Study On Formation Of Fe-Rich Intermetallics And Mechanical Properties Of Squeeze Cast Al-5.0Cu Based Alloys

The Al-Cu cast alloys with high strength and toughness are widely used in the aerospaceand transportation industry. The impurity of Fe is inevitable and considered to be the mostdetrimental to mechanical properties. The Fe content is strictly controlled (mostly below0.15%) in order to get high performance properties. This will make the alloys expensive toproduce and limit the use of recycled aluminum alloys. Finding an advanced process toincrease the allowable Fe content of Al-Cu alloys has been became an important way todevelop low cost aluminum alloys with high performance.In this study, the Al-Cu alloys are prepared by combining squeeze casting and Mnmodification. Various test techniques, including optical microscopy (OM), image analysis,scanning electron microscope (SEM), X-ray diffraction (XRD), differential scanningcalorimeter (DSC),water quenching, deep etched technology, electron probemicro-analyzer (EPMA) and transmission electron microscopy (TEM) are used to examinethe formation of Fe-rich intermetallics in squeeze cast Al-Cu alloys and the effect of Mn andFe content, applied pressure and heat treatment on microstructures and mechanical propertiesat room and elevated temperature of the alloys. The experimental results are as followed:When the Fe content increases from0.1wt%to1.5wt%, the Fe-rich intermetallics ofas-cast Al-5.0Cu-0.6Mn alloys are-Fe (Al15(FeMn)3(CuSi)2) and β-Fe (Al7Cu2(FeMn)) inlow Fe content, while it will change into Al6(FeMn) and Al3(FeMn) in high Fe content. Theelongations of gravity die cast and squeeze cast alloys decreases gradually with increasing theFe content and there is a peak value of ultimate strength and yield strength for the alloy with0.5wt%Fe. The reason is that the small Chinese-script Fe-rich intermetallics in low Fecontent alloys are useful to improve the alloy strength through second phase hardening,although these brittle second intermetallics will cause a reduction in ductility. The elongationof the alloys with different Fe content increases more than2times when the applied pressureincreases from0MPa to75MPa. The improvement of mechanical properties is due to theporosity reduction,(Al) dendrite refinement and the decreasment of intermetallics. Thechemical composition and the needle-like shape of β-Fe (Al7Cu2Fe) solidified from the Alalloy melt have no change after T5heat treatment. However, the Chinese script-Fe,Al6(FeMn) and needle-like Al3(FeMn) transform partially to a new Chinese script (CuFe)-Al7Cu2(FeMn). When the Fe content increases from0.1wt%to1.5wt%, the mechanicalproperties of both the gravity die cast and squeeze cast alloys decrease gradually with increasing of Fe content. The Fe-rich intermetallics leads to the decrease of mechanicalproperties due to the increase amount of (CuFe), decrease of precipitation particles in (Al)matrix and the increase of (Al) dendrite size. The elongation of the alloys with different Fecontent increases more than2times when the applied pressure increases from0MPa to75MPa. The squeeze cast alloys with different Fe contents have superior mechanical propertiescompared to the gravity die cast alloys, which is mainly attributed to the reduction of porosity,the refinement of Fe-rich intermetallics and (Al) dendrite and the decrease of needle-likeFe-rich intermetallics.The mechanical properties at elevated temperature of Al-5.0Cu-0.6Mn alloys with differentFe content have been investigated. The ultimate tensile strength and yield strength ofAl-5.0Cu-0.6Mn alloys with different Fe content decrease with incresing the tensiletemperature, while the elongation increases obviously. The ultimate tensile strength and yieldstrength of Al-5.0Cu-0.6Mn alloys with different Fe content decrease with the incresing theFe contetn. The mechanical properties at different temperature increase with increasingapplied pressure, especially the elongation. However, the increasment becomes slightly whenthe temeperature increases.The effect of Mn/Fe ratio on the microstructure and mechanical properties has been studied.The Fe-rich intermetallics, which may present in Al-5.0Cu-0.5Fe alloys with different Mn/Feraito, including Chinese script AlmFe,-Fe, Al6(FeMn), and needle-like-Fe. The optimalMn/Fe mass ratios is1.6for the best mechanical properties when the applied pressure is0MPa due to the needle-like β-Fe is completely converted to the Chinese script-Fe and theminimum volume percent of prosity. Only Mn/Fe mass ratio of0.8is needed for the completeconvert of β-Fe phase to-Fe phase when the applied pressure is75MPa. After T5heattreatment, the Chinese script AlmFe,-Fe and Al6(FeMn) transform to a new Chinese script(CuFe). The optimal Mn/Fe ratio for the best mechanical properties of heat-treated alloy isdetermined by the morphology of Fe-rich intermetallics, volume fraction of θ’ and T(Al20Cu2Mn3), size of (Al) dendrite and porosity.The formation of Fe-rich intermetallics during solidification of Al-Cu alloys has beenstudied. When the Fe content varies from0.1%to1.5%, β-Fe, AlmFe,-Fe, Al6(FeMn) andAl3(FeMn) are all possible to be the dominant Fe-rich intermetallic phases in the solidifiedmicrostructure of Al-Cu alloys, depending on different Mn, Fe contents and applied pressures.The Mn addition will promote the formation of-Fe and prevent the formation of AlmFe andβ-Fe. The primary Fe-rich intermetallics which formed from the Al alloy melt are AlmFe,Al6(FeMn) and Al3(FeMn) through observing the microstructures of the water quenched specimens. AlmFe and Al3(FeMn) preferably precipitates in low Mn (below0.2%) and Felevels (below0.5%) alloys. Al6(FeMn) preferably forms at high Mn (above0.4%) contentalloys， they will partly or completely change into β-Fe or-Fe, respectively, aftersolidification. In the Al-5.0Cu-0.6Mn alloys with high Fe content (above1.0%), theAl6(FeMn) and Al3(FeMn) are considered to be the dominate stable Fe-rich intermetallics.The applied pressure is favor to the formation of Chinese script Fe-rich intermetallics AlmFe、-Fe and Al6(FeMn) and prevents the precipitate of needle-like β-Fe and Al3(FeMn) becausethe high cooling rate and the growth rate of the Fe-rich intermetallics with different crystalstructure. The applied pressure will promote the nucleation rate of Fe-rich intetmetallics anddecrease the diffusion coefficient of elements, which leads to growth inhibition of Fe-richintermetallics. The formation temperature increases for-Fe and AlmFe but decreases forβ-Fe with the increase of applied pressure.The evolution of Fe-rich intermetallics characteristics during solution heat treatment (SHT)of Al-Cu alloys has been studied. It is found that the SHT temperature and time, the appliedpressure promote the transformation of AlmFe、Al6(FeMn)、-Fe and Al3(FeMn) to new β-Fe.The new β-Fe preferentiallynucleates various points on the interface between the AlmFe,-Fe,Al6(FeMn), Al3(FeMn) and the (Al) matrix, then grows into the Fe-rich intermetallics.