US2477503A - Processes of treating magnesiumbase castings and improved magnesium-base alloys produced thereby - Google Patents

Description

Patented July 26, 1949 enocassjas or. 'raEATiNo nasi: casamos AND. IMraovlin.lyiatimn-4v SIUMvBasc ALLQXS econoom incitent Edward .L Vargto, Warrensville Heights, Qhio, as` signor to The Wellman Bronzeand; Aiunrninum` Gemeente. Moreland, ohio, annotations ct NoDrawing. Application Deeem SeralNo. 65,7441" seriales-a.

2e @was 01- leans).

The invention relates. to. alloys, and: particularly to alloys having a magnesium` base.

ofA treating magnesium-base` castings to produce the improved alloys. 5 stated, little. difficulty has` been, encountered in This application` on said inventionis a continumeetingthaplfopenties specied.f.or.ANM-36, the ation-in-part application of my pending applica.- diifverenceof,-2000zp.s.i.inltensilestrength which is tion Serial No. 586,024,1iled. March B1, 194,5, now specified for the aged condition. over the` heatedA abandoned, on Magnesium-.base alloys and proccondition hasi not been maintainable. On the essof treating thesame, and my pending applica- 1o contrary, the. ane-ragey tensile strength found` in a tion SerialI No. 18,858, ligledr April 3, 1948., now` large numberoi foundry-produced composition abandoned, on Processes oftreating magnesium- A magnesium` alloys tested was some 500`p.s.i. base castings and improved magnesium-.baseA allower-for.the.aged, condition than for the heated loys produced thereby. condition onlyi This average for the aged" con- Primarily, the invention comprises improveeition was about 39,000; whereas, the expected ments in aging magnesium-.base castings by difierentialshould have produced an average ofl continuous heating at stepped temperatures about:41',500lp.s.i, whereby the ultimate` strength, elongation, yield It. has been found that testA bars cast under strength, and hardness are improved. close laboratorycontrol yielda tensile strength of Insofar as I am informed, known processes of approximately k1,000p.s.i1. andan elongation of heat-treating magnesium-base alloy castings substantiallye`161% forithe; solution-heat-treated consists in giving them a solution heat treatcondition, while. the tensile strength is approxiinent immediately followed by an air quenchrnatelye42,-500lp.s.i. andthe elongation 7% after ing, and then aging them by heating at about the-bar hadfbeen agedfonsixteen hours at 350 F. 350 F. for about sixteen 16) hours, and then re- 25 However, lessrieidly controlled castings of test moving them from the heat treatment furnace bars in. commercial practice yield: a tensile anclletting themair-.cool at room temperature. strength of approximately-40,000 psi. for the it has longbeen well recognizedin` commercial heet leetedf metal; andf39l000 D-S-- for the .metal practice that, although a difference of 2000 p.s.i. agedfor Sixteen-hours'ot 35-0F1, With- IleSlUeC'LVe in tensile strength between the solution-heatelongatorls Off approximately 14o11d 5%j `This treated and the solution-heat-treated and aged disceparloy` between tensile StrenthS vof the conditions of the commercial magnesium casting laboratory specimenanclf the commercial pracalloys such as A263 and AZ92 is indicated by the tice Specimen is. explained byttljie respective difspecications to be feasible, there is a persistent ferenGeS-inelongaton. ThetWoheoii treatments, inability to maintain this difference. However, the solution heattreatmentA and the aging the increase in yield strength and the decrease heat. treatment, elle Chefreioteljzedbywell defined in elongation do correspond in commercial pracstress-strain. curves, and-- they cast quality 0f a tice to those ofthe specifications. The commer- @est bei@ does not affect the shape of the curve cial magnesium castingalloys A263 and A292 butti 011157detellflflllesly its terminal point. This are, respectively, those in which aluminum conio elmrlalpoint has coordinateswhich represent minutes about 6 and 9%, Zincabout 3 and 2%, theelongationandthetensile strength. Therewith `from 0.15 to 0.20% maganese, and traces fore, the elongationof 16%in the laboratory bars of other foreign elements such as silicon, iron, oo1\l^eS-l&0IlC-S-` t0 elJ-Si- 0f` 413000, and the Cometc., the remainder of the casting alloy being marcial elongation of,z 14%- produces-a` ns-iof magnesium. Manganese is added to al1 com- 45 401,000, Whichu is on1y`1000 poiloWer than the mereial magnesium alloys to im'ppove the cor.. laboratory value. the laboratory.- bar, in the rosion resistance and is, therefore, disregarded aged Condition, theelongatiorlofe't; determines herein, a tensile strength .of-:42,500 p sci., and for the com- Because in commercial practice the specified inertialv bell', the elongation of 5% Produces a properties are relatively low, there has been entensile. strength of.3'9,000.p.s.i., which is3500 p.s.i. counteredlittle diiiiculty nmeeting the properlower than the` laboratory: Value. Therefore, Cleties `specified for AN-M-B, in either the heatfectstin or` a decrease in the.- casting quality retreated or the.heat.-treated and `aged condition. duces the tensilestrengthof the agedcondition AN-M-e is an Army-Navy aeronautical. speciconsiderably..more.,V than', that of=li the solutionfication, one type of which known as composi.- heat-.treated condition! However, the allowance The; in". vention also includes improvements. in processes` tion, fAt cor-nainsA4 substantially, 6%. aluminum,

