Study On Friction Stir Welding Of In-situ TiB₂/7050 Aluminum Matrix Composite

In-situ particle reinforced aluminum matrix composites have the advantages of good interface bonding between reinforced phase and matrix,fine particle size and clean interface,and have high mechanical property and good processability.They are widely used in aerospace,automobile manufacturing and other fields.Among them,In-situ TiB₂/7050 aluminum matrix composites have outstanding mechanical properties,including high specific modulus and specific strength,good fatigue resistance and high temperature resistance,etc,which is a promising engineering application material.However,in-situ TiB₂/7050 aluminum matrix composites are difficult to weld,such as aluminum alloy matrix is prone to oxidation during welding,joints will appear softening zone,and the physical and chemical property of TiB₂particles and aluminum alloy matrix are quite different Lead to welding defects.Therefore,it is a key problem to further expand the application field of the composite material to select the appropriate welding method,explore the optimal welding process and improve the strength of the welded joint.Aiming at the welding difficulties of particle reinforced aluminum matrix composites,the welding research of in-situ self-generated TiB₂/7050 aluminum matrix composites is carried out by friction stir welding(1)friction stir welding process exploration;(2)single-sided friction stir welding;(3)double-sided friction stir welding.The results show that the small diameter of the shaft shoulder and the concave shaft shoulder with the diameter of 3:1 are more suitable for friction stir welding of the composite.When the welding parameters are certain,the small diameter mixing tool can make the welding heat input more concentrated.When the rotating speed of the mixing needle is 2500?3000 r/min,the welding speed is 500?700 mm/min,and the pressure between the axes is 0.4 mm,the weld surface of the composite is well formed,the microstructure of the welding core is fine,and there is no obvious welding defect.After friction stir welding,the TiB₂ reinforced phase in the raw material was dispersed,individual particles were broken,and distributed evenly in the weld.With the increase of welding heat input,the degree of plastic deformation and recrystallization of weld metal increases,and the weld microstructure is significantly refined.However,the resistance of weld zone to chloride corrosion is worse than that of non-welded composite.The results of single-sided friction stir welding show that with the increase of the rotation speed of the mixing tool,the heat input in the welding zone increases,the plastic flow of the weld metal increases,the arc spacing of the weld surface decreases,and the forming quality of the weld is improved.When the rotating speed of the mixing tool is 2200?2500 r/min,the welding speed is 700 mm/min,the axial shoulder pressure is 0.4 mm,the microstructure of the base alloy can be refined,and the TiB₂ particle dispersion is uniform.The hardness of the weld core zone is lower than that of unwelded composite and weld forward and backward.When the heat input is small,the hardness is mainly affected by fine crystal strengthening and particle strengthening,and the hardness of the forward side is higher than that of the backward side.The hardness of the forward side is slightly smaller than that of the backward side.The maximum tensile strength of single side welded weld is 513.0MPa,which is 75.4%of the unwelded composite.The joint breaks in the weld zone,showing brittle cleavage fracture characteristics,and the plastic dimple structure can also be observed in the fracture microzone.The mechanical property of the weld are mainly attributed to the fine grain strengthening of aluminum alloy matrix and the Orowan strengthening of TiB₂ particles.The results of double-sided friction stir welding show that the morphology of the joint is positive and negative symmetrical"hourglass shape".Nanometer ultrafine crystal structure is obtained by plastic deformation and dynamic recrystallization in the weld core area;only partial dynamic recovery occurs in the heat engine affected area,and subgranular appears in the large grain;because of insufficient experimental design,The front and back welds failed to lap,and only some plastic deformation occurred in the joint area.From the base metal,the forward side,the core zone,the backward side,and then to the base metal,the microhardness of the weld appears"w"shape,and the hardness of the whole joint core zone is the lowest.When the rotating speed of the mixing needle is 3000 r/min,the welding speed is 700 mm/min,the shoulder pressure is 0.4 mm,the maximum tensile strength of the double-sided welded joint is 469.0 MPa,which is 69%of the unwelded composite.In this paper,the failure of double-sided welded joints to realize the uneffective lap is the main reason that the maximum tensile strength of the joints is less than that of single-sided welded joints.