Microstructure And Mechanical Property Of Al-Fe Eutectic Alloys By Laser Powder Bed Fusion

Aluminum alloys are widely used in lightweight bearing structures of automobile and aerospace aircraft due to their advantages of light weight and high specific strength.There is an urgent need for the heat-resistance components with the increase of the power of engine and the heating of the electronic equipment in the aircraft cabin.The traditional heat-resistant Al-Si and Cu alloys are prone to failure owing to the coarsening of strengthened phase at elevated temperature above 250℃.The essential reason for this phenomenon is that high diffusion rate of Si and Cu elements in aluminum matrix.The development of new aluminum alloys based on alloy elements with low diffusion coefficient is an important direction in the field of heat-resistant aluminum alloys.In this proposal,Fe element with low diffusion rate and low price is selected as the main alloying element.In this paper,we aim to form Al-Fe eutectic alloys by laser powder bed fusion（L-PBF）.The Zr and Ce elements with low solid solubility and low diffusion coefficient are added in Al-8Fe alloys,respectively.The nano-reinforced precipitates with excellent thermal stability are introduced to construct Al-8Fe heat-resistant aluminum alloys framework.The effects of Zr,Ce addition and Fe content on the microstructure evolution and strengthening mechanism of Al-Fe eutectic are discussed.The main content and relevant results of this paper are as follows:（1）The microstructure evolution and precipitation of Al-8Fe hypereutectic alloy during L-PBF solidification and subsequent ageing treatment are studied.The fraction deformation mechanism of as-built and aged Al-8Fe alloys is revealed.The microstructure of Al-8Fe alloy shows a typical fishscale morphology at melt pool.The melt pool center is composed of cellular structure.Numerous of granular precipitates with the size of about 95.0 nm are distributed at melt pool boundaries.The size of cellular structure is about 89.0 nm.The supersaturated aluminum solid solution serves as the cell inner and Al₁₃Fe₄ nanocrystalline forms envelope structure at cell wall.When the ageing temperature rise to 400℃,the cell wall of the cellular structure changed into the nano-scale rods-like Al₁₃Fe₄particles.The rod-like Al₁₃Fe₄ particles grow along the{100}_Al planes,showing a novel coherent Berger vector relationship with the matrix,（001）_Al13Fe4//（002）_Al,[100]_Al13Fe4//[200]_Al.The Al₁₃Fe₄ nanocrystalline splits the adjacent aluminum matrix,resulting in ultimate tensile strength of 680 MPa but near-zero ductility.These discrete rod-like particles enable wide-range slipping of dislocations and deformation coordination with aluminum matrix.Excellent mechanical property of Al-8Fe alloys aged at400℃is achieved with tensile strength of 480 MPa and elongation of 5.5%.（2）Zr element is introduced into Al-8Fe alloy to improve the thermal stability.The effect of Zr on microstructure,mechanical properties and thermal stability of Al-8Fe alloy prepared by L-PBF are investigated.The as-built Al-8Fe alloys do not change the morphology of the cellular structure with 1wt.%Zr addition.The cellular microstructure still consists of continuous nanocrystalline Al₁₃Fe₄ at cellular wall and susaturated solid solution at cell inner.Besides,the nano-particles Al₃Zr with size of below10 nm are found at the cell inner.After ageing treatment at 400℃,the nanocrystalline Al₁₃Fe₄ at cell wall in Al-8Fe-1Zr alloys also transform into rod-like Al₁₃Fe₄ particles.Al₃Zr particles provide heterogeneous nucleation sites for rod-shaped Al₁₃Fe₄ particles.Meanwhile,Al₃Zr particles refine the size of Al₁₃Fe₄ particles.The Al₃Zr particles show a good orientation relationship with Al₁₃Fe₄and aluminum matrix at the initial nucleation stage.The novel coherent Berger vector relationship can expressed as[100]_Al13Fe4//[100]_Al3Zr//[110]_Al and（100）_Al13Fe4//（001）_Al3Zr//（002）_Al.Apart from these coherent relationship,the Al₁₃Fe₄ particles formed directly from the aluminum matrix still grow along the{100}_Alplanes.The coarsening of rod-shaped Al₁₃Fe₄ particles began at 500℃.With the temperature rising to 600℃,the coarsening of rod-shaped Al₁₃Fe₄particles is accompanied by serious spheroidization.Al₃Zr particles play a key role in grain refinement in as-built Al-8Fe alloys.The ultimate tensile strength and elongation of as-built Al-8Fe-1Zr alloy is 601 MPa and 3.0%,respectively.Compared with Al-8Fe alloy,the ultimate tensile strength and elongation of Al-8Fe-1Zr alloy aged at 400℃is 525 MPa and 7.5%respectively due to the coupling effect of sub-micron Al₁₃Fe₄and nano-scale Al₃Zr particles.（3）The effect of rare earth Ce on microstructure and mechanical properties of Al-8Fe alloy prepared by L-PBF are discussed.The effects of the introduction of Ce element on the microstructure and ambient/high temperature mechanical properties of Al-8Fe eutectic alloy at high temperature are explored.The addition of 1wt.%Ce significantly changed the morphology of the cell structure at melt pool center and the Al₁₃Fe₄particles at melt pool boundaries of as-built alloys.The cell wall of the cellular structure changes from continuous envelope to discontinuous pearl chain.The cell wall is composed of discontinuous distribution of Al₁₃Fe₄and Al₁₁Ce₃ spherical particles.The spherical Al₁₃Fe₄ particles at melt pool boundaries transform into rod-like particles.After ageing at 400℃,spherical Al₈Fe₄Ce particles are formed in Al-8Fe-1Ce alloy,and a small amount of Al₁₃Fe₄ particles are attached to the Al₈Fe₄Ce particles for nucleation and growth.Most of the Al₁₃Fe₄ particles still grew along the{100}_Alplane.The discontinuous cellular structure can effectively avoid the cleavage of the adjacent aluminum matrix,which is conducive to coordinate plastic deformation,achieving good mechanical properties with tensile strength of 620 MPa and elongation of 3.6%.The Al-8Fe-1Ce alloy ageing at 350℃exhibits excellent high-temperature strength with ultimate tensile strengths of 391 MPa and 210 MPa at 250°C and 400°C respectively,which are 47%and 100%higher than that of the Al-8Fe alloy under the same conditions,respectively.（4）The effect of Fe content on microstructure and mechanical properties of Al-Fe eutectic alloy prepared by L-PBF are investigated.The effect of aging temperature on phase transformation is revealed.The discontinuous cellular structure is formed at melt pool center in as-built Al-2Fe alloy.Besides,the cell wall is enriched in Fe elements and no intermetallic phase is formed.The rods-shaped metastable Al₆Fe particles are formed at melt pool boundaries in as-built Al-2Fe alloys.The Al₆Fe particles grows along the{100}_Alplanes.The Al₆Fe particles obey the Burgers orientation relationship with aluminum matrix of（046）_Al6Fe//（2 02）_Al and[032]_Al6Fe//[1 11]_Al.The transformation of metastable Al₆Fe into Al₁₃Fe₄phase starts at 350℃.With Fe content decreasing from 8wt.%to 2wt.%,the ultimate tensile strength of as-built Al-Fe alloy decreases from 680 MPa to 287 MPa at room temperature,and the elongation increases from zero to 12%.