Consideration of Core Segregations on the Formability of Bearing Steel

In this paper the influence of core segregations on the formability of workpieces made of bearing steel is analysed and discussed on the basis of compression tests. First the necessary material properties and flow curves, taken at different regions of the bar stock material, were investigated. The core segregation material shows significantly higher flow stress at higher strain rates than the surrounding segregation‐free material. Further a strong influence of the forming temperature on the flow stress was found. On basis of these findings an FEM‐model was developed, that considers the core segregation properties, its shape and position. The results of the compression simulation with this model show a clear impact of the inhomogeneous material properties on the stress distribution.     

Simulation based microstructural optimization of thermo‐mechanically treated steel components

A simplified approach for the simulation based estimation of the phase distribution in a thermo‐mechanically treated steel component is presented. A key aspect of the approach is the time‐temperature relation for each volume element. Based on a forming simulation with a commercial tool the numerically calculated temperature evolution in the component is analyzed with an in‐house code. The code allows estimating the local phase distribution after the forming process with the help of the continuous‐cooling‐diagram of the material used. A first validation fits well with the existing phase distribution in the component, even though the phase transition in the component is critical in terms of time, deformation and local chemical composition of the material used.     

Einfluss der Al2Cu‐Phase auf die Superplastizität der AlCuMn Legierung

Influence of the Al₂Cu‐phase on the superplasticity of AlCuMn alloy High‐temperature creep‐resistant AlCuMn wrought alloy has been investigated and optimised with respect to their superplastic deformability; a maximal elongation ε of 850 per cent was thus attained at a deformation temperature of 530°C. Prerequisites for superplastic deformation behaviour and for the associated high elongation values of these aluminium alloys are an especially fine‐grained structure as well as a decrease in the amount of Al₂Cu phase and a uniform distribution of this phase in the structure. Superplastic deformation (SPD) results in a pronounced change in the shape of the large particles of the θ‐phase; the particles of this phase thereby form veins along the boundaries of adjacent grains. During deformation, the grains lose their equiaxial shape and elongate in the direction of tension as a result of pronounced intragranular sliding dislocation in the microstructure. Transmission electron micrographs of the deformed structure have revealed a pile‐up of dislocations in the grains of the aluminium alloy. The grain size of commercially available sheets of AlCuMn wrought alloys with a thickness of 1 mm is approximately 30 μm. After optimising, the grain size of the sheets produced by the new method was on 12 μm until 5 μm. The new technique differs only slightly from industrial manufacture.     

Design and manufacturing of a human standardised hip cup out of titanium

Background: In recent years, the use of hip prostheses has become a routine procedure. Despite this experience and good clinical results different complications arise which have a negative influence on the lifetime of prostheses. Especially the migration or loosening of the hip cup prosthesis due to strain adaptive bone remodelling is still a problem. Patient‐individual prostheses represent a possible solution to this problem. Individual hip cups, however, are just implanted for the treatment of massive deformities or tumours. This study aimed at developing an innovative concept for the production of patient‐specific human hip prostheses made of titanium plates by sheet metal forming. Methods: For the realisation of this innovative concept, a reproducible design method for the generation of standardised human hip cup prosthesis was generated based on 13 original human geometries. By means of this methodology a hip cup was designed. Based on this design a human hip cup was produced by a developed high‐pressure sheet metal forming process. The development of the process was accompanied by a numerical preliminary design. Results: A comparison between the simulation and the fabricated hip cup leads to a standard deviation of 0.404 mm. Furthermore, an implantation of the prosthesis in a synthetic bone model shows a satisfactory fit accuracy at the edge of the prosthesis. Conclusion: The high‐pressure sheet metal forming process is suitable to manufacture the designed standardised hip cup. However, further optimisation is necessary.     

Lokale Hochverformung zur Erzeugung von ultrafeinkörnigen Zonen

Local severe plastic deformation for producing ultrafine‐grained regions The methods of severe plastic deformation are able to produce semi‐finished products with a homogenous ultrafine‐grained microstructure. An alternative option is the formation of ultrafine‐grained layers or rather a gradation of the grain size. The qualification of incremental bulk forming processes is concluded from an analysis of methods for producing ultra fine‐grained materials and the kneading in cyclic forming. Spin extrusion is investigated regarding the formation of ultra fine‐grained regions. Tests are carried out to analyse the grain refinement in cyclically deformed regions.     

Modeling of the material behavior during hot hydroforming

For the purpose of numerically simulating metal forming processes, material data are necessary, determined by testing procedures similar to the particular process. The new technology of hot tube bulge tests has been introduced recently, fulfilling the requirements of material data determination for hot hydroforming. Based on measurement data gained by this technology, selected constitutive relations for approximating the flow stress depending on temperature, strain rate and logarithmic strain were parameterized applying linear regression analysis. Using the material law with the best approximation quality among the regarded equations, a numerical simulation of an exemplary forming process was accomplished. A comparison between the experimentally obtained geometry after a hot hydroforming process and the prediction by numerical analysis is used for evaluating the quality and applicability of the determined material data for this kind of process. Additionally, a process simulation, using extrapolated material data from compression tests is presented to visualize the influence of the testing procedure on the resulting part geometry prediction.