Role of Ca additive on the effect of phase composition and microstructure of BaZrO3 refractory and its interaction with titanium alloys

In this study, the effect of different Ca additives (Ca(OH)₂, CaO, and nano-CaCO₃) on the composition and microstructure of the fused BaZrO₃ crucible were investigated, as well as their interaction with Ti₂Ni alloy during vacuum induction melting. Results showed that the crucibles had the same phase compositions when the doping amounts of Ca additives were the same. Three kinds of Ca additives provided three different grain sizes of CaO precursors to participate in the solid solution reaction. When the doping amount of the Ca additive was 5.3 wt.%, the ZrO₂ phase in the fused BaZrO₃ was disappeared, and the CaZrO₃ phase was founded. The composition of the crucibles doped with 7.7 wt.% Ca additives only consisted of Ba_1-xCa_xZrO₃ and CaZrO₃ phase. The relative densities and erosion resistance of the crucibles were improved effectively with the increase of Ca additive content. Moreover, the crucible doped with 7.7 wt.% nano-CaCO₃ additive exhibited the highest density and the thinnest erosion layer. The Ba_1-xCa_xZrO₃ and CaZrO₃ phases were dissolved by the alloy melt, according to the interaction analysis. In addition, the crucible doped with nano-CaCO₃ additive had a higher content of refractory element concentration in the alloy melts in comparision with the other two kinds of crucibles due to their higher number of pores.     

Dissolution of BaZrO3 refractory in titanium melt

Although numerous investigations have studied BaZrO₃ as a crucible refractory for melting titanium alloys, the interaction mechanism between them has not been clarified. In this study, a set of three designed alloys with different Ti composition (TiNi, Ti_1.5Ni, and Ti₂Ni) were melted in the BaZrO₃ crucibles. By using the X‐ray diffraction, optical, and scanning electron microscopy analysis, the interactions between the BaZrO₃ crucibles and the titanium melts were investigated. It was found that dissolution of the BaZrO₃ refractory into the titanium melts resulted in the crucible erosion and the melt contamination, the degree of which were both increased with the increasing of Ti content in the melts. The dissolution reaction could be determined as follows: . The oxygen content dissolved in TiNi, Ti_1.5Ni, and Ti₂Ni melts was thermodynamically calculated as 0.0055, 0.1922, and 0.2263 wt%, respectively, which were in agreement with the experimental results.     

A novel potential ceramic material for melting Ti6Al4V alloy: A solid solution of BaZrO3 and CaZrO3

Vacuum induction melting technology is a promising low-cost method for producing high-quality titanium alloy. The key challenge lies in the development of ceramic crucibles with excellent chemical stability for titanium alloy corrosion. In this work, (Ba_1−x,Ca_x)ZrO₃ ceramic was designed and synthesised via pressureless sintering of a mixture of BaZrO₃ and CaZrO₃ powders. X-ray diffraction and scanning electron microscopy analyses showed that a new phase, Ba_0.8Ca_0.2ZrO₃, was formed after heat treatment at 1700 °C. Vacuum induction melting experiments of the Ti6Al4V alloy were carried out using a Ba_0.8Ca_0.2ZrO₃ crucible. Compared with the original BaZrO₃ crucible and CaZrO₃ crucible, the erosion layer of the Ba_0.8Ca_0.2ZrO₃ crucible was significantly reduced by approximately 85∼92.5%. The interface between the crucible and the alloy was clearly visible, and there was no obvious element diffusion between the alloy and the material. This shows that Ba_0.8Ca_0.2ZrO₃ is highly promising as a crucible material for melting Ti6Al4V alloys.     

BaZrO3 refractory crucibles for vacuum induction melting of industrial Zr-based bulk metallic glass master alloys with Y addition

The absence of appropriate melting method and expensive cost of high-purity Zr raw material limit the commercial application of Zr-based bulk metallic glass. In the present study, using high oxygen industrial grade sponge Zr as raw material and the metal Y as additive, the low-cost and high-purity master alloys were successively prepared using a VIM method with a BaZrO₃ refractory crucible. The results indicate that the BaZrO₃ refractory exhibited good erosion resistance to the alloy melt, the Y additive formed the Y₂O₃ barrier layer on the surface of crucible, which prevented the melt permeation into the crucible, then effectively reduced the thickness of the erosion layer. In addition, the metal Y deoxidizer could remove the oxygen of melts, finally the low oxygen Zr-based master alloy (about 0.02 wt%) was prepared. These results may provide a promising preparing technique prototype of low-cost Zr-based bulk metallic glass.     

