Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C1/00—Making non-ferrous alloys
  • C22C1/10—Alloys containing non-metals
  • C22C1/1036—Alloys containing non-metals starting from a melt
  • C22C1/1068—Making hard metals based on borides, carbides, nitrides, oxides or silicides
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C1/00—Making non-ferrous alloys
  • C22C1/10—Alloys containing non-metals
  • C22C1/1036—Alloys containing non-metals starting from a melt
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C1/00—Making non-ferrous alloys
  • C22C1/04—Making non-ferrous alloys by powder metallurgy
  • C22C1/05—Mixtures of metal powder with non-metallic powder
  • C22C1/059—Making alloys comprising less than 5% by weight of dispersed reinforcing phases
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C32/00—Non-ferrous alloys containing at least 5% by weight but less than 50% by weight of oxides, carbides, borides, nitrides, silicides or other metal compounds, e.g. oxynitrides, sulfides, whether added as such or formed in situ
  • C22C32/0047—Non-ferrous alloys containing at least 5% by weight but less than 50% by weight of oxides, carbides, borides, nitrides, silicides or other metal compounds, e.g. oxynitrides, sulfides, whether added as such or formed in situ with carbides, nitrides, borides or silicides as the main non-metallic constituents
  • C22C32/0052—Non-ferrous alloys containing at least 5% by weight but less than 50% by weight of oxides, carbides, borides, nitrides, silicides or other metal compounds, e.g. oxynitrides, sulfides, whether added as such or formed in situ with carbides, nitrides, borides or silicides as the main non-metallic constituents only carbides

