Research On Hot Corrosion And High Temperature Oxidation Behavior Of Selective Laser Melting Forming IN738LC Alloy

In recent years,with the increasing demand for the performance and efficiency of the engine in the complex environment in the aviation and aerospace fields,higher requirements have been put forward for the performance of nickel-based high temperature alloy materials used in its key components.IN738 alloy is a precipitation-strengthened nickel-based superalloy.Because of its excellent high-temperature mechanical properties and corrosion resistance,it is widely used in hot end parts such as aero engines and gas turbine blades.However,the use of conventional techniques to prepare IN738 alloy often has problems such as coarse dendrites,uneven chemical composition,and long processing cycles,which limit its use in industry.The rapid development of selective laser melting(SLM)technology has great advantages in the preparation of structurally complex parts and low-segregation alloys,which has accelerated the development of new nickel-based superalloys and the further expansion of application fields.At present,the research on SLM forming IN738LC alloy mainly focuses on the forming process parameters and the influence of heat treatment on its microstructure and mechanical properties,Other important properties such as high-temperature corrosion resistance are insufficient research.The hot corrosion behavior has not yet received full attention,which is very unfavorable for the application of IN738LC forming parts in the actual industry.Therefore,according to the service environment of IN738LC alloy,this research focuses on the hot corrosion behavior and high temperature oxidation characteristics of two batches of deposited and heat treated IN738LC forming parts under 75%Na₂SO₄+25%Na Cl mixed salt at 900?.At the same time,Combined with acid-alkali fusion theory,the mechanism of heat-resistant corrosion is discussed.The research work provides theoretical basis and data support for the application of IN738LC formed parts prepared by additive manufacturing method in actual industry.This work uses OM,XRD,XPS,and SEM characterization methods to analyze the corrosion behavior of two batches of as-deposited IN738LC forming parts under high temperature molten salt conditions,and discuss their heat-resistant corrosion mechanism.The results show that the corrosion weight gain rate of Sample 1#is linear,and the weight gain reached 3.99 mg/cm² after 60 hours of corrosion,and the thermal corrosion performance is extremely poor,The thermal corrosion mass weight gain rate of Sample 2#is equivalent to that of the traditional deformed GH738 alloy,all showing a parabolic law.The weight gain after 120 h is less than 1 mg/cm2,and the thermal corrosion performance is very good.The corrosion products formed are mainly Ni O,Cr₂O₃,Ni Cr₂O₄ and Ni₃S₂,but the corrosion products are constantly changing as the corrosion progresses.In the later stage of corrosion,the corrosion layer formed by Sample 1#is mainly composed of Cr₂O₃,and the corrosion layer formed by Sample 2#is composed of an outer layer of Cr₂O₃,a small amount of Ti O₂,and an inner continuous Al₂O₃ layer.The corrosion layer of GH738 alloy is composed of a single Cr₂O₃ layer.And the phenomenon of internal vulcanization is serious.The corrosion rate of the sample is proportional to the crack density and inversely proportional to its density,and the boundary of the molten pool with poor thermal stability is preferentially corroded.Because the XOY surface and XOZ surface of the samples prepared by the additive manufacturing method have a large difference in grain boundary width and area,the final alloy exhibits obvious anisotropy of corrosion resistance.Two batches of as-deposited IN738LC were heat treated,and then the relationship between the microstructure evolution and corrosion performance was studied.The results showed that only a few molten pool boundaries of HT 1#disappeared,and the internal cracks did not change significantly.The molten pool boundaries of HT 2#almost completely disappeared,and the grain morphology was further grown,so its chemical composition distribution was more uniform.The corrosion performance of HT 1#and HT 2#under molten salt conditions has been improved,but the types of corrosion products are basically the same as those of the deposited samples.The smaller the?'phase and the larger the quantity,the better the corrosion resistance.Because the sample forms a double protective corrosion layer,the fine grain structure exerts a"positive size effect",which is conducive to the improvement of corrosion resistance.For Sample 2#,which has better thermal corrosion resistance,the oxidation resistance at different temperatures was studied.The results show that the oxidation behavior of Sample 2#at different temperatures is the same,the corrosion kinetics curves are all parabolic,and the oxide layer is a mixed oxide layer of Cr₂O₃ and Al₂O₃.Since the Cr element undergoes further oxidation at 1000°C to generate volatile Cr O3,which makes the oxide layer discontinuous,the oxidation resistance of Sample 2#at900°C is better than 1000°C.