Process design for the forming of organically coated sheet metal
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Date
2011-07-26
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Abstract
Der Einsatz organisch beschichteter Bleche hat viele Vorteile im Vergleich zu
konventionellen Blechhalbzeugen, wie z.B. die Reduzierung der Anzahl der notwendigen
Fertigungsschritte, die Verringerung der Produktionskosten und die Vermeidung
umweltbelastender Lackierbearbeitungsschritte. Während der Umformung verändern sich die
Eigenschaften der dünnen und relativ weichen Beschichtung abhängig von den
Umformbedingungen, wie z.B. Werkzeugradien sowie Reibungskoeffizient zwischen dem
Blech und dem Werkzeugsystem. Bei großen Formänderungen des Blechwerkstoffs kann es
beispielsweise zu einer Änderung der Oberflächentopografie des Substrates führen. Dies kann
bis zu einer Beschädigung der Beschichtung und somit zu einem Versagen des Produktes
führen. Daher ist es wesentlich, die Formänderungsgrenzen der Beschichtung zu kennen, um
die ursprüngliche Funktionalität der Beschichtung, beispielsweise die optischen Eigenschaften
und den Korrosionsschutz, nach der Umformoperation zu erhalten. Das bedeutet, dass
unerwünschte Defekte wie bzw. eine Rissbildung oder eine Delamination der Schicht vom
Substrat durch die Anwendung geeigneter Umformverfahren und durch die Ermittlung
optimaler Prozessparameter vermieden werden müssen. Hierzu ist eine Methodik zur
Prognose der Änderung der funktionalen Schichteigenschaften in Abhängigkeit von
Formänderungszuständen notwendig.
In dieser Arbeit wird die Umformung von organisch beschichteten Blechen grundlegend
untersucht. Zunächst werden experimentelle Untersuchungen zur Umformbarkeit organisch
beschichteter Bleche durchgeführt und der Einfluss der Prozessparameter auf die
Produkteigenschaften (insbesondere des Glanzgrades) von organisch beschichteten
Blechformteilen geklärt. Darauf aufbauend werden Einsatzmöglichkeiten der FE-Simulation
zur Vorhersage der Veränderung der Oberflächeneigenschaften bei der Umformung
beschichteter Bleche untersucht. Die FE-Modellierung ermöglicht schließlich eine
halbzeuggerechte Auslegung und Optimierung des Umformprozesses.
Die Untersuchungsergebnisse zeigen, dass die Umformung organisch beschichteter Bleche
grundsätzlich einen Abfall des Glanzgrades der Beschichtung bewirkt, wobei der Glanzverlust
sowohl von der Höhe des Umformgrades als auch von den auftretenden
Formänderungszuständen abhängt. Um das Versagen der Beschichtung sicher prognostizieren
zu können, wird ein Grenzformänderungsdiagram für die Beschichtung aufgenommen, um
das Formänderungsvermögen des organisch beschichteten Blechs bei der Auslegung des
Umformprozesses zu berücksichtigen. Ferner wird auch die Abhängigkeit der optischen
Eigenschaften, wie die Glanzabnahme, von den Formänderungszuständen berücksichtigt. Die
gewonnenen Erkenntnisse werden schließlich anhand des hydromechanischen Tiefziehens für
eine praxisrelevante Bauteilgeometrie überprüft. Hierzu wird zunächst eine analytische
Beschreibung des Einflusses von Geometrie- und Prozessparametern auf die
Formänderungszustände durchgeführt. Auf Basis dieser Erkenntnisse wird die Prozessführung
des hydromechanischen Tiefziehens hinsichtlich der Erzielung günstiger
Oberflächeneigenschaften der Beschichtung der Blechformteile optimiert.
