Investigation On Processing And Characterization Of Friction Stir Welding 7B52 Aluminum Laminated Armored Thick Plate

7B52 laminated armored aluminum alloy is a new type of laminated material with excellent anti-elasticity.Compared with other welding methods,friction stir welding has significant advantages.In this paper,FSW experiments of 7B52 laminated armored aluminum alloy plate with a thickness of 16 mm was studied.By analyzing the weld temperature characteristics and metal flow,metal powder was chosen to plant into weld in order to improve mechanical properties of welded joint.The microstructure and mechanical properties of the welded joints were investigated.Firstly,the friction stir welding process of 16 mm thick 7B52 laminated aluminum alloy was studied.The orthogonal test was designed.The relationship between thermal input and tensile strength of welded joint was combined to determine the welding parameters.The optimal welding parameters are that:stir tool rotating counterclockwise,which has a needle with a right-handed thread and three planes on its surface.The welding speed was 50 mm/min.The rotation speed was 300 r/min.Under this parameter,the tensile property of welded joint was 383 MPa,and the impact toughness was 13.8 J/cm².By testing tensile strength of each layer in the thickness of welded joint,the lowest strength was found to be located in the first layer,valued 353.6 MPa.Then,the metal flow of welding process was studied.Two onion rings were observed in Nugget Zone（NZ）.It was proved to be produced by two metal flow cycles alongside the thickness of weld,which was related to the geometry of stir pin.The laminated structure on the retreating side of NZ is relatively complete.While that was destroyed in advancing side of NZ.By analyzing the metal flow in thickness direction,horizontal direction and welding direction,a metal flow model for stabilization stage was established.In order to improve tensile strength of the first sliced layer,Cu and Ti were chosen as planted powder.Combined the tested temperature and metal flow,the bonding mechanism between planted powder and matrix was studied.Optimized size of planting hole was:1 mm in depth,3 mm in diameter and 6 mm in center distance between two adjacent holes.Under the thermal-mechanical effect of FSW,Cu combined well with base metal.Cu dispersed mainly in the surface layer of NZ with a diffusion layer of 5.4μm in thickness.Hardness phase such as Al₂Cu was detected.Finally,the mechanical properties of metal planted welded joints were studied.It was found that the mechanical properties of welded joint were improved by planting Cu powder with a grain size of 10μm.Its tensile strength reached 396 MPa and impact toughness was14.1 J/cm².The microhardness of metal planted area in NZ reached 90%of the base metal.With the increase of Cu size,the mechanical properties of welded joints decreased.Then the fracture mechanism of welded joint was studied.FSW destroyed original laminated structure,formed various soft and hard interface.Cracks initiated from those interface.When conducting impulse test,lots of energy was consumed when cracks run through hard layer,and growth orientation deflected when run through soft layer.This special structure improved tensile strength,resulted in a“crystalline-fibrous-crystalline”morphology on its impact fracture.