A method for establishing the plastic constitutive relationship of the weld bead and heat-affected zone of welded tubes based on the rule of mixtures and a microhardness test

The plastic forming of welded tubes is becoming more widespread in the aircraft and automotive industries owing to the manufacturing advantages of low cost, short production period and great variety. To obtain a more precise constitutive model of a welded tube with a straight weld line, a method of establishing constitutive models of the weld bead and heat-affected zone (HAZ) metal based on a mixed material tensile experiment and microhardness test is proposed by introducing the influence of the HAZ metal and considering the variation in flow stress across the HAZ. In addition, it is proposed to use cross-sectional samples of the tube in measuring the tube microhardness so that the characteristics of the microhardness throughout the weld zone and HAZ can truly be represented. Constitutive models of a Q215 welded tube are established using the proposed method. The proposed method is assessed by comparing results with those obtained using the original method based on the rule of mixtures. The effects of the sample size on the constitutive model of a welded tube have been investigated by simple tensile testing simulation and experiment. The results show that tensile specimens of sufficient width to include the weld bead, HAZ, and parent metal are suitable considering the constraint among the weld bead, HAZ, and parent metal.