Roll bonding materials with with highly unequal material strengths - Analyses of the joining mechanism and modeling new manufacturing procedures

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During roll bonding of metals with greatly differing yield strength, the different flow behavior of the materials can impede the bond creation. Hence in hot roll bonding of slabs long and complex rolling schedules are required, while during cold roll bonding the differences can lead to local thinning and cracks in the harder layer. In the first two funding periods the bond formation in hot roll bonding of slabs was investigated. Using a subroutine to describe the bond formation, calibrated by laboratory experiments, the influences of height reduction on bond strength, and the influence of sheet thickness ratio on length differences between the layers after rolling were investigated. These simulations proofed that unfavorable geometric parameters and sheet thickness ratios can lead to stress states, which cause length differences between the layers even when using the same material for both layers during roll bonding. These findings were subsequently used to demonstrate the possibility to find appropriate rolling parameter for materials with different yield strength that overcome the length differences between the layers. Hence the works regarding the hot roll bonding of slabs will be finished successfully in the second funding period. In the present proposal possibilities to optimize cold roll bonding of coiled thin strips will be investigated in detail.. In discussions with producers of roll bonded strips the combination of a thin hard top-layer with a thick soft core-layer was identified to be error-prone, because in this combination the harder top-layer can experience periodical local thinning and cracking. These errors depend on the different flow behavior and the used height reduction. A great height reduction supports a firm bond formation, but can lead to failure of the top-layer due to the induced stresses. Therefore the main goal of the third funding period is to develop an understanding for the dependencies between the required height reduction and the occurrence of failure in the top-layer. Build on this understanding the aim is to overcome the present process limitations via improving the bond strength in the cold roll bonding process. Therefore the present findings will be transferred and applied to the specifics of thin strips. Initially a parametric study will be performed to achieve an understanding for the predominant effects and dependencies. This requires an adjustment and calibration of the previously mentioned subroutine. Options to influence the flow behavior and the failure characteristics of the layers via an appropriate roll gap geometry and additionally thermal activation will be investigated in simulations and experiments. Furthermore the possibility to support the bond formation via electrochemical reduction will be investigated in cooperation with the LUF Paderborn.