Definitions

• the present invention relates to an in situ process for producing aluminium alloys containing a uniform dispersion of fine titanium carbide particles and to the resulting alloy.
• Aluminium alloys containing refractory carbide particles also referred to as particulate- reinforced aluminium composites, possess a spectrum of advantages with respect to their monolithic counterparts and have received a great deal of attention recently, owing to their competitive prices, easy implementation and characteristic isotropic behavior.
• Aluminium alloys reinforced with titanium carbide particles are a prominent example of these hybrid materials which combine the ductility, toughness, electrical and thermal conductivity of the aluminium matrix with the strength, stiflhess, hardness and wear resistance of the refractory carbides, leading to a unique combination of properties which cannot be achieved in either of the individual components by alloying and/or thermomechanical processing alone.
• Titanium carbide-reinforced aluminium composites are attractive candidates for various applications, particularly in the automotive sector, as they offer, in addition to favorable mechanical properties, substantial weight savings.
• Pistons and connecting rods are examples of such automotive applications where titanium carbide reinforced aluminium alloys are receiving serious consideration with such benefits as reduced weight and improved wear resistance.
• aluminium alloys containing titanium carbide particles technologically very important is not only the promise they hold as structural composite materials with superior mechanical and physical properties but also the potential they have as grain refiners for aluminium-based metals.
• A.Banerji and W.Reif, Metall.Trans., 17A(1986)2127-2137. This "carbide theory" has been generally accepted and has prompted many attempts to synthesize master alloys with a fine dispersion of titanium carbide particles in an aluminium matrix.
• a variety of ceramic particles including carbides, borides and nitrides have been produced with this technique in a number of matrix materials including aluminium.
• Another process for producing composite materials disclosed in U.S. Pat. No. 4,402,744, relies on sintering a mixture of carbon particles, aluminium powder and the powder of an aluminium-transition metal intermetallic compound, the transition metal being a refractory carbide-former such as titanium.
• the intermetallic phase is reduced by carbon to aluminium and a carbide such as titanium carbide to yield aluminium-based composites which are reinforced by carbon particles in addition to refractory carbide particles. While the attributes of the Powder Metallurgy route have long been recognized, it is more expensive, more sophisticated and pose serious health and safety hazards as it involves handling of fine metal/ceramic powders.
• Solidification processing of aluminium alloys containing ceramic particles is particularly attractive as it is economical and practical. It is carried out in practice generally by direct introduction of the particles into aluminium melts, by mechanical means. A variety of techniques have been proposed to introduce ceramic particles into metal melts. P.K.Rohatgi et.al., Int.Met.Rev., 31(1986)115-139. This, however, is not a simple task as the ceramic particles are hardly wet by aluminium melts. Hence, either the ceramic particles or the aluminium melts ought to be treated by thermal, chemical and/or mechanical means to promote wetting. These pre-treatments increase the number of manufacturing steps involved and, thus, the cost of processing.
• One such approach involves the formation of titanium carbide particles in aluminium melts by a gas-molten metal reaction.
• a carbon-bearing gas is intoduced into a molten composition comprising aluminium as the matrix metal and a transition metal as the refractory carbide- forming component, allowing the transition metal in the melt to react with the carbon released from the injected gas to form a dispersion of carbide particles.
• carbon is introduced into the melt in the form of a carbonaceous solid suspended in the gas.
• the composite materials of this patent reveal several phases other than titanium carbide, the reinforcing phase of choice in the product when the matrix liquid is molten aluminium and the refractory carbide- forming component is titanium, including unreacted carbon and titanium and thus suffer from a low recovery of the carbon source as well as the carbide forming-transition metal.
• the unreacted titanium crystallizes in the form of titanium aluminide needles which impair the mechanical properties of the product.
• Ternary Al-Ti-C carbides which form in these materials have yet another adverse effect on the recovery situation in regard to the population of titanium carbide particles.
• gas injection of the carbonaceous component in the process of this invention introduces into the melt a quantity of gas which is extremely difficult to remove since the viscosity of the melt increases with increasing volume fraction of particles.
• the process of this patent also fails to achieve complete conversion of titanium to titanium carbide. So, the product reveals excess titanium in the form titanium aluminide needles in addition to a number of phases, namely aluminium carbide and ternary Al-Ti-C phases, which have a deleterious effect on its grain refining capacity.
• the vortices generated in the melt during the process of this invention leads to gas entrapment with an adverse effect on the quality of the product.
• titanium carbide is the only phase which, being a refractory carbide, can be utilized as a reinforcing component in aluminium alloys. It is also fair to conclude from the so-called "carbide theory" that titanium carbide is the only (or the most effective) phase in the ternary Al-Ti-C system which is capable of refining the grain structure of aluminium-based metals.
• the present invention provides an in situ process for producing aluminium alloys containing a dispersion of fine titanium carbide particles with improved homogeneity.
• the method comprises introducing into an aluminium-titanium melt, graphite in bulk form, manufactured preferably in the form of a rotor, under favorable conditions which cause titanium in the melt to react with graphite spontaneously to form a dispersion of titanium carbide particles until the melt is depleted of titanium.
• the process of the present invention utilizes graphite in bulk form as the source of carbon, which is withdrawn from the melt once titanium is entirely carburized, thus avoiding any excess quantities of carbon entrapped in the cast alloy and also eliminating any risk of the supplied carbon being expelled out of the melt during the process. That this process does not require a precise balancing of the supply of carbon with respect to the titanium concentration in the melt is a further advantage offered by the present invention.
