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This paper mainly focuses on the folding which is the main defect on the inner surface of the inner grooved copper tube during ball spin forming. The material model and two-dimensional geometrical model of finite element simulation of multi-step ball spinning for forming inner grooved copper tubes have been built. The folding defects formed both on one side of the fin and on the bottom of inner grooves of the copper tubes had been analyzed according to the finite element results such as strain distribution, metal flow and contact state between the copper tube and the plug. Efficient measures have been proposed to prevent the folding defects. The SEM observation on the inner surface of the finished inner grooved copper tubes has been performed to illustrate the folding distribution and shape. Metallographic observation on the microstructure of cross-section of the copper tubes has been performed to analyze the forming process of folding. The results of the finite element simulation agree well with the experimental observation of the folding defects on the inner surface and the microstructure of cross-section of the inner grooved copper tubes. (c) 2006 Elsevier B.V. All rights reserved.