Magnesium (Mg) alloy wheels are widely concerned for their satisfactory energy saving and emission reduction effects and increased driving comfort. However, the current single-step back extrusion (BE) forming process of Mg alloy wheels uses a relatively small billet height-diameter ratio (HDR), which results in coarse grains and uneven grain size distribution of the spokes, seriously affecting the service performance of the wheel. A numerical model of the Mg alloy wheel was constructed based on the actual single-step BE process of the Mg alloy wheel in this study, and the effect of billet HDR on the physical field and dynamic recrystallization (DRX) grain size during the forming process of Mg alloy wheel was explored through numerical simulation. Finding reveals that as the HDR of the billet increases, the overall effective strain of the wheel increases, the overall DRX volume fraction increases, and the grains are refined. When the HDR of the billet is greater than 1, the overall grain size distribution of the wheel tends to be stable. Compared with the traditional small HDR, when the billet HDR is greater than 1, the grain size at the spokes is reduced from the original 60 similar to 120 to 30 mu m. During the forming process of the wheel, the upper rim is prone to cracking. With the increase of the HDR of the billet, the damage value at the upper rim of the wheel increases, i.e., the cracking tendency at the upper rim increases. Simultaneously, the cracking tendency of the outer side of the rim increases with the HDR of the billet. Considering it comprehensively within the scope of this study, it is recommended to set the HDR of the wheel billet between 0.7 and 1.
