METALLURGICAL ENGINEERING
Year:2018 | Volume:21 | Issue:3 (71)|Papers Count:7

In this research, the effect of severe plastic deformation by high pressure torsion process on the microstructure, mechanical properties and pseudoelastic behavior of the Fe-10Ni-7Mn (wt. %) alloy after annealing at intercritical temperatures was investigated. The results revealed that applying high pressure torsion process for 20 turns on the solution annealed specimen and intercritical annealed samples at two different temperatures, 580 and 600° C for 2 hours, caused reasonable ductility with significant increase in micro-hardness, ultimate tensile strength and reduced grain size to nanometer. Phase evaluation by XRD and EBSD analysis showed that high pressure torsion process led to reverse transformation of martensite to austenite in solution annealed specimen and austenite to martensite transformation in the intercritical annealed samples. Among studied samples conducted in this research, the intercritical annealed sample at 600° C and subsequently deformed by high pressure torsion, revealed the most significant mechanical properties. Moreover, in this sample due to the higher fraction of austenite, fine grain size and high tensile strength, and easier reversible movement of the fcc/hcp interface, more amount of pseudoelasticity was achieved in comparison to the others samples. The d-spacing of the specimen without heat treatment, calculated byX-Ray Diffraction, had a little increase due to clustering. In addition, the surface morphology by optical microscope confirmed this behavior of Cu– Ni alloys in the different heat treatment.

In this study، the effect of casting porosity on the strength of cast nickel aluminum bronze (NAB) was investigated. For this purpose، four-point fatigue tests were carried out under R =-1 and 50 Hz on this alloy. Experimental results and plotted fatigue curve indicated high values of scattering in fatigue data، increased at applied stress levels higher 150 MPa. To reduce scattering and to obtain a more realistic fatigue strength، pore sizes were measured from the fracture surfaces of the specimens by using image processing technique. The fatigue strength was، then، calculated based on the effective area and fatigue curves were re-plotted. Results indicated that S-N curves obtained by this method are independent from casting defects and could present a better index for fatigue strength of the casting alloy. Therefore، it was concluded that the strength was obtained from this method could be an appropriate reference value when considering the fatigue properties of cast nickel aluminum bronze.

The purpose of this study is to demonstrate the advantages of computer simulation and parametric studies in improving the copper electroforming process. For this purpose, the finite element model for a cone geometry was prepared using the Comsol software, and the effect of key parameters including applied current density, solution electrical conductivity, electrode spacing, and anode length, on the thickness uniformity. In order to validate the model, a conical shell was produced in the laboratory by electroforming method, and the thickness distribution was compared with the simulation results. The comparison of the results showed that using the tertiary current distribution method to simulate the electroforming process is a precise and efficient model and can be used for parametric studies. Finally, after parametric study, it was determined that all selected variables had a significant effect on the thickness uniformity. In addition, it was found that the most to least significant variables were applied current density, solution conductivity, anode-cathode spacing, and anode length, respectively.

In this research، the influence of various weight percentages of reinforcing particles on the microstructure and mechanical properties of composites was investigated. Aluminum alloy (A356) matrix composites reinforced with 0. 5، 1 and 1. 5 wt. % nano-particle SiC (about 80nm) were fabricated via stir casting method. Fabrication was performed at 610° ? by the injection of reinforcing particles into molten alloy. The composites were characterized by field emission scanning electron microscope (FESEM) equipped with image analyzer and energy dispersive spectroscopy (EDS) and، Optical microscope (OM). Microstructure evaluation revealed a uniform distribution of nano particles with reduced in weight percentages and the average size of dendritic arms has decreased at least 50 percent. . Brinell hardness measurement showed that addition of reinforcing particles gives rise to hardness compared to matrix alloy and the highest increase was for the sample contain 0. 5% SiC with 40 percent increase. The porosity percentage in the materials was calculated according to the difference between the theoretical and measured density and revealed that porosity increased about 3% by increasing in amount of SiC particles.

Basically, the limitation on the grinding capacity of iron ore mines is one of the challenges of increasing the capacity of pelletizing units along with the program of increasing steel production in our country. Therefore, the purpose of this research is to investigate the operational solution for obtaining appropriate quality of green pellets of fin e concentrate with low specific surface area (less than 1500 cm2/g). Pilot pelletizing process was carried out by mixing fine concentrates of Chadermallo and Bafgh mine with specific surface area number from 1220 to 1392 cm2/g and various additives such as bentonite, sodium carboxymethylcellulose (CMC), sodium hydroxide and sodium carbonate. Wettability of iron ore particles was evaluated with distilled water, process water, CMC solution and sodium hydroxide. The particle size analyzer and specific surface area analyzer were used for determination of particle size distribution and specific surface area of concentrate powder, respectively. The drop number, wet strengthanddry strength were measured for evaluation of the quality of green pellets produced. High wettability was measured when Carboxymethyl cellulose organic binder solution was used. To maintain the qualitative properties of green pellet, proportional to reduction of specific surface number of concentrate powder from 1396 to 1220 cm2/g and as well as avoid the increasing bentonite consumption, the CMC organic viscose solution has been added to the mixture of 0. 01 to 0. 03 %wt. the dry strength of the pellets to 5. 5 kg/pellet was increased, by addition of sodium carbonatewithout of NaOH.

Sub-zero treatment is an efficient method for increasing the efficiency of metal alloys. In addition، aging is a common method for increasing the ratio of the strength to weight of aluminum alloys 2024 and 7075 for using in a passenger aircraft. In this research، the effect of sub-zero treatment at temperature of-196° C (deep cryogenic treatment) and time of 4 hours on the hardness of aluminum alloys 2024 and 7075 are studied. For this purpose، scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) have been applied to study the microstructure. Also، tensile and hardness tests have been applied to study mechanical properties. The results show that the formation of precipitation around the particles (especially for iron particles) is facilitated by performing sub-zero treatment. Because the difference in the coefficient of iron contraction with the aluminum matrix at sub-zero treatment causes elements (such as copper in 2024 and magnesium in alloy 7075) are easier to absorb the areas around iron particles at aging temperature of 100 ° C. The new precipitates increased the yield strength of aluminum 2024 and 7075 compared to the control specimen، 32 and 20 MPa، respectively، and the tensile strength of aluminum 2024 and 7075 increased 26 and 21 MPa، respectively. While in both specimens، the change of hardness was not noticeable.

In this research، the effect of cold rolling and short-term heat treatment on the microstructure and hardness of A356 aluminum alloy were investigated. Microstructural observations by optical microscopy، quantitative analysis using Clemex software and hardness test by Brinell macrohardness were performed. The results showed that the rolling process led to refining، increasing the sphericity، and decreasing the aspect ratio of the silicon particles، as well as removal of porosity in the casting sample. The distribution of silicon particles at the RD-ND plane was more uniform than the RD-TD and section because during the rolling process، the dimensional changes occurs only in the RD and ND directions، resulting in more plastic deformation at the RD-ND plane، and the distribution of silicon particles which is totally dependent on the amount of plastic deformation became more uniform. Also، rolling and heat treatment caused the grain refinement of aluminum through activation of continuous recrystallization and particle stimulated nucleation (PSN) mechanisms. The presence of silicon particles resulted in formation of finer grains and also suppression of the grain growth. Rolling process increased the hardness of the sample، but the heat treatment up to 300 s reduced the hardness. Finally، increasing the heat treatment time to 600 s resulted in an increase in hardness of 26% due to the completion of the continuous recrystallization mechanism.