Laser Remelting And Additive Manufacturing Of Beryllium-aluminum Alloy

The Beryllium aluminum(Be-Al)alloy combines the low density,high strength and stiffness of beryllium,with good formability and machinability of aluminum.Furthermore,processing costs and the amount of beryllium resources are reduced,so the Be-Al alloy is more competitive than beryllium in some applications for which performance is not the most prominent requirement.Be-Al components are usually fabricated by powder metallurgy or investment casting.However,the powder metallurgy technology process is complex and has limited net-shape capabilities.Investment casting is a near-net-shape fabrication process,while the micro structure of large Be-Al alloy castings is relatively coarse due to a large solidification interval and a slow solidification rate,which limits its mechanical properties.Laser additive manufacturing(LAM)is a kind of advanced processing technology that allows direct fabrication of near-net-shape metal components.LAM offers several potential benefits,including an increased raw material utilization ratio and a reduced number of machining operations,leading to a decrease in the cost and time.LAM has recently been widely performed to produce high performance components of Titanium alloys,Nickel base alloys and Aluminum alloys for aerospace,medical,energy and automotive applications.Therefore,LAM is particularly attractive for fabrications of expensive and small-lot Be-Al components due to the unique advantages in material saving,unrivalled design freedom and short lead times.To the best of our knowledge,no studies have been carried out on Be-Al alloys by LAM.In the present work,an optimized model for the composition of Be-Al alloy is established by considering the intrinsic strength of the material,the interfacial bonding strength,and the volatilization and oxidation behavior during the LAM process.The relationship between laser remelting process and microstructure and mechanical properties of Be-Al alloy was established.A typical Be-38wt.%Al alloy thin walled sample was fabricated by LAM process.The anisotropic tensile behavior and the relationship with the microstructure were investigated.The effect of alloying elements on microstructure and mechanical properties of LAM Be-Al alloy was discussedThe main contents and resluts are as follows:(1)The composition design method of Be-Al alloy suitable for LAMBased on the modified shear lag model in the composite material,the yield strength of the Be-Al alloy is predicted.The results show that increasing the strength of the aluminum matrix and refining the grain size of the beryllium particles can significantly improve the yield strength of the Be-Al alloy.The effects of alloying elements and oxides on the interface bonding strength of Be-Al alloy are considered from the perspective of solid-liquid interface wettability.The results show that the addition of Mg,Si,Ag and other elements can improve the wettability between the aluminum liquid and beryllium,and improve the interface bonding strength of the Be-Al alloy.Reducing the oxidation of Be-Al alloy is beneficial to improve the Be/Al interface bonding strength.From the perspective of vapor pressure and thermodynamics,the selective volatilization and oxidation behavior of alloying elements during the LAM process were discussed,respectively.The results show that increasing the laser scanning speed is beneficial to reduce the volatilization loss of alloying elements.The content of the most volatile Mg element needs to be increased during the composition design.The oxidation of Be-Al alloy is inevitable during the LAM process.(2)The effect of laser remelting on defects,micro structure and properties of Be-Al alloyThe experimental results demonstrated that a pore-free refined microstrucutre of remelted layer was obtained by controlling the parameter of effective laser energy input.Focused ion beam/scanning electron microscope(FIB/SEM)reconstruction reveals that the microstructure consisted of a continuous network of Al phases wrapping the columnar Be phase.The columnar Be phases grow from the substrate towards the remelting zone and are oriented in different directions at the top of the molten pool.The gradient solidification rate explains well the gradual size reduction of the Be phase from the bottom to the top of the molten pool.The Vickers hardness in the remelted zone(approximately 210 HV)was approximately 3 times that of as-cast Be-Al alloy.Analysis of the Vickers hardness and the Be phase size showed a good agreement with a Hall-Petch equation.The results obtained in this study indicate that the laser remelting allowed refining the microstructure and further strengthening the properties of Be-Al alloy.(3)The typical microstructure characteristics and mechanical properties of Be-Al alloy fabricated by LAMA laser additive manufacturing system for toxic and harmful powders was built.The spherical Be-Al pre-alloy powder was obtained for the first time by the plasma rotating electrode atomization method.Thin walled Be-38wt.%Al samples have been produced using LAM for the first time in this study.The results show that no cracking or significant porosity are observed in the thin walled samples.The microstructure of LAM Be-Al alloy is composed of columnar Be grains delineated by continuous Al networks.The growth direction of the columnar Be grains inclines to the scanning direction at an angle of approximately 70°.The widths of the Be columnar grains increase gradually as the deposition height increases.The tensile properties of the Be-Al alloy show higher ultimate tensile strength and yield strength,but smaller elongations for the longitudinal samples than that of the transverse samples.BeO and Al2O3 particles are observed in the Be phase,Al phase,and the Be/Al interfaces in the fracture surfaces.The oxide particles lead to a lack of bonding or a crack during the tensile test.(4)The influence of alloy elements on the microstructure and properties of Be-Al alloy fabricated by LAMThe microstructure and properties of Be-AlSi1OMg alloy was investigated.The addition of Si and Mg elements did not change the biphasic structure of the Be-Al alloy,and the Mg element was mainly dissolved in the polycrystalline Al phase.The addition of more Si elements leads to the coarsening of the microstructure and the formation of massive silicon particles in the Al phase.It is proposed that the size of the solidification interval of Be-Al alloy determines its cracking tendency during the LAM process.The addition of eutectic Si increases the solidification interval of Be-Al alloy,which leads to the occurrence of intergranular thermal cracking in Be-AlSi1OMg alloy during the LAM process.Based on the composition design theory and experimental results,the composition of Be-Al alloy(Be-37.4A1-0.2Si-0.4Mg)was optimized,and a defect-free sample was obtained.The yield strength and tensile strength of Be-37.4Al-0.2Si-0.4Mg alloy fabricated by LAM were 175 MPa and 225 MPa,respectively,which is higher than the international as-cast Be-Al products with the same beryllium content.