7050-T7451铝合金搅拌摩擦焊接头组织和性能与疲劳断裂

Microstructure,properties and fatigue fracture of 7050-T7451 aluminum alloy friction stir welded joints

文中主要对7050-T7451铝合金搅拌摩擦焊焊接接头组织性能和疲劳断裂进行研究,对焊件进行金相组织、硬度、疲劳试验,并根据试验结果进行分析。试验结果表明,焊核区和热力影响区组织有明显的分界,整个接头表面和截面的不同区域的组织有明显的不同,焊核区经历高温热循环,并且受到强烈的搅拌作用,发生了显著的动态再结晶,组织均匀细小,没有明显的方向性。热力影响区组织在搅拌摩擦焊接过程中主要受到热循环作用的影响,同时一定程度上受到了机械外力的作用,产生了较小的塑性变形,但储能不够,最终未出现动态再结晶。热影响受到热循环的作用,组织比母材要粗大,但没有发生明显塑性变形,仍具有母材纤维组织的特点。硬度试验的结果表明,母材区和焊核区的硬度比较大,而热影响区的硬度偏低,热力影响区的硬度在它们之间。通过对疲劳试验分析,应力水平为200 MPa时,试样在前进侧热影响区发生断裂,断口的表面较为粗糙。应力水平为250 MPa时,试样在焊核区发生断裂,断口表面较为平整。应力水平为300 MPa时,试样在母材处发生断裂,断口表面十分平整。

Microstructure, properties and fatigue fracture of 7050-T7451 aluminum alloy friction stir welded joints were mainly studied in this article. Tests of metallographic structure, hardness and fatigue of weldments were carried out, which were analyzed based on the test results.The test results showed that there was a clear boundary between weld nugget zone and thermo-mechanically affected zone, microstructure of surface and different areas of cross-section of the entire joint had obvious differences. Weld nugget zone underwent high-temperature thermal cycles and was subjected to strong stirring, and significant dynamic recrystallization occurred, which resulted in uniform and fine microstructure without obvious directionality. Microstructure of thermo-mechanically affected zone was mainly affected by thermal cycle in friction stir welding process, meanwhile, it was affected by mechanical external force to a certain extent, which resulted in a small plastic deformation, but energy storage was not enough, dynamic recrystallization did not appear in the end. Heat-affected zone was affected by thermal cycle, and its microstructure was larger than that of base material, but there was no obvious plastic deformation, and it still had characteristics of fiber microstructure of base material. The results of hardness test showed that hardness of base material and weld nugget area was relatively higher, while hardness of heat-affected zone was lower, and hardness of thermo-mechanically affected zone was between them.Through analysis of fatigue test, when stress level was 200 MPa, samples fractured on the forward side of heat-affected zone, and surface of fracture was relatively rough. When stress level was 250 MPa, samples fractured in weld nugget area, and surface of fracture was relatively flat. When stress level was 300 MPa, samples fractured at base material, and surface of fracture was very flat.