resistance and fluidity of AZ91 alloy melt have been investigated. For this purpose, samples of AZ91 alloy were prepared with different amounts of calcium. Afterward samples were stored at 700 ° C for different periods of time in furnace atmosphere .Microstructural, GI-XRD and surface oxidation tests showed that, a dense and compact oxide film composed of CaO is formed on surface, which performs melt protection and prevents penetrating of oxygen into the melt .As a result, the melt stables at a temperature of 700 ° C without presence of a protective gas .The most important advantage of use of calcium in magnesium alloys is not applying protective gases such as SF6. To investigate the fluidity of AZ91 alloy in the presence of calcium, the fluidity test was performed by using spiral pattern. The test results showed that by adding calcium, length of fluidity increased from about 100 mm to 340 mm.

In this research, the effect of cast microstructure on the recovery and recrystallization process of 2304 duplex stainless steel has been investigated. The specimens were hot-formed at low temperatures of 850°C and 950 °C and high temperatures of 1050 and 1150°C by using hot compression test machine at strain rates of 0.001 to 1 s−1. The microstructure changes were studied using optical microscopy equipped by image analyzer. In addition, the apparent activation energies were determined based on the stress-strain hot compression curves. The results show that during of hot deformation of 2304 two-phase stainless steel, the restoration mechanisms in two phase are different, it is performed by dynamic recovery in the ferrite phase and by recrystallization process in the austenite phase. The apparent activation energy Q of the cast microstructure at 850  and 1150  temperatures of hot working is 325 and 150 kJ/mol respectively, which is lower than the applied wrought microstructure.