Study On Fatigue Property Of AZ31 Magnesium Alloys Welding Joint And Improving Techniques

Magnesium alloy has a series of advantage and there are abundant inmagnesium in the world. It is important and significant to develop magnesiumalloy for sustainable development. Nowaday magnesium and processingmagnesium have been comprehensively reserached by boffins in many country.Generally, the magnesium alloy products are manufactured by casting ordie-casting, but they are small and simple. Because weld is one of mostimportant interconnection technique, the property of magnesium alloy weldingjoint must be critical factor that will determine application of magnesium alloylarge parts.It is reported that 70%～90% of welded structure failure due tofatigue crack in welding joints, so that, it is very important to study on fatigueproperty of magnesium alloy joint.In this thesis, AZ31 magnesium alloy plates with 8 mm thickness werewelded by TIG A.C. arc welding. The mechanical properties of magenesiumalloy joint were tested in order to know the fundamental characteristics of themagnesium alloy. The result indicated that the grains seriously grew up in HAZof joint, grain diameter became 15～80μm from 6～35μm. There is the lestimpact absorption work in the center of welding seam, which is only 30.5 J·cm^-2about 67% of base metal. The tensile strength of AZ31 is 285.2 MPa. Comparedwith base metal, the tensile strength of transverse cross joint (258.0 MPa) andlongitudinal cross joint (255.9 MPa) reduced about 10%.The fatigue properties of base metal, transverse cross joint and longitudinalcross joint were tested by high frequency fatigue machine, R=0.1 andf=97～99Hz. The S-N curves were drawn by least square fitting fatigue test data.Base on the S-N curves, the fatigue resistance (N=2×10⁶) of specimens wasgot: That of base metal is 36.0 MPa, that of transverse cross joint is 17.2 MPa and that of longitudinal cross joint is 20.0 MPa.Ultrasonic impact treatment (UIT) was applied in magnesium alloywelding joint in order to improving fatigue property of them. After toes of weldwere treated by UIT equipment of Tianjin University, the fatigue property ofwelding joint was improved in some degree, fatigue resistance of transversecross joint (2×10⁶) improves 24.7 MPa and fatigue resistance of longitudinalcross joint (2×10⁶) improves 30.3 MPa. The fatigue life of joint is 3 times thanthe specimen as welded.The fatigue fracture of magnesium alloy welding joints consist of densecleavage surface, sector pattern fracture and river pattern fracture. There are lotsof bigger or smaller secondary crack on the fatigue fracture, which shows thatfatigue failure of magnesium alloy joint is a sort of brittle quasi-cleavagefracture. The equiaxial grains of welding seam were turned into rolling tissue incertain thickness by Ultrasonic impact treatment, which led to improving fatigueproperty of welding joint, but fatigue crack came of weld line all the same.The local approach fatigue assessment was applied in AZ31 magnesiumalloy transverse cross joint without regard to residual stress, and the parameterswere amended toα′=0.62,β′=8.42 MPa, so that the assessment result wasconsistent with the test result, then the correctional parameters were used in thelocal approach fatigue assessment for AZ31 magnesium alloy longitudinal crossjoint. Compared assessment result with test result, it is found that the newparameters are amended further.