Raoul Plettke

Computer Assisted Design of Actuator Systems for Laser Micro Adjustment

The adjustment of micro system components with laser forming using especially designed sheet metal actuator systems is a new and promising technology. However, due to the complexity of the design challenge and the contradicting targets that have to be considered computer assistance for the design of the actuator systems is needed. In order to build such a system several steps have to be taken. First, the actuators have to be modeled with all necessary data. Second, quality criteria have to be defined and fully automated assessment modules for every single objective have to be implemented. And third, an optimization system which utilizes the assessment modules must be developed to improve an initial design. This paper presents a solution for each of these steps. It closes with first results of a reduced version of the system as well as an outlook on the next development steps.

A ROUND ROBIN STUDY FOR LASER BEAM MELTING IN A METAL POWDER BED

With recent developments in additive manufacturing, there has been a keen interest in understanding its possibilities and limitations specifically with respect to the conventional engineering and manufacturing standards. Although coined as a prototyping technology at the time of its inception, Additive manufacturing with its characteristic layer by layer fabrication methodology is now the focus of end product manufacturing for many niche applications. One of the key additive manufacturing processes leading this evolution is the process of Laser Beam Melting in metal powder bed. With its ability to fabricate fully dense 3-dimensional structures by selectively melting micro-sized metal powder, Laser Beam Melting is being considered by many as a significant complimentary technology to the conventional forming and subtractive manufacturing processes. In order to completely understand the abilities and limitations of the Laser Beam Melting process, a detailed analysis of the system technology, process and user induced variations in relation to the characteristics of the resultant part needs to be performed. With the above motivations in mind, an initiative at the Collaborative Working Group, Lasers in production at the International Academy of Production Engineering (CIRP) was undertaken to conduct a comparative study in the form of a Round Robin test by analyzing the mechanical characteristics of samples fabricated by various users of the Laser Beam Melting technology from volunteers within the members of the academy. The presented paper illustrates the design and methodology of the round robin test in addition to some preliminary results and makes an attempt to connect these results with the various phenomena occurring in the Laser Beam Melting process. Authors of the paper gratefully acknowledge the contributions from the various members of the Collaborative Working Group, Lasers in production at the International Academy of Production Engineering (CIRP) who volunteered for providing the samples for the conducted round robin test.

Mechanical Testing of Additive Manufactured Metal Parts

The quality of additive manufactured parts however depends pretty much on the workers experience to control porosity, layer linkage and surface roughness. To analyze the robustness of the Laser Beam Melting (LBM) process a Round Robin test was made in which specimens from four institutes from different countries were tested and compared. For the tests each institute built a set of specimens out of stainless steel 1.4540. The aim of this work is to analyze the influence of the process parameters on the mechanical properties. The results show that there is a high potential for additive manufacturing but also a lot of further research is necessary to optimize this technology.

Manufacturing of Sheet Metal Components with Variants Using Process Adapted Semi-Finished Products

Manufacturing of functional sheet metal products by forming can be realised with the application of conventional bulk forming operations on sheet metals. The challenges of those sheet bulk metal forming processes are high resulting forming forces and the demand on a specific control of material flow. To meet these challenges well-directed thinning of blanks as well as accumulations of material to form functional elements is employed. Due to local loads, simultaneous 2D and 3D stress and strain states occur. Process adapted semi-finished products, containing a defined sheet thickness characteristic, are formed in the presented work by the technologies upsetting and orbital forming. Orbital forming is an incremental bulk forming operation to decrease the forming zone extension and consequently the required process force. Afterwards a process combination of deep drawing and upsetting in order to manufacture a cup-shaped workpiece with external gearing is presented. The results of this integrated single-stage forming process are discussed and subsequently the potential to enhance the process limits is shown by using process adapted semi-finished products.

