ABSTRACT Resistance spot welding was used to join austenitic stainless steel sheets. Mechanical properties of the spot welds were evaluated using tensile shear test. Mechanical behaviour was described by peak load, failure energy and... more
ABSTRACT Resistance spot welding was used to join austenitic stainless steel sheets. Mechanical properties of the spot welds were evaluated using tensile shear test. Mechanical behaviour was described by peak load, failure energy and failure mode. The relationship between weld fusion zone attributes and failure behaviour was studied. Generally, it was observed that increasing fusion zone size is accompanied by an increase in load carrying capacity and energy absorption capability. However, when expulsion occurs, despite almost constant weld fusion zone size, energy absorption capability reduces significantly due to increase in electrode indentation depth. Considering the failure location and failure mechanism in the tensile shear test, minimum required fusion zone size to ensure the pull-out failure mode was estimated using an analytical model. According to this model, in addition to sheet thickness, ratio of fusion zone hardness to failure location hardness is the key metallurgical factor governing failure mode of spot welds during the tensile shear test.
Research Interests:
Research Interests:
Research Interests: Materials Engineering, Condensed Matter Physics, Materials Science, Microstructural Evolution, Rietveld refinement, and 12 moreRefinement (Formal Methods), Crystal structure, High Resolution Transmission Electron Microscopy, Mechanical Alloying, HAADF, HRTEM, Rietveld, Nanocrystallines, Elsevier, X Ray Diffraction Analysis, Lattice Deformation, and Solid Solutions
Research Interests:
Research Interests:
Research Interests:
ii Mathematical h,fodelling of Gas Tungsten Arc Welding (GTAW) and Cas Metal Arc Welding (GMAW) Processes Doctor of Philosophy, 1997
Research Interests:
In the current research, an attempt was made to study the effect of shielding gas composition and operational parameters on the microstructure of austenitic stainless steel. Fulfilling this purpose, a 4 mm austenitic stainless steel sheet... more
In the current research, an attempt was made to study the effect of shielding gas composition and operational parameters on the microstructure of austenitic stainless steel. Fulfilling this purpose, a 4 mm austenitic stainless steel sheet was provided. Pulsed current gas tungsten arc welding was carried out using nitrogen gas with the volume percent of zero, 0.5, 1, 2, 5, and 10 in addition to argon as the shielding gas and under pulsed current with frequencies of 40, 80, 120, 160, and 200 Hz. After welding, samples were cut, and the metallographic study was done on weld metal by optical microscope and scanning electron microscope. Furthermore, a ferrite-scope test was performed on the weld metal, and the results were evaluated. Mechanical properties were investigated, and fracture surface was studied. Results showed that increasing the frequency of pulsed current leads to a decrease in ferrite amount in the microstructure, and the area fraction of ferrite decreased to 23% by increasing the frequency. Moreover, it was proven that the addition of nitrogen to the shielding gas resulted in an ascending change in the heat input in the weld pool. Also, with the increase in welding metal nitrogen, ferrite at frequencies of 40 and 200 Hz decreased to 56% and 62%, respectively. In addition, the morphology of the remaining ferrite transformed from mixed lacy-vermicular to completely vermicular. The hardness of the weld metal increased to 66% and 37% at the frequencies of 40 and 200 Hz, respectively. Similarly, the yield strength increased to 11% and 10% at similar frequencies.
Research Interests:
Article Information In this research paper, principal attention is given to the effect of interstitial nitrogen on the precipitation of secondary phases and tensile strength of gas tungsten arc welded (GTAW) Inconel 718. Welding was... more
Article Information In this research paper, principal attention is given to the effect of interstitial nitrogen on the precipitation of secondary phases and tensile strength of gas tungsten arc welded (GTAW) Inconel 718. Welding was performed using Ar+(0-5%)N2 shielding gas mixtures. Secondary phases were characterized by optical microscope, field-emission gun scanning electron microscope (FE-SEM) and transmission electron microscope (TEM). The results revealed that with increment of nitrogen content, the volume fraction of Nb-bearing phases like Laves and carbide increased due to increased microsegregation of Nb in interdenderitic region. Moreover, nitrogen was found to have negative role in the size of gamma double prime particles within γ core. According to the tensile test results, Ar+1%N2 weld samples showed the maximum ultimate tensile strength and yield strength compared to other samples. While more nitrogen leads to reduction of both strengths due to increased Laves phase qu...
