Materials Science Forum Vols. 783-786

Abstract: Stationary (or Static) Shoulder Friction Stir Welding (SS-FSW) is a variant of FSW that was developed primarily to improve the weldability of titanium alloys by reducing the through thickness temperature gradient. Surprisingly, SS-FSW has been largely ignored by the Al welding community because it is widely supposed a rotating shoulder is an essential aspect of the process and that the higher conductivity means the surface heating effect of the shoulder is generally beneficial. In the work presented it is shown that SS-FSW has major advantages when welding high strength aluminium alloys; including a reduction in the heat input, a massive improvement in surface quality, and a narrower and more symmetric temperature distribution, which leads to narrower welds with a reduced heat affected zone width and lower distortion. The reasons for these benefits are discussed based on a systematic study aimed at directly comparing both processes.

Abstract: FSW process has been evaluated on three shipbuilding steels (DH36, S690QL and 80HLES steels) by fully penetrated butt welds on 8mm thick plates. Non destructive tests were carried out to highlight the presence of intrinsic defects known for the welding process (eg kissing bond). The validated inspection methodology (volume and surface testing) confirm the integrity of welds and the absence of geometrical defects for examinations and mechanical tests as part of a qualification procedure.

Abstract: Ultrasonic welding is conducted using pure aluminum sheets to investigate effect of the transitional welding processes comprised of three transitional stages on the formation of the weld microstructure. In the first stage of ultrasonic welding, the relative motion mainly occurred between the workpieces, and a partially bonded region was observed in the weld interface. In the second stage, the relative motion at the weld interface was suppressed by the formation of the partially bonded region, while relative motion between the weld tool and the workpiece in contact with the weld tool. The relative motion at the weld tool/workpiece caused weld temperature rise with a penetration the weld tool edge. In the third stage, a plastic deformation zone generated by the relative motion at the weld tool/workpiece spread into the lower side of weld part as the welding time increased. It is proposed that the formation of weld microstructure in ultrasonic welding is attributed to the thermo-mechanically effect of the relative motion of the weld tool and workpiece.

Abstract: In order to industrialize friction stir welding (FSW) processes, not only the machine concept itself needs to be evaluated, but also the robustness of the process application being carried out on the machine. Especially for FSW of medium-sized and larger parts, a small degree of misalignment can have an increasing influence on the weld quality. Therefore an exemplary tolerance study for friction stir welded butt joints was conducted. The intentional introduction of gaps between the sheets to be welded can limit the welding process and thus the weld quality. However, for the considered experimental set-up it can be shown that the introduction of a well-defined gap can support the welding process and weld quality. The experimental procedure was carried out on a parallel kinematic machine - a so-called Pentapod. This machine is suitable for large and complex three-dimensional structures. Although the machine is able to record the process forces (F_x, F_y and F_z) acting on the tool–work piece interface, the forces acting perpendicular to the clamping system are still unknown. Therefore additional load cells were integrated into the clamping system to measure the in-process reaction forces. The combined results of the force measurements give a nearly complete overview of the internal loads during the process. In conjunction with knowledge about gaps and misalignment, the data gained in this study can help to understand and predict the clamping behaviour, and thus design rules for future clamping systems can be derived.

Abstract: Friction stir welding (FSW) is an advanced solid state joining technology, which was invented by TWI in 1991. During the process, large amount of heat is generated due to the friction between the tool and the material. As a result, the metal around the tool is softened as the temperature rises, and significant plastic flow occurs. So FSW is a complex process with multi-field coupled phenomena. Material flow plays a central role in FSW. But it is still difficult to reveal the material flow regime and joining mechanism during FSW process. Numerical simulation is a powerful tool for investigating the metal-flow-related complex phenomena during FSW. Meanwhile, numerical simulation could also help to optimize FSW tool design and FSW parameters. In this paper, we review the recent development in simulation of material flow during FSW. Then, the important issues in modeling multi field coupled phenomena during FSW are summarized, which include the heat generation mechanism, the temperature and strain rate dependent material’s behavior, and the interaction between tool and material. Finally, a comprehensive simulation model is presented, which enables advanced study on the coupled phenomena of heat generation, temperature distribution, material flow, and defects formation. This model has shown potential applications in simulating the relation between the transport of material and the macrostructure formation or defects formation. In spite of these progresses, simulation of material flow during FSW still need quite a lot of researches to fulfill industry requirement.

