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A device for fabricating metal-matrix composites by liquid infiltration under a protective atmosphere
Research report A device for fabricating metalmatrix composites by liquid infiltration under a protective atmosphere A. S. GULEC and D. H. BALDWIN
This note describes an apparatus developed specifically for the laboratory scale fabrication of aluminium matrix, stainless steel fibre-reinforced composites by the liquid inf'tltration technique. The composites and their fracture characteristics have been described elsewhere. ! The basic device, however, would be of general applicability wherever it is necessary to melt and cast small quantities (up to a few kilograms) of metal under a protective atmosphere. Its simple design and use of low-cost materials are its chief advantages. Basically the device consists of a valved melting chamber from which the molten metal flows under gravity, through a filter, into a casting chamber or mould containing a prewound mandrel designed to maintain filament alignment during heating and infiltration. The entire process is conducted under an inert gas atmosphere. The choice of materials for our application was governed by the fact that only aluminium/silicon alloys were to be melted. Clearly, materials compatible with the metal or alloy to be processed must be employed.
application, which involved melting and casting of an aluminium/12% silicon alloy, a filter plate containing 16 holes, each approximately 2.5 mm in diameter proved adequate. Secondly, the filter plate needs to be accessible for removal of the filtered oxide skins after each run. The casting chamber E is of split design to provide for easy removal of the finished casting. The bottom cap F for the casting chamber was deliberately made thin in relation to the vertical walls in order to promote heat flow through the
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Fig. 1 is a schematic diagram showing the essential details of the melting and casting device. In our version, all components except the outer casing were machined from graphite. The outer casing was fabricated from type 304 stainless steel tube and plate. The melting crucible A incorporates a 45 ° semi-apex conical section at its lower end. A corresponding taper on the plunger B provides sealing of the melting chamber until melting is complete and the liquid has attained the desired temperature. At that time plunger B is lifted and the melt drops by gravity through a filter plate C, into a conical collector and then into two feeder passages D which divert the liquid metal to the desired regions of the mould. The filter plate C was made as a separate piece of the apparatus for two reasons. First, the plate itself is simply a flat graphite disc containing several drilled holes. For different melt sizes or for melts of various metals or alloys it may be necessary to use plates with larger or smaller holes. If holes are too small it is difficult or impossible to establish flow through these holes, due to the surface tension of the liquid. If the holes are too large, filtering efficiency will be reduced and the probability of oxide skins passing through the filter and becoming entrapped in the casting is increased. In our Mr GLIlec is with ITU Makina Fak, Teknoloji Kursusu, Gumussuyu, Istanbul, Turkey, and Dr Baldwin is with Kaman Aerospace Corporation, Bloomfield, Connecticutt, USA
COMPOSITES. MAY 1975
Fig.1 Schematic diagram of melting and casting device. Key as follows: A - graphite crucible; B -- plunger (graphite); C -- filter plate (graphite); D -- feeders (in graphite block); E - casting chamber (graphite); F - bottom cap (graphite); G -- heat sink (stainless steel); H -- heating coil ( N i - C r ) ; I - c o n t a i n e r tube (stainless steel); J - top cap (graphite); K -- wire pre-form to be liquid infiltrated
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bottom of the apparatus during the freezing portion of the cycle. The entire apparatus was placed on a water-cooled heat sink G which was activated immediately after the casting operation was completed. Heating was accomplished by a 5 kW resistance heating coil H spirally wound around the outside of the stainless steel tube 1, and secured with refractory cement. In our work, it was found necessary to locate 50% of this heating element over the lower third of the apparatus, in order to compensate for conductive losses to the heat sink and the relatively large thermal mass of the wire preforms and positioning devices associated with the composite fabrication. A simple separate radiation shield outside the apparatus improves thermal efficiency without making the device unwieldy. During the entire heating and casting operation, the system was purged with dry argon introduced through the longitudinal hole in plunger/9. Argon was used because of its inertness and its greater-than-air density over the temperature range involved. The argon was pumped into the casting chamber and filled the remaining volume of the system by leakage through the various graphite/graphite joints. The graphite melting crucible and stainless steel tube were both sealed with a single, loose-fitting graphite top J. The length of the plunger was such that, when in its lowest position (crucible sealed for melting), approximately 5 cm of the shaft project above the top of the cap. This projection could be grasped with tongs and lifted to release the
melt at the appropriate time, without opening the melting chamber. Finally, it was found advantageous to fill the narrow annular space between the graphite components and inside of the stainless steel casing with clean, granular silica. This improved the heat transfer from tube to graphite by eliminating long convection currents and also restricted argon leakage through this space and forced a greater portion of the argon to flow upwards through the melting chamber.
