A Study On Ultrasonic Testing Technology Of Friction Stir Welded Joints Of 2A70 Aluminum Alloy

FSW-3LM-006 type friction stir welding equipment was employed to produce a series of T-type FSW joints of 2A70 aluminum alloy under the optimum welding processing conditions. The optical microscope, scanning electron microscope, and the universal tensile testing apparatus were used to observe the morphology, microstructure of typical weld defects and to evaluate the mechanical properties of T-type FSW joints. Different ultrasonic testing apparatus in combination with ultrasonic C-scan imaging system and corresponding analysis softwares were applied to quantitatively characterize the types and locations of defects at the FSW joints of 2A70 aluminum alloy.In combination of formation process of FSW joints with the results of metallographic observations and evaluation of mechanical properties, the weld joints consist of nugget zone, thermomechanically affected zone and heat affected zone, corresponding to the heat history under the stirring action of rotating FSW tool. Five typical joints defects observed in the welds include lamination defects, kissing bond, tunnels/pores, inclusions and abnormal growth of grains. The formation mechanisms and the corresponding precaution methods of different joints defects are also proposed to further improve the propeties of FSW joints.Based on theoretical analysis and experimental study, different methods such as direct contact process of vertical wave probe, water-immersion focusing method, the ultrasonic C-scan imaging and direct contact of horizontal wave declining probe are used to evaluate the ability of ultrasonic testing to FSW joints defects. The technological parameters of UT testing plan, selectively configured supersonic instrumentation, ultrasonic C-scan imaging examination system, ultrasonic probe, and testing medium are further determined. Four kinds of reference blocks are carefully prepared to cater the needs of system debugging and product detection. By means of determination and analysis of the weld acoustics propagation characteristics, the UT correlation parameters, including the testing side, scan area, scan way, sampling space of ultrasonic C-scan image examination, pulse repeat frequency, scan speed and testing sensitivity are ascertained.In combination with the results of metallographic analysis and XRD nondestructive testing, we characterize the reflexive wave curves of defect-free joints, kissing bond, lamination defects, incomplete penetration (or incomplete fusion) defect, crack-like defects, inclusions/pores, etc. to obtain a better understanding on the relationship between the ultrasonic testing spectra and typical defects characterization.Based on the analysis of static and dynamic characterization of defect reflexive waves, the time-frequency features and wavelet analysis parameters of various defects are extracted to obtain the qualitative, quantitative and positioning methods for defect ultrasonic echo. By selecting theф1.2mm diameter pore defects as a typical case, the frequency range and the wavelet analysis parameters of different defects are extracted and filtered to determine the positions of defects, which provides data support for automatic distinguishing of weld joint defects by using ultrasonic testing technology.