Defect detection is essential for quality assurance in metal forming, but traditional methods often suffer from process complexity and imbalanced datasets. This study proposes a GAN-based defect detection framework integrating transfer learning to improve accuracy and robustness. A pre-trained GAN is first employed to augment non-defective metal samples, enhancing data diversity, followed by transfer learning-based defect classification. Experimental results demonstrate that the improved auxiliary classifier GAN achieves an average classification accuracy of 95.98%, outperforming traditional auxiliary classifier GAN (91.08%) and ResNet-50 (88.65%). Moreover, the proposed model obtains a Mode Score of 1.386, higher than Res-Wasserstein GAN (1.374) and Res-ACGAN (1.361), indicating improved quality and diversity of generated samples. The model is further applied to defect detection of aluminum profiles and hot-rolled steel strips, where it effectively identifies and classifies various defect types with enhanced detection efficiency and reliability. These results demonstrate that the proposed AI-driven approach can improve intelligent quality control in metal processing, reduce potential safety risks caused by undetected defects, and provide an effective solution for advanced manufacturing applications.