Composite ring rolling - method for manufacturing seamless composite rings on a radial axial ring rolling mill

• Applicant: Professor Dr.-Ing. Gerhard Hirt
• Subject Area: Primary Shaping and Reshaping Technology, Additive Manufacturing
• Term: since 2015
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 284309777

Composite ring rolling is a process in which two concentrically arranged metallic rings are hot ring rolled to establish a material bond between inner and outer ring similar to roll bonding. This allows production of seamless rolled rings, for which the choice of materials allows tailoring of the product even to conflicting requirements.The aim of the first funding period to establish a fundamental understanding of the complex relationships between geometry and process parameters was reached. To achieve this, a fully closed-loop controlled Finite Element (FE) ring rolling model was extended by a routine to consider generation and rupture of bonds between parts and was calibrated using a bond strength experiment to determine the bond generation behaviour between two materials. For the considered material combination of a 1.7335 construction steel inner ring and a 1.4401 stainless steel outer ring simulations showed the separation of the rings at the start of the process and the generation of a non-symmetrical bond line, both of which lead to a non-successful composite ring. The cause of these effects was analysed and measures to prevent these were developed.The above-mentioned material combination shows a large difference in thermal expansion between inner and outer ring as well as the generation of an oxide layer at the stainless steel. Experiments showed that even for favourable process conditions these effects impede the generation of a material bond between the rings, which lead to only a small number of successfully rolled composite rings. The generated bonds were analysed by electron microscopy as well as mechanical methods, in which the bonds showed strengths larger than the softer material’s tensile strength.Based on this knowledge the second funding period aims to enable the reliable generation of material bonds between rings of different materials. On the basis of three different material combinations the impeding influence of the above-mentioned effects shall be investigated in detail. By model bond strength experiments the extended FE model is calibrated for each material combination and by a Design of Experiments ideal process parameters for ring rolling will be identified for wall thickness ratios between inner and outer ring of 1:1 as well as with regards to possible applications with a minimum ratio of the more expensive material. Rolling experiments prove reproducibility of the process for all combinations. Furthermore, the effect of consecutive heat treatments on the generated bond strengths shall be investigated. To conclude, the collected knowledge of both funding periods will be used to generate composite rings of a fourth material combination of the bearing steel 100Cr6 and the stainless steel 1.4401 with regard to a possible use case of a roller bearing. Those rings are also investigated in detail.

• DFG Programme: Research Grants