Specialized metallurgical processes – compositions for use therei – Processes – Free metal or alloy reductant contains magnesium
Reexamination Certificate
1997-09-30
2001-09-18
King, Roy (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Free metal or alloy reductant contains magnesium
C075S309000, C420S590000
Reexamination Certificate
active
06290748
ABSTRACT:
This invention relates to the production of TiB
2
ceramic particulate reinforced Al-alloy metal-matrix composites.
The benefits of light alloy materials for structural engineering applications have been realised for their strength, toughness and above all for specific modulus. Consequently the aerospace and automotive industries have reaped a considerable incentive: fuel economy and longevity of components in service. In the last two decades or so, a new type of material has emerged which is based on the reinforcement by low density, high temperature ceramic materials: namely silicon carbide, alumina and carbon fibres. The reinforcement has been achieved with these materials either in the form of particulates or as fibres, resulting in a substantial reduction in the density, coefficient of thermal expansion and improvement in the value of Young's modulus. The combinatorial effect of properties of matrix and reinforcement is therefore observed in the metal-matrix composites. Based on laboratory-scale experiments, novel metal-matrix composite fabrication techniques namely spray-forming of Al-alloy/SiC, squeeze and infiltration casting of fibre reinforced metal-matrix composites, including powder mixing and extrusion processing techniques, have emerged. See, for example, the article by T. W. Clyne and P. J. Withers:
An Introduction to Metal
-
Matrix Composites
, Cambridge Solid-state Science Series, Cambridge University Press, 1993, pp 318-359.
These methods offer potential benefits both in terms of profitability and materials properties. Also the laboratory methods have now been available for small-scale commercial production of materials and hence the above-described metal-matrix composite fabrication methods compete with each other.
Experimental data also points to several problems leading to the formation of defect structures such as void formation during liquid metal infiltration and fibre-metal reaction, or fibre misorientation during squeeze casting. In the spray-forming process, which is a rapid quenching of a two-phase mixture, namely liquid metal and fine ceramics, the cost of material production is high. Additionally, the spray-formed ingot requires further processing because it has a wide range of porosity, and the ingot cannot be formed into complex shapes during the spray-forming process. The cost comparison indicates that the powder extrusion route produces materials of prohibitively high cost. The new technology has nonetheless been used in the fabrication of a wide range of consumer sports items for which high production cost has so far been justified. See, for example, the article by T. W. Clyne and P. J. Withers: An Introduction to Metal-Matrix Composites, Cambridge Solid-state Science Series, Cambridge University Press, 1993, pp 459-470.
Using the above techniques, the cost of automotive and aerospace components has not yet been justified and for this reason the metal-matrix market for automotive, aerospace and other engineering applications still remains uncertain. The fabrication cost of automotive and aerospace structural engineering components however remains unfairly high, hence the market for these metal-matrix composite components has been virtually non-existent.
Apart from the high production cost of materials made by the above routes, a much more fundamental problem, related to the long-term reliability of Al—SiC components, remains unsolved, particularly for the high temperature applications. With prolonged exposure to high temperature service conditions, aluminium matrix has a tendency to react with SIC over a period of time. Aluminium carbide, which also forms readily as an embrittling layer at the matrix-reinforcement interface during liquid-state processing, is detrimental for high temperature toughness of the composite materials. Aluminium carbide is also susceptible to moisture attack and hydrolyses to aluminium hydroxide, and methane is a gaseous reaction product. This attack with moisture is known to cause corrosion around the particulates of SiC and carbon fibre-matrix interface. As a result, the component can considerably weaken. Material toughness and fatigue, being the most important properties of engineering components in motion, suffer adversely due to the presence of the embrittled layer of aluminium carbide phase. This therefore leaves a question as to the long-term high temperature structural reliability of aluminium/SiC and Al/carbon fibre composites.
Additional problems of recycling Al/SiC and Al/carbon composites also arise due to undesirable presence of silicon and carbon in the metallic phase. This is expected to create a stock-pile of non-recyclable aluminium alloy composites which will also contribute to the overall cost of the composite materials.
More recently, titanium based materials have been recognised as a promising candidate in the fabrication of metal-matrix composites. Titanium diboride and carbide have been traditionally used for grain refinement in aluminium alloys. The ceramic phase is known to adapt microstructurally with the metallic matrix, providing a significant improvement in the mechanical properties of the alloy, which is unlikely to be achieved with SiC and carbon fibre reinforcement. The diboride ceramic phase does not aggressively react with the liquid metal to form an intermediate layer of embrittled phase. The diboride phase dispersion technology using melting and casting of aluminium alloy in air is a well-proven technique for the last 50 years in aluminium industries for the fabrication of grain-refined master alloy and fine grain-size Al-alloy castings for shape forming. The grain-refining reaction is:
4Al
liq
+TiB
2
=Al
3
Ti+AlB
2
(1)
which is an important aspect of TiB
2
and related ceramic phase dispersion in the metallic phase. Both AlB
2
and/or mixed diboride (Al,Ti)B
2
, which form as a result of the grain-refining reaction, are isostructural with TiB
2
and hence from the Hume-Rothery rule exhibit extended solubility.
This solid-solution boride phase, having an identical crystal structure as TiB
2
, is interfacially and crystallographically compatible with the alloy matrix. This is one of the reasons that the grain-refined Al-alloy exhibits better fatigue properties because of the interlocking of grain boundaries and dislocation by complex boride phase, a feature also commonly seen in high temperature superalloys. As a result of the favourable interfacial reaction and lower solubility of complex borides in the matrix, Al—TiB
2
composite is microstructurally a far superior composite material capable of exhibiting better high and low temperature fatigue and fracture properties. Some of the mechanical properties of as-cast and annealed Al-alloy metal-matrix composites with TiB
2
are discussed in reference GB-A-2,259,308. Titanium carbide favours the improvement in the properties in the same way as TiB
2
but to a lesser degree.
London Scandinavian Metallurgical (LSM) Company has recently developed an in situ ceramic dispersion technique reported in GB-A-2,257,985, GB-A-2,259,308 and GB-A-2,259,309. This method uses a flux mixture of K
2
TiF
6
and KBF
4
in contact with molten aluminium. The chemical procedure for dispersing TiB
2
in aluminium alloys is an extension of grain-refining reaction:
K
2
TiF
6
+2KBF
4
+(3+⅓)Al=TiB
2
+(3+⅓)K
3
AlF
6
+2AlF
3
(2)
In this in situ technique, also referred to as the reactive casting technique, the ceramic phase (TiB
2
) forms via chemical reaction (2) and is subsequently dispersed in the molten alloy.
The patent publications point out that the procedure has resulted in the development of cast aluminium/TiB
2
product with a maximum of 9 volume percent of the ceramic phase (see GB-A-2,257,985). So far there has been no further reported improvement in the volume fraction of titanium diboride phase dispersion by any other research group in the world.
According to an aspect of the present invention, there is provided a method of producing a ceram
Cannon Stuart M.
Dometakis Chris
Jha Animesh
Troth Elisabeth
King Roy
McGuthry-Banks Tima
Merck Pateng GmbH
Millen White Zelano & Branigan P.C.
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