US1865089A - Corrosion-resistant aluminum alloy articles and method of making the same - Google Patents

Description

nese 0.6; (B)same as Patented June 28, 1932 UNITED STATES PATENT OFFICE EDGAR H. DIX, JR., 0]! OAKHONT,

PENNSYLVANIA, ASBIGNOB TO ALUIIHUM comm VAJTIA- CORROSION-RESISTANT ALUMINUM ALLOY ARTICLES AND IMHO!) 01' MAKING m BAKE 1T0 Drawing.

Aluminum base alloys are for many purposes superior to pure aluminum but at least some of them are subject to more or less rapid deterioration by reason of corrosion or like action, which militates against their use in situations where they are exposed to agencies conductive to or producin such effects. Impure aluminum is also a acted by the same agencies though eiierally to a less extent. Among the most 1m ortant and valuable alloys are those of higli tensile strength, as for example alloys containing copper, magnesium, silicon, manganese, and zinc, in various combination and proportions as components in addition to aluminum. Of these I may mention specifically the well known alloys containing one or more of the above-named elements in up roximately the following percentages, an usually containing small amounts of impurities, as for example iron: (A)-copper 3 to 5, magnesium 0.5, manga- (A) but without magnesium and with copper 45.5; (C)- magnesium 0.7, silicon 0.9.

The high-strength alloys, including the three specifically mentioned above, require heat treatment to bring out their desirable physical properties to the fullest extent. The

eat treatment usually consists in heating the finished article to a temperature of around 500 C. for a suitable period and then uenching, followed by aging. Alloy described above, is a ed at room temperature, but alloys (B) an (C) may be aged-(after quenching at a temperature between 100 0., and 150 approximately. This is known as artificial aging. Alloys (B) and (C) have greater resistance to corrosion before aging than after, and experience in general indicates that artificial or high temperature aging decreases resistance.

The deterioration of alloys by corrosion is a serious matter in various arts, especially where replacement of corroded parts would mean virtual rebuilding, or where failure of even a single part may mean disaster, as for example in aircraft. Moreover, the deterioration does not always betray itself by excessive surface corrosion, but may be and not infrequently is due to internal or inter-granu- Appllcation med January 82, 1987. Serial Ho. 169,880.

lar action, leaving the surface of the part with no more than the eii pected amount of oxidation or the like. he result is that metal which to all appearances is unharmed ma nevertheless be seriously weakened, thus ma ing it diflicult to detect imperfect parts.

I have accordingly been led to devise my present invention, which has for its chief object to protect the impure aluminum or aluminum alloy article against corrosion by a non-porous surface layer or coating of substantially pure aluminum or of corrosion-resistant aluminum alloy. This object can be attained by casting the alloy or impure metal against the protective aluminum, preferably in the form of sheets or plates, and subjecting the'ingot thus formed to pressure and heat. By such method the coating and the underlying metal can be bonded together with a tenacity exceeding the tensile strength of the coating.

In practicing the process in the preferred manner it is convenient to arrange t e aluminum plates or sheets in the form of a lining for the mold in which the ingot or other article is to be cast, but it is not necessary, and indeed it is not in neral desirable, to have all inner surfaces 0 the mold lined with the alumium plates. On the contray it usually is sufiicient and in most cases advisable to line only two opposite surfaces. I have found that if an ingot so surfaced is rolled down to a thin sheet or plate, say a sixteenth of an inch thick, the alloy exposed at the edges in many cases is not appreciably subject to weakening corrosion, if at all, even under severe conditions. Apparently the pure aluminum with which the alloy is surfaced exerts a protective action on the uncovered ed s. On the other hand, in casting ingots or illets for drawing tubes, rods, wire etc., the entire inner surface of the mold ma have a lining in the form of a tube{ which is to be considered the equivalent 0 one or more plates or sheets.

In order that the ingot may be rolled down or otherwise shaped by rolling, drawing, or like operations, the castin of the alloy against the aluminum moldmn must prw duce an initial bond which wi withstan shearing or similar stresses produced in such operation tending to cause the surface plates to slip on the underlying allo To insure this initial bond without harm y affecting the surfacing sheets against which the alloy is cast it is advisable to observe certain precautions.