A CKNOWL EDGEMEN TS
The authors gratefully acknowledge the help and encouragement of Dr R. L. Sierakowski of the Department of Engineering Science, Mechanics and Aerospace Engineering, University of Florida. This work was supported by the Air Force Armament Laboratory, Eglin Air Force Base, Florida, under Contract No F08635-72-C-0206, and by the Department of Materials Science and Engineering, University of Florida, Gainesville.
REFERENCE
1 Baldwin,b. H. and Sierakowski,R.L. 'Fracture characteristics of a metal matrix composite', Composites 6 No 1 (Jan 1974) pp 30-34
QualityTechnology Handbook
~o.~ Edition Compiled by the NDT Centre, Harwell
Contents: UK Directory Western European Directory Non European Directory International Directory European and International Standards Safety Guide
Legislation, liability and patents sources Details of education, training and certification Technical section List of books, periodicals and reports Index A5 500 pages ISBN 0 902852 37X £14 ($37.00)
ipc science and technology press IPC House, 32 High Street, Guildford, Surrey, England, GU1 3EW Telephone: Guildford (0483) 71661 Telex: Scitechpress Gd. 859556
130
COMPOSITES. MAY 1975
This note describes an apparatus developed specifically for the laboratory scale fabrication of aluminium matrix, stainless steel fibre-reinforced composites by the liquid inf'tltration technique. The composites and their fracture characteristics have been described elsewhere. ! The basic device, however, would be of general applicability wherever it is necessary to melt and cast small quantities (up to a few kilograms) of metal under a protective atmosphere. Its simple design and use of low-cost materials are its chief advantages. Basically the device consists of a valved melting chamber from which the molten metal flows under gravity, through a filter, into a casting chamber or mould containing a prewound mandrel designed to maintain filament alignment during heating and infiltration. The entire process is conducted under an inert gas atmosphere. The choice of materials for our application was governed by the fact that only aluminium/silicon alloys were to be melted. Clearly, materials compatible with the metal or alloy to be processed must be employed.
