US3234053A - Improved method forming aluminum magnesium sheet - Google Patents

Description

United States Patent 3,234,053 IMPROVED METHOD FGRMING ALUMENUM- MAGNESIUM SHEET Michael J. Pryor, Hamden, Conn., assignor to Olin Mathicson Chemical Corporation, a corporation of Virginia No Drawing. Filed May 14, 1963, Scr. No. 280,443

Claims. (Cl. 148-115) This application is a continuation-in-part of co-pending application Serial No. 162,986, filed December 28, 1961, by M. Pryor, now abandoned.

This invention relates to the formation of improved properties in sheet and strip aluminum-magnesium alloys by controlled cooling. More particularly it relates to a process for the improvement of certain physical properties of aluminum-magnesium alloys, which are suitable for bright anodizing in sulfuric acid after prior treatments such as bufiing and/ or bright dipping, by controlled cooling of selected portions of the sheets or strips, and is directed toward the achievement of greatly simplifying the production of such material in a form highly suited to subsequent bright anodizing processing.

Of the many sheet metal products formed of rolled metal, a large percentage desirably have a surface appearance which is highly lustrous and generally pleasing to the eye, and also have functional metallurgical properties of hardness, formability, or other desirable combinations of physical properties. The production of sheet aluminum having these esthetic characteristics combined with physical properties is a highly developed art and constitutes a very substantial portion of the existing capacity of the aluminum industry.

In the art of producing sheet and strip material from aluminum containing small additions of magnesium which are suitable for bright anodizing, serious difiiculties arise from the relationship of the combined factors of hot rolling temperature and pressure. For example, suitable and desirable surface, as well as bulk, characteristics in sheet and strip materials made from the aforementioned alloys is normally achieved through the conjoint action of a moderate hot rolling temperature combined with an essentially high separating force on the hot mill rolls. The factor of temperature is critical because of the necessity of accomplishing the strain induced precipitation of intermetallic compounds such as Mg Si and BMg Al at relatively low temperatures so that a very fine dispersed precipitate is obtained rather than at higher temperatures such as 700 to 800 P. where an undesirable coarse dispersion of these compounds is obtained.

However in the lower temperature range 450 to 650 extreme pressure is required to make significant Warm reductions due to the substantial strain hardening of these alloys. More specifically as hot rolling temperature decreases, particularly with regard to aluminum-magnesium alloys, separating force on the hot mills increases sharply requiring hot mills of great power and stiffness. In addition, extremely careful control of temperature at any given reduction, as well as high eflicient removal of oxide coatings from the work roll surface and extremely effective lubrication are needed. As a result of these factors commercial production of acceptable bright sheet products by lower temperature hot rolling tends to be low in comparison to the rate of production resulting from high temperature hot rolling unless costly multiple stand tandem mills of great power, stiffness and cost are available. On the other hand high temperature hot rolling frequently produces a product substantially inferior in bright anodizing characteristics to that produced by low temperature hot rolling and this results in a commercially unacceptable degree of waste.

3,234,953 Patented Feb. 8, 1966 It has been recently found that the properties obtained by hot rolling in the lower temperature ranges are desirable or even necessary only in a surface layer of the metal sheet or strip, this surface layer frequently constituting only a very minor portion of the total thickness of the sheet. The remainder of the thickness, which is not involved in the bright anodizing process, is accordingly, susceptible to maintaining the physical properties which are desirable for greatest ease of hot rolling and for lowest cost in this operation. Accordingly, an article is achieved which possesses a gradient of physical properties through its thickness which result from more suitable processing than heretofore known and which yield desirable characteristics for all portions of the thickness of the sheet or strip.

In connection with this processing it is well known that there are two types of constituents or intermetallic compounds which influence the bright anodizing characteristics of aluminum-magnesium alloys. Firstly, there are those which are nearly insoluble during anodizing and become included in the anodic film; these obviously reduce the light which is transmitted through the anodic film, and include such constituents as FeAl MnAl and uAlFeSi. Maximum bright anodizing characteristics are obtained if these constituents are present in a coarse particulate form.

The other type of constituents which influence bright anodizing characteristics are those which are soluble during anodizing such as Mg Si and ,8 phase Mg Al Their distribution affects the roughness of the reflecting oxide film-metal interface; but because they dissolve during anodizing they do not interfere with the oxide film clarity to a significant degree. A given quantity of these constituents dispersed in fine particulate form produces a smooth oxide film-metal interface having a high specular reflectivity. A similar quantity of these constituents dispersed in coarse particulate form produces a roughened oxide metal interface having a low specular reflectivity and poor image clarity.