Abstract In the present study, the pulsed laser utilized to synthesize the in-situ Fe-TiC hard layer on carbon steel. In this respect, ilmenite, as an inexpensive raw material, was used for the first time in laser cladding. The effect of... more
Abstract In the present study, the pulsed laser utilized to synthesize the in-situ Fe-TiC hard layer on carbon steel. In this respect, ilmenite, as an inexpensive raw material, was used for the first time in laser cladding. The effect of pulse parameters on the microstructure and hardness of the clad layer was investigated. The solidification of the weld pool was started in planar form and then transformed to columnar, dendritic, and finally lamellar eutectic. Thus, an appropriate structure with a high density of hard particles was obtained near the surface. The effects of laser scan speed and pulse duration on the evolution of the solidification structure were investigated. It was found out that the volume fraction of regions with different solidification modes can be adjusted. The properties of the hard layer can be manipulated in this regard. Increasing the laser scan rate from 1 to 5 mm/s reduced the thickness of the clad layer from 135 to 70 µm, while increased the fraction of the planar growth region from 7% to 46% of the clad thickness. Numerical simulation by finite element method was also implemented for better realizing the influence of parameters. To achieve this aim, the variations of temperature and temperature gradient were analyzed. The results revealed that the effect of the laser scan speed is more significant than the pulse duration. Eventually, 2 mm/s and 8 ms were defined as the optimized process parameters. The average hardness of the produced clad is improved eight times higher than the substrate.
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
The microstructural characteristics of mechanically milled (MM) iron (Fe) and copper (Cu) powders are investigated by means of various X-ray crystallography analysis methods. The conventional Williamson–Hall and Warren–Averbach methods... more
The microstructural characteristics of mechanically milled (MM) iron (Fe) and copper (Cu) powders are investigated by means of various X-ray crystallography analysis methods. The conventional Williamson–Hall and Warren–Averbach methods are used besides the modified Williamson–Hall, the modified Warren–Averbach, and the Variance approaches, in proper cases. Afterward, the obtained crystallite size and dislocation density are used to calculate the stored energy in the nanostructured powders. For this aim, a new geometrical approach is developed which can consider three-dimensional crystallites and the thickness of boundaries between them. Moreover, the released energy during annealing of MM Cu and Fe powders is measured using differential scanning calorimetry. The results of line broadening analysis and geometrical modelling are combined to the calorimetry of a room temperature aged Cu powder. In this way, the thickness of grain boundary in the nanostructured Cu is calculated to be 1....
Research Interests:
Research Interests:
ABSTRACT In the present research, the modern fiber laser beam welding of newly-designed precipitation-strengthened nickel-base superalloys using various welding parameters in constant heat input has been investigated. Five nickel-base... more
ABSTRACT In the present research, the modern fiber laser beam welding of newly-designed precipitation-strengthened nickel-base superalloys using various welding parameters in constant heat input has been investigated. Five nickel-base superalloys with various Ti and Nb contents were designed and produced by Vacuum Induction Melting furnace. The fiber laser beam welding operations were performed in constant heat input (100 J mm−2) and different welding powers (400 and 1000 W) and velocities (40 and 100 mm s−1) using 6-axis anthropomorphic robot. The macro- and micro-structural features, weld defects, chemical composition and mechanical property of 3.2 mm weldments were assessed utilizing optical and scanning electron microscopes equipped with EDS analysis and microhardness tester. The results showed that welding with higher powers can create higher penetration-to-width ratios. The porosity formation was increased when the welding powers and velocities were increased. None of the welds displayed hot solidification and liquation cracks in 400 and 1000 W welding powers, but liquation phenomenon was observed in all the heat-affected zones. With increasing the Nb content of the superalloys the liquation length was increased. The changing of the welding power and velocity did not alter the hardness property of the welds. The hardness of welds decreased when the Ti content declined in the composition of superalloys. Finally, the 400 and 1000 W fiber laser powers with velocity of 40 and 100 m ms−1 have been offered for hot crack-free welding of the thin sheet of newly-designed precipitation-strengthened nickel-base superalloys.
