Advanced Materials Research Vol. 769

Abstract: Due to increasing demands for lightweight structures in automotive applications the use of sheet metal components made from aluminium alloys is a promising approach for weight reduction. The combination of steel and aluminium in car bodies may be an interesting alternative compared to a monolithic material design. The weight of structural parts of a car body shell can be reduced if dedicated parts consist of aluminium instead of steel. This approach allows for an optimal exploitation of the material properties of both materials, bringing high strength into highly loaded areas while areas subject to lower loads are equipped with lower strength and weight. However, a multi-material design combining steel and aluminium demands for suitable joining methods, especially if a forming operation is applied to the welded sheets. In conventional fusion welding processes the formation of intermetallic phases due to the metallurgical affinity of aluminium and iron is a serious problem. Recent developments in regulated cold metal transfer (CMT) welding technologies at the Institute of Welding Technology and Joining Technology (ISF) at the RWTH Aachen promise an appropriate solution to this problem. Due to a digitally regulated arc technology, the heat input in CMT is reduced to a minimum. However, the inevitable formation of a welding bead in arc processes with filler material is a criterion of exclusion in the case of production of welds for car body shells. To achieve an optimal appearance of the body shell, the welding beads need to be removed from both sides of the sheet in a second manufacturing step. Hence, to avoid further costs, it seems expedient to search for alternative welding technologies. Friction stir welded (FSW) joints show relatively even welding beads. Furthermore, this joining method is characterised by a low process temperature, which is considerably below the melting temperature of the base materials. Hence, FSW is a promising joining technique to produce tailored blanks out of aluminium and steel. The main objective of the present paper is the evaluation of suitable process parameters for the production of FSW butt joints with a thickness of 1 mm made from the aluminium alloy AA6016-T4 and the mild steel DC04. Welding experiments using a varying rotational speed, tool offset, tool velocity, tool plunge depth and tool tilt angle were carried out. To identify the best parameters in terms of the strength of the joint, tensile tests were performed. It is shown, that an amount of approximately 85% of the tensile strength of the base material AA6016 can be achieved. Using SEM the formation of the fracture surfaces was analysed. Different fracture types were identified and the possible reasons for their occurrence are discussed. It is shown that in the case of optimal joining procedure the failure occurs in the thermomechanically affected zone in the aluminium sheet, were the plastic deformation is low. Additionally, thermography has been employed to evaluate the temperature distribution during the process. In metallographic investigations it was found that during welding the microstructure of the aluminium base material changes due to plastic deformation and temperature increase in the area of the weld seam. Using hardness measurements the change of the mechanical properties in the contact zone of both base materials and in the heat affected zone was examined. Finally, an outlook is given with respect to the possibilities of producing FSW welded sheets that can be formed using conventional deep-drawing.

Abstract: Mechanical surface treatments like machine hammer peening and deep rolling can substitute an essential part of the manual polishing time in the conventional process chain of die and mold production. However, the increasing use of high strength steels in the automotive industry and the associated wear of deep drawing tools require further wear-protection methods. In this context it is still unknown if hammer peened and deep rolled surfaces can ensure a sufficient adhesive strength of a coating. Therefore, in the present work different coatings are applied on hammer peened and deep rolled surfaces. Finally, the wear behavior is examined in the strip drawing test. The evaluation of the experimental results proves the potential for an industrial application of the mechanically treated and coated tools.

Abstract: Handling, welding or painting are currently the main fields of application for industrial robots. Due to their high flexibility and low investment costs industrial robots are increasingly used for machining processes in production environments. Robotic milling is one example of these processes, which nowadays can only be applied for tasks with low accuracy requirements and minor cutting forces. The main reason for this is the low stiffness of the robot structure and hence the huge deflection of the tool caused by the cutting forces. Robotic milling tests of aluminum show deviations of the programmed track in the millimeter range even with moderate depth of cut. To harness high possible savings of milling robots, a new method to increase the machining accuracy was developed at the Institute of Machine Tools and Industrial Management (iwb). The core of the method is a model-based controller for the compensation of deviations that are caused by the cutting forces. The input variables of the controller are the axis angles of the robot (provided by the robot controller) and the cutting forces (measured by a three-component force plate). Based on the cutting forces and the axis angles, the deflection of the Tool Center Point (TCP) is calculated by means of a simulation model. The calculated offset is transmitted to the robot controller so that the tool path is corrected. To implement the compensation strategy, a real-time model of the robot which includes all major compliances of the structure needs to be developed. Besides the real-time requirement, the model needs to be valid for the main working area of the robot. A major challenge in this regard is the determination of the relevant compliance parameters of the robot. In addition to the stiffness values of the gears and bearings the elasticities of the robot links need to be identified. The paper presents a novel method to determine the relevant stiffness parameters of a robot by measurements with a 3D-Scanning-Laser-Doppler-Vibrometer (LDV). In these measurements the robot is loaded with a defined force induced by an actuator at its TCP. During this process, the deflection of the robot is detected by the LDV at a multitude of measuring points. From the relative movements of the measuring points, the tilting-angles of the gears, bearings, and the structural components are calculated. Using the known torques caused by the defined load the stiffness parameters are calculated. In order to minimize the experimental effort it is aspired to identify all necessary parameters by one single measurement. To achieve this goal, the best measurement setup consisting of the position and the orientation of the TCP as well as the direction of the actuator force, is identified by a multibody system (MBS) to ensure sufficient torques in every axis of the robot and all directions (transmission direction and perpendicular to it). The simulation shows that such a measuring setup exists, so that the required parameters, which were validated in additional experiments, could be determined with a single measurement. The determined parameters are used in a controller model to calculate the displacement of the TCP due to the cutting forces during the machining process. Since this model needs to be very efficient regarding the computation time, a MBS cannot be used so that an analytical model must be developed. The analytical model is based on conventional forward kinematics, which is used for determining the position and orientation of the TCP of the robot. In conventional forward kinematics, the rotation of an axis is described by a transformation matrix, which also takes the (constant) dimensions of the robot arms into account. This description only includes a single degree of freedom to the joint angle of the axis and is extended to provide additional degrees of freedom to represent the elasticity of the gear and the bearing. To be able to consider the elasticity of the robot arms, additional transformation matrices are introduced in the center of the arm and the link arm. The computing time of this analytical model is in the range of 1 to 2 ms, so that the model is suitable for the control. In initial machining experiments with a robot of type KR 240 R2500 prime the proposed approach was validated. Milling tests with aluminium showed a significant reduction of the process-related path deviations using the presented control strategy.

