نشریه فرآیندهای نوین در مهندسی مواد
پیاپی 55 (زمستان 1399)

In the current research, the effect of Parameters on friction stir welding of aluminum 6061 metal is carried out using with and without an interlayer. After the welding process, microscopic tests, traction and microscopic examination were performed using optical microscope and scanning electron microscope. Among the welded samples with 4043, 5556, and 2024 aluminum interlayers and without interlayer, the welded specimen with a rotational speed of 1250 rpm and a linear velocity of 50 mm / min with an aluminum interlayer of 2024 has the highest tensile strength 192 MPa and has the most hardness of 154 Vickers. The minimum tensile strength of the welded specimen with the aluminum 4043 as an interlayer at the rotational speed of 800 rpm and the linear velocity of 31.5 mm / min is 166 MPa, and with 96 wickers, it also has the least hardness in the weld button in between All samples were. The microstructure study also showed the larger grain size in the welded samples with the interlayer and without interlayer, with a rotational to linear ratio of 32 rpm as compared to the rest of the specimens. The sample was welded to the aluminum with interlayer of 4043 at a speed of 800 rpm and a linear speed of 31.5 mm / min with the smallest grain size of 9 μm in the samples welded to the interlayer. The results of the tests show that the use of aluminum 5556 and 2024 as an interlayer improves the mechanical properties of the bonding zone.

Friction stir welding process is solid state welding method that does not have many common defects in fusion methods. In this method for creating optimum weld, some parameters should be optimized, such as welding tool geometry, rotational speed and travel speed. The aim of this study was to investigate the effect of rotational speed on microstructure and mechanical properties of friction stir lap welding AA5456 in rotary state to optimize the parameter values. For this purpose, Welding process was performed in rotating state, rotating tool was plunged from the cold-worked tube (AA 5456-H321 with 5 mm thickness) surface into the surface of Annealed tube (AA 5456-O with 2.5 mm thickness) and lap joints were produced by rotational speeds of 300, 500, 700 and 900 rpm and welding speed of 45 mm/min. Macro and microstructure of weld cross sections by optical microscopy (OM) and scanning electron microscopy (SEM) were studied. Then the hardness profile and tensile shear test were obtained and compared to another. Finally the fracture surfaces of some samples were examined by using a scanning electron microscope (SEM). The Macro and microstructure results show that increasing of rotation speed, increases the vertical flow of material, the height of hook as well as fine-grained sediments in the nugget zone. Increasing the rotational speed, decreases hardness of weld nugget. The results of tensile shear test show that the welding parameter of (700 rpm- 45 mm/min) is the optimal combination of parameters in this study.

In present research, the effect of tool transverse speed on the microstructure and mechanical properties of friction stir welded DP700 dual-phase steel has been studied. Welding process conducted at a rotational speed of 800 rpm and tool transverse speeds of 50 and 100 mm/min. Optical and scanning electron microscopy were used for microstructural examinations, and mechanical properties were evaluated using microhardness measurements and tensile test. Microstructural investigation revealed that the stir zone consists of bainite, acicular ferrite and polygonal ferrite. It was also revealed that the heat-affected zone (HAZ), based on the peak temperature (Tp), can be subdivided into three different regions: 1) inner HAZ, where Tp is higher than Ac3, 2) Middle HAZ, where Tp lies between Ac1 and Ac3, 3) Outer HAZ in which Tp is lower than Ac1. It was also found that the martensite phase tempers in OHAZ and the degree of tempering decreases with the increment of tool transverse speed. This results confirmed by microhardness measurements where the hardness reduction of the softened zone decreased from 28 to 20HV with an increment of tool transverse speed. The highest hardness of the joints corresponded to the stir zone, and its value increased from 345 to 375HV with rising tool transverse speed. Tensile test results showed that the ultimate strength of the joints was lower than the base metal (723MPa) and it increases from 662 to 671MPa with rising tool transverse speed. It was also revealed that increasing transverse speed improves the total elongation by 2.6%.

