UFG-Microstructures by Linear Flow Splitting

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.

Further processing of ultrafine-grain materials

“…The same behaviour (identical hardness distribution) was found for the whole zone of constant hardness (Fig. 3) [2]. …”

“…UFG-microstructures are generated in the flange by the process. These microstructures are characterized by an increased stiffness and a high surface hardness [2]. In direction perpendicular to the flange surface the split profiles show a steep hardness gradient (Fig.…”

“…2) for measurements at the flange centre towards the web of the profile. The highest hardness measured is at the surface with a value of 365 HV0.05 [2]. With increasing distance to the surface the hardness decreases and reaches a constant level of 225 HV0.05 in a depth of 1 mm.…”

Manufacturing‐induced material properties of linear flow split and linear bend split profiles

“…The resulting grain size decreases with increasing strain of the material but reaches a lower limit where further deformation does not lead to a decreasing grain size anymore . The fact that severe plastic deformation processes can reach a steady state where further deformation is feasible but does not lead to significant microstructural changes, supports the hypothesis that the maximum splitting depth of linear flow splitting and linear bend splitting is not limited by exhausted formability of the material . A second important consequence of severe plastic deformation processes is a significant grain refinement and according to the Hall–Petch correlation a pronounced increase of the yield strength, and, as a consequence an elevated (high cycle) fatigue strength .…”

Analysis of different fatigue damage accumulation theories and damage parameters based on experiments with the steel HC340LA

“…A steep microstructural gradient is present in the flange thickness, with the presence of highly elongated grains on the flange surface, while moving through the thickness the grains become gradually coarser. At the flange surface, Ultra‐fine grained microstructures (UFG) are present . These changes induced by the manufacturing process imply also a change in the mechanical properties, with increased hardness and tensile strength at the surface and a property gradient.…”