Mechanism of increasing spinnability by multi-pass spinning forming – Analysis of damage evolution using a modified GTN model

Multi-pass spinning forming is one of the major approaches to manufacture rotationally symmetric thin-walled components, which is strongly influenced by the spinnability of materials. But it remains an open question regarding the formability limit in the multi-pass spinning process. According to the spinning experiment phenomena under different spinning passes and thinning rates, a modified GTN model was incorporated into the FE simulation to analyze the damage evolution and cracking mechanism during single-pass tube spinning and multi-pass spinning. The results show that the thinning rate and spinning pass have an important influence on the forming limit of 2024 aluminum alloy. Smaller thinning rates in multi-pass tube spinning led to earlier crack compared to single-pass tube spinning, which was induced mainly by void damage, while the cracking under larger thinning rates was primarily caused by shear damage. When the pass thinning rate reached ∼21%, no crack occurred at the total thinning rate of 38% in multi-pass spinning, which was much higher than the forming limit (no more than 29.85%) in single-pass spinning. In this case, both the insufficient plastic deformation and low stress triaxiality suppress the damage accumulation, contributing to the increase of the spinnability of materials in multi-pass spin forming. This study provides an effective guidance for establishing the thinning strategy for spin forming practice.