US2734820A - Process and composition for treating - Google Patents

Description

United States Patent PROCESS AND COMPOSITION FOR TREATING MOLTEN ALUMINUM Hugh S. Cooper, Shaker Heights, ()hio, assignor to Walter M. Weil, Cleveland, Ghio No Drawing. Application May 29, 1953, Serial No. 358,577

3 Claims. (CI. 75-68) This invention relates to the production of aluminum and aluminum alloys containing small amounts of bothtitanium and boron as grain refining constituents and for other purposes. The invention involves both a novel method of introducing the titanium and boron into an aluminum melt and a novel combination of titanium and boron in gaseous form in a pressure tank or cylinder from which the gases may be discharged directly into molten aluminum.

Titanium has long been added to aluminum and its alloys as a grain refining alloying agent. More recently, the addition of boron to aluminum and its alloys for similar purposes has been proposed and has been found to possess certain advantages over titanium. One of the advantages obtained by the use of boron is that the resulting aluminum alloys retain the grain refining effects to a greater degree after several remeltings.

Titanium has been added to aluminum and its alloys in commercial operations by two principal methods. One of the methods was to prepare a master alloy of aluminum and titanium containing a relatively high percentage of titanium and to add the master alloy to an aluminum melt. The other method was to introduce the titanium into the aluminum melt in the form of gaseous titanium tetrachloride. The master alloy method is expensive to employ in commercial production because of the difliculty of preparing and the high cost of the master alloy. The titanium tetrachloride method has the great advantage that it effectively degases and cleans the melt so treated. However, it has not been a convenient method to employ in commercial operations because titanium tetrachloride boils at about 136.4 C. at normal pressure, and it is normally necessary to maintain this reagent substantially above that temperature to introduce it into a melt in gaseous form. Since safety regulations require the use of safety plugs in gas cylinders which fuse at about 72 C., the required gas temperature in a titanium tetrachloride tank or cylinder far exceeds the upper limit imposed by safety regulations and prevents commercial use of this reagent from gas cylinders.

While boron trichloride boils at about 13 C. and is thus easily supplied as a gas from pressure tanks or cylinders at temperatures providing a large margin of safety, this reagent is many times more expensive than titanium tetrachloride. Its use in treating aluminum melts has probably been limited by its cost more than by any other consideration, and it has been little used in commercial operations, to the best of my knowledge. The solubility of boron in aluminum is so low that the master alloy method of introducing it into an aluminum melt is not feasible, and other methods of introducing the boron, such as by the inclusion of boron salts in a melting flux, have been difficult to control because of the tendency of the boron salts to remain in the flux rather than to be reduced and alloyed with the aluminum.

Some information has been published on the value of incorporating both titanium and boron in aluminum and "ice its alloys. Use of both of these alloying elements has been shown to produce a finer grain structure than when either element alone was used. However, the problem of introducing the two alloying elements in a satisfactory manner for commercial operations has involved many difiiculties. Insofar as I am aware, introduction of the two alloy metals has only been successfully performed by adding boron to an aluminum alloy in which the desired amount of titanium has already been incorporated. This procedure has involved the same objections as the incorporation of boron alone into aluminum, and is additionally objectionable because it involves a second operation.

In view of the above, I have sought to develop a satisfactory method for introducing titanium and boron together directly into an aluminum melt in the form of their gaseous chlorides (TiClr and BCls). By substituting a substantial amount of titanium tetrachloride for the expensive boron trichloride, the cost of the latter becomes less significant.

Since titanium tetrachloride and boron trichloride are respectively normally liquid and normally gaseous and have widely different boiling points (about 136 C. and 13 C., respectively), the problem of introducing them into an aluminum melt together in fixed proportions had no obvious solution. This problem also involved the diificulty of supplying the two gases at a pressure high enough to force them into large aluminum melts against the pressure existing adjacent the bottoms of the melts without requiring that the supply tanks be held at a temperature too close to the limit imposed by the melting point of th safety plugs required by safety regulations.

In the course of working on this problem, I made the surprising discovery that a mixture of liquid titanium tetrachloride and liquid boron trichloride, in fixed proportions, can be discharged from the same gas cylinder as mixed gases at temperatures considerably below the boiling point of titanium tetrachloride alone. However, I also found that the proportions in which the two gases are discharged are not necessarily the same as the propor tions in which the compounds are present in the cylinder and that the proportions discharged and the cylinder pressures vary with the temperature at which they are released and also with time as the proportions of the compounds left in the cylinder change during the process.

Finally, I discovered that mixtures of the two compounds, when maintained at the proper temperature, will release a mixture of the two compounds in' gaseous form in substantially the same proportions in which they are contained in a gas cylinder and that these proportions will remain substantially constant if this temperature is maintained, until the cylinder is substantially exhausted. For example, when a mixture of titanium tetrachloride and boron trichloride in about equal proportions by weight is held in liquid form in a cylinder and the cylinder is maintained at a temperature of about 60 C., the two compounds can be released from the cylinder in gaseous form in substantially the same proportions until the cylinder is substantially exhausted. Under these conditions, the liquid mixture maintains a gas pressure in the cylinder of about 39 pounds per square inch, which is more than ample for forcing the mixed gases through large aluminum melts in most commercial operations.

remains principally in liquid form until the tank approaches exhaustion.

