Effect Of ? On Hot Deformation Behavior,Microstructure And Mechanical Properties Of IN718 Alloy During DP Process

IN781 alloy is a typical nickel-chromium-iron-based deformed superalloy,which has become the material of choice for national defense in various countries.At present,the forming process of complex IN718 alloy parts adopts Delta Process(DP)process,and this process has achieved good results in industrial applications.The conventional DP process uses direct temperature ? phase peak temperature insulation to cause uneven precipitation of the ? phase.In this paper,by changing the precipitation heat treatment method of ? phase in DP process,it is named Advanced Delta Process(ADP)process,and the DP process and ADP process are systematically studied.First,different ? phase precipitation heat treatment methods are applied to the IN718 alloy.Traditional precipitation method: 900 °C insulation,improved precipitation: 720 °C insulation 8h + 900 °C insulation.The morphology of the ? phase was different under the two precipitation modes by OM and SEM.In the traditional precipitation mode,the acicular ? phase is precipitated along the grain boundary and the twin boundary,and the crystal is granular.In the improved precipitation mode,the ? phase is uniformly precipitated in the grain boundary and the crystal,and grows from the granular shape to the long acicular.The microstructure of the indirect precipitation method for 1 h and 8 h by TEM showed that the granular ? phase was transformed from the ,,phase.And through EPMA found that the improved precipitation method eliminates the segregation defects of Nb and Cr elements.Then,the alloy after the ? phase conventional precipitation method is called an aged alloy,and the alloy under the improved precipitation mode is called a double-aging alloy.The effects of ? phase morphology on flow stress and deformation activation energy were studied by analyzing the stress-strain curves under different deformation conditions during the hot deformation of aged alloys and double-aging alloys.The results show that the flow stress of double-aging alloys It is lower than the flow stress of the aged alloy,and the deformation activation energy of the double-aged alloy is greater than that of the aged alloy.The flow stress equations of the two alloys were obtained by regression of experimental data and compared with the actual experimental results.Secondly,by analyzing the microstructure and deformation of the alloy under different deformation conditions and the microstructure of the double-aging alloy,it is found that the ? phase in the double-aging alloy is pinned at the grain boundary during medium and low temperature deformation.However,the ? phase of the aged alloy is mainly deformation,so the grain size of the double-aging alloy is smaller than that of the aging alloy;the recrystallization positions of the two alloys are also different,and the aging alloy mainly along the original grain boundary,the double-aging alloy The grain boundary and the crystal occur simultaneously,so the recrystallization of the dual-aging alloy is more sufficient;the microstructure of the double-aging grain after the deformation is also finer,which is related to the evolution of the ? phase.When deformed at a high temperature and kept warm,there is almost no difference in the microstructure of the two alloys due to the large amount of dissolution of the ? phase.Finally,the mechanical properties of IN718 alloy formed by ADP process and DP process were tested by room temperature tensile test.The results show that the mechanical properties of forged parts after ADP forming are better.Through the analysis of the stress-strain data,the stretching process is elastoplastic deformation,and the work hardening index of the two alloys has almost no effect by fitting the tensile data.By observing the fracture morphology,it is found that the fracture mechanism of the two alloys is microporous aggregation.The alloy formed by the DP process has a large number of dimples and a large size,and there are large particles of NbC in the dimple.The alloy formed by the ADP process has a small number of dimples and no large granular carbide.