Effect Of Thermal Cycling On Corrosion Behavior Of A7N01 Aluminum Alloy With Different Content Of Mn And Cr Elements

A7N01S-T5 aluminum alloy which is a kind of heat-treated aluminum alloy has benne widely used on high speed train body.Trace elements play an important role in the properties of aluminum alloys,the corrosion properties can be greatly improved by addition of Mn and Cr elements.During welding process,the weld heat input affect microstructure,mechanical properties and corrosion resistance on aluminum alloy.In this paper,the influence of Mn,Cr content and weld heat cycle on properties of aluminum alloy has been studied.Two kinds of A7N01S-T5 aluminum alloy with different Mn and Cr content were employe to study their changes on mechanical properties and corrosion properties after thermal cycling at different peak temperature by weld thermal simulation technology.On the basis of previous research in our group,the A7N01S-T5 aluminum alloy H with high content of Mn and Cr,and Lwith low content of Mn and Cr were selected.According to actual weld process,four kinds of thermal cycling parameters were established form thermal simulation experiments,eight kinds of specimen H1,H2,H3,H4 and L1,L2,L3,L4 were obtained?Among them1,2,3,4 were stand for four thermal cycling curves with four peak temperature 458?,415?,360?,260?respectively?.The results showed that the tensile strength and yield strength of four H sampleswere higher than those of the four L samples after thermal cycling whichsamples has no change compared with H and L base metal.The difference of tensile strength and yield strength between L3 and H3 is H3 larger than L3.TEM results showed that the content of?phase of on grain boundary of L3was low,and its PFZ was wide,the grain boundarywas clean.There were precipitated phases in the grains and around the grain boundaries of H3,the existence of these phases made the material more easily to crack,which was harmful not to mechanical properties ofut the materials.The TEM results of the six materials H1,H2,H4 and L1,L2,L4 were opposite to H3and L3,thus the tensile strength and yield strength are opposite too.The results of dynamic potential scanning showed that open circuit potential of L was lower than that of H,corrosion current density of Lwais higher than that of H,Cyclic polarization hysteresis ring of H was smaller than that of L,values of capacitive arc resistance,pitting resistance and transfer resistance ofHwerehigher than those of L,which showed that the corrosion resistance of H was superior to that of L.The main reason was that Mn,Cr existed in the form of intermetallic compound which contained impurity elements.These compounds could reduce the content of impurity elements and improved the corrosion resistance of the alloy.After different thermal cycles,open circuit potential of H3was lower than that of L3,Corrosion current density of H3 was higher than that of L3,hysteresis ring of H3 was larger than that of L3,Electrochemical impedence spectroscopy values of capacitive impedance,pitting resistance and transfer resistance of L3were higher than those of H3,Electrochemical test results show that the corrosion resistance of L3 is better than that of H3.However the corrosion resistance of samples which experienced parameter1,2,4 was contrary to the sample which experienced 3.That was,corrosion resistance of L1 was weaker than that of H1,L2 wass weaker than that of H2,and L4 was weaker than that of H4.The recrystallization occurred in the process of high temperature thermal cycling with parameter 1 and 2,when the temperature continues to rise to 440?,it was advantaged to refine the size of grains.Mn and Cr could inhibit grain refinement to prevent recrystallization,the fine grains were harmful to corrosion resistance about the aluminum alloy.Therefore,the corrosion resistance of H was better than that of L.The content of?phase to L3 on grain boundary was low,the PFZ was wide,and there were precipitated phases in grains and around the grain boundaries of H3.The potential of these precipitates were higher than that of Al substrate.The aluminum substrate with low potential dissolved preferentially to cause the corrosion channel which made corrosion ocurr.So the corrosion resistance of H3 was weaker than that of L3.There were intermittent precipitates on the grain boundaries of L4,anddispersive phase in the crystal with the size of100nm²00nm,while the distribution of precipitates in H4 was similar to L3,so the corrosion resistance of H4 was better than that of L4.Electrochemical noise technique can accurately respond the occurrence about pitting corrosion on the materials surface,the destruction and repair process of passivation film.The results of time domain spectra showed that the trend of noise potential to base metal and thermal simulated specimen were unchanged,the pit corrosion resistance of aluminum alloy was the same as dynamic potential scanning results.The results to frequency spectrum analysis showed that the pitting corrosion resistance after thermal cycling with different parameters was consistent with time-domain spectrum analysis after immersion in solution NaCl-H₂O₂ for 96 h.The content of Mn and Cr and the weld thermal cycle have an interactive effect on the corrosion resistance to A7N01S-T5 aluminum alloy.On different welding input condition,the effect of acontent of Mn,Cr elements to the corrosion properties of AN01S-T5 aluminum alloy.At the peak temperature of 360?,the content of Mn-Cr elementhas the greatest influence on A7N01S-T5 aluminum alloy.