【作者】 刘敏；

【导师】 卓龙超； 尹恩怀；

【作者基本信息】 西安理工大学 ， 材料工程（专业学位）， 2020， 硕士

【摘要】 传统减材制造是经铸锻车铣刨磨等工序进行工件制备,其中涉及周期长、加工工艺繁琐、原材料浪费等问题,3D打印不仅可以明显缩短制备周期节约材料,其特殊的加工方式使材料获得细致均匀的组织和优异的性能。本文针对一种典型的难加工高温合金,采用激光选区熔化(SLM)成型IN 718合金和WC增强IN 718合金,并对其进行720℃/8h+620℃/10h直接双级时效处理,对比研究SLM成型和直接双级时效工艺及WC的添加对合金微观组织、力学性能和高温稳定性能的影响,同时探究合金中各析出相的演变规律,本论文通过结合模拟和实验获得了以下主要结果:SLM成型的IN 718高温合金在XY方向上组织呈沿扫描方向的长条状,Z方向呈鱼鳞状且存在穿越打印层的各向异性长柱状晶粒。合金中主要存在树枝晶基体γ相,枝晶间粒状γ’增强相,晶界处长棒状δ相及不规则形状的硬脆Laves有害相,重熔区的亚晶胞可产生亚结构强化,为析出相提供形核位点,提高合金力学性能。时效处理促使γ’、γ"的析出和Laves相的溶解,组织发生均匀化转变,晶界数量减少相位差减小。力学性能实验获得打印态的IN 718合金室温抗拉强度为1162MPa,σ0.2为834MPa,断后延伸率为7.9%:双级时效处理后抗拉强度为1608MPa(相比热处理前提高了38.4%),σ0.2为1472MPa(相比热处理前提高了76.5%),断后延伸率为9.2%(相比热处理前提高了,16.5%),断裂机制以韧性断裂为主。空气气氛下1000℃/8h的热重分析实验揭示时效前后失重量由0.15～0.20 mmg/cm2降低到 0.08-0.15 mmg/cm2,氧化膜组织由团状 Cr2O2、片状 NiCr2O4及米粒状NiFe2O4形成,且更加均匀致密,高温稳定性能进一步提高。SLM成型的WC增强IN 718高温合金中,添加的WC颗粒与基体生发生化学反应产了MC碳化物,高熔点WC阻碍了凝固时的晶界移动从而细化晶粒,具有更低热导率的WC在凝固散热时产生热封作用,促使晶粒内析出更多δ、Laves相及MC碳化物。双级时效处理促进WC增强IN 718合金中Nb元素在晶粒间扩散形成NbC,这种对Nb元素的消耗减少了δ和Laves相的形成。在扫描速度为800mm/s的工艺下,由于体积能量密度不不足材料在拉伸实验测试中发生脆断,抗拉强度为567.9MPa,延伸率为0.5%,时效后的抗拉强度为731.7MPa,σ0.2为472MPa,延伸率为0.9%,材料性能发生一定程度改善;进一步改进工艺增加体积能量密度的样品,抗拉强度为1071.9MPa,延伸率为4.5%,从而有利于指导后续工艺优化。时效前后失重量由0.15～0.17mmg/cm2减少到0.12～0.15mmg/cm2,以 Cr2O2,NiCr2O4、NiFe2O4 及 WO3、CrWO4 新相为主要成分的氧化膜层中,结构疏松的片状WO3相不利于高温稳定性能提升,致密的Cr2O2,NiCr2O4和NiFe2O4相是合金耐高温腐蚀的真正原因。更多还原

【Abstract】 Traditional subtractive manufacturing is the preparation of workpieces through casting,forging,milling,milling,and other processes.It involves long cycles,cumbersome processing techniques,and waste of raw materials.3D printing can not only shorten the preparation cycle signicantly and save materials,but its special processing method makes the materials Obtain a fine and uniform structure and excellent performance.In this paper,focusing on a typical superalloy hard-to machine,laser selective melting(SLM)is used to form IN 718 alloy and WC reinforced IN 718 alloy,and it is subjected to 720℃/8h+620℃/10h direct two-stage aging treatment to study SLM molding and direct two-stage aging process and WC The effect of the addition on the microstructure,mechanical properties and high-temperature stability of the alloy,as well as the evolution of each precipitated phase in the alloy.The following main results based on simulation and experiements were obtained as the followings:The structure of IN 718 superalloy formed by SLM is long in the direction of the scanning track in the XY direction,ish-shaped in the Z direction and there anisotropic long columnar grains passing through the printed layer,and the structure is anisotropic.There are mainly dendritic matrix γ phases,intergranular granular γ ’reinforced phases,long rod-shaseaped δ phases at grain boundaries and irregular and hard and brittle Laves harmful phases in the alloy.Sub-cells in the remelting zone can produce sub-structure strengthening.Provide nucleation sites for the precipitated phase,improve the mechanical properties of the alloy.The two-stage aging process promoted the precipitation of γ’,γ",and the dissolution of Laves phase,and the structure is homogenized more,the phase difference of the grain boundary decreases and the subcell increases;the room temperature TS of the IN 718 alloy after aging increases from 1162MPa to 1608MPa Increased by 38.4%,PS increased from 834MPa to 1472MPa increased by 76.5%,elongation after fracture increased from 7.9%to 9.2%increased by 16.50/n.and ductile fracture occurred;thermogravimetric analysis experiment at 1000℃/8h+air In the meantime,the weight loss before and after aging is reduced from 0.15～0.20mmg/cm2 to 0.08～0.15mmg/cm2.The dense oxide film formed by lumpy Cr2O2,flake NiCr2O4 and rice granular NiFe2O4 is more uniform and fine,and the high temperature stability can be further improved.In the WC-reinforced IN 718 superalloy formed by SLM,the added WC particles react wtith the matrix to produce MC carbides such as W1-xC,W2C,CrNiFeC,CoCx,NbC,and Ni2W4C.forming a chemical interface with a certain strength and high melting point.WC hinders the movement of grain boundaries during solidification to refine the grains.WC with lower thermal conductivity produces a heat seal during solidification and heat dissipation,which promotes the precipitation of more δ,Laves phase and MC carbides in the grains.The two-stage aging process promotes the diffusion of Nb element in the WC-reinforced IN 718 alloy to form NbC between grains.This consumption of Nb element reduces the formation of δand Laves phase.Under the SLM+direct two-stage aging process with a scanning speed of 800mm/s,the alloy has insufficient input energy density to cause brittle fracture,TS is 731.7MPa,PS is 472MPa,and the elongation is only 0.9%.In the SLM process with a scanning speed of 600mm/s,the WC-reinforced IN 718 alloy has a good rigidity under the SLM forming process with a scanning speed of 600mm/s,a tensile strength of 1071.9MPa,a yield strength of 872.5MPa,an elongation of 4.5%.the weight loss before and after aging is from 0.15～0.17mmg/cm2 is reduced to 0.12～0.15mmg/cm2.In the oxide film layer with Cr2O2,NiCr2O4,NiFe2O4 and new phases of WO3 and CrWO4 as the main components,the loosely structured sheet-like WO3 phase is not conducive to the improvement of high temperature stability,dense Cr2O2,NiCr2O4 And the NiFe2O4 phase is the real reason for the high temperature corrosion resistance of the alloy.更多还原