Browsing by Author "Muvunzi, Rumbidzai"

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    Application of additive manufacturing for improved thermal management of hot sheet metal forming tools
    (Stellenbosch : Stellenbosch University, 2020-03) Muvunzi, Rumbidzai; Matope, Stephen; Harms, T. M.; Dimitrov, D. M.; Stellenbosch University. Faculty of Industrial Engineering. Dept. of Industrial Engineering.
    ENGLISH ABSTRACT: In hot stamping, a blank or sheet of metal at high temperature (800-900 °C) is formed and cooled simultaneously by the tools. The rapid cooling of the blank causes transformation to a martensitic microstructure with high tensile strength (1 500 MPa) which enables the parts to acquire crash resistant properties. Accordingly, the process is used to produce vehicle components for improving safety of passengers. However, the hot stamping tools are exposed to high thermal load as they come into contact with the hot blanks. To aid in the cooling, the tools have a network of drilled channels in which a coolant circulates to extract heat. Due to machining restrictions, the straight drilled channels are unable to ensure consistent cooling of geometrically complex parts. If the tools are not evenly cooled, thermal stresses are induced and this compromises the tool service life and quality of parts (hardness properties). Moreover, the average cooling time in hot stamping occupies at least 30 % of the total cycle time. Thus, one of the major challenges in hot stamping research is to find ways of reducing the cycle time. The above mentioned challenges can be resolved through exploiting the design freedom offered by Additive Manufacturing technologies in the producing of tools with cooling channels which conform to the shape of tools. This has already been extensively investigated in the injection moulding and die casting tooling industry. However, there is limited information on the design and manufacturing parameters of hot stamping tools with conformal cooling channels. The aim of this research was to apply Additive Manufacturing as a tool for improving thermal management of hot stamping tools. The first objective was to identify the parameters required for an effective thermal management system of hot stamping tools. A method for identifying the structural conformal cooling system parameters was developed based on the technical limitation of the Selective Laser Melting process, principles of mechanics and heat transfer. The developed method was validated using finite element analysis simulation on a typical benchmark component. The second objective of the study was to develop a model for predicting minimum cycle time in hot stamping under ideal conditions. The model was developed using heat transfer principles and study of the stages in hot stamping. The model is a useful benchmark tool which is applicable in cycle time prediction. The third and fourth objectives were to design and manufacture a hot stamping tool with conformal cooling channels for a benchmark part. The fifth objective was to investigate the impact of the tool with conformal cooling channels on cycle time. In view of that, experiments were conducted to compare the performance of the optimised tool and the conventional one under typical industry like conditions. According to the results, the conformable tool shows the potential of reducing cooling time by 29%.