Papers by Author: M.K. Besharati Givi

Abstract: Die shape plays a key role in extrusion process through widely affects on the extrusion pressure and product quality. Therefore, prediction of the optimal die shape is the main objective for an effective extrusion process. In this study, the notion of Equi-Potential Lines (EPLs) was applied to 3D-die designing in extrusion process for the first time. To implement the analogy in the extrusion, the initial and final shapes were considered and two different potentials were assigned to them, and then EPLs were drawn between two shapes that show the minimum work path between the entry and exit cross sections. The drawn EPLs were connected to build up a 3D-die. The effectiveness of the proposed method was examined experimentally, by comparing the results between the designed die and the linear die (with the linear curve for the deformation zone). It was found that there was acceptable reduction in extrusion pressure for the designed die.

Abstract: As a solid-state welding technology, friction stir welding (FSW) can join dissimilar materials with good mechanical properties. In this paper, friction stir welding between 304L stainless steel and commercially pure copper plates with thicknesses of 3 mm was performed. A number of FSW experiments were carried out to obtain the optimum mechanical properties by adjusting the rotational speed to 1000 rpm and welding speed in the range of 14-112 mm/min and with an adjustable offset of the pin location with respect to the butt line. Microstructural analyses have been done to check the weld quality. Cross-sectioning of the welds for metallographic analysis in planes perpendicular to the welding direction and parallel to the weld crown was also performed. The mechanical properties of the welds were determined using a combination of conventional microhardness and tensile testing. From this investigation it is found that the offset of the pin is an essential factor in producing defect free welds in friction stir welding of copper and steel.

Abstract: In this study, friction stir processing (FSP) was applied to fabricate a Cu/SiC surface composite layer by incorporation of 5 µm SiC particles. Effects of the traverse speed and SiC volume fraction on the microstructure, hardness and powder distribution pattern of the developed surface layer were investigated. Optical and scanning electron microscopy (SEM) was employed to carry out the microstructural observations. Results show that increasing the volume fraction causes an intense decrease in the grain size and increase in the hardness of the developed surface. To achieve a uniform distribution of particles and uniform microstructure, the traverse speed should decrease as far as possible.

Abstract: Friction stir processing (FSP) was applied to modify the microstructure of pure copper and Cu/SiC nanocomposite layers. Optical and scanning electron microscopy (SEM) was employed to investigate the microstructure on the modified surface. Also, the wear resistance and friction coefficient behavior of specimens were investigated. FSP homogenizes and refines the copper structure and creates a microstructure with nano-sized SiC particles (30 nm) distributed in the pure copper matrix. Also, it is found that the traversal speed of tool significantly influence the microstructure of developed zone in pure copper. Generally, higher tool traverse speed leads to a more homogeneous microstructure and SiC particles dispersion. This means that higher traverse speeds result in agglomeration of SiC particles which reduces the microhardness values.

Abstract: Friction stir welding (FSW) is a novel solid-state welding process and has been employed in several industries such as aerospace and automotive. Several parameters such as rotational speed, welding speed, axial force and attack angle play critical roles in FSW process in order to analyze the weld quality. The aim of this study is to investigate the effects of different rotational speeds and attack angles on the weld quality of high density polyethylene (HDPE). In the optimum welding condition, 75% of the base material strength is achieved. SEM micrographs show the changes of the weld microstructure which result in the reduction of the strength and the percent of elongation.

Abstract: High strength, high ductility and low thermal conductivity cause the austenitic stainless steels to be hard materials to machine so that heat concentrates at the tool cutting edge due to their properties. This study aims to optimize turning parameters of AISA 304 stainless steel. Turning tests have been performed in three different feed rates (0.2, 0.3, 0.4 mm/rev) at the cutting speeds of 120, 160 and 200 m/min. An analysis of variance (ANOVA) has been made to determine the effects of each parameter on the tool wear and the surface roughness using the Taguchi approach. It is being inferred that cutting speed has the main influence on the flank wear and as it increases to 200 m/min, the flank wear decreases. The feed rate has the most important influence on the surface roughness and as it decreases, the surface roughness decreases as well.