zinc, and; 0.20% manganese, with traces of.

other foreign. elements; It-` is. the. specication heneinbeiore; referred. to. as AZ63. Although, as

made by the specifications in commercial practice for an inferior casting is considerably larger for the heat-treated condition than for the condition both heat-treated and aged. It is evident that these specincations should set aV minimum tensile Y Vstrength of the aged condition at an equal or smaller value than that of the solution-heattreated condition. The stepped aging treatment herein presented eliminates this necessity, and also produces improved characteristics of the resultant casting alloys. It also'eiects a greatY saving in total heat-treatment time. i

I have discovered that the inability to maintainj the 200@ p. s. i. difference in tensile strength bei; tween the two conditions of the alloy is causedj by a prematureltermination in the stress-strainY curve resulting from casting defects. 'IY-:his premature termination results in a greater loss in' the tensile strength for the heat treated and aged condition casting than for the casting which hasVY been heat treated only. Therefore, the invention disclosed herein-comprises an fimproved aging treatment for which the curve indicating the stress-strain is not prematurely'terminat'ed, such improved aging treatment being effective to improve the characteristics of the resultant alloy such as its tensile strength, yield strength, elonga tion, and hardness. Y

Occasional'diiculties with temperature control when producing the` composition A magnesium alloy resulted ir certain heats being subjected to a stepped aging suchV as at 250 F. for three hoursand at 35 F. for thirteen hours. The product o-.these heats hada strength value greater than that resulting from the normal heating at a uniform temperature of 350 F. over the same totalgime. Therefore, the whole production was tentatively subjected to a stepped aging treatment. It was found thatthe tensile strength was decidedly' higher than either the solution-heat-treated product or the normally aged product. Comparison of test bars subjected to solution heat treatment vonly showed-identical properties; whereas, comparison of the aged test bars showed the increased strength values. Therefore, the benefitswere attributable to the differential aging treatment. f-

Primarily, the improvements consist in a stepped aging treatment havingdifferent temperature and time factors. For instance, it has been ascertained by laboratory tests that an aging treatment which utilizes various times of heating at two temperatures with a total timerequal to that of the regular commercial practice at one temperature results inga combination of tensile strength, per cent elongation, and yield strength which is superior to that obtained in regular commercial practice. Also, it has been ascertained that an aging treatment which utilizes two periods of heating time, employing a higher temperature.

in the second period of the agingtreatment,results in properties which are equal to those obtained @in commercial practice, even when the total time of the improved?Y stepped aging treatment isY only about:25% of the total time required by commer-f cial practice at one temperature.

A part of the improvements-presented herein consists in aging the magnesium-base castings, after the solution heat treatment including the air quenching, by heating them for about `three (3) hours at 250 F., :1 -15, and then continuing the heating for about thirteen (13). hours at 350 F., +30-10. The castings are then removed irom the heat treatment furnace and air cooled at and to room temperature. This im-A Y then at a temperature of from about 350 F. to

about 400 F., during which heating the aging time at the lower temperature does not exceed one-half theltotal aging time.W Preferably, the total aging time is aboutfour hours, during which the heating jat the lower temperature is from about one to about one and one-half hours, and the heating at the higher temperature is Yfrom about two and one-half toabout three hours.