Performance of BaZrO3/Y2O3 dual-phase refractory applied to TiAl alloy melting

Vacuum induction melting is a potential process for the preparation of TiAl alloys with good homogeneity and low cost. But the crucial problem is a selection of high stability refractory. In this study, a BaZrO₃/Y₂O₃ dual-phase refractory was prepared and its performance for melting TiAl alloys was studied and compared with that of a Y₂O₃ refractory. The results showed the dual-phase refractory consisted of BaZr_1-xY_xO_3-δ and Y₂O₃(ZrO₂), exhibited a thinner interaction layer (30 μm) than the Y₂O₃ refractory (90 μm) after melting the TiAl alloy. Although the TiAl alloys melted in the dual-phase and Y₂O₃ refractory exhibited similar oxygen contamination (<0.1 wt%), the alloy melted in the dual-phase refractory had smaller Y₂O₃ inclusion content and size than that in the Y₂O₃ refractory, indicating that the dual-phase refractory exhibited a better melting performance than the Y₂O₃ refractory. This study provides insights into the process of designing highly stable refractory for melting TiAl alloys.     

On the Modification of Hydration Resistance of CaO with ZrO2 Additive

Various ZrO₂/CaO samples were fabricated by cold isostatic pressing and sintered at 1750°C for 4 h. It was observed that the sample with 12% ZrO₂ additive possessed the good hydration resistance and had the lowest apparent porosity of about 0.75%; its weight additive stored after 56 days was less than 0.6 wt%, and it contributed to the occurrence of CaZrO₃ on the surface of CaO. The CaO crucible with 12 mol% ZrO₂ additive did not react with titanium melt during melting TiNi alloy. This provides a support for searching a new refractory with the good hydration resistance for induction melting titanium alloys.     

CaZrO_3耐火材料的制备及其与钛合金熔体的界面反应

合成了一种CaO-ZrO_2系复合耐火材料,用模压法成型,分别在1 650℃和1 680℃烧结制成坩埚.使用此坩埚在感应炉中熔化钛合金(TiNi),在真空条件下, 1 500℃保温5min.对在1 680℃烧结成的坩埚进行了抗水化性能测试.用X射线衍射仪对CaZrO_3材料进行物相分析.用扫描电子显微镜观察钛合金与此材料的界面反应层的微观形貌,结合能谱仪进行微区成分分析.结果显示:1 680℃烧成的CaZrO_3材料具有良好的抗水化性;用此坩埚熔化TiNi合金后,液态钛合金和耐火材料的界面反应层厚度约为30μm,界面反应层中,Ca,Zr,Ti,Ni等元素无明显扩散.     

Sintering behavior of Y-doped BaZrO3 refractory with TiO2 additive and effects of its dissolution on titanium melts

In this study, the sintering behavior of Y-doped BaZrO₃ with TiO₂ additive and effects of its dissolution on titanium melts were investigated. In order to overcome the difficulty of Y-doped BaZrO₃ sintering performance, the 0-5 wt% TiO₂ was added into Y-doped BaZrO₃ and its densification was investigated by density analyzer, scanning electron microscope (SEM) and X-ray diffraction (XRD). Thereafter, the interface reaction between two crucibles (without and with 2 wt% TiO₂) and titanium alloys, the thermodynamics and kinetics of dissolution reaction were also investigated. The results showed that Y-doped BaZrO₃ was rarely dense without TiO₂ additive and its relative density was just 88%, while after doping 2 wt% TiO₂, the relative density was more than 97%. However, with the excessive TiO₂(>2 wt%) doping, the secondary phase was observed by SEM and XRD. After melting titanium-rich alloys (Ti₂Ni, 66 mol% Ti) by Y-doped BaZrO₃ crucibles without and with 2% TiO₂ additive, the erosion layer of the two crucibles was approximately 4000 and 1700 μm, respectively. It was also found that the dissolved reaction rate was related to the density and grain size of Y-doped BaZrO₃ ceramic; the higher density and larger grain size ceramics can effectively prevent the crucible from being eroded by Ti₂Ni melt.     