Abstract: During a recent upgrade to the U.S. Department of Energy’s Princeton Plasma Physics Laboratory operates an experimental nuclear fusion reactor known as the National Spherical Torus Experiment (NSTX) a new center stack design was selected that required an innovative joining method to fabricate the bundled copper extrusions found in the center stack. Each of the 20-ft long copper extrusions used in the center stack was made from a quarter hard copper alloy (CDA10700) joined to lead extensions made from a high-strength copper alloy (CDA 18150). A team consisting of PPPL, Major Tool, and EWI selected Friction Stir Welding (FSW) to join these two materials together as its solid-state nature would provide a higher joint efficiency while minimizing distortion and preserving properties of the heat treatable lead extension flag.

Abstract: To realize modern light weight constructions it is more and more necessary to combine the advantages of dissimilar materials. Fusion welding of dissimilar metals is in the most cases difficult or even impossible as a result of different melting points of the materials and the development of undesirable brittle intermetallic phases in the welding zone. This often leads to joint strengths considerable below the tensile strength of the base materials. By using Friction Stir Welding (FSW) as a pressure welding method, it is possible to reduce the development of the intermetallic phases of Al/Mg-joints significantly. But as calculated phase diagrams and high resolution microscopic SEM-investigations have shown it is not feasible to avoid them completely. The intermetallic phases form in the contact area very small continuous layers between the joining partners. On the other side it is known that ultrasonic energy can crack oxide layers. Hence a hybrid welding system at the Institute of Materials Science and Engineering (WKK) at the University of Kaiserslautern was developed called “Ultrasound Supported Friction Stir Welding (US-FSW)” with the aim to shatter the brittle interlayer lines and to scatter fragments in the welding area during the FSW process. Pre-investigations have shown that for US-FSW-joints between Al wrought alloys and Mg cast alloys the strength can be increased up to 30% in comparison to conventional friction stir welds. Currently, further investigations are carried out with joints between AC-48000 and AZ80.

Abstract: Friction Stir Welding (FSW) is an innovative welding process increasingly used by industry for the welding of aluminum alloys. In order to reduce the high investment costs of a dedicated FSW’s machine and in order to offer more flexibility to weld complex geometry, high payload robots may be used. A serial kinematics robot meets these specifications but under the stresses generated during welding, its structure readily deforms. The consequences are deviations of the tool nominal position with respect to the seam. The work presented here proposes to study the process tolerances with tool positioning defect. An experimental study enables to evaluate the influence of the tool position disorientation on weld quality, the travel force and torque generated. The objective is to estimate the impact of the disorientation on the tool mechanical interactions when welding using a serial kinematics robot.

Abstract: We conducted in situ Raman spectroscopy study on ammonia borane loaded in diamond anvil cell (DAC). The ammonia borane was decomposed at around 140 degree Celsius under the pressure ~0.7 GPa. Raman spectra show the hydrogen was desorbed within 1 hour at 140 degree Celsius. The hydrogen was sealed in the DAC well and cooled down near to room temperature. Applying higher pressure up to ~10 GPa indicates interactions between the products and loss of dihydrogen bonding. No rehydrogenation was detected in the pressure range investigated.Keywords: Ammonia borane; Diamond anvil cell; High pressure; Phase transition

Abstract: High-pressure studies on strongly correlated-electron systems allow the study of the relationship between structural, elastic, electronic, and magnetic properties of d-and f-band systems. The High Pressure Science and Engineering Center (HiPSEC) at UNLV performs interdisciplinary research on a wide variety of materials at high pressures. One such system, YbB₂ displays antiferromagnet order at ambient pressure. We present heat capacity measurements at high magnetic fields to 9 T and structural measurement at pressures up to 5 GPa on YbB₂.