在线稳恒磁场对激光选区熔化成形GH3536组织和性能各向异性的影响

Effects of Online Static Magnetic Field on Anisotropy of Microstructure and Mechanical Properties of GH3536 Fabricated bySelective Laser Melting

研究了外加在线稳恒磁场对激光选区熔化(Selective Laser Melting,SLM)成形GH3536合金组织及力学性能各向异性的影响。设计了可作用于激光选区熔化成形台面的在线磁场发生装置,可产生最大强度为0.3T、竖直向上的稳恒磁场。当无磁场和施加0.1T、0.3T磁场时,对成形试样进行了微观表征和拉伸试验。结果表明:随着磁场强度的增加,试样致密度由96.4%升高至98.8%,晶粒取向逐渐由强(001)取向转变为(001)、(101)和(111)的均匀取向。在0.1T磁场下,试样沿沉积方向的织构指数由6.843下降至6.363;在0.3T磁场下,织构指数下降至5.718。在稳恒磁场的作用下,试样沿扫描和沉积方向的抗拉强度及延伸率差异有明显改善。在0.1T磁场下,强度和延伸率差异相对于无磁场时分别减小了14.1%和11.1%;在0.3T磁场下,则减小了47.5%和55.3%。理论分析与计算认为,晶粒取向和性能各向异性的改变主要是由于外加磁场产生了10^(5)N/m^(3)量级的热电磁力,进而枝晶生长方向发生改变。

Objective The heat transfer is downward along the solidified substrate during the selective laser melting(SLM)process and results in the bottom-up growth of columnar crystals.The oriented columnar structure causes the anisotropy in the mechanical properties of materials and hinders the application of SLM.For this reason,many researchers have studied the causes and suppression methods of the anisotropy of SLM parts,mainly from the perspectives of heat treatment and SLM process.However,both heat treatment and SLM adjustment have certain limitations.Heat treatment requires new exploration for different materials,however the adjustment of the SLM process cannot change the nature of the directional heat transfer in the melt pool.The application of electromagnetic fields is an effective way to influence the metal solidification process.Recent studies have found that magnetic fields can induce the microstructures of SLM parts to change from columnar to equiaxed crystals,thus improving their mechanical properties.However,in general,the studies on the influence of magnetic fields on the microstructures of SLM parts are very limited,and the mechanism of a magnetic field influences the SLM process is still unclear.In particular,there are not studies involving the influence of magnetic fields on the anisotropy of SLM parts.In this paper,a device for generating a static magnetic field is built.Using GH3536,a typical aerospace material,as a representative material,we investigate the influence of a static magnetic field on the microscopic and macroscopic properties of the SLM-formed GH3536 parts,especially the influence of magnetic fields on the anisotropy of the parts.This study provides a theoretical and technical reference on the potential of using a superimposed magnetic field during the SLM process and the associated benefits to improve the performance of SLM parts.Methods In this paper,a static magnetic field generator that can be integrated into the self-developed equipment in our research group to generate a static magnetic field is designed.The direction of the magnetic field is upright,and the intensity can be adjusted arbitrarily from 0to 0.3 T.The gas atomized GH3536powder with a particle size distribution of 15--53μm is employed for the experiments.After the parameter optimization of laser energy,the cubic specimens and the tensile samples are formed under magnetic field strengths of 0,0.1,and 0.3 T.An optical microscope and a scanning electron microscope equipped with a detector are employed to examine the microstructures of the cubic specimens.A material testing machine is used for the mechanical property tests with a strain rate of1.33×10^(-3) s^(-1).Results and Discussions With the increase of magnetic field strength,the relative density of the specimen increases from 96.4%to 98.8%(Fig.5),and the grain orientation gradually changes from strong(001)orientation to(001),(101),and(111)uniform orientations.The texture index of the specimen along the deposition direction decreases from 6.843to 6.363at 0.1T,while at 0.3T,the texture index further decreases to 5.718(Figs.6and 7).At 0.1Tand 0.3T,the differences in tensile strength along the scanning and deposition directions decrease by 14.1%and47.5%,respectively,while the differences in elongation decrease by 11.1%and 55.3%,respectively(Fig.8).Theoretical analysis and calculations suggest that the magnetic field suppresses the intense Marangoni convection in the melt pool to a certain extent,thus alleviating the air entrapment defects and improving the relative densities of the specimens.At the same time,the magnetic field generates a thermoelectric force of 10^(5) N/m^(3),which acts on the dendrites and changes the direction of grain growth,thus weakening the anisotropy of grain orientation and mechanical properties.Conclusions In this work,an online static magnetic field system for the SLM process is designed.An optical microscope and a scanning electron microscope are employed to examine the microstructures of the specimens,and a material testing machine is used for the mechanical property tests.Our findings show that applying a magnetic field can significantly improve the relative density of the formed sample.As the magnetic field strength increases from 0to0.3T,the grain orientation changes from the strong(001)orientation to(001),(101),and(111)uniform orientations,while the texture index decreases from 6.843to 5.718.Thus,to a certain extent,the anisotropy of grain orientation of GH3536alloy is weakened.The results suggest that the change in grain orientation is mainly due to the generation of thermoelectric forces acting on the dendrites.Besides,the applied magnetic field makes the tensile strength and elongation of the sample along the scanning and deposition directions tend to increase,especially along the scanning direction.The tensile strength along the scanning direction under the 0.1 Tand 0.3 T magnetic fields increases by 20 MPa and 70MPa,respectively,and the elongation increases by 0.8%and 2.9%,respectively.At 0.1Tand 0.3T,the differences in tensile strength along the scanning and deposition directions decrease by 14.1%and47.5%,respectively,while the differences in elongation decrease by 11.1%and 55.3%,respectively.The static magnetic field can significantly weaken the anisotropy of the mechanical properties of SLM parts since the static magnetic field changes the grain orientation during the SLM process.