Study On Corrosion Resistance Of AZ31 Magnesium Alloy CMT Welded Joint By Cryogenic Treatment

Light weight, excellent mechanical properties, environmental friendly make magnesium and its welding weldment one of widely-used material in aerospace,railway transportation and civil areas.However,the oxidation film of magnesium is highly vulnerable,can hardly protect the base,especially when the weldment are in a Cl—environment,corrosive pitting happens very quickly, which then expand to be a large area of corrosion,disabling the weld joint. The poor corrosion resistance of weld joint has become the bottleneck of the use of magnesium weldment.Therefore,research on the corrosion resistance of magnesium weld joint is significant. Deep cryogenic treatment will improve the microstructure of material.This dissertation will research the use of deep cryogenic treatment to improve the microstructure of AZ31 CMT joint weld joint,to further improve corrosion resistance thereof.AZ31 Wrought magnesium alloy plate which after Hot rolled annealed state treatment as base metal, employing Cold Metal Transfer(CMT) as welding method, with the aim to find out the techniques of postweld treatment to enhance the corrosive resistance of AZ31 magnesium CMT weld joint,this dissertation is researching the corrosion behavior and pattern of the weld joint in the simulated marine environment( 3.5wt.%NaCl medium), through microscopical observation of OM, SEM, EDS X-ray diffraction, and chemical corrosion method like EIS, potentiodynamic polarization curve.The results shows that with the cryogenic temperature getting lower and cryogenic time getting longer,the joint inside produce amount of thermal stress and deformation energy,rebuilding the microstructure.The isometric crystal and columnar in the welded joint are uniform, β-Mg17Al12 particles are dispersively distribute in α-Mg base. different deep cryogenic parameter has different effect on the isometric crystal, columnar crystal and β-Mg17Al12 of the weld zone. Generally speaking, under the circumstance of this experiment, The β phase are tiny and disperse in microstructure after-140 ℃, 4h cryogenic treatment.According to the principle of material corrosion, the internal microstructure and surface characteristic affect the electrochemical corrosion characteristics of the material. Deep cryogenic treatment enhance the internal microstructure of the weld joint, serving the function of balanced electrode potential and enhanced the impedance of oxidation film, and thereby enhance to some extent the resistance to corrosion of AZ31 magnesium CMT joint under marine conditions.Using different deep cryogenic treatment,the increase of charge transfer impedance and film impedance, the increse of corrosion potential and the decrease of corrosion current density of AZ31 magnesium CMT joint in the simulated marine environment are corelated to combined action of refined microstructure and disperseed β phase. Compare to the non-cryogenic treatment joint, chargetransfer impedance of during-140℃,4h’s cryogenic joint increases 72.6%, film impedance increases 61.5%, corrosion potential increases 6.7%, pit potential increase 18.47% and the corrosioncurrent density decreases 8.03%,contributing the most to the corrosion resistance.