Elsevier

CIRP Annals

Volume 69, Issue 1, 2020, Pages 153-156
CIRP Annals

Development of a process signature for electrochemical machining

https://doi.org/10.1016/j.cirp.2020.04.078Get rights and content

Abstract

Electrochemical machining (ECM) principally enables a highly productive and virtually wear-free production of components with simultaneously high surface quality. However, the process generates changes concerning both the geometry as well as the rim zone of manufactured components, so that the entire process design currently runs through several heuristic cycles. As a result, the cost-effectiveness of the process is often only given in large-scale production. The paper therefore mechanistically links the material modifications and the process-induced material loads for electrochemical processes to predict rim zone properties. Inverted components of the resulting process signature can finally be used for virtual process design.

Introduction

Increasing loadings during application mean it is no longer sufficient that components are manufactured within geometric tolerances, but must also have defined properties, especially in the rim zone of the component. The deterministic setting of rim zone properties by production processes is described as the inverse problem of manufacturing technology. Nowadays, an a priori solution to this problem is only possible in exceptional cases. A possible generic solution is the concept of process signatures [1,2], describing modifications in the rim zone based on local material loads induced during a machining process.

Electrochemical machining (ECM) principally enables a high surface integrity due to its mechanically contactless working principle. However, the chemical loads during the process generate a series of changes in the geometry and the rim zone of a manufactured component. As a result, the process design has to go through heuristic cycles and thus, the cost-effectiveness of the process is often only given in large-scale production [2,3].

The principal physical and chemical mechanisms of ECM and its variants have been known for decades [4,5] but modelling has mainly focused on the achievable geometrical precision [6]. Latest works in this context focussed on interdisciplinary “multi-physics” modelling to incorporate fluid dynamics, temperature, electrical field and local chemical material dissolution aspects [7,8]. They also considered inverse approaches but did not focus on the modelling of process induced surface modifications. However, for a comprehensive ECM process design the surface integrity should be addressed, Fig. 1.

The importance of the microstructure during ECM processes has been established a long time ago, cf. [9]. Only due to the availability of advanced surface measuring techniques this topic re-entered the research focus in the last years, e.g. [10,11].

The concept of process signatures was already successfully applied to surface grinding and deep rolling in [12]. Therefore, this work focusses on the transfer of this concept to ECM processes. The according causal sequence for ECM is shown in Fig. 2. While most analyses focus on the correlation between machining parameters and resulting surface modifications (correlation A), the ECM Process Signature is linking the material loads with the resulting modifications in a more generic way. This fundamental process description facilitates the transfer of results to other machining tasks or process variants [13]. A first example on this transferability between ECM and Laser-chemical machining (LCM) is given in [14] and successful correlations to functional properties can be found in [15,16].

The subject of this work is therefore the mechanistic linking of material modifications and process-induced material loadings for electrochemical processes to predict rim zone properties [2]. A special focus will be given to the analysis on changes of local phase concentration and the development of flow grooves, which are often encountered in ECM. However, the numerical prediction of changes in the rim zone alone is not sufficient for the deterministic adjustment of the rim zone, since the process design would remain iterative. Therefore, the invertibility (inverse problem) of the found signature components will be examined and it will be shown how the inverted components can be used for a virtual process design – thus achieving the intended overall added value of this new approach.

Section snippets

Analysis on change of local the phase concentration

The change of the local phase concentration is a consequence of the different electrochemical dissolution behaviours of individual material phases on the microstructure level. This means that the physical mechanism that causes this modification is the electrochemical reaction. In addition to the change in phase concentrations, this also determines the topography and the achievable roughness of ECM surfaces. Since the electrolysis conditions depend largely on the selected process parameters, the

Analysis on development of flow grooves

Flow grooves are process-induced shape deviations that lead to a texturing of the surface and an increase in surface roughness. On the basis of electron microscopic images of the grooves [21], the hypothesis is established that there is a local failure of the oxide layer at these points. High electric field strength, corrosion, oxidation, inhibitors like chloride ions, and cathodic collapse due to hydrogen formation can be excluded by the locality of the occurrence and the boundary conditions

Inversion concept for a-priori adjusting processes

The goal of the process signature is to solve the "inverse problem" of manufacturing technology. With signatures, it is possible to determine the necessary process parameters for a required component functionality a priori using a virtual process design. In order to determine the required process variables, first a correlation with corresponding modifications is necessary (I). Then, the individual components must be invertible and provide clear solutions for loads (II) and process parameters

Summary and conclusions

The development of a characteristic process signature for electrochemical manufacturing processes (ECM) was successfully carried out for the change of the local phase concentration and the formation of flow grooves. The detailed analysis of these modifications formed the basis for the mechanistic linkage to the material loadings. The limit loads of the individual phases of 42CrMo4 steel were measured on the basis of the involved pure substances. For this purpose, pure cementite was

Acknowledgements

The work was funded by Deutsche Forschungsgemeinschaft DFG – 223500200 – TRR 136 “Process Signatures”, project F03.

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