In the first place the tendency of the molten alloy to fuse or dissolve the aluminum sheets should be restricted so that the fusion or solution produced is only superficial. A certain amount of diffusion of one metal into the other is often desirable and in some cases essential, but this diffusion can be produced by subsequent heating and working of the ingot. and if the initial diffusion or penetration of alloy-components extends too close to the outer surface of the aluminum sheets or plates the subsequent difl'usion, combined with the thinning of the protective aluminum layer by working, may in effect result in converting such layer into a corrodible alloy. While care in rolling or other working. and avoidance of too high temperatures in heating the ingot for working, will aid in preventing the result alluded to, care in such operations is not always suflicient and hence the casting operation should be properly carried out. It is therefore desirable to use a mold which will conduct, absorb, or dissipate heat rapidly, and the lining sheets should be in close contact therewith, so that the heat imparted to the sheets or plates by the larger body of molten alloy, preferably at a. temperature as low as possible consistent with proper casting and initial bonding, will flow to the mold walls too rapidly to permit more than surface fusion of the aluminum. Iron molds having smooth inner surfaces have been found satisfactory for the purpose, and these are referably water-cooled Even with molds of the kind just described it is advisable to use lining plates of substantial thickness not only because too thin a plate is more easily melted or raised to a dissolving or diffusing temperature but also if the plate is not of suflicient thickness originally' the subsequent reduction in thickness by working may make it too thin to afford the desired degree of protection.

It is also advantageous, in casting the ingot, to avoid pouring the molten alloy against any part of the mold lining, as it has been found that a stream of molten alloy flowing on the aluminum plate is apt to make a hole or a cavity therein. Such effect may be due in part, at least, to erosion or some analogous action. Avoidance of splashing. of the molten alloy is also desirable. In general I try to have the alloy fold itself, so to speak, into contact with the plates, as a liquid poured gently into a vessel rolls or folds into contact with the walls. Then if there is any substantial fusion of the aluminum plate the fused portion is not moved out of of lace, and deeper or more extensive fusion is t erefore less likely to occur. A good method of casting is to pour the molten alloy in a gentle stream down an unlined surface of the mold, tilting the mold at first so as to incline the unlined surface and gradually straightening it up as the pouring proceeds. In this way, with the temperature of the alloy, the mass of the alloy and the mass of the lining sheets, and the rate of pourin so related that the alloy freezes against t e lining surface at about the same rate as its contact therewith, the disadvantageous conditions mentioned can be avoided. What I aim at is to produce a union which is predominantly one of cohesion as distinguished from adhesion.

Contrary to expectation, my observation is that the surface oxidation ordinarily found on aluminum does not prevent the necessary initial bonding, and hence, although dirt and grease should, obviously, be removed, it has not been necessary to clean the aluminum by etching with acid or alkali, or by sand-blasting or wire-brushing, or the like abrasion. It is probable that this oxide film is one cause of the failure of attempts to bond plates of aluminum and aluminum allow together by hot-rolling of the two in contact. I believe that in my process the molten alloy penetrates the oxide film at many points and thereby comes into cohesive or molecular contact with the metallic aluminum, and may, conceivably, dislodge or penetrate under some of the oxide particles. In strengthening this initial bond or union by rolling or other working, the numerous polnts or minute areas of cohesion are extended or spread out, but intervening oxide particles are not. Hence in the aggregate the area of molecular cohesion so greatly exceeds the aggregate area of the island-like noncohering points that the latter are in effect non-existent. Whether the theory just outlined be true or not, a union comparable to such a bond in completeness and tenacity is obtainable when my process is carried out with care and intelligence.

The coating plates may be held against the inner surfaces of the mold clamps, or they may have their edges seated in grooves. Making the lining in two or more parts is advantageous in that it permits expansion with less or no tendency to buckling or warping during heating by the hot alloy. In many cases annealing the coating material minimizes such tendency.

The initial bond of the surfacing plates to the alloy or impure aluminum core is made complete by heat and pressure, preferably rolling pressure, repeated as may be necessary to bring about complete union and, if desired, reduce the ingot or billet to a plate or sheet of the desired thickness. In such treatment there is more or less diffusion of alloycomponents into the aluminum plates, and

