| 1. |
- Ashrafi, Hamid, et al.
(författare)
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Damage Micromechanisms in Friction Stir-Welded DP600 Steel during Uniaxial Tensile Deformation
- 2022
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Ingår i: Journal of materials engineering and performance (Print). - : Springer. - 1059-9495 .- 1544-1024. ; 31, s. 10044-10053
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Tidskriftsartikel (refereegranskat)abstract
- In this study, damage initiation micromechanisms in friction stir-welded DP600 steel sheets during tensile deformation were studied by scanning electron microscopy (SEM) and electron backscatter diffraction. For this purpose, DP600 steel was welded using friction stir welding with two combinations of rotational and transverse speed, to prepare joints with low and high heats. Microhardness measurements on the cross section of the weldments revealed the formation of a softened zone in the HAZ as a result of the tempering of the martensite, which led to the localization of strain and failure during the tensile testing. SEM observations on the cross section of tensile tested specimens showed that ferrite–martensite interface decohesion and martensite fracture are the main void nucleation mechanisms in the DP600 steel. For the sample welded with low heat input, ferrite–martensite interface decohesion started at higher strains compared to the DP600 steel. A new void initiation mechanism including plastic deformation of tempered martensite, necking, separation of martensite fragments and formation of a void between the separated segments was also suggested for this sample. For the sample welded with high heat input, formation of void at the ferrite–cementite interface was the main void nucleation mechanism and ferrite–martensite interface decohesion was an inactive mechanism.
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| 2. |
- Ashrafi, Hamid, et al.
(författare)
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Void formation and plastic deformation mechanism of a cold-rolled dual-phase steel during tension
- 2020
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Ingår i: Acta Metallurgica Sinica (English Letters). - : Springer. - 1006-7191 .- 2194-1289. ; 33, s. 299-306
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Tidskriftsartikel (refereegranskat)abstract
- The void formation and plastic deformation micromechanisms of a cold-rolled DP600 steel during tensile loading were studied by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The SEM observations revealed that the main void nucleation mechanism in the DP600 steel is decohesion at the ferrite–martensite interfaces. The voids were mostly observed between the closely spaced martensite islands situated at the boundaries of relatively finer ferrite grains. The EBSD results indicated a strain gradient developed from the ferrite–martensite and ferrite–ferrite interfaces into the interior of ferrite grains during the tensile deformation, which led to a stress concentration at these interfaces. Moreover, it was demonstrated that local misorientation inside the finer ferrite grains surrounded by martensite islands was higher than that for the coarser ferrite grains, which made the former more prone to void initiation.
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| 3. |
- Hajiannia, Iman, et al.
(författare)
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A microstructure evaluation of different areas of resistance spot welding on ultra-high strength TRIP1100 steel
- 2018
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Ingår i: Cogent Engineering. - : Cogent OA. - 2331-1916. ; 5:1
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Tidskriftsartikel (refereegranskat)abstract
- In this study, the microstructure of resistance spot welds of advanced ultra-high strength TRIP1100 steel was investigated. For this purpose, welding was performed after determining the best welding parameters. Four sections of the heat-affected zone (HAZ) regions were selected in the regions where the heat exchange was used to control the microstructure. Then, they were used with EBSD by scanning electron microscopy (SEM). The results showed that the TRIP1100 steel microstructure consisted of polygonal ferrites, bainites, residual austenite (RA) and martensite/austenitic islands (M/A). They also showed that the melting zone (FZ) has a lath martensite structure, and the grains are larger in packets. The structure of the martensite and different orientation grains are located in the Upper-critical area (UCHAZ). In the inter-critical region (ICHAZ), the high carbon martensitic content is higher due to the presence and the structure of ferrite and martensite. In the sub-critical region (SCHAZ), due to the tempering of martensite at a temperature below A(C1), the structure is similar to the base metal (BM), with the difference that the RA degradation reduces its structure by 50%. It was found that the RA in the BM had completely transformed. The results showed that with the movement of the BM to the weld metal, the boundaries with a low angle were increased.
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| 4. |
- Hajiannia, Iman, et al.
