Investigation And Characterition On Top Surface Microstructure Of Friction Stir Welded Joint For High Strength Aluminium Alloys

The microstructures and properties on the top surface of FSW joint were characterized by optical microscopy and the Vickers hardness tester. SEM and TEM were employed to investigate the dimension,types and distributing of precipitate particles in different regions of FSW joint. The behavior of dissolution and precipitation of precipitate particles of FSW joint was investigated by combining TEM and DSC.The results of metallographic observation show that the nugget zone happened to dynamically recrystallize and the refined crystal grains were obtained and the distributing of precipitate particles are heterogeneous; The cladding layer of pure aluminium is destroyed during FSW, and the pure Al is enriched in retreating side. The results of SEM and TEM reveal that the main precipitates of AA2524 in the FSW joint areθphase (Al2Cu),S phase (Al2CuMg),a spot of Al6Mn and Fe-containing phase. The main precipitates of AA7050 in the FSW joint isηphase (MgZn2).The results of DSC and TEM observation of the behavior of dissolution and precipitation of precipitate particles show that precipitate particles of all the regions in the FSW joint distribute along definite orientation in the AA2524. In the heat affected zone, the S phase are coarsened andθphase partly dissolved, and shows severely softening in this area. In TMAZ, precipitate particles are still elliptoid and club-shaped, but the density of these particles has decreased. Precipitation phases in the nugget are dissolved intosolution completely and are re-precipitated during the weld cooling process.Precipitation phase has been varied in different regions of FSW joint in the AA7050, coarseηprecipitate in HAZ. The precipitates in HAZ on retreating side are larger than the one on advancing side. There is minimum hardness value in the HAZ on retreating side, which shows severely softening in this area. In TMAZ, the strengthening precipitates phase are coarsened and partly dissolved. Precipitation phases in the nugget are dissolved into solution mostly. Precipitate particles that are continuous and coarse in grain boundaries of basic metal , heat affected zone and nugget zone. The width of precipitation free zone (PFZ) is about 70 nm. The grain boundary precipitates (GBP) distribute almost continuously, which can reduce corrosion and mechanical properties.