by means ofnaeaooo care should be taken not to cause or permit this alloying action to extend too close to the outer surface. It is therefore desirable in most cases to kee the working temperature below that at w ich too rapid solution of copper or other non-aluminous metal or metals resent takes place. For this reason, especially in the case of alloys whose physical properties are to be improved by heat-treatment, it is desirable that the temperature to which it is heated for rolling or other working should be well below the melting point of the eutectic or eutectics. For example, with. an alloy of type A above referred to for which a heat-treating temperature of 520 C. is satisfactory. a working temperature between 480 and 500 C. gave perfect results, the surface sheets or layers being so completely united to the alloy that they could not be stripped off nor could separation be even started. In this example an iron mold was used, 8 inches deep, 7 inches wide, and 1.5 inches from front to back. inside measurements. .The side walls were inch thick, edge walls 1 and bottom 3 inches thick. The lining sheets, smooth on both sides and 0.1 inch thick, were of annealed electrolytically refined aluminum 99.95 per cent pure, and were cut to allow for the expansion incident to the heat to which they were subjected in casting the alloy. The alloy contained copper 3.33 per cent. silicon 0.33 per cent, magnesium 0.49 per cent, manganese 0.40 per cent, iron 0.34 per cent, and the rest aluminum. The alloy, at a temperature of about 730 C., was poured into the open top of the mold in the ordinary manner. After removal from the mold the ingot was heated to 480 to 500 C. and while at this temperature was rolled to 0.5 inch thickness. It was then reheated to 400 C. and rolled to 0.1875 inch. The sheet was then annealed at 250 C. and cold-rolled to 0.064 inch, and finally heat-treated by holding it for twenty minutes in a fused bath of sodium and potassium nitrate at 520 C., followed bv quenching in cold water and aging at room temperature.

The surface appearance of the sheet was better than that of a sheet produced according to the usual method and there was no evidence of blistering or like defects. Microscopic examination showed there were no discontinuities between the alloy and the pure aluminum coating, and indicated perfect union. Diffusion of copper from the alloy into the aluminum coating, 0.0045 inch thick, was slight but easily seen.

After aging for one week, five specimens of the sheet were tested for corrosion, along with similar pieces of the same alloy uncoated, by continuous subjection for eight weeks to a mist produced by spraying a 20 per cent solution of common salt. At the end. of this time the tensile strength and elongation of the coated articles had suffered no impairment. On the other hand the average of the five uncoated specimens showed a loss of 13.6 per cent in tensile strength, or more than 4 tons per square inch, and the elongation had decreased from 20.4 per cent to 7.0 per cent in 2 inches, or a loss of 65.7 per cent. Larger ingots of type A alloy, approximately 4 by 12 by 24 inches in size, with surfacing sheets inch thick, have also been cast and worked with completely satisfactory results. I have also found that fabrication of coated ingots into other articles is notably easier and can be effected at less cost than with uncoated ingots of the same alloy, as the coating eliminates or greatly decreases many of the difliculties hitherto encountered in rollmg.

In speaking of heating and working the ingot after casting I do not mean to imply that the ingot must be allowed to cool below the working temperature and then reheated. On the contrary it is only when the casting has cooled too far that reheating need be resorted to, and in some cases it is advantageous to begin the working without allowing the casting to cool far enough to necessitate reheating. This is especially true with alloys of high magnesium content. or, in general. with alloys which oxidize readily. With such alloys, it appears that if the core and the coating sheets do not stick at all points air finds its way in between, with the result that in the reheating the oxide or nitride film, or whatever it may be, is materially increased and becomes too great to permit eventual attainment of a bond complete enough to prevent separation. l have found, however. that if the ingot be subjected to pressure. say a light pass between rolls. before it has cooled too far, preferably as soon as it can be taken from the mold and before it has cooled appreciably, the union can be improved to such an extent that adequate initial bond can be obtained.

I have found that alloys containing copper, say from 2 to 6 per cent, are most suitable for my process. On the other hand magnesium tends to be disadvantageous and apparently should be less than about 1 per cent to give in the casting step alone the bond needed to prevent excessive oxidation or the like between the alloy and the aluminum surfacing plates. but by pressing the plates firmly on the alloy core before the ingot has cooled down too much, preferably applying the pressure by rolling as soon as the ingot can e taken from the mold, as described just above, the desired bond can be obtained with alloys containing much higher amounts of magnesium. For the surfacing plates I prefer the high purity electrolytically refined aluminum now available on the market but ordinary commercial aluminum may be used, as may also aluminum alloys having adequate resistance to corrosion, and in the claims the term aluminous metal is intended to include commercial aluminum and corrosionresistant aluminum alloys as well as aluminum of high purity. The alloys are generally harder than aluminum and are therefore advantageous where a harder surface is desired. Alloys of aluminum and manganese, and aluminum and beryllium, are suitable for such purpose.

I am aware that it has bee proposed, in British Patent No. 25,380 of 1898, and in Swiss Patent No. 18,292 of 1899, to provide an article of aluminum alloy with a coating of pure aluminum by rolling the aluminum and the alloy together, hot or cold; but such method has been found to be unsuccessful, it being impossible to produce thereby the cohesive bond whichcharacterizes the product of my process and'makes the two metals, the alloy and the aluminum, an integral unit. The lack of cohesion between the two bodies in the product of the method referred to is evidenced by the fact that if the article so made is heated the aluminum layer pufi's up in the form of blisters, indicating that although the two layers may have been close together they were 1n fact not in actual molecular metal-to-metal contact.

