Key Engineering Materials Vols. 611-612

Abstract: The aircrafts fuselage structure is usually composed of an assembly of stringers and frames made of cold-worked aluminium profiles. In particular, frames need of a forming process that shapes the profile into the frame’s curved shape. To do this, both profile ends are clamped, and then the profile is simultaneously stretched and pressed against the mould so that the material is plastically deformed. Industrial experience shows that most of times the resultant frame does not fulfil neither curvature nor planarity tolerances. These defects are mainly due to spring-back, residual stresses, and some technologic restrictions related to the machinery. The lack of understanding has led industry to reduce the automation level, and thus the forming process is frequently interrupted to perform verifications and adjustments that make the process to be time-consuming and very much dependent on the know-how of the machine operator. Aiming to improve the frame’s industrialisation, this work first analyses the influence of several parameters in the final shape. Then, we propose a computer-aided forming process based on the concept of Computational Vademecum (CV), which is also introduced in this work. It allows reducing the dependence on the operator know-how, while reliability and repeatability of the process can be improved.

Abstract: This paper presents a fully automatic parameter classification procedure in order to identify the most influencing parameters together with locally interesting parts of the component considered in a certain processing step. The results of this classification approach are used for a parameter space reduction in order to minimize the computational effort for subsequent analysis and optimization tasks based on forecast models. In particular, an outlook on the evaluation of radial basis function metamodels for a robust parameter identification is given. We demonstrate the classification procedure and its benefits by an industrially relevant deep drawing process of a pan with secondary design elements.

Abstract: Product properties for innovative materials, e.g. dual phase steels, require precise control of production processes. Difficulties in optimization of process parameters correspond with large number of control variables, which should be considered in the technology design. Sensitivity analysis allows evaluating the importance of all process inputs on the final properties of material. Information on the most important inputs is crucial for further design of the process. Application of sensitivity analysis requires detailed knowledge of the process phenomena as well as the definition of the mathematical model of the thermomechanical process. Furthermore, some sensitivity analysis algorithms are of the high computational cost. Presented work concerns possibility of the application of data exploration approach in evaluation of the importance of process inputs as the alternative for sensitivity analysis. Use of data mining algorithms eliminates necessity of mathematical model development, it also does not require any apriori knowledge about the process. Authors presents the comparison of sensitivity analysis and data exploration approach in evaluating relationships between inputs and outputs of the hot rolling for dual phase steel strips. The presented approach and the perspectives of the practical application could lead to significant decrease of time necessary for the computations of process design. The theoretical considerations are supplemented with the results of both types of analysis.

Abstract: Different parameters are used to evaluate the machined surface quality; roughness, residual stress and white layer are the most common factors that affect the surface integrity. Residual stress, in addition, are one of the main factors that influence the component fatigue life. Superficial residual stresses depend on different factors, such as cutting parameters and tool geometry. This article describes the development of an automated optimization procedure that allows the matching of a residual stress Target Profile by varying process parameters and tool geometry for a typical aeronautic superalloy, such as Waspaloy, for which a reliable numerical model has been developed for comparison to experimental data. The objective of this procedure is to maximize the Material Removal Rate under physical constraints represented by appropriate limits assigned to: Cutting Force, Thrust Force, Tool Rake Temperature and residual stress Target Profile. The developed optimization procedure has shown its effectiveness to match a given residual stress profile in accordance to process responses numerically evaluated.

Abstract: Inverse form finding aims to determine the optimum blank design of a workpiece whereby the desired spatial configuration that shall be obtained after a forming process, the boundary conditions and the applied loads are known. As a verification of the optimal material configuration a subsequent direct FEM computation has to result in the desired shape of the deformed workpiece. In this contribution the inverse FEM, which formulates the mechanical weak form in the spatial configuration and solves it with respect tothe material configuration, is used to obtain an admissible blank configuration. An example of applying this inverse method to a contact forming simulation is presented.To this end a FE-code applicable to use the inverse mechanical formulation is coupled with the commercial software MSC.Mentat. In the inverse computation the contact problem is approximated by displacement control.

