The Effect Of Micro-alloying On The Microstructure And Corrosion Properties Of 5A12 Aluminum Alloy

Al-Mg alloys are weldable wrought alloys with low density,good corrosion resistance and weldability,which are generally used in transport and ship structural applications.Owing to a lack of age-hardening response,5xxx alloy strength is usually improved through a combination of work hardening and solid solution strengthening.As the main alloying element in 5xxx alloys,Mg can appreciably improve the work-hardening abilities of solute Mg atoms.Our group has showed that the room temperature strength of 5A12(Mg content is 9.2wt.%)alloy sheet with optimized treatment can reach 648 MPa,however,alloys with Mg content above3.5wt.%tend to be susceptible to stress corrosion cracking(SCC)and intergranular corrosion(IGC)when exposed to moderately elevated temperatures(60-180°C).The latter effect can be mainly ascribed to?phase formation(Al₃Mg₂)along the grain boundaries.The precipitation of the?phase has a small impact on the mechanical properties of Al-Mg alloys,as no precipitation hardening effect is involved.It has become the key problem to limit the engineering application of 5A12 aluminum alloy.At present,the systematic effects of element addition on the microstructure and comprehensive properties of Al-Mg alloys,in particular,superhigh-Mg-content 5xxx alloys have scarcely been investigated.Super-high-Mg-content 5xxx alloys exhibit excellent mechanical properties,though they are subject to greater?phase precipitation from the?-Al matrix.In the present paper,the effects of Ce,Y and Nd addition on the microstructure and properties of the 5A12 alloy were investigated using a variety of experimental methods,to determine the optimal approach to improving its corrosion properties at no expense to its mechanical properties.The main results are shown below:1.The effect of Ce addition(not higher than 0.9 wt.%)on the microstructure,corrosion behavior,and mechanical properties of the Al-9.2Mg-0.7Mn alloy was systematically investigated to assess its potential use in engineering applications.Ce can easily diffuse into the Al₆Mn compound to form a special segregation during the solidification process,and the common Ce-rich phase did not appear in the structure.For the as-cast and as-homogenized stated,the average grain size of alloys with0.1-0.3wt.%Ce addition can reduce about 20%.Ce addition had a significant effect on pitting corrosion resistance due to i ts segregation in the Mn-rich phase.In particular,the pitting corrosion rate of the alloy in the sensitization state decreased substantially with Ce addition,and was approximately 48%and 53%lower than that of the alloy matrix with 0.1wt.%and 0.3wt.%Ce,respectively.In addition,its strength was improved through 0.3wt.%Ce addition(providing an increase in ultimate tensile strength of approximately 30 MPa)at no expense to intergranular corrosion resistance,indicating that Ce micro-alloying could comprehensively improve the properties of Al-Mg-Mn alloys.2.The influence of Y content(0.1,0.2 and 0.4 wt.%)on the microstructure,corrosion behavior and mechanical properties of 5A12 alloy were investigated and Y was added in the form of the Mg-25Y master alloy during melting.The generation of the?-Al₃Mg₂ phase along the grain boundaries is suppressed in the as-cast alloy due to the formation of the Al Mg Y(Al₁₄Mg₅Y)ternary phase.The average intergranular corrosion mass loss of the alloy with 0.1wt.%Y addition decreases about 53.1%almost at no expense of mechanical performance in the as-rolled alloy after annealing.Moreover,the alloy with 0.1wt.%Y addition shows the corrosion mass loss about30.2%lower than the Y-free alloy in the sensitized state.The enhanced corrosion resistance of the alloy can be ascribed to the red uced?-Al₃Mg₂ precipitation along the grain boundaries associated with Y addition.3.There was a Al₆₇Mg₃₀Nd₃ phase formed in the as-cast structure of 5A12 alloy by introducing Nd element through the Mg-Nd master alloy,and which was not same as the common phase reported such as Al₁₁Nd₃.The Al₆₇Mg₃₀Nd₃ phase with a high melting point could exist stably after heat treatment,which could refine the grain size of as-cast and as-homogenized alloys because of an attachment interface in nucleation process.Especially,the average dendrite spacing and the average grain size of two stated alloys with 0.1wt.%Nd addition decreased about 27.3%and 33.9%,respectively.For cold-work alloy sheets,the pitting corrosion mass loss rate of alloys with 0.1-0.2wt.%Nd addition reduced to 0.118mg/cm²d,which was about 62.3%lower than that of the matrix.And the mechanical properties of 5A12 alloy with 0.1wt.%Nd addition improved about 20MPa without IGC deteriorated,so it had superior comprehensive properties.