Abstract: The studies presented in this paper show a concept for setting up a milling process with intentionally invoked chatter vibrations. The process is used for the generation of surface structures on forming tools. Wear of the milling tool is investigated as well as the tribological characteristics of the surfaces. The results lead to the conclusion that this method is suitable for machining challenging materials like high speed steel.

Abstract: Piezoelectric ceramics can be used as sensors, as well as actors. The concept of a self- sensing-actuator tries to use both modes of operation in one device, allowing the economic integration of mechatronic systems. Possible fields of application are ball screws of machine tools, where wear-induced degradation of the preload can be compensated. Furthermore, the signal processing part of such a system can be used to gather information related to the condition of the ball screw. Both excitation signal generation and filtering of the measured signal have to offer high flexibility and signal fidelity. In this article the concept of a power amplifier and its corresponding signal processing system are presented.

Abstract: For environmental and economic reasons, energy- and resource- efficient operations of cutting machines are increasingly important. The determination of properties and functions of machine tools, which affect future energy consumption in operation, essentially takes place within the design phase by combining required components. Therefore, it is necessary to develop approaches to find an efficient optimum between energy consumption, productivity, acquisition costs and operating costs within the design phase of a machine tool. However, the energy consumption of a machine tool depends on the application scenario. In addition to that, it is difficult to forecast the energy consumption of several components because of their mutual interaction. Existing approaches to forecast the energy consumption of a machine tool within design phase are based on complex simulation or mathematical models which are difficult to parameterize for the design of a machine tool and thus, for the comparison of various configuration alternatives. An alternative for forecasting energy consumption is the use of empirical information. That information can be acquired by measuring the energy consumption of machine tools in operating production systems. This paper presents an approach to forecast the energy consumption of machine tools within the design phase, which will be developed by the Institute of Production Engineering and Machine Tools. It will be based on the data feedback (empirical information) from a machine tool operating in an existing manufacturing system. For this purpose, a logger module will be developed, which continually captures the energy consumption by means of the machine integrated sensors. That information will be sent back to an energy navigator module, which processes that information in order to forecast the energy consumption of a new designed machine tool. Also, the lifecycle costs will be calculated in order to rate cost and benefits of each machines lifecycle in terms of energy consumption.

Abstract: In order to provide higher accuracy and an increased life time for bearings in kinematic drives of servo-driven stroke controlled presses this article presents a combination of a roller and a plain bearing. With an example of the 3D-Servo-Press, the applied design is illustrated and the advantages are discussed. A technological solution is presented which enables a specific pretension for each individual bearing according to its tribological requirements. Using a developed matlab-programme the behaviour of a conventional plain bearing is opposed to the new bearing combination by its illustrated displacement paths of the shaft in the bearing shell under certain conditions of a common balancer system. The feasibility of this initial tension system is shown by means of a bearing prototype. Finally measurement results of a prototype will be shown which proof the manufacturability of the presented bearing combination.

Abstract: Enabled by their size, small machine tools manufacturing small workpieces allow for a leap of improvements and therefore overcome current limitations in micro manufacturing. Nevertheless, to achieve those benefits the classically engineering mindset and machine tool design have to be overcome. Those ideas, the reasoning behind them and the corresponding conditions are laid down in the so called theory of small machine tools. The generic theory includes a framework approach to help in the process of machine development resulting in a new kind of miniaturized and function integrated machine tools. This conceptual contribution describes the idea, aspects, objectives and methodology pursued by the theory of small machine tools.

Abstract: To solve the supply chain decision problem of technology selection for electric drive production, the authors chose a game theoretic approach to model and solve the bargaining situation of manufacturer (OEM) and supplier. The OEM can decide between a same part strategy and a differentiating strategy with increasing sales potential as well as higher complexity and production costs. The supplier has to choose a production technology and can decide between a specialized technology (e.g. punching) and a flexible technology (e.g. laser). Using the specialized technology the variable costs per piece are lower but every additional variant causes additional tool investments (e.g. punching dies). The interdependence of a supplier’s choice of production technology and its customer’s decision, regarding the number of different product variants that he wishes to source, is modeled as a non-cooperative game in strategic form. The resulting Nash equilibriums in pure and mixed strategies are determined with respect to the investment costs faced by the supplier, the target market conditions and the cost share passed on to the supplier’s customer.

Abstract: Purpose and problem. Today, business organisations are facing turbulent markets that are characterised by an increasing individualisation of products and customer specifications. This desire for individualisation leads to a growing number of complex tasks to fulfil requirements. Those tasks require very different competencies that exceed the competencies and the knowledge of a single person. As a result, many people within the company have to work together to meet the requirements of the market and the customer and to be able to react fast on changes. For that to happen, the employees need to exchange a lot of information efficiently. Additionally the need to communicate effectively increases while working together, so there has to be an environment that supports efficient and effective communication processes.