It is to be understood that the invention is not limited to the specific procedure herein described but can be carried out in other ways without departure from its spirit. Nor it is confined to alloy, strictly so called, as the coated metal or metal to be coated, since it may, in general, be employed with impure aluminum as the equivalent of an alloy and the subjoined claims are intended to be so interpreted.

I claim- 1. Process of coating corrodible aluminum alloy with corrosion-resistant aluminous met- :11. comprising casting a body of the alloy in contact with a plate of the aluminous metal while dissipating the heat of the alloy at a rate adapted to cause freezing of the alloy on the plate with not more than superficial fusion of the plate, whereby an initial surface-bond between the two is produced, and completing the bond by heat and pressure with the article at a temperature suificiently below the eutectic melting point to prevent excessive intor-diffusion.

2. Process of coating corrodible aluminum alloy with corrosion-resistant aluminous metal, comprising casting a body of the alloy in contact with a plate of the aluminous metal while dissipating the heat of the alloy at a. rate adapted to cause freezing of the alloy on the plate and produce an initial bond with not more than superficial fusion of the contiguous plate surface, hot-working the article to complete the bond without causing suflicient inter-difl'usion to bring non-aluminous alloy components to the surface of the alumiworked article.

3. Process of coating corrodible aluminum alloy with corrosion-resistant aluminous metal, comprising lining with the aluminous metal a mold having thermally conductive walls, casting therein a body of the alloy and by regulation of the alloy temperature and pouring rate causing the alloy to freeze on the mold lining with cohesion-contact without more than superficial fusion of the lining, hot-working the article at a temperature sufliciently lower than the eutectic melting point to prevent such diffusion of non-aluminous alloy-components into the coating metal as to destroy the corrosion-resistant repertiels thereof, and heabtreating the wor ed artic e.

4. Process of coating corrodible aluminum alloy with corrosion-resistant aluminous metal, comprising casting a body of the alloy against a plate of the aluminous metal without erosive movement of the alloy thereon and by regulation of the temperature and pouring rate according to the rate of heat dissipation from the contiguous surface of the plate causing the alloy to freeze on said surface and produce an initial bond with not more than superficial fusion, completing the bond by hot-working, and heat-treating the worked article. a

5. Process of coating corrodible aluminum alloy with corrosion-resistant aluminous metal, comprising lining a metal mold with the aluminous metal, casting in the lined mold a body of the aluminum alloy and by regulation of the temperature and pouring rate of the alloy according to the rate of heat transfer through the mold lining and the mold walls causing the alloy to freeze in cohesive contact with the lining with not more than superficial fusion thereof whereby an initial bond is produced, hot-working the article to complete the bond, and heat-treating the worked article.

6. Process of coating corrodible aluminum alloy with corrosion-resistant aluminous metal. comprising casting against a plate of the aluminous metal a bodv of aluminum alloy containing copper and by regulation of factors determinative of the solidification of the alloy causing the same to freeze on the plate and produce an initial bond with not more than superficial fusion thereof, completing the bond by hot-working at a temperature too low for excessive difi'usion of copper into the coating metal, and heat-treating the worked article.

7. Process of coating corrodible aluminum alloy with corrosiomresistant aluminous metal, comprising casting a body of aluminum alloy containing copper against a lining of the aluminous metal in a mold having heatconducting walls and by regulation of other casting conditions causing the alloy to freeze tic to prevent excessive penetration of copper into t e aluminous coating metal.

8. Process of coating corrodible aluminum alloy with corrosion-resistant aluminous metal comprising casting a body of the alloy against a plate of the aluminous metal and by regulation of the casting conditions causing the alloy to freeze on the plate and produce an initial bond with not more than superficial fusion of the plate, and hot-working the article under conditions preventing destruction of the corrosion-resistant properties of the aluminous metal by diffusion of non-aluminous alloy components into the same.

9. Process of coating corrodible aluminum alloy with corrosion-resistant aluminous metal, comprising casting a body of the alloy against a plate of pure aluminum and by regulation of the casting conditions causing the alloy to freeze on the aluminum and produce an initial bond with not more than superficial fusion of the aluminum, completing the bond by hot-working under conditions preventing destruction of the corrosion-resistance of the aluminum by diffusion of non-aluminous alloy components into the aluminum, and heat-treating the worked article.

10. Process of coating corrodible aluminum alloy with corrosion-resistant aluminous metal, comprising casting a body of aluminum alloy containing copper against a late of pure aluminum and by regulation 0 the casting conditions producing an initial bond between the alloy and the aluminum with not more than su erficial fusion of the latter or penetration 0 copper into the same, hotworking the article at a temperature too low for penetration of copper far enough to destroy the corrosion-resistant properties of the coating, and heat-treating the worked article.

