Research On Microstructure,Mechanical Properties And Flow Characteristics Of Friction Stir Additive Manufacturing For Aluminum-Lithium Alloy

Up to now,additive manufacturing?AM?technology for metal material is mainly based on the principles of fusion welding processes.Due to its principle,it has limitations especially in the fabrication of light alloys such as aluminum alloy.Friction stir additive manufacturing?FSAM?,as one of the innovative solid-state AM technologies,has great potential to additive manufacture light alloy.The FSAM experiment was conducted with 2 mm thick sheets of 2195-T8 Al-Li alloy,and the selection of tools,process optimization,interfacial defects,and material flow characteristics were mainly investigated in the paper.The results show that there is no obvious mixing of the materials across the interface of the builds manufactured by the cylindrical pin and the conical pin with three flats.However,the Trivex^TM pin and the pin with three concave arc grooves are good for mixing of the materials across the interface and reducing hooking defects.Dense and defect-free metallurgical connection occurs at the interface on the advancing side?AS?of the nugget zone?NZ?.However,the materials across the interface on the retreating side?RS?are not sufficiently mixed,hence the hooking defect on the RS is easy to extend into the NZ,and the weak-bonding defect is originated from the hooking defect on the RS.It is because the material flow on the AS is more intense than that on the RS.The temporary cavity formed after the material on the AS moving promotes the vertical flow of the material,and the material in the NZ always flows from the RS to the AS,which leads to the asymmetric interface characteristics.The back and forth double passes welding is one of the effective methods to eliminate the hooking defects extending into the NZ,improve the weak-bonding defect and increase the NZ size.However,this method reduces manufacturing efficiency and increases manufacturing costs.The method that the welding directions of two adjacent lapped layers are opposite increases the efficiency of multilayered manufacturing,and improves the interfacial defects in other layers except the top layer.The faster the tool rotation speed,the greater the deformation degree of the AS interface.There are cavities in the five-layered build with high tool rotation speed.When the tool rotation speed is 800 rpm and the welding speed is 100 mm/min,the five-layered build has the best formation.In the five-layered build,compared with the top layer,the grain size in the NZ of the fourth layer is smaller,but there are white strip structures.The grain coalescence and growth in the NZ near the bottom plate further soften the material.The weak-bonding defect on the RS is the weakest area of the mechanical properties,and the cracks always originate from it in the tensile process.The layer which is closer to the top of the build has the poorer toughness and the lower tensile strength due to the worse improvement of the weak-bonding defect.