Corrosion resistance of calcium zirconate crucible to titanium-copper melts

The corrosion resistance of refractory materials to the titanium alloy melts is vital for the production of titanium alloys by vacuum induction melting. In this study, the corrosion behavior of calcia-stabilized zirconia, solid state synthesized calcium zirconate, and fused calcium zirconate refractory suffering Ti-5 wt% Cu melts were investigated at 1680 ℃ for 15 min of soaking time by the cup test method. It was found that the three crucibles directly dissolved into the titanium melt, then generated Ti (Zr, O) and CaZrO₃ in the infiltration layer, and eventually developed a porous Ti₃O layer in the lining. Besides, the contamination of Ti-5 wt% Cu alloy (oxygen: 5.3 wt%; zirconium: 6.01 wt%; calcium: 0.42 wt%) by fused calcium zirconate crucible was significantly less than the solid state synthesized one (oxygen: 5.83 wt%; zirconium: 6.14 wt%; calcium: 0.43 wt%), implying that the production method of calcium zirconate notably affected the impurity of titanium alloys.     

Comparison of directional solidification of γ-TiAl alloys in conventional Al2O3 and novel Y2O3-coated Al2O3 crucibles

The effects of novel Y₂O₃-coated Al₂O₃ (Y₂O₃/Al₂O₃) crucibles on the microstructure and composition of directionally solidified TiAl alloys were investigated and compared with those of single layered Al₂O₃ and Y₂O₃ crucibles, based on which the corresponding alloy–crucible interaction mechanisms were discussed. The DS alloys exhibited a fully lamellar γ/α₂ structure interspersed with some Al₂O₃ or Y₂O₃ particles. Differently from that in the case of using Al₂O₃ crucibles, no interfacial interaction layer was found in the ingots prepared using Y₂O₃/Al₂O₃ crucibles. Dissolution and erosion were the main mechanisms responsible for the alloy–crucible interactions which increased with the heating temperature and interaction time. Nevertheless, the interaction extents when using Y₂O₃/Al₂O₃ crucibles were much lower than using Al₂O₃ crucibles, making the former promising candidate crucibles for the high quality DS of highly reactive TiAl alloys.     

Preparation and application evaluation of La2O3-doped Y2O3 crucible materials for melting TiAl alloys

A new La₂O₃-doped Y₂O₃ crucible materials was fabricated and evaluated by TiAl alloys melting test. Microstructure and properties of the La₂O₃-doped Y₂O₃ ceramics were systemically investigated. In addition, interfacial reaction mechanism of the La₂O₃-doped Y₂O₃ crucible materials and TiAl alloys, together with oxygen content of TiAl alloys were discussed. Solid solution of La³⁺ in the crystal lattice of Y₂O₃ significantly improved sintering properties of the La₂O₃-doped Y₂O₃ crucible materials and decreased the open porosity. Compared with pure Y₂O₃, when adding 15 wt% La₂O₃, the open porosity and strength retention ration after thermal shock test of the La₂O₃-doped Y₂O₃ crucible materials changed from 10.8% to 3.9% and from 64% to 78%, respectively. The interfacial reaction between La₂O₃-doped Y₂O₃ crucible materials and TiAl alloys belongs to physical dissolution, and no reaction products were found during the melting of TiAl alloys. When using the 15 wt% La₂O₃-doped Y₂O₃ crucible materials to melt TiAl alloys, oxygen content of the TiAl ingot declined to 530 ppm, which was only one fourth of that using pure Y₂O₃ materials.     

