精密铸造熔体测温热电偶断裂机理分析及对策

Rupture Mechanism of Thermocouple for Melt Temperature Measurement in Investment Casting and Solution Strategy

航空发动机叶片精密铸造过程中熔体测温用B型热电偶(PtRh30-PtRh6)出现鼓包和频繁断裂的问题,导致叶片铸件报废。采用辉光放电质谱仪、扫描电子显微镜等方法,研究热电偶丝及陶瓷套管的化学成分和金相组织,并分析热电偶丝经1500℃/100 h热暴露后的组织热电势稳定性,确定热电偶断裂原因和机理。结果表明:热电偶断裂的主要原因是偶丝中出现富含Al、Si的低熔点组成相;同时发现该B热电偶的组织稳定性差,热暴露后表面组织和测温精度出现明显变化,严重影响热电偶的使用寿命和测温精度。建议控制热电偶原材料成分和组织,以减少热电偶的非正常断裂。

During the investment casting of aeroengine blades,the swelling and the fracture phenomena appear frequently in the Btype thermocouple(PtRh30-PtRh6)for melt temperature measurement,which results in the scrapping of castings.The chemical composition and microstructure of the B-type thermocouple with swellings and fractures,as well as the ceramic insulator and sheath,were detected by glow discharge mass spectrometry and scanning electron microscopy.At the same time,the thermal potential stability related to the microstructure of the B-type thermocouple after thermal exposure at 1500℃/100h was analyzed according to national standards.All the investigations were to obtain the fracture reason and mechanism of thermocouple.The results showed that the main reason for the fracture of the type B thermocouple was the presence of the phased with Al and Si rich contents in the thermocouple wire.Aside from that,it was found that the microstructure stability of the B-type thermocouple was poor,and there were significant changes in the surface structure and temperature measurement accuracy after thermal exposure,which seriously affect its service life and temperature measurement accuracy.Finally,based on the above results,the suggestions including the controlling the composition of raw materials and the microstructure stability for B-type thermocouples are proposed,which could reduce the abnormal fracture trouble of the thermocouple.