Investigation Of Fatigue Properties Of Friction Stir Welded Aluminium Alloys LY12CZ And LD10CS

Solid state Friction Stir Welding (FSW) is of major in the welding of aluminium since it improves the joint properties. Many applications where Al-alloys are used are subject to varying load conditions, making fatigue failure a critical issue. In the scope of this thesis, the fatigue performance of friction stir welded aerospace aluminium alloys, LY12CZ and LD10CS, has been investigated. Fatigue properties of different butt joints produced with varying process parameters and surface conditions have been determined. In addition, microstructural evolution and the internal defects of friction stir welds have been studied at the same time, and the bend test and Micro-hardness test have been conducted on friction stir welds. Finally, the S-N curves of each weld have been drawn out, and the fatigue characteristic values of each weld have been calculated by using a statistic method recommended by International Institute of Welding (IIW).FSW made a sound joint without any voids or distortion and the microstructure of the welded joint was formally divided into four zones: base material, heat affected zone (HAZ), thermo-mechanically affected zone (TMAZ) and weld nugget. There is an obvious micro-defect"kissing bond"at the root region of LY12CZ(1) welds the vertical length of which is about 0.3mm, while the LD10CS(1) welds contain another defect"lack of penetration"at the root region the vertical length of which is about 0.2mm. This indicates the weld condition (1) is not appropriate for welding aerospace aluminium alloys, but both of these defect could not be detected by X-ray radiographs method. During the bend test, all FS welds with different conditions fractured from the weld root.The fatigue test results indicate that the micro-defect"kissing bond"at the root region of LY12CZ(1) welds and the"lack of penetration"defect at the root region of LD10CS(1) welds make severe reduction in crack initiation life under fatigue loading and decreases the fatigue characteristic values of FS welds to a large degree. The fatigue characteristic values of FS welds produced with optimized parameters (3), (4) and (5) are 69.5MPa for LY12CZ and 77.1MPa for LD10CS, and the slope of S-N curves is 8.58 for LY12CZ and 6.70 for LD10CS, respectively. Although there were not any defects found in FS welds under optimized parameters (3), (4) and (5) by metallographic examination, the fatigue properties of FS joints in (3), (4) and (5) are much worse than those of sound welds on the reference documents. Therefore, it is...