Friction Stir welding of 3.175 mm (0.125 in.) thick plates of AZ31-H24 magnesium alloy was performed using several travel velocities and tool-rotation speeds. After production the welds were cross-sectioned and a metallurgical characterization was performed using optical microscopy, and scanning electron microscopy. Assessment of the weld nugget or ''stirred zone'' shows evidence of dynamic recrystallization and the start of grain growth in some spots of this region compared to the parent metal. Recrystallization was identified in the thermomechanically affected zone (TAZ) as well. The mechanical properties of the weld are correlated with the corresponding microstructures present in the weld nugget and TMAZ. Corrosion resistance of the weld was assessed using Electrochemical Impedance Spectroscopy (EIS) techniques and immersion tests in a corrosive environment; it showed better corrosion resistance than the base metal.
Friction stir welding (FSW) of electrolytic tough pitch copper plates was conducted using a conventional CNC milling machine. The microstructure evolution of the weld was correlated with the process parameters used in the study and in conjunction with increasing temperatures during processing. When the optimum process parameters were achieved, a sound weld joint was obtained. The weldments were evaluated by microstructural analysis, using optical and scanning electron microscopes, and in terms of mechanical properties. At early stages of FSW and/or when using less than optimum welding parameters low temperatures result, metal does not plasticize effectively producing defects, such as large cavities, porosity, and poor bonding, due to the lack of plasticized material. Cavities were found at the advancing region of the weld, and in this area the finest grains were observed from the entire weld. The cavities were reduced, and the grain size increased further along in the weld as the temperature increased also. The typical weld nugget found in the friction stir welding of other metals was not observed in this case. Dynamic recrystallization was observed in the “stirred zone” of the weld; considering that the strain rate in this region was the same in all three cases, the difference in grain size was attributed to the differences in process temperature.
There is a compelling desire by power generating plants to continue running existing stations and components for several more years, despite many of them have surpassed their design service life. The idea is to avoid premature retirement, on the basis of the so-called design life, because actual useful life could often be well in excess of the design life. This can most readily be achieved by utilizing nondestructive monitoring methods to monitor the degradation of the microstructure, either when a station is down for maintenance or preferably when it is under operation. This study evaluates the use of quasi static hysteresis measurements as a possible procedure to evaluate creep in a 410 martensitic stainless steel, a material utilized in power plant components. The creep rupture tests were conducted at stresses of 100 and 200 MPa, temperatures of 500°C and 620°C, and the times varied between 48 and 120 hours. Following the creep tests all specimens were evaluated magnetically and then metallurgically by optical and scanning electron microscopy, x-ray diffraction (XRD) and by energy dispersive spectroscopy (EDS). The microstructural changes were compared with the magnetization changes. It was determined that the changes in the hysteresis curves were clearly detectable and correlated with the creep-induced damage.
The creep evolution was followed by conducting magnetic measurements on ferromagnetic steel samples exposed to different creep strains at a constant temperature. 410 stainless steel (410 SS) rods were submitted to creep at 625°C applying a constant stress of 124 MPa for different creep times. A magnetic hysteresis curve was generated for every sample. It was found that the shape of the hysteresis curves varied with creep time. The extent of creep was assessed by measuring magnetic saturation, coercivity, and remanence. The changes in microstructure due to creep are related to variations in magnetic properties, which are explained in terms of possible magnetic domain pinning. It was observed that the microstructural changes due to creep are better correlated with the coercivity of the material. In summary, it is feasible to use a magnetoelastic sensor to detect the partial level of creep in a ferromagnetic material by nondestructive examination.
There is need for efficient energy conversion systems based on domestic fossil or biofuels. Solid Oxide Fuel Cells (SOFC's) are attractive in this case, however sealing is a critical issue in SOFC development. The purpose of this investigation is to find a procedure to seal yttria stabilized zirconia (YSZ) electrolyte to the stainless steel electrical interconnect or gas manifold. The seal is usually exposed to high temperatures in the range of 500 to 1000°C. Brazing by in-situ alloying of nickel and titanium foils was performed to braze zirconia to 444- stainless steel. Different combinations of nickel/titanium foils were used; brazing was done in vacuum at 6 x 10-6 torr at 960°C, 1010, and 1030°C for different brazing times. The braze and interfacial microstructures were characterized by optical microscopy, scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). This paper assesses the effect of process parameters on the development and stability of the braze metal and the interactions of the filler metal with the two substrates.
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