It is also well known that the distribution of soluble constituents of Mg Si and Mg Al can be controlled in the hot rolling process. This is accomplished in an intermediate temperature range of 450 to 700 F. under the conjoint action of temperature and roll deformation. The lower the temperature, the finer will be the dispersion of the constituent particles and therefore the higher the reflectivity after bright anodizing.

In view of the above mentioned factors, and others that Will become apparent hereinafter, it is a principal object of this invention to provide an improved method forming aluminum-magnesium sheet or strip material which is suitable for bright anodizing.

It is another object of this invention to provide a method of forming aluminum-magnesium sheet or strip material having metallurgical characteristics in at least a surface layer thereof which are suitable for bright anodizing.

It is a still further object of this invention to form a bright anodizing aluminum-magnesium sheet or strip material having a gradient of metallurgical properties throughout the thickness thereof which are desirable both in subsequent bright anodizing.

It is still another object of the present invention to provide a process of manufacturing bright anodizing aluminum-magnesium sheet or strip material which facilitates the achievement of certain desired metallurgical properties by hot rolling in low temperature ranges with substantially reduced separating forces on the hot mill rolls.

Other objects and advantages of the present invention will become apparent from a consideration of the following description and examples.

In accordance with the principles of this invention it has been discovered that an improved method of manufacturing bright anodizing aluminum-magnesium sheet material is achieved, in which the sheet has a high thermal conductivity and has magnesium present in the form of at least one precipitated intermetallic magnesium compound.

uniformly dispersed in fine particle size in a surface layer of the sheet, by (1) providing a sheet of aluminum-magnesiu-m alloy having magnesium in the form of at least one intermetallic magnesium compound as a soluble constituent of the alloy, (2) dissolving the constituent into solid solution in the alloy (3) establishing a thermal gradient through the thickness of said sheet,and (4) precipitating the constituent through the conjoint action of said thermal gradient and mechanical deformation to produce, in a surface layer only of said sheet, a fine particle size uniform dispersion of said constituent and a coarse particulate dispersion of said constituent in the remainder of the thickness of said sheet. As an adjunct of this process we have discovered a novel article of manufacture in the form of an integral bright anodizing aluminum-magnesium sheetwhich is characterized by having a gradient of metallurgical properties throughout the. thickness thereof, the gradient being constituted by at least one surface layer of the sheet having magnesium in the form of at least one precipitated intermetallic magnesium compound uniformly dispersed throughout this layer in fine. particle sizewith the remainder of the thickness of the sheet having this compound dispersed in .a coarse particle size in relation to the aforementioned fine particle size.

Aluminum alloys suitable for subsequent bright anodizing after being formed into sheet or strip material fall generally into two categories, those containing around 2% to 1.8% magnesium and those containing around 2% to 3.2% magnesium, both with Fe and Si as impurities in amounts less than .4%. Included within the first group are alloys 5257 (.2%-.6%) magnesium, 5357' (.8%1.2% 5457 (.8%1.2% 5557 (.4%-.8% 5657 (.6%-1.80%), 5757 (.6%1.80%), 5857 (.5%8%), 5957 (.4%.8% among the second group are such alloys as 5252 '(2.2%-2.8%), 5652 (2.2%-2.8%), and 5053 (3.2%). These aluminum-magnesium alloys contain magnesium in sufficient quantity to form .intermetallic magnesium compounds which are soluble in high temperature ranges. In the case of the .2% to 1.8% mag: nesium group the operative intermetallic compound is Mg Si which ,is'soluble in the temperature range of 750 to 850 F. and is precipitated upon cooling to below this temperature range. In the case of the 2% to 3.2% magnesium group two intermetallic compounds of magnesium are formed, Mg Si which is soluble in the temperature;

range previously stated, and Mg Al which is soluble in the temperature range of 850 to 950 F. and which precipitates upon subsequent cooling below this latter temperature range. In either case. the lower temperature precipitation of these compounds is accelerated by deformation at lower temperatures.

