Preparation And Corrosion Resistance Of Micro-arc Oxidation Coatings On Laser-MIG Hybrid Welded Joint Of Dissimilar Aluminum Alloys

Welding technology plays an important role in industrial production,but traditional welding methods have their own specific application fields and shortcomings.In order to eliminate or alleviate the defects caused by the welding process,the laser beam and arc are coupled into a process,which is named laser-arc hybrid welding.It can integrate the advantages of laser welding and arc welding,and avoid or reduce their shortcomings.However,the poor corrosion resistance of Al alloy welded joints limits the wide application of laser-arc hybrid welding.Therefore,it is of great significance to study the corrosion protection of welded joints.In this paper,laser-MIG hybrid welding of 6061/7075 dissimilar Al alloy was carried out using RC-YLS fiber laser and Fronius TPS5000 digital welding machine.Based on the previous experiments,the effects of different welded gaps on the microstructure,phase composition,microhardness and tensile properties of welded joints were investigated,and better welding parameters were obtained.After that,micro-arc oxidation coatings and graphene/micro-arc oxidation composite coatings were prepared on the surface of welded joints.The surface morphology,cross-section morphology,coating structure,phase composition and corrosion resistance of micro-arc oxidation coatings and graphene/micro-arc oxidation composite coatings were compared and studied.The effects of micro-arc oxidation coatings on corrosion performance was analyzed.The effects of micro-arc oxidation coatings on corrosion performance was analyzed.The effects and mechanisms of graphene addition on the corrosion resistance of micro-arc oxidation coatings were preliminarily discussed.The results are as follows:?1?The microstructures of laser-MIG Hybrid welded joints of dissimilar aluminium alloys were obviously different,mainly consisting of five zones:6061 base metal zone?6-BM?,6061 heat-affected zone?6-HAZ?,weld metal zone?WZ?,7075 heat-affected zone?7-HAZ?and 7075 base metal zone?7-BM?.Among them,the microhardness of HAZ of the welded joint was lower than the corresponding base metal,and the hardness order is:7-BM>7-HAZ>6-BM>WZ>6-HAZ.When the welding gap was 0.2 mm,the tensile strength and elongation of the welded joint were slightly higher,and the welded parts processed under this parameter had better mechanical properties.?2?The corrosion resistance of the welded joints showed that the corrosion resistance of the HAZ was lower than that of the corresponding base metal.Among them,the corrosion resistance of 7-HAZ was the worst.The corrosion resistance of WZ was better than that of 7-HAZ.The corrosion galvanic reactions were formed by different potential in different zones of welded joints.Among them,7-HAZ was corroded by galvanic corrosion due to the lowest potential,while its adjacent WZ and 6-BM were protected by cathodic protection.The micro-galvanic corrosion between the second phase and matrix also aggravated the corrosion of the welded joints.T?AlZnMgCu?phase was found to be corroded as an anode in 7-HAZ.Al-Si-Fe and Mg₂Si phases in 6-HAZ were considered to be cathodes that corrode the surrounding?-Al matrix.Mg₂Si phase which also existed in WZ caused pitting corrosion of the surrounding matrix,which resulted in irregular corrosion morphology.?3?After micro-arc oxidation treatment,the immersion corrosion sample had no obvious corrosion products,and the corrosion current density in each zone of the welded joints was reduced by two orders of magnitude compared with that of the samples without coatings.The corrosion potential of the coated samples moved forward and distributed more centrally than that of the welded joints before micro-arc oxidation treatment,and the difference of corrosion resistance between different zones decreased.Micro-arc oxidation treatment could effectively improve the corrosion resistance of laser-MIG hybrid welded joints of dissimilar aluminum alloys.?4?Comparing the structure and corrosion behavior of micro-arc oxidation coatings and graphene/micro-arc oxidation composite coatings,it was found that the main phases of the two coatings in different zones were?-Al₂O₃ and a small amount of mullite phase.Both kinds of micro-arc oxidation coatings were composed of a dense inner layer and a porous outer layer.The dense inner layer could isolate the contact between external corrosion medium and aluminium matrix,which played a major role in corrosion protection.The densification and uniformity of the composite coatings were improved by adding graphene.The corrosion current density of the composite coatings was further reduced and the corrosion potential distribution was more concentrated than that of the non-graphene coatings.The corrosion resistance of the graphene/micro-arc oxidation composite coatings was slightly higher than that of the micro-arc oxidation coatings.