Effects Of Trace Sc And Heat Treatment On Corrosion Resistance Of Casting Al-Li-Cu Alloy

Cast Al-Li-Cu alloy has the characteristics of low density,high specific strength and high specific stiffness,and can be used to prepare complex structural castings by liquid forming.it has a broad application prospect in high load lightweight shell parts for aerospace vehicles,underwater weapons and other weapons.However,at present,the research on cast Al-Li-Cu alloy is mostly focused on composition design,microstructure and property analysis,and there are few reports on the corrosion properties of cast Al-Li-Cu alloy.Corrosion performance is a key index in the actual service process of components,so it is of great value to study the corrosion behavior of cast Al-Li-Cu alloy.In this paper,the effects of trace Sc and heat treatment on the microstructure,pitting behavior and intergranular corrosion behavior of cast Al-2Li-2Cu-0.5Mg-0.2Zr alloy were systematically studied by means of immersion test,intergranular corrosion test,XPS analysis,electrochemical analysis,electrochemistry and transmission electron microscope.The mechanism of the effect of Sc content on the corrosion resistance of Al-Li-Cu alloy was revealed,and the effect of heat treatment and casting process on the corrosion properties of Al-Li alloy was explored,which laid a foundation for the application of high performance cast Al-Li-Cu alloy.By observing the microstructure of the alloy,it is found that the addition of a small amount of Sc element can significantly refine the alloy grain and precipitate the square primary Al-Sc-Zr phase.The coarse Cu-containing compound can only be observed at the three-forked grain boundary,and the regular highly faceted Al-Sc-Zr primary phase can also be observed in the alloy containing Sc.Sc element can refine T₁ and S'precipitates during aging,promote the formation of rod-like core-shell composite phase,and reduce the width of grain boundary non-precipitation zone.Therefore,pitting corrosion and grain boundary corrosion of aged alloys can be prevented in alloys containing appropriate amount of Sc.The grain structure of sand casting alloy is thicker than that of metal mold casting alloy.And more defects such as porosity and sand entrainment is observed.The results of corrosion morphology and corrosion product analysis show that Sc element microalloying can significantly improve the pitting resistance of the experimental alloy and greatly reduce the corrosion products on the sample surface,but excessive Sc can accelerate the corrosion of the alloy matrix around the primary phase of Sc.The corrosion of the experimental alloy mainly occurs at the matrix around the Cu-containing primary phase and the Al-Sc-Zr primary phase at the grain boundary,and some slight pitting corrosion occurs in the intragranular matrix.The main corrosion products on the surface of the experimental alloy are Cu Cl₂,Al(OH)₃,Mg(OH)₂,Li OH,etc.,and Sc₂O₃ is also formed on the surface of the experimental alloy containing Sc.The types of corrosion products of the experimental alloy did not change after heat treatment.The corrosion products of sand casting alloys are similar to those of metal mold casting alloys,but the grain boundary products are thicker.The results of intergranular corrosion experiments show that the addition of Sc can effectively reduce the intergranular corrosion susceptibility of cast Al-Li-Cu alloy,0.1Sc alloy has the smallest intergranular corrosion depth.After heat treatment,the intergranular corrosion type of cast Al-Li-Cu alloy gradually changed from grid to pit,and the intergranular corrosion depth of the alloy decreased significantly after aging treatment.The base alloy reaches the minimum in the underaging state,and the 0.2Sc alloy reaches the minimum in the peak aging state.The intergranular corrosion resistance of sand casting alloy is higher than that of metal mold casting,the intergranular corrosion depth is the highest in solid solution state,and the intergranular corrosion resistance is the best in obsolete state.The results of electrochemical analysis show that Sc microalloying can increase the electrochemical impedance of cast Al-Li-Cu alloy,the content of 0.1wt.%Sc can reduce the corrosion current density of aged alloy.The cast Al-Li-Cu alloy after solution treatment has the best corrosion resistance.With the extension of aging time,the corrosion current density increases gradually,and the corrosion resistance decreases gradually.The longer the aging time is,the higher the bonding impedance is and the lower the impedance value of 0.2Sc alloy is.The experimental results show that in terms of the effect of trace Sc content,the experimental alloy with 0.1wt.%Sc content has the best corrosion resistance.The corrosion resistance of solid solution Al-Li-Cu alloy is the best.Compared with metal mold casting,sand casting has larger grain size,more pores and inclusion defects and poor corrosion resistance.