After casting the ingot and scalping it, the alloy is reheated prior to breakdown hot-rolling, to a temperature of sufficient magnitude to redissolve the intermetallic.

magnesium compound into solid solution in the base metal. Thi temperature is within the range of 750 to 950 F. depending upon which of the aforementioned groups of aluminum-magnesium alloys is selected for processing. Desirably the scalped process ingot is heated to approximately 850 F. in order to drive the Mg Si into solid solution, or to approximately 900 E, which tem-, perature is required to drive the Mg Al into solid solution, in the event that one of the 2% to 3.2% magnesium alloys is selected for the processing. Within these tem-. perature ranges the alloys have a high degree of plastic flow and accordingly can be rolled with considerably high reductions per pass with far less separating force on the hot mills than would be required for corresponding rolling rolling in the tandem mill the material is reduced to lower 4 gages and the alloy cool within the temperature range where precipitation of magnesium intermetallic compounds can occur under the conjoint action of temperature and roll pressure. As the alloy becomes cooler its resistance to deformation or reduction increases strongly thereby limiting the degree of reduction in thickness that can be obtained in a single pass; This condition becomes more severe the lower the bulk temperature of the strip. Attempts to alleviate this resistance to, deformation by final hot rolling at'a high-er bulk temperature, where resist ance to deformation isless, 'is accompanied in these aluminum-magnesium alloys by a coarser dispersion of precipitated intermetallic compounds and poorer bright= anodizing characteristics. a

To overcome this diificulty, and to obtain the'desired fine dispersion of magnesium intermetallic compounds which result in excellent bright anodizing characteristics, While at the same time maintaining the production advantages of higher temperature hot rolling, the sheet or strip is subjected to a very. rapid one side cooling process immediately before the final hot reduction or reductions.

It has been found that heat can be removed rapi dly from one side of a continuously advancing high temperature strip to establish a steep temperature gradient through the thickness of the strip immediately before'its'entry into the last hot reduction. This gradient is established by the application of a cooling medium directed at an extremely high velocity to the side of the strip which it is desired to.

be'cooled, and must be maintained .until the strip enters the .roll bite and becomes plastically deformed. It shouldv be noted thatthe aluminum-magnesium .alloys suitablef'for use in connection withthis invention havean extremely high thermal conductivity, in the order of B.t'.u. per

hour per square f ootper degree Fahrenheit; accordingly in order to develop an effective thermal differential at the instant of reduction by the rolls it is necessary to use a:

high capacity cooling systemvsuch as .that described in copending application for patent S.N. 156,119, filed No-' vember 30,1961, assigned to a common assignee as the instant application. According to the novel cooling process of that application a liquid medium is expressed from an orifice as a highvelocity jet-which is directed against the surface to be cooled substantially perpendicular thereto. Rapid cooling is achieved. even though vaporization of the cooling liquid form on the hot metal surface,

I thereby constituting a vaporons thermal barrier, due to the fact that the cooling medium moves at such a high velocity that it penetrates :the vbarrier and brings the cooling liquid into direct contact with the high temperature surface.

Since theabove mentioned alloys, which are used in connection with this invention,;have -arrelatively high thermal conductivity, specialized problems arise with respect to maintaining the thermal differential until the moving strip enters the roll bite. Withirespect to the rate of cooling of the surface-layer, it hasbeen found that in a metal sheet or strip in the order of-a quarter inch or less, itiis possible to develop a temperature gradient of as much as 400 F. where the temperature of the sheet was originally at 800 F. by using a spray which removes heat at a coefii'cientatabout 8,000. 3.t.u. per hour per square foot ,per degree Fahrenheit. In order to establish such a heat transfer coefiicient at the metal surface, spray must be delivered at a volumetric .rate of PPIOXimately 10 gallons per minute under a pressure of at least 300 1 psi. through a spray developing nozzle located at about 12" from the hot metal surface Lower. temperature coeflicients cause lower-gradients to be established. Thus where the coefficient is about4,000 in the above example a temperature gradient of about 225 F. is developedin a sheet initially at 800 F.

