Dennis Freiburg

Influence of Surface Modifications on Friction, Using High-Feed Milling and Wear Resistant PVD-Coating for Sheet-Metal Forming Tools

Increasing technological requirements, as well as the demand for an efficient production demands high performance materials and enhanced manufacturing processes. The development of a new manufacturing process, sheet-bulk metal forming (SBMF), is one approach to produce lightweight forming parts with an increased number of functional properties while, at the same time, combining the advantages of sheet and bulk metal forming. For SBMF processes, the specific adjustment of the friction between tool and workpiece for a specifically designed material flow, which is called tailored friction, is of great importance. The reduction of friction is essential in order to ensure a homogeneous forming zone. However, a higher friction can be used to control the material flow to increase the local thickness of the work piece for additional functional integration. This paper shows the development of surface structures for SBMF tools by means of high-feed milling. Process parameters like the tilt angle or the feed are varied to influence the surface parameters of the structures, which results in different tribological properties of the forming tool. The structured surfaces are subsequently coated with a wear resistant CrAlN coating, processed by a magnetron-sputtering process (PVD) to enhance the lifetime and performance of the forming tool. Finally, a ring compressing test is used to investigate the tribological behavior of the coated structures.

Tribological measures for controlling material flow in sheet-bulk metal forming

Sheet-bulk metal forming (SBMF) is characterized by successive and/or simultaneous occurrence of quite different load conditions regarding stress and strain states. These conditions significantly influence the material flow and thus the geometrical accuracy of the components. To improve the product quality a control of the material flow is required. An appropriate approach is given by locally adapted tribological conditions due to surface modifications of tool and workpiece, so-called tailored surfaces. Within the present study different methods to adapt the surfaces are presented and investigated with respect to their tribological effectiveness in SBMF. In a first step, requirements regarding necessary adaptions of the friction values for two SBMF processes are numerically defined. Based on the requirements different tailored surfaces are presented and analyzed regarding their tribological influence. Finally, the potential of surface modifications to improve SBMF processes is shown.

Analysis of Residual Stress States of Structured Surfaces Manufactured by High-Feed and Micromilling

Abstract The performance of technological surfaces can be optimized via tailored characteristics according to their specific field of usage. These high performance surfaces are needed for the new technology of Sheet-Bulk Metal Forming (SBMF), which is a combination of sheet metal and bulk forming operations. Due to the high surface loads of bulk forming operations, tool surfaces need to be capable to withstand high stress states. Additional to a high wear resistance, the friction coefficient of these surfaces is an important criterion for the material flow of the sheet material. Surface characteristics can be adjusted by using technologies such as high-feed and micromilling processes resulting in different friction coefficients optimizing functional performance of the tools. In dependency of these different manufacturing processes, different residual stresses are induced into the subsurface of the forming tool. Reliability of residual stress measurements via X-ray diffractometry for microstructured surfaces produced through high-feed milling and micromilling is investigated.

Determination of Force Parameters for Milling Simulations by Combining Optimization and Simulation Techniques

Milling is a machining process in which material removal occurs due to the rotary motion of a cutting tool relative to a typically stationary workpiece. In modern machining centers, up to and exceeding six degrees of freedom for motion relative to the tool and workpiece are possible, which results in a very complex chip and force formation. For the process layout, simulations can be used to calculate the occurring process forces, which are needed, e.g., for the prediction of surface errors of the workpiece, or for tool wear and process optimization examinations. One limiting factor for the quality of simulation results is the parametrization of the models. The most important parameters for milling simulations are the ones that calibrate the force model, as nearly every modeled process characteristic depends on the forces. This article presents the combination of a milling simulation with the Broyden–Fletcher–Goldfarb–Shanno (BFGS) optimization algorithm for the fast determination of force parameters that are valid for a wide range of process parameters. Experiments were conducted to measure the process forces during milling with different process parameters. The measured forces serve as basis for tests regarding the quality of the determined force parameters. The effect of the tool runout on the optimization result is also discussed, as this may have significant influence on the forces when using tools with more than one tooth. The article ends with a conclusion, in which some notes about the practical application of the algorithm are given.

Simulation of surface structuring considering the acceleration behaviour by means of spindle control

Due to the increasing demands on surface quality of machined surfaces, deviations of the feed velocity, which can occur in complex 5-axis milling processes and are caused by the insufficient acceleration and deceleration behaviour of the machining centre, have to be avoided. This is crucial in the case of surface structuring by means of high-feed milling. The high-feed rate can be used to generate quasi-deterministic surface structures on forming tools. Applying surface structures for forming processes, the friction can be tailored to improve the form filling of small cavities. However, in order to generate homogeneous surface structures, it is important to ensure a constant feed per tooth during the milling process. In this work, a novel approach for the predicting surface structures using a geometric physically-based simulation system is shown. Furthermore, an empirical model was developed which represents the acceleration and deceleration behaviour of the used machining centre for predicting the deviations of the feed rate. Therefore, it is possible to take the alternating feed rate into account when simulating the milling process. In addition, a control approach, for adapting the spindle speed online, is used to keep the tooth feed constant.