带热障涂层镍基单晶高温合金的激光制孔研究

Laser Drilling of Ni-Base Single-Crystal Superalloy through Thermal Barrier Coatings

摘要:

采用毫秒激光和皮秒激光在带热障涂层的镍基单晶合金上加工了气膜孔，对比研究了长脉冲与超短脉冲加工对热障涂层及金属基体孔壁形貌的影响。实验发现，波长1064 nm的毫秒激光在试样表面产生的能量密度直接影响到陶瓷层的加工。以2866 J/cm2的能量密度从陶瓷面加工，陶瓷面的熔化所需要的热积累时间长，热量会传导至高温金属，产生类似熔池的热影响；而从金属面加工则由于陶瓷是最后加工的材料，有足够的热积累时间熔化陶瓷涂层，从而直接打通小孔。当毫秒激光的能量密度提高至6369 J/cm2时，热量在涂层中的积累速度加快，陶瓷材料能够快速熔化，从而避免了金属基体先于陶瓷熔化的现象，同时，加工过程中熔化后的陶瓷会经过孔通道，从而出现附着在孔壁上的现象。采用皮秒激光加工陶瓷涂层仅需要能量密度达到32 J/cm2，皮秒激光旋切制孔是将小孔圆周上的材料全部剥蚀掉，直至孔打通，而孔内的材料会从孔中掉出。皮秒激光加工中产生的等离子体冲击力会引起涂层的开裂，由于热障涂层制备方法不同引起涂层中的裂纹方向有所不同，等离子喷涂制备的涂层为层状结构，裂纹易沿平行于表面方向生长，而EB-PVD制备的涂层为柱状晶结构，裂纹多出现在柱状晶的间隙

Abstract:

Millisecond laser and picosecond laser were used to drill in Ni-based single crystal alloy with thermal barrier coatings for processing film cooling holes. The morphology of the thermal barrier coating and the metal substrate processed by long pulse laser and ultrashort pulse laser has been studied. Results show that when the energy density generated by millisecond laser of λ=1064 nm is 2866 J/cm2, the process from the top coatings requires long time for melting and heat accumulation will be conducted to the alloy, and thus the heat affected zone appears in the form of molten pool. However, millisecond laser drilling from the metal surface avoids obvious heat affection, because there is enough time to accumulate heat to melt the ceramic coating but not affect the alloy. When the energy density of millisecond laser has been increased to 6369 J/cm2, the accumulation of heat in the coating is very rapid, the ceramic material is molten rapidly, so heat transmitted to the metal substrate is avoided, while there is ceramic chip is attached to the hole wall when the molten ceramic pass through the hole. The picosecond laser requires only energy density of 32 J/cm2 for ceramic coating processing. The process of picosecond laser trepanning drilling is that the material on the circumference of the hole is ablated until the hole is opened up, and the rest material of the hole falls out from the hole. The microcracks in the coating are generated by plasma impact force caused by picosecond laser processing. The preparation methods of TBCs cause the different directions of microcracks. The TBCs prepared by plasma spraying is a layered structure and the microcracks grow up along the direction parallel to the deposited layers. However, the EB-PVD coatings is columnar crystal structure, therefore microcracks appear in the gap of columnar crystals.