CA2087993A1 - Process for removing hydrogen gas and non-metallic inclusions from molten aluminum or aluminum matrix composites - Google Patents

Abstract

Abstract Molten unreinforced aluminum or aluminum matrix composites are treated to remove hydrogen gas and non-metallic inclusions. A melt is placed in a treatment vessel having a vaned rotary mixing impeller and a mixture of NH4Cl powder and inert gas is injected into the melt beneath the surface of the melt with mixing and is preferably followed by a period of inert gas injection. This results in the formation of fine bubbles which are well distributed throughout the melt, thereby causing the contained hydrogen gas and non-metallic inclusions and alkali/alkaline earth metal compounds to rise to the surface of the melt. Dross is regularly removed from the surface leaving a purified aluminum.

Description

2~879g3 Process for Removing Hydrogen Gas and Non-Metallic Inclusions from Molten Aluminum or Aluminum Matrix Composites Backqround of the Invention This invention relates to a proc~ss for treating molten unreinforced aluminum or aluminum matrix composites to remove hydrogen gas and non-metallic inclusions, such as solid particulates, oxides, alkali/alkaline earth metals, etc., from the melt.
Molten aluminum prior to casting may contain many impurities which, if not removed, can cause high scrap losses in the casting, or otherwise poor metal quality in products fabricated therefrom. In molten aluminum base alloys, the principal objectionable impurities are dissolved hydrogen and suspended non-metallic particles such as the oxides of aluminum and magnesium, refractory particles, etc.
The solubility of hydrogen in aluminum alloys decreases by about an order of magnitude when the metal solidifies.
Consequently, hydrogen gas is released from the metal during casting if the hydrogen content of the molt~n metal is not reduced below the solid solubility limit of hydrogen in the metal~ Hydrogen causes pinhole porosity in rapidly solidified metal such as direct-chill cast ingots, or fills shrinkage cavities in slowly solidified metal. Even hydrogen remaining dissolved in the metal after solidification may be harmful, since it diffuses during heat treatment into voids and other discontinuities in solid metal, thereby aggxegating the harmful effects of these defect points in the properties of the metal.
Solid, non-metallic particles suspended in the molten metal cause serious difficulties during casting and fabrication of aluminum alloys. These particles consist mainly of oxides which are introduced into the melt with the scrap during the melting operation, or ar~ produced by direct oxidation with air, water vapour and other oxidizing gases while the metal is processed in the molten state. Fine, broken-up oxide films stirred into the molten metal are , ......... . . , . . , ~ .

particularly harmful since in contrast to the more macroscopic oxides in other solid particles, they cannot be skimmed off as dross and remain suspended in the molten metal.
Also, in processes in which the aluminum matrix composites are utilized, there i5 a proportion of waste or scrap materials. The scrap from these metal matrix composites represents a serious disposal problem since metal matrix composites cannot be incorporated into standard alloys on account of the danger of contamination by the reinforcing particles present. For instance, a metal matrix composite containing even a very low concentration of ceramic particulates could have disastrous effects i~ mixed with metal destined for thin gauge rolled products, such as can stock.
One technique for removing the ceramic particulates is described in Provencher & Riverin, U.S. Patent No. 5,080,715.
Hydrogen gas, alkali~alkaline earth metals or compounds and non-metallic inclusions are commonly removed from molten aluminum by introducing an inert gas or chlorine gas into the molten metal in the form of bubbles. Although chlorine gas has been highly satisfactory in terms of its ability to remove hydrogen and non-metallic inclusions from most aluminum alloys, the use of chlorine presents serious problems due to its corrosiveness and toxic nature. Thus, although the use of chlorine for fluxing aluminum alloys has in the past been considered a commercially acceptable practice, increasing concern with air pollution and handling ha~ards in plants has stressed the need for its elimination. As a result, numerous attempts have been made to avoid the use of chlorine gas for the treatment of molten aluminum.
One solution has been to utilize the chlorine in the form of a powdered flux containing a chloride as the main component. When ths flux is introduced into the molten metal at high temperatures, it undergoes thermal decomposition to generate corresponding gaseous chlorine or produce a liquid salt, which carries out the refining action. In practice, a powdered flux is fed through a blowing device which blows in both inert gas and the flux simultaneously. Inert gas acts as ', ' ~ . ~ ' ... . .
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2~87993 a carrier, transporting the flux. This can typically be done by blowing the flux and inert gas into the molten metal through a lance. Japanese published application 82-42118 describes such a method of blowing a flux-inert gas mixture into molten aluminum.
It is also well known to feed refining gases into molten aluminum by means of rotating impellers. These are described, for instance, in U.S. Patents 3,839,019, 4,772,319, 4,992,241, 5,080,715, etc.
A widely used cleaning/degassing agent is hexachloroethane (C2Cl6) in tabletted form. However, this has the disadvantages of forming a wet dross and contaminating reclaimed particulate.
It is also known from Japanese patent publication JP
89210156A that NH~,Cl can be used together with a degassing agent selected from a fluoride, chloride or sulphate as a refining agent for molten aluminum. This procedure has the disadvantage that the mixture of NH4Cl and the fluoride, chloride or sulphate produces a liquid salt residue on the melt surface, which is highly undesirable.
It is the object of the present invention to find an improved method of removing from molten aluminum solid inclusions, oxides, alkali/alkaline earth metals and hydrogen while avoiding the use of chlorine gas.
Summary of the Invention The invention relates to a process for treating unrein~orced aluminum or aluminum alloy metal melts as well as metal matrix composites thereof, to remove hydrogen gas and non-metallic inclusions therefrom. In the procedure of the present invention, a mixture of NH4Cl powder and inert gas is injected into the molten metal beneath the surface thereof through a rotating impeller or through a lance. With this novel process, it is possible to remove significant quantities of solid inclusions. For instance, the procedure is capable of removing 20% by volume of alumina particles present in a metal matrix composite. Hydrogen is also very significantly reduced.