All of the aging treatments comprehended by thelpart of the improvements last-described produce an improved 'combination of properties in the'resultant alloys over those of similar alloys heated for the whole period at about 350 F. for about sixteen (15) hours, the particular properties improved being hereinafter designated for certain of the 2stage heating procedures which will be hereinafter described.

Reference to the results which have been obtained by a 2-stage aging'treatment of about three (3) hours at 250F., m15, and then acontinued heating for about thirteen (13) hours at 350 F., +3010, for which results there have been treated a magnesium-base casting containing 6% aluminum,l3% zinc, about 0.15% inanganese, and the balance magnesium; and also a casting containing 9% aluminum, 21% zinc, 015% manganese, and the balance magnesium, shows that the ultimate strength by the improved 2-stage procedure had been increased by about 2000 p. s. i. over the ultimate strength produced in test-pieces from the same casting operation by an aging thereof for sixteen hours at 350 F.; and that the hardness, bythe Brinell table, had been increased about four V(4) hardness numbers, and the percentage of elongation in 2 gauge length test-pieces had been increased about 1.5.

The temperatureV at which the iirst phase of the aging operation is Yeiiected, over a heating period of about three hours, is quite critical, viz., 250 F., as stated,.but I have ascertained that a range of temperature within i;15 of 250'F. produces improved ultimate strength,`hardness, and elongation characteristics. The temperature at which the second'phase of the aging operation is effected,

Y 017er a period of about thirteen (13) hours, viz.,

350 F., islalso quite critical, having, however, a permissible range withinV Which the improved results are obtained, this range being +30 and -l0 from 350 i Y In MgAlZn castings, as also in-other magnesium-base castings, there will be traces of foreign elements other than those which-"enter into the compounds which are found concentrated in the alloys and which precipitate during the aging yoperation in pearlitic structures, :such asc-manganese, silicon, iron, nickel, etc.1 Inasmuch as these other elements are not present in the pearlitic structures precipitated by the aging operation and,Y thereforefhave no effect in the production of the improved' characteristics, according to my theory of the rproduction of the latter, no further reference to such other elements is made herein.

The results of the second part ofmy improve- Y ments which involve a shorter period of total aging time, viz., from about four to about eight hours at initial temperatures of from about 225 F. to about 275 F., and second heating temperatures of from about 350 F. to 400 F., were obtained by aging, and noting the improved results, the Army-Navy Aeronautical Specification AN- M-BS, known as composition A. This is the composition hereinbefore 'referred to as containing substantially aluminum, 3% zinc, and 0.15% manganese, with traces of other foreign elements, the balance being magnesium. This specificais referred to as A253 and its maximum and minimum limits are: aluminum `,5.3 `to 6.7%, zinc 2.5 `to 3.5%, manganese 0.15% minimum, and silicon 0.30% maximum, the balance being magnesium.

This specication covers the most common heattreated magnesium casting alloy. However, the

improvements are abplicableto and the improved characteristics obtained `from the various magnesium casting alloys containing from 6 to 9% aluminum, 3 to '2% zinc, 0.15 to 0.20% manganese, with traces of silicon, iroL etc., the balance ybeing magnesium.

Various closely-controlled laboratory tests of the conventional agin-gtreatment have been conducted for vdifferent total times, and different temperatures. Conventional aging has been tested for various periods upto 500 hours and at temperatures from 250?. to 400 F. For each variation of the aging treatment .many specimens of the product have been tested for yield strength, tensile strength, `elongation (per 2 inches), and Brineli hardness. of the results withresults of similar tests using the stepped `aging treatment produced a better combination of properties 4by the stepped aging treatment using different temperatures than by the conventional aging treatment over the whole .-g

