Effect Of C On The Sand Adhesion And Microstructure And Mechanical Properties Of Directionally Solidified Ni-based DZ417G Superalloy

The effect of carbon(0.05 wt.%,0.09 wt.%,0.17 wt.%)on the interface reactions between DZ417 G molten alloy and the ceramic material and the effect of carbon addition on the microstructure and the mechanical properties of DZ417 G alloy was investigated in this dissertation.In investment casting of superalloys,the interface reactions between the molten alloy and the ceramic mould would cause superficial defects on the casts,such as sand adhesions or reaction layers.The sand adhesion and reaction layers would lower the surface quality and affect the subsequent machining of the casts.The wettability between the molten alloy and the ceramic material is correlated with interface reaction.For example,interface reactions usually improve the wettability and facilitate the infiltration of the molten alloy to the microcracks or pores on the ceramic mould.The main factors that affect the wettability include the active element in the alloy,the active component in the ceramic,atmosphere as well as the casting temperature.Accordingly,it’s of great importance to investigate the interface reactions and the wettability between the molten superalloy and the ceramic material.The influence of the active element in the alloy and the active component in the ceramic on the influence reactions was studied carefully and the research findings have both great theoretical and practical significance.The effect of C on the interface reactions were investigated by the method of sessile drop experiment.The microstructures of the reaction product were observed by SEM and the phase identification was analysed by XRD.The main results are that with C content increasing the wetting angle become decreasing and the the interface reaction become severely.The C as a role of catalyst for Cr reaction,Cr would react and produce Cr2O3 if C content beyond 0.09 wt.% and 9Al2O3·Cr2O3 was ultimately formed due to the combination of Cr2O3 and Al2O3 in the ceramic.3 samples with different C content were produced by using directional solidification furnace with the method of Bridgeman.The results showed that the addition of carbon did not significantly alter the primary and secondary dendrite arm spacing,however the tertiary dendrite is the most developed at the carbon levels of 0.09 wt.%;with increasing carbon addition,the degree of segregation of Ti and the volume fraction of γ-γ’ eutectic both decreased,meanwhile the volume fraction of carbides increased;the carbides formed in all alloys appeared script-type morphology and the orientation relationship between the carbides and the matrix was(001)γ//(001)Ti C and <001>γ//<001>Ti C.Higher carbon content leads to a greater supersaturation in the later stage of solidification,which induce the growth of well-developed secondary and tertiary dendrite arms.In addition,with the carbon addition increasing,the number and the volume fraction of porosity represent the trend of decreasing first and increasing afterwards.It is correlation between the solidification microstructure and the precipitation behavior of MC during solidification process.After heat treatment(1220℃/4h,AC+980℃/16 h,AC),The effects of carbon addition on the solidification microstructure and heat treatment microstructure and rupture life were investigated in three different carbon level of DZ417 G superalloys through scanning electron microscopy(SEM)and transmission electron microscopy(TEM).The relationship between microstructure and rupture property was discussed.The results showed that with the increasing of carbon level,the volume fraction of eutectic decreased and the volume fraction of carbides increased.After heat treatment the γ’ become uniform cubical and the γ-γ’ eutectic were dissolved mainly;MC carbides were dissolved slightly,and M23C6 was appeared after heat treatment.With the increasing of carbon content,the rupture life of the DZ417 G superalloy decreased.The change of rupture life was mainly effected by the un-dissolved eutectic and carbides of alloy.The volume of eutectic and carbides were controlled by carbon addition,therefore,the carbon content should be controlled in 0.05 wt.%,which was benefit to the rupture property mostlyThe present work indicates that C add to the alloy could strengthening grain boundary and refining alloy melt;on the other hand,as an active element,C would react with Si O2 and released gases as long as C content was high enough.Interface reactions could improve the wettability and facilitate the infiltration.Therefore it may reduce the interface reaction between superalloys and ceramic mould,but also improve the mechanical properties if the C content could control reasonably.