Research Interests:
Research Interests:
Resistance spot welding was used to join austenitic stainless steel and galvanized low carbon steel. The relationship between failure mode and weld fusion zone characteristics (size and microstructure) was studied. It was found that spot... more
Resistance spot welding was used to join austenitic stainless steel and galvanized low carbon steel. The relationship between failure mode and weld fusion zone characteristics (size and microstructure) was studied. It was found that spot weld strength in the pullout failure mode is controlled by the strength and fusion zone size of the galvanized steel side. The hardness of the
Research Interests:
ABSTRACT In part I, the effect of the electrode tip angle on the arc properties was investigated. In part II, a mathematical model for the weld pool is developed in order to study the effect of the electrode tip angle on the weld pool... more
ABSTRACT In part I, the effect of the electrode tip angle on the arc properties was investigated. In part II, a mathematical model for the weld pool is developed in order to study the effect of the electrode tip angle on the weld pool properties. The information required to simulate the flow in the weld pool including the heat flux to the workpiece, the input current density and the gas shear stress, was derived from the arc model. By individually examining the various driving forces in the weld pool, it is found that the buoyancy and electromagnetic forces do not play major roles in determining the flow pattern in the weld pool for a 200 A arc. Instead, the relative magnitude of the gas shear stress and the surface tension and also the sign of the surface tension determine the flow pattern in the weld pool. The electrode tip angle which alters the gas shear stress and especially the heat flux to the workpiece can produce a significant change in the overall shape and size of the weld pool. With a very sharp tungsten electrode, the heat-flux distribution over the weld pool tends to be flatter. In addition, there is a very strong shear stress due to gas flow at the top of the workpiece. The overall result of these two effects is a wider and shallower weld pool.
Research Interests:
By developing mathematical models for the arc and the weld pool in the GTAW process, the effect of the electrode tip angle on both arc and weld pool was studied. The present paper is concerned with the model for the arc. By applying a... more
By developing mathematical models for the arc and the weld pool in the GTAW process, the effect of the electrode tip angle on both arc and weld pool was studied. The present paper is concerned with the model for the arc. By applying a variable cathode surface area, the effect of the electrode tip angle (in the range of 10 to 0022-3727/30/19/013/img1) on the arc properties, especially on the anode current density, heat flux and gas shear stress over the weld pool, was investigated. Comparison of the calculated results with the available experimental data for 200 A arcs of different lengths showed that the model predictions for temperatures higher than 10 000 K are in very good agreement. For temperatures less than 10 000 K, some modifications were necessary to take into account the absorption of heat by the cooler parts of the arc. It was found that by increasing the electrode tip angle, the anode spot at the weld pool surface tended to be more localized. This led to a higher maximum heat flux and anode current density. On the other hand, the gas shear stress increased on decreasing the electrode tip angle.
Research Interests:
ABSTRACT The synthesis of ZrO2 by mechanochemical reaction using ZrCl4 and CaO as raw materials and subsequent annealing of the products were investigated. The effect of thermal treatment on the structural evolution and morphological... more
ABSTRACT The synthesis of ZrO2 by mechanochemical reaction using ZrCl4 and CaO as raw materials and subsequent annealing of the products were investigated. The effect of thermal treatment on the structural evolution and morphological characteristics of the nanopowders was studied by X-ray diffractometry, Raman spectroscopy, transmission electron microscopy, scanning electron microscopy, differential thermal analysis and Rietveld refinement. The results showed that the average crystallite size of ZrO2 was less than 100 nm up to around 1100 °C. The activation energy for ZrO2 nanocrystallite growth during calcination was calculated to be about 13,715 and 27,333 J/mol for tetragonal (t-ZrO2) and monoclinic (m-ZrO2) polymorphs, respectively. Mechanism of the nanocrystallite growth of the ZrO2 polymorphs during annealing is primarily investigated.