The Study On BN Based Investment Shell Mold Used For Casting Ti Alloy And Its Preparation Process

Titanium alloys are one of the excellent metallic materials, which have high properties and wide usages. The investment casting technology is the one of most important forming technologies for Ti alloys parts. Currently the key of the development of the investment technology for Ti alloy casting still is to search a kind of shell mould material which has lower cost, good technologic properties and high chemical inertness for investment casting of Ti alloys and to explore its preparation technology and to manufacture excellent Ti alloy castings with the investment shell mould.In this paper, the interaction between the Ti melt and some refractory such as hBN, ZrO₂ were studied. From those studies, it is found that hBN has enough chemical inertness for the melting Ti. So we tried to adopt hBN particles to wax pattern surface as the mold material to reduce the thickness of reaction layers on Ti castings.Based on the interaction study, we developed a new kind of investment shell mold,which is BN based shell mold used for the Ti casting. The face coatings of BN based investment shell mold are made of pretreated hexagonal Boron nitride (hBN) particles with a few yttria powders and colloidal yttria as binder. And the back up coatings of the shell mold are composed of mullite powders and colloidal silica.Moreover, some facts of that shell mold were also studied including all kinds of typical properties of those shell mold, interaction between the melting Ti and shell mold, so as to improve both of the properties and preparation process of those shell mold, and clarify whether that shell mold could be used for investment casting Ti.The slurry used for manufacturing face coatings of shell mold should have an appropriate value of filler/binder ratio with a low viscosity which is needed to achieve better surface of shell mould. Otherwise, the cost of those BN particles is another important impact concerned, which has to be cheap enough to low the cost of shell mold. There are different grades of BN with various properties such as tap density, area, flexibility and cost, but some kinds of BN are not suitable enough for making the shell mold, such as t-BN and high purity hBN. In this study, it is found that pretreated BN particles which were treated in high temperature and high pressure could be used to making the face coating of BN based investment shell mold.In order to produce shell mold with enough glabrous and compact inner surface, the slurries made of different BN content and filler/binder ratio were studied in this paper. The results show that the slurries made of Boron nitride particles, yttria powders and colloidal yttria have yield and psendoplastic property which means the appearant viscosity decreased with increasing shear rate. The BN particles content have great influence on appearant viscosity, suspension rate and thickness of slurry. Meanwhile the increase of yttria powders content will lead to improve filler/binder ratio, density of the slurries.Because bind yttria colloidal used in shell mold will resolve when it meets water. To keep the shell mold from cracks, a new kind of dewaxing method—microwave dewaxing was studied. The results show that microwave dewaxing is an effective dewaxing process, and some influence which mainly impact dewaxing efficiency are quantity of back-up coatings, design of wax mold and the way shell mold located in microwave.Sintering process is one of the most important process in investment casting processes. The DSC, TG and some other analysis were employed to study appearance of inner surface, blending strength and composition of those BN based shell mold sintered in different heat process. The physical and chemical changes of those shell mold occurred during sintering process were also studied. The results show that during the sintering process in air, in the face coating of shell mold the BN particles reacted with yttria powders and formed YBO₃, which has a low melting temperature and low chemical inertness to molten Ti.Considered with cost of shell mold, the sinter process of BN based shell mold should be as fellow: The sinter temperature should be 1000℃, and according to the size of shell mold, and heat preservation time is different. When the sinter temperature is lower than 900℃, shell mold can be heated in air; when the sinter temperature is over 900℃, it should be heated in protective atmosphere such as N2. Moreover, the interaction between the shell mold and melting TiNi alloy and Ti-6Al-4V alloy were also studied, so as to clarify whether this kind of shell mold could be used for investment casting Ti alloys. The TiNi alloy casting process was done in a controlled atmosphere induction melting furnace, and TiNi alloy was melted in CaO crucible, and the Ti-6Al-4V alloy was melted in a controlled atmosphere induction furnace with a high purity BN and a segment water-cooled copper vessel crucible (ISM). The cross-section of reaction intersurface between Ti alloys and BN based shell mold was investigated by electron microscopy (SEM), electron probe micro-analyzer (EPMA) and mircohardness analysis. The results show that the reaction between melting TiNi and shell mold is slight and the thickness of the reacting layer is only about several micrometers. The results also show that when the casting alloy was 1750℃Ti-6Al-4V alloy, the thickness ofα-case in Ti casting is about 180²00μm.The activity of TiNi and Ti-6Al-4V alloy, and theα-case formation mechanism were also discussed. The resluts indicated that activity of Ti-6Al-4V alloy was higher than that of TiNi alloy, and the mold-metal interation was a dual diffusion process which would be influenced by Ti alloys activity, pouring temperature and quality of shell mold。Except the study of mold-metal interation, the comparison of the making cost of these shell mold with that of the yttria one was also studied. And the result shows that Ti parts casted in BN based investment shell mould with a low cost and a low thickness of"α-case".In brief, the Ti investment casting technology with the BN based investment shell mould is a development potential technoiogy. It not only has the dominance of the Ti investment casting technology over other forming techniques, but also has strickingly features of lower cost and environment friendship, comparing to the conventional Ti investment casting technology. However it is still not the end and our research on the Ti investment casting technology does not finish. There are a lot of things to be done, such as the property modification of the BN based investment shell mould, the manufacture technology improvement, the appreciation about its value in use and so on, which make a solid base of the industrial appication of the BN based investment shell mould for Ti alloy casting. Once its industrial appication has been realized for the future, it is possibly as a substitute for the conventional Y2O₃ based investment casting technology. Thus it may become an environment friendship technology which greatly decrease the production cost and make a continuation of the Ti alloy application from the military to the civil use.