A Study of the Diffusion Brazing Process Applied to the Single Crystal Superalloy CMSX-4
A Study of the Diffusion Brazing Process Applied to the Single Crystal Superalloy CMSX-4
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单晶高温合金CMSX-4扩散钎焊工艺研究
DOI：
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发表时间：
2006
期刊：
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通讯作者：
E. Marcos
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作者：
A. Schnell;A. Stankowski;E. Marcos
镍基高温合金扩散钎焊是燃气涡轮机工业中广泛采用的标准连接工艺。DB技术使用低熔点镍基填充材料，其含有快速扩散熔点金属（MPD），例如硼。在钎焊时，钎焊填料首先熔化，然后通过硼从液体钎焊合金扩散到固体母体材料中而等温固化。完全等温凝固的接头呈现无脆性相的显微组织，并显示出优异的力学性能。DB过程的动力学由从液体到周围散装材料的“有效MPD通量”控制。有效MPD通量是硼扩散速率和母体材料的热力学参数（例如MPD溶解度）的函数。在钎焊高温合金的情况下，关于控制有效MPD焊剂的热力学参数，钎焊填料和母材之间的复杂相互作用没有完全理解。在目前的工作中，DB过程应用于单晶多元高温合金的研究相结合的实验和数值模拟结果。选择了一种有代表性的高温合金/钎焊填料系统，即CMSX-4/D-15。主要任务包括研究控制有效MPD（硼）流量进入母体高温合金的热力学参数。由于基质相对硼的低溶解度和硼化物形成元素如Cr、W和Mo的存在，硼扩散到超合金材料中导致硼化物的形成。因此，在等温凝固过程中，硼通过原位硼化物沉淀从高温合金基体相中稳定地消耗。因此，进入母材的有效硼通量增加。母体超合金中的总硼含量高于硼溶解度极限所允许的。发现存在一个最佳钎焊温度，在该温度下，由于硼化物的形成，进入CMSX-4材料的有效硼焊剂最高。随着钎焊温度的升高，在母材中稳定的硼化物的体积分数稳步下降。在最佳钎焊温度以上，硼化物形成对进入高温合金材料的有效硼熔剂的影响很低。DB过程则主要由基质相中的低硼溶解度控制。由于大多数高温合金含有硼化物形成元素，因此关于有效硼焊剂和最佳钎焊温度的结果可以应用于大多数高温合金系统。本文的工作为高温合金扩散钎焊工艺中钎焊循环的选择提供了依据。
Diffusion Brazing (DB) of nickel-based superalloys is a standard joining process widely used in gas turbine industry. The DB technique uses a low melting nickel-based filler material, which contains a rapidly diffusing melting point depressant (MPD) such as boron. Upon brazing, the braze filler firstly melts and then solidifies isothermally through the diffusion of boron from the liquid braze alloy into the solid parent material. Fully isothermally solidified joints exhibit a brittle-phase free microstructure and show excellent mechanical properties. The kinetics of the DB process are controlled by the "effective MPD flux" from the liquid into the surrounding bulk material. The effective MPD flux is a function of the boron diffusion rates and thermodynamic parameters of the parent materials such as the MPD solubility. In the case of brazing superalloys, the complex interactions between the braze filler and the parent material are not fully understood with regard to thermodynamic parameters that control the effective MPD flux. In the present work, the DB process applied to a single crystal multi-component superalloy is studied by combining experimental and numerical modelling results. A representative superalloy/braze filler system has been selected, namely CMSX-4/D-15. The main task consisted of studying the thermodynamic parameters that control the effective MPD (boron) flux into the parent superalloy. Boron diffusion into the superalloy material causes the formation of borides due to the low solubility of the matrix phase for boron and the presence of boride forming elements such as Cr, W and Mo. Boron is therefore steadily consumed from the superalloy matrix phase during isothermal solidification by in-situ boride precipitation. As a consequence, the effective boron flux into the parent material is increased. The overall boron content in the parent superalloy is higher than the boron solubility limit would allow. It has been found that there exists an optimum brazing temperature at which the effective boron flux into the CMSX-4 material due to boride formation is highest. With an increase in the brazing temperature the volume fraction of borides that are stable in the parent material steadily decreases. Above the optimum brazing temperature, the effect of boride formation on the effective boron flux into the superalloy material is low. The DB process is then mainly controlled by the low boron solubility in the matrix phase. The results regarding the effective boron flux and an optimum brazing temperature can be applied to most superalloy systems as most superalloys contain boride forming elements. The present work supports generally the selection of appropriate brazing cycles for the diffusion brazing of superalloys.