Research On Microstructure,Performance,Strengthening And Toughening Mechanism Of AlMg14ScZr Aluminum Alloy Printed By Selective Laser Melting

With the development of aviation industry,the components applied in this field tend to be lightweight,personalized,complex and functional integration.There are many limitations for the manufacture of these parts using t raditional subtracting processes,such as long production cycle,high manufacturing cost,low dimensional precision and poor overall mechanical properties.These features can be better realized by structural optimization and personalized customization of aerospace parts through additive manufacturing technology,especially for the honeycomb structural components and functional system components.Adopting additive manufacturing technology to manufacture these parts as a whole can not only improve the overall performance of components but also significantly shorten the processing cycle and manufacturing cost.Selective laser melting（SLM）is an additive manufacturing process based on powder bed.It can be used to prepare complex structural parts in one time and has a broad application prospect in the manufacture of aerospace components.Aluminum alloys have large solidification interval,thus the porosity and liquefaction crack defects are easily formed during rapid solidification condition of SLM,which significantly reduce the mechanical properties of the printed component.The optimization of powder composition can effectively refine the grain size,eliminate the crack defects and improve the comprehensive performance of printed components.In this paper,a new type AlMg14ScZr alloy powder was adopt for the SLM process,and the forming characteristics and overall performance of printed parts w ere studied in detail.Firstly,the forming quality of AlMg14ScZr alloy during the SLM process was studied,including the effect of SLM process parameters on the surface forming and internal quality of printed samples.The experimental results showed that the surface appearance of block samples was related to the forming quality and stacking arrangement of single tracks.Under reasonable processing conditions,the balling and pitting defects on the surface of printed samples disappeared,and the minimum value of surface roughness was 7.1μm.The maximum density of block sample（99.4%）was obtained when the energy density was 267 J/mm³.At lower energy density,the porosity defects with irregular shapes dispersed inside the sample due to the insufficient melting of metal powder,and poor spreading and wetting of liquid metal.At higher energy density,for the born loss of Mg element,the porosity defects were mostly round and irregularly distributed inside the printed sample.The density of printed samples still had some differences even at the same laser energy density.Through comparing porosity defect in different samples,it was found that more porosity defects were formed under the condition of high laser power and scanning speed due to the unstable SLM process.As long as printing efficiency permits,the forming quality of AlMg14ScZr alloy can be improved by using small laser power and low scanning speed.Secondly,the microstructure and comprehensive properties of printed AlMg14ScZr alloy were analyzed.For conventional aluminum al loys,it is easy to form coarse crystal and crack defects under the condition of rapid solidification.With the optimizing of alloy composition,the grain coarsening and cracking tendency can be reduced with the decrease of solidification interval.Alterna tely distributed fine grain zone and coarse grain zone formed in side the AlMg14ScZr alloy.The fine grain zone distributed near the melt pool boundary,while the coarse grain zone formed inside the melt pool.At a laser power of 160 W and a scanning speed of 200 mm/s,the average grain sizes of the top surface and side surface were1.82μm and 2.67μm,respectively.Based on the fine grain strengthening effect,the printed AlMg14ScZr alloy has high corrosion resistance,abrasion resistance and mechanical properties.The corrosion current density and polarization resistance of the printed alloy were 0.148μA cm^-2 and 20.1×10⁴Ω*cm²,respectively.And good passivation behavior was observed in the range of-750m V to-850 m V.The friction coefficient and wear rate of SLM printed AlMg14ScZr alloy were 0.1～0.14 and 1.81×10^-4～3.64×10^-4 mm³/Nm.With a scan strategy of single scan and 67°layer rotation,the tensile strength and elongation of printed sample were 488 MPa and 17%.Compared with other SLM-fabricated Al alloys,the AlMg14ScZr alloy exhibited good comprehensive properties.Finally,the microstructure,mechanical properties and strengthening and toughening mechanisms of AlMg14ScZr alloy after aging treatment were studied.With an aging treatment of 325℃,the average grain size of AlMg14ScZr alloy was almost unchanged with the increase of aging time.The printed samples have good thermal stability.At the aging time of 24 h,the average grain sizes of printed samples were 2.84μm,which was only 6.4%larger than the printed condition（2.67μm）.Furthermore,the printed samples also show good mechanical properties in the medium temperature range.The tensile strengths of samples were 394 MPa and 254 MPa at the temperatures of 473 K and 523 K.After aging treated at325℃,the mechanical properties of the printed alloys increased first and then decreased.At an aging time of 12 h,the tensile strength reached to the maximum value（580.0 MPa）.While the fracture elongation of AlMg14ScZr alloy did not decrease,which was 18%at this condition.Through the characterization of microstructure,it was found that the strengthening and toughening mechanisms of the AlMg14ScZr alloy were solid solution strengthening,fine grain strengthening,second phase strengthening and stcking fault strengthening.Among them,solid solution strengthening is the main part,and the strength increase induced by solid solution strengthening was 165～171 MPa.The SLM printed AlMg14ScZr alloy has a low density of 2.54 g/cm³,and the comprehensive properties of high strength,high toughness,heat resistance,abrasion resistance and corrosion resistance can be obtained in the same time.This alloy will provide a new alternative for the preparation of complex aerospace structures.