(författare)
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The assessment of second pulse effects on the microstructure and fracture behavior of the resistance spot welding in advanced ultrahigh-strength steel TRIP1100
- 2019
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Ingår i: Iranian Journal of Materials Science and Engineering. - : Iran University of Science and Technology. - 1735-0808. ; 16:2, s. 79-88
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Tidskriftsartikel (refereegranskat)abstract
- In this study, the effects of the second pulse resistance spot welding on the microstructure and mechanical properties of transformation induced plasticity 1100 steel were evaluated. The thermal process after welding was designed to improve metallurgical properties with pulse currents of 6 kA, 9 kA, and 12 kA after initial welding with 10 kA current. The effect of the second pulse on mechanical and microstructural properties was investigated. The fracture of the welds was for pulsed samples of 6 kA and 9 kA pull out with mechanical test. Due to the existence of the microstructure including the equiaxial dendritic and finer in fusin zone in the pulsed current of 9 kA, the maximum fracture energy, and maximum force were observed. A significant decrease in the FZ hardness in 6 kA current was observed in the nano-hardness results, which was attributed to the existence of martensitic and ferrite temper. The highest ratio of CTS/TSS was obtained for 6 kA and 9 kA, respectively, and force-displacement evaluation was maximum in 9 kA. The fracture surfaces included dendrites and dimples. The results of partial fracture revealed separation in the coherent boundaries of the coarse grain of the annealed region.
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| 5. |
- Naalchian, Mojtaba, et al.
(författare)
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Phase Formation during Heating of Amorphous Nickel-Based BNi-3 for Joining of Dissimilar Cobalt-Based Superalloys
- 2021
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Ingår i: Materials. - : MDPI. - 1996-1944. ; 14:16
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Tidskriftsartikel (refereegranskat)abstract
- Phase transformations and the melting range of the interlayer BNi-3 were investigated by differential scanning calorimetry, which showed three stages of crystallization during heating. There were three exothermic peaks that indicated crystallization in the solid state. The cobalt-based X-45 and FSX-414 superalloys were bonded with interlayer BNi-3 at a constant holding time of 10 min with bonding temperatures of 1010, 1050, 1100, and 1150 °C using a vacuum diffusion brazing process. Examination of microstructural changes in the base metals with light microscopy and scanning electron microscopy coupled with X-ray spectroscopy based on the energy distribution showed that increasing temperature caused a solidification mode, such that the bonding centerline at 1010 °C/10 min included a γ-solid solution, Ni3B, Ni6Si2B, and Ni3Si. The athermally solidified zone of the transient liquid phase (TLP)-bonded sample at 1050 °C/10 min involved a γ-solid solution, Ni3B, CrB, Ni6Si2B, and Ni3Si. Finally, isothermal solidification was completed within 10 min at 1150 °C. The diffusion-affected zones on both sides had three distinct zones: a coarse block precipitation zone, a fine and needle-like mixed-precipitation zone, and a needle-like precipitation zone. By increasing the bonding temperature, the diffusion-affected zone became wider and led to dissolution.
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| 6. |
- Rashtchi, Hamed, et al.
(författare)
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Performance of a PEM fuel cell using electroplated Ni–Mo and Ni–Mo–P stainless steel bipolar plates
- 2017
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Ingår i: Journal of the Electrochemical Society. - : Electrochemical Society. - 0013-4651 .- 1945-7111. ; 164:13, s. F1427-F1436
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Tidskriftsartikel (refereegranskat)abstract
- The performance and durability of 316L stainless steel bipolar plates (BPP) electroplated with Ni–Mo and Ni–Mo–P coatings are investigated in a proton exchange membrane fuel cell (PEMFC), using a commercial Pt/C Nafion membrane electrode assembly (MEA). The effect of the BPP coatings on the electrochemical performance up to 115 h is evaluated from polarization curves, cyclic voltammetry and electrochemical impedance spectroscopy together with interfacial contact resistance (ICR) measurements between the coatings and the gas diffusion layer. The results show that all the coatings decrease the ICR in comparison to that of uncoated 316L BPP. The Ni-Mo coated BPP shows a low and stable ICR and the smallest effects on MEA performance, including catalyst activity/usability, cathode double layer capacitance, and membrane and ionomer resistance build up with time. After electrochemical evaluation, the BPPs as well as the water effluents from the cell are examined by Scanning Electron Microscopy, Energy Dispersive and Inductively Coupled Plasma spectroscopies. No significant degradation of the coated surface or enhancement in metal release is observed. However, phosphorus addition to the coating does not show to improve its properties, as deterioration of the MEA and consequently fuel cell performance losses is observed.
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