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2~79~3 The fluxing and degassing process of this invention is based on the co-injection of the inert gas and reactive chemical agent in fine powder form, preferably followed by an inert gas injection period. Non-metallic inclusions, alkali/alkaline earth and hydrogen are removed by floatation.
Tha inert gas/NH4Cl mixture should be injected well beneath the surface of the melt and in such manner that fine gas bubbles are well distributed throughout the melt. Thus, the mixture is preferably injected close to an impeller so that there is a strong shearing action to break up the gas into small, well distributed bubbles. For instance, the gas may be injected through a hollow rotor with an outlet for the gas at the bottom end of tha impeller rotor or the gas may be injected by being discharged through an opening in the rotor in the vicinity of the impeller bladesO Alternatively, a separate injection lance may be used having an outlet close to the impeller blades. For best results the melt is preferably maintained at a temperature in the range of 690~C to 750C.
The inert gas may be any gas which is substantially non-reactive toward liquid aluminum at reaction temperatures, e.g. argon or nitrogen. However, argon is preferred.
The mixing device is preferably made from graphite, silicon carbide or a ceramic material which is inert to the molten metal. It comprises a crucible with an impeller consisting of a central hub portion with radially projecting vanes set to provide substantial shearing action. The vanes may be vertically mounted or they may be inclined up to 45 to the vertical. Typically, about 4 to 8 vanes are used and the impeller is typically rotated at a rate of about 100-400 rpm.
The process of the invention functions to de-wet non-metallic inclusions. Thus, the chemical agent wets the non-metallic inclusions but is, itself, non-wetted by the liquid aluminum phase. The reduction of surface tension between the chemical agent wetted non-metallic and liquid aluminum is the main mechanism of the non~metallic in~lusion removal by floatation.

