S. Buchkremer

Finite-element-analysis of the relationship between chip geometry and stress triaxiality distribution in the chip breakage location of metal cutting operations

Abstract Chip breakage is a major machinability criterion of metal cutting operations. Favorable broken chips enable their efficient removal and prevent mechanical damages to the machined surface. Ductile failure on the chip free surface initiates chip breakage. The ductility of most materials depends on the stress triaxiality. Its relationship to the manufacturing parameters has to be understood in order to develop predictive methodologies of tool/process design. The problem can be approached by assessing the relationship between triaxiality and chip geometry, which is an integral representation of all tool/process parameters and material properties. This work presents a novel Finite-Element (FE) modeling approach of the relationship between the 3D chip geometry and the distribution of the stress triaxiality in the chip breakage location. For the derivation of the proposed approach it is shown that the stress state in the chip breakage location mainly develops during the chip bending phase. The approach enables for the first time to study the separate impacts of chip helical radius, helical angle, helical pitch and the shape of the deformed chip cross section on the triaxiality distribution. A sensitivity analysis of all input parameters is conducted and it is shown that all parameters but the helical pitch have characteristic impacts on the triaxiality distribution. The computational effort of the proposed modeling approach is significantly lower of than of available FE-models of metal cutting processes. The validation includes longitudinal turning experiments on steel AISI 1045 and 3D FE-process simulation modeling.

From Orthogonal Cutting Experiments towards Easy-to-Implement and Accurate Flow Stress Data

Despite recent advances in numerical modeling of machining processes showing a significant potential for shortening product and process design activities, the broader application of the Finite Element Method (FEM)–based modeling approaches in the manufacturing industry is still limited by the expensive and time-consuming techniques involved in obtaining accurate material flow stress data. This study proposes a new efficient approach consisting of a combined numerical-empirical methodology for inversely identifying the thermomechanical material behavior of AISI 316L stainless steel from machining experiments. In order to establish the work materials flow stress properties under the extreme conditions of machining, the Johnson–Cook (JC) constitutive equation is chosen to consider the impacts of strain, strain-rate, and temperature. Due to the unstable material behavior of AISI 316L stainless steel under certain thermomechanical conditions in the primary shear zone, the calibration of a damage model is integrated into the process for evaluating the flow stress data. The methodology approximates the material constants of the JC relationship by systematically comparing experimentally measured machining results with those predicted equivalents of a 2D implicit FEM process model. The methodology is experimentally verified on AISI 316L stainless steel with respect to cutting forces, chip geometries and temperatures in the tool-chip interface.

Ecological lifecycle assessment of an electric drive for the automotive industry

The growing consequences of global warming and rising prices for fossil fuels are calling for a broad substitution of combustion engines by electrical machines and an energy- and resource-efficient design of manufacturing systems. The present work proposes a conceptual approach for the holistic product lifecycle assessment (LCA) of an electric drive for the automotive industry. The proceeding is part of the NRW Bank funded Project KERME (engl.: Competitive Electro-Mobility through Resource Efficiently and Modularly Designed E-Drives). KERME aims at the eco-efficient production of a scalable E-Drive, which finally enables the manufacturers to realize economies of scale, which in return is a key condition for a successful adaption of environmentally friendly transportation concepts by the end users due to reduced sale prices. The investigated electric drive will be designed to meet the load spectrum of a compact vehicle for close-by traffic in an urban environment. Accordingly, the engine is supposed to provide short but intensive phases of acceleration followed by long downtimes. Suitable engine concepts are the synchronous and asynchronous layout with an internal rotor and liquid cooling (P = 30 kW, nmax = 9000-12000 rpm, V = 300-400 V, dimensions (length × external diameter of box): 480 × 270 mm). Following the guidelines in ISO 14040/44, the analysis will determine the ecological impact of an E-Drive during all of the following lifecycle stages: Production of raw materials, manufacturing phase (including final assembly) and utilization phase. However, the focus of the analysis will be the manufacturing processes, completed by assumptions and mathematical models about the remaining phases. The analysis will break down the absorbed flows of resources and energy to the applied manufacturing processes per manufactured E-Drive. By doing so, the contribution of each production step to the over-all energy consumption, global warming potential or any other factor of ecological impact (e.g. eutrophication, eco- and human-toxicity) can be determined. Accordingly, those processes can be identified, which are the most promising to be modified in order to improve the energy efficiency of the production system and thus to lower the manufacturing costs while protecting the environment at the same time. In detail, the proposed work investigates the manufacturing and assembly of the main engine parts lamination stack, housing, rotor, stator and shaft. Thus, the E-Drive production includes a great variety of manufacturing technologies mentioned in DIN 8580: Primary forming (e.g. casting of housing), forming (e.g. stamping of magnetic strips in lamination stack), cutting (e.g. machining of shaft), joining (e.g. adhesive joint of box and stator), coating (electrical insulation of stator-grooves), modifying material properties (e.g. heat treatment of shaft). Physical measurement techniques will later be applied in order to determine the absorbed flows of electrical power, cutting fluids and material of each machine tool. By implementing and validating the production chain into a software tool (GaBi 5) possible scenarios with alternative manufacturing processes or materials can be analyzed concerning their impact on the ecological impact of the engine without conducting costly and time-consuming experiments.

Impact of the Heat Treatment Condition of Steel AISI 4140 on Its Frictional Contact Behavior in Dry Metal Cutting

In this work, the impact of the heat treatment condition of steel AISI 4140 on its frictional contact behavior with coated cemented carbide and cubic boron nitride (CBN) in dry metal cutting is experimentally investigated. Two different kinds of tests were performed. The frictional behavior was investigated under conditions very similar to metal cutting on a frictional test bench, which was installed on a broaching machine. Additionally, orthogonal cutting processes with linear workpiece geometries were conducted on the same machine. The cutting experiments included observations of cutting forces, high-speed filming of chip formation, chip thickness ratio analysis as well as a comprehensive metallographic characterization of the chips and workpiece surfaces. The impacts of the undeformed chip thickness and cutting speed were investigated individually for coated cemented carbide and CBN as cutting materials. The frictional examinations delivered the Coulomb friction coefficients for all four combinations of work and cutting materials as a function of the relative velocity. The identified frictional behaviors explain the dependencies of forces, chip thicknesses, and surface microstructures on the tool and process conditions during the cutting tests.