The Fatigue Failure Mechanism Research On The TA1 Titanium Alloy Clinched Joint

With the continuous development of modern science and technology,the lightweight structure has become the main direction of development because the research on various new materials,new technologies and new techniques has received universal attention.Research and develop the new lightweight high-strength materials has become the most effective way to ensure the needs of lightweight structure under the premise of ensuring structural strength.As a new material with high strength,good corrosion resistance and high temperature resistance,titanium alloy has been widely used in the automobile,aerospace and defense industries in recent years.However,it is hard to connect with dissimilar materials by traditional connect technology.With the development of technology and people's awareness of environmental protection,mechanical clinching has been valued and developed.It has been used in automobile manufacturing and other industries such as home appliances gradually.The research on the fretting wear mechanism of the clinched joints can fundamentally analyze the fatigue failure mechanism of clinched joints.In conclusion,realizing the effective connection of titanium alloy clinched joints and researching its fatigue performance and fretting wear mechanism would provide a strong theoretical significance and application value for its popularization and application in aviation and automotive fields.This paper intends to achieve the effective connection of titanium alloy sheets by mechanical clinching and to study the mechanical properties and fretting failure mechanism of the clinched joints.The specific researches are as follows:(1)In this paper,the connection process of the titanium alloy sheets by mechanical clinching was explored referring to the forming mechanism of clinched joints and the mathematical model of the mechanical clinching.The titanium alloys were connected to similar and dissimilar sheet by mechanical clinching through the joint cross-sectional joint quality evaluation parameters.Then the connection process parameters were obtained.(2)The tensile-shear static experiments were performed on the prepared clinched joints.The results showed that the titanium-titanium clinched joints had the highest static strength and energy absorption capacity which represents good load carrying capacity and energy absorption capability.In addition,the static strength of the titanium/galvanized steel joints were secondly,the titanium/copper joints were the last.(3)The fatigue tests were carried out to the titanium/galvanized steel joints and the titanium/copper joints to obtain the F-lgN curves.Among them,the titanium-galvanized steel clinched joints have the best fatigue performance.The fatigue life and strength of titanium/galvanized steel joints are better than that of titanium/copper joints.It can be seen that the fretting wear debris is a necessary and insufficient condition for cracking during the fatigue of the clinched joints through the SEM analysis of the typical fractures of the four different kinds of joints.Results showed that the wear debris were severely oxidation.Therefore,the fretting wear can be alleviated by reducing the surface roughness of the sheets,or plating and oiling can be adopted to the sheets in order to avoid the oxidation of the clinched joint as much as possible.Thus,the wear and fracture of the specimens could be delayed.(4)The tensile-shear and tensile fatigue tests were conducted to the titanium alloy clinched joints,self-piercing riveted joints and spot-welded joints.Results showed that the spot-welded joints have the highest tensile-shear strength but poor fatigue performance.The tensile-shear strengths of clinched joints and self-piercing riveted joints were close,but the fatigue life of clinched joints under high fatigue loads is more advantageous than that of self-piercing riveted joints.With two million as the fatigue limit,the fatigue strength of the three kinds of joints arranged from highest to lowest were: the clinched joints were the highest,the self-piercing riveted joints were the second and the spot welding joints were the lowest.(5)This article defines two types of fretting wear areas of the clinched joints: neck fretting wear and indentation-surrounding fretting wear.The results showed that the proportions of the two kinds of fretting wear is inversely proportional.With the decrease of the applied load,the angle of neck fretting wear is getting smaller and smaller while the angle of indentation-surrounding fretting wear is getting larger and larger.Fretting fatigue cracks tend to occur on the more severely worn side.Fatigue cracks tend to occur at the junction of the two wear areas when the two wear conditions are equivalent.The fatigue fracture positions of the clinched joint were changed with the load level.As the applied load decreases,the proportion of the neck fretting wear decreases while the fatigue life increases.Therefore,the fatigue life of clinched joints can be improved by reducing the proportion of the neck fretting wear in practical applications.(6)The annealing treatment were carried out to the clinched joints in titanium alloy.The metallographic experimental results showed that the size and shape of the specimens were more stable due to the annealing treatment which refined the microstructure and reduced the residual stress of the titanium alloy.In addition,the annealing treatment slightly increased the microhardness of titanium alloy of the clinched joints.The tensile-shear test results showed that the load-bearing capacity of the annealed titanium alloy clinched joints was slightly reduced while the energy absorption capacity was increased.The F-lgN curves of titanium alloy clinched joints before and after annealing treatment were obtained from fatigue tests.The results showed that the fatigue life of the annealed joints was longer at higher loads.But the fatigue strength of the untreated joints was higher than that of the annealed joints.In a word,the annealing treatment could improve the plasticity and toughness of the titanium alloy clinched joints.Then the material's deformability and energy absorption capacity could be improved.However,at the same time it would reduce the dislocation density of the titanium alloy metal crystals and decrease the tensile-shear strength as well as the fatigue strength of the titanium alloy clinched joints.Therefore,whether to anneal the titanium clinched joints or not depends on the operating conditions in practical applications.