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 similar to 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.