| The 7xxxx aluminum alloy has the advantages of high specific strength and high specific stiffness.It is widely used in the main industrial fields,such as automobile,aerospace,national defense and military industry.It is an indispensable key material in production and life.Selective laser melting is a new type of near-net manufacturing technology.It uses high-energy laser to directly melt metal powder,which can manufacture high-performance metal parts with nearly full density and complex structures that cannot be processed by traditional processing methods or multi-component assembly.Therefore,it has broad application prospects in aerospace,personalized biological manufacturing and complex molds.Due to the wide solidification temperature range,selective laser melting 7xxx aluminum alloy is easy to crack and difficult to manufacture.There is great theoretical significance and engineering application value to study the cracking mechanism of selective laser melting 7xxx aluminum alloy design and develop 7xxx high-strength aluminum alloy suitable for selective laser melting.In this paper,the selective laser melting 7xxx high strength aluminum alloy was studied.And 13 kinds of Al-Zn-Mg(-Cu)-Sc-Zr alloys with different contents of Zn,Cu,Sc and Zr were prepared by composition design,ingot melting,gas atomization and powder particle screening.Through selective laser melting seven kinds of alloys,the formability,cracking mechanism and the law,principle of Sc and Zr elements inhibiting cracking were studied.The existence form of alloying elements in as-printed Al-Zn-Mg(-Cu)-Sc-Zr alloy was analyzed.The evolution mechanism of alloying elements in Al-Zn-Mg(-Cu)-Sc-Zr alloy during solution and aging heat treatment and its effect on mechanical properties were revealed.Through the above research and analysis,the design ideas and principles suitable for selective laser melting 7xxx aluminum alloy were obtained.Then,Al-8.5Zn-3Mg-1Cu-0.8Sc-0.6Zr alloy was designed.Through process optimization and microstructure performance analysis,a 7xxx high-strength aluminum alloy suitable for selective laser melting,processing parameters and heat treatment parameters was finally obtained.The main results of this paper are as follows.(1)The formability of Al-Zn-Mg(-Cu)-Sc-Zr alloys with different compositions were studied.It was found that when the laser power and scanning spacing were kept constant,and the energy density was changed by changing the scanning speed,the formability gradually improved from low energy density to high energy density.This is because when the bulk energy density is low,the laser scanning speed is too fast,and the laser residence time in the molten pool is short,which will cause the molten pool to absorb less energy and lower temperature.Due to the poor fluidity of Al-Zn-Mg(-Cu)-Sc-Zr alloy melt,uneven melting channel of Al-6.8Zn-2Mg-0.35Cu-0.6Sc-0.36Zr alloy is formed due to the poor fluidity of molten Al-6.8Zn-2Mg-0.35Cu-0.6Sc-0.36Zr alloy in the molten pool,and finally accumulates layer by layer to form a large-sized gully surface.The optimization range of volume energy density is 110-115 J/mm3.(2)It is found that when the addition of Sc and Zr in Al-Zn-Mg(-Cu)-Sc-Zr alloy is 0.2%and 0.12%,respectively,the Sc and Zr elements will be completely dissolved in the Al matrix at a high cooling rate,and will not precipitate in the form of primary Al3(Sc,Zr)phase as a heterogeneous nucleation point to refine the grains.The microstructure is coarse columnar grains,and can pass through the bottom of the molten pool to show obvious epitaxial growth.The cracks are distributed along the grain boundary and can continue to expand outward through the bottom of the molten pool.(3)The relationship between the content of Sc and Zr and the microstructure of Al-Zn-Mg(-Cu)-Sc-Zr alloy manufactured by selective laser melting was studied.It was found that the microstructure gradually changed from coarse columnar crystals to the bimodal structure of micro-nano equiaxed crystals at the bottom of the molten pool and columnar crystals growing along the temperature gradient direction in the middle of the molten pool with the increase of Sc and Zr content from zero.With the refinement of the grains,the grain boundary feeding channel will be shortened in the later stage of solidification and the cracking tendency will be reduced.At the same time,the smaller grain size means that the liquid film between the grains is thinner,so the capillary pressure to be overcome before tearing propagation is greater.The fine equiaxed grains can more effectively coordinate the stress and strain during solidification than the coarse columnar grains,thus reducing the cracking tendency.(4)It is found that most of the Sc and Zr elements in the Al-6.8Zn-2Mg-0.35Cu-0.6Sc-0.36Zr alloy exist in the form of supersaturated solid solution during the solidification under the action of high cooling rate,and some of them are precipitated as fine primary Al3(Sc,Zr)phase.Primary Al3(Sc,Zr)is coherent with the matrix and acts as a heterogeneous nucleation point during the solidification of Al-6.8Zn-2Mg-0.35Cu-0.6Sc-0.36Zr alloy to refine grains.After 470℃/1h solid solution+120℃/24h aging heat treatment,the relatively coarse T phase is fully dissolved while the large quantities of fine Al3Sc/Al3(Sc,Zr)secondary phase precipitates.However,due to the weak diffusion ability of Zr atoms,the aggregation degree of Zr atoms is not as obvious as that of Sc atoms.With the increase of solution time,the secondary phase of Al3Sc/Al3(Sc,Zr)grows to a certain extent,and the mechanical properties decrease slightly.This is because the migration and diffusion ability of Zr atoms in Al3(Sc,Zr)grains is weak and has good thermal stability.Zn and Mg atoms tend to aggregate around Al3(Sc,Zr)phase.(5)Based on the study mentioned above,Al-8.5Zn-3Mg-1Cu-0.8Sc-0.6Zr high-strength aluminum alloy powder suitable for selective laser melting was finally designed and prepared.The optimized processing parameters is:laser power 250 W,scanning speed 758 mm/s,scanning spacing 0.1 mm,layer thickness 0.03 mm,and volume energy density 110 J/mm3.There are no cracks and large pores in the sample.After 470℃/1 h+120℃/24 h solution-aging treatment,the tensile strength reaches 647.3±5.5 MPa,the yield strength reaches 639.7±2.3 MPa,the elongation after fracture is 6.2±2.8%,and the reduction of area is 8.7±2.3%.Finally,the part with good appearance and internal quality was manufactured. |
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