Composition Design Based On Crack-Inhibiting And Strengthening Mechanisms Of High-Performance Scandium Modified Aluminum-Magnesium Alloys Fabricated By Additive Manufacturing

In recent years,the additive manufacturing（AM）technology based on laser powder bed fusion（LPBF）has significant advantages in the integral forming of complex lightweight components,but its application in the fields of high-strength aluminum alloys still faces many challenges.Due to the influence of the non-equilibrium solidification of LPBF,the traditional casting and forging high strength aluminum alloys are easy to crack during the LPBF process,and the mechanical properties of commercial-grade Al-Si alloys for AM are inadequate.At present,the basic research of crack inhibiting,strengthening mechanisms and special composition design of high-performance aluminum alloys under the special solidification process of LPBF is weak,which greatly limits the wide application of LPBF high-performance aluminum alloys in aviation,aerospace and rail transit fields.This study focuses on the weldable Al-Mg series alloy.Firstly,the laser single-layer scanning melting experiments were carried out to study the formation of liquid-solid cracks in the molten pool,and the variation trend of"crack susceptibility-compositions"was analyzed combined with the solidification path calculation.Then,the thermal stress accumulation caused by the reciprocating thermal history of multilayer LPBF process was revealed,and the influence of alloy composition on liquid-solid and solid-solid cracks was clarified.The crack-free LPBF aluminum alloy composition was designed.Finally,the effects of Sc and other alloy elements on the microstructure and mechanical properties were studied.The LPBF aluminum alloy with a high strength-plastic of～550 MPa was developed,and the new strengthening mechanism of 9R and QC was revealed.The main contents and innovations of this study are summarized as follows:（1）The formation mechanism of the liquid-solid crack in LPBF rapidly solidified Al-Mg alloy was revealed,and the prediction criterion of crack susceptibility and composition correlation was established.It was found that the crack susceptibility factor|d T/df_s^1/2|increased first and then decreased with the increase of Mg and Si content.And the crack size measured by LPBF laser single layer scanning is consistent.The crack susceptibility factor|d T/df_s^1/2|value decreased to 0 when the Si element was 2.0 wt.%,and the experimental results also show that there is no crack in the composition.In addition,the idea of combining grain refinement of the primary phase and eutectic reinforcement at the end of solidification was put forward to suppress liquid-solid cracks.The elements Sc and Zr effectively promote the formation of the primary phase,refine grains and shorten intergranular travel,and effectively inhibit the formation of liquid-solid cracks.At the same time,Si elements can narrow the solidification temperature range,formingα（Al）-Mg₂Si eutectic,promoting intergranular feeding,and inhibiting the formation of liquid-solid cracks.（2）Based on the optimization of aluminum alloy composition to inhibit liquid-solid cracks by LPBF single-layer scanning,the LPBF multi-layer additive experiments were carried out,and the characteristics,formation and inhibition mechanism of solid-solid cracks under the effect of stress accumulation in the reciprocating thermal cycle were elucidated.The simulation results show that the higher the hardness of the alloy,the greater the thermal stress of the sample.Experiments results show a similar pattern,when Mg content increases from 1.5 wt.%to 6.0 wt.%,microhardness increases from 64.03 HV to 93.38 HV,and the cracks change from columnar intergranular liquid-solid cracks in the single molten pool to liquid-solid and solid-solid mixed cracks propagating across multiple molten pools along the building direction.The crack density increased from 1.665×10^-3μm/μm² to 3.068×10^-3μm/μm².The average grain size decreased from 163.6μm to 11.7μm.When 1.3 wt.%Si is added,the crack of multi-layer additive is suppressed.The boundary of the molten pool is clearly visible and the grain is refined,and the average grain size decreased to 6.9μm.By calculating the non-equilibrium solidification Q value,the mechanism of grain size reduction by adding solute atoms Mg and Si is explained.Solute atoms not only shorten nucleation latency,but also form enough supercooling at the solidified solid/liquid front to promote nucleation,inhibit grain growth and prevent crack formation.（3）The high-strength aluminum alloy of LPBF～550 MPa was designed by fine grain,precipitation and solution strengthening,and the microstructure and mechanical properties of LPBF high-strength aluminum alloy in print and after heat treatment were systematically studied.The average grain size was reduced from 6.9μm to 2.8μm by adding Sc+Zr and Mn elements.The addition of the Mg element increased the average grain size to 3.1μm,but the volume fraction of fine grain increased to 40.71%.After the design of alloy to improve the mechanical properties,the microhardness of LPBF aluminum alloy increased from104.35 HV to 157.74 HV,the tensile strength enhanced to～500 MPa.After aging,the tensile strength can reach 506-550 MPa,and the fracture elongation was 8-17%.The microstructure of as-LPBF manufactured sample was a bimodal grain structure,the boundary of the molten pool was very fine equiaxed grains with a size of 0.3-0.6μm.There wereα（Al）-Mg₂Si eutectic with a diameter of 50-200 nm,and Al₃（Sc,Zr）precipitated particles at 2-15 nm in the interior of the grain.After aging,the grain boundary was blurred or even disappeared,and nano-precipitations L1₂superlattice Al₃（Sc,Zr）,about 4 nm,were dispersed in the grains.（4）A new strengthening mechanism in LPBF high-strength Al alloys:nanotwin and Quasi-crystal（QC）strengthening have been revealed.The existence of the 9R phase and twins,and the existence ofΣ3{111}coherent twin boundary andΣ3{112}incoherent twin boundary at the interface were characterized.The stable existence of the 9R phase after ageing was expounded.And the formation of long period stacking order（LPSO）9R phase arranged as...ABACACBCB....was described.After rapid solidification in LPBF,the formation sequence of QC phases in the printed alloy was determined as follows:clusters（QC-nucleus）→Guinier Preston（GP）zone→LPSO（6H）→QC(Al₈₁Mn₁₉).After ageing treatment,Si atoms move to QC phase Al₈₁Mn₁₉ through reaction-diffusion.The QC evolution can be described as:Al₈₁Mn₁₉+Si→Al₇₄Mn₂₀Si₆+Al₆Mn.