Tailored Heat Treated Profiles - Enhancement of the Forming Limit of Aluminum Profiles under Bending Load

Aluminum profiles are well-established components in lightweight constructions. However, these profiles have a small forming capability in comparison to steel profiles, which leads to a limitation in their application. Within this paper a new and innovative approach for the enhancement of the forming limit of aluminum profiles under bending load called Tailored Heat Treated Profiles (THTP) is presented. With THTP the mechanical properties of the material are locally modified by a short-term heat treatment. By this local modification the material flow during the following cold bending operation can be influenced. For the design of the heat treatment layout, the correlation between the heat treatment parameters and the material properties has to be investigated. Tensile specimens were cut out of the profile and were subsequently completely heat treated with a laser. The changes of the mechanical properties caused by the heat treatment were analyzed by tensile tests. However, with a complete softening of the profile, the formability could not be improved. To increase the formability a local heat treatment, which leads to partial softening of the profile, has to be investigated. In order to characterize the heat-affected zone (HAZ) of the laser treatment, thermal camera and microhardness measurements were carried out. Appropriate heat treatment layouts have to be found to enhance the forming limit. Different layout strategies were developed and afterwards validated by the heat treatment and forming of profiles. This paper will present the findings of this investigation and show that THTP can be used to improve the formability of aluminum profiles for bending operations.

Investigation on the Process Parameters and Process Window of Three-Roll-Push-Bending

Three-roll-push-bending is a highly flexible method for the manufacturing of free-form bent tubes. The bending radius as target value is mainly determined by the position of the setting roll, defined by its axes P and Y. In industrial application the process design is carried out incrementally by trial-and-error. To avoid machine occupancy and to save expenditure of human labor, a lean offline process design is desirable. Furthermore, the technically possible parameter range is not fully used in common production processes. The machine’s behavior should be investigated in the complete range to find the optimal choice of process parameters for each case. For a methodical process design a reliable data basis had to be gained about the resulting bending radius depending on the position of the rolls. To achieve this, a parameter study was run where samples were taken covering the entire theoretically feasible parameter range. To determine the influence of the roll position, the measured data have been approximated by thin plate splines. The new data covers a considerably enlarged process window, which proved to have yet unused regions featuring low scatter.

Multi-objective optimization of actuator system design for laser micro adjustment

Complex design processes require a high level of expertise and are time consuming. By assisting the engineer with a computer aided design system the design process can be accelerated and be made more reliable. Actuator system design for laser micro adjustment is complex and its challenges may be a hindrance for the application of laser micro adjustment. To overcome this obstacle a computer aided design system was developed which utilizes a multi-objective optimization algorithm to automatically improve actuator design. In this paper, the system and its components are presented. A special focus will be upon the assessment functions which allow the efficient assessment of an actuator design. An application example will be given to demonstrate the functionality of the design system.

A New Process Chain for Joining Sheet Metal to Fibre Composite Sheets

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.

Comparison of new analytical and numerical approaches to model the three-roll-push-bending process

Freeform bent tubes can be produced with the highly flexible three-roll-push-bending. The process has been fundamentally investigated in recent research by numerical simulation using shell or volume elements. However, because of the large aspect ratio of tube length to wall thickness, computation time required is high. This prohibits the integration in analysis routines to predict the resulting geometry online. In this contribution, the basics of two new approaches are presented to compute the resulting bending radius of two-dimensional freeform bends requiring significantly less computation time. The first approach is based on an analytically derived moment-curvature relationship considering realistic flow behaviour. In the second one, a numerical method is developed using beam elements. The results are compared to experimental data and to numerical results using shell elements for steel and aluminium tubes.

Geometrical variations of tubes and their impact on freeform bending processes

In bending processes for tubes and profiles, the produced bending geometry depends on the characteristics of the semifinished product. Tubes are subjected to scatter and deviations from the standard’s dimensions caused by their fabrication process. This results in deviations of the forming behaviour. Depending on the bending process, the effect of this phenomenon is even larger, if the deviation of process forces leads to a significant deflection of the die elements. Particularly kinematic bending processes, e. g. the 3-roll-push-bending process have shown to be highly sensitive to low machine stiffness. The effect of this factor has not been identified among the influences of other disturbance factors so far. As an additional aspect, the tube dimensions affect the material characterization which leads to inaccurate material properties in numerical simulations. This results in wrong input data for the process design and has to be corrected in lengthy adjustment procedures. In this contribution the scatter of some geometrical tubes properties and its impact on the 3-roll-push-bending process is investigated. The tube geometry is measured by a tactile measurement system and is compared to the product standards. Observed deviations are categorized according to DIN 4760. The impact of the deviations on the bending process is examined by numerical variant calculations. Besides the tube dimensions, the stiffness of the machine is varied and the influence determined. By considering the errors resulting from wrong input data in the material characterization the overall error is quantified.