US2242944A - Thermal treatment of aluminum base alloys - Google Patents

Description

Patented May 29, 1941 THERMAL TREATMENT or ALuMINUM BASE ALLOYS r Edgar H. Dix, Jr., Oakmont, and Joseph A. Nook, Jr., Tarentum, Pa., assignors to Aluminum Company of America, Pittsburgh, Pa a corporation of Pennsylvania.

No Drawing. ApplicationiMay 5, 1939,

Serial No. 271,932 I 4 Claims. (01.1485211") This invention relates to an improvement in the art of heat treating certain aluminum base alloys, and it is especially concerned with improving their resistance to corrosion insofar as this property is affected by thermal treatment.

It is an object of our invention to improve the resistance to corrosion of certain solution heat treated and artificially aged aluminum base alloys. A particular object is to provide a method of improving the resistance to corrosion of precipitation hardened aluminum base alloys containing copper, magnesium and silicon. Still another object is to provide a method of controlling the initial precipitation of dissolved constituents in aluminum base alloys quenched from the solution heat treating temperature.

Certain aluminum base alloys are greatly improved in strength and hardness by subjecting them to the Well known thermal treatments in the neighborhood of 500 C., rapidly cooling to ordinary temperatures, and finally causing some of the dissolved constituents to precipitate by an aging treatment. The resistance to corrosion of alloys treated in this manner is greatly affected by the character of the thermal treatment. In general it has been believed that the more drastic the cooling from the elevated temperature, the better is the resistance to corrosive attack, all other factors remaining constant. Wherever corrosion resistance is important we have strongly advocated quenching in cold water. For instance, although it would be highly desirable to quench thin sheet, for aircraft construction, more slowly as in an air blast, in order to prevent distortion, we find that this practice is not permitted by reliable manufacturers. The resistance to corrosion of heavier sections is often not so important as that of thin sheet and slower quenching is sometimes permitted to minimize distortion. Even with cold water very heavy sections are not cooled as rapidly as lighter sections. The lowered resistance to corrosion sometimes encountered on heavy sections is attributed to this fact.

The precipitation treatment also exerts a pronounced influence upon the resistance to corrosion. It has been observed in general that an alloy containing a fine uniformly dispersed precipitate possesses a better resistance to attack than the same alloy with non-uniformly dis- 7 The lower resistance to corrosion of copper bean ing alloys which must be reheated to temperatures in the neighborhood of 120 to 180 C. after being quenched from the solution temperatures, is frequently ascribed to the size and arrangement of precipitated constituents and inequalitributed larger particles of coalesced precipitate.

ties in the amount of constituent remaining in solution at the grain boundaries and Within the grains. On the other hand alloys that age spontaneously ordinarily have small sized uniformly dispersed particles of precipitate and a better resistance to corrosion. On this account the latter alloys are sometimes preferred where resistance to corrosion is of primary importance. However, the alloys which must be reheated or aged artificially, as the precipitation hardening treatment is called, usually develop a higher strength and hardness which is very desirable for many purposes. It is particularly desirable that such alloys should also have a high degree of resistance to corrosion.

Another form of corrosion to which certain aluminum base alloys are sometimes subjected is that known as stress corrosion. This type of corrosion differs from the ordinary type of attack in that it is influenced by the stress, and it also proceeds at a more rapid rate. It often develops" certain characteristic features in the alloy structure which can be identified by microscopic examination. Solution heat treated and artificially aged alloys of the kind herein described are sometimes subjected to conditions which promote acceleration of corrosion under stress, and hence it becomes important to minimize or eliminate this type of attack as far as possible.

We have now discovered a means of improving the resistance to corrosion of artificially aged aluminum base alloys containing copper, magnesium and silicon as the principal soluble hardening constituents. The improvement is effected by controlling the temperature of the quenching medium in such a manner as to induce an initial precipitation of the dissolved constituents. The alloys so treated are then aged in the usual mam nor to develop the desired strength and hardness. Our method is particularly applicable to aluminum base alloys containing from 3 to 6 per cent copper, 0.5 to 2 per cent silicon, 0.1 to 1 per cent magnesium and 0.1 to 1.5 per cent mananese.

.. perature.

' loys, yet this general practice is not desirable in the case of alloys herein described. In particu lar, artificially aged aluminum base alloys con-- taining from 3 to 6 per cent copper, 0.5 to 2 per cent silicon, 0.1 to 1 per cent magnesium, and V 0.1 to 1.5 per cent manganese do'not attain "their maximum resistance to corrosion when quenched in the manner heretofore employed.