Phase and microstructural evolution of BaZrO3-CaZrO3 refractory and its interaction with titanium alloy melt

In this study, the effect of CaZrO₃ additives on the phase and microstructure of BaZrO₃ refractory were investigated as well as the interaction with titanium alloy melts. Results revealed that no second phase was observed in the BaZrO₃ refractory with 10 and 20 mol% CaZrO₃ additives, respectively, and the incorporation CaZrO₃ additives promoted the densification and the growth of grains due to the generation of BaZrO₃-CaZrO₃ solid solutions. Whereas, the appearance of CaZrO₃ phase was in the BaZrO₃ refractory with 30 mol% additive, indicating that the additive content excessed the solid solubility limit in the BaZrO₃ refractory. Interaction analysis indicated that CaZrO₃ additive could improve the performance of BaZrO₃ refractory for resisting the penetration of Ti₂Ni alloy melt. Meanwhile, an increasing extent of melt contamination by the BaZrO₃ refractory was also detected with the increasing content of CaZrO₃ additive. According to the thermodynamic calculation, an obvious increase in Gibbs free energy of formation for the BaZrO₃ refractory was confirmed with the CaZrO₃ additive, resulting in the increasing extent of the dissolution-corrosion between the refractory and the titanium alloy melts.     

Preparation of a novel Sr-Zr oxide refractory for induction melting of high-activity alloy

In this study, a new Sr-Zr oxide refractory was designed and prepared through a solid-state reaction method according to the mole ratio of n(SrO): n(ZrO₂) = 2:1. The results reveal that the Sr-Zr oxide refractory consisted of three phases, i.e., Sr₂ZrO₄, Sr₃Zr₂O₇ and SrO. The thermodynamically stable phase Sr₂ZrO₄ was hard to form completely due to the limitation of reaction kinetics between SrCO₃ and ZrO₂, and thus Sr₃Zr₂O₇ and SrO coexisted in the Sr-Zr oxide refractory. The composite refractory exhibited a good corrosion resistance during the melting of TiNi alloy melts, and only a slight interaction occurred in the refractory crucible after melting, with the formation of a reaction layer (28 μm in thickness). In addition, the oxygen concentration of the prepared TiNi alloys was determined to be only 0.046 wt.%. These results indicate that the Sr-Zr oxide composites have potential to be used as a refractory candidate for the induction melting of high-activity alloys.     

The effect of synthesis conditions on the phase composition and structure of thorium bearing murataite ceramics

Samples of thorium-bearing ceramic with a target composition (wt%) 5 Al₂O₃, 10 CaO, 55 TiO₂, 10 MnO, 5 Fe₂O₃, 5 ZrO₂, 10 ThO₂ were produced by melting in glassy carbon crucibles in a resistive furnace and by cold crucible inductive melting (CCIM) at a vibration power of 10 kW and operation frequency of 5.28 MHz. All the samples contained 85–95 vol% murataite polytypes with 5- (5C), 8- (8C), and 3-fold (3C) elementary fluorite unit cell composing core, intermediate zone and rim of the grains, respectively, and minor crichtonite, perovskite, pyrochlore, rutile, etc. A feature of the ceramics obtained by melting in glassy carbon crucibles is formation of Fe (II) titanate whereas the inductive-melted ceramics contained traces of vitreous phase due to melt contamination with a cold crucible putty material. Melting rate in the cold crucible of up to 350 kg/(m² × h) has been achieved. The ceramics obtained have excellent chemical durability.     

Reaction mechanism between MgO crucible and AerMet100 steel during vacuum induction melting

AerMet100 steel has strict composition and inclusion requirements. Therefore, its reaction with MgO refractory during vacuum induction melting cannot be ignored. In this study, the reaction mechanism between the MgO refractory and AerMet100 steel during the refining stage was investigated using a MgO crucible. The influence of the MgO crucible on AerMet100 steel composition and inclusions under refining vacuum pressures of 50–100 and 5–10 Pa was compared. The results indicate that SiO₂, Al₂O₃, and MgO in the crucible decompose and are reduced by C in the liquid steel, which results in the increase of Si, dissolved Al (Als), and dissolved Mg (Mgs) content in the liquid steel. The increase of Ca content is due to the reduction of CaO in the crucible by C in the liquid steel. The reaction of Al₂O₃ inclusions and Mgs in the liquid steel is the primary generation method of MgO·Al₂O₃ spinel inclusions. As the Mgs content in the liquid steel increases, Al₂O₃ inclusions transform into MgO·Al₂O₃ spinel inclusions along the path Al₂O₃ + Mgs → Al₂O₃ with a small amount of MgO + Mgs → MgO·Al₂O₃ spinel. In contrast, the vacuum pressure of 50–100 Pa is more effective at controlling the composition and inclusions of AerMet100 steel and is a more appropriate choice for the refining vacuum pressure.     