period of time at a :single temperature. it was also ascertained that for the given aging temperatures and total time there `results `from the stepped aging an almost constant or slightly increased yield strength and tensile strength and an elongation from 1 to 2% above the values obtained by aging for the entire period at the higher temperature. Thus, stepped aging temperatures of 250 F. and 350 F. over the usual total commercial time of sixteen hours has been tried; also, a total aging time of eight hours at 250 F. and 400 F.; and a total agingtime of four hours at 250 F. and 400 F. The first aging time of 250 F. was tried in each series from zero hours to the total hours. For each series of tests, a better combinationof properties than was obtainable by a single `temperature aging was obtained by stepped aging in which a fraction of the aging time is consumed at 250 F. However, it was further ascertained that a reduction in yield strength, whichcannot be tolerated, made it necessary `that the lower temperature aging time, i. e., the aging timeat which lsubstantially 250 F. is utilized, `whilea .substantial portion of the entire aging period, should not exceed onehalf Of the total time. Also, itis knownfthat the aged condition improves with lower `temperatures but a second-stage temperatureimaterially below 350 F. is impracticable in standard practice because of `the great length .of time necessary `for satisfactory aging. Apracticable feasible aging time should not exceed twenty-four'hours.

It has been further ascertained-.that `the lowest `combination'of temperatures utilized, i. Ve., 250

and `350" F., for atOtnl.lagingperiodof nsix- For every test, a comparison teen hours, yields the best combination of Aprop-- erties except that a total aging time of fourhours with the use of temperatures of 250 F. and 400 F. yields an equal improvement or possibly slightly larger. For this stepped aging use of 250 F. and 400 F., a rst aging treatment of 250 F. for one and one-half hours, and a second aging treatment of 400 F. for two and one-half hours .is indicated, resulting, y'as compared with conventional aging at 350 F. for sixteen hours, in a tensile strength only slightly lower, in markedly increased elongation, and in an insignificant decrease (readily tolerated) in yield strength. However, an aging Iperiod of eight hours, using 250 F. and 400 F., yields properties only slightly improved over those obtainable by lan aging 'heat of 400 lor the entire eight hours, indicating that -overaging is a deleterious factor, markedly more so than underaging. It is further yascertained that an increase of the time required for aging to obtain a maximum tensile strength produces a considerablyless favorable combination of strength and elongation than does underaging, Furthermore, the complete aging time decreases with increasing temperature but the properties simultaneously decrease.

Application to commercial practice of the stepped aging treatment above-mentioned has given even better results than those obtained by laboratory tests of the improved stepped aging treatment.

A large number of bars of commercial production have been tested over a 4-hour aging period consisting of one hour at 250 F, and three hours at 400 F. This stepped aging treatment produced properties distinctly superior to commercial procedure of sixteen hours Aat 350 F. The tensile strength was equivalent, and the :yield strength yand elongation were higher, despite the reduction of in total aging time.

The results indicate that otherpromising combinations of temperatures for stepped Iaging are 250 F. and 375 F., 275 F. and 375 F., and 225 F. and 400 F., utilized for a total aging time of about `four hours, in which the aging time at the lower temperature does not exceed one-half the total aging time. Results obtained indicate that the best combination of properties is obtained 4by a total aging time of approximately four hours, one and one-half hours at about 250 F., and twoand one-half hours at about 400 F.

My theory of the cause of .the improved characteristics obtained by the improved 2-stage-aging treatment above-described is as follows:

It is known that, in the cast condition, in a magnesium-base alloy casting containing aluininum and zinc as major alloying elements, an aluminum, magnesium, and Zinc compound iis concentrated in scattered areas at the grain boundaries, and that, when the casting is given the solution heat treatment, this scattered AlXMgyZnZ compound passes into solid solution. When this solution is aged by the known processes above-mentioned, viz., heating at sixteen hours at 350 F., the `consequent pearlitic precipitation or" the aluminummagnesium-Zinc compound is a coarse formation produced by a comparatively rapid action. During the preliminary aging period, namely, about three (3) hours heating at 250 Fil", the aluminum-magnesium-zinc com'- po'und comparatively slowly precipitate'sin aime pearlitic structure, and during the later treatment for about thirteen (13) lhours at 350 vnaam,.-10

the precipitation is accelerated but continues on the basis of the fine precipitation which takes place during the earlier aging period at the lower temperature. Thus the characteristics of the final product are bettered to substantially the respective amounts hereinbefore mentioned. The improvements are noted in the other specific procedures described.

