Tool surface with a supporting plateau of hard particles for deep drawing of high alloy steel

Abstract

Sheet metal forming normally requires the application of lubricants to protect the tool and the sheet against wear. The parts must be cleaned to remove the lubricants before joining and coating. This process step wastes energy and water resources. In the case of non-lubricated sheet metal forming, cleaning processes would not be necessary anymore and the process chain could be optimized regarding ecological and economical aspects. However, forming without lubrication leads to an intensive contact between the tool and the sheet. Thus, higher wear occurs and process reliability cannot be ensured for industrial mass production. High alloy steels are applied for mass-market products e.g. for appliances. Because of the higher strength, strain hardening and galling effects the austenitic steels are comparatively difficult to form. For dry metal forming of high alloy steels new tool concept must be developed to withstand the higher loads. In this work, a laser generated tool surface with a supporting plateau of hard particles (metal matrix composite (MMC-surface)) is presented. Spherical fused tungsten carbides were injected into the surface by laser melt injection. The metallic matrix of the composite was rejected by applying laser ablation. In consequence, the hard particles stood out of the matrix and were in direct contact with the sheet material. The surface of hard particles had a high hardness about 3000 HV and less metallic character. Cold working steel and aluminium bronze were tested as reference tool materials. Dry and lubricated forming experiments were carried out by strip drawing with bending and deep drawing of cups. Dry deep drawing of cups was not possible by using cold work tool steel. This can be traced back to the occurrence of wrinkles and cup base fracture at the same time. Applying aluminium bronze as tool material for dry metal forming resulted in high adhesive wear. Within this work the feasibility of dry metal forming of high alloy steel could be demonstrated by applying the MMC-surface whereby adhesive wear could be reduced.