Processing of vacuum heat-treated MgO-densified silicon nitride crucible for molten cast iron handling

Silicon nitride with 3% MgO powder mix is uniaxially and cold isostatically pressed to form a green Si₃N₄ crucible. Liquid phase sintering was applied to the green Si₃N₄ crucible at 1600 °C for 30 min under the nitrogen atmosphere. Intergranular Mg–Si–O–N glass remained between the silicon nitride grains which reacted with the molten metal during melting. This grain boundary glass was removed by vacuum heat treatment at 1575 °C for 5 h. The vacuum heat treated crucible was used to melt cast iron to examine reactions between the molten metal and Si₃N₄ ceramic crucible. EDX spectra across the Si₃N₄–cast iron interface and XRD for silicon nitride sample after cast-iron melting side surface analysis were carried out. Optical microscopy and SEM image analysis were made to examine the interaction between Si₃N₄ crucible and cast iron melt. Surprisingly, no reaction was observed between the vacuum heat treated crucible and melted cast iron.     

Chemical reactivity of oxide materials with uranium and uranium trichloride

A graphite crucible is used for the manufacturing of uranium ingots in the uranium casting equipment of the electrorefining process. Uranium and uranium alloys are typically induction melted in graphite crucibles under a vacuum condition; however, due to the chemical reactivity of uranium and most alloying elements with carbon, a protective ceramic coating is generally applied to the graphite crucibles. To investigate the most suitable ceramic coating material for application to graphite melting crucibles used for the melting uranium in uranium casting equipment, firstly, a thermodynamic analysis using HSC software was performed to examine the chemical reactivity of ceramic oxide materials with uranium and uranium trichloride, and also, experiments concerning the reactivity of molten uranium in some ceramic coated crucibles were performed at 1,300 °C. From the results, yttria was finally selected as the most suitable ceramic coating material for application to graphite crucibles for melting the uranium.     

Vacuum heat-treatment of MgO-densified silicon nitride ceramics and their compatibility with molten aluminium and copper

Silicon nitride with 3% MgO powder mix was compacted with cold isostatic pressing followed by uniaxial pressing. Pressureless sintering of the compacted silicon nitride (Si₃N₄) crucibles was at 1600 °C for 30 min in the carbon furnace. The densities achieved after this process are 3140 kg/m³. One of these crucibles was vacuum heat treated at 1575 °C for 5 h to remove grain boundary glass. Both this crucible and the as-sintered crucibles were used for melting aluminium and copper by heating in, air atmospheres to 700 °C and 1100 °C, respectively. Vacuum heat-treated glass free ceramic crucible was successfully used for the molten aluminium and copper handling. On the other hand, the chemical bond occurred for as sintered Si₃N₄ crucible. No adherence was observed after examining of interface between Al and the heat-treated ceramic crucible with SEM and EDX analysis.     

Reaction of metallic titanium with zirconia crucibles

Conclusions The different nature of the destruction of crucibles made from ZrO₂ with additions of CaO and Ti during the melting of titanium was established, and also the influence of the accompanying processes on the microhardness of the molten titanium.Wider testing of ZrO₂ crucibles containing additions of titanium for melting metallic titanium and its alloys should be made in laboratory and industrial conditions with a detailed investigation of the quality of the metal obtained.     

Interaction between CA6-MA crucible and molten wrought Ni-based superalloys

The interfacial behavior between CaAl₁₂O₁₉-MgAl₂O₄ spinel (CA₆-MA) crucible and molten wrought Ni-based superalloys during the vacuum induction melting process have been comprehensively investigated, with Al₂O₃ crucible included for comparison. Experimental and theoretical results suggest that CA₆-MA crucible exhibits higher thermodynamic stability and poor wettability to the superalloy melt. Benefited from these merits, the superalloy ingot with cleaner surfaces and more metallic luster is obtained in the CA₆-MA crucible. Furthermore, the total oxygen content in the superalloy ingots obtained from CA₆-MA crucible is 17.9 ppm, which is considerably lower than that from Al₂O₃ crucible (29.7 ppm). These results indicate that CA₆-MA crucible can be a promising candidate for melting wrought Ni-based superalloys.