【作者】 邱宇；

【导师】 张秉刚；

【作者基本信息】 哈尔滨工业大学 ， 材料科学与工程， 2023， 硕士

【摘要】 空间核反应堆电源技术是目前核领域及航天领域重要的研究方向,热离子发电元件中的燃料元件封接件端部采用耐高温和中子辐照稳定性好的Nb-1Zr合金材料,在难熔金属之间采用过渡材料4J29可伐合金,从而实现燃料元件封接。封接件中会通入碱金属蒸气作为热离子发电元件的动力来源,因此对于接头高温下的气密性提出了严格要求。然而,Nb与4J29可伐合金焊接过程中存在脆性相析出问题,易产生裂纹缺陷,极大限制了Nb与可伐合金焊接接头在空间领域内的应用。本文对焊接接头中裂纹产生机制进行了研究,通过偏束焊接及添加中间层的方式,成功消除了焊接裂纹。首先对Nb-1Zr合金与4J29可伐合金焊接过程中裂纹产生机制进行了研究。Nb与可伐合金由于热物性参数差距较大,在焊接过程中,Nb元素与Fe元素会发生强烈相互作用,形成各种脆性金属间化合物,焊缝区显微组织为γ-Fe、Fe+Fe₂Nb共晶组织、Fe₂Nb及Fe₇Nb₆组成的金属间化合物反应层。反应层硬度极高,焊缝主体中α-Fe与Fe₂Nb点阵常数差异较大,相界面处存在大量位错塞积,致使其呈现低塑、韧性,高硬、脆性特征,在焊接纵向热应力的作用下引起横向开裂。在Nb与可伐合金直接对中焊情况下,接头会产生贯穿焊缝的横向裂纹。通过偏束焊接方法进行了Nb与可伐合金焊接接头显微组织调控。随着偏束量从0.1mm增加至0.5mm:Nb元素熔化量不断降低,焊缝中脆性相组织生成得到抑制;全共晶组织区域厚度从100μm不断缩小至完全消失,成功改善了焊缝显微组织;接头中宏观裂纹产生数量逐渐由11条减至完全消失,但是当偏束量为0.5mm时产生未熔合缺陷。偏束焊接可以降低裂纹产生倾向,但并不能完全消除焊接裂纹。选择添加纯V、纯Cu作为中间层材料,对接头显微组织、力学性能及裂纹产生倾向的影响进行了研究。当选择V作为中间层时,焊缝呈显著脆性,沿纵向开裂。当选择Cu作为中间层时,接头成形良好,无裂纹产生。进一步探究中间层厚度对焊缝组织性能影响,随着中间层厚度由0.1mm增至0.8mm,焊缝中主要显微组织由Fe₂Nb化合物逐渐向Cu基固溶体改变,接头抗拉强度呈先上升后降低趋势,当中间层厚度为0.5mm时,接头抗拉强度达到最高,为180MPa。更多还原

【Abstract】 The power supply technology of space nuclear reactor is an important research direction in the nuclear field and aerospace field at present.The end of fuel element seals in the thermionic power generation element is made of Nb-1Zr alloy with high temperature resistance and neutron irradiation stability,and the transition material 4J29Kovar alloy is used between refractory metals to achieve fuel element sealing.Alkali metal vapor is introduced into the sealing component as the power source of the thermionic power generation component,therefore strict requirements are put forward for the airtightness of the joint at high temperatures.However,there is a problem of brittle phase precipitation during the welding process of Nb and 4J29 Kovar alloy,which is prone to crack defects,greatly limiting the application of Nb and Kovar alloy welding joints in the space field.This article investigates the mechanism of crack generation in welded joints,and successfully eliminates welding cracks through beam bias welding and the addition of intermediate layers.Firstly,the mechanism of crack generation during the welding process between Nb-1Zr alloy and 4J29 Kovar alloy was studied.Due to the significant difference in thermal physical parameters between Nb and Kovar alloys,during the welding process,Nb and Fe elements will undergo strong interactions,forming various brittle intermetallic compounds.The microstructure of the weld zone isγ-The intermetallic compound reaction layer composed of Fe,Fe+Fe₂Nb eutectic structure,Fe₂Nb,and Fe₇Nb₆.The reaction layer has extremely high hardness,and the main body of the weld seamγ-There is a significant difference in lattice constants between Fe and Fe₂Nb,and there is a large amount of dislocation accumulation at the phase interface,resulting in low plasticity,toughness,high hardness,and brittleness.Under the action of longitudinal thermal stress in welding,it causes transverse cracking.In the case of direct alignment welding between Nb and Kovar alloy,the joint will generate transverse cracks that run through the weld seam.The microstructure control of Nb and Kovar alloy welded joints was carried out through beam bias welding method.As the deviation of the beam increases from 0.1mm to 0.5mm,the melting amount of Nb element continuously decreases,and the formation of brittle phase structures in the weld seam is suppressed;The thickness of the fully eutectic structure area ranges from 100μm Continuously shrinking to complete disappearance,successfully improving the microstructure of the weld seam;The number of macroscopic cracks in the joint gradually decreases from 11 to completely disappear,but when the deviation is 0.5mm,incomplete fusion defects occur.Electron beam offset can reduce crack tendency,but it does not completely eliminate weld cracks.The effects of adding pure V and pure Cu as intermediate layer materials on the microstructure,mechanical properties,and crack generation tendency of the joint were studied.When V is selected as the intermediate layer,the weld seam exhibits significant brittleness and cracks along the longitudinal direction.When Cu is selected as the intermediate layer,the joint is well formed and there are no cracks generated.Further explore the influence of interlayer thickness on weld microstructure and properties.With the increase of interlayer thickness from 0.1mm to 0.8mm,the main microstructure in the weld gradually changes from Fe₂Nb compound to Cu based solid solution.The tensile strength of the joint increases first and then decreases.When the interlayer thickness is0.5mm,the tensile strength of the joint reaches the highest,180MPa.更多还原