Process for Obtaining Y-Tial Pieces by Casting

Abstract

The present invention concerns the development of a process for obtaining γ-TiAl castings, with a maximum thickness of 20 mm, starting from melting charges constituted by commercially pure Ti and Al. In this process the metallic charge is induction melted in a ZrO2 ceramic crucible inside coated with Y2O3, the pouring being performed by centrifugation in ZrO2 moulds, with an Y2O3 contact coating, obtained by the lost wax moulding process. The whole sequence of operations is accomplished under argon atmosphere. The use of this process, besides avoiding the use of previously melted charges needing very specific furnaces, allows to obtain parts with homogeneous chemical composition and very low contamination with residual elements, without surface oxidation, with a hardness quite uniform from the surface to the inside of castings, which reveal a surface finishing identical to that recommended by the international standards for steel castings obtained by the same moulding process.

Description

1.1 BACKGROUND OF THE INVENTION [0001] This invention respects a foundry process for reactive alloys, namely titanium alloys, using conventional melting and moulding techniques, to which small alterations were made, either in terms of equipment, and in respect to the materials being used. [0002] From the beginning of its industrial use, in the middle of the XXth century, titanium alloys have been finding application in areas in which a good relationship mechanical strength / density and the ability to keep high mechanical properties at high temperature are the most significant selection factors, and the relevance of the parameter cost is tiny, or even non considered—in the military, aerospace and aeronautic industries. However, after the end of the Cold War, the demand of these sectors of activity has abruptly decreased, and there are references to production decreases over 40% by the beginning of the nineties. [0003] Nowadays, an effort of widening the domain of application of titani...

AI Technical Summary

Benefits of technology

[0031] In that context, the invention suggests the melting of γTiAl in low cost ceramic crucibles, suitable to achieve the main task of the invention. The developed ceramic crucibles reveal high thermal-shock resistance and are thermodynamically stable facing γTiAl alloys. When compared with CaO crucibles used up to now at laboratory scale, the developed crucibles are not hygroscopic, which makes easier their manipulation.

[0032] In order to reduce significantly the present production cost of γTiAl, this invention suggests the use of melting stocks consisting in commercially pure titanium and aluminium, in order to avoid pre-melting operations needed to obtain sub-products (billets) with the desired final chemical composition.

[0035] The developed technique allows the elimination, or significant decrease, of those problems, namely the absence of casting porosities and very low contamination of the base metal in elements known to be harmful to the performance of any part (oxygen for example). The “alpha-case” extension is lower than 25 μm, nevertheless the hardness value is still in agreement with the existent standards for γ-TiAl, and therefore being not needed any later operation of removal of the surface metal. The developed technique relies upon low cost equipments, so decreasing the global production costs, and allows its use by any foundry that can use the traditional lost wax moulding process, without need of great modifications of its production processes.

Problems solved by technology

The use of traditional foundry techniques presents, however, a lot of difficulties, in consequence of the characteristics of the metal itself, namely its high melting temperature, low fluidity at the pouring temperature and high reactivity with almost all the elements.

In face of these imperatives, the resources needed for the production of titanium castings are different, and present more sophistication, than those needed for casting parts made of traditional alloys, what strongly contributes to the high cost of those castings.

On the other hand, the technical difficulties and the equally high cost of post-casting machining operations, when necessary, limit the number of moulding processes liable to be used, so becoming necessary to use processes leading to “near net shape products”, that means, precision casting processes

The main limitation to the development of a ceramic crucible induction melting technique suitable for reactive alloys concerns mainly the refractory material to use on the crucibles production.

Results obtained so far with the purpose to develop a ceramic crucible suitable for titanium alloys are not very encouraging.

Reports usually refer poor thermodynamic stability, with negative consequences on the final castings, such as: Metal contamination with different elements, as a consequence of metal-crucible interaction; Presence of gas porosities and non-metallic inclusions; Crucible destruction or deterioration during the melting operation.

On the first case, contaminants usually appear forming interstitial solid solutions with titanium, leading to poor mechanical and metallurgical properties, namely hardness increase and structure heterogeneity.

On the third case, the causes of crucible destruction are usually related with poor thermal-shock resistance of the crucible material, but erosion phenomena, metal-crucible reaction or even dissolution of the crucible material may also be present.

The use of CaO is, however, delicate, given the high hygroscopicity it presents.

However, the use of this process for the obtaining γ-TiAl is not viable, since the decrease of the contamination of the alloy with oxygen is reached by significantly altering the chemical composition of the alloy itself.

The process still reveals other drawbacks, like the high production cost of the crucible, due to the high cost of the refractory materials used on its production.

Besides those referred, other oxides have been tested in the manufacturing of crucibles for Ti alloys, namely MgO, ThO2 and Dy2O3, for which oxygen absorptions between 0.5 and 3.4% in weight were registered [7]-Its use in crucible production presents, however, problems of different nature: the radioactive nature of the ThO2, the low thermal shock resistance of MgO and the high cost of Dy2O3.

This process doesn't avoid the surface oxidation of the molten aluminium during the melting, transport and pouring operations.

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