The application of organically coated sheet metal (OCSM) possesses many advantages in comparison to the conventional sheet metal such as reducing the number of necessary manufacturing steps, reducing the production costs and a more environmental friendly production process. During forming processes, original properties of thin and relatively soft coating layers may be altered depending on forming conditions such as die radii and friction coefficient between blank and die components. Additionally, the forming processes induce large strains in materials that lead to topographical surface changes of steel substrate. As a result, the coating layer may be easily damaged and this is one of the major reasons which unexpectedly lead to the loss of protective and optical properties of the OCSM products. Therefore, it is essential to know the formability of OCSM in order to maintain all original functions i.e. optical and corrosion properties of the coating layer after forming processes. That means the undesirable defects such as cracking or delamination must be avoided by choosing an appropriate forming technique and by determining optimal process parameters. Furthermore, if these coating failures can be predicted, the process parameters of the processing may be adjusted in advance in order to prevent those damages. Therefore, a methodology for prediction of the change of functional coating layer properties as a function of plastic deformation is necessary. In this research work, the process design for forming of organic coated sheet metal has been investigated. The project aims firstly at the investigation of the forming behavior of the OCSM, focusing on changes of the optical properties i.e. gloss degree depending on process parameters. Consequently, the application of Finite Element Method (FEM) to simulate the forming process of the OCSM and to predict the change of the surface properties is discussed. The FE modeling allows, finally, a material-based process design and optimization of forming process. The experimental results indicate that the gloss reduction of the coated surface is basically caused by the strain states and strain level imposed on both steel substrate and the coating layer. In order to predict exactly the coating failures the forming limit diagram of coating (FLDC) should be used instead of the forming limit diagram (FLD) of steel substrate. By using FLDC, the formability concerning the optical property of the coating layer is considered in the process design of OCSM. Furthermore, the dependence of the gloss reduction on the strain states can also be taken into account. The obtained results are finally validated by hydro-mechanical deep drawing (HDD) for forming of complex practice-oriented geometry. The influences of process parameters on the surface property of OCSM products in HDD process are also investigated. For this purpose, an analytical model is first investigated in order to evaluate the influences of geometrical and process parameters on the strain states and strain distributions over the forming part. Based on the conclusions obtained by the analytical results, the process design for the HDD process using OCSM can be optimized with regard to the best surface characteristics of organically coated layers.
The application of organically coated sheet metal (OCSM) possesses many advantages in comparison to the conventional sheet metal such as reducing the number of necessary manufacturing steps, reducing the production costs and a more environmental friendly production process. During forming processes, original properties of thin and relatively soft coating layers may be altered depending on forming conditions such as die radii and friction coefficient between blank and die components. Additionally, the forming processes induce large strains in materials that lead to topographical surface changes of steel substrate. As a result, the coating layer may be easily damaged and this is one of the major reasons which unexpectedly lead to the loss of protective and optical properties of the OCSM products. Therefore, it is essential to know the formability of OCSM in order to maintain all original functions i.e. optical and corrosion properties of the coating layer after forming processes. That means the undesirable defects such as cracking or delamination must be avoided by choosing an appropriate forming technique and by determining optimal process parameters. Furthermore, if these coating failures can be predicted, the process parameters of the processing may be adjusted in advance in order to prevent those damages. Therefore, a methodology for prediction of the change of functional coating layer properties as a function of plastic deformation is necessary. In this research work, the process design for forming of organic coated sheet metal has been investigated. The project aims firstly at the investigation of the forming behavior of the OCSM, focusing on changes of the optical properties i.e. gloss degree depending on process parameters. Consequently, the application of Finite Element Method (FEM) to simulate the forming process of the OCSM and to predict the change of the surface properties is discussed. The FE modeling allows, finally, a material-based process design and optimization of forming process. The experimental results indicate that the gloss reduction of the coated surface is basically caused by the strain states and strain level imposed on both steel substrate and the coating layer. In order to predict exactly the coating failures the forming limit diagram of coating (FLDC) should be used instead of the forming limit diagram (FLD) of steel substrate. By using FLDC, the formability concerning the optical property of the coating layer is considered in the process design of OCSM. Furthermore, the dependence of the gloss reduction on the strain states can also be taken into account. The obtained results are finally validated by hydro-mechanical deep drawing (HDD) for forming of complex practice-oriented geometry. The influences of process parameters on the surface property of OCSM products in HDD process are also investigated. For this purpose, an analytical model is first investigated in order to evaluate the influences of geometrical and process parameters on the strain states and strain distributions over the forming part. Based on the conclusions obtained by the analytical results, the process design for the HDD process using OCSM can be optimized with regard to the best surface characteristics of organically coated layers.
Description
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Keywords
Forming limit diagram, Gloss reduction, Hydromechanical deep drawing, Organically coated sheet metal