Investigation of bond formation behaviour in composite ring rolling

Ring rolling is an incremental bulk metal forming process used to produce seamless rings with a wide variety of sizes as well as processible materials. Combining the principle of roll bonding and ring rolling could enable production of seamless composite rings with different materials on the inside and outside, therefore giving the opportunity to manufacture rings tailored to specific applications. A fundamental condition to achieve this is a stronger growth tendency of the inner ring compared to the outer ring, which is satisfied at respective forming temperature by choosing a material combination of a stainless steel outer ring and a tempering steel inner ring. A closed-loop controlled Finite Element ring rolling model is used in conjunction with a contact interaction subroutine for modelling generation and rupture of local material bonds by implementation of an empiric model for bond generation. Efforts to prevent oxidation of the joining surface for ring rolling trials were successful but did not lead to a lasting material bond after rolling. A lasting material bond between both rings was only achieved after adjusting the process strategy to initially allow free lateral spread of the rings in the radial roll gap without reduction to the original ring height in the axial roll gap. FE simulations provided the crucial hints for this adjustment, as they were able to show repeated bond formation and rupture during the process as well as the impact of the adjustment on the combined stress state between both rings.