The development of temperature differentials in the range of 200 to 400 F. is necessary in order to, establish the gradient of metallurgical properties ultimately desired inthe .finished sheet suitable for bright anodizing. Thus. the effect of the :one side cooling and maintaining this temperature differential until the strip is subjected to deformation is to promote rapid precipitation of the intermetallic compound, or compounds as the case may be, as a fine particle size uniform dispersion in a substantially cooler thin surface layer only of the sheet when the sheet is subjected the mechanical deformation, while simultaneously permitting the precipitation to occur, upon deformation, as a coarse particulate dispersion in the much hotter bulk of the thickness of the sheet. In the case of the .2% to 1.8% magnesium alloys, for example, precipitation of fine Mg Si will occur within the temperature range of 450 to 550 I? and under the exemplary conditions set forth above this will produce a surface zone of approximately /2 to 1 mil in thickness, in a final sheet of L050" in thickness, this .0005 to .001 thickness being all that is required in the final sheet to generate good brightness after bright dipping (electrobrighting and polishing) and anodizing, with or without mechanical bufiing. Normally about of the thickness of the final sheet is required as the surface layer for these operations. Con-fining the desirable fine precipitates of magnesium intermetalliccompounds to the outer 10% of the strip during hot rolling results in an average bulk temperature that is close to 'thatof the strip before the application of the coolant. Little average temperature drop results under .these conditions which results in easy hot rolling with large permissible reductions which is characteristic of the unquenched strip with the coarse dispersion of magnesium intermetallic compounds and the poor bright anodizing character istics.

'In the case of the 2% to 3.2% magnesium group of alloys which contain not only Mg Si which is precipitated in the above-mentioned temperature range, but also 'Mg Al which is fully precipitated in a somewhat higher temperature range in the order of 550 to 700 F. A similar depth of precipitation is desired with these latter alloys as with the former.

'It is usually not advisable to develop two side temperature gradients on metal substantially less than a half inch in thickness Where the metal has a high temperature coefficient because substantial bulk cooling then occurs and resistance to roll deformation increases sharply. Distinct advantages are readily obtained, however, from two side cooling of thicker metal .sheets during rolling operations because the roll acts-on the lower temperature surface by the reduction produced while a given mill roll separating orceis that characteristic of the high temperature core.

Regardless of whether one or both sides of the strip are cmled, the strip rolls as a strip at a higher temperature but he finished product on the surface layer, which is substantially cooler than the bulk of the strip, crystallizes during hot rolling inamanner characteristic of the lower temperature range to produce the desired fine particle size uniform dispersion of precipitate. It is noteworthy that usually, although not necessarily, the surface subjected to the high pressure coolant will be the upper face of the strip since this is less subjectto mechanical scratching and other handling damage.

.The high velocity coolant, under transient conditions only leads to a steep temperature gradient in approximate'ly the surface 10% of'the strip as mentioned before, with little bulk cooling of the remainder of the metal. High hot rolling speeds in the order of 250 feet per minute favor the maintenance of these transient conditions until the strip enters the roll bite. Since the separating force on the hot mill is a function of the average temperature of the strip at the instant of strip deformation this temperature differential will enable similar hot rolling to be accomplished at speeds and reductions characteristic of the average higher temperature of the strip. However, the upper surface layers being as much as 200 to 400 F. cooler than the bulk" material will recrystallize during hot rolling in a manner characteristic of the lower temperature range, thereby producing the uniform fine particle size dispersion of the intermetallic magnesium compounds.

In further connection with the establishment of the thermal gradient throughout the thickness of the sheet,

it .should be pointed out that, while it is preferred to employ a very rapid cooling system such as the high pressure spray system mentioned above, it will be apparent that other systems may be employed. For example, the desired thermal gradient may be established by applying a cooling effect to one surface of the sheet while simultaneously applying a comparably efficient heating effect to the opposite surface.

Precipitation of the intermetallic magnesium compounds in the desired particle size and uniformity of dispersion is effected through the conjoint action of temperature differential and mechanical deformation of the hot sheet or strip. Customar'ily, although not necessarily, this deformation is accomplished by pressure rolling of the sheet immediately after the one side cooling to effect a reduction in the thickness of the metal. Where the control of the magnesium compound precipitation is of primary concern, the pressure rolling reduction should be accomplished on the final hot rolling pass. This is generally, although not necessarily, true where other considerations such 'as grain size and texture control are of primary importance, these factors being discussed in more detail hereinafter; however, of these two stages of hot rolling, th ultimate pass is the most important and the most critical in order to achieve the desired metallurgical prop- In order to obtain the desired particle size and uniformity of dispersion of the magnesium precipitates in the finished pro-duct, it is desirable that the final hot rolling reduction occur while the outer surface layer of the sheet or strip is at a temperature within the range of 450 to 550 F. in the case of the .2% to 1.8% magnesium alloys, and within the range of 550 to 700 F. in the case of the 2% to 3.2% magnesium alloys. By

