Microstructure And Properties Of Dissimilar Friction Stir Welded Joints Of 6061 Aluminum Alloy And T2 Copper

At present,friction stir welding has successfully solved many welding problems that are difficult to be completed by traditional welding methods,and has become the first choice for welding light metals such as aluminum and magnesium.Dissimilar metal welding is one of the research hotspots and difficulties in recent years.Aluminum-Copper dissimilar metal friction stir welding has been widely studied at home and abroad because of its huge potential industrial value.Through previous studies,we found that the stir tool wear can be reduced when materials thickness are different.So,the thickness of Al plate is 2.5mm and Cu is 2mm in this paper.The effect of process parameters on welding joint quality was studied.Results show that the influence of process parameters on welding is mainly reflected in the following two aspects.First,the process parameters affect the heat input in welding.High heat input will cause the welding tool to adhere to the metal,welding parts back adhesion;too low heat input will lead to insufficient fluidity of materials,forming welding defects.Second,the material flow is also influenced.The spindle speed directly affects the material flow in welding.Welding speed affects material flow by affecting heat input and thus plasticizing degree of material.When the spindle rotates 25 r/s,the welding speed is 40 mm/min and a 2.25 mm shoulder plunge depth,the best welding quality is got.Further research was carried.All the mechanical test were tested at 25?.TMAZ area hardness decreased,SZ area hardness is similar to the base metal area.So the distribution of hardness is"W"shape.Further microscopic observations were made on different regions.The fracture microstructure results show that the fracture mode is mixed fracture mode and copper particles exist at the bottom of the dimple.EDS scan shows that the thickness of Aluminum-Copper diffusion layer is about 2?m.Based on the Analysis of Holpech formula,it is concluded that the intermetallic compounds formed by copper particles and diffusion together affect the joint properties.The material flow was studied.The SZ area is divided into three areas:welding tool shoulder influence area,stir pin influence area and forging area.In the horizontal direction,the material flow is mainly affected by the inclination of the welding tool and the properties of the welding material.Compared with the back side,the flow velocity on the front side is higher.The flow velocity is influenced by the different Properties between Al and Cu.In the vertical direction,the material flow is mainly affected by the dip Angle of the stir pin,the spiral groove of the stirring pin and the rotation direction.The existence of inclination Angle causes the material to receive vertical force when flowing around the stirring needle.The spiral groove of stir pin leads to the increase of the vertical force,resulting in the strengthening of the vertical flow of material.The rotation direction of the welding tool and the spiral groove of the tool act together to affect the flow direction of the material in the vertical direction.The effect of temperature on joints was studied.The tensile properties of welded joints decrease with the increase of temperature,and the fracture mode changes to brittle fracture.The SZ microhardness increased,and the base metal hardness decreased.The thickness of diffusion layer increases gradually with the increase of temperature,intermetallic compounds are detected in welded joints.It is concluded that the increase of diffusion layer thickness leads to the transformation of fracture form.The form of intermetallic leads to the increase of hardness in the SZ region.The base metal suffers recovery under thermal conditions,which decreases the hardness.The relationship between the thickness of diffusion layer and the tensile properties was deduced.When the thickness of diffusion layer was less than 4?m,the tensile properties of the weldment decreased sharply with the increase of the diffusion layer.It was found that Al₂Cu was easily formed when the Cu to Al ratio was lower than 37%.Al₄Cu₉is easily formed when the Cu to Al ratio is higher than 65%.Al Cu is easily formed when the ratio of Cu to Al is between 37%and 55%.