Investigation of Friction in Closed-Die Warm Forging of Aluminium Alloys

Bernhard Buchner

Research output: ThesisDoctoral Thesis

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Aluminium forged parts play a significant role as components of light weight constructions in the automotive and aerospace industry. The fundamental knowledge of the friction behaviour at the tool-workpiece interface is necessary due to the fact, that it influences material flow, die filling, wear and workpiece quality. In order to understand the tribological processes and interactions in the tool-workpiece interface systematically, basic experiments that allow an independent variation of influencing parameters are necessary. This thesis presents an investigation of friction in hot forging of aluminium by employing a modified ring-on-disc test. The experiments were performed without lubrication and with commercial graphite-based lubricants and at various loads, sliding velocities and specimen surface conditions. It was found, that dry sliding conditions result in sticking and - at low normal loads - in galling at the die-workpiece interface. Micrographs confirmed that at dry friction (most of) the relative motion was done by shearing in subsurface layers of the workpiece. When employing lubricants, the shearing was restricted to the graphite layer. Generally, the friction coefficient decreased with increasing normal pressure at all investigated lubricants. In the observed range, the sliding velocity had no significant influence on the tests. However, the scatter of the friction coefficients at various velocities got smaller with increasing normal pressures. The results were compared to the results of a study employing a pin-on-disc test for lubricant evaluation, and good qualitative agreement was found in terms of friction coefficient and friction evolution during the tests. In accurate finite element analysis, friction has to be faced by numerical models that consider the real conditions at the die-workpiece interface. In this context, physical approaches based on a contact model and a local friction law allow the formulation of friction models that take into consideration the complexity of the tribosystem. This thesis presents a new contact model that takes into consideration the material properties and real asperity shapes, and simplification is achieved by making use of the statistical character of real surfaces. The main idea of the new concept was to obtain the real contact area-load relation by combining the bearing area curve and a model asperity with correct representation of the mean asperity slope. Experiments with aluminium specimens showed excellent correspondence with the numerical results.
Translated title of the contributionUntersuchung der Wirkflächenreibung beim Gesenkschmieden von Aluminiumlegierungen
Original languageEnglish
  • Eichlseder, Wilfried, Assessor B (internal)
  • Buchmayr, Bruno, Assessor A (internal)
Publication statusPublished - 2008

Bibliographical note

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  • friction forging aluminium ring-on-disc
  • test pin-on-disc
  • test contact model

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