Agglomerated tungsten carbide: A new approach for tool surface reinforcementMittwoch (26.06.2019) 11:56 - 11:59 Uhr
Metal matrix composites (MMC) fulfill the requirement of a wear resistant material. MMC consist of two different materials: A metal matrix material reinforced by a second harder phase. MMC surfaces can be produced by laser melt injection (LMI) and improve the abrasive wear resistance of matrix materials. In the LMI process the metal surface of the matrix material is melted and the hard particles are fed into the molten zone. This reinforced surface can be applied on deep drawing tools and facilitates a forming of high alloyed steels without lubricants. Tribological tests of tungsten carbide reinforced bronze showed that the abrasive wear resistance is significantly higher compared to regular bronze in ball on plate tests. Nonetheless the coefficient of friction of this MMC tool surface is equal or even higher to bronze in strip draw tests depending on the contact pressure. Also, adhesive wear of bronze is still existing and to prevent this adhesion a plateau of hard particles is necessary. The matrix can be set back by laser ablation. However, the contact pressure between laser ablated tool surfaces and metal sheet influences the friction as well. A higher contact pressure up to 20 MPa leads to an increase of the forming force and the friction coefficient. Moreover, the surface roughness of the metal sheet is increasing because the hard particles are penetrating the surface. These results are also observed for deep drawing of cups with MMC tools. The formed cups showed scores due to the higher contact pressures in the radius area of the tool.
In consequence these higher contact pressures must be reduced to realize a Dry Metal Forming with MMC tools. A new approach is to increase the contact area between tool and workpiece surface for reduction of the stress level. For the first-time tungsten carbide particles were fused locally together to one agglomerate. After a regular laser melt injection, the laser melted a local spot with additional tungsten carbide particles for a short pulse time and the result is a tungsten carbide agglomerate. In this work, the influence of laser power, laser spot diameter and powder feeding rate on the height, width and cross-sectional area of the tungsten carbide agglomerates is presented. The envelope of this agglomeration process will be shown to generate locally a larger hard particle surface.
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