Research On The Eddy Current Testing Rules Of Lack Of Penetration In Friction Stir Welding

As a solid state joining technology, friction stir welding (FSW) technique is more and more widely applied to aviation and spaceflight, especially FSW of aluminum alloy was widely used. The improper welding parameters can lead inner discontinuities, such as lack of penetration (LOP) which has fatal effect on mechanical properties of the FSW joint. Compare with the conventional welding defects, LOP has these features, they are cling and minuteness, which bring us difficulties in nondestructive testing. Eddy current testing is mainly used in testing surface and near-surface, at the same time, compare with other testing methods, it is more suitable for testing LOP. This article mainly aimed at lack of penetration for LY12aluminum alloy with FSW, testing LOP by using the method of eddy current conductivity and eddy current impedance, study on the depth and width of LOP respectively versus the influence rule of detection signal, found the minimum LOP which by eddy impedance meter can test; in order to contrast with the effect of testing LOP by eddy current, then detect that LOP by using ultrasonic phased array and penetrant testing. The results show that:The eddy current conductivity distribution of various aluminum alloy weld of FSW is different, the conductivity distribution of LY12CZ states aluminum alloy weld showed M type; the conductivity distribution of LY12M and LF6M states aluminum alloy weld showed V type; when there is LOP in weld, the conductivity distribution of the nuclear area of weld showed V type, and the angle of V type decreased gradually with the increase of the depth of LOP. The conductivity decreased gradually with the increase of the depth of LOP and there exist a liner relationship between LOP depth and the conductivity; the rate of conductivity of the LOP at different depth in LY12CZ states aluminum alloy is similar to the defect of standard, and it is more than the rate of conductivity of the LOP center in LY12M states aluminum alloy, and the rate of conductivity of the LOP center in LY12aluminum alloy is more than the rate in LF6M states aluminum alloy. The conductivity meter cannot test the changes of width of LOP in Friction stir welding, because the changes is within10μm.When the depth of LOP is bigger than1.02mm, the component of resistance and reactance increased with the increase of the depth of LOP, and there exist a liner relationship between LOP depth and the component of resistance and reactance. When the frequency of testing from250KHz to500KHz, the lowest depth of LOP is0.5mm and the maximum width of the minimum LOP is3μm which be detected by eddy impedance meter, the LOP cannot be detected which depth is smaller than0.5mm.The LOP which depth is more than0.79mm can be detected better by ultrasonic phased array and penetrant testing, but there does not exist a liner relationship between depth of LOP and the testing signal. The detection sensitivity of LOP by eddy current testing is more than that by ultrasonic phased array and penetrant testing.