Effect Of Aging Treatment And Friction Stir Processing On Microstructure And Fatigue Properties Of 7050 Alloy

As a deformable high-strength aluminum alloy,7050 aluminum alloy has many advantages of the good fracture toughness and excellent corrosion resistance,which is widely applied in bearing structural parts such as the wing skin,fuselage frame and bulkhead in the aerospace field.With the continuous development of this field,the traditional brand aluminum alloys can’t fully satisfy the demand of industrial purpose any longer,so it’s necessary to improve them.A variety of aging heat treatment,friction stir processing（FSP）at different rotating speeds and its subsequent artificial aging（FSP-AA）process were mainly adopted to improve 7050 aluminum alloy in this paper,the analysis on effect of microstructural changes in each process on the alloy conductivity,mechanical properties,and fatigue crack propagation（FCP）was conducted with the detection methods of optical microscopy,scanning electron microscopy,and transmission electron microscopy.The research result showed that:（1）The fine GP zones andη’phases of the crystal made the alloy show good mechanical properties in the state of peak aging heat treatment（T6）.Compared with T6,higher density of the intragranular precipitation phases in the retrogression and re-aging（RRA）state,and the mechanical properties were further enhanced.The intragranular was mainly theη’phases with the larger size,and the yield strength of the alloy was a little higher than T6 of the C120 state of non-isothermal aging（NIA）.The poor mechanical properties of the alloy in the double stage aging（T74）state was caused by both intragranular thickη’phases andηphases.With the influence of the intracranial precipitation phases,the grain boundary precipitation phases,and the precipitate free zone（PFZ）,the T74>C120>RRA>T6 was shown in alloy conductivity.（2）FCP threshold(ΔK_th)of both T74 and C120 states was lower.T74 showed a higher FCP rate in the stable propagation stage;Theη’phases in the C120 state could inhibit crack propagation effectively,while theΔK goes up,the tendency of crack propagation along the grain boundary would increase.When theΔK was about 29 MPa·m^1/2,the da/dN value would exceed T6 in the C120 state,and the crack propagated forward at a faster speed.Cracks were more inclined to intragranular expansion in the RRA state,maintaining at a lower FCP rate.（3）After FSP deformation,the grain size of the alloy was fine and equiaxed.With the increase of the rotational speed of FSP,the grain size of the alloy increases slightly.In addition,the matrix supersaturation would also rise as the rotating speed went up,the mechanical properties of the alloy would improve gradually,but the conductivity would tend to drop.After artificial aging treatment,there was no significant change in grain size of the alloy.The high-density GP zones andη’phases re-precipitated in the matrix could improve the mechanical properties of FSP-AA samples significantly.Moreover,the density of the precipitation phases at a high rotating speed was higher,and the alloy mechanical properties would be better.As the lattice distortion was reduced,the alloy conductivity would be better.（4）TheΔK_th value of base metal（BM）was lower.The FSP samples at the same rotating speed was higher than FSP-AA samples.The FCP rate was higher for BM samples in the stable propagation stage.The FCP resistance of the alloy significantly increased after FSP deformation While the lower FCP resistance than the FSP samples was caused by the reduction of residual stress for the FSP-AA samples and the weakening influence of plasticity-induced crack closure.There was almost the same FCP rate for FSP or FSP-AA samples at each rotating speed in the stable propagation stage.