Multi-pass intermittent local loading process of large-scale rib–web component: Forming characteristics and implementing apparatus

The multi-pass intermittent local loading process, which features a more flexible processing path, can further enhance the second material distribution during local loading, improve the formability of components, and reduce forming loads. However, the absence of compatible forming equipment makes it difficult to control the constraint in the unloaded zones during the forming process. This difficulty complicates coordination and control of deformation, particularly for asymmetric rib–web components. Additionally, the current implementation involves multi-fire heating, a long process flow, and high energy consumption, which limits the popularization and application of the local loading process. In this study, a new multi-pass local loading hydraulic forming apparatus that can quickly and reliably switch between heavy-load deformation and low-load constraint for different local loading sub-dies was developed. A 10-tonne laboratory prototype was developed, and the forming characteristics during the forming process as well as the response characteristics of the hydraulic system during the multi-pass intermittent local loading of rib–web component were investigated using numerical simulations and physical experiments. Results indicated that, compared to a whole loading process with the same initial geometry of billet, the total forming load (i.e., the sum of loaded and restrained loads) is reduced by more than 40% with the local loading process, and by nearly 50% with multi-pass local loading. The multi-pass local loading process allows for more effective control of material flow compared to single-pass local loading, leading to improved cavity filling and reduced flow line disturbance. For a large-scale, complex titanium alloy bulkhead, the cavity filling problem was addressed by optimizing the multi-pass local loading path with an unequal thickness billet. The dynamic performance of the multi-pass local loading hydraulic system was found to be robust, with stable pressure transitions during motion and load switching for the sub-die(s). The dynamic characteristic of the hydraulic cylinder when switching from non-moving/unloaded state to a moving/loading state are consistent whether a load is present or not. However, the dynamic characteristics differ when switching from a moving/loading state to non-moving/unloaded state, showing opposite behavior. The developed hydraulic drive mechanism provides a way for implementation of multi-pass local loading without auxiliary operation and extra heating. The results of the study provide a foundation for the industrial production of large-scale, complex components with reduced force requirement and low-energy consumption.