Structure And Corrosion Characteristics Of AZ31B Mg/6061Al Heterogeneous Alloy Friction Stir Weld

Friction stir welding(FSW)technology is used to weld 6061-T6 Al alloy and AZ31B magnesium alloy plates.With the help of optical microscope(OM),scanning electron microscope(SEM),energy spectrometer(EDS)and X-ray diffraction analyzer(XRD),the microstructure,element distribution and phase composition of heterogeneous welds were analyzed;the hardness distribution of heterogeneous welds and base metals was studied by using a microhardness tester;the electrochemical workstation was used to test the open circuit potential,self-corrosion potential,self-corrosion current density and impedance value of the heterogeneous weld zone and the base metal;the immersion corrosion test method was used to study the corrosion behavior of the weld zone and the base metal,and the corrosion morphology and corrosion products of the weld zone and the base metal were analyzed by SEM and EDS;the differences and reasons of corrosion resistance of weld zone and base metal were compared and analyzed.The main findings are as follows:1.The microstructure of Mg/Al heterogeneous FSW weld was observed and analyzed.The heterogeneous weld can be divided into the following areas according to the different microstructure characteristics:the thermo mechanical affect zone(TMAZ)and the heat-affected zone(HAZ)on Mg side,nugget area(NZ),TMAZ and HAZ on Al side.The HAZ of the weld is affected by heat during the welding process.The grains of the HAZ on the magnesium side are recrystallized and refined relative to the magnesium base material,while the grains of the HAZ on the aluminum side grow up relative to the aluminum base material.The TMAZ of the weld is also affected by force on the basis of heat.The grain size of the TMAZ on the magnesium side is further refined compared to the HAZ on the magnesium side,and the grains of the TMAZ on the aluminum side are gradually refined from the HAZ to the NZ.The NZ can be further divided into Al side and Mg side.The microstructure of Al side is more uniform,while that of Mg side is heterogeneous.The grain size of NZ is further refined than that of the TMAZ.The EDS test shows that the structural composition of most areas on the Mg side of the NZ is:?(Mg)+Al₁₂Mg₁₇,only a few areas are organized as Al₃Mg₂+Al₁₂Mg₁₇,and only?(Al)structure exists on the Al side of the NZ.The XRD test proved the production of IMC phases Al₃Mg₂and Al₁₂Mg₁₇in the weld.It is also due to the presence of the IMC phase on the magnesium side of the NZ,and the hardness distribution on the magnesium side of the NZ is significantly higher than the hardness of the base metal on both sides.2.The structural characteristics of different areas of Mg/Al hetero-alloy FSW welds at different welding speeds are compared and analyzed.With the decrease of welding speed,the grains of the HAZ on the magnesium side and the aluminum side have grown;the grain size of the TMAZ on the magnesium side has not changed significantly;and on the magnesium side of the weld nugget,more aluminum strip was observed.The increase in welding heat input caused by the decrease of welding speed leads to the grain refinement of the HAZ on both sides of the weld;and the decrease of the welding speed will also cause the stirring effect of the stirring head to increase,causing more aluminum alloys on aluminum side of the NZ to be stirred to the magnesium side;under the dual effects of heated growth and crushing refinement,the grain size of TMAZ on the magnesium side did not change significantly with the decrease of welding speed.EDS composition test and XRD phase test show that an IMC layer composed of Al₃Mg₂+Al₁₂Mg₁₇is formed on the Mg/Al interface in the weld NZ,and the size distribution of the thickness of the IMC at different parts of the Mg/Al interface is the largest in the middle,second in the upper part,and smallest in the lower part;as the welding speed decreases,the welding heat input increases,and the thickness of the IMC at the Mg/Al interface gradually increases.3.The electrochemical properties of Mg/Al heterogeneous FSW weld in different areas and base metal were measured and analyzed,and the variation trend of electrochemical properties of weld in different areas and base metal at different welding speeds was compared and analyzed.The measuring results of the potentiodynamic polarization curve show that the corrosion rate of each area of the weld and the base metal is:Al/NZ transitional area<Al base metal<NZ<Mg base metal<Mg/NZ transitional area;the impedance spectrum test results show that the impedance value distribution of each zone of the weld and the base metal is:Al/NZ transitional area>Al base metal>Mg base metal>Mg/NZ transitional area>NZ.As the welding speed decreases,there are more Al alloys and IMC on the Mg side of the weld,the corrosion resistance of the NZ area gradually increases,the corrosion resistance of the Mg/NZ transitional area gradually weakens,and the Al/NZ transitional area is due to passivation occurs at medium and high welding speeds,and the corrosion resistance first increases and then weakens.4.The immersion corrosion behavior of Mg/Al heterogeneous FSW weld in different areas and base metal was measured and analyzed,and the corrosion resistance of weld and base metal was compared and analyzed.The immersion corrosion results show that the corrosion resistance of each zone of the weld and the base metal is distributed as follows:before immersing for 1 hour,the Mg side HAZ>weld nugget Al side>Al base metal?Al side HAZ>weld nugget Mg side>Mg Base material:After immersing for 1h(including 1h),the weld nugget Al side>Al base material?Al side HAZ>weld nugget Mg side>Mg side HAZ>Mg base material.EDS analysis shows that the corrosion products in each area of the weld are mainly Mg O/Mg(OH)₂and Al₂O₃/Al(OH)₃.5.The corrosion resistance mechanism of different areas of Mg/Al heterogeneous FSW weld and base metal was analyzed.Since the grain size of the weld nugget area in the weld is smaller than the base material area,the corrosion resistance of the Mg side of the weld nugget is improved relative to the Mg base metal,and the Mg base metal has the worst corrosion resistance;compared with the Mg base metal,the grain size of the HZA on the Mg side is refined,and its corrosion resistance is better than that on the Mg base metal,but lower than that on the Mg side of the nugget.However,in the early stage of immersion corrosion,due to the protection of the Mg base metal,the corrosion degree of the HAZ on the Mg side is the lightest in all areas of the weld.Since the Al base material and the HAZ on the Al side are at the cathode position in the immersion corrosion of the weld,the corrosion degree of the Al base material and the HAZ on the Al side is lighter than that of the Mg base material and the Mg side of the weld nugget.There is no obvious difference during the immersion corrosion.Because the grains on the Al side of the weld nugget are refined relative to the Al base material,passivation occurs during the immersion corrosion,and its corrosion resistance is improved compared to the Al base material.