| In today's society,energy conservation and environmental protection have become the topic of common promotion among people all over the world.As an important representative in the field of energy conservation,lightweight materials such as aluminum alloys have attracted widespread attention.7075 aluminum alloy is a typical Al-Zn-Mg alloy,widely used in aerospace,high-speed train structural parts.Laser-arc hybrid welding is a hot spot for aluminum alloy welding in recent years.This welding method absorbs the advantages of both laser and arc welding,both the high energy density of the laser and the high heat input of the arc.In addition,through the interaction of the two,the coupling characteristics of the laser energy and the stability of the arc are improved to obtain a comprehensive effect.In this paper,laser-arc hybrid welding is performed on 2mm thick 7076-T6 aluminum alloy.The microstructure,microhardness,tensile properties and fatigue properties of welded joints were studied.Then it analyzes the fatigue crack initiation and propagation mechanism of welded joints,which has certain theoretical and practical significance.The weld center metal is mainly melted from the ER5356 filler material to form a coarse equiaxed as-cast structure.The weld edge has sufficient heat dissipation and a fast cooling rate.The molten metal grows directly on the wall to form a columnar crystal arranged along the heat dissipation direction at the edge of the weld,and is vertically distributed along the isotherm until it extends to the center of the weld.Affected by the welding heat cycle,the heat affected zone is sequentially solid solution and overaged.The grains in the solid solution zone grow under the action of thermal cycling,and some of the strengthening phase particles dissolve and are unevenly distributed.In the aging zone,the strengthening of the phase is insufficient,and the undissolved strengthening phase is also aggregated and grown due to the influence of the welding heat process.The tensile strength,yield strength,and elongation after fracture of the welded joint were 355 MPa,240 MPa,and 2.35%,respectively.The weld is the weakest area of the entire welded joint.The fracture feature exhibits a mixed fracture of brittle fracture and ductile fracture.The fatigue limit of the welded joint is 100 MPa.After the sample undergoes multiple low-stress and high-cycle fatigues,multiple slip systems alternately move,forming intrusion and extrusion on the surface,resulting in rough surface roughness.The initiation of fatigue cracks in welded joints is mainly caused by welded pores.Under fatigue loading,the three sides of the air hole are subjected to radial normal stress,circumferential normal stress and circumferential shear stress.At the boundary of the circular hole perpendicular to the direction of the stress,the hoop normal stress is three times the applied stress,and therefore,the crack is more likely to sprout there.Fatigue cracks mainly expand in a transgranular manner,and are not a straight line but a snake-shaped distribution,which is a combination of tearing and shearing,and is mainly caused by tearing,which is the result of mixed loading of type I and type II.The forward expansion of the crack is caused by the passivation of the crack tip.Under the action of tensile stress,plastic passivation at the crack tip causes the crack to extend forward a distance,re-sharpening under the subsequent reverse stress,and passivation again in the subsequent process.The crack tip is subjected to high stress to cause local yield of the material to produce a plastic zone of a certain size. |
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