Research On Stir Friction Welding Of TiB₂ Particle-reinforced Aluminum Matrix Composites And Weld Metal Migration Behavior

In-situ TiB₂ particle-reinforced aluminum matrix composites have the advantages of high mechanical properties,good processing performance,low process cost and easy preparation,and have become popular materials in many fields such as aerospace,rail transportation,military industry and shipbuilding.The study of stir friction welding of this type of material,through a combination of analytical tests and computer numerical simulations,reveals the intrinsic connection between tissue evolution and welding process,which can lay a certain theoretical and technical foundation for the application of in situ autogenous TiB₂ particle-reinforced aluminum matrix composites for joining.This paper investigates the stir friction welding behavior of in situ autogenous TiB₂/7050 aluminum matrix composites in the rolled state.The specific research is divided into three parts:（1）the study of the effect of stir friction welding process parameters on the weld formation,microstructure and mechanical properties;（2）the numerical analysis of the migration behavior of the weld metal using hydrodynamic analysis software;（3）the study of different stir head shape and welding process of the weld plastic metal flow behavior change law.Stir friction welding process results show that:within the scope of this thesis experimental research,when the rotation speed is too fast,the heat input is large,making the weld deformation increases,but the plastic metal flow stability at the weld becomes poor,resulting in a reduction in the surface finish of the weld.When the amount of downward pressure is too large,the plastic metal will overflow and produce flying edge defects.Analysis of the microstructure and mechanical properties after welding shows that:stir friction welding makes the weld tissue refinement,grain size reduction,TiB₂ particles in the weld nucleus no longer agglomerate and present a uniform distribution;T6 heat treatment after the joint grain size increased significantly,grain boundaries are more obvious,TiB₂ particle distribution did not change significantly.The hardness of the weld core zone is the highest in the weld state,the hardness of the hot machine affected zone is higher than that of the heat affected zone,and the hardness of the base material zone is the lowest.The hardness of the weld decreases sequentially from the top to the bottom.After heat treatment,the overall hardness of the weld increases compared to the weld state,and the hardness of the entire weld is uniform.Weld state tensile specimens are broken in the base material,the average tensile strength of 241MPa,indicating that the strength of the weld is higher than the base material;heat treatment after the base material tensile strength increased significantly,the weld tensile strength is lower than the base material,the average tensile strength of the weld is 578MPa,85%of the strength of the base material after heat treatment.Numerical simulation analysis results show that:in the horizontal direction where the end face of the shoulder is located,the highest flow rate of plastic metal on the outside of the shoulder,in the direction of the thickness of the weld,the region closer to the shoulder,the flow rate of plastic metal is larger and the flow range is wider;the flow rate of plastic metal in the region farther away is lower and the flow range is smaller;plastic flow metal diverges in the region near the forward side in front of the stirring head,and the forward side plastic The flow of plastic metal is better on the forward side than on the backward side.Shoulder shape is different,the plastic metal flow rate under the plane shoulder is higher,but taking into account factors such as increased welding heat input and avoid welding defects,concentric circular shoulder is more favorable;stirring needle shape is different,the metal flow range is basically the same,but the stirring needle plastic metal flow in the thread cut out of three planes is the best;when the stirring head rotation speed and the amount of pressure increases,the overall flow rate of plastic metal accelerates,the welding speed increases on the overall plastic metal flow is less affected.Part of the numerical simulation work can provide some practical reference and guidance for the subsequent optimization of the welding process and the selection of the stirring head.