Abstract: Friction stir processing (FSP) is a metal-working technique that causes microstructural modification and change in the upper surface of metal components. In this work the effects of tool pin profile on the microstructure and mechanical behavior of reinforced SiC particles metal matrix composites (MMCs) produced by friction stir processing were studied. Optical microscopy (OM) and Scanning electron microscopy (SEM) was employed to carry out the microstructural observations. Vickers Microhardness Machine used for microhardness evaluation. Results show that, tool pin profile play a major role in improvement of the surface quality, SiC particles dispersion in pure copper matrix, hardness behavior and wear resistance. Two different tool pin profile (straight cylindrical and square) were used to perform the process. It was found that, straight cylindrical tool pin profile led to finer grains, uniform dispersion of SiC particles, higher microhardness and wear resistance values.

Abstract: In this paper, a modified up-quenching heat treatment method to the ASSAB 705M steel (ultra high strength steel) is proposed. A low alloy steel (0.33%C), was used to study the effect of isothermal austempering, successive austempering and modified up-quenching austempering heat treatment on the mechanical properties. The specimens, were cut from a bar with 25mm diameter and after achieving the best temperature and time of austenitizing, austenitized at for 60 min and followed by quenching at for the high austempering temperature to achieve the upper bainite morphology and at for the lower austempering temperature to achieve the lower bainite morphology. In the case of successive austempering, the specimens were first austempered at for different periods (500 sec and 60 sec) and then austempered at for 1000 sec to achieve the mixed structure of upper bainite and lower bainite morphology. The specimens selected for up-quenching, after austenitization were quenched to below ( ) for 120 sec. followed by heating at to achieve the mixed structure of tempered martensite and lower bainite and to achieve the mixed structure of tempered martensite and upper bainite for 1000 sec. All of the processes were performed in the salt bath furnaces. Experimental results are presented and the advantages of the modified method are discussed. As well, it is shown that the best combination of strength and ductility can be achieved by the proposed heat treatment method. This modified method, can offer techniques that simultaneously improve not only strength 12 %( compare with results of strength after other heat treatment methods), but also ductility 38 %( compare with results of ductility after other heat treatment methods). While, conventional heat treatment of ultra high strength steels (UHSS) cannot always meet the strict engineering requirements for improved strength and ductility simultaneously. It has been shown that the mixed structure of tempered martensite and lower bainite that has been suggested in this investigation offers a good combination of strength and ductility. The technical reason for this superiority returns back to the fact that it has increased dislocation density. As a result of the increment of the dislocation density, in the morphology, the inter lath carbide (e.g. cementite) decreases, and the intra lath carbide increases. This modified austempering is applicable to all the ultra high strength steels, has noticeable economic advantages because it is simple. Use of this modified austempering for heavy parts, leads to the lightness of heavy parts and combination of the thermomechanical methods with this modified austempering can yield even much more improvements.

Abstract: Compared to the many fusion welding processes that are routinely used for joining stainless steel 316L, the friction stir welding (FSW) process is an emerging solid state joining process in which the material that is being welded does not melt and is being recast. The welding parameters play a major role in deciding the weld quality. In this investigation an attempt has been made to understand the influences of rotational speed and traverse speed of the tool on the microstructure of the friction stir processing zone in stainless steel 316L. Five different tool rotational speeds have been used to fabricate the joints at four different traverse speeds from this investigation which is the optimum for the tool speed and higher or lower amounts of these parameters are not useful for the process.

Abstract: Friction stir processing (FSP) is a solid state process to modify microstructure and mechanical properties of sheet metals and as-cast materials. In this process stirring action of the tool causes the material to intense plastic deformation that yields a dynamical recrystalyzation. In this study the effect of FSP and process parameters on hardness, and microstructure of stainless steel 316L has been investigated. Also by using of FSP, a composite layer of 316L/SiC has been produced. Results show that FSP leads to a finer and homogenized grain structure, as well as increased hardness, strength, toughness, and elongation at failure of the material. The composites produced by FSP have a uniform distribution of SiC particles between the grains of the base metal.