喷丸+滚压辅助高强铝合金FSW接头疲劳行为研究

Facing the lightweight requirement of vehicles such as cars, railway vehicles, aerospace vehicle, this project proposes a modified refining method based on shot peening and rolling composite on the surface of the weld where fracture is more easily to happen in the thermo-mechanically affected zone (TMAZ) , whick caused low strength coefficient of joint deduced by the microstructrure heterogeneity of high strength aluminum alloy components for friction stir welding technology. This project intends to use 7050 aluminum alloy to finish friction stir welding experiment, The experiment of composite surface modification with different microstructure welds is carried out by shot peening and rolling. The strengthening mechanism, intensifying degree and the improvement of mechanical properties after modification are studied..It is well known that shot peening and rolling reinforcement are mechanical reinforcements achieving complementary advantages on the substrate reinforcement, but for non-uniform microstructure of friction stir welding, namely weak connection thermo-mechanically affected zone mixing micro area shape and its size changing with welding parameter，located between the coarsening of heat affected zone and fine equiaxial grains, has a influence on the surface of the weld reinforcement effect. Combining with the characteristics of that microstructure, how to develop shot peening / rolling composite surface modification technology is to reduce the number of the adverse grain orientation of the TMAZ, to make weld microstructure of district granular layer thickness homogeneous, to control effectively of fatigue source shifting from surface to subsurface or more deeper and to rich FSW modified theory to full extent.

面向汽车、轨道车辆、航空航天器等交通装备轻量化的需求，针对搅拌摩擦焊技术在连接高强铝合金构件时导致焊缝组织不均匀，断裂更容易在热-机影响区发生，引起接头强度系数较低的难题，本项目提出一种基于喷丸+滚压轧制复合改性细化焊缝表面的方法。本项目拟使用7xxx系7050铝合金开展搅拌摩擦焊接实验，对不同微结构焊缝进行喷丸+滚压复合表面改性工艺实验，研究其强化机理、强化程度，改性后的力学性能的提升程度。喷丸和滚压强化都属于机械强化，在强化基体上实现优势互补，但对非均匀的搅拌摩擦焊缝组织，弱连接热-机影响区横截面夹在粗化的热影响区与细小的等轴晶之间，微区结构、尺寸随焊接参数而改变，影响焊缝表面的强化效果。结合这种组织特点，弄清喷丸+滚压复合表面改性工艺如何实施，使热-机影响区处于软位向的晶粒数量降低，同时焊缝组织各区细化层厚度均匀，有效控制疲劳源，使起裂从表层转移到次表层更深处，丰富焊缝改性理论。

搅拌摩擦连接铝合金后虽然接头的连接强度显著高于熔化焊的接头强度，但是接头微区薄弱区的存在导致连接件强度系数不高，使得基体材料的应用范围受限。特别是7xxx系列的高强铝合金，由于其重量轻、比强度高的优势，适合在航空航天、轨道交通、汽车制造等领域应用，连接接头优良的性能是推广其广泛应用的保证。本课题利用表面改性的技术，通过喷丸或滚压单一的表面改性工艺、喷丸与滚压两种工艺复合的表面改性工艺，在焊缝上下面进行表面改性。改性后的样件分别进行接头横截面上二维硬度测试，以焊核区为研究对象，对表层及亚表层的应变程度、晶粒的组织进行表征分析，测板材不同深度处的残余应力，对接头试样进行静态拉伸试验，针对焊核区、热-力影响区、热影响区、母材区分别进行各微区低周疲劳试验、相同循环频率下进行疲劳裂纹扩展试验。试验结果为喷丸表面改性、滚压表面改性两种单一的表面改性均有效的改善了接头处的组织，表现为焊接态时接头组织为热影响区、热-力影响区、焊核区三个微区显著不同的微观组织被上下表面改性后的纳米化组织所包裹，形成了类似“三明治”的组织结构。上下两表层及次表层组织纳米化，使得表层组织均匀化，回避了焊接态时不均匀组织暴露在表层的不足。表层组织经历塑性变形不仅使表层晶粒实现了纳米化，同时引入了比较大的残余压应力，与喷丸改性相比较，滚压改性后引入的残余压应力数值更大，且最大值移到更板更深处。EBSD表征了表层不同深度处的应变量，从表层向板厚方向由纳米层、过渡层、微变形层组成，应变层厚度最大达到633um。表面改性后接头的疲劳寿命均得到提高，提高的效果是复合改性最好，滚压次之，喷丸的略低。疲劳裂纹扩展速率均有降低，复合改性中先喷丸后滚压的试件其裂纹扩展速率最慢，稳态扩展阶段断口形貌特征与试样裂纹的扩展速率相一致，扩展速率慢的疲劳纹间距明显变窄，二次裂纹数量少且分散。研究取得了理想的实验数据，为工艺的推广提供了有价值的参考。