A microscopic examination of the test bars has been made. Solution-heat-treated magnesiuin alloy consists of uniform solid solution crystals with some heterogeneous phases (MgzSiMn.) both at the grain boundaries and within the grain. The structure of all commercially usable aged condition metal consists of '75 to 50% of apparently retained solid solution crystals and 25 to 50% precipitated areas, similar in appearance to pearlite in steel. Such grain structure is in agreement with X-ray investigation which indicates that during aging an increasing portion of the solid solution decomposes (almost) to its equilibrium condition while the remainder is subject to a very slow precipitation process. The completely precipitated material apparently has only minor commercial significance because it possesses both lower tensile strength and elongation than the intermediate stages, although possibly associated with a higher yield strength. The microstructure of the partially (and completely) precipitated conditions when aged at the same temperature throughout the entire aging period is distinctly different for different aging temperatures. The lamellae in the precipitated areas are thinner' and closer together the lower the aging temperature. And at lower aging temperatures the aging time appears to be of little iniiuence in this regard. At the higher aging temperatures, comparatively long periods of aging time result in some breaking up and coagulation of the lamellae.

The microstructure of the bars subjected to the improved stepped aging also exhibits distinctly nner lamellae than that of metal aged during the entire aging period at the higher of the two temperatures used in the stepped aging. To this distinctly finer lamellar precipitate is attributed the superior physical properties of the product obtained by stepped aging. stepped aging results in a structure and .in properties very similar to those obtained over a considerably longer 'period 'by an aging at a single temperature intermediate the two temperatures of the two stages of stepped aging. Hence, a major benefit derived from stepped aging is a reduction in aging time.

What I claim is:

l. The process of improving the ultimate strength, hardness, and elongation characteristics of a magnesium-base alloy, consisting in solution-heat-treating a casting thereof, containing a proportion of aluminum of from about 6% to about 9%, and a proportion of Zinc in percentage of from about one-half to about one-nfth the percentage of aluminum, substantially all of the remainder being magnesium, the solution heat treatment including a quenching; then aging the alloy, by heat treatment thereof at 250 F. and continued heat treatment thereof at 350 2. The process of improving the ultimate strength, hardness, and elongation characteristics `of a magnesium-base alloy, consisting in solutionheat-treating a casting thereof, containing a proportion or aluminum of from about 6% toabout 9%, and a proportion of zinc in percentage of from about one-half to about one-fth the percentage of a1uminum,substantially all or the remainder lbeing magnesium, the solution heat treatment including a quenching; then aging the alloy, by heat treatment thereof for approximately three hours at 250 F. and continued heat reatment thereof for approximately thirteen hours at 350 F.

3. An aging treatment to improve the ultimate strength, hardness, and elongation characteristics of a magnesium-base alloy, a casting of whichhas been solution-heat-treated and air-quenched, said casting having from 6 to 9% aluminum and 2 to 3% zinc, substantially all of the remainder being magnesium, consisting in rst heating such alloy at from 235 F. to 265 F., and then continuing the heat treatment at from 340 F. to 380 F.

4. An aging treatment to improve the ultimate strength, hardness, and elongation characteristics of a magnesium-base alloy, a casting oi which has been solution-heat-treated and airquenched, said casting having from 6 to 9% aluminum and 2 to 3 zinc, substantially 0.15 manganese, the remainder being magnesium, consisting in first heating such alloy for approximately three hours at from 235 F. to 265 F., and then continuing the heat treatment for approximately thirteen hours' at from 340 F. to 380 5. improvements in treating a magnesium-base alloy containing from 6 to 9% aluminum,` from 2 to 3% Zinc, substantially all of the remainder being magnesium, consisting in solution-heattreating a casting of such alloy, including an air-quench; then aging the alloy by heat treatment thereof for approximately three hours at from 235 F. to 265 F.; continuing the heat treat ment for approximately thirteen hours at from 340 F. to 330 F.; and then air-cooling at and to room temperature.

6. Improvements in treating a magnesiumbase alloy containing from 6 to 9% aluminum, from 2 to 3% zinc, substantially all of the remainder being magnesium, consisting in solution-heat-treating a casting of such alloy, including quenching thereof; then aging the alloy by heating it at from 235 F. to 265 F. to elect comparatively slow precipitation of an aluminum-magnesium-zinc compound having a fine pearlitic structure, and then continuing the heat treatment at a higher temperature to accelerate precipitation.

