Materials Science Forum Vols. 783-786

Abstract: Solid oxide electrolyser (SOE) has been receiving increasing attention due to its potential applications in large-scale hydrogen production and carbon dioxide recycling for fuels. Improving the performance of SOE cell through oxygen electrode development has been of main interest because the major polarization loss of the SOE cell is at the oxygen electrode during high temperature electrolysis (HTE). In the present study, Ag was doped into (La_0.75Sr_0.25)_0.95MnO_3+δ(LSM) based oxygen electrode of Ni/YSZ cathode-supported SOE cell through a solid state method enhanced by ball milling. Short stacks were manufactured using doped and undoped cells and tested under HTE of steam at 800°C up to 150h for in situ comparative study of doping effect. The cells with doped oxygen electrodes showed less polarization loss, lower resistance and improved performance by comparison with the undoped cell. Post-mortem examination revealed Ag migrated from the current collecting layer to the electrolyte/anode interface, which may promote the cell performance.

Abstract: Fundamental studies on durability and reliability of SOFC modules/stacks had started as one of the new NEDO project from FY2005 and the organization had been partly modified at FY2008 and FY2013[1]. The main target is to make sure the long term durability within 10 % of degradation rate in the range of 40,000 h (5 years), and reliability during 250 times of starting up and ending operations. In this project, the effect of impurities on degradation of SOFCs was examined for 5 different stacks/modules (Kyocera, Tokyo gas, TOTO, MMC-KEPCO and MHI) at AIST. During the operation of SOFC stacks and modules, several kinds of impurities are carried continuously with air and fuels and react with cell components. The concentration of impurity in cell components was carefully investigated before and after long term operation by using a secondary ion mass spectrometer (SIMS) as a main analytical tool with conventional analytical techniques such as a scanning electron microscopy (SEM) and an electron probe micro analyzer (EPMA). Many approaches were carried out to investigate the effect of gaseous impurities as listed below. (1)The concentration of impurity in cell components was carefully investigated before and after long term operation. (2)Chemical reactivity between impurities and cell components are examined from the view point of thermodynamics. (3)In order to understand the mechanism of the degradation due to impurities, the effect of impurities on degradation of laboratory scale button cell were examined. (4)Cation diffusion were carefully investigated in some diffusion couples to clear the mechanism on formation of secondary phases between the cathode and electrolyte. (5)Acceleration tests on degradation by impurities were also carried out by using a button cell to predict the degradation rate depending on the contamination of impurities in the cathode. The results of the project is partly introduced in this report.

Abstract: Friction stir processing (FSP) has successfully evolved as a technique in fabricating surface composite. An alternative technique on fabrication of a SiC-reinforced Al6061 aluminum matrix composite (AMC) by stirring copper-coated SiC particles into matrix to form a reinforced zone was developed. Copper film was deposited onto the SiC particles by electroless plating and by photodeposition processes. The copper coating serves as an adhesion and diffusion layer to enhance the cohesion between the particles and the matrix. It is to expect that the strength of the AMC could be improved. The uniformity of particle distribution in the stir zone (SZ) was improved by adjusting the location of particle insert and by a double-pass stir. T5 post weld heat treatment (PWHT) was conducted to retrieve the hardness and the strength of the SZ to the strength level of the matrix. While the submicron-thick Cu-coating was partially separated from SiC particles after FSP, photodeposition Cu-coating less than 100 nm thick exhibited a better adhesion to the SiC particles. The EPMA analysis across the interface shows evidence of interdiffusion between copper and aluminum which implies an enhanced cohesion between the particles and matrix. After PWHT, while the SZ containing photodeposition Cu-coated SiC exhibited the highest hardness among different SZs, the SZ containing electroless Cu-coated SiC exhibited the highest strength. The possible mechanisms for the improvement of the hardness and strength were discussed. In summary, the purpose of fabricating a locally particulate-reinforced Al6061 AMC by stirring Cu-coated SiC particles into Al6061 matrix was achieved. Keywords:Friction stir processing (FSP), particulate reinforced AMC, electroless plating, photodeposition, copper-coated SiC particles

Abstract: Friction Stir Welding is a well known solid state joining technology. Many processing conditions and materials properties affect the microstructural evolution and mechanical behavior of the produced joints. The main parameters involved in the welding process have been studied and the results presented in the present paper. The fatigue life and crack behavior of several aluminum alloys FSW joints have been presented. The analysis was conducted through a multi-objective optimization tool capable of correlating all the material properties and processing parameters to the final mechanical performances of the welds.

Abstract: A high pressure die cast (HPDC) magnesium alloy was friction stir processed (FSP) at high rotation rates with different advancing speeds. The AZ91 plate was 3 mm thick and the pin had a truncated cone tip. The friction stir processing induced the disappearance of porosity (typical of the HPDC process) in the nugget zone but some tunnel defects were introduced. The presence of characteristic FSP zones is not obvious due to the low plastic deformation of this alloy. The stirred zone is very narrow around the tool and this makes the FSP difficult to occur in the whole thickness of the plate. Microhardness values and electrical conductivity are sensitive to stirred zone and differences arise according to FSP parameters. The grain size is refined and homogenized by FSP due to partial solubilisation and disappearance of the eutectic phase surrounding Mg-alpha. X-rays diffractometry was performed on FSP samples to index phases and calculate peak shifts of Mg-alpha.