The artificial aging treatment which follows the quenching should be such as to cause a pr'ecipitation within the grains, and thus make the precipitation more nearly uniform through the alloy. In general the aging is to be accomplisedby heating to between 120 and 180 C. for 5 to 24 hours, the shorter time being employed if'the temperature is in the upperportion of the'permissible range. V I V Y Y I The effect of controlling the temperature of the quenching bath in improving the resistance to corrosion of the artificially aged alloy ma be explained by the fact that a relatively uniform precipitation throughout the alloy minimizes, if if it does not eliminate, differences in solution potential between the interior of the grains, the

boundaries, and the precipitated constituent.

percent silicon, 0.5 per cent 1ronand' the balj an'c'e aluminum. All the samples were give'n'a Byway of contrast, if precipitation largely confined to the grain boundaries, and the alloy is exposed to a corroding medium, the attack will occur at the grain boundaries, because of the aforesaid differences in potential.

1 The rate of cooling of anarticle is determined byits mass, the size of the load being quenched, the nature of the quenching medium, its temperature, and perhaps other factors An article of thin section such as a sheet 0.064 inch in thickness may be quenched inhot oil, or alow melting point salt bath having a temperature of 225" 0., whereas an article of heavy section should bequenched in a medium at a lower tem- As the thickness and mass of "the article increase, progressively more drastic quenching media should be employed. For example, to obtain a comparable'efiect in'a .plate 0. inch in thickness, it should be quenched in fused salt bath at about 200? C. In the table below are given various thicknesses of sheets or plates and the fused salt bath temperatures that were used in quenching single pieces of ,these sheets or plates in order'to givethe best resistance to corrosion where the samples were made 1 Thickness Temperature of'sh'e'et' ofiused salt Inches g'ra'ile article.

Because of the wide variety in size and shape of articles and the number to be treated at one time, only an approximate rule can be given as to the temperature of the quenching bath and the temperatures given above may be altered, depending On the size of the load, that is to say the number of pieces quenched at one time. The precise conditions requisite to producing the maximum resistance to corrosion in a particular case can be determined by tests. In general, less drastic quenches should be used than would have heretofore been employed for the same sized The article need not be left in the quenching medium longer than sufiicient to at- -tain the temperature of the bath, say about 5 minutes. bath and cooled to room temperature either in It should then be removed from the air or'byim'mersing in water. The latter practice has been found to be especially beneficial where'articles of heavy cross section are being treated.- The cooling efiected by immersing in constituents in solution. The immersion in Water also serves to dissolve any coating of salt left entire article by the quenching bath.

'The eifectof the quenching bath temperature upont'he resistanceto stressless'co'rrosionof an aluminum base 'alloy containing copper, magnesiumandsilic'on, may be seen in the following examples. For the purpose of these tests samples of "s'he'et10.064 inch in thickness were used. The alloy from which'the sheet wasinade had a nominal'composition off4.4 per cent 'c'o' p-per, 0.75 per cent manganese, 0.35 .per cent magnesium, 0.8

. solution heat treat'ment 'a't'49'8" C. for 15 minutes and then were divided into four "groups for being cooled to 'room temperature in the air after remov'algfrom the quenching bath. All of the quenched samples were then aged at 171 C. for lo hiours. The samples were subjected'to a 48 hour accelerated corrosiontest consisting of a1- ternately immersing in and removing them from a'n'aqueous'solution containing 5, per cent NaCl byweight'and 0.3 per cent H202 by volume. The mechanical propertiesof the corroded 'anduncorroded specimens we're compared, and "the losses "in tensile strength and elongation expressed in "terms of per cent were calculated.

rosion, and that the most drastic quench does not. produce the bestresistance 'to attack .as

expressed in terms of per cent .aregiven in the following table.

