Univ. Prof. Dr. Ing.

Bergs, Thomas

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  • Publication
    Hybrid Wire- and Powder-based Laser Metal Deposition for Modification of Local Properties of H11 Dies
    ( 2019-05-13)
    Bergs, Thomas
    ;
    Arntz, Kristian
    ;
    Teli, Mahesh
    ;
    Klocke, Fritz
    ;
    Winands, Kai orcid-logo
    ;
    Kammann, Sebastian
    ;
    Riepe, Jan
    Hot work tool steel X37CrMoV5.1 (AISI H11) is largely used to produce highly stressed hot forging dies due to its high levels of hardness, temper resistance and toughness. This steel is exposed to high thermo-mechanical loads, which reduce its hardness and toughness and lead to the failure by wear and surface cracks. To address such kind of failures, the present study was performed to implement the novel wire- and powder-based laser metal deposition (WP-LMD) process and realize its potential to modify the local mechanical properties of H11 dies. The process was employed to fabricate standard tensile specimens (DIN50125) out of H11 wire and Nb powder. These specimens were subjected to various heat treatment sequences, and subsequently tested for microhardness and tensile properties. The mechanical properties were compared for samples with and without addition of Nb. This study revealed the potential of WP-LMD process to refine the microstructure, to reduce carbides' size, and to modify the local mechanical properties of H11 dies.
  • Publication
    Experimental investigation of contact heat transfer coefficients in nonisothermal glass molding by infrared thermography
    ( 2019)
    Vu, Anh Tuan orcid-logo
    ;
    Vu, Anh Ngoc
    ;
    Liu, Gang
    ;
    Grunwald, Tim
    ;
    Dambon, Olaf
    ;
    Klocke, Fritz
    ;
    Bergs, Thomas
    Nonisothermal glass molding has recently become a promising technology solution for the cost-efficient production of complex precision glass optical components. During the molding process, the glass temperature and its temperature distribution have crucial effects on the accuracy of molded optics. In nonisothermal molding, the glass temperature is greatly influenced by thermal contact conductance because there is a large temperature difference between the glass and mold parts. Though widely agreed to be varied during the molding process, the contact conductance was usually assumed as constant coefficients in most early works without sufficient experimental justifications. This paper presents an experiment approach to determine the thermal contact coefficient derived from transient temperature measurements by using infrared thermographic camera. The transient method demonstrates a beneficially short processing time and the adequate measurement at desirable molding temperature without glass sticking. Particularly, this method promises the avoidance of the overestimated contact coefficients derived from steady-state approach due to the viscoelastic deformation of glass during the inevitably long period of holding force. Based on this method, the dependency of thermal contact conductance on mold surface roughness, contact pressure, and interfacial temperature ranging from slightly below-to-above glass transition temperature was investigated. The results reveal the dominance of interfacial temperature on the contact conductance while the linear pressure-dependent conductance with an identical slope observed for all roughness and mold temperatures. The accurate determination of the contact heat transfer coefficients will eventually improve the predictions of the form accuracy, the optical properties, and possible defects such as chill ripples or glass breakage of molded lenses by the nonisothermal glass molding process.