Research On Microstructure And Mechanical Properties Of Al-Cu-Mn Alloys Containing High Fe And Si Contents Prepared By Ultrasonic Vibration And Applied Pressure

High-strength Al-Cu alloys have been widely used in the fields of aeronautics,astronautics and transportation due to good mechanical properties,good forming properties,excellent heat-resistance properties.The high-strength Al-Cu alloys are limited by the impurity elements,such as Fe and Si.Therefore,large amounts of electrolytic aluminum with low impurity content must be used.With increasing scrape aluminum,it is more difficult to monitoring the impurity elements.To achieve recycling of waste aluminium,green casting,energy-saving emission reduction and sustainable development trends,it is important to develop new Al-Cu alloys with high performance and low cost.Exploring the new technology for material preparation and forming,plays an important role in the application of Al-Cu alloys.In this paper,casting Al-5.0Cu-0.6Mn alloys with high Fe and Si contents were produced by squeeze casting,ultrasonic treatment,neutralization and combined with squeeze casting and ultrasonic treatment.Optical microscopy?OM?,scanning electron microscopy?SEM?,transmission electron microscopy?TEM?,differential thermal analysis?DSC?,X-ray diffraction?XRD?,and synchrotron radiation X-ray computed tomography?SRXCT?were used to study the influence of Si content,squeeze pressure and ultrasonic treatment on the microstructure and mechanical properties of the alloys.The formation characteristics of Fe-rich intermetallic phases and their role on the alloys were studied.The main conclusions are as follows:?1?Si and applied pressure have a significantly impact on the microstructure and mechanical properties of the as-cast and heat-treated Al-5.0Cu-0.6Mn alloys containing high Fe content.For the alloy having Fe content of 0.7%or 1.2%,the Fe-rich intermetallic phases in as-cast alloys change from Al3?FeMn?and Al6?FeMn?to?-Fe when the Si content increased from 0%to 1.1%.The strength of gravity casted alloys decreases with the increase of Si content.While,the strength of the alloys produced by squeeze casting increased slightly with the increase of Si content.The main reason is that the pressure eliminates the pores of alloys and refined?-Fe grains which retard dislocation movement.?2?In heat-treated alloys containing 0.7%and 1.2%Fe Fe-rich intermetallic phases change from??CuFe?to?-Fe when the Si content increasing from 0%to 1.1%.The strength of the alloys increases with increasing Si content,which is mainly owing to the Si addition promotes the formation a compact and refined?-Fe and to have a dispersion distribution of precipitated T phase.With an applied pressure of 75 MPa,T5 heat-treated Al-5.0Cu-0.6Mn-0.7Fe-0.5Si alloy have an optimum mechanical property,in which ultimate tensile strength?UTS?,yield strength?YS?and elongation are 370 MPa,270 MPa and 8.7%,respectively.Compared with conventional high strength and tough cast aluminum alloy,the minimum content of Fe and Si impurities is greatly increased.?3?Ultrasonic treatment and applied pressure have significantly effects on the microstructure and mechanical properties of Al-5.0Cu-0.6Mn-0.6Fe alloys.Comparative study of gravity field,ultrasonic field,pressure field and compound field on microstructure and properties of the alloys.Using the compound field during casting,?-Al dendrite,Al₂Cu and Fe-rich intermetallic phases of alloys were refined,pore were eliminated,removal of bimodal structure,obtained uniform and refined microstructure.Compared with the alloys produced with applied pressure,UTS,YS and elongation of alloys with compound field increases by 8.1%,5.5%and 33.6%,respectively.Compared with gravity casting alloys,UTS,YS and elongation of alloys produced with compound fields increases by 64.5%,58.7%and311.9%,respectively.?4?The treatment time of the melt in the compound field can change the microstructure of Al-5.0Cu-0.6Mn alloys.The alloy treated by 120 s has more homogenous and refined microstructure than those treated by 30 s.The reason is that the alloys under long time treatment,the cooling rate of alloys under compound field is higher than those of other alloys under individual field.?5?Pouring temperature also has a certain effect on the microstructure of Al-5.0Cu-0.6Mn-0.5Fe alloy under compound field.In the alloys containing 0.5%Fe and0.5%Si,when the casting temperature is 710oC,the grain size and secondary dendrite arm spacing of alloys is smaller,which posses the best mechanical properties.?6?The 3D morphology of Fe-rich intermetallic phases and Al₂Cu of Al-5.0Cu-0.6Mn alloys containing high Fe content has been characterized by synchrotron radiation X-ray computed tomography.It is found that Fe-rich intermetallic phases and Al₂Cu are interconnected in 3D with complex network structures.The positive mean curvature alloy is higher than those of in 0.5Fe alloy.The node length of Fe-rich intermetallic phases of 1.0Fe is higher than those of in 0.5Fe alloy,while Al₂Cu shows opposite trend.This is due to that with increase of Fe content,the Fe-rich intermetallic phases transform from compact Chinese-script shape to long-rod shape.The formation of Al₂Cu is affected by eutectic reaction of?-Al and Fe-rich intermetallic phases at the late stage of solidification.?7?The 3D morphology of pores of Al-5.0Cu-0.6Mn alloys containing high Fe content has been characterized by SRXRT.Pores in alloys are mainly composed of equiaxed and interconnected shrinkage pores and near-spherical hydrogen pores.The sphericity of pores is mainly distributed in the range 0.4-1.0 and the equivalent diameter is between 10-20?m.The relationship of spherical and the equivalent diameter of pores are followed by power function.The positive mean curvature of 1.0Fe alloy is higher than that of 0.5Fe alloy.The part with high mean curvature of shrinkage pores is contacted with eutectic reaction,while the part with negative mean curvature is contacted with the tip of aluminum dendrite.?8?The 3D morphology of microstructure of Al-5.0Cu-0.6Mn alloys contained high Fe and Si contents under compound field of ultrasonic and applied pressure has been characterized.It is more comprehensive to explain that the compound field of ultrasonic and applied pressure can significantly refine the Fe-rich intermetallic phases??-Fe?and Al₂Cu in the alloys,elimination of pores,and make the microstructure distributed finer and more uniformly.