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.,,~ : -20~7~93 The removal of alkali impurities is accomplished by areaction with the halide-containing treatment gas. When the NH4Cl is injected into the molten aluminum, it decomposes into volatile compounds N2, H2 and HCl. It also reacts with alkali present at temperatures above 660~C to form NaCl, LiCl or CaCl2. The product of the reaction is then entrained to the surface by the gas bubbles and then trapped in the dross.
The hydrogen dissolved in the liquid aluminum is removed by diffusion into the gas bubbles which are lifted to the melt surface. The dross is regularly remov~d from the surface of the melt, removing with it the contaminants and leaving a purified aluminum.
The process of the invention can be used in either a batch mode or in-line continuous mode for either foundry or wrought alloys. The term "aluminum" as used herein is intended to include the alloys thereof. It has been found to be an effective means for cleaning molten aluminum which is safe and environmentally sound without chlorine fluxing.
Moreover, with the process of this invention, it is possible to remove very significant amounts of solid inclusions. For instance, it has been demonstrated that a complete and efficient removal of 20% by volume of alumina particulate having particle sizes of 5-75 ~m from the aluminum matrix composite material, DUR~LCAN~, can be achiev~d. Also, hydrogen is typically reduced from about 0.27 ml/lOOg to about 0.10 ml/lOOg.
Brief Description of the Drawinqs The invention is illustrated by way of example with reference to the drawings in which:
Figure 1 is a schematic illustration of a treatment system for carrying out the invention; and Figure 2 is a plot showing calcium and hydrogen removal as a ~unction of time.
Description of the Preferred Embodiments A suitable system for carrying out the invention is depicted in Figure 1 where a vessel 10 is provided which may conveniently be a furnace in the form of a 300 kg crucible :. ~ .. .-: .

2087~3 electrically heated from the sides. It is filled with a molten metal or metal matrix composite to level 14.
Extending down into the molten material is a rotatable mixer consisting of a hollow rotatable drive shaft 11 with an impeller 12 with radially projecting vanes at the bottom thereof. One embodiment of the impeller has six blades set at a ~5 angle, with a diameter of 25 centimetres and a thickness of 8 centimetres. The bottom of the impeller 12 is also preferably positioned about 8 centimetres from the bottom of the vessel. The impeller may be located at the centre of the crucible or it may be off-set from the centre.
The mixture of NH4Cl and inert gas is fed through the hollow shaft 11 by a feed line 13. This feed line 13 connects to a reservoir 17 for holding NH4Cl powder. A rotating feeder 18 is connected to an electric motor 19. The inert gas is fed in through inlet line 20 and is divided into lines 21 and 22 feeding to the top and bottom of the reservoir 17 to maintain a constant pressure. The NH4Cl powder feeds out of the bottom of vessel 17 into line 22 and is carried along by the inert gas into line 13 through safety valve 23. A mixing action takes place in the vessel 10 as shown with gas bubbles 16 being formed.
Dross forms on the surface of the melt and this is regularly removed from the surface, removing with it the contami~ants and particulate and leaving a purified aluminum.
Preferred embodiments of the invention are further illustrated by the following non-limiting examples.
Example 1 A test was carried out using the equipment described in Figure 1 to remove ceramic particulate from scrap DURALCAN
metal matrix composite material. The impeller had six blades set at a 45 angle, with an impeller diameter of 25 centimetres and a thickness of 8 centimetres. The bottom of the impeller was about 8 centimetres from the bottom of the vessel and ~as positioned off-centre at one-quarter of the vessel diameter. The composite was an AA 6061 aluminum alloy containing 20% by volume of alumina particles havin~ particle ... . .
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sizes in the range of 5-75 ~m. An amount of 214 kg of the scrap composite was added to the furnace and was heated to a temperature of 720C. Argon was fed in at a rate of 6 l/min.
and two additions of NH4Cl were made, each addition being in the amount of 15g mixed with 200g of pure alumina powder. The alumina powder was used only for security reasons to prevent any excessive solid/gas transformation during the addition into the molten metal, which can cause metal splashing.
The total time of the test was 23 minutes and all 20% by volume of alumina particulate was removed.
The results obtained are shown in Table 1 below:

Chemical Composition of the Aluminum Metal Matrix ELEMENT BEFORE AFTER
TREATMENT TREATMENT ¦
_ _ - - I
~Cu 0.~9 _ 0.28 Fe 0.16 0.13 I _ _ I
¦Mg 0.71 0.55 ¦
¦Mn 0.002 0.002 Ni 0.019 0.002 l _ I
ISi 0.58 0.57 ¦Ti O. 034 0.024 ¦ Cd < 0.001 < 0.001 Co < O.001 < O.001 I - _ I V 0.008 0.008 25 Alkali/Alkaline Earth Removal Sr _ 0.16 0.078 Na 0.0057 0.0017 Sr removal: 51%
Na removal: 70%