might be ordinarily "expected. The 19 percent Table I loss in strength and 66 per cent loss in elongationrepresents the results which will ordinarily 5 Oyiging] 'lg g Period Pelee/105595. be'obtained following thermal treatments as pre-v 221 g gg g h oftest .viously known whereas. the 4 per cent loss in minches g hrs strength and 39 per cent loss in. elongation rep- S r g resents the desired improvementfito. be obtained 7 by the present invention. Although we generlo 813%: mt -eat: 2 2 2 5:. 3 ally prefer to use a quenching medium maing- I}; tained at a temperature. between 190 and,225 0:250 225 72 -1a -54. C., yet an improvement in the corrosion resist, g- 3; ance of the alloys becomes evident under certain p conditions when the quenfhmg medium is at Fo'rpurposes of comparison we include also in temperatute as low as 100 as Seen Table'II the results obtained from a companion the foregomg table set of samples; similar in all respects except that The controlled quenching of aluminum base althey were unstressed loys of the kind herein described .also has a J V marked beneficial efiect upon the resistance to Table II stress corrosion. As mentioned hereinabove, the emp .Pemnmsses solution heat treated and artificially aged .alumii j Quenching P0. of Period num base alloys containing from 3 to 6 per cent olsheet medium" f fg Tensile F1011 copper, 0.5 to 2 per cent silicon and 0.1 to 1 per mmches medium strength tiofi cent magnesium are sometimes subject to this p form of corrosive attack, and henceit is import- 0.064 Water..." 20 24 -23 -72 ant to eliminate, if possible, even the chance of gzggi g; :2: such an attack, The improvement inresistance 225 72 r -11 -55 to stress corrosion that can be efiected by con- :g trolled quenching is illustrated in the: following .50 200 72 s -15 64 test results. For the purpose of this test sheets were used which were of different thicknesses and It Should be noted that Owing the xp made from the same alloy as that mentioned in and time involved in this method o es these the preceding corrosion test. All of the sheets results are obtained on v dual specim s and were first heat treated at about 500 C. A-portion are not averages o a number of Specimens as is of the sheets 0.064 inch in thicknesswas quenched generally desirable in corrosion testing, hence from 500 C. in water at room temperature in ac. small difierences canv not be considered too sigcordance with the usual practice of cooling'the Ilificant. It y o e Observed not o y that heat treated alloy as rapidly as possible, The ree Specimens q e n e fu ed salt bath mainder f t Sheets was quenched i fused 40 and corroded for 72 hours sustained lower losses salt bath at the temperatures indicated in-the than the Water quenched s p c r ded for table below. The quenched samples were withonly 24 hours, but that in most cases no acceleradrawn from the salt bath and immediately imtion of corrosion is produced by a stress equivalent mersed in water at room temperature. All of' the t fr'p r nt f t y ld. st i the sh et samples were then aged at 160 C; for 18 hours. quenchedin fused salt. In the case of sheets thicker' than 0.064 inch, It is true that there is no acceleration of the specimens were cut from the sheetsand machined rate of corrosion by stress shown in the case of to a thickness of 0.064 inch in order toprovide the water quenched sheet. This however is the specimens of the same size. These specimens result of the very low resistance to corrosion of were then subjected to'an accelerated corrosion this material which causes the specimens to rapidtest consisting of immersing them in an aqueous ly yield under the load until it touches the botsolution of 5 per cent byweight of sodium chloride tom of the vessel containing the corrosive soluand 0.3 per cent by volume of hydrogen peroxide. tion and in this way substantially relieves the The specimens were supported at both ends and stress. v a load producing a stress equivalent to '75 per cent Under less severe conditions of corrosion, as for of the yield strength of the alloy appliedin the instance exposure to an industrial atmosphere, central portion of the specimens thus stressing the difierential between the stressed and unthem as simple beams. They were exposed to the stressed specimens quenched in cold water heattack for 24 to 72 hour periods as indicated in comes more evident as shown from Table III. the table below. The tensile strength and elonga-i The specimens in this case were all 0.064 inch in tion of the corroded specimens were then deterthickness'and were exposedto the atmosphere for mined and compared with the properties prior to a period of 6 months. The stressed specimens corrosion. The losses resulting from corrosion were stressed at 75 per cent of their yield strength.

Table III 7 emp ggfg Stressed samples fil g (ft h l i Aging treatment Percent Percent @meim mine iiit isehis lissstrength strength Water.... 20 l8hts. at C... -7 -56 -13 so Oil 225 do +1 --l6 +1 '-13 Watch... 20 10 hrs. at 17190... -8 59 -16 -83 011...," 2 25 -----de.; +2 -11. .0 i -17 It .may be again zobserved zthat iwith. specimens quenched in cold water and unstressed, :the ief-i fect of corrosion is quiteevident, and that under the mildly corrosive .e'fiect of an industrial atmosphere, coupled with an artificially :applied stress, the losses inducedby corrosion have'been accelerated to a marked degree. It may-also1be observed that with the material quenched in oil at 225 0., the losses sufiered in the unstressed 'condition are negligible as compared with the losses suffered by the water quenched material; furthermore this same material, vunder an equivalent stress and subjected for thesame period of time to an industrial atmosphere suffered; substantially .no greater losses than-in the unstressed condition. ,This clearly indicatesthe improvement-that can-be obtained by-the .prac:

tice of our invention.