Abstract: Facing a decreasing amount of resources on the one hand and an increasing demand for comfort on the other, more and more attention is being paid to sustainability and care for the environment. Particularly in the automotive sector, lightweight design principles continue to prosper rapidly. As a result, adjusted materials for different applications were developed. Due to the formation of intermetallic phases, most multi-material mixes cannot be welded and require adapted joining technologies. Mechanical joining technologies such as self-piercing riveting and mechanical clinching have proven effective methods of joining lightweight materials like aluminium and ductile steels. New high-strength steels are increasingly used in crash-sections, where limited deformation under impact load is required. These hot stamped steels have a very low elongation at break and therefore a low formability. Currently there is no joining by forming technology without pre-punching available using these grades of steels on die-side. The newly developed shear-clinching process is one possible method of joining this kind of material without additional elements. The fundamental idea of shear-clinching is a single-stage process in which pre-punching of the die-side material is performed by indirect shear-cutting and subsequent forming of the upper layer into this hole. This would immensely enlarge the application segment of mechanical clinching even if hot stamped steels are positioned on die-side. Fundamental studies are required to ensure process reliability and it is necessary to break down the joining process into fragments, like pre-punching and clinching with pre-punched sheet, and superpose them to form the combined procedure shear-clinching. This paper presents a detailed investigation of the sub-process clinching with pre-hole.

Abstract: The friction stir welding of lap sheets can be performed considering different variables in terms of process parameters, tool configuration, welding typology and so on. The proposed investigation deals with the friction stir welding of blanks, with the same thickness, performed under lap configuration with the sheets welded, in one-side and in both sides as well, with different tool geometries and tool rotation-wise. The double side allows to extend the weld through the whole thickness leading to better mechanical welding properties at the blank to blank interface. The weld morphology has been investigated through microstructure observations performed on the transverse area, with respect to the welding velocity, of each joint. The tensile shear strength of the joint in one-side weld is generally lower than that detected in two side weld.

Abstract: Hybrid components made of steel and aluminum sheet metal are a promising approach for weight reduction for automotive applications. However, lightweight components made of steel and aluminum require suitable joining technologies, particularly if forming operations follow after the welding process. Friction Stir Welding (FSW) is a promising solid-state welding technology for producing dissimilar joints of steel and aluminum. Within this work dissimilar butt joints were produced using sheet metals of mild steel DC04 and the aluminum alloy AA6016 with a thickness of about 1 mm. The FSW joints show approximately 85 % of the tensile strength of the aluminum base material. In metallographic investigations of the produced FSW blanks it was found that the microstructure in the area of the weld seam changes in the aluminum alloy due to the process temperature and plastic deformation. Due to temperature dependent changes of precipitations of the aluminum alloy, temperature measurements have been carried out during the welding process. To find an explanation of the reduction in tensile strength of the FSW joints, short time heat treatment experiments in the temperature range between 250 °C and 450 °C were performed using the aluminum base material. Heat treatments in the temperature range of the measured process temperature result in a reduction of the tensile strength of about 20 % regardless the annealing time.

Abstract: Mechanical joining technologies are becoming increasingly important with the trend towards light and multi-material designs in the automotive industry. Providing robust connection techniques will be of particular importance. Thus, rejection rates are reduced and costs are cut in the parts production. This paper discusses the example of clinching and its potentials and limits concerning FEM based sensitivity analysis and optimization for the joining by forming technology. By determining the sensitivity of the design for relevant connection parameters, the most important values for the optimization of the forming die are derived. On this basis, both, appropriate tools for a particular material combination and other tools for the joining of different thicknesses and sheet materials are designed. Furthermore, a sensitivity analysis of uncertain variables allows to evaluate the robustness of the clinching process in production. Based on these results, methods to increase process robustness or process monitoring in terms of quality assurance can be derived.

Abstract: For aerospace applications, light-weight boxes to protect and carry electronic equipment need to be sealed. The main requirements on the components are low thermal expansion and gas tightness. The common material for such an application is a metal matrix composite (MMC). The MMC suggested here consists of A356 aluminum alloy matrix with 15 vol.% SiC particle reinforcement. A safe limit for the electronic component inside the boxes during sealing is determined to be 180°C. Due to the boundary conditions gas tightness and low heat input, Friction Stir Welding (FSW) might be an alternative to the employed joining techniques. For the FSW process the T-Joint is the most appropriate joint geometry in respect to the box design. The geometry of the lid has to ensure the backing system for the stir zone inside the box. A successful welding of the box was done after a joint geometry optimization. The examination of the welded box concerns material characterization with microscopic methods, measuring thermal expansion in base material and stir zone and temperature measurement while FSW.