The Study On Microstructures And Mechanical Properties Of Al-Li-Cu Alloy Prepared By Squeeze Casting

In recent years, the lightweight/high-performance Al-Li alloy has been greatly developedin aviation and aerospace fields. Although the wrought Al-Li alloys have been widely used inindustry, the application of casting Al-Li alloys is extremely limited. Apart from the sameproperties of low density, high elastic modulus and high fatigue crack growth resistance aswrought Al-Li alloys, wider range of lithium content in casting Al-Li alloys can be allowedand the microstructures are more isotropic. So, the parts with lower density and higherresistance to stress corrosion can be obtained. In addition, the casting processing has certainadvantages in terms of forming complex components. However, complicate melting process,poor fluidity and the lower mechanical properties result in the stagnation of development ofAl-Li castings.In this paper, thermodynamic analysis of chemical reactions and smelting attempts revealthat high purity graphite crucible is suitable only for melting Al-Li alloy at vacuum oranaerobic conditions for the energetic chemical reaction among the three Li, O and Celements, while silicon carbide crucible is suitable for use at atmosphere. The loss of Liduring melting can be controlled below20%by using the75%LiCl and25%LiF mixedsolvents to protect the melt with the dose of0.8~1%of the total melt weight.The effects of applied pressure and pouring temperature on the microstructures andmechanical properties of squeeze casting Al-Li-Cu alloy were investigated. The effects ofpressure on the microstructures and mechanical properties of Al-2.47Li-1.49Cu alloy showthat pressure can dramatically improve the surface quality, inhibit the development ofcolumnar crystal area, obtain dense microstructure without shrinkage, refine the grain size anddisperse the second phase. The distribution of Li and Cu elements shows positive segregationunder squeeze casting condition. Under pressure of50MPa, Cu distribution and the density ofthe casting are most uniform and the mechanical properties reach the climax. When pressureincreases to75MPa, macro-segregation deteriorates the mechanical properties.The effects of pouring temperature on the microstructures and mechanical properties ofAl-2.75Li-1.52Cu alloy show that pouring temperature has great influence on macro- crystalline structure. When pouring temperature is above700℃, the macroscopic structure ofAl-Li-Cu alloy ingot prepared by squeeze casting is consist of columnar crystal in the surfacelayer and equiaxed grain in center. When the pouring temperature drops to670℃, onlyequiaxed grain exists in the ingot. High pouring temperature is easy for columnar crystal andgrain boundary segregation which decreases the properties of the alloy. At pouringtemperature of670℃, the best mechanical properties of the squeeze castings are tensilestrength of246MPa, yield strength of336MPa, elongation of4.8%at r=27mm position andtensile strength of267MPa, yield strength of350MPa, elongation of5.0%at r=15mm,respectively.