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Over the last years, the weight of modern car bodies has risen significantly due to the increasing customers’ demand for comfort and safety equipment. However, this ongoing trend leads to an increasing fuel consumption and thus to higher carbon dioxide emissions. In order to counteract these problems, hot stamping has been established in the automotive industry as a key technology for lightweight construction, regarding the manufacturing of safety-relevant car body components. Hot stamped parts are commonly made out of boron-manganese steel 22MnB5, which is initially austenized and subsequently formed and quenched in one process step. As a result, geometrical complex structures with an ultimate tensile strength of 1500 MPa are generated. The surfaces of the workpieces are coated with an Al-Si layer to avoid oxide scale formation and to ensure corro-sion protection. However, the coating system leads to an increased adhesive wear on the tool sur-face due to the high thermo-mechanical tool stresses. Therefore, a time and cost consuming rework of the hot stamping tools is required. The aim of this study is to increase the tribological perfor-mance of hot stamping tools by using a laser implantation process. This technique allows the ma-nufacturing of separated, elevated and dome-shaped microstructures on the tool surface in conse-quence of a localized dispersing of hard ceramic particles by pulsed laser radiation. The generated surface features offer great potential for reducing the tribological load, due to their high hardness and wear resistance. For this purpose, the friction coefficient of unmodified and laser implanted tool surfaces were examined and compared by using a modified pin-on-disk test. In addition, the surfaces were analyzed by optical measurements in order to quantify the amount of wear.
Over the last years, the weight of modern car bodies has risen significantly due to the increasing customers’ demand for comfort and safety equipment. However, this ongoing trend leads to an increasing fuel consumption and thus to higher carbon dioxide emissions. In order to counteract these problems, hot stamping has been established in the automotive industry as a key technolo-gy for lightweight construction, regarding the manufacturing of safety-relevant car body compo-nents. Hot stamped parts are commonly made out of boron-manganese steel 22MnB5, which is initially austenized and subsequently formed and quenched in one process step. As a result, geo-metrical complex structures with an ultimate tensile strength of 1500 MPa are generated. The surfaces of the workpieces are coated with an Al-Si layer to avoid oxide scale formation and to ensure corrosion protection. However, the coating system leads to an increased adhesive wear on the tool surface due to the high thermo-mechanical tool stresses. Therefore, a time and cost con-suming rework of the hot stamping tools is required. The aim of this study is to increase the tribological performance of hot stamping tools by using a laser implantation process. This tech-nique allows the ma-nufacturing of separated, elevated and dome-shaped microstructures on the tool surface in consequence of a localized dispersing of hard ceramic particles by pulsed laser radiation. The generated surface features offer great potential for reducing the tribological load, due to their high hardness and wear resistance. For this purpose, the friction coefficient of un-modified and laser implanted tool surfaces were examined and compared by using a modified pin-on-disk test. In addition, the surfaces were analyzed by optical measurements in order to quantify the amount of wear.
The laser implantation technique bases on a localized dispersing of ceramic particles in metallic surface layers for changing their micro geometry as well as their material properties. Using TiB2 and NbC hard particles, tailored microtextures consisting of elevated dome-, ring-, or line-shaped Features can be manufactured. Especially TiB2 implants feature high hardness values up to 1800 HV1 and ceramic particle contents up to 55 %, indicating superior wear resistant properties.
AISI D2 is one of the most applied cold-working tool steel for deep drawing operations, due to the materials good toughness and high wear resistance. However, since the lubricant-free deep drawing as well as the processing of high strength and ultra-high strength steel sheets remaining ongoing trends in the automotive industry, the tool surfaces need to be improved regarding their friction and wear behavior. For this purpose elevated micro textures can be applied. Since protruded micro features are predominantly affected by wear, a structuring process enabling the formation of suitable textures with high wear resistance is needed.
In this paper surface texturing at microscale was conducted by laser implantation. This technique allows the fabrication of highly wear resistant, separated and elevated features (implants) on steel surfaces via a discontinuous dispersing of ceramic particles by pulsed laser radiation. The aim of this study was to investigate the techniques potential for the creation of wear resistant elevated micro features on AISI D2 tool steel surfaces by the application of TiB2 particles. The laser parameters (pulse power and length) were varied and a comparative material analysis on AISI D2 laser remelted spots and TiB2 localized dispersed zones was examined. High-speed camera recordings allowed the description of the particle insertion mechanism from pre-coatings during laser processing. Mechanical properties were analyzed by (micro-) hardness measurements at top and cross sections. The microstructure was investigated by optical microscopy, scanning electron microscopy (SEM), energy dispersive Xray
spectroscopy (EDX) and X-ray diffraction (XRD). The implants geometrical properties (diameter, height, depth) were investigated by white light interferometry and optical microscopy. The experiments reveal that a pulsed laser remelting of AISI D2 leads to a significant hardness drop due to high retained austenite (γR) contents. Contrary, a localized dispersing of TiB2 particles leads to defect free dome- or ring-shaped elevated features with hardness values up to 1800 HV1. The high hardness values result from the dispersed TiB2 particles and an in-situ precipitation of new hard particle phases, which lead to a reduction of the γR content within the matrix.