7. A magnesium-base alloy produced by a 2- stage aging treatment consisting in first heating a solution-heat-treated and quenched casting containing 6 to 9% aluminum and 2 to 3% zinc, substantially all of the remainder being magnesium, at from 235 F. to 265 F. for a substantial portion oi the entire aging period, and then continuing the heat treatment at from 340 F. to 380 F., the alloy being characterized by improved ultimate strength, hardness, and elongation, as compared with alloys aged for substantially the same period of total aging time at the higher temperature of the 2-stage aging.

8. A magnesium-base alloy produced by a 2- stage aging treatment consisting in first heating a solution-heat-treated and quenched casting containing substantially 6% aluminum and substantially 3% zinc, substantially all of the remainder being magnesium, at from 235 F. to 265 and then continuing the heat treatment at from 340 F. to 380 F., the alloy being charackterized by. precipitated aluminum-magnesiumzinc compound having a fine pearlitic structure imparting thereto improved ultimate strength, hardness, and elongation.

fganese;.thesremainderibing'magnesium, at from AV235 to.;265iFOraaiSubStantial A.portion 'of `the 'entire:agingiperiodeandizthen ,continuing the heat treatment at from 340 F. to 3380F., the alloy being `characterized cby .improved ultimate ..A:str.en.:th,` hardness, iandifelongation, dasV compared "with rralloysiagel uzfor 1 substantially vthe same perriodiof ltotal:againgttime;` .atthe higher temperatureroff thef2estage aging.

.11. .n magnesium-:base alloy u producedby `a 2- stage -agin'g-treatmentnconsisting in first heatling v a: slolution-heatetreated and i quenched casti ingr containingfsubstantiallyi -alumintmfi, `sub- `:stantiallyfZ zinc, andlsubstantially 0.15% man- ;ganesathe remainderzbeingfmagnesium, at fromv 235 F. to 265 F. for a substantial portion of fthe .1 entire-aging period, and fthen continuing the heat treatment atffromi'liilbli to 380? the -alicy `.being characterized 'by improved `ultimate strength; hardness, f1 and elongation, as compared `Withalloys.iagedfortsubstantially the same period of l total aging iti-mo fat fthe Ahigher temperature ofitheT2-stage--aging- 12.1Improvements' in itreating a magnesiumbase alloy-'containingfiromito 9%` aluminum, `from"2 to 31% 4tiin'c,-substsultiallyfall `of theremaindenbeing magnesium,consisting in solution- 'heat-treating fa' casting-of such alloy, including quenching-thereof;ftheniaging the-alloy by heatinglit iatrabout`250iF-ito e'iiectpornparativelyslow precipitation #of an aluminum-magnesium-zinc compoun'dh' ing 1a'peailiti-c' structure, and then ncontiniiin-g'g the; heat"V treatment 'at about 350 F. to accelerate Mprecipitation fthe heatitreatment at about 250 F. being 'f'or a substantial portion yof theAi entiI-'ef heat-treating period.

` lf3. AAfinanganeserbase:alloyJ produced `by a 2- stage agingtreatmentrconsisting in heating a .eolution-heatiti-eated'andY quenched casting con- "tainingf to'-9 %faluininum` and i2* to 3 zinc, subetantially' allffof'- the remainder Abeing magnesium, foverejtotal heatingrperiodoffromfour to eight hours.wrst at from 225 F. to 275"'F. for a subrtantialT portioniofthe Aentire heating period but not exceeding fone-half ;of:the1"total time, and then continuing the heat treatment for the balance of the heating period at from 340 F. to 400 the alloy being characterized by precipitated aluminum-magnesium-zinc compound having a ne pearlitic structure imparting thereto an improved combination of properties over those of similar alloys heated for a substantially longer total heating period at the temperature at which the second of the above-mentioned heat treatments is effected.

