مجله مواد نوین
سال یازدهم شماره 1 (پیاپی 39، بهار 1399)

In this research, the effect of high-frequency resistance welding (HFRW) parameters on microstructure and mechanical properties of seamless 2.25Cr-1Mo ferritic steel as spiral (helical) finned tube used in industrial boilers was investigated. So HFRW was implemented on actual (Industrial) samples by changing multiple parameters including Current, Electric Potential, Welding Speed (Rotation Speed) and Pitch, then metallography, fusion weld, tensile strength and hardness tests were performed on various sections of the samples according to ASTM and international standard. The results of metallographic and mechanical properties tests indicated that in order to obtain a minimum of 90 percent welding diffusion, depends on optimum welding parameters, fin tip and tube tip position of resistance welding, hydraulic pressure jack setting on squeeze roller and pitch FPI (fin per inch) to dissimilar joints between chromium-molybdenum low alloy steels tube to chromium high alloy steels of coil strips. Furthermore as the lower pitch and fin thickness are selected, the higher quality of fin tube welding diffusion is achieved.

Designing scaffolds that possess superior physical properties with cell adhesion and proliferation capabilities significantly promotes bone tissue repair and regeneration. In this study, polycaprolactone (PCL)/keratin/ carbon nanotube (CNT) scaffold was fabricated using the electrospinning method for bone tissue engineering and the effects of CNTs on bone cells growth was evaluated. For this purposes, surface morphology, porosity, specific surface area, mechanical properties and functional groups of scaffolds were examined by scanning electron microscope (SEM), displacement liquid method, brunauer-emmett-teller (BET)test, tensile strength test and Fourier transform infrared spectroscopy (FTIR), respectively. Due to the presence of CNTs in nanofibers, the average fibers diameter in scaffolds containing CNTs in comparison to the scaffold without CNT reduced from 138 nm to 75 nm and the specific surface of nanofibers containing CNT increased. Pore size of scaffold containing CNTs was calculated 680 mμ that this size can be proper for bone cell growth. Biodegradable behavior of scaffolds was determined by immersing samples in the PBS solution for 6 weeks that the results showed 50% degradation in CNT scaffolds. Also, bone cell viability and cell adhesion on scaffolds surface were evaluated via the MTT test and SEM. The presence of CNTs and keratin in scaffolds increased osteoblast cells' growth and proliferation. In conclusion, the findings demonstrated that PCL/keratin/CNT scaffolds can be suggested for bone tissue engineering applications.

In this research, effects of the replacement vanadium to niobium at E347-16 electrode on microstructure and corrosion behaviors of weld metal have been studied. For this reason, two types of electrodes have been produced with ferrovanadium and ferroniobium containing at the coat of the electrodes. Specimens were welded by SMAW according to AWS A5.4, then the samples were cuts and prepared from the weld metal for the chemical analysis, corrosion test and metallographic examination. The result of the chemical analysis test and matching with the schaeffler diagram indicated that the micro-structure of the weld metals was composed of the ferrite and austenite. The the metallographic examination results indicated that the weld metal of the electrode containing vanadium had a higher volume percentage of ferrite than that the electrode containing niobium. Also, the results of corrosion tests showed that with replacing vanadium to niobium at the E347-16 electrode, susceptibility of the intergranular corrosion was at the acceptable range, and the resistance to pitting corrosion was not significantly affect, as well as the electrochemical behavior of the weld metals is not that much difference. The final results determined that the replacement of vanadium to niobium at the E347-16 electrode is possible to metallurgical standpoint.

The Copper/Graphene oxide composite sheets, containing 2% graphene oxide were made by accumulative roll bonding method in four steps for the first time. The process was performed at ambient temperature and non-lubricating conditions. The initial materials were commercial pure copper and graphene oxide. In order to evaluation the produced composites the mechanical, microstructural, electrical and corrosion behavior of the produced composites were investigated at different ARB cycles. The mechanical properties of the composite were investigated by tensile, micro hardness and fractography tests before and at different stages of the process. To observe structural changes a field emission scanning electron microscopy (FESEM) equipped with an EDX spectrometer were used. The results have shown that no new phase has been produced in this composite, and only the main peak of the copper, graphene and oxygen elements could be observed in the EDX patterns.The observation of microstructure showed that in lower cycles, graphene oxide powders were more agglomerated and had non-uniform distribution and in the final stages the powders distribution was more uniformly. The fractography results revealed ductile fracture of the produced composites. The corrosion resistance and electrical conductivity of composites increased compared to pure copper

