Microstructure And Property Controlling Of IN718 Alloy Bar By Triple Melting Processing

As the first-choice material for aero-engine turbine discs,IN718 alloy is increasingly strictly required with its metallurgical quality and uniformity.Therefore,GE,R-R and other aero engine companies require IN718 alloy billets for turbine discs to be prepared by VIM+ESR+VAR triple melting process in recent years,and the average grain size should be ASTM 6 or finer.In order to meet the requirements of aero engine development in China,the microstructure and property controlling of IN718 alloy bar for turbine discs by triple melting processing was studied in this paper.The main results are as follows:The effects of different melting rates on the macrostructure and microstructure of IN718 alloy 0430mm argon gas protection ESR ingots were studied.No macrostructure defects such as gas cavity,freckle and so on was observed in the ESR ingots.The depth of the melting pool,the equiaxed crystal ratio,and the secondary dendrite spacing can be effectively controlled by controlling the melting rate.There is microscopic loose in the ingots and the loose size is reduced as the melting rate is lowered.By using the self-developed‘BMPS-ESR' electroslag simulation software,the whole process simulation calculation of the ESR process was carried out,and was compared with analysis result of the ingot.The melting process of ?430mm IN718 alloy ESR ingot was predicted and simulated,and the variation of solidification and process parameters was systematically analyzed.The calculation indicates that the radial dimension of the shrinkage cavity plays the predominant role on the distributions of the Joule heat and electromagnetic force in the slag pool;the shrinkage cavity radius should be controlled as less than 0.05m in order to stably control the parameters such as melting power and current,and the result was experimentally verified.The effects of different melting rates on the macrostructure and microstructure of IN718 alloy 0508mm VAR ingots were studied.The growth modes of dendrites on both sides of the different melting rate ingots are basically the same,and the edges are fine columnar dendrites.The Laves phase content in the central,R/2 and edge part is basically consistent with the different melting rates,while the Laves phase in central and R/2 part is significantly higher than the edge part in an ingot.With the increase of the melting rate,the depth of the molten pool is obviously increased,the ingot symmetry becomes worse,the angle between the columnar crystals growth direction and axis increases,and the grain structure is transformed from columnar crystal to equiaxed crystal in the center of ingot.The melting rate becomes a key factor in controlling the dendritic morphology of the ingot.The effects of phosphorus and boron addition on the microstructure and properties of IN718 alloy bars were studied.The macrostructure of the IN718G alloy 0508mm VAR ingot with 0.02wt%phosphorus and 0.008wt%boron addition is similar to the IN718 alloy ingot,and is uniform,and no‘freckles',‘white spots' or other macro-segregation related defects was observed.The grain size,precipitated phase and tensile properties at room temperature and at 650?of IN718G alloy have no obvious difference compared with conventional IN718 alloy which was produced by the same high-alloy semi-continuous rolling process,but the stress rupture life at 650?/725MPa of the IN718G alloy is significantly longer,about 2?3 times,and the plasticity is also good.It is found that the segregation of phosphorus and boron at the grain boundary improves the grain boundary state,promotes the coordinated deformation of the crystal and the grain boundary,and significantly improves the long-term performance.The microstructure and properties of fine grain 0250mm bar of triple melting IN718 alloy were studied.The microstructure,tensile and stress rupture properties of the bar can meet the special requirements of aviation turbine discs.This is due to controlling the forging temperature and the amount of deformation,the average grain size of the bar can reach ASTM 6?7,and short rod like ? phases are mainly precipitated along the grain boundary.During long-term aging at 650?,the ?" phase gradually coarsens,and the y" phase depleted area of the grain boundary ? phase gradually widens.Although both the tensile properties(at RT&ET)and stress rupture life decreased after aging for 5000h,the stress rupture life is still far longer than the requirement in specification.