基于改进GTN损伤模型的镁合金镦粗成形损伤及裂纹预测研究

Prediction of Damage and Cracking of Magnesium Alloy Upsetting and Forming Based on Improved GTN Damage Model

镁合金在成形过程中极易产生开裂,精准的损伤预测可以为塑性成形工艺提供理论支撑。为此,基于含连续介质剪切损伤因子的Gurson-Tvergarrd-Needleman(GTN)损伤模型被应用于预测镁合金成形时的损伤演化,通过压缩试样的力-位移曲线标定了AZ31B镁合金的流动应力和剪切损伤参数,预测并验证了AZ31B镁合金在镦粗工艺下的表面损伤及开裂,扩展了模型对低应力三轴度下的成形工艺损伤预测的适用性。在此基础上,对AZ31B镁合金在受压条件下损伤成因、裂纹扩展进行了分析。结果表明,随着压缩位移的增加,镁合金侧面中心区域的切应力不断增加,在此作用下基体内部孔洞沿非垂直压缩方向伸长、聚合,最终形成宏观裂纹;镦粗后的试样侧面裂纹走向与垂直压缩方向所成角度范围为34°~46°,改进的GTN模型模拟结果为44°~54°,原始GTN模型模拟结果均呈90°;改进后的GTN模型可应用于预测镁合金在轧制、热冲压等工艺下的损伤演化行为,为后续工艺优化奠定基础。

Magnesium alloys are highly susceptible to cracking during the forming process,and accurate damage prediction can provide technical support for the plastic forming process.To this end,the Gurson-Tvergarrd-Needleman(GTN)damage model with a continuous medium shear damage factor was applied to predict the damage evolution of magnesium alloys during forming,and the flow stress and shear damage parameters of AZ31B magnesium alloy were identified by force-displacement curve with compressed specimen to predict and verify the surface damage and cracking of AZ31B magnesium alloy under upsetting process.The applicability of the model for predicting damage in the forming process under low stress triaxiality was extended.On this basis,the causes of damage and crack propagation were analysed for AZ31B magnesium alloy under compression conditions.The results showed that with the increase of compression displacement,The shear stress in the center area of magnesium alloy side increased,under the effect of which the voids inside the matrix grew and coalesced in the non-vertical compression direction,and eventually formed macroscopic cracks;the lateral crack direction of the upset specimen was in the range of 34°-46°with respect to the vertical compression direction,and the simulation results of the improved GTN model were 44°-54°,while the simulation results of the original GTN model were 90°;The improved GTN model could be applied to predict the damage evolution behaviour of magnesium alloys under rolling,hot stamping and other processes,laying the foundation for subsequent process improvement.