Causes of micro-crack formation in extraction hole of aluminum shell

Serious irregular deformation occurred in the side extraction hole flange of a welded aluminum shell after hydraulic test, and micro-cracks were generated at the extraction hole part. The causes of micro-cracks were analyzed by means of macroscopic observation, chemical composition analysis, mechanical property test, macro-examination, hardness test and metallographic examination. The results show that the preheating temperature was too high when the shell was extracted, resulting in overburning of the structure. The coarse grain and local grain boundary remelting had great influence on the tensile strength and toughness of the material. Finally, a control method was proposed for the deformation and overburning of the shell to prevent the air leakage accident during operation.