Study On The Microstructure Evolution And Oxidation Behavior Of Laser Remelted Nb-Si Alloy

The rapid development of aviation technology requires better performance of high thrust turbine engines,which means that the materials used for engine blades must have better overall performance.Compared to Ni-based superalloy,Nb-Si based ultra-high temperature alloy becomes an important candidate alloy as new generation structural material,because of its higher melting point and lower density.However,the imbalance of mechanical properties and the lack of oxidation resistance limit the application of Nb-Si based ultra-high temperature alloy.Novel techniques such as laser remelting could form a remelted layer with fine microstructure on the surface of the alloy,change the diffusion of elements in the alloy,and promote the formation of a protective oxide film.Meanwhile,the refinement of the microstructure could also improve the adhesion of the oxide film.Based on this,the laser remelted treatments of Nb-Ti-Si-B alloy and Nb-Ti-Si-Al alloy were carried out in this work,and the laser remelted microstructure evolution of Nb-Si alloys was studied.Moreover,oxidation mechanism of as-cast Nb-Si alloy and laser remelted Nb-Si alloy was compared and analyzed.Firstly,the finite element simulation of the laser remelted process of Nb-Si alloy was conducted.It was found that the depth,the width,the length and length-width ratio of the molten pool were all increased along with the increase of laser power and the decrease of laser scanning rate.When the laser power density was in the range of 56?75W/mm²,it was expected to obtain a stable molten pool and good surface melting quality.The laser power and scanning rate of Nb-Ti-Si-B alloys were determined based on the simulated temperature field of the molten pool and the pr-experimental results of Nb-22Ti-17Si-4.5B alloy.Nb-18Ti-14Si,Nb-18Ti-14Si-4.5B and Nb-18Ti-17Si-4.5B were remelted with laser power of900W,scan rate of 6mm/s and spot diameter of 2mm.After laser remelting,the microstructures in thesealloys were significantly refined.The microstructure of Nb-18Ti-14Si alloy was Nbss+Nb₃Si eutectic in the remelting zone;the microstructure of Nb-18Ti-14Si-4.5B alloy and Nb-18Ti-17Si-4.5B alloy were Nbss+?-Nb₅Si₃ eutectic in the remelting zone,and the eutectic microstructure of Nb-18Ti-17Si-4.5B alloy had a higher content of silicide.Oxidation experiments were carried out at 850°C/3h,and the oxidation resistances of as-cast microstructure and laser-remelted microstructure were compared for each alloy.As-cast Nb-18Ti-14Si-4.5B alloy had the best oxidation resistance,and its oxide film was still intact,which was related to its Nbss+T2(?-Nb₅Si₃)eutectic structure.There was no primary Nbss or T2 phase in this alloy.These oxide films in the remelted area of Nb-18Ti-14Si,Nb-18Ti-14Si-4.5B and Nb-18Ti-17Si-4.5B alloys were smooth and dense without flaking,which was related to the grain refinement caused by laser remelting.For the Nb-18Ti-14Si-9Al alloy,laser remelting experiments were carried out with scan rates of 6mm/s and 8mm/s and laser power of 800W and 1000W,respectively.Under different processes,the microstructure of the Nb-18Ti-14Si-9Al alloy in remelting zone was composed of fine Nbss+T2 eutectic cells.However,the laser process parameters affected the refinement efficiency of the eutectic cells.As the scan rate increased or the power decreased,the size of the eutectic cell decreased accordingly.When the power was 800W and the scanning speed was 8mm/s,the minimum average size of the Nbss+T2 eutectic was 4.77?m in the center of remelting zone.After the Nb-18Ti-14Si-9Al alloy was oxidized at 850°C for 3 hours,an oxide film was mainly composed of Ti₂Nb₁₀O₂₉ on the surface of the as-cast alloy,and cracks and holes existed in the oxide film.The oxidation film on the surface of remelting zone was quite thin,and the microstructure followed the eutectic structure characteristics of the remelting zone.There were a few pores in the oxide film.The O content in the internal oxidation zone of the remelting zone was lower than that of the as-cast zone.After 13h(3h+5h+5h)oxidation,the oxide film in the remelted zone was still dense and intact,and no cracks or holes appeared.From the analysis of microstructure,Nb-18Ti-14Si-9Al alloy was expected to maintain good oxidation resistance for a longer time after laser remelting.This was related to the refining eutectic structure,which could accelerate element diffusion and shorten the lateral growth distance of oxides.