Papers by Author: Roland Golle

Abstract: The formability requirements of high-strength steels are increasing as a result of progressive lightweight construction. An innovative two-stage shear cutting process has been developed in order to meet these requirements. It significantly reduces the edge crack sensitivity of the material in the cutting zone. Studying the effect of worn tool elements on process safety is a key focus for the ongoing improvement of this production process. A production-based tool condition was simulated by creating different cutting edge radii on the active tool elements. Collaring tests showed a reduction of residual formability through wear of up to 65% for high-strength heavy plates (plate thickness > 3 mm).

Abstract: Nowadays, lightweight design is a relevant issue in the automotive industry and in the field of electric mobility. The utilization of Press Hardened Components (PHC) made of manganese-boric steels enable weight saving through the reduction of the sheet metal thickness with sufficient component reliability. Because of the hard martensitic structure, by default, laser cutting is applied for piercing and trimming the contour after the Press Hardening Process (PHP). Due to the high energy consumption, in combination with long cycle times, this process is cost intensive.Instead of laser technology, shear cutting would be an interesting alternative for Press Hardened Steels (PHS) but this process poses substantial challenges for the industrial application. In order to cope with the difficulties of shear cutting operations of PHS, this work presents an alternative for processing PHS. This new method is called notch shear cutting.

Abstract: The edge cracking sensitivity of AHSS and UHSS is quite challenging in the cold forming process. Expanding cut holes during flanging operations is rather common in automotive components. During these flanging operations the pierced hole is stretched that its diameter is increased. These flanging operations stretch material that has already been subjected to large amounts of plastic deformation, therefore forming problems may occur. An innovative cutting process decreases micro cracks in the cutting surface and facilitates the subsequent cold forming process. That cutting process consists of two stages, which produces close dimensional tolerance and smooth edges. As a result the hole expanding ratio was increased by nearly 100% when using thick high strength steels for suspension components

Abstract: Due to the development of corrosion-resistant lightweight, todays automotive manufacturers typically use zinc coated sheet metals in the forming process. However, zinc abrasion in industrial presses decreases the process stability and often causes interruption of the whole process. The application of high strength steels leads to a significant increase of the temperature due to the plastic work. So far a detailed, quantitative analysis of the relation between temperature and zinc abrasion is not available. Therefore, this paper examines the impact of the temperature on abrasion behaviour in sheet metal processes. To achieve this, a progressive die was built. The deep drawing stage of this tool is connected to a cooling / heating system in order to obtain a constant temperature during the forming process. A variety of different galvanized sheet metals compared to commonly used tool materials has been tested. For each combination of materials five experiments at different temperatures were performed to determine the effect of the temperature on the zinc abrasion. Applying the method of total reflection x-ray fluorescence (TXRF) the quantity of zinc abrasion was measured. A relation between low temperatures and reduced zinc abrasion can be clearly observed. Industrial experiments revealed that temperature exerts a high influence on the zinc abrasion. The new insights into the impact of the temperature show a significant way to lower the zinc abrasion and therefore increase the process stability in deep drawing processes.

Abstract: The thermoelectric phenomenon can be used for a wide application spectrum. Typically, the Seebeck effect of metallic materials is used for temperature measurement in common thermocouples. However, there is also a high potential for adapting the effect in nondestructive testing due to a high sensitivity of the thermoelectric properties to a variety of material characteristics.Different studies point out an influence of plastic deformations on the thermoelectric behavior of metals, but a detailed and quantitative analysis and description is not provided yet. There is an increasing interest in detecting the changes of properties by nondestructive characterization of plastically deformed materials. Especially in metal forming, the knowledge about the correlation between the degree of deformation and the thermoelectric behavior can help to check formed metal parts. On that account, the influence of plastic deformations on the thermoelectric behavior, in particular the Seebeck coefficient, of four steel alloys is introduced in this paper.An apparatus based on the integral measuring method was built up to measure the relative Seebeck coefficient to a reference material at different temperature gradients and for several degrees of plastic deformation. Well defined values of plastic deformation are realized by cold rolling. With this compressive forming technology a logarithmic deformation φ up to 2.11 was set for all steel alloys. Besides a high degree of deformation, a uniform strain over the sample thickness can be obtained by rolling.With increasing plastic deformation a significant change of the relative Seebeck coefficient can be observed in a defined temperature range for all investigated steels. The plastic deformation is accompanied by an increase of dislocation density into the lattice structure of the metal. These line defects provoke the formation of new scattering centers and thus the electron motion is disturbed. By the combination of metallurgical examinations and measurements of micro hardness, the relation between the thermoelectric behavior and the plastic deformation of steels can be clearly illustrated.

Abstract: Blanking is one of the most widely used manufacturing technologies in sheet metalprocessing, because nearly each sheet must be trimmed out of a semi-finished part or has to beblanked after a forming process to get the precast part in the manufacturing chain. In general, a highquantity of blanked parts should be manufactured without reworking the tool. Therefore a capableprocess is indispensable to avoid inadequate part quality or premature failure of the tool because ofwear. The blanking process is affected by tool parameters, the press and the material properties ofthe blanked part.However, another important factor is the occurring temperature in the shearing zone of the sheetmetal due to the dissipation of nearly 95% of the plastic work during blanking and, in addition,frictional heating. This temperature impacts the blanking process features such as tool-wear andresulting cut edge quality. It has been presumed to be negligible by a lot of authors yet. In contrastsome publications with experimental and analytical research assume that the temperature reachesvalues up to 1000°C. Therefore, this report outlines a thermoelectric method to measure theresulting temperature distribution during the blanking process on the cutting edge of the blankingpunch. The feasibility of the investigated measurement concept is shown on a concrete example.

Abstract: The required number of punched and blanked parts in the electronic industry, such as leadframes, contact pins or plugs often reach several million pieces. According to this, the production process has been sped up to frequencies up to 2000 parts per minute. At this production rate parts of thin steel or copper sheets are produced with high-speed stamping presses and blanking tools. Because of the high gating velocities in the blanking tool there is a recurring acceleration at the moment, when the blank holder contacts the sheet metal. A second shock in the blanking tool arises, when the sheet cracks. This is also known as the impact shock during the blanking process. Trough these two impulses and through the cycle of the plunger movement a periodic oscillation arises in the tool. Horizontal vibrations can lead to an undefined position between punches and die-plate, while vertical movement leads to increased wear because of friction forces between the blanked surface and the lateral area of the punches. The goal of this project was to minimize these oscillations in cutting tools by the usage of lightweight materials in the flux of forces. An experimental cutting tool was designed with alternative top and bottom plates for the comparison of the oscillation status in the cutting process when using plates of steel, aluminium or magnesium. The centre plate of the tool remains constant. Experiments were accomplished for several velocities and tool setups with different plate materials. To determine the influence of the materials with different density and elasticity acceleration, force and acoustic emission sensors were integrated in the tool. The set of problems was investigated by analyzing the measured data and by determining the wear in practical tests.