| As one of the lightest structural alloys,magnesium alloys are increasingly widely applied in today's industry,and effective connection between Mg alloy and steel has great effects on the development and application of automotive lightweight.Corrosion property of Mg/steel joints can affect its industrial application seriously because of large corrosion resistance difference between Mg alloy and steel.It is necessary and urgent to study the corrosion behavior of Mg/steel joints,but few research on corrosion mechanism of Mg/steel joints was carried out.In this article,with adding AZ61 Mg filler metals or Ni interlayer,AZ31B Mg alloy and Q235 steel were butt welded by laser-arc hybrid welding,and joints with good formation and mechanical properties were obtained.Microstructure evolution and corrosion behavior of two kinds of Mg/steel butt joints were studied detailed in this project.Microstructure evolution of welding seam and Mg/steel interface was observed,microstructure evolution rules were studied,corrosion behavior of different zones for the butt joints was tested,corrosion mechanism was explored,the factors affecting joints corrosion properties were discussed,and these efforts can provide theoretical basis to further research and application of dissimilar metal joints.For the joints with adding AZ61 filler wire,the weld seam is composed of?-Mg substrate and?-Mg17Al12 intermetallic compounds?IMCs?.An"interface-like"structure forms between blocky steel splatter and its adjacent Mg substrate.Among the joints,welding seam around Mg/steel interface and around"interface-like structure"owns the worst corrosion resistance,with largest corrosion current density--2.821×10-4 A·cm-2.It is large potential difference between weld substrate and steel base metal that deteriorates the corrosion property of welding seam,however,clustered splatter shows less effect on that of welding seam.Micro galvanic corrosion occurs between?-Mg substrate and?-Mg17Al12 IMCs,which shows pitting corrosion morphology macroscopically.The enrichment of Al,Mn elements at the Mg/steel interface enforces the corrosion resistance of interface.The welding seam of Mg/steel laser-arc single butt welding joints with adding Ni interlayer is composed of?-Mg substrate and AlNi IMCs.At the Mg/steel interface,there is FeNi solid solution on the top half part,while?-Mg+Mg2Ni eutectic structure with low melting point forms on the bottom half part.The amount of steel splatter in the welding seam is large and these splatter distributes intensively near Mg/steel interface.The grain size of heat affected zone?HAZ?for Mg alloy is larger than that of AZ31B base metal,and the corrosion resistance is worse than that of parent metal.Corrosion occurs easier at partial melting zone rather than its adjacent alloys?HAZ and welding seam?.Welding seam around Mg/steel interface owns the worst corrosion resistance,with lowest corrosion current density(2.549×10-4 A·cm-2).Around the Mg/steel interface,?-Mg+Mg2Ni eutectic structure owns weak corrosion resistance,while it has little deteriorating effect on the corrosion property of adjacent welding seam.Large potential difference between FeNi phase and welding seam substrate aggravates the corrosion of the seam.With the increase of distance from Mg/steel interface,the amount of steel splatter and AlNi phase distributing in the welding seam gradually decreases,and the corrosion of the seam was slowed down.Potential difference between FeNi and welding seam with Ni interlayer is larger than that between Q235 and welding seam with adding AZ61 welding wire,and eutectic structure owns inferior corrosion resistance,so the corrosion properties of Mg/steel interface with adding AZ61filler metal is superior.Blocky steel splatter and IMCs in the welding seam deteriorate the corrosion resistance of welding seam. |
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