Key Engineering Materials Vols. 611-612

Abstract: Lightweight materials, such as aluminum die castings, are used more and more for automotive applications. Due to the limited weldability, joining these materials by self-pierce riveting has been established. The challenge in this regard is that these materials, especially new high strength aluminum die castings, have a limited ductility, while the joining processes locally induce large plastic deformations. Consequently, joining by forming of these materials can be accompanied by cracks, which develop during the forming operation. This paper shows the experimental and numerical investigation of a new die concept for self-pierce riveting materials with limited ductility. At the new tool concept the riveting die is separated and a movable die element is used. This element allows that the parts are superimposed with compressive stresses during the self-pierce riveting process. In the paper it can be shown, that in contrast to the conventional process crack-free joints can be generated by using the new tool concept. Determination of the joining parameters and the die design was supported by simulative investigations. Additionally, the new and the conventional self-pierce riveting process are compared on the basis of results from the experimental investigations.

Abstract: In order to use a steel/aluminum tailored blank in actual practice, galvanic corrosion due to the contact of dissimilar metals should be examined. However, few studies have reported the influence of galvanic corrosion on a steel/aluminum laser butt-welded joint. In this study, the effect of galvanic corrosion on the strength of SPCC/A1100-O laser butt-welded joints was investigated by immersion tests in air, distilled water, and salt water. The appearance was observed and tensile tests were conducted. In addition, the joint interface and the surface near the joint interface were observed and analyzed by FE-SEM and EDS. During the immersions in distilled water and in salt water, specimens were covered with an oxide film and the joint strength decreased. In the salt water immersions, the strength of the SPCC base materials decreased. And a large reduction of the cross-sectional area of the A1100-O side of the joints was seen in the salt water immersions. From the results of the observations and analyses by FE-SEM and EDS, a layer of brittle intermetallic compounds formed in the joint interface during the immersion test, and corrosion progressed as the oxide film sloughed off.

Abstract: Mixed-Materials parts have great light-weight potential for the automotive application to reduce the carbon footprint. But the joining of fibre composite plastic sheets to metal sheets is in practical application limited to adhesive bonding or mechanical joining with additional fastener elements due to the large differences in physical properties. A new process chain based on plastic joining without fastener elements is proposed and first results on the mechanism and on the achievable strength of the new joints are shown. The process chain consists of three steps: First joining pins are added to the sheet metal by an additive manufacturing process. In a second step these pins are pierced through the fibre composite sheet with a local heating of the thermoplastic in an overlap setup. In the third and last step the joint is created by forming the pins with the upsetting process to create a shape lock. The shear strength of the joined specimens was tested in a tensile testing machine. The paper shows that even with a non-optimized initial setup joints can be realised and that the new process chain is a possible alternative to adhesive bonding.

Abstract: Friction stir welding (FSW) is an innovative solid-state process, patented in 1991 by The Welding Institute, which avoids solidification problems associated with conventional fusion welding, providing joints having excellent mechanical and metallurgical properties. Due to its many advantages, FSW is successfully applied to weld various aluminium, magnesium and copper alloys. In recent years, FSW of high melting temperature materials such as steels and titanium alloys has become a key research topic. However, the high softening temperature of high melting temperature materials result in extreme stress of the pin tool during FSW, which adds to the difficulty in creating defect-free joints, especially for titanium alloys. This work is a preliminary investigation of FSW application on titanium alloy Ti6Al4V. The research focused on optimization of tool material and geometry, as well as process parameters. Complete microstructural and microhardness evaluations were conducted in addition to surface examinations. In order to evaluate the service behaviour of the joints realized with the optimized parameters, an in-depth investigation was also conducted on the fatigue properties of FSW joints.

Abstract: For an effective application of polymers, it is essential to have good adhesion behaviour to ensure good mechanical properties and durable components. Unfortunately, in general terms, polymers are characterized by high chemical inertness, which leads to very low surface energy values and, consequently, poor adhesive properties; this is particularly true for polyolefins. In this study, the effects of low pressure plasma treatment on surface roughness of polyethylene and polypropylene samples and on shear properties of adhesive bonded joints based on these substrates have been investigated. In particular, the optimization of three plasma process parameters, exposure time, voltage and working gas, were studied performing roughness measurement, contact angle evaluation and lap-shear tests. The experimental results show that the optimized plasma process may remarkably change the surface morphology, increasing wettability properties of the surfaces and shear strength of the bonded joints. These good properties remain almost unchanged even after some days of storage in the laboratory.