effecting the hot rolling reduction with the outer surface layer at a temperature within these ranges with the bulk of the material still within the higher temperature range of 750 to 950 -F., a differential or gradient of metallurgical properties is established within the thickness of the material which yields desirable characteristics of the finished product for the purpose of both the surface layer appearance upon bright anodizing and greatest ease and economy "of hot rolling. More specifically under these circumstances of temperature differential during final hot rolling, the intermetallic magnesium compound or compounds are precipitated in a manner characteristic of low temperature rolling to yield the desirable very fine particle size uniform dispersion, while retaining its coarse particle size during precipitation which is characteristic of higher temperature rolling. At the same time the separating force on the rolls need only be that which is character istic of that required for rolling the strip with the entire thickness thereof at the higher temperature level.

For purposes of definition, distinction between fine and coarse particle size is made by defining fine particle size as that which is not resolvable under a conventional optical microscope at a magnification of 500 diameters.

In order to effect the final hot rolling of the strip with the desired thermal gradient established therein, it is necessary that a minimum amount of time elapse between the development of the temperature gradient and the subsequent rolling. It has been found that a suitably steep temperature gradient in the order of 150 to 250 F., but not less than F., nor more than 350 1 should be established not lower than .4 second and preferably .1 second before the strip enters the roll bite. It is necessary to confine this time lapse to the shortest possible period due to the high thermal conductivity of the metal sheet or strip. If longer periods of time elapse the temperature gradient will become very shallow; this may in some cases be offset by the one side heating and one side cooling as mentioned above. In either case, however, the average temperature of the strip should not 7. be reduced more than 50 F. from a desirable and economical average hot rollingtemperature.

As a practical matter, in order to effectthe final hot rolling reduction in as short a time as possible after the.

establishment of. the desired temperature gradient, the cooling is applied at a point close as possible to the contact point of the strip with the hot mill, and this is generally in the order of 1 foot or less. Of course, it should be kept in mind that this distance is a function of the linear speed of the strip as it passes through the hot mill, and since the temperature differential is transient, i.e., can be maintained for only very brief periods of time, it isdesirable to maintain a fairly high m-ill speed on the strip. At these short distances from the striphot roll contact point, water soluble oil or other hot mill cooling lubricant used as a cooling medium is suitable for the achievement of the temperature gradient. HOW! ever, the use of other cooling fluids such as Water is contemplated where additional measures are employed to remove the cooling liquid, as by air or water Wipe devices, before the strip enters the hot roll surface.

The following are illustrative examples of this invention:

Example I A 5457 aluminum-magnesium alloy was direct chill cast into a 20" thick ingot, cooled to room temperature and scalped to remove surface imperfections. It was then reheated to 875 F., and reduced in a reversing mill.

from approximately 19% inches to approximately half inch strip. The stripwas then fed into the-first stand 01' a tandem continuous hot mill press for a further re duction to a gage suitable for processing underthe princi- Since the solution heating was unnecessary. Between the penultimate and ultimate hot rolling passes, the strip was subjected to a high velocity stream of a cooling medium on the upper surface thereof to cool-the surface and the adjacent of the thickness of the metal to about 500 F., thereby establishing the desired thermal gradient. The cooling medium nozzles were located aboutone foot away from the surface of the strip and directed. at right angles thereto. The distance between the point of application of the cooling medium and the hot roll nip was about 10 inches.

At a linear speed of about250'ft. per minute, the strip passed into the final stand of hot rolls and subjected to a further reduction to about .20 to precipitate the, intermeta-llic compound under the conjoint action of temperatur-e differential and mechanical deformation due to roll pressure. The sheet was then examined and found to have the desired gradient of metallurgical properties through its thickness of fine particle size uniform precipitate dispersion and excellent bright anodizing characteristics in the surface 10%, with a relatively coarser particulate precipitate dispersion and poorer bright anodizing characteristics in the unquenched remainder of the thickness.

Example II A sample of 5252 aluminum alloy was prepared and i processed in the manner of Example I with the exception that the bulk temperature of the metal was at 875 F. at the moment of the final hot rolling and the. surface layer temperature after one side cooling was at 600 F. Good results were obtained.

