镍基合金焊接热影响区液化裂纹形成机制的多尺度原位研究

Nickel-based alloys are widely applied in aviation, nuclear power and other fields. However, the liquation cracking in the welding heat affected zone (HAZ) is always a difficult problem restricting its application. Further research is needed on the formation mechanism of liquation cracking. In this project, the dynamic process of liquid film and thermal stress in HAZ of nickel-based alloys is studied with multi scale in-situ methods, and the formation mechanism of liquation cracking is elucidated. First, with an equivalent welding condition established, thermodynamic simulation test is applied to in-situ study the influence of cooling rate on the type, distribution, size and content of the precipitates of low melting point in HAZ, and the precipitation mechanism of forming phase for liquid film under thermal stress conditions is then revealed at the micro-scale; Secondly, in-situ observation of the dynamic characteristics of the distribution, size and content of the liquid film under thermal cycle conditions is carried out by a high temperature confocal scanning laser microscope (H-T CSLM), then the influence of factors such as element segregation, grain size and grain boundary orientation on the evolution and migration of liquid film are analyzed, which explore the evolution and migration regularities of liquid film at the mesoscale. Finally, dynamic distribution of thermal stress in the HAZ under the equivalent welding condition is in-situ evaluated by laser ultrasonic technique, the function relation of the width of the liquid film and crack with the change of the thermal stress is established for the constitutive model of cracking behavior, the coupling cracking mechanism of liquid film and thermal stress is then clarified for the liquation cracking under macro-scale. Research of this project provides theoretical support for improving the welding theory of nickel-based alloys and promoting its reliable application in aviation, nuclear power and other fields.

镍基合金在航空、核电等领域应用广泛，但焊接液化裂纹一直是限制其应用的难题，液化裂纹的形成机制有待深入研究。本项目拟多尺度原位研究镍基合金焊接热影响区液膜、热应力的动态演变过程，阐明液化裂纹形成机制。建立等效焊接条件，采用热力学模拟试验原位研究热影响区冷速对低熔点析出相种类、分布、尺寸及含量的影响，从微观尺度揭示热应力条件下液膜形成相的析出机理；采用高温共聚焦扫描激光显微镜原位观察热循环条件下液膜分布、尺寸及含量的动态特征，分析元素偏聚、晶粒尺寸、晶界取向等因素对液膜演变及迁移的影响，从介观尺度探究液膜的演变及迁移规律；采用激光超声技术原位探测等效焊接条件下热影响区热应力的动态分布，建立液膜/裂纹宽度随热应力大小变化的函数关系，构建开裂行为的本构模型，从宏观尺度阐明液化裂纹的液膜-热应力耦合致裂机制。项目研究为完善镍基合金焊接理论，并促进其在航空、核电等领域的可靠应用提供理论支撑。

镍基合金在航空、核电等领域应用广泛，但焊接液化裂纹一直是限制其应用的难题，液化裂纹形成机制研究有待进一步深入。本项目主要研究内容：1）针对镍基合金焊接接头，提出了焊接裂纹敏感性量化评价方法，研究了焊接接头的组织结构，重点分析了热影响区的裂纹缺陷特征，结合Gleeble系统采用STF试验研究不同焊前热处理状态以及合金不同HAZ区域的组织和力学性能，从而探索影响镍基合金产生HAZ裂纹的影响因素。2）围绕焊接接头裂纹敏感性影响因素，提出调整镍基合金焊前热处理状态、优化焊接热工艺参数、改变焊接填充材料，研究焊接热影响区裂纹的行程机制，提出抑制焊接裂纹的方法。3）围绕高可靠性高性能镍基合金材料开发，基于固溶强化、析出强化、细晶强化等综合强化机制，系统研究了镍基合金成分设计、热处理等工艺对组织及力学性能的调控，开发了低裂纹敏感性的高性能镍基合金。项目研究结果表明：1）通过热延性试验可测定材料的延展性曲线。曲线测定的零强度温度NST和延性恢复温度DRT的差值表示HAZ出现液化膜的最大温度范围，该值可以用来确定液化裂纹温度范围（LCTR）。Inconel617的LCTR为150℃，表明其HAZ液化裂纹敏感性低于625合金、A-286合金，但高于310型和690合金。2）固溶态850合金的裂纹敏感性要大于过时效态。主要表现在固溶态的850合金在STF试验的塑性和断裂时间都要小于过时效态。固溶态850合金在模拟HAZ试验中组织基本不变，而过时效态的合金则会发生γ'相和Eta相的回溶。峰值温度越高，回溶程度越大。靠近晶界的γ'相主要通过共晶液化的形式回溶，而晶内的γ'相通过局部溶解的形式回溶。Eta相则通过局部溶解的形式回溶。在STF试验中，850合金在850℃-950℃温度范围内出现塑性和断裂时间最低值，表明在该温度范围内裂纹敏感性较大。随着热循环峰值温度提高，过时效态850合金的塑性和断裂时间逐渐降低。3）减小焊接电流的能够抑制850合金产生焊接HAZ液化裂纹。当焊接电流为70A时，在固溶状态下对850合金进行焊接，HAZ中没有形成液化裂纹，当焊接电流超过90A时，液化裂纹数量随电流的增大而增加。850合金接头的焊接残余应力随着焊接电流的增大而增大。固溶态的850合金焊接残余应力要大于过时效态，因此对镍基合金开展焊前热处理及选用合理的焊接参数能够抑制液化裂纹产生。