Deposition of the Cr3C2-NiCr cermet by the HVOF process results in the dissolution of Cr3C2 in the NiCr metal phase and decreasing the hardness and the elastic modulus of the resulting coating. In this study, Cr3C2-25wt% (Ni-20Cr) powder was applied to Hastelloy X super alloy substrates by high velocity oxy-fuel (HVOF) process. Influence of heat treatment on Cr3C2 phase recovery and improvement of hardness and elastic modulus of coatings was investigated. For this purpose, microstructural examination performed by field emission scanning electron microscopy and phase composition analysis by X-ray diffraction (XRD) analysis on the coating before and after heat treatment. Analysis of back scattered electron microscopy images showed that up to 11% of the dissolved carbide phases in the NiCr alloy recovered during heat treatment. In the XRD patterns of the heat-treated coatings, the amorphous regions disappeared and the NiCr peaks pronounced more in the background phase pattern. Also, the hardness and elastic modulus of coating after heat treatment increased by 156 HV0.3 and 98 GPa, respectively.

One of the best ways to improve the abrasion resistance and toughness of stainless steels is to apply surface coatings. Among these coatings are nickel base alloy and composite coatings. In this research, nickel-phosphorus-titanium oxide coatings were developed using electrical plating technique and the effect of pH (3, 3.5 and 4) on microstructure and their wear and tear behavior were studied. In this research, nickel phosphorus-titanium oxide coating was deposited onto the AISI 430 steel using electrical plating technique and the effect of TiO2 particles concentration on microstructure and wear behavior was studied. X-ray analysis (XRD) was used to determine the available phases and calculate grain size. Characterization of the coating was performed using SEM (Scanning Electron Microscopy). The michardness was measured by Vickers microhardness device. To test the abrasion resistance of the phosphorus-titanium oxide coated and uncoated samples, a pin on the disk test was used. The results of X-ray analysis showed that the increase of pH causes the increase of grain size. Also the results of microhardness and pin on disk tests showed the increase of pH causes decrease of microhardness and abrasion resistance. The highest hardness (618.18 Vickers) was related to the coating created at pH =3 and TiO2 =40 gr / L. The highest wear resistance and lowest weight loss (0.15 mg) were also observed in the same coating.

In this research, a double layer thermal barrier coating was applied and then an alumina diffusion barrier layer was deposited on the YSZ by two solution precersore plasma and solution precersore flame spraying. High temperature oxidation and thermal shock resistance tests were done at 1100˚C. Microstructure of coatings were studied by optical Microscopy and Field Emission Scanning Electron Microscopy. Comparison of the microstructures of coatings showed that applying of Alumina with the solution precursor flame spray process upgrades the thermal properties. High temperature oxidation and thermal shock resistance of YSZ/Al2O3 coatings with Alumina applied by the solution precersoure thermal spray with the same compound were studied. Findings showed that applying alumina with the solution precursor flame spray process leads to increase the amount of the deposited splats and proper contact between them, causes to decrease the diffusion of O2 and as a result TGO thickness decreases and also thermal shock resistance increases.

In this study, cerium oxide coatings were fabricated by electrochemical deposition method on copper substrates. The effect of applied current density on morphology, crystallographic structure, surface chemistry, surface roughness & wetting property of coatings was investigated by scanning electron microscopy, X-ray diffractometry, Fourier transform infra-red spectroscopy, atomic force microscopy & static water contact angle measurement methods. The results showed that, by increasing the applied current density, cerium oxide coatings become thicker & rougher including more cracks. Also decreasing of applied current density lead to enhanced growth of (002) crystallographic planes & crystallite size in the microstructure of cerium oxide coatings. More hydrophilic cerium oxide coatings were fabricated at higher applied current densities. Although as-deposited cerium oxide coatings were hydrophilic but their behavior changed to hydrophobic as a result of long exposure to atmosphere & hydrocarbon adsorption. The hydrocarbon adsorption was higher in the case of cerium oxide coatings fabricated at higher applied current densities.