The foregoing method of the present invention is particularly adapted to the manufacture of sheet "or strip' or zone adjacent at least one side of' the, shee t or strip and the .bulk remainder: of the .materiaL. The .surface' layer is characterized .by having .a .fine.v particle. size uniform dispersion of the .intermetallic'magnesium compound, 'or compounds'as the case maybe, precipitatedv invention have application in fields other thanthe produce tion of sheet; or. strip materialfsuitable for'subsequent bright anodizing. .For examplejan'inherem characteristic of the one sidecoolir'ig process of this inventionis that the surface layer. of the' material, being as much 'as 350? cooler than. the bulk material during final hot rolling.

recrystallizeslduring hot'rol-ling in a manner characteristic of the lower temperature,"therebyproducing a fine grain size and so called. warrn rolled. texture'in'the-base metal in addition/tothe precipitate.characferistics desired for bright anodizing. These characteristics of fine grainsize' and warm rolled texture, while they promote'maxirnuin bright anodizing qualities, are beneficial to the performance of aluminum alloys subsequently used in the produc-, tion of specificarticles under the process described in"U.S.

Patent No. 2,690,002.I I

This inventionmay be embodied in other forms. or

carried out in 'other ways without departing from the: spirit or essential characteristics thereof. The present embodiment of the invention 'is therefore to be considered i.

in all respects as illustrative and not restrictive, thev scope of the invention being indicaed by the appended claims and all changes which come within the meaning and range are intended to be embraced 1" of equivalency of the claims therein.

What I claim and desire to secure .by Letters'Patent is: 1. A process vofproducing an aluminum-magnesium sheet having a high thermal conductivity and having magnesium'presentin the form of "at least one precipitated intermetallic magnesium compound uniformly dispersed in fine particle size in a surface layer of said sheet, said process comprising the steps of A. providing a sheet of aluminum-magnesium alloy having magnesiumin the form'of at least one intermetallic .magnesium' compound as a soluble con stituent of said alloy, 7

alloy by heating said sheet, C: establishing a thermal gradient through the thickness ofsaid sheet characterized 'by developing in asur- B.' dissolving said constituent into solid solution in said:

face layer only of said. sheetp a temperature lower than that developed in the remainder of the thickness of said sheet, andv D. precipitating said constituent through the conjoint action-.of-said thermal gradient and mechanical deformation to produce,.in said surface layer only, a fine particle size uniform dispersion of said constituent-and a coarseparticulate dispersion of said constituentin said remainder of the thickness of sai sheet. 2. A .process of producing an aluminum-magnesium sheet having a high thermal conductivity and having magnesium .present in the form of at least one precipitated intermetallic magnesium compound. uniformly dispersed in fine particle size in a surface layer of said sheet, said.

process comprising the steps of A. providing 'a sheet of aluminum-magnesium alloy having magnesium in the form of at least one intermetallic magnesium compound .as. a soluble constituent of said alloy,

B. dissolving said constituent into solid solution in said alloy by heating said sheet substantially uniformly through the thickness thereof,

C. establishing a thermal gradient through the thickness of said sheet by rapidly cooling at least one surface of said sheet whereby said thermal gradient is characterized by a surface layer only of said sheet being at a substantially lower temperature than the remainder of the thickness of said sheet, and

D. precipitating said constituent through the conjoint action of said thermal gradient and mechanical deformation by pressure rolling to produce, in said surface layer only, a fine particle size uniform dispersion of said constituent and a coarse particulate dispersion of said constituent in said remainder of the thickness of said sheet.

3. A process of producing an aluminum-magnesium sheet having a high thermal conductivity and having magnesium present in the form of at least one precipitated intermetallic magnesium compound uniformly dispersed in fine particle size in a surface layer of said sheet, said process comprising the steps of A. providing a sheet of aluminum-magnesium alloy having magnesium in the form of at least one intermetallic magnesium compound as a soluble constituent of said alloy,

B. heating said sheet substantially uniformly through the thickness thereof thereby dissolving said constituent into solid solution in said alloy,

C. rapidly cooling at least one surface of said sheet thereby establishing a thermal gradient through the thickness of said sheet characterized by a surface layer only of said sheet being at a substantially lower temperature than the remainder of the thickness of said sheet, and

D. pressure rolling said sheet immediately after said surface cooling to precipitate said constituent through the conjoint action of said thermal gradient and mechanical deformation as a fine particle size uniform dispersion in said surface layer only and in a coarse particulate dispersion in said remainder of the thickness of said sheet.