The preferred method of treating melts of aluminum and its alloys in accordance with the present invention is to employ a conventional gas cylinder as a pressure tank and to charge the cylinder with measured quantities of the two chlorides while holding them below the boiling point of the boron trichloride, i. e. below 13 C. Such cylinders, are conventionally equipped with outlet conduits having control valves therein, and a gas line may be connected to the outlet conduit and run downwardly into a molten metal bath in a furnace to a region adjacent the bottom thereof. The gas cylinder is preferably maintained at an operating temperature of 50 to 60 C. by immersing the tank in a water bath, oil bath, or the like, equipped with suitable thermostatically controlled heating elements, whereby the liquid contents of the cylinder may be maintained at a substantially constant temperature in spite of the cooling effect of evaporation as the mixed gases, are discharged.

When a molten metal bath to be treated has been prepared in the furnace, the gas line from the gas cylinder may be inserted therein, and the valve of the cylinder may be cracked to bleed the gas mixture into the melt.

The mixed gases discharged into the melt bubble upwardly therethrough and act to degasify and clean up the melt by removing hydrogen and other impurities. Some of the titanium and boron remain in the melt and exert a profound grain refining effect. Since there is a distinct limit on the amount of titanium and boron that can be retained in the melt by this procedure, there is no limit on the length of time the gassing operation can be continued. The amount of gassing required in any particular case will be determined by the nature and purity of the product sought. Also, difierent alloys require ditlerent amounts of gas treatment to be cleaned up and refined by this procedure; and the initial purity of the metal from which the melt is made greatly afiects the extent of gas treatment required to produce a satisfactory product.

The invention has been employed according to the above-described mode of operation with outstanding success in the production of aluminum alloy castings. The castings resulting from use of the invention in this manner have shown a very line and uniform grain size and have been unusually free from the porosity commonly caused by gas inclusions.

Depending upon the gas pressures required, which are in turn determined by the depth of a molten metal bath to be treated, the proportions of the two reagents in the pressure cylinder may be varied to some degree. The temperature required to evaporate the mixed gases in substantially the same proportions as the reagents are present in the cylinder will also vary with the proportions employed. Within the practically useful range of pressures, however, and while operating well below the safety plug temperature limit of about 72 C., the proportions of the two reagents must not depart too far from equal proportions, though some variation is permissible. For example, a mixture of about 44% titanium tetrachloride and 56% boron trichloride by weight worked satisfactorily at about 25 pounds per square inch.

For obvious practical reasons, it isdesirable for most purposes that the two gases be discharged from the cylinder in substantially the same proportions as the liquid reagents are contained in the cylinder. However, to the best of my knowledge, it has never before been recognized that the two reagents could be discharged together from the same pressure compartment in any proportions. Because it may be desired in some instances to discharge the two gases in different proportions from a given cylinder, and because of the difliculty of accurately proportioning the two chemicals in a pressure cylinder in the first instance, considerable departure from the approximately equal proportions of the two gases, preferred for most purposes, may be accidentally or intentionally encountered in practice. Such departures, nevertheless, still involve use of the basic discovery on which this application is based, i. e. that the two reagents may be discharged as mixed gases from a liquid mixture held under pressure well below the boiling temperature of the titanium tetrachloride. By proper regulation of the temperature both gases will be discharged simultaneously until the cylinder is substantially exhausted.

The invention involves evaporating a liquid mixture of the boron and titanium chlorides at a temperature which is substantially below the boiling point of one of the compounds alone. Insofar as I am aware, the possibility of doing this is unpredictable from anything heretofore known about the compounds, and the temperature at which the two compounds will evaporate in the same proportions in which they are present in the liquid mixture can only be determined empirically for the particular liquid mixture to be employed.

Having described my invention, I claim:

1. The method of treating melts of aluminum and its alloys comprising maintaining a mixture of liquid titanium tetrachloride and liquid boron trichloride in a pressure chamber in about equal proportions, heating said liquid mixtureto a temperature in the range of about 45 to 60 C., and maintaining said temperature wln'le withdrawing a stream of vapor from said chamber and introducing it into the metal melt.

'2. A composition consisting essentially of a liquid mixture of titanium tetrachloride and boron trichloride in about equal proportions maintained in a confined space under the vapor pressure of said mixture.

3. A composition consisting essentially of a liquid mixture of titanium tetrachloride and boron trichloride in about equal proportions maintained in a confined space under the vapor pressure of said mixture and being present in amounts sufficient to maintain a substantial amount of both components of said. mixture in liquid form.

References Cited in the file of this patent UNITED STATES PATENTS

Claims (1)

1. THE METHOD OF TREATING MELTS OF ALUMINUM AND ITS ALLOYS COMPRISING MAINTAINING A MIXTURE OF LIQUID TITANIUM TETRACHLORIDE AND LIQUID BORON TRICHLORIDE IN A PRESSURE CHAMBER IN ABOUT EQUAL PROPORTIONS, HEATING SAID LIQUID MIXTURE TO A TEMPERATURE IN THE RANGE OF ABOUT 45* TO 60* C., AND MAINTAINING SAID TEMPERATURE WHILE WITHDRAWING A STREAM OF VAPOR FROM SAID CHAMBER AND INTRODUCING IT INTO THE METAL MELT.