مجله مهندسی متالورژی
پیاپی 67 (پاییز 1396)

In this paper the effects of thickness ratio on pseudoelastic, shape memory and thermal recovery properties of NiTi/NiTiCu bi-layer composite was investigated by using FEM simulation and experimental results. The NiTi/NiTiCu bi-layer composites with different thickness ratios designed and analyzed in ABAQUS software using SMA constitutive 3D model presented by Lagoudas et al. The pseudoelastic and shape memory strain values for different composites were studied and measured as well as thermal recovery strain and its rate. For validating the results of the FEM study NiTi/NiTiCu bi-layer composites were made and tensile tested. The results indicate a high correlation between two methods that leads to reduce the number of experiments to optimizing the composite behavior, also, considering the results of the FEM method, the NiTi/NiTiCu bi-layer composite shows high potential in adjusting pseudoelastic and shape memory properties and also deformation parameters such as stress and strain plateau with changing the thickness ratio that can be considered in SMA components design.

In this study, the effect of cold rolling on the reverse transformation in Fe-18Ni Martensitic alloy were studied by means of microstructure development, mechanical properties by means of tensile and hardness tests. One of promising way to obtain fine grained materials is using reverse martensitic transformation in specific in martensitic steels. This means reverse the alpha phase to the parent phase of gama and quench rapidly. In the present research, the studied alloy was subjected to different amount of cold rolling consisting of 10, 20 and 50%. Then the cold rolled samples was conducted to reverse transformation heat treatment at 700 C for different cycles. It si found that the 50% cold rolled sample followed by reverse transformation at 7000 C for 0 second demonstrated considerable elongation and relatively high strength of 760 MPa and harness of 270 HV. It should be noted that the starting material has strength of 620 MPa and hardness of 223 HV.

In this research, Welding of D2 Cold Work Tool Alloy steel have been evaluated. A-TIG technique has been utilized to fine an appropriate solution in order to increase the depth of penetration and welding width compared to TIG. Active oxide flux and current intensity have considered as a main variables and their effects on penetration depth, welds width and welding surface appearance were investigated. The fluxes used in this study were ZrO2 and SiO2 and welding process was carried out at 85, 100, 115, 130 and 145A current intensities.
The results showed that arc deviation and spattering due to electrical resistance of ZrO2 flux creates improper appearance on weld surface. Welding by SiO2 flux make welds with good surface appearance and increase the depth of penetration and also decrease weld width. SiO2 flux shows the best performance by increasing of penetration depth up to 5.4 mm at 145A current intensity.

The effect of stir casting process on the modification of the Fe-containing intermetallics formed in a cast in-situ composite based on Al-319 matrix alloy was studied in the present work.
Microstructural observations using optical and scanning electron microscopes showed that the undesirable needle-like shape of the Fe-containing intermetallics (β) and also the coarse star-like α phase were modified into the disc and spheroid shape particles with much less length to width ratio. The effect of parameters such as stirring temperature, cooling rate and Fe contents on the shape, size and distribution of intermetallic particles and eutectic Si blades were also studied. Results showed that the best condition to improve the above-mentioned microstructural features can be achieved at a stirring speed of 1200 rpm for 5 minutes at the vicinity of β needles nucleation temperature followed by casting into a metallic mold. Based on the results obtained from the current work, it can be concluded that the harmful morphology of the β needles can be properly modified by applying a shearing force during stirring the molten alloy in the semi-solid state.

In this study, AZ91C magnesium matrix in-situ nano-composites reinforced with oxide particles were produced by addition of 2 wt% nano silica in the form of a 50wt.%Al.%SiO2(np) activated powder mixture to the melt using stir casting method. For this purpose, the activated powder mixture was embedded in AZ91C ingots and their temperature was stabilized at 800°C. Then the melt temperature was reduced to 750°C and the composite slurry was stirred for 15 min. Finally the slurry was cast in a preheated die at 720°C. Control samples were also cast under the same conditions. Improved microstructure, reduced porosity and increased hardness, tensile strength and yield strength of the composite sample were revealed by microstructural and mechanical investigations. The hardness, yield strength and tensile strength values increased from 67 BHN, 84 MPa and 168 MPa for the monolithic samples to 80 MPa, 120 MPa and 230 MPa for the in-situ formed cast composites, respectively. Microstructural and EDS analyses suggested the in-situ formation of Al2O3, MgAl2O4 and MgO oxide particles by in-situ reaction of Al, Mg and SiO2 in the melt.

In this paper, effects of porosity of pure Al and Al2024 matrix nanocomposite reinforced with carbon nanotubes, sintering parameters such as temperature and consolidation methods of nanocomposite on its properties have been studied. For this purpose, powder composite was prepared using flake powder metallurgy, dispersing particles with ultrasonic and chemical mixing. Consequently, nanocomposites were produced based on two strengthening procedures of spark plasma sintering and cold press-sintering. Finally, the samples were treated under hot extrusion in order to improve mechanical properties. Densitometry, hardness test and metallography were performed on final products. It was concluded that properties of nanocomposites produced by SPS were better than those manufactured by cold press-sintering and also porosity of nanocomposites, which were hot extruded, has been decreased and their hardness has been increased. Hardness and porosity of Al-1.5%wt.CNT, produced by SPS and hot extruded, were measured 62.45 HB and 1% respectively. In order to investigate the formation of aluminum carbide in the interface, Raman spectroscopy and XRD were used and no peak for carbide appeared in both of the experiments.

The Production of iron- bronze bimetallic artifacts is one of the techniques of the Bronze Age to the Iron transitional period. In this research, in order to assess the alloy composition and method of production, two iron-bronze bimetallic objects in the site of Ziviyeh have been studied as one of the important Iron Age sites in Iran. The microstructure of the cross section of objects, the phases and metallic and nonmetallic inclusions was studied by the optical microscope (OM) and the scanning electron microscope equipped with an elemental analysis (SEM-EDX). The results shows the ferrite-pearlite microstructure and preferred orientation of inclusions in substrate of base metal that is decorated with bands of tin- bronze alloy. The amount of carbon in different parts of the body of objects is not the same. Dendritic microstructure and distribution of gas holes in copper phase of bronze strips confirmed the use of casting-on technology in production of bronze layers. Fine grains and twin lines in the outer edges of bronze strips are the result of final hot working.