Studies On Corrosion Resisting Property Of Friction Stir Welded Joint Of Ultrafine-grain Aluminium Alloy

Friction stir welding is a revolutionary solid-state connection technology,which was proposed and applied for the patent by The Welding Institute in 1991.aluminum, magnesium alloy,copper alloy,titanium,steel,metal matrix composites and heterogeneous metallic materials can be connected by FSW,it has broad potential and foreground for industrial applications.Corrosion behavior of the FSW welding line is one of the extremely important indicators to assess its performance.Now the study on corrosion behavior of FSW has just started in our country,there is no international system to conduct in-depth study in the world.In this paper,I researched the FSW process on aluminum alloy materials which have been widely used in our current life,focusing on the corrosion performance and its influencing factors of the ultra-fine grain welding joints.The principal works in the thesis are as follows:In accordance with the characteristics of 2A11 aluminum alloy material,select the appropriate process parameters to eight times ECAP processing,refined grain;Carries on the friction stir welding on aluminum alloy material after ECAP processing,research its mechanical properties using metallography and scanning method;Researching influence factors of the joint's mechanical property,mainly the ratio of welding parameters,to get the optimum welding parameters;Researching the stress corrosion performance of welded joints,to assess its sensitivity to stress corrosion cracking.The results show that after 8th pass of EcAP processing,the internal organization of 2All aluminum alloy is equiaxed ultrafine-grain,with an average grain size of 200nm; Friction stir welding can achieve connection of ultrafine-grain 2A11 aluminum alloy, when the ratio of rotation speed and welding speed is between 30 to 40,the forming surface and mechanical properties of welded joints can be excellent;In SSRT test, elongation,tensile fracture time and fracture strength of friction stir welded Ultrafine-grained 2All aluminum alloy joints in 3.5%NaCl solution are lower than that in dry air,stress corrosion sensitivity parameter shows the joints have the stress corrosion sensitivity in 3.5%NaCl solution.Tensile strain rate,the polarization potential,solution pH value and the material microstructure work on the stress corrosion sensitivity of FSW joints;Anodic dissolution and hydrogen embrittlement are important factors leading to stress corrosion cracking,electrochemical methods can not apply to the stress corrosion damage protection of 2A11 aluminum alloy friction stir welded joint.