Papers by Author: Clemens Müller

Abstract: The magnetic hardening of ARMCO^® and FeCo17 in a severe plastic deformation and an incremental forming process is presented. The enhancement of the coercivity, which depends on the strain induced by the forming process, is investigated. Strain induced during the incremental forming process are analysed in FE-simulations.

Abstract: Metal forming processes often involve large strain gradients which results in heterogeneous deformation and consequently residual stresses. Furthermore the strain gradients also generate variations in the deformation texture and related properties. For materials with a significant crystallographic elastic anisotropy such as ferritic steel, these textures may have a substantial effect on the reliability of the determination of residual stresses. In the present investigation this influence is examined for the hole drilling method by a combination of experiments and finite element simulations.

Abstract: 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.

Abstract: 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.

Abstract: 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.

Abstract: Equal Channel Angular Swaging (ECAS) is a new severe plastic deformation process which combines the conventional equal channel angular pressing (ECAP) and the incremental bulk forming process rotary swaging. The tool system consists of two rotary swaging dies with an angled channel that contains four shear zones, generating severe plastic strain per pass. The crucial advantages compared to conventional ECAP are a significant reduction of friction and axial forces plus the potential to be extended to continuous processing. Thus, ECAS has high potential for a cost-efficient production of bulk UFG materials. In the present paper the principles of ECAS are introduced and first experimental results for the processing of copper are presented.

Abstract: Linear flow splitting is a new cold forming process for the production of branched sheet metal structures. It induces severe plastic strain in the processing zone which results in the formation of an UFG microstructure and an increase in hardness and strength in the flanges. Inbuilt deformation gradients in the processing zone lead to steep gradients in the microstructure and mechanical properties. In the present paper the gradients in the UFG microstructure and the mechanical properties of a HSLA steel (ZStE 500) processed by linear flow splitting are presented, as well as a calculation of local strength from hardness measurements on the basis of the Ludwikequation. In order to investigate the thermal stability of the UFG microstructure heat treatments below the recrystallization temperature were chosen. The coarsening process and the development of the low angle to high angle grain boundary ratio in the gradient UFG microstructure were monitored by EBSD measurements. It is shown that heat treatment can lead to a grain refinement due to a strong fragmentation of elongated grains while only little coarsening in the transverse direction occurs. A smoothing of the gradients in the UFG microstructure as well as in the mechanical properties is observed.

Abstract: Linear flow splitting is a new continuous cold forming process where the edge of a sheet metal is formed into two flanges by splitting and supporting rolls. Thus the production of bifurcated profiles from sheet metal without lamination of material becomes feasible. The production of such structures takes place incrementally in a modified roll forming machine. Experimental investigateons on a HSLA steel show, that even at a surface increase of the sheet edge of about 1800% no cracks were nucleated in the profiles. EBSD measurements in the splitting centre reveal that similar to other SPD processes UFG microstructures develop in the processing zone. Thus a steady state is reached in the processing zone where increasing strain has no more (or little) influence on the materials properties i.e. its deformability, as it is typical for SPD-processes. The formation of UFG microstructures is considered to be a mandatory condition for the linear flow splitting process, as it improves the formability of the material to the extremely high level required for this process. The mechanical properties of profiles produced by linear flow splitting are characterised by large gradients, depending on the local deformation and the resulting microstructure. Very high hardness is measured at the former processing zone, i.e. the splitting centre and the flange surface, where severe plastic deformation takes place and UFG microstructures are present. In direction to lower deformation i.e. with increasing distance to the splitting ground or flange surface the hardness decreases close to the level of the undeformed material. In the present paper the linear flow splitting process is introduced and the microstructural development in the process zone is discussed on the base of EBSD measurements on profiles of the steel ZStE 500. The repartition of mechanical properties in a bifurcated profile is demonstrated by detailed hardness measurements.