Manuel Steitz

Potential of mechanical surface treatment for mould and die production

Abstract The use of mechanical surface treatment methods can extraordinarily increase the productivity in mould and die making processes. The present paper shows how deep rolling and machine hammer peening can smoothen machined surfaces in order to substitute manual polishing processes. Initial roughness of Ra > 3 μm can be smoothed to Ra < 1 μm, independent from the treated material. For a further improvement of the surface quality, a closer look is given to the influence of geometric effects in hammer peening. Both procedures also increase the surface hardness by work hardening. The influence on the attainable work hardening depth is examined in detail. When combined with thermal hardening operations, hardness and smoothness are still influenced positively, although this combination may be constrained by crack nucleation beneath.

Influence of Process Parameters on Surface Hardening in Hammer Peening and Deep Rolling

This paper focusses on the statistical evaluation of process parameters in the mechanical surface treatments deep rolling (DR) and machine hammer peening (MHP) on the hardness increase. In MHP process a spherical hard metal tool is repeatedly accelerated onto the material surface. Just as the shot peening process MHP is an impact treatment although in MHP the impact area can be controlled, leading to the desired impact density. In DR the contact between spherical tool and work piece is quite different to MHP as the spherical is in sliding contact as it is moved along the surface. Although the material loading of both surface treatments vary, the resulting surface structure is the same. Both lead to a cold worked, smooth surface including compressive residual. Technically DR and MHP parameters have been part of researches but there still is a lack of statistical validation of every single process parameter leading to a hardened surface. This paper tries to close this gap. DR and MHP are conducted on different materials, containing tool steel 1.2379 and grey cast iron EN-JS-2070. Using a fractional factorial test design an experimental matrix was created able to examine the influence of every single process parameter. Which were for DR: rolling pressure, line spacing between hammer traces, diameter of roller ball and the travelling speed. For MHP the influence of the following process parameters was investigated: angle between hammering direction and surface normal, line spacing between hammering traces, diameter of hammering ball, hammering energy, travelling speed and hammering frequency. On every single sample ten Brinell hardness indents are made which give the statistical coverage needed to calculate the effect of every single process parameter within a confidence interval of at least 95 %. For all mentioned materials the effect of every single process parameter has been calculated with respect to hardening. It could be shown that especially the loading of the cast iron is quiet complex as a high amount of impact energy (MHP) or contact pressure (DR) can lead to overloading of the material leading to a degradation of the surface. At least an explanatory approach which describes the different influence of the tool diameter on the surface hardness is given using FEM simulations. These FEM simulations contain an advanced material model in which the Bauschinger-effect of 1.2379 is implemented. It can be clearly shown that a larger tool diameter in DR produces a higher amount of cold working in the material surface leading to harder surfaces compared to the smaller tool diameter. In contrast to DR the contact pressure in MHP is determined by the Hertzian pressure distribution. Here smaller tool diameters create larger Hertzian pressure and therefore a higher amount of cold working.

Influence of Hammer-Peened Surface Textures on Friction Behavior

Although the tribological advantages of textured surfaces in sheet metal forming are well known, the texturing of drawing tools is not yet established due to the intensive processing efforts. With the surface treatment technology of machine hammer peening (MHP), deterministic macro- and micro-textures can be machined on free-form surfaces, while simultaneously the time need for tool production is reduced significantly compared to the conventional process chain. Therefore, this paper investigates the influence of hammer-peened surface textures on the friction behavior. First, the general texturing process is optimized and suitable process parameters are identified. Using a strip drawing test, the friction behavior of different geometrical textures, varying ratio of surface textures (area) to the total surface and different lubrication systems are determined. It is shown that MHP with surface textures can reduce friction in sheet metal forming compared to conventionally polished tools.

Effect of Process Parameters on Surface Roughness in Hammer Peening and Deep Rolling

The challenges in die and mold making industry to increase productivity and reduce costs can be addressed by expanding the automation in the process chain. Conventionally the final surface quality is produced by manual polishing operations. This expensive time-consuming production step can be reduced significantly by using machine hammer peening (MHP) and deep rolling (DR). For both processes the emphasis of each process parameter on the resulting surface topographyis largely unknown. This gap of knowledge about significant and non-significant parameters needs to be closed in order to allow a fast process optimization and more economic use of both methods. Therefore this study focuses on figuring out the statistically secured effect of each process parameter on the attainable surface smoothing on cast iron and tool steel. Based on a fractional factorial test design the results of an experimental parameter study are presented and significant parameters are identified. Using a high-speed camera, it may also be proved why an inclination angle between the hammering direction and surface normal is advantageous with regard to the resulting surface quality. Finally, the results are discussed and advices for an industrial use of MHP and DR are given.

Coating of Deep Rolled and Hammer Peened Deep Drawing Tools

Mechanical surface treatments like machine hammer peening and deep rolling can substitute an essential part of the manual polishing time in the conventional process chain of die and mold production. However, the increasing use of high strength steels in the automotive industry and the associated wear of deep drawing tools require further wear-protection methods. In this context it is still unknown if hammer peened and deep rolled surfaces can ensure a sufficient adhesive strength of a coating. Therefore, in the present work different coatings are applied on hammer peened and deep rolled surfaces. Finally, the wear behavior is examined in the strip drawing test. The evaluation of the experimental results proves the potential for an industrial application of the mechanically treated and coated tools.