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Adjusting a resulting surface integrity prior to a manufacturing process is a big challenge in production engineering. One approach to solve this challenge is the methodology of process signatures. To develop a comprehensive process signature for Sinking EDM (S-EDM) it is necessary to describe the relationship between the applied material loadings and resulting modifications in the workpiece rim zone. It is generally considered that S-EDM is an electro-thermal process, which is significantly characterized by the heat flux due to the spark plasma. Accordingly, the main loadings during S-EDM are the arising temperatures and their temporal as well as spatial gradients in the workpiece. These loadings lead to varied surface integrity modifications. Thus, the task is to investigate the surface integrity alteration according to the initial material condition. Consequently, in this paper the effect of different annealing and heat treatment states of 42CrMo4 (AISI 4140) on the S-EDM process were investigated. Hence, changes of state variables depending on different machining parameters and the varying dissipated energy were considered. Therefore, the resulting microstructures were analyzed by scanning electron microscope (SEM). Additionally residual stress was determined and compared to the initial state. The identified changes of investigated state variables are the described modifications.

The surface and subsurface conditions of components are significant for their functional properties. Every manufacturing process step changes the surface condition due to its mechanical, chemical and/or thermal impact. The depth of the affected zone varies for different machining operations, and is predetermined by the process parameters and characteristics. Furthermore, the initial state has a decisive influence on the interactions that lead to the final surface conditions. The aim of the investigation presented here is to compare the influence of the load characteristics over the depth applied to manufactured components by several different machining operations and to determine the causing mechanisms. In order to ensure better comparability between the surface modifications caused by different machining operations, the same material was used (AISI 4140; German steel grade 42CrMo4 acc. to DIN EN 10083-3) and annealed to a ferritic-pearlitic microstructure. Based on interdisciplinary cooperation within the collaborative research center CRC/Transregio 136 “Process Signatures”, seven different manufacturing processes, i.e., grinding, turning, deep rolling, laser processing, inductive heat treatment, electrical discharge machining (EDM) and electrochemical machining (ECM), were used, and the resulting surface zones were investigated by highly specialized analysis techniques. This work presents the results of X-ray measurements, hardness measurements and electron microscopic investigations. As a result, the characteristics and depths of the material modifications, as well as their underlying mechanisms and causes, were studied. Mechanisms occurring within 42CrMo4 steel due to thermal, mechanical, chemical or mixed impacts were identified as phase transformation, solidification and strengthening due to dislocation generation and accumulation, continuum dynamic recrystallization and dynamic recovery, as well as chemical reactions.

In contrast to most other manufacturing technologies, in electrochemical machining processes only slight changes in material characteristics in the rim zone of workpieces are stated in the literature. Due to the physical active principle, no thermo-mechanically induced phase changes or the evolution of a so-called white layer were ever observed. Aside of this fact, a not inconsiderable number of smaller modifications in the rim zone were found in the past. The most common effects occurring during electrochemical machining are the generation of a passive layer on the surface by changing the local chemical composition of the material, the selective dissolution of one metallic phase, or the occurrence of flow marks. Consequently, the last two effects also change the surface roughness as the marks and dissolved phases represent ditches in the surface. Therefore, in this article, material modifications occurring during electrochemical machining are presented. Their influence on the surface integrity is exemplarily analyzed for the heat-treatable steel 42CrMo4. In addition, first steps for a correlation of material loadings that promote these changes, the so-called process signature, are made. Based on this, the influence of different machining parameters can be compared to set up rim zone properties purposefully.