Thermomechanical Processing for Creating Bi-Metal Bushing Bearings

Bi-material machine components are fabricated by joining two individual components which are already given their near-final or final form. These are then put into operation either directly or upon a finishing process. Contrary to that, researchers of the Collaborative Research Centre “CRC 1153 Tailored Forming” are investigating novel process chains, in which different materials are joined in the first step and then subjected to further processing, i.e., forming, machining and heat treatment. By this means, the joining zone properties, which are adversely affected due to the joining process, can be treated and improved via thermomechanical processing during forming. On the other hand, process-specific challenges arise especially for workpieces consisting of dissimilar materials, i.e., steel and aluminum. In order to obtain a favorable flow behavior of the materials in the vicinity of the joining zone, a near step-function temperature distribution in the bi-material billet is desirable. Induction heating is viewed as the most promising method to be used for this purpose. The current study involves the potential for creating bi-metal bushing bearings. In order to investigate the influence of tailored heating and subsequent forming on the final bonding quality workpieces were assembled without metallurgical bonding. Two concentric cylinders were joined by shrink fitting (steel on the internal diameter and aluminum on the external diameter). After assembly, induction heating is used to heat the steel on the internal diameter. After heating, the component is moved to closed-die forging operation. Results of the initial computer modeling and experimental validation of the thermomechanical process will be presented. Besides the achieved temperature gradients used for the forging experiments, forged hybrid bushings and investigations regarding material flow, microstructure and the joining zone will be demonstrated. Additional considerations will be given on how to further improve the process and move towards a production capable process.