基于FEM分析轧制预变形对AZ31B镁合金热轧板材边部损伤的影响规律

Edge Damage of Hot Rolled AZ31B Magnesium Alloy Sheets with Pre-Rolling Based on FEM

基于FEM(finite element method)研究了轧制预变形对AZ31B镁合金热轧板材边部损伤的影响规律。选用Normalized Cockcroft&Latham损伤模型,在轧制温度为400℃、轧制速度为0.5 m·s^(-1)的条件下,对规格为50 mm×20 mm×15 mm的AZ31B镁合金板材预先使用凸度轧辊制备不同形状的板坯,使板坯中部的变形量一致,边部比中部分别高出2,4和6 mm,然后分别进行多道次、小压下率和单道次、大压下率平辊轧制模拟仿真。结果表明,轧制预变形能够显著降低镁合金板材边部的损伤,经多道次轧后板材边部的拉应力减小,应力三轴度降低,边部与中部的应变差值减小,边部金属与中部金属流动趋于同步,且在预设仿真方案范围内边部凸度越大,轧后板材边部的损伤值越小,最小损伤值为0.729。对镁合金板材预变形后可实现单道次、大压下率轧制,板材的边部温度和应变速率均有所增加,有利于降低轧制过程中的边部损伤。研究结果可为少或无边裂镁合金板材轧制工艺制定提供理论依据。

Magnesium alloy is one of the key metal structural materials developed in recent years.It has the advantages of light weight,high specific strength,and large elastic modulus.It also has good shock absorption,noise reduction and impact resistance,economics and industrial production.It is of great significance to carry out in-depth research on key technical problems of magnesium alloys to solve the outstanding problems of the current energy structure and industrial structure.Rolling is one of the common methods for producing magnesium alloy sheets.This production process can refine the microstructure of magnesium alloy sheets,improve their mechanical properties,and has the advantage of continuous mass production.However,the magnesium alloy sheet is prone to edge cracks during the rolling process,and the resulting large amount of trimming waste seriously affects the yield and material utilization rate,which limits the further development of the material.Aiming at the problem of edge cracks during the rolling process of magne⁃sium alloy sheets,in recent years,many scholars have adopted methods such as on-line heating,cladding rolling,lining rolling,asyn⁃chronous rolling,prefabricated crown,model prediction,etc.These techniques have been validated effectively in the laboratory.This provides the possibility to prepare magnesium alloy sheets with no edge cracks or less edge cracks,and its industrial production will bring huge economic benefits.However,these technologies have process or equipment complexity,which brings certain difficulties to actual production.Whether pre-deformation affects the damage evolution at the edge of magnesium alloy sheets and how it affects the edge damage should be focused.Firstly,simulation schemes with different pre-deformation degrees were designed.On the basis of veri⁃fying the correctness of the finite element model,several sets of simulation schemes were carried out to analyze the changes of physical quantities such as tensile stress,stress triaxiality and plastic strain,exploring the causes of edge cracking in magnesium alloy sheet rolling in the manufacturing process.Based on finite element method and the Normalized Cockcroft&Latham,the parameters used in the thermal-mechanical coupling simulation were shown in Table 1.The dimensional comparison between the simulation and the test of the magnesium alloy sheet under the same rolling parameters were shown in Figure 1.The results showed that the simulation results had higher shape prediction accuracy,and the model had higher accuracy.The effect of pre-rolling on the edge damage of hot rolled AZ31B magnesium alloy sheets was studied.Under the conditions of rolling temperature of 400℃and rolling speed of 0.5 m·s-1,the magnesium alloy sheets of 50 mm×20 mm×15 mm were pre-formed with crown rolls to prepare slabs of different shapes.The deforma⁃tion in the center of slabs was the same,and the edge was 2,4 and 6 mm higher than the center,respectively.Then the simulations of multi passes with small reduction and single pass with large reduction flat rolling were carried out.The results showed that the pre-roll⁃ing could significantly reduce the edge damage of magnesium alloy sheets.The positive and negative values of the stress triaxiality val⁃ue indicate the load condition of the material.When it was subjected to tension,its value was positive,in this case,the growth of the hole was accelerated;when it was subjected to compression,its value was negative,and the growth of the hole was suppressed,so as to delay the occurrence of cracks.After multi passes,the tensile stress and the stress tri-axiality at the edge of the sheets were re⁃duced,and the strain difference between the edge and the center was decreased.Meanwhile,the flow of edge metal and center metal tended to be synchronous.In the range of the simulation scheme,the greater crown formed at the edge,the minor edge damage of the sheets after rolling,and the minimum damage value was 0.729.After pre-rolling,the magnesium alloy sheets could be rolled with sin⁃gle pass and large reduction,and the edge temperature and strain rate of the sheets increase,which is beneficial to reduce the edge damage during the rolling process.The stress state of the edge of the rolling deformation zone of the magnesium alloy sheet was two-di⁃rectional compressive stress and one-directional tensile stress,and the middle part was three-directional compressive stress.The predeformation process could reduce the tensile stress on the edge.The rolling pre-deformation process could reduce the stress triaxiality of the magnesium alloy sheet,and the greater the convexity formed at the edge after pre-deformation,the more obvious the reduction of the stress triaxiality,and the strain difference between the edge and the middle after pre-deformation was reduced,the flow of edge metal and middle metal tended to be synchronized.After pre-deformation,the magnesium alloy sheet could be rolled with a single pass and a large reduction rate.The edge temperature of the sheet increased and the strain rate increased,which was beneficial to reduce the edge damage during the rolling process.The research results could provide theoretical basis for rolling process formulation of mag⁃nesium alloy sheets with little or no edge cracks.