Composition Design Of Al-Mn-Mg High-strength Aluminium And Microstructure Optimazation For Selective Laser Melting

In many fields such as aerospace and transportation,further improvements in performance and cost savings are being achieved mainly through lightweight substitution of structures and materials.Lightweight structures are often complex,and the recent development of selective laser melting（SLM）technology offers unparalleled advantages in forming such structures.Using SLM technology to form high specific strength aluminum alloys into lightweight structures can achieve optimal light-weighting results.However,the forming process is complex and the forming principle is very different from that of conventional reduction processing,which makes conventional alloys formed by SLM not only impossible to achieve the design strength,but also hard to fully exploit the advantages of SLM.Therefore,the development of new composition alloys for SLM can maximize the advantages of this technology,which is necessary and urgent for lightweight applications and to continue to promote aerospace and other fields to a higher level.The main objective of this project is to design a new lightweight and high-strength aluminum alloy for SLM characteristics and explore the potential mechanism of micro-melt pool melting behavior,defect formation and tissue evolution in SLM,and finally obtain a dense and reasonable structure of microstructure through regulation and make the tensile strength of the new composition alloy reach more than 600 MPa.The effect of each element on the formability of the alloy was first investigated by means of pseudo-binary phase diagrams.It was found that a single increase in Mg content would not only lead to a higher susceptibility to hot cracking,but also have a limited improve on the strength of the alloy.The addition of Mn/Mg elements can not only improve the alloy formability,but also improve the mechanical strength more significantly.However,the solid solution capacity of Mg and Mn elements are mutually constrained,and their ratio needs to be further balanced.To this end,the new high-strength aluminum alloy composition was determined as follows:Al-4.8Mn-2.6Mg-0.8Sc-0.5Zr-0.44Cu-0.3Cr（wt.%）by using the"cluster-connected atom"model and adjusting the microalloy element composition.Afterwards,the formability of the new composition alloy was verified,and a rapid prediction model of the deposition organization was established based on the melt pool size information and the melt pool superposition law.It was determined that a suitable combination of process parameters for the new alloy needed to meet the bulk energy density of 65-100 J/mm³ and the normalized enthalpy of 3.3-4.5,respectively.The specimens prepared with all combinations of parameters within this process window will have comparable relative densities（>99.75%）.Then,the effect of microalloying elements on the structure and precipitation phase of the alloy was investigated,and the results of the microstructure analysis combined with the numerical simulation of the temperature field of the multiple heating showed that the solidification conditions with a wide variation range during solidification of the melt pool led to a bimorphic heterogeneous structure.The whole structure consists of alternating0.3～1μm fine equiaxed crystals and 3～25μm coarse columnar crystals.Differences in solidification conditions due to process parameters also affect the thickness of the equiaxed crystal area,and the final percentage of the fine crystal zone is influenced by both the solidification behavior of the melt pool and the spatial stacking relationship.The process parameter with the highest percentage of fine crystalline area was selected for forming the tensile specimen,which was 370 W-1459 mm/s-0.13 mm.The bulk energy density（Ev）was calculated to be 65 J/mm³,and the normalized enthalpy（ΔH/hs）and average remelting number（NMC）were 4.1 and 3.32,respectively.The composite addition of Sc and Zr elements leads to the generation of a large number of Al₃(Sc_xZr_1-x)phases at the grain boundaries in a non-co-grained relationship with theα-Al matrix.The phase diagram and first-principles calculations suggest that the incipient Al₃(Sc_xZr_1-x)is formed by the nucleation of Al₃Zr followed by the replacement of Zr by Sc atoms,and the lattice parameters and interfacial relationships are transformed by the change of the spatial symmetry structure after atom replacement.By comparing the calculated results with the experimental results,it is estimated that the lattice constant is about 0.5402 nm and the interfacial mismatch withα-Al is about 34.57%,and this phase may lead to grain refinement in a way that hinders grain growth.In addition,the high cooling rate of SLM forming leads to the existence of icosahedral Mn-containing quasicrystalline phase with symmetry in the new composition alloy.Based on the valence electron concentration and atomic radius criterion for quasicrystalline composition,the chemical composition of the quasicrystalline phase in this alloy is Al80Cu2.83Mn17.16（at%）with e/a of 1.9849 and R_av of 1.4152（?）.The quasicrystalline phase with this characteristic has not been found in other related reports.The diffraction peak angle ofα-Al in the heat-treated specimens decreased and approached the standard diffraction peak angle in the absence of solid solution,which was due to the reduction of the grain surface spacing caused by the previous solid solution of Mn atoms and the reduction of solid solution caused by the precipitation of Mn atoms after heat treatment.In comparison with the deposited state,the precipitated phase did not grow significantly after the aging treatment at 325°C-4h,while the second phase particles grew significantly when the aging temperature was further increased to 380°C.Moreover,the Mn-containing quasi-crystalline phase might have formed a stable Al6Mn phase by the inclusion reaction with the matrix（α-Al）during the heat treatment.The mechanical properties of the alloy in the deposited state were tested using room temperature unidirectional tensile tests,and the results showed that the room temperature yield strength(Rp_0.2)and ultimate tensile strength（σ_b）of the new composition alloy were about 504±2.3 MPa and 555±5.4 MPa,respectively,and the elongation was about 7.54±1.0%.The strength contribution calculation revealed that the most important strengthening mechanism of the new composition alloy in the deposited state is solid solution strengthening.The room-temperature tensile stress-strain curves of the deposited specimens exhibited dynamic strain aging characteristics,in which the PLC band of the deposited specimens was type A,while the PLC band of the aging-treated specimens transitioned from type A to type B.By studying the changes of microstructure and precipitation phase under different heat treatment processes and combining with the hardness test results,the optimal heat treatment process was determined as 325°C-4h direct aging treatment.The ultimate tensile strength of the alloy increased from about 556MPa to about 613 MPa after aging,however,the elongation of the alloy decreased from about 7.76%in the deposited state to about 4.55%.This may be due to the tendency of a large number of precipitated phases generated by heat treatment to precipitate along the grain boundaries,which reduces the co-deformation ability with the grains at the grain boundaries and eventually leads to the deterioration of plasticity due to the concentration of deformation stress.