Components subject to rolling contact fatigue, such as gears and rolling bearings, are among the fundamental machine elements in mechanical and vehicle engineering. Rolling bearings are generally not designed to be fatigue-resistant, as the necessary oversizing is not technically and economically marketable. In order to improve the load-bearing capacity, resource efficiency and application possibilities of rolling bearings and other possible multi-material solid components, a new process chain was developed at Leibniz University Hannover as a part of the Collaborative Research Centre 1153 “Tailored Forming”. Semi-finished products, already joined before the forming process, are used here to allow a further optimisation of joint quality by forming and finishing. In this paper, a plasma-powder-deposition welding process is presented, which enables precise material deposition and control of the welding depth. For this study, bearing washers (serving as rolling bearing raceways) of a cylindrical roller thrust bearing, similar to type 81212 with a multi-layer structure, were manufactured. A previously non-weldable high-performance material, steel AISI 5140, was used as the cladding layer. Depending on the degree of forming, grain-refinement within the welded material was achieved by thermo-mechanical treatment of the joining zone during the forming process. This grain-refinements lead to an improvement of the mechanical properties and thus, to a higher lifetime for washers of an axial cylindrical roller bearing, which were examined as an exemplary component on a fatigue test bench. To evaluate the bearing washers, the results of the bearing tests were compared with industrial bearings and deposition welded axial-bearing washers without subsequent forming. In addition, the bearing washers were analysed micro-tribologically and by scanning acoustic microscopy both after welding and after the forming process. Nano-scratch tests were carried out on the bearing washers to analyse the layer properties. Together with the results of additional microscopic images of the surface and cross-sections, the causes of failure due to fatigue and wear were identified.

Hybrid cylindrical roller thrust bearing washers of type 81212 were manufactured by tailored forming. An AISI 1022M base material, featuring a sufficient strength for structural loads, was cladded with the bearing steel AISI 52100 by plasma transferred arc welding (PTA). Though AISI 52100 is generally regarded as non-weldable, it could be applied as a cladding material by adjusting PTA parameters. The cladded parts were investigated after each individual process step and subsequently tested under rolling contact load. Welding defects that could not be completely eliminated by the subsequent hot forming were characterized by means of scanning acoustic microscopy and micrographs. Below the surface, pores with a typical size of ten µm were found to a depth of about 0.45 mm. In the material transition zone and between individual weld seams, larger voids were observed. Grinding of the surface after heat treatment caused compressive residual stresses near the surface with a relatively small depth. Fatigue tests were carried out on an FE8 test rig. Eighty-two percent of the calculated rating life for conventional bearings was achieved. A high failure slope of the Weibull regression was determined. A relationship between the weld defects and the fatigue behavior is likely.

As a part of the newly established Collaborative Research Center 1153 (SFB 1153) „Process chain for the manufacturing of hybrid high-performance components by tailored forming” at the Leibniz Universität Hannover, the Institute of Forming Technology and Machines (IFUM) examines the influence of thermo-mechanical stresses on the reduced Young’s modulus as well as the hardness of hybrid (steel-steel compound) joined semi-finished products. Currently the expertise in the production of bulk metal formed parts is limited to mono-materials. For manufacturing parts of hybrid materials and also for the methods of the new process routes, practical experience has to be gained. The subproject C1 within the collaborative research center 1153 with the short title “Failure Prediction” deals with the question, if the hybrid semi-finished products fulfill the thermo-mechanical demands or if they fail at the joining zone (JZ) during forging. For this purpose, stresses similar to those in the process were imposed on hybrid semi-finished products by torsion tests by using the thermo-mechanical test system Gleeble 3800. Afterwards, the specimens were examined metallographically and by nanoindentations with the help of a TriboIndenter TI950. Thus, first knowledge on the behaviour of thermo-mechanical stresses on the reduced Young’s modulus and the hardness of hybrid joined semi-finished parts was gained.

Multi-material bulk metal components allow for a resource efficient and functionally structured component design, with a load adaptation achieved in certain functional areas by using similar and dissimilar material combinations. One possibility for the production of hybrid bulk metal components is Tailored Forming, in which pre-joined semi-finished products are hot-formed using novel process chains. By means of Tailored Forming, the properties of the joining zone are geometrically and thermomechanically influenced during the forming process. Based on this motivation, forming processes (die forging, impact extrusion) coupled with adapted inductive heating strategies were designed using numerical simulations and successfully realised in the following work in order to produce demonstrator components with serial or coaxial material arrangements. The quality of the joining zone was investigated through metallographic and SEM imaging, tensile tests and life cycle tests. By selecting suitable materials, it was possible to achieve weight savings of 22% for a pinion shaft and up to 40% for a bearing bush in the material combination of steel and aluminium with sufficient strength for the respective application. It was shown that the intermetallic phases formed after friction welding barely grow during the forming process. By adjusting the heat treatment of the aluminium, the growth of the IMP can also be reduced in this process step. Furthermore, for steel-steel components alloy savings of up to 51% with regard to chromium could be achieved when using low-alloy steel as a substitute for high-alloy steel parts in less loaded sections. The welded microstructure of a cladded bearing washer could be transformed into a homogeneous fine-grained microstructure by forming. The lifetime of tailored formed washers nearly reached those of high-alloyed mono-material components.

Lightweight multi-material components are of great importance for the transport industry. Not only the component’s weight can be decreased, but also its local properties can be adapted to different loading profiles. Tailored Forming is a novel concept for producing multi-material components. By using a joining process, the creation of a bond between different materials takes place in the first step of the process chain. In the subsequent steps, multi-material workpieces are processed in their joined state while maintaining or improving the joint strength. This study focuses on steel-aluminium joints, which were created by friction welding and further processed by induction heating and impact extrusion. A counter pressure superposition mechanism was implemented in the extrusion tooling to control the stress state during plastic deformation. Flow behaviours of steel and aluminium are largely different at a given temperature, which necessitates a near step-function temperature distribution in the hybrid billet to obtain matching flow stresses. An inductive heating strategy was developed which led to a temperature gradient in the billets before extrusion. Extruded billets were analysed by destructive testing methods and metallography. The bond could be maintained after extrusion when counter pressure superposition was used; but no improvement could be obtained. Without counter force superposition, however, cracks were observed in the joining interface and the joint strength decreased. This paper discusses the aforementioned findings in the current process design and makes suggestions on how the involved processes should be reconfigured to improve the joint strength.

Forming of already joined semi-finished products is an innovative approach to manufacture components which are well-adapted to external loads. This approach results in an economically and ecologically improved production by the targeted use of high-quality materials in component areas, which undergo high stresses. One possible production method for hybrid semi-finished products is friction welding. This welding method allows for the production of hybrid semi-finished products made of aluminium and steel as well as steel and steel. In this paper, the thermomechanical tensile and shear stresses causing a failure of the joined zone are experimentally determined through tension tests. These tests are performed with specimens whose joint zones are aligned with different angles to the load direction.