Laser Surface Melting Treatment On Ni-base Superalloys And The Research On Selectively Chemical Dissolution Of The Recast Layer

Ni-base superalloys are widely used in aero-engine and power plant because of their excellent high temperature mechanical and chemical properties.But they demonstrated very poor weldability in practice.During laser drilling process,the inner wall of film holes is easy to form remelt recast layer.There are more metallurgical defects within recast layer,such as voids and microcracks,which will cause stress concentration,directly impacting on the fatigue strength of the blade,thus causing a serious threat to the safety and reliability of the entire engine.Currently,the process for removal of laser recast layer is not easy to apply to the actual production.To achieve the removal of recast layer efficiently and reliably,An experimental investigation on the microstructural characteristics of Ni-base superalloys recast layer by rapidly melting is described in the paper.Based on it,the process of recast layer removal was studied and a thorough study of the mechanism was made.In order to provide the technical guideline for chemical milling,microstructural characteristics of DZ125?IN625?GH4648 recast layer was investigated,the formation and propagation mechanism of the premature cracks of the recast layer was also researched.The research result shows that the microstructure of the recast layer is significantly refined,consisting of cellular crystal and dendrite structure.For DZ125 recast layer,however the prime(? + ?')eutectic disappeared and ?' precipitate reduced.And solidificdation microcracks can be observed in that recast layer,which in turn will be harmful to the laser treated components,but rare crack extends to the substrate.The electrochemical corrosion properties for recast layer and the substrate in a chemical milling solution was investigated.The results reveal that different alloys recast layer and substrate in the same solution demonstrates different electrochemical corrosion characteristics,the corrosion behavior of recast layer and substrate is greatly different.Thickness of recast layer does not affect the electrochemical corrosion behavior.CuSO4 can enhance alloy substrate passivation,FeCl3 can accelerate recast layer corrosion.During a certain concentration,DZ125 alloy substrate corrosion is much better than recast layer.Since recast layer is heat treated,recast layer precipitate out ?' phase,its corrosion resistance.significantly enhanced.Thus after laser drilling,chemical milling should be carried before heat-treatment.In order to achieve industrial production of chemical milling,investigation of recast layer dissolution kinetics was conducted.The research shows that optimum conditions of chemical milling,it is that chemical milling solution is incubated at 60? or more,more than 90%concentration.During the opitimum conditions,recast layer is removed efficiently and reliably,and bring no damage to the substrate.The study results showed that the recast layer will be effectively removed when approitate chemical solution and optimized technique are applied.These will generate smooth holes,thus do not reduces metal mechanics,and the thermal fatigue and high temperature stress rupture performance of holes has also improved.