Any quenching medium is suitable for our purposes which will afford the desired control of the quenching rate. Allowance must of course be made for the difference in. cooling capacity of the various media Withcorresponding adjustments in bath temperature. Where a normally drastic quenching-medium is' used,and

equalized between the body and boundary of the 40,

grains. Ingeneral, a longer time-atalow "tom perature is preferred to a shorter "time at-a higher temperature to obtain the best resistance to corrosion. However, the composition oifthe alloy must also'be considered,sinceprecipita tion is more easily induced in 'some"cases.than in others. The range of aging temperatures that produces the desired efiect varies "between about 120 and 180 C.an'd the timeof exposure usually varies between 5 and .24lhours depending upon the temperature .at which the aging .-is done. To produce equivalent strengthandhardness in two lots of the same material, ga longer period of exposure is needed at a low temperature than-is required if a higher/temperature is used. ..i V

AlLaluminum base alloys which -'-are susceptible to improvement in physical properties by solution heattreatment and aging are not amenable-to improvement in resistance to corrosion in--accordance with the method described' here inabove. our invention is thus'limite'd'to ar-tifiicially aged aluminum base alloys'containing copper as the principal alloying constituent together with lesser amounts of magnesium and silicon. Under someconditions it is desirable to increase the strength of the alloy, and this may be :done by adding thereto "from-0.1--to-4 per cent of zinc. 'The addition of zinc may also serve toenhance other properties of the base composition. When 'zinc'is added it maybe zd'esirab'le .to use up to 2 per cent zinc in the alloy. A preferred range of composition of .anaalloy containing zinc'is as follows:

4-: messa es copper, .011 to 12 mar :cent magnesium, v-0.5-.to :2 percent silicon, sand llzfirto -3 percent .zinc. The

alloys "containing zinc may also contain .from" 0.05;to. 1.5 per cent of-one or more of a'group of .high melting point elements consisting .01

chromium, titanium, cobalt, nickel, molybdenum; tungsten, zinconium, manganese, and, .iron.

Where :two or v more et these elements are in- Ixzin'c, 0i4- aper-cent;iron, -and 0.8' per cent manganese, :the balance being aluminum.

This application is a continuation-in-part' of ourcopen'ding application, Serial vNo. 139,940,

filed Api-il 30, Z1937.

tain aembodiments'fthereof, we claim:

sistance to.- corrosion :in a solution heat treated j and artificially a ed aluminum base alloy containing :up ;'to 6 :per .cent :copper as the chief added-alloyingzconstituent, 0.1fto4 per cent zinc, and magnesium and silicon in smaller amount than copper, said Y method comprising quenching the:-;solution.;heat Ktreated alloy in the quenching medium maintained atrsuchartemperature above C. aslto induce an initial precipitation of dissolved constituents, removing said alloy from the quenching mediumand thereafter artificially aging-the alloy- --:2. .In .t-he I art 1 of heat treating aluminum base alloys, :the :methodof producing maximum resistance :-t;o. corrosion in a solution heat treated and-.artificially agerl.aluminum base alloy containing. from .3 .to-6=gper cent copper, 0.1 to 4. per

cent :zinc, 10:1 to 2. per cent magnesium, and 0.5110 2 ,per :centsilicon, said method comprising quenching tthe solution heat treated alloy in a quenching medium maintained at such a-temperature above; 100 .C. as tov induce an initial precipitation of dissolved constituents, removing the ;alloy kiromzthe guenching medium, cooling to,.iroom.-temperature, and thereafter artificially aging' thealloy.

,3. In rthe artcf ..heat treating aluminum base alloys, :the method of producing maximum resistance ,to corrosion in a solution .heat treated and artificially aged aluminum base alloy containingrfromfi :to firper cent-copper,0.1 to 4-per cent lzinc, .021 to'2 per cent magnesium, and 05 to 2- 'per-cent silicon, said method comprising quenching the solution heat treated alloy-in -a quenching "medium maintained at such axtemperature between "190 and 225" C. as to induce an initialfprecipitation ofdissolved constituents, removing-the *alloy"fr,om the quenching medium, cooling; to room temperature and thereafter re-'- heating the alloy to between and C.'i'or 2 to 24 hours to complete the aging.

4. In the art of heat treating aluminum base added alloying constituent, 0.1 to 4 per cent vzinc, and magnesium and silicon in smaller :amount than copper, and from 0.05 to 1.5 per "content at least one of the group of elements composed of chromium, titanium, cobalt, nickel,

F-Havingithusxlescribed our invention and certhe quenching medium, cooling to room temperature, and thereafter artificially aging the alloy.

EDGAR H. DIX, JR. JOSEPH A. NOCK, JR.