Effects Of Pr_xO_y And La_xO_y Multi-modification On The Microstructures And Properties Of The Cast Al-Cu Alloy

Aluminum-copper （Al–Cu） alloys have attracted much attention because of theiradvantages such as lightweight, high strength and toughness. Up to now, many Al–Cu alloyssuch as ZL205A with high strength and toughness have been developed and applied to actualapplication. However, they can still not meet the fast growing demands of automotive,aerospace and military on the material properties. Therefore, the mechanical properties of theAl–Cu alloys need further improvement. In the cast Al–Cu alloys, the copper-rich secondphases are the primary strengthening phases, and they usually exist in the form ofintermetallic compounds. Owing to the differences in the microstructure and electrodepotential between these intermetallic compounds and the Al matrix, the corrosion resistanceof the cast Al–Cu alloys was severely reduced, which limits the application of the cast Al–Cualloys. In present, the study on the simultaneous improvement of the strength, toughness andcorrosion resistance of the casting Al-Cu alloys are rather limited, and the modificationmechanism of Al–Cu alloys with using rare earth （RE） or RE oxides is still unclear.Therefore, the study on the effects of rare earth oxides （Pr_xO_yand La_xO_y） on themicrostructure evolution and mechanical properties of ZL205A is of great significance.In this thesis, the composition of the multi–modifier of rare earth Pr_xO_yand La_xO_ywasoptimized using the orthogonal test; the effect regularity of the multi–modifier （Pr_xO_yandLa_xO_y） on the microstructure evolution, mechanical properties and corrosion resistance werediscussed; and the effect regularity of different factors on the tensile strength was revealed,which provides an experimental basis and builds the application foundation for theimprovement of the mechanical properties and corrosion resistance of the casting Al–Cualloys. The main conclusions are as follows:（1） The best comprehensive performance is gained at0.4wt.%La_xO_yand0.6wt.%Pr_xO_y, theultrahigh tensile strength and elongation are560MPa and10.45%, respectively, whichwas increased by21.81%and49.29%than those of the unmodified alloy. The La_xO_ycontent is the prominent influencing rare earth oxides on the tensile strength, with fixed content of Pr_xO_y, the tensile strength increases firstly and then decreases with theincrease in the La_xO_ycontent, and the highest tensile strength is always gained at0.4wt.%La_xO_y. The simultaneous improvement of the tensile strength and elongation ofthe modified Al–Cu alloy is mainly due to the refined α–Al grains, the finer and moreuniformly distribuded nano θ’ precipitates and the Al–RE intermetallic compoundsformed at grain boundaries and interdendritic which can restrict the movement of thedislocation.（2） Within a certain range, the higher the solution temperature and the longer the solutiontime, the more completely the Al₂Cu existed at the grain boundaries and interdendriticdissolve into the α–Al dendrites. Then, the solid solution strengthening effect becomemore satisfying. The best solid solution strengthening effect was obtained when thealloys were kept at540oC for15h, water-quenched and then held another5h at540oC.（3） Compared with the unmodified alloy, the Pr_xO_yand La_xO_ymulti–modified alloys havesmaller and more developed α–Al grains and dendrites. Moreover, the θ′precipitates inthe modified alloys become finer and more uniformly distributed. The average width andlength of θ′phase in the modified alloy were about2–5nm and40–150nm, respectively.However, there were only a few θ′phases with a width of5–25nm and length of80–400nm in the unmodified Al–Cu alloy.（4） The addition of multi–modifier of Pr_xO_yand La_xO_yin the casting Al–Cu alloys canpromote the production of the passivation, improve its corrosion potential and rupturepotential and reduce the corrosion tendency, inducing the improvement of the corrosionproperties of the casting Al–Cu alloys. It was found thatâ‘ In the3.5wt.%NaClsolution, the corrosion potential Ecorr（–1.09V） of the modified alloy by0.6wt.%Pr_xO_y+0.8wt.%La_xO_yshifted positively about370mV compared with that of theunmodified alloy. With fixed Pr_xO_ycontent, the corrosion potential and rupture potentialincrease when the La_xO_ycontent increases and the average corrosion weight lossdecreases.â‘¡In the acid solution of pH=4, the corrosion potential Ecorr（–0.2306V） ofthe modified alloy by0.6wt.%Pr_xO_y+0.8wt.%La_xO_yshifted positively about170mVcompared with that of the unmodified alloy. With fixed Pr_xO_ycontent, the corrosionpotential and rupture potential first increase and then decrease when the La_xO_ycontentincreases, and the average corrosion weight loss decreases first and then increases. Thepositive shift of the corrosion potential indicates the decrease in the corrosion tendencyof the modified alloys. The increase in the rupture potential indicates that the generationof pitting corrosion is more difficult, and thus the corrosion resistance is increased.