Abstract: It has previously been shown that dissimilar Al-alloy and un-coated steel automotive sheets can be successfully welded with a cycle time of less < 1 s using a novel solid state joining approach; Abrasion Circle Friction Stir Spot Welding (ABC-FSSW). The ABC-FSSW technique provides an effective strategy for exposing a larger clean steel surface during welding than conventional FSSW, allowing high failure energy joints to be produced. In this study the influence of different steel zinc coatings on the viability of the ABC-FSSW process has been systematically investigated - focusing on the effect on the process window, weld temperatures, and joint performance. The effect of a galvanised coating on the weld formation, zinc dispersal in the joint, and interface intermetallic reaction behaviour are discussed.

Abstract: The effective joining of aluminium with copper is one of the central technical goals involved in electro mobility. However, the joining of both metals by conventional fusion welding is challenging because of poor weldability arising from different chemical, mechanical and thermal properties of the materials and especially from the massive formation of hard and brittle intermetallic compounds (IMC) weld interface. In order to accomplish the difficult task of joining aluminium and copper several new joining technologies and strategies such as Laser Beam Welding (LBW) using highly dynamic beam deflection, Friction Stir Welding (FSW), Laser Induction Roll Plating (LIRP) and Electromagnetic Pulse Welding (EMPW) are under development at the Fraunhofer IWS. The current work describes the different technological approaches to the dissimilar joining of aluminium and copper. Thereby, the different joining technologies are compared with respect to weld quality. Special consideration is given to the study of interface morphology and microstructure of the welding zone. It will be shown that, depending on the joining method chosen the kind and extension of intermetallic phase formation differs considerably. Conclusions are drawn with respect to the applicability of the different joining methods.

Abstract: Friction Stir Welding (FSW) is being investigated as a method to fabricate a partial penetration closure weld of the steel vessel of a copper-coated used fuel container. The hemi-head is made of A516 Grade 70 steel and the cylinder body is made of A106 Grade C steel. In this initial feasibility study, the objective is to use FSW to demonstrate the merits of FSW using flat steel plate in concert with a closure weld joint designed specifically for a cylindrical container. To complete this objective, there are two initial feasibility demonstrations. First, demonstrate the capability of FSW to create defect free welds in a corner joint design with specific dimensions (10 mm deep weld nugget). Subsequently, verify the weld quality by ultrasonic inspection and metallography. Further, characterize weld zone properties by establishing mechanical properties and hardness at room temperature, and impact toughness at-5°C. Second, demonstrate the ability to use FSW to repair defects that might occur in the initial friction stir weld. Weld repair was accomplished by intentionally creating tunnel defects by FSW, performing metallography and ultrasonic inspection to characterize the morphology of the defects, performing a second friction stir weld repair over the defects, and verifying the subsequent weld integrity by repeating the metallography and ultrasonic inspections. Results on these initial two phases of this program are presented herein.

Abstract: Friction stir welding (FSW) offers both economic and technical advantages over conventional fusion welding practices for welding line-pipe. For offshore line-pipe construction, the economic savings has been shown to be considerable, approaching a calculated 25%. Offshore pipe is relatively small diameter but heavy wall compared to onshore pipe. One concern is the ability to achieve consistent full weld penetration in an on-site offshore FSW operation, e.g., on a lay-barge. Further, depending on the size and morphology of the unwelded zone, lack of penetration at the weld root can be difficult if not impossible to detect by conventional NDE methods. Thus, an approach to assure consistent full penetration via process control is required for offshore line-pipe construction using FSW. For offshore construction, an internal structural mandrel can be used offering the opportunity to use a sacrificial anvil FSW approach. With this approach, a small volume of sacrificial material can be inserted into the structural anvil. The FSW tool penetrates into the sacrificial anvil, beyond the inner diameter of the pipe wall, thus assuring full penetration. The sacrificial material is subsequently removed from the pipe inner wall. In the work presented herein, FSW studies were completed on both 6 mm and 12 mm wall thickness line-pipe. Post-FSW evaluations including radiography, root-bend tests, and metallography demonstrated the merits of the sacrificial anvil approach to achieve consistent full penetration.

Abstract: The friction stir welding (FSW) is known as non-melting joining. It used widely in the field of industry. Numerical analysis models for FSW also have been developed. In these models, the most frequently used method is a grid method (finite element method or finite difference method). However it is difficult or troublesome to calculate the advective term both for momentum and temperature employing these methods. It is also difficult to calculate the big deformation of the material's free surface. Moreover, complex process is required to analyze the dissimilar joining with respect to dealing with substance transfer. In this paper, to avoid these difficulties, particle method is adopted for FSW simulation. In particle method, advective term, substance transfer, and surface deformation are calculated automatically mainly because that Lagrangian approach is used. To verify the effectiveness of this method, fluid motion around the tool is examined by particle trace. As a result, relations between the rotating speed of the tool and area of plastic flow is evaluated.