4. A process of producing an aluminum-magnesium sheet having a high thermal conductivity and having magnesium present in the form of at least one precipitated intermetallic magnesium compound uniformly dispersed in fine particle size in a surface layer of said sheet, said process comprising the steps of A. providing a sheet of aluminum-magnesium alloy having magnesium in the form of at least one intermetallic magnesium compound as a soluble constituent of said alloy,

B. heating said sheet substantially uniformly through the thickness thereof to a temperature within the range of 750 to 950 F. thereby dissolving said constituent into solid solution in said alloy,

C. rapidly cooling at least one surface of said sheet to a temperature within the range of 450 F. to 700 F. thereby establishing a thermal gradient through the thickness of said sheet characterized by a surface layer only of said sheet being at a substantially lower temperature than the remainder of the thickness of said sheet, and

D. pressure rolling said sheet immediately after said surface cooling and while said thermal gradient is at least F. to precipitate said constituent through the conjoint action of said thermal gradient and mechanical deformation as a fine particle size uniform dispersion in said surface layer only and in a coarse particulate dispersion in said remainder of the thickness of said sheet.

5. A process as set forth in claim 4 wherein A. said alloy contains magnesium in the form of Mg -Si as a soluble constituent thereof,

B. said heating is within the temperature range of 750 to 850 F., and

C. said surface cooling is within the temperature range of 450 to 550 F.

6. A process as set forth in claim 4 wherein A. said alloy contains magnesium in the form of Mg Si and Mg Al as soluble constituents thereof,

B. said heating is within the temperature range of 850 to 950 F., C. said surface cooling is within the temperature range of 550 to 700 F.

7. As an article of manufacture, an integral aluminummagnesium sheet containing from 0.2 to 3.2% magnesium, balance essentially aluminum, said sheet being characterized by having a gradient of metallurgical properties through the thickness thereof, said gradient comprising at least one surface layer of said sheet having magnesium in the form of at least one precipitated intermetallic magnesium compound uniformly dispersed throughout said layer in fine particle size with the remainder of the thickness of said sheet having said compound dispersed in coarse particle size relative to said fine particle size.

8. An article as set forth in claim 7 wherein said precipitated magnesium compound is Mg Si.

9. An article as set forth in claim 7 wherein said precipitated magnesium compounds are Mg Si and Mg Al 10. An article as set forth in claim 7 wherein said surface layer comprises approximately 10% of the thickness of said sheet.

References Cited by the Examiner UNITED STATES PATENTS 2,177,711 10/1939 Graham 148-32 3,042,555 7/1962 George et a1 148-325 3,164,494 1/1965 English 148-315 DAVID L. RECK, Primary Examiner.

H. F. SAITO, Assistant Examiner.

Claims (1)

1. A PROCESS OF PRODUCING AN ALUMINUM-MAGNESIUM SHEET HAVING A HIGH THERMAL CONDUCTIVITY AND HAVING MAGNESIUM PRESENT IN THE FORM OF AT LEAST ONE PRECIPITATED INTERMETALLIC MAGNESIUM COMPOUND UNFORMLY DISPERSED IN FINE PARTICLE SIZE IN A SURFACE LAYER OF SAID SHEET, SAID PROCESS COMPRISING THE STEPS OF A. PROVIDING A SHEET OF ALUMINUM-MAGNESIUM ALLOY HAVING MAGNESIUM IN THE FORM OF AT LEAST ONE INTERMETALLIC MAGNESIUM COMPOUND AS A SOLUBLE CONSTITUENT OF SAID ALLOY, B. DISSOLVING SAID CONSTITUENT INTO SOLD SOLUTION IN SAID ALLOY BY HEATING SAID SHEET, C. ESTABLISHING A THERMAL GRADIENT THROUGH THE THICKNESS OF SAID SHEET CHARACTERIZED BY DEVELOPING IN SURFACE LAYER ONLY OF SAID SHEET, A TEMPERATURE LOWER THAN THAT DEVELOPED IN THE REMAINDER OF THE THICKNESS OF SAID SHEET, AND D. PRECIPITATING SAID CONSTITUENT THROUGHT THE CONJOINT ACTION OF SAID THERMAL GRADIENT AND MECHANICAL DEFORMATION TO PRODUCE, IN SAID SURFACE LAY ONLY, A FINE PARTICLE SIZE UNIFORM DISPERSION OF SAD CONSTITUENT AND A COARSE PARTICULATE DISPERSION OF SAID CONSTITUENT IN SAID REMAINDER OF THE THICKNESS OF SAID SHEET.