| Selective laser melting(SLM)technology has developed rapidly because of its high precision in producing objects of the required shape and of the low porosity of the final products.However,the effects of the high temperature involved in SLM forming on the microstructure of the alloys remains to be clarified.Duplex stainless steels have a good application prospect in the fields of energy,aerospace,electric power and so on because of their good resistance to corrosion,fatigue,creep and intergranular corrosion.However,there are still few studies on the high temperature oxidation properties of super dual phase steel.This thesis examined the oxidation behavior of three commercial alloys of classical forged type including the nickel base superalloy IN718,IN625 and the super duplex steel F55,plus the IN625 produced by additive manufacturing in 1 atm oxygen at 900℃,1000℃ and 1100℃.The aim of this work is to study the characteristics of their microstructure and to clarify their high-temperature oxidation behavior.In particular,the high temperature oxidation behavior of the two types of the alloy IN625,produced by rolling and by laser selective melting,was studied to clarify the influence of the microstructures induced by the two different forming methods on their oxidation resistance.The results are as follows:The phase composition,grain size,texture and high temperature oxidation properties of the alloy Inconel 625,produced both by rolling and by SLM,were studied.Both materials have the phase structure of the γ-Ni phase.However,the alloy prepared by SLM has polycrystalline structure and is mainly composed of cellular crystals and columnar dendrites,while the grain size is uneven and the dislocation density is small.The proportion of ∑ 3 in the CSL boundaries of the SLM alloy is much smaller than that of rolled alloy.There is a great difference in grain orientation between XY plane and XZ plane of the SLM alloy,while the grain(001)orientation of XZ plane is stronger in the construction direction.The kinetic results show that at 900℃ the oxidation rate of the SLM type is slightly higher than that of the rolled type,since the former variety can form more rapidly Cr2O3 layers which protect the alloy from further oxidation;However,at 1000℃ and 1100℃ the oxidation rate of the SLM material increases more rapidly;The thermal stability of SLM alloy is poor at high temperature.In particular,the high oxidation rate at 1100℃ leads to a poor compactness or even peeling of the oxide film,to the formation of cavities in the sub-surface layer of the alloy and to the formation of large amounts of NbCrO4 at the alloy/scale interface.The difference in the number of dislocations and twins affects the oxidation resistance of the two alloy types to the high temperature oxidation environment,resulting in a better oxidation resistance of the rolled samples as compared to the samples produced by SLM.The oxidation weight gains of the duplex stainless steel F55 at 900℃ and 1000℃ are lower than those of the alloy IN718,but the oxidation rate of F55 increases after 36 hours of oxidation at 1000℃;at 1 100℃,the oxidation rate of the duplex stainless steel F55 exceeds that of IN718 alloy.By oxidation at 900-1100℃ both alloys form protective Cr2O3 oxide scales,developing a Cr-depletion zone in the underlying alloy substrate.At the same time,according to Wagner’s theory,the rate of the intergranular oxidation of IN718 alloy is controlled by the diffusion of oxygen along the oxide/alloy interface and the diffusion of Al and Ti diffusion in the bulk.Finally,Oxidation of IN718 produced the metallic compound continuous TiO2Ni3Nb and TiO2 located at the scale/alloy interface which acted as a barrier and gave a good protection to this alloy at high temperatures. |
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