مجله مهندسی متالورژی
پیاپی 76 (زمستان 1398)

Studies have shown that magnesium-based compounds can be used as solid state hydrogen storage materials. In this study, the Al3Mg2 intermetallic compound was prepared by casting method. Then samples of this compound were ball milled under hydrogen atmosphere at different times. Results of thermo-gravimetry and X-ray diffraction analyses showed that the highest amount of hydrogen storage was achieved after 2 h of ball milling under hydrogen atmosphere. A weight loss of about 2.4% was achieved from mentioned sample that is the highest among the samples. This weight loss occurred during three steps at temperatures of 150, 220 and 360 °C. In the next step, Al3Mg2 ingot along with TiO2 (as catalyst) was ball milled at different times under hydrogen atmosphere. The results of thermo-gravimetry and x-ray diffraction tests showed that the highest amount of hydrogen storage in this sample was achieved after 2 hours hydrogenation and a weight loss of about 2.6% by weight was obtained due to hydrogen desorption, which is close to the theoretical value.

In this research, grain size variation and grain distribution and the effect of different factors such as friction, tool rotational speed, linear velocity of the tool, cooling rate, tool geometry and tool penetration depth on the temperature change and grain size during the friction stir welding process of the aluminum alloy Al-2024 sheet with a thickness of 7.8 mm has been investigated. In order to simulate the model of finite element friction stir welding, the Deform 3D software has been used. The tool is a rigid body and a sheet is considered as a formable plastic material. In order to validate, a comparison between simulation data and experimental results was performed. The effect of effective factors on the temperature of the welding and the amount of grain size variation in the cross section of the weld line have been discussed. Among the variables studied, increasing the cooling rate and the linear velocity of the tool reduce the temperature and grain size, and increase the temperature and grain size with increasing other variables. The results show that with the method presented in this paper, precise prediction of the effect of variation of the variables affecting temperature and grain size can be obtained. In the following, these results can be used to determine the optimum conditions for the friction stir welding process

In the present study, ASTM A516-Grade 70 steel sheets were coated using E410 and E309 electrodes via Shielded Metal Arc Welding (SMAW) process in different number of welding passes. To evaluate the microstructure and properties of the clad layer, metallographic examinations, hardness test, wear test and scanning electron microscope (SEM) from the worn surfaces were used. Microstructural examinations revealed that, all the samples contain ferrite and martensite in the microstructure with different volume fraction. The results showed that, the volume fraction of martensite increased by increasing the number of welding passes. Also, the results of hardness test revealed an increase in the hardness by increasing the number of the number of welding passes. Wear test results showed that, for some samples with higher hardness value, wear resistancewas decreased. Evaluation of the worn surfaces revealed that, in the case of the sample welded using E410 electrode without interlayer, abrasive and oxidative wear mechanisms were dominated. In the case of the sample welded using E309 electrode interlayer and E410 overlay, abrasive, oxidative and galling were observed.

In the present study, the effect of chromium carbide element on microstructure, hardness and abrasive properties of welded coating on the DIN-25CrMo4 substrate was investigated. For this purpose, three groups of roughing electrodes with different chromium content (0-13 wt%) were made and roughing operation was performed by hand electrode arc process (SMAW). Optical and scanning microstructural studies (SEM) along with XRD fuzzy analyzes showed that with increasing chromium content the microstructure of the coating is varied from a ferritic field with needle and morphology morphologies into a rich austenite field of carbide phases such as (FeCr) 7C3. The evaluation of the characteristics of roughened samples showed that the coating hardness increased by about 350% with increasing chromium content (the hardness of the chromium-plated coating and the hardness of the metal substrate were 875 ± 5HV and 200 ± 5HV respectively). Examining the wear behavior of the samples indicates a significant improvement in the wear resistance of the chromium-rich coating (an 80% reduction in the coefficient of friction and a 90% reduction in the amount of lost weight relative to the metal-tensile properties) The SEM study of washed surfaces implying a change in the wear mechanism from the two-body scrubbing wear to the cutting mechanism by increasing the volume fraction of carbide compounds in higher-chromium specimens.

The microstructure of Inconel 718 superalloy in the as-cast (vacuum arc remelted and cast in a water-cooled copper mold) and homogenized conditions was studied. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were used for microstructural studies and elemental analysis, respectively. The presence of austenite/Laves eutectic constituent and carbides was discussed based on the phase diagram and the microsegregation of niobium and molybdenum between dendrite arms of the austenite matrix. In the eutectic structure, the amount of Nb was estimated to be ~ 20 wt%, which is much higher than the corresponding value of ~ 5 wt% in the original chemical composition of the alloy. Based on the elemental analysis, the Laves phase and carbides were characterized as Ni2Nb and MC, respectively. The dissolution of the Laves phase and the disappearance of the dendritic structure as a result of the elevated temperatures homogenization were also studied and it was revealed that the carbide particles remain in the microstructure after homogenization.

In this study, cobalt ferrite powder (CoFe2O4) and cobalt ferrite-hydroxyapatite composite were synthesized by three-step co-deposition method. The synthesized powders were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and infrared spectroscopy (FTIR) techniques. The XRD results confirm the formation of the hydroxyapatite phase along the cobalt ferrite peaks. The FTIR spectra showed the formation of phosphate bonds in the compound, confirming the formation of hydroxyapatite along the cobalt ferrite phase. The results of the dye adsorption experiments on the synthesized nanocomposite methylene blue showed that with increasing pH the amount of dye adsorption increased. Also, optimum adsorption values were obtained using 0.03 g of adsorbent at concentration of 10 mg/L of methylene blue and pH=9 at about 100% for 45 min. The results also showed that the adsorption of methylene blue on the synthesized composite was much higher than that of cobalt ferrite and also at lower adsorption time than pure hydroxyapatite.

Nowadays, the use of ultra high strength steels has been used extensively in the country’s sensitive and strategic industries such as defense, nuclear, aerospace and automotive industries due to it’s high special strength (strength to weight ratio). Among the ultra high strength steels, the use of ultra high medium carbon low alloy steels in sensitive structures such as pressure vessels, solid fuel rocket motor shell, some component of pumps, landing gear, gears and fittings are expanding because of their advantages such as weldability and proper formability, relatively low production costs and easy accessibility. The most important limitations of these steels are elongation, impact energy and low fracture toughness in yield strength higher than 1300MPa. Lower bainite/martensite two phase microstructures have the ability to simultaneously combine strength and high elongation in these steels. Therefore, in the present study, the microstructural evolution of low temperature austempering heat treatment on tensile mechanical properties and impact energy of AISI 4340 steel sheet has been investigated. After evaluation and analysis parameters such as microstructural evolutions, tensile mechanical properties and impact energy of heat treated samples, the sample with 30 minutes transformation time, due to the mixed structure of martensite and lower bainite and having the yield strength, ultimate tensile strength, elongation, impact energy and calculated fracture toughness, 1380MPa, 1490MPa, 12.5%, 52J and 118MPa√m respectively, was selected as the optimum sample in terms of mechanical properties and fracture toughness.