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Typical hydrogen removal was 50-80% depending upon the initial level of hydrogen. ~ typical hydrogen concentration after treatment was found to be 0.10 ml/lOOg.
Example 2 Another test was carried out for cleaning and degassing of AA3004 aluminum alloy. For this test, a different impeller was used, this one having eight blades set at right angles with an impeller diameter of 15 centimetres and thickness of 4 centimetres. The impeller was located at the centre of the vessel about 8 centimetres from the bottom.
An amount of 204 kg of the AA3004 aluminum alloy was added to the furnace and the temperature of the melt was maintained at 720C. Argon gas was injected at a rate of 8 l/min. and the NH4Cl powder was injected at a rate of 1 g/min.
The injection time was 15 minutes.
The results obtained are shown in Table 2 below:

_ I
ELEMENT BEFORE AFTER
TREATMENT TREATMENT
. . _ _ Cu .16 .16 I
Fe .33 .33 I
Mg .99 .96 I_ . _ Mn 1.07 1.08 I _ _ _ Ni .004 .004 Si .14 .14 ~
. I
Ti .007 .007 l _ ¦ Cd 0002 0001 ~ .008 .007 I _ _ .
C~ .0008 O
Na .0001 O

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Example 3 Using the same basic procedure as in Example 2, a 200 kg batch test was conducted with an injection of 0.075 kg/ton of NH4Cl during a period of 15 minutes. This resulted in complete removal of calcium from an initial concentration of 8 ppm. No significant loss of magnesium or other alloying elements was observed. The dross generated was dry, indicating a good reactivity in the melt during the fluxing.
Example 4 The procedure and equipment of Example 2 were again used with 204 kg of AA3004 aluminum alloy being added to the furnace. This was heated to 720C and argon was injected at rates of 8 and 10 l/min. The NH4Cl was added at amounts between 0 and 0.075 kg/ton. The results obtained are shown in Tables 3 and 4 below:

.
Argon NH4Cl _ Ca (ppm) Removal flow rate Injection efficiency (L~min) (kg~ton) Before 10 min 20 min(~) _ _ ~
8 0 210520.5_ 19 12 8 0.037 34.523.5_ 16 54 8 0.075 _7.5 1 0 100 8 0.075 24 8.5 1.5 94 _ 11 ~0 0.075 26 12 8 69 0.075 26.5 7 74 Argon NH~Cl Inclusion (PoDFA)RemOva flow rate ~njection efficiency I
(L/min) (kg/ton)BeforeAfter (%) ¦¦
_0.075 1.44 0.37 74 0.075 0.94 0.3~ 64 ,: :
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Exam~le 5 Again using the same procedure and equipment as in Example 2, the removal of calcium and hydrogen was determined as a function of time. The furnace was charged with 204 kg of AA3004 aluminum alloy and heated to 7~0C. Argon was injected at a rate of 10 l/min and NH4Cl was injected in an amount of 0.075 kg/ton. The results obtained were measured over a treatment time of 45 minutes and the results obtained are shown in Figure 2.

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Claims (8)

1. A process for treating molten unreinforced aluminum or an aluminum matrix composite to remove hydrogen gas and non-metallic inclusions therefrom, comprising the steps of:
(a) providing a melt of the unreinforced aluminum or aluminum matrix composite in a treatment vessel, (b) injecting a mixture of NH4Cl powder and inert gas into the melt beneath the surface thereof such that fine bubbles are well distributed throughout the melt, thereby causing the hydrogen gas and non-metallic inclusions to rise to the surface of the melt, and (c) recovering a melt of substantially pure aluminum.

2. A process according to claim 1 wherein inert gas is injected into said melt after completion of the injection of NH4Cl powder.

3. A process as claimed in claim 1 or claim 2 wherein the inert gas is argon.

4. A process as claimed in claim 1 wherein the melt is mixed by a vaned rotary impeller during injection of the NH4Cl powder/inert gas mixture.

5. A process as claimed in claim 4 wherein the NH4Cl powder/inert gas is injected through an opening in the impeller.

6. A process as claimed in claim 4 wherein the NH4Cl powder/inert gas is injected through a lance having an outlet close to the impeller vanes.

7. A process as claimed in claim 4 wherein the melt comprises unreinforced aluminum or aluminum alloy.

8. A process as claimed in claim 4 wherein the melt comprises an aluminum or aluminum alloy matrix composite.