Corrosion Behavior Of Refilled Friction Stir Spot Welded Joint Between Aluminum Alloy And Steel

With the development of lightweight in automobile industry,more and more attention has been paid to the aluminum/steel composite structure,since replacing traditional steel structures with aluminum alloy can effectively reduce the weight of automobiles,save energy and reduce carbon dioxide emissions.Refill friction stir spot welding(Refill FSSW)is a solid-state welding method,which can well join aluminum alloy and steel.However,the joints made of different corrosion potential materials will face severe galvanic corrosion problems in electrolyte solution.Therefore,based on the application background of aluminum/steel composite structure and the advantages of Refill FSSW technology,this paper focuses on the corrosion behavior in different areas of aluminum/steel Refill FSSW joint,explores the influence mechanism of galvanized layer on the corrosion behavior of the joint.In this paper,the experimental materials were the aluminum alloy plate(Aleris Superlite200 ST aluminum alloy plate)and low carbon steel plate(ST16 steel plate and ST06 Z galvanized steel plate)specially used for automobile.First,the connection of aluminum alloy plate and steel plate was completed by Refill FSSW.Then the microstructure and corrosion morphology of aluminum/steel joint were characterized by means of scanning electron microscope(SEM),electron probe micro analyzer(EPMA),and laser scanning confocal microscope.Finally,the corrosion behavior and corrosion mechanism in different zones of aluminum/steel dissimilar metal welded joint were investigated by design immersion corrosion,in-situ observation and electrochemistry experiments.The influence mechanism of galvanized layer on the corrosion behavior of the joint was further explored.Based on the analysis of the microstructure of ST-aluminum alloy/ST06 Z galvanized steel joint,the cross section of the joint was divided into five different zones,i.e.,the pin-affected zone(PAZ),the sleeve-affected zone(SAZ),the thermal-mechanical affected zone(TMAZ),the heat-affected zone(HAZ)and the base metal(BM).The results of immersion corrosion experiment and electrochemical experiment showed that the sequence of the corrosion resistance in each region of the refilled friction stir welded joint between aluminum alloy and galvanized steel was TMAZ>BM>HAZ>SAZ>PAZ.Both SAZ and PAZ were severely corroded and became the weakest part of the entire joint.In order to further explore the corrosion mechanism of the seriously corroded Al-Zn mixed layer,the corrosion behaviors of two different joints(ST-Al/ST06 Z galvanized steel and ST-Al/ST16(uncoated)steel)were compared.Compared with ST-aluminum alloy/ST16 steel joint,the corrosion in ST-aluminum alloy/ST06 Z galvanized steel joint was more serious.Because in the process of welding,the galvanized layer on the surface of galvanized steel flowed to the nugget zone(NZ)with the agitation of the stirring needle and mixed with the plastic aluminum alloy,forming the Al-Zn mixed layer.Compared with Al-Fe-Si phase,aluminum alloy matrix and steel matrix,the potential of the mixed layer was the lowest.In the NZ of ST-Al/ST06 Z galvanized steel joint,the mixed layer was corroded as anode,and ST06 Z galvanized steel was protected as cathode in saline solution.The surface ratio between the anodic and cathodic sites was about 1:15,forming a severe corrosion system with small anode and large cathode.However,in the NZ of ST-Al/ST16 steel joint,ST-Al was corroded as anode,while ST16 steel was protected as cathode,and the surface ratio between the anodic and cathodic sites was about3:2.The galvanic corrosion degree is small.In addition,defects such as micro-holes and micro-cracks were found in the mixed layer,which led to pitting easily nucleated at these locations.The segregation of Zn and Mg elements with low corrosion potential at the grain boundary was found in the mixed layer,which resulted in serious intergranular corrosion.Therefore,the galvanized layer has a negative effect on the corrosion resistance of friction stir spot welded joints between aluminum alloy and galvanized steel.In addition,in-situ observation showed that two types of pitting corrosions seriously attacked the nugget zone of ST-aluminum alloy/ST06 Z galvanized steel joint,which were related to the destruction of the passive film by Cl~-and the local micro-galvanic couples formed by Al-Fe-Si precipitates with base metal,respectively.However,pitting corrosion in the nugget zone of ST-aluminum alloy/ST16 steel joint was only related to Cl~-,and the base metal around Al-Fe-Si precipitates did not dissolve,so the pitting corrosion degree was slight.Severe galvanic corrosion can promote pitting to nucleate around Al-Fe-Si phase,but the scope of galvanic corrosion is limited.