14. An aging treatment to improve the ultimate strength, hardness, and elongation characteristics of a magnesium-base alloy, a casting of which has been solution-heat-treated.and airquenchedsaid cast-ing.ltiavngiromhV to 9% aluminum and 2 to 3% .-zinc,substantially 0.15% manganese,.the remainderbeing magnesium, consisting in rst heatingsuchalloy for-approximately three hours at about 250? F.,.andthen continuing the heat treatment forapproximately thirteen `hours at about 350. F.

l5. An improvement in agingcommercial magnesium castingA alloys .comprised ci from 5.3 `to 0.7% aluminum, from..2.5 to 3.5% zinc, aminimum of 0.15% manganese, and a .maximum of 0.30% silicon, the balance being magnesium, consisting in heating the `solution-heat-treated vcondition thereofiromfaboutene to about one and one-half hours at from225F..to275 F., and` then stepping-up the `heating `temperature to from about 375 F. to about 400F. and continuing the heating at thestepped-uptemperaturefor from about two and. one-half lhours tovabout three hours.

16. An improvementin aging commercial magnesium casting alloys comprised of'from y5.3to 6.7% aluminum, fromi2.i5to 3.5% zinc,-a minimum of 0.15% manganese, andra maximum of 0.30% silicon,` the balance being. magnesium, consisting in continuouslyiheating the solution-heattreated condition thereof at two stepped temperatures of about 250:F.-and. 400.F., respectively, for a total periodv ofabout.four.-hours,.of which the initial heating atV about 250.F.,-is from about one to about one and one-half hours.

17. An improvementin aging `commercial magnesium casting'alloysccomprised-of from 5.3 to 6.7% aluminum, from f2.5.to 3.5% zinc, a minimum of 0.15%fmanganese,-and a-maximum of 0.30% silicon,.the balance being magnesium, conssting in continuously. heatingrthe solution-heattreated condition thereof at twostepped temperatures of about 250.F.and 400'F.,-respectively, for a total period of about four hours, ofwhich the initial heating at.250 F. is about one hour.

18. A magnesium-base alloy produced by Van aging treatment 4lconsistingin` continuously heating a solution-heat-treated casting containing from 6 to 9% a1uminum,.from2 to 3% zinc, about 0.15% manganese,` the balance-being magnesium, at two stepped temperatures offabout 250 F. and from about 340F: toabout -380:F., respectively, in which the initial lower heating'tempe-rature at about 250 F. is utilized` everasubstantial portion of the entire heating period but notexceeding one-half of the totaltimegthe falloy being `characterized by an improved combination of` properties over those of similar ffalloys heated for the Whole period at thehghertemperature of the stepped temperatures.

19. A magnesium-basera'lloy produced by an aging treatment consistngintfirst heating a solution-heat-:treated:casting :containing from 5.3 to 6.7% 'aluminumgfrom 2.5:.to 3.51% zinc, ay minimum `ci 0.15% s.manganese,` andea i maximum of 0.30% silicon;:the'balance'beingxmagnesium for from one to one and one-half hours at atemperature of from about 225 F. to 275 F. and then stepping-up the heating temperature to from 375 to 400 F. and continuing the heating at the stepped-up temperature for from tWo and onehalf to three hours, the alloy being characterized by a structure and combination of properties comparable to those of similar alloys aged by standard commercial practice, viz., for periods materially longer than four hours and at a single temperature intermediate 225 F. and 400 F.

20. A magnesium-base alloy produced by an aging treatmentV consisting in continuously heating Va Vsolution-heat-treated casting containing from 5.3 to 6.7 aluminum, from 2.5 to 3i5% zinc, a minimum of 0.15% manganese, and a maximum of 0.30% silicon, the balance being magnesium, for about four Vhours total time at two stepped temperatures orf; about 250 F. and 400 Fg respectively, in which: the lower heating temperature is utilized over a period of about one and one-half hours, the alloy'being characterized by markedly increased elongation, ascompared with that of a similar alloy aged by a single conventional temperature heating at abc-ut 350 F. over a period of sixteen hours.

