Effect Of Supergravity On Solidification And Inclusion Behavior Of IN 718 Superalloy

IN 718 superalloy is a key metal structural material which is most widely used in aerospace,power energy,national defense technology and other fields.Although the IN 718 alloy produced by domestic smelting can meet the application requirements by introducing foreign production equipment,there is still a gap in product quality and performance compared with the foreign advanced level,which is mainly manifested in the high content of inclusions and more structural defects.Due to the high degree of alloying of IN 718,the main problem of the ingot structure is the inhomogeneity of the composition caused by solute redistribution during solidification.And it harms the subsequent hot processing performance and the final product properties.During the independent research and development of high-cleanliness and homogenization of IN 718 alloy,it is noted that the supergravity separation technology has the characteristics of enhanced mass transfer and phase separation,and plays a unique role in the removal of impurity elements from the metal and changing the alloy solidification behavior.In addition,there is no direct contact with high-temperature alloys in the supergravity environment which demonstrated that supergravity is a green and clean processing technology.Moreover,the basic theory of the influence of high gravity on the solidification and inclusion behavior of IN 718 alloy melt is established by the experimental methods and theoretical analyses,which provides a guidance for the subsequent research work and the preparation of high-quality IN 718 alloy.Firstly,the solidification process of IN 718 alloy in the duplex process smelting was simulated by Thermo-calc software,combining with the DTA data and the high-temperature confocal results,it is determined that the liquidus and solidus temperatures are 1330? and 1125?,respectively.Also,the corresponding two reactions are L??+NbC and L??+Laves.Secondly,the Stokes formula is derived based on the force analyses of the inclusion particles in the supergravity field,and the influences of different experimental factors on the movement of the inclusion particles are analyzed theoretically.Subsequently,a series of experiments on the influence of different gravity coefficients(G)and holding times(t)of Al2O3 and TiN inclusions in IN 718 alloy were carried out.Based on these,the theoretical precipitation temperatures of Al2O3 and TiN inclusions in IN 718 alloy were calculated by Thermo-calc software.Besides,the precipitation process of TiN inclusions during solidification was observed in-situ according to high temperature confocal,and the following conclusions are drawn;the removal effect of Al2O3 and TiN inclusions from IN 718 alloy is remarkable.The number density and average size of the inclusions present an obvious gradient along the direction of supergravity,and the gradient becomes steeper with the increase of gravity coefficient and separation time.When the gravity coefficient G=210 and t=10 min,the total oxygen content at the best position of F(6 mm from the bottom of a sample)is 13.3 ppm,the content of nitrogen is 36.8 ppm,and the maximum removal rates of oxygen and nitrogen are 78.7%and 79.1%,respectively.Subsequently,the influences of gravity coefficient and cooling rate on the element distribution trend and micro-segregation characteristics of IN 718 alloy during solidification were investigated.The results show that the Nb and Mo are the most important segregation elements in the solidification process of IN 718 alloy.And the contents of Ni,Cr,Fe,and Al in the primary dendrite of the as-cast alloy decrease with increasing the gravity coefficient and cooling rate,while the contents of Nb,Mo,and Ti increase under the same conditions.In the as-cast state,the solidification structure of IN 718 alloy at room temperature transforms from dendrites to equiaxed crystals.Simultaneously,both the grain size of equiaxed crystals and the average length of the brittle Laves phase between dendrites decrease as the increase of gravity coefficient.In the heat treatment state,the number of strengthening phase ?" in the IN 718 alloy is increased,and the size of which is smaller,the needle-shaped 8 phase is slenderer.On the contrary,the content of the Laves phase is significantly reduced,which is beneficial to the improvement of the high-temperature strength of IN 718 alloy.After forging and heat treatment,when G=360,the high-temperature tensile strength,yield strength,elongation and the reduction of the area were increased by 17.9%,11.02%,12.5%and 25.4%,respectively.Finally,based on the above main results,the kilogram scale experiment was carried out of the IN 718 alloy obtained from the duplex and triple processes under the supergravity field.The effect of ultimate oxygen and nitrogen content of IN 718 alloy after supergravity treatment was predicted combined with thermodynamic calculations and Thermo-calc simulation analyses.And it reveals the law of removing Al2O3 and TiN inclusions from the IN 718 alloy by supergravity.The results show that after the IN 718 alloy obtained from the duplex process is treated by supergravity,the O content at the best position G(14 mm from the bottom of a sample)is 8.28 ppm and the N content is 22.08 ppm;In contrast,the O content at the best position G(14 mm from the bottom)is 3.98 ppm and the N content is 14.25 ppm when the IN 718 alloy smelted by the triple process.Therefore,the removal of inclusions from the IN 718 alloy by supergravity technology is feasible.