New insights into combined thermal and vibration softening of magnesium alloy in rotational vibration assisted incremental sheet forming

With the generation of both localised thermal and vibration in incremental sheet forming (ISF) by novel tool designs, rotational vibration assisted ISF (RV-ISF) can achieve significant force reduction and material softening. However, the combined thermal and vibration softening in RV-ISF is unclear. By evaluating the similarities and differences of friction stir ISF (FS-ISF) and RV-ISF, this study develops a novel approach to decouple and quantify the thermal and vibration softening effects in RV-ISF of AZ31B-H24, providing new insights into underlying thermal and vibration softening mechanism. Experimental results reveal that in RV-ISF of AZ31B-H24 the thermal softening due to frictional heating dominates with 45 ∼ 65 % of softening, while the vibration effect only contributes up to 15 % of softening, from the conventional ISF, depending on the tool designs and tool rotational speed. The double-offset tool (T2) produces greater vibration softening than the three-groove tool (T3) owing to the higher vibration amplitude of the T2 tool. An increase in tool rotational speed primarily enhances thermal softening with only marginal changes to the vibration effect. Microstructural analysis suggests that with average grain size of 0.94 µm at the top layer, RV-ISF with T3 and 3000 rpm is more effective for microstructure refinement than that by FS-ISF, especially on the tool-sheet contact surface, which confirms the occurrence of surface shearing. This refinement is a result of the reduced recrystallisation degree, 71.8 % at the top bottom layer. Compared with FS-ISF, RV-ISF can lead to not only higher geometrically necessary dislocation density, but also higher fraction of low-angle grain boundaries, indicating that softening mechanism due to localised vibration effect is resulted from the enhanced rearrangement and annihilation of dislocations. These findings contribute to new understanding of the thermal and vibration softening effects in RV-ISF of AZ31B-H24 and offer a theoretical foundation for the tool design and process optimisation.