Effect Of Y And Al₃Ti On Microstructure And Mechanical Properties Of Heat-resistant Al-Cu Alloys

The high-temperature strength and heat fatigue properties of traditional cast heat-resistant aluminium alloy are approaching the limit state,which can not meet the development requirements of the new high-power engine.It is a fundamental approach to further improve the mechanical properties of the aluminium alloy at high temperatures to introduce fine dispersion strengthening phases with high strength,hardness and high thermal stability into the matrix or improve the thermal stability of existing strengthening phases by microalloying method.This paper used melting and casting methods to add rare earth element Y to Al-Cu-Mn alloy and Al-Cu-Mg-Ag alloys.Then adds K₂TiF₆ to the Y-containing alloy after optimization of composition and performance,and high heat stable Al₃Ti reinforcement particles were introduced by melt stirring in-situ reaction method.Study the effects and mechanisms of Y and Al₃Ti on the microstructure,thermal stability of the strengthening phase,room temperature,and high-temperature mechanical properties.The main research results are as follows:（1）The microstructure of Al-7Cu-0.4Mn-nY alloy is mainly composed ofα-Al,Al₂Cu,Al₂₀Cu₂Mn₃ and Al₈Cu₄Y phases.With the increase of Y content,α-Al primary dendrites were gradually refined,and the continuity and quantity ofα-Al+Al₂Cu eutectic structures decreased.After solid solution-ageing heat treatment,the networkα-Al+Al₂Cu eutectic structure disappeared,α-Al grain coarsened,Al₈Cu₄Y phase increased significantly,θ′-Al₂Cu dispersion precipitated from the matrix.The introduction of Y promoted the precipitation and refinement ofθ′-Al₂Cu.（2）The microstructure of Al-7Cu-0.5Mg-0.6Ag-nY alloy is mainly composed ofα-Al,Al₂Cu,Al₂Cu Mg and Al₈Cu₄Y phases.After adding Y,theα-Al and Al₂Cu dendrites are refined.After heat treatment,the network dendrites of Al₂Cu are fused,andθ′-Al₂Cu nano-precipitated phase appears in the matrix.Different from Al-7Cu-0.4Mn-nY alloy,the size and quantity ofθ′-Al₂Cu nanocrystalline precipitates in the matrix decreased with the increase of Y content.The heat-treated alloy′s high-temperature tensile strength reaches168.96 MPa whenY content is 0.15 wt.%,which increases by 24.99%compared with the base alloy.（3）After adding K₂TiF₆ to alloys,Al₃Ti phase appeared in the composite material.For the heat-treated Al₃Ti/Al-7Cu-0.4Mn-0.15Y alloy,with the increase of K₂TiF₆ addition,the quantity of Al₃Ti phase in the composite gradually increases,and the size of the Al₃Ti phase decreases first and then increases.The size ofθ′-Al₂Cu nanocrystalline precipitates phase in the matrix decreases,and the quantity slightly decreases.The high-temperature tensile strength of the composite reaches a peak value of 145.52 MPa when the content of Al₃Ti is 3 wt.%,which is 6.69%higher than that of the matrix alloy.For Al-7Cu-0.5Mg-0.6Ag-0.15Y alloy,with the increase of K₂TiF₆ addition,the formation of large size Al₃Ti in the matrix also increases;The number ofθ′-Al₂Cu nano precipitates has significantly decreased,neither is conducive to improving the tensile properties of the material.（4）It was verified by thermal exposure that Y is beneficial to improve the high-temperature thermal stability ofθ′-Al₂Cu nanocrystalline precipitates in Al-Cu-Mn alloys.The addition of Al₃Ti particles by the in-situ reaction method promoted the maturation and coarsening ofθ′-Al₂Cu nanophase during thermal exposure of the two alloys,leading to a significant reduction in the number ofθ′-Al₂Cu nanophase particles.Therefore,after introducing micron-grade high thermal stable Al₃Ti particles into Al-Cu alloy,the strengthening phaseθ′-Al₂Cu will accelerate its maturation and coarsing due to the presence of Al₃Ti particles,which is not conducive to the improvement of mechanical properties at high temperatures.