Frederik Knauf

Investigations on Thermal Influences for Thixoforging and Thixojoining of Steel Components

At the Institute of Metal Forming (IBF) current investigations within the framework of the Collaborative Research Centre 289 are mainly concentrated on forming of semi-solid precursor material of the steel grades X210CrW12 and 100Cr6. One important task is the precise temperature for the composition of solid and liquid phase fraction in the preheated billet. Experimental measurements and numerical simulations show significant heat losses during transport of the billet and after its insertion into the die. These developing temperature gradients influence strongly the resulting temperature field in the formed component. In case of the forming tools the critical increase of the temperature depending on the tool material is shown. As fundamental research in the field of thixojoining the temperature development of the inserts is analysed and demonstrates the feasibility of joining higher and lower melting materials into the semi-solid matrix.

Rheoforging of Steel

Rheoforging is a modification of semi-solid forging starting directly from the liquid phase instead of reheating of a billet. In this process semi-solid slurry is prepared by pouring slightly overheated melt over a slope into a holding cup to induce homogenous nucleation. By controlling the casting temperature and holding time in the cup the desired semi-solid slurry with fine, globular microstructure is successfully created. The inline forging trials using this slurry show the principal feasibility of the process.

Influence Of Tool Geometry, Tool Coating And Process Parameters In Thixoextrusion Of Steel

Thixoextrusion could become one possibility to enlarge the complexity of extruded profiles made of steel. Accordingly semi‐solid extrusion experiments of X210CrW12 tool steel using round dies of approximately 15 mm diameter were performed in order to achieve first information concerning possible process windows and process limits. For liquid fractions between 38% and 10%, extrusion press velocities from 10 mm/s to 50 mm/s and dies with novel PVD‐coatings no complete solidification during extrusion was achieved. However the collected pieces of the extruded bars showed a fine and evenly distributed globular microstructure.

Semi-Solid Extrusion of Complex-Shaped Steel Rods Using Ceramic Dies

Isothermal semi-solid extrusion experiments using steel grade X210CrW12 as work material were performed on an industrial forging machine. An improved and up-scaled tool design was applied, based on the concept of self-heating ceramic dies tested in previous laboratory-scale studies and allowing for die preheating temperatures of up to 1400°C. Steel rods of complex cross sectional geometry were formed at low extrusion forces. Shape accuracy of as-formed rods is accurate and metallurgical examination yields no evidence of liquid phase separation. With a view on the intended industrial implementation construction of ceramic tools has to be improved in order to avoid rupture of ceramic parts due to thermo-mechanically induced stresses and to benefit from the advantages inherent to ceramics in metal forming.

Process Window Determination for Thixoextrusion Processes Using a Steel Alloy

A direct semi-solid bar extrusion process is characterised by inserting a feed stock in a container and extruding through a forming die with a punch. Compared to conventional bar extrusion the use of semi-solid material complicates the process due to the requirement of solidification of the material. To achieve the solidification of the semi-solid bar, different basic tool concepts are presented. With a combination of these concepts experiments were carried out using the steel alloy X210CrW12 to detect the influence of the most influencing parameters press velocity, extrusion channel diameter, length and geometry. Numerical simulations enable a better understanding of the process mechanics like temperature development in the billet and forming die as well as the material flow in the deformation zone.