Abstract: In the field of materials forming processes, the use of simulation coupled with optimization is a powerful numerical tool to support design in industry and research. The finite element software Forge®, a reference in the field of the two-dimensional and three-dimensional simulation of forging processes, has been coupled to an automatic optimization engine. The optimization method is based on meta-model assisted evolutionary algorithm. It allows solving complex optimization problems quickly. This paper is dedicated to a specific application of optimization, inverse analysis. In a first stage, a range of reverse analysis applications are considered such as material rheological and tribological characterization, identification of heat transfer coefficients and, finally, the estimation of Time Temperature Transformation curves based on existing Continuous Cooling Transformation diagrams for steel quenching simulation. In a second part, a novel inverse analysis application is presented in the field of cold sheet forming, the identification of the material anisotropic constitutive parameters that allow matching with the final shape of the component after stamping. The advanced numerical methods used in this kind of complex simulations are described along with the obtained optimization results. This article shows that automatic optimization coupled with Forge® can solve many inverse analysis problems and is a valuable tool for supporting development and design of metals forming processes.

Abstract: In this paper authors present joining by forming method for screws and nuts. Self-clinching nut process was compared to a traditional nut welding method after drawing the requested shape in order to analyse quality of junction. That process is very often carried out for High Strength Steels which usually contain decreased plastic properties and increased strength. It usually causes a problem to set up the self-clinching process. Currently, there are a few companies specialising only in this kind of processes, however correct designing of such tool requires taking into account many factors such as choose of joining method by forming and proper connector type. Despite many difficulties, this method allows for joining hard welded steels and most of all for minimization of operations amount in mass production. Operations of joining by forming of nuts and screws, gives significant savings of anticipated costs with simultaneously guarantees high static and dynamic strength.

Abstract: Linear Friction Welding (LFW) is a solid-state joining process used for non-axisymmetric components. LFW involves joining of materials through the relative motion of two components undergoing an axial force. In the process, the heat source is given by the frictional forces work decaying into heat and determining a local softening of the material and eventually the needed bonding conditions. In the paper, an experimental and numerical campaign is proposed for AA2011 aluminum alloys welding. Different case studies are considered with fixed oscillation frequency and varying pressure at the interface between the specimens. Constant oscillation amplitude and specimens geometry is used. The calculated results permitted to highlight the effects of the process parameters on the material flow determining the soundness of the weld.

Abstract: At Esaform 2013 a hierarchical metamodeling approach had been presented, able to combine the results of numerical simulations and physical experiments into a unique response surface, which is a “fusion” of both data sets. The method had been presented with respect to the structural optimization of a steel tube, filled with an aluminium foam, intended as ananti-intrusion bar. The prediction yielded by a conventional way of metamodeling the results of FEM simulations can be considered trustworthy only if the accuracy of numerical models have been thoroughly tested and the simulation parameters have been sufficiently calibrated. On the contrary, the main advantage of a hierarchical metamodel is to yield a reliable prediction of a response variable to be optimized, even in the presence of non-completely calibrated or accurate FEM models. In order to demonstrate these statements, in this paper the authors wish to compare the prediction ability of a “fusion” metamodel based on under-calibrated simulations, with a conventional approach based on calibrated FEM results. Both metamodels will be cross validated with a “leave-one-out” technique, i.e. by excluding one experimental observation at atime and assessing the predictive ability of the model. Furthermore, the paper will demonstrate how the hierarchical metamodel is able to provide not only an average estimated value for each excluded experimental observation, but also an estimation of uncertainty of the prediction of the average value.

Abstract: The numerical simulation of Friction Stir Welding (FSW) is carried out using an Arbitrary Lagrangian or Eulerian (ALE) formulation. Its computational cost is reduced by appealing to the parallel version recently developed within the Forge software. The accuracy of the numerical model is increased by enhancing the state variables remapping algorithm and by introducing a better suited time integration scheme based on the cylindrical coordinates. The pin and shoulder threads are modelled in order to account for this crucial phenomenon on material heating. The developed model provides quite satisfactory temperature fields for the FSW of a butt joint of 6061 aluminium, as compared to experimental results. It allows simulating welding defects such as tunnels holes or flashes. The study then focuses on numerical simulations and experimental measurements of a lap joint of a 7175 aluminium sheet on a 2024 aluminium sheet for an aeronautical application.