A cryogenic incremental sheet forming process for improving the formability of AA6061 to reveal the dual enhancement effect and microstructure evolution mechanism

Based on the dual enhancement effect of hardening and plasticity in aluminum alloys at cryogenic temperatures, the cryogenic incremental sheet forming process is introduced in this paper for manufacturing complex components. Experimental results indicate that incremental sheet forming at cryogenic environments results in a remarkable enhancement of formability. The ultimate forming height of specimen at 113K presents an increase of 32.4% compared with the specimen at 295K. Meanwhile, it is confirmed that cryogenic conditions increase the work-hardening ability and reduce the microcrack generation on the specimen surface. Moreover, the mechanism of dual enhancement effect on 6061 alloy during cryogenic incremental forming was studied. At 295 K, the dislocation distribution was localized due to significant cross-slip in the specimens. It was also observed that dislocation entanglement occurred at grain boundaries, which tends to cause stress concentration and therefore reduced formability. In contrast, at 113 K, the decrease in stacking fault energy leads to suppression of cross-slip, and the uniform slip leads to a significant increase in dislocation density. As a result, the cryogenic temperature exhibits enhanced work-hardening ability and formability. The rolling texture evolves mainly along the α and β orientation lines during the forming process, the 295K specimen generates a large number of Goss textures in the final forming region due to the uneven deformation which makes it difficult for the texture to evolve further. In contrast, at 113 K the texture fully evolves and intersects in the Brass texture along the two orientation lines due to the enhanced formability.