• the aluminium melt with a dispersion of in situ formed titanium carbide particles is finally solidified so as to retain the uniform distribution of carbide particles in the cast alloy.
• the aluminitim melt containing titanium for reaction with graphite to form titanium carbide particles once the graphite is introduced into the melt may be prepared by one of several ways known in the art, for example by melting a pre-existing Al-Ti master alloy, by melting an aluminium-based solid and a titanium-based solid together and mixing the two in the liquid state or by melting an aluminium-based solid and introducing into this melt a suitable titanium salt.
• the titanium concentration in the melt may be selected according to the level of reinforcement, i.e. the volume fraction of titanium carbide particles, desired in the final alloy. It is preferable to use a titanium concentration of more than 5 weight % in the melt in order to generate a population of titanium carbide particles which has a notable effect on the mechanical properties of the final alloy when destined to be used in structural applications and which offers sufficient grain refining-effect when destined to be used as a grain refiner for aluminium-based metals.
• the source of carbon for the carburization of titanium in the melt in the process of the present invention is bulk graphite introduced into the melt in the form of a rotor.
• the rotor is designed with certain features to increase its surface area and thus its contact with the melt. It is fixed to a motor-driven shaft, preferably also made of graphite and is rotated inside the melt until the melt is depleted of dissolved titanium and is saturated with titanium carbide particles.
• the shaft may be manufactured out of clay/ceramic coated steel rod as well. Its size and dimensions depend on the size of the melt to be treated and the dimensions of the crucible used.
• the total weight of carbon introduced into the melt in this fashion is far greater than that required to carburize the titanium in the melt.
• the remaining graphite is withdrawn from the melt at the end of the process.
• the shaft has to be regarded as a consumable component as it is partially consumed during the process, very much like the rotor head, when manufactured out of graphite.
• An inert vessel may be used in melting of the alloy and in holding of the melt during the process.
• a graphite crucible may also be used.
• the reaction of the melt with the crucible has to be accounted for when a graphite crucible is used.
• Experience has shown that the generation of titanium carbide particles is accelerated in the case of a graphite crucible with a favorable effect on the duration of the treatment.
• the duration of the treatment is affected by a number of other process variables as well: namely, the intensity of stirring, the titanium concentration of the original alloy, the temperature of treatment and the design and dimensions of the graphite rotor. It can be easily estimated for a given set of conditions with a trial run during which small melt samples are quenched in the course of the treatment. Metallographic evaluation of these samples provides very useful information as to when the proces shall be terminated for the set of conditions which have prevailed during that particular run.
• a C 3 starts to form when the melt is nearly saturated with titanium carbide particles, i.e. when it is nearly devoid of dissolved titanium. Since Al-tC 3 is not desired in the final alloy as it is a brittle compound which decomposes readily when exposed to atmospheric moisture, it is best to leave some titanium unreacted to avoid the risk of getting A--tC 3 in the cast alloy. So, we prefer to terminate the process just before the melt is depleted of dissolved titanium.
• Typical microstructure of the alloys produced by processes practiced according to the present invention consists of a uniform dispersion of titanium carbide particles in the alpha-aluminium matrix.
• the in situ-formed titanium carbide particles are very small, generally less than 3 microns in size and are very nicely wet by the matrix phase with no detrimental interaction zones at the interface.
• FIG. 1 shows a sketch of the set-up used for producing aluminium alloys containing titanium carbide particles
• FIG. 2 shows the optical micrograph, at a magnification of 40:1, of the resulting alloy produced in accordance with the present invention.
• the rotor was manufactured out of a cylindirical graphite block, 70 millimeters in diameter and 35 millimeters in length, by machining four evenly spaced grooves along its entire length, each having a maximum depth of 15 millimeters.
• the rotor was fixed to a motor-driven shaft also made of graphite, having a diameter of 12 millimeters.
• the temperature of the melt was monitored by a chromel-alumel thermocouple and was maintained at about 1150 degrees Celsius throughout the process.
• the surface of the melt was covered with a salt flux.
• the rotor was rotated inside the melt at 100 revolutions per minute. The operation was terminated after 60 minutes and the melt was skimmed and thoroughly stirred with graphite rods. It was then poured into a permanent mold and squeezed into rectangular plates with a plunger.
• the cast plates were subjected to microstructural examination by optical and scanning electron microscopy which revealed a homogeneous distribution of particles on every section taken from the rectangular plates.
• the particles were identified by x-ray and Energy Dispersive Spectroscopy (EDS) analysis as titanium carbide.
• EDS Energy Dispersive Spectroscopy
• the titanium carbide particles were generally smaller than 3 microns and occasionally formed rather small clusters. Couple of very small titanium aluminide particles could also be detected only at higher magnifications. Only the reflections of the alpha aluminium solid solution and of the titanium carbide phases could be identified in the x-ray spectrum of these plates with no evidence of ternary carbides and the AL ⁇ C 3 phase.

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• Manufacture Of Alloys Or Alloy Compounds (AREA)

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• 1997
  • 1997-11-20 WO PCT/TR1997/000018 patent/WO1999027146A1/en not_active Application Discontinuation
  • 1997-11-20 AU AU51444/98A patent/AU5144498A/en not_active Abandoned
  • 1997-11-20 EP EP97946231A patent/EP1034315A1/de not_active Withdrawn

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