蠕变时效成形曲率对7075合金疲劳裂纹扩展的影响

Effect of creep aging forming curvature on fatigue crack growth of 7075 alloy

为了探明蠕变时效成形过程中内应力消减对铝合金韧性的影响,采用蠕变时效成形、室温拉伸、疲劳裂纹扩展速率实验等方法和扫描电镜(SEM)、电子背散射衍射(EBSD)、X射线衍射(XRD)等表征手段研究2.5 mm厚的T6态7075合金薄板在300,600和1800 mm曲率半径下蠕变时效成形后(对应的试样编号分别为CA300,CA600和CA1800)的疲劳裂纹扩展速率。研究结果表明:T6态薄板的残余应力(初始值为517.1 MPa)在蠕变时效成形后下降明显,CA300样品的残余应力下降为54.0 MPa,其疲劳裂纹扩展速率最低,在裂纹稳态扩展阶段,相对于T6态样品降低了28.2%,CA600和CA1800样品的疲劳裂纹扩展速率也分别降低了21.7%和16.0%。蠕变成形后CA300试样的几何必要位错密度更为均匀,疲劳裂纹不仅在晶界处发生偏转,也在晶内发生偏转,相比于其他试样疲劳裂纹路径更曲折,相邻晶粒实际激活滑移面之间的夹角α增大,疲劳裂纹扩展阻力更大,因此其疲劳裂纹扩展速率最低。

In order to investigate the effect of internal stress reduction on the toughness of aluminum alloy during creep aging forming,experimental methods such as creep aging forming,room temperature stretching,and fatigue crack growth rate experiments,and scanning electron microscope(SEM),electron backscatter diffraction(EBSD),X-ray diffraction(XRD)and other characterization methods were used.The fatigue crack growth rate of T67075 alloy sheet with thickness of 2.5 mm was investigated at curvature radius of 300,600 and 1800 mm,the corresponding sample numbers of which are CA300,CA600 and CA1800,respectively.The results show that the residual stress of T6 thin plate(initial value is 517.1 MPa)decreases obviously after creep aging forming,and the residual stress of CA300 decreases to 54.0 MPa.Accordingly,the fatigue crack growth rate of CA300 is the lowest.The fatigue crack growth rate of CA300 decreases by 28.2%compared with that of T6 state sample,and that of CA600 and CA1800 samples decrease by 21.7%and 16.0%,respectively.The geometrically necessary dislocation density of the CA300 specimen is more uniform,and the fatigue crack is not only deflected at the grain boundary,but also deflected within the grain,compared to other samples.The fatigue crack path is more tortuous,the angleαbetween the actual activated slip surfaces of adjacent grains increases,the fatigue crack growth resistance is larger,and so the fatigue crack growth rate is the lowest.