application, which involved melting and casting of an aluminium/12% silicon alloy, a filter plate containing 16 holes, each approximately 2.5 mm in diameter proved adequate. Secondly, the filter plate needs to be accessible for removal of the filtered oxide skins after each run. The casting chamber E is of split design to provide for easy removal of the finished casting. The bottom cap F for the casting chamber was deliberately made thin in relation to the vertical walls in order to promote heat flow through the
I.~A
rgon
supply
~flllliilllllllllli/iJ
A
l'i
Fig. 1 is a schematic diagram showing the essential details of the melting and casting device. In our version, all components except the outer casing were machined from graphite. The outer casing was fabricated from type 304 stainless steel tube and plate. The melting crucible A incorporates a 45 ° semi-apex conical section at its lower end. A corresponding taper on the plunger B provides sealing of the melting chamber until melting is complete and the liquid has attained the desired temperature. At that time plunger B is lifted and the melt drops by gravity through a filter plate C, into a conical collector and then into two feeder passages D which divert the liquid metal to the desired regions of the mould. The filter plate C was made as a separate piece of the apparatus for two reasons. First, the plate itself is simply a flat graphite disc containing several drilled holes. For different melt sizes or for melts of various metals or alloys it may be necessary to use plates with larger or smaller holes. If holes are too small it is difficult or impossible to establish flow through these holes, due to the surface tension of the liquid. If the holes are too large, filtering efficiency will be reduced and the probability of oxide skins passing through the filter and becoming entrapped in the casting is increased. In our Mr GLIlec is with ITU Makina Fak, Teknoloji Kursusu, Gumussuyu, Istanbul, Turkey, and Dr Baldwin is with Kaman Aerospace Corporation, Bloomfield, Connecticutt, USA
COMPOSITES. MAY 1975
Fig.1 Schematic diagram of melting and casting device. Key as follows: A - graphite crucible; B -- plunger (graphite); C -- filter plate (graphite); D -- feeders (in graphite block); E - casting chamber (graphite); F - bottom cap (graphite); G -- heat sink (stainless steel); H -- heating coil ( N i - C r ) ; I - c o n t a i n e r tube (stainless steel); J - top cap (graphite); K -- wire pre-form to be liquid infiltrated
129
bottom of the apparatus during the freezing portion of the cycle. The entire apparatus was placed on a water-cooled heat sink G which was activated immediately after the casting operation was completed. Heating was accomplished by a 5 kW resistance heating coil H spirally wound around the outside of the stainless steel tube 1, and secured with refractory cement. In our work, it was found necessary to locate 50% of this heating element over the lower third of the apparatus, in order to compensate for conductive losses to the heat sink and the relatively large thermal mass of the wire preforms and positioning devices associated with the composite fabrication. A simple separate radiation shield outside the apparatus improves thermal efficiency without making the device unwieldy. During the entire heating and casting operation, the system was purged with dry argon introduced through the longitudinal hole in plunger/9. Argon was used because of its inertness and its greater-than-air density over the temperature range involved. The argon was pumped into the casting chamber and filled the remaining volume of the system by leakage through the various graphite/graphite joints. The graphite melting crucible and stainless steel tube were both sealed with a single, loose-fitting graphite top J. The length of the plunger was such that, when in its lowest position (crucible sealed for melting), approximately 5 cm of the shaft project above the top of the cap. This projection could be grasped with tongs and lifted to release the
melt at the appropriate time, without opening the melting chamber. Finally, it was found advantageous to fill the narrow annular space between the graphite components and inside of the stainless steel casing with clean, granular silica. This improved the heat transfer from tube to graphite by eliminating long convection currents and also restricted argon leakage through this space and forced a greater portion of the argon to flow upwards through the melting chamber.
A CKNOWL EDGEMEN TS
The authors gratefully acknowledge the help and encouragement of Dr R. L. Sierakowski of the Department of Engineering Science, Mechanics and Aerospace Engineering, University of Florida. This work was supported by the Air Force Armament Laboratory, Eglin Air Force Base, Florida, under Contract No F08635-72-C-0206, and by the Department of Materials Science and Engineering, University of Florida, Gainesville.
REFERENCE
1 Baldwin,b. H. and Sierakowski,R.L. 'Fracture characteristics of a metal matrix composite', Composites 6 No 1 (Jan 1974) pp 30-34
QualityTechnology Handbook
~o.~ Edition Compiled by the NDT Centre, Harwell
Contents: UK Directory Western European Directory Non European Directory International Directory European and International Standards Safety Guide
Legislation, liability and patents sources Details of education, training and certification Technical section List of books, periodicals and reports Index A5 500 pages ISBN 0 902852 37X £14 ($37.00)
ipc science and technology press IPC House, 32 High Street, Guildford, Surrey, England, GU1 3EW Telephone: Guildford (0483) 71661 Telex: Scitechpress Gd. 859556
130
COMPOSITES. MAY 1975