Hot Cracking Mechanism And Performance Control Of 2195 Aluminium-lithium Alloy Fabricated By Laser-powder-bed-fusion-based Additive Manufacturing

With the increasing demand for lighter and stronger alloys in aerospace,Al-Li alloy has been widely used in aviation,aerospace and weapon equipment fields due to its advantages of lower density,higher specific strength,damage tolerance,and corrosion resistance.At present,most Al-Li alloy components used in aviation aircraft and space vehicles are fabricated by riveting or welding.This splicing method not only will sacrifice part of the mechanical properties of components,but also cannot meet the development trends of complexity and functional integration of aerospace structures.Therefore,it is urgent to develop a new integrated forming technology for Al-Li alloy to realize its wider application.Laser powder bed fusion（LPBF）,as an important branch of Additive Manufacturing（AM）technology,is an approach to the three-dimensional（3D）near-net-shaped formation of complex-shaped high-performance metal parts.It not only ensures the overall performance of components,but also significantly shortens the processing cycle and manufacturing cost.However,the AM processability has not been fully considered during the development of Al-Li alloy.Due to the introduction of Li element with a low boiling point and Cu element with high hot cracking susceptibility（HCS）,it is difficult to restrain the vaporization of Li element,improve the densification of samples,and regulate the strength during the LPBF of 2195 Al-Li alloy.If the above difficulties are not effectively solved,the mechanical performance and application level of LPBFed Al-Li alloy components cannot meet the actual application requirements.Therefore,our research focuses on the element vaporization and hot cracking formation mechanism during the LPBF of 2195 Al-Li alloy.The strengthening regulation strategy will be explored to achieve better comprehensive performance.Firstly,to realize the appearance quality control of Al-Li alloy fabricated by LPBF,the process windows of single-tracks,multi-tracks,and cubic-samples were preliminarily explored by referring to the point-line-plane-body progressive research idea.It was found that the forming characteristics of single-tracks were determined by the linear energy density and melting mode in the laser-powder interaction process.At the same linear energy density,the fusion track size of keyhole mode was 2-8 times that of heat conduction mode.It was proved that the forming characteristics of single-and multi-tracks were inherited by the cubic samples.High-quality cubic samples with smooth surface appearance and dense interior structure could be obtained only at low speed and low power due to the high HCS of Al-Li alloy.When P=200 W and v=100 mm/s,the minimum surface roughness was 2.59μm and the maximum density was 99.24%.However,the vaporization loss ratio of Li was up to 19%,resulting in a low ultimate tensile strength（UTS）of 337 MPa.Secondly,to reveal vaporization mechanism of Li element in LPBF,the melting and solidification behaviors were calculated based on the DEM-FVM coupling model of molten pool dynamics.The simulated molten pool geometry and temperature characteristics were plugged into the non-equilibrium vaporization model based on the Knudsen layer.In this way,the vaporization characteristics and control strategy of vaporization loss ratio of Li element under different energy densities were interpreted clearly.It was found that the maximum temperature of molten pool was lower than the boiling point of 2195 alloy,and the saturated vapor partial pressure of Li element would not exceed 10³ atm in the heat conduction mode.While in the keyhole mode,the intense material/energy exchange caused by the laser-material interaction led to the formation of local high-temperature zone in the keyhole wall,and the saturated vapor partial pressure could reach 10⁴-10⁵ atm.A huge pressure gradient was generated between the upper and lower boundary of the non-equilibrium Knudsen layer near the vaporization interface,resulting in the strong vaporization rate of Li element.Meanwhile,the expansion of molten pool volume was promoted by high heat accumulation and molten pool flow,which increased the vaporization loss of Li element of the whole single-track.The dual strategy of heat conduction mode and high scanning speed was adopted to achieve vaporization suppression.In this way,the maximum temperature of molten pool was lower than the boiling point of the alloy,and the existence time of molten pool was reduced as much as possible.Finally,the vaporization loss of Li element could be controlled within 5%.Thirdly,to clarify the hot cracking formation mechanism in LPBF,the crack morphology and distribution characteristics were reconstructed and analyzed by3D tomography technology.Based on the solidification theory,the relationship between the microstructure evolution and the HCS was established to reveal the hot cracking initiation and propagation behavior.The results showed that the solute diffusion between Al₂Cu phases and the matrix accompanied by the Cu segregation in the residual liquid phase led to the discontinuous precipitation of Al-Cu eutectic liquid film along the large angle grain boundaries（HAGBs）at the last stage of solidification.Calculation demonstrated that the liquid film at the HAGBs was more stable than that in the interior of the grains,confirming that the hot cracks tended to initiate and propagate along the HAGBs.It had been concluded that the main crack sources consisted of the tearing of low-melting-point eutectic liquid film,the insufficient filling of the liquid channel,and local slip voids.Driven by internal tensile stress,the crack initiation and propagation along the grain boundaries during the layer-by-layer forming process were activated,causing the formation of interconnected hot cracks with a 3D network structure.Finally,to inhibit the crack initiation and improve the mechanical properties of LPBFed Al-Li alloy,continuous microstructure control was achieved by double heat treatment to reduce the HCS and expand the process window.The proposed control strategy significantly improved the mechanical properties of Al-Li alloys.The results showed that the continuous precipitation transformation of grain boundary precipitates from（Al₂Cu+Al-Cu eutectic phase）to T₂（Al₆Cu Li₃）phase and then to T₁（Al₂Cu Li）phase was realized by in-situ heat treatment（substrate preheating at 175℃）and T6 heat treatment（solid solution at 515℃/30 min+artificial aging at 170℃/6 h）.Finally,the high-strength Al-Li alloy parts with UTS of 574 MPa were obtained.It also could be confirmed that the precipitation strengthening caused by T₁ phase was the main strengthening mechanism of T6heat-treated Al-Li alloy.Specifically,the shearing of T₁ phase within the grains resulted in two ledges of precipitate/matrix interface,further causing a higher energy barrier for the subsequent dislocation movement.At the same time,the presence of continuously distributed T₁-cells along the grain boundaries was not only capable of providing a pinning effect on dislocations movement and boundary migration to produce a strain hardening,but also able to shorten the pile-up distance on its slip plane to restrain the planar slip due to the rapidly decreasing precipitation-free zones.Therefore,the obtained high-strength Al-Li alloy fabricated by LPBF will play a positive role in promoting the transformation of lightweight design and manufacturing of complex components in the Chinese aerospace manufacturing industry.