Investigation on Sustainable Refractory Systems for Investment Casting Manufacturing of Titanium Alloys

轻质钛合金具有密度低、强度高、耐腐蚀性能好等优点，在高温环境下具有广阔的应用前景。然而，铸造钛合金的困难来自于它们与氮和氧的反应性，即使在低浓度的标准真空炉中也是如此。熔化和铸造需要在真空非常高或惰性气体的炉中进行，以避免熔融钛中溶解氧气和氮气。除了气态元素和物质外，熔模铸造面临的主要挑战是熔融钛和硅基陶瓷模具之间的高反应性。人们发现，熔融的钛合金会与二氧化硅反应，释放出氧气，氧气溶解到溶液中，冷却后形成脆化的表面层，通常称为α情况。越来越多的惰性耐火材料系统正在设计以适应这些要求，这项研究将使人们了解一种拟议的成本有效的途径，以尽量减少α病例的形成。传统上，材料的选择是基于埃林厄姆图中氧化物形成的自由能。虽然氧化钇似乎是取代二氧化硅作为表面涂层材料的最佳候选材料，但限制这种氧化物使用的因素是氧化钇溶胶异常不稳定且非常昂贵。纯氧化物如氧化铝和氧化锆仍然与合金有明显的相互作用，需要通过化学铣削去除。所提出的研究将导致一种可持续的非二氧化硅基多矿物陶瓷系统，减少金属/陶瓷相互作用，用于铸造活性合金。为了支持这项研究，还将开发能够处理多组分、多相（即多矿物）系统的工具。这些模型允许计算合金成分和多种氧化物的实际活动。所有必要的信息都可以从吉布斯能量中获得，就像在卡尔帕德方法中使用的那样。虽然Ellingham图涵盖了纯元素和氧形成的纯氧化物，但calphhad方法将超越此范围，并处理非理想体系。这些信息在优化新浆料和壳系统的反应性时是无价的，在为特定合金定制壳系统时也是至关重要的。研究所选体系的胶体特性将对当前和未来浆料体系的稳定性有基本的了解，这对熔模铸造界至关重要。

The light-weight titanium alloys are promising for high temperature application owing to their low density, high strength and excellent corrosion resistance. However, the difficulty of casting titanium alloys arise from their reactivity with nitrogen and oxygen even when in low concentrations in standard vacuum furnaces. Melting and casting needs to be done in furnaces with very high vacuum or inert gas to avoid dissolving oxygen and nitrogen in the molten titanium. In addition to gaseous elements and species, the major challenge for the investment casting foundry is the high reactivity between molten titanium and silica based ceramic moulds. It has been found that molten titanium alloys will react with silica, freeing oxygen which dissolves into solution and forms an embrittled surface layer upon cooling, commonly referred as alpha case. Increasingly inert refractory systems are being devised to accommodate these requirements and this study would bring understanding to a proposed cost effective route to minimizing alpha case formation. Traditionally the material selection is based on free energy of formation of the oxides from Ellingham diagrams. Although yttria appeared to be the best candidate for replacing silica as a face-coat material, limiting factors on the use of this oxide are that yttria sols are exceptionally unstable and very expensive. Pure oxides such as alumina and zirconia still have significant interaction with the alloy which need to be removed by chemical milling. The proposed investigation will lead to a sustainable non-silica based multi-mineralic ceramic system with reduction in metal/ceramic interaction for casting reactive alloy. In order to support this study, tools capable of handling multicomponent, multi-phase (i.e. multi mineral) systems will also be developed. These models allow the calculation of realistic activities of alloy components and multiple oxides. All the necessary information can be obtained from Gibbs energies, as used in the Calphad approach. While the Ellingham diagram covers formation of pure oxides, from pure elements and oxygen, the Calphad method will go beyond this and treat non-ideal systems. This information is invaluable when rationalising the reactivity of new slurry and shell systems and crucial in tailoring shell systems for specific alloys. Studying the colloidal properties of selected systems will give fundamental understanding on the stability of current and future slurry systems, which is crucial for the investment casting community.