Addition of rare earth (RE) elements to magnesium alloys has developed the Mg-RE alloys having high specific strength to be used in the aerosapace and automobile industries. Deformability of Mg alloys is limited due to hexcagonal closed packed (HCP) structure and the alloys are routinely processed by hot deformation. The aim of this study is investigation of hot deformation behavior and developed a con-stitutive model for Mg-7.1Gd-2.8Y-1.3Nd-0.4Zn (wt%) alloy. Therefore, hot compression tests were performed at temperature interval of 400ºC-475ºC and at strain rates in the range of 0.001s-1-1s-1 with maxiumum strain of 0.6 on as-extruded specimens. The hyperbolic-sine analysis showed that the stress power (n) and the activation energy (Q) of the investigated alloy are 4.92 and 307.42 kJ.mol-1, respectively. The processing maps were drawn based on the dynamic material model (DMM) in the study of the alloy. The maps suggest two stable domains at: (1) 475ºC-0.001s-1 and (2) 425ºC-460ºC at the strain rates of 0.005s-1-0.05s-1, and indicate a wide unstable region below 425ºC.

The purpose of this work is to develop a new type of calcium silicate nanocomposite, with low setting time, good workability and high strength. This new nanocomposite is prepared by mixing Nano Fast Cement, Nano Hydroxyapatite, polyvinyl alcohol and colloidal nano silica. The effect of three additives (hydroxyapatite nanoparticles, colloidal nano silica and polyvinyl alcohol solution) on its physical and mechanical properties was investigated. Using Taguchi design method, the effect of different levels of additives and optimum percentages of each additive on having nano composite with high compressive strength, low setting time and good workability were determined. According to the results, the most effective factor on the mechanical properties of nanocomposite (compressive strength, flexural strength and flexibility) is polyvinyl alcohol, that level of three (6%) has the highest signal to noise ratio, indicating that the optimal level for this factor is level three. Also, factor A, which represents the hydroxyapatite nanoparticles, is the signal-to-noise value of all surfaces almost close to each other, indicating that this factor does not have much effect on the mechanical properties. And according to the test results, the most effective factor on the setting time is percentage of hydroxyapatite nanoparticles. The optimal level for this factor is level one (zero for hydroxyapatite). Thus, the optimum percentages for nanocomposite production with the highest strength and the lowest setting time and good workability are 6% polyvinyl alcohol, 0% hydroxyapatite and 0.5% nanosilica.

In this paper, the effect of bonding temperature and time on the microstructure of transient liquid phase bonding of GTD-111 nickel-based superalloy was studied. The bonding process was performed at temperatures of 1080, 1120 and 1160 °C at different times and the microstructure of the various bonding regions was analyzed by light and scanning electron microscopy. The results show that by increasing the bonding temperature from 1080 to 1160 °C, solidification time was reduced from 195 to 90 min and the dissolution rate of the base metal and the bonding width increased. Also, at all holding times, the bonding zone containing secondary phases included nickel-rich and chromium-rich borides and nickel silicide in a  matrix. These phases were observed in the centerline and adjacent of the interface. By increasing the bonding time, the volume fraction of the precipitates in the bonding zone decreased and the brittle boride phases were completely removed. This process is due to the strong dependence of the diffusional behavior of the TLP-joint on temperature and time. It was observed that with increasing the bonding temperature, the bonding width and the rate of dissolution of the base metal increase. The results showed that with increasing holding time at all three bonding temperatures, the thickness of the ASZ zone and the volume fraction of precipitates in the bonding area decreased and the DAZ width increased.

In this research hybrid nanocomposite coating of Ni-P/Al2O3-SiC and Ni-P coating were codeposited and deposited by electrodeposition on copper substrate. Electrodepositions for both cases was done by DC current. Sic nanoparticles sizes and Al2O3 nanoparticles sizes which were used for preparing nanocomposite bath, were respectively 55nm and 50nm. Charactrizaton and surface morphology of coating were done by scanning electron microscopy and energy dispersive spectroscopy.The tribological behavior of the coatings were evaluated by pin-on-disc test, and microhardness of coating were also evaluated by Vickers microhardness test. Morphology of surface coating was evaluated by optical electron microscopy. Results show that adhesion of hybrid nanocomposite coating on copper substrate is perfect. Participation of nano Al2O3 particles and nano SiC particles, causes improvement in the microhardness and wear resistance of hybrid nanocomposite coating. High strength of nano Al2O3 particles and nano SiC particles, improved the microhardness and wear resistance of hybrid nanocomposite coating. Also, increases in current density for electrodeposition improve microhardness of Ni-P coating and Ni-P/Al2O3-SiC coating.