11. Process of coating corrodible aluminum alloy with corrosion-resistant aluminous metal, comprising casting against a plate of pure aluminum a body of aluminum alloy containing copper and magnesium and by regulation of temperature and pouring conditions according to the rate of conduction of heat from the surface of the aluminum causing freezing of the alloy on the aluminum and producing thereby an initial bond with not more than superficial fusion of the aluminum plate or penetration of copper into the same, completing the bond by hot-working the article under conditions preventing destruction of the corrosion-resistant properties of the aluminum by penetration of copper into the latter, and heat-treating the worked article.

12. A corrosion-resistant aluminous metallic article comprising a relatively thick main body of corrodible aluminum alloy and a relatively thin wrought non-porous protective coating of corrosion-resistant aluminum directly cohesively united with the main body, the article being capable of being heat treated without blistering.

13. A corrosion-resistant aluminous metallic article comprising a relatively thick main bod of heat-treated aluminum alloy and a re atively thin wrought non-porous protective coating of annealed corrosionresistant aluminum directly cohesively united with the main body, the article being capable of being heat-treated without blistering.

14. A corrosion-resistant aluminous metallic article comprising a relatively thin main body of heat-treated aluminum alloy containing between 2 and 6 per cent of copper, and a relatively thick wrought nonporous protective coating of annealed corrosion-resistant aluminum, directly cohesively united with the main body and free from blisters.

15. A corrosion-resistant aluminous metallic article comprising a relatively thick main body of heat-treated aluminum alloy containing between 2 and (3 per cent of copper, and a relatively thick wrought non-porous protective coating of annealed corrosionresistant aluminum, directly cohesively united with the main body and free from blisters, said article being capable of withstanding continuous contact with mist from a 20 per cent salt solution for a period of at least'eight weeks without appreciable loss of tensile strength and ductility.

16. An article composed of a relatively thick body of corrodible aluminum alloy having a relatively thin wrought non-porous coating of corrosion-resistant aluminous metal directly cohering with a tenacity exceeding the tensile strength of the coating.

17. An article composed of a relatively thick body of corrodible aluminum alloy having a relatively thin wrought non-porous coating of corrosion-resistant aluminous metal directly cohering with a tenacity exceeding the tensile strength of the coating, said article being capable of withstanding continuous contact with mist from a 20 per cent salt solution for a period of at least eight weeks without appreciable loss of strength and ductility.

18. A corrosion-resistant aluminous metallic article comprising a relatively thick main body of heat-treated aluminum alloy and a relatively thin wrought-non-porous protective coating of annealed corrosionresistant aluminum, directly cohesively united with the main body and free from blisters, said article being capable of withstanding continuous contact with mist from a 20 per cent salt solution for a period of at least eight weeks without appreciable loss 1,ses,oae

June 28, 1932.

page 2, line 93, for "allow" read it; page 5, line 85, claim 14, for

nt. should be read with these corto the record of the case in the M. J. Moore,

of strength and ductility.

In testimony whereof I hereto aflix my signature.

EDGAR H. DIX, JR.

CERTIFICATE or CORRECTION.

PatentNo. 1,865,089.

EDGAR H. DIX, JR.

it is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 1, line 79, for the misspelled word "contray" read contrary; alloy; page 4, line 32, for "it is" read is "thin" read thick. and lines 88 and 97, claims 14 and 15 respectively for "thick" read thin; and that the said Letters Pate rections therein that the same may conform Patent Office.

Signed and sealed this 25thday of October, A. D. 1932.

(Seal) Acting Commissioner of Patents.

blisters, said article being capable of withstanding continuous contact with mist from a 20 per cent salt solution for a period of at least eight weeks without appreciable loss 1,ses,oae

June 28, 1932.

page 2, line 93, for "allow" read it; page 5, line 85, claim 14, for

nt. should be read with these corto the record of the case in the M. J. Moore,

of strength and ductility.

In testimony whereof I hereto aflix my signature.

EDGAR H. DIX, JR.

CERTIFICATE or CORRECTION.

PatentNo. 1,865,089.

EDGAR H. DIX, JR.

it is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 1, line 79, for the misspelled word "contray" read contrary; alloy; page 4, line 32, for "it is" read is "thin" read thick. and lines 88 and 97, claims 14 and 15 respectively for "thick" read thin; and that the said Letters Pate rections therein that the same may conform Patent Office.

Signed and sealed this 25thday of October, A. D. 1932.

(Seal) Acting Commissioner of Patents.