21. A process of aging commercial magnesium casting alloys consisting in heating the solutionheat-treated condition'of a casting thereoiV containing a proportion ot aluminum of from about 6% to about 9%, and a proportion of zinc of from about 3%: to 2%, substantially all of the remainder being;V magnesium, at stepped-up temperatures, viz., rst of from about 235 F. to about 265 F., and then of from about 340 F. to about 380F., the first-mentioned heating in the temperature range of from about 235 F. to about 265 F. being utilized over a substantial proportion of the entire heating period but not exceeding one-half the total aging time, the aged alloys being characterized byan improved combination of properties over those of similar alloys heated for thewhole period at a single'temperature within the range of Vfrom about Y235 F. to about 380 F. if i 22. A process ofaging commercial magnesium casting alloys consisting in heating the solutionheat-treated condition of a casting thereof containing a proportion of aluminum of from about 6% to about 9%, and a proportion of zinc of from about 3% to 2%, substantially all of the remainder being magnesium, at stepped-up temperatures, viz. rst ofabout 250 F., and then of from about 350 F. to about 400 F., the total aging Vtime being about four-hours, and the first-mentioned heating of about 2507F. being from about 25% to about 371/2 of the total aging time.

23. A process of aging commercial magnesium casting alloysfconsisting in heating the solutionheat-treated condition of a casting thereof containing from 5.3% to` 6.7% aluminum, from 2.5% to 3.5% zinc, a minimum of .15% manganese, and a maximum of 0.30% silicon, substantially all of the remainder being magnesium, at stepped-up temperatures;viz., first ofr about 250 F., and then of from about-350 F. to about 400 FL, the total agingftime being about four hours, and the firstmentioned heating of about 250 F. being from about 25% to about 37%, of the total aging time.

24. A process of aging commercial magnesium casting alloys consisting in heating the solutionheat-treated conditionof a casting thereof containing a` proportion of aluminum of from about 6% to about 9%, and a proportion of zinc of from about 3% to 2%, substantially all of the remainder being magnesium, at stepped-up temperatures, Viz., first of about 250 F.; and then of about 375 F., the total aging time being about four hours, and theV first-mentioned heating of about 250F. being not to exceed two hours.

25. A magnesium-base alloy containing from about 6% to about 9% aluminum, and from about 3% to 2% zinc, substantially all of the remainder being magnesium, produced by an aging treatment consisting in heating a solution-heattreated casting thereof for from about four hours to about eight hours total time at steppedup temperatures, viz., rst of from about 225 F. to about 275 F., and then of from about 350 F. to about 400 F., in which the first-mentioned heating inthe temperature range of from about 225 F. to about 275 F. is utilized over a period not exceeding one-half of the total aging time, the alloy Vbeing characterized by an improved combination of properties over those of similar alloys heated for the whole period at a single temperature Within the range of from about 225 F.'to about 400 F. Y V

26. A magnesium-base alloy containing from about 6% to about 9% aluminum, andffrom about 3% to 2% zinc, substantially all of the remainder being magnesium, produced by an aging treatment consisting in heating a solutionheat-treated casting thereof at stepped-up temperatures, Viz., rst of about 250 and then of from about 350 F. to about 400 F., the total aging time being about four hours, and the iirstmentioned heating of about 250 F. being from about 25% to about 371/2% of the total aging time.

27. A magnesium-base alloy containing from 5.3% to 6.7% aluminum, from 2.5% to 3.5% zinc, a minimum oi .15% manganese, and a maxi-mum of 0.30% silicon, substantially all of the remainder being magnesium,r produced by an aging treatment consisting in heating a solutionheat-treated casting thereof at stepped-up temperatureaviz., rst of about 250 F., and then of about 375 F., the total aging time being about four hours, and the rst-mentioned heating of about 250 F. being for about one hour. t,

n28. A magnesium-base alloy containingfrom 5.3% to 6.7% aluminum, from 2.5% to:3.5% zinc, a minimum of .15% manganese, and a'maximum of 0.30% silicon, substantially 'all of the remainder being magnesium, produced by an aging treatment consisting inheating a solutionheat-treated casting thereof, at stepped-up temperatures, viz., irst of about 250Y F., and then of about 400 F., the total aging time being about four hours, and the first-.mentioned heating of about 250 F. being for aboutone hour, the alloy being characterized by increased yield strength and elongation with no substantial loss in tensile strength as compared with similar alloys aged at a single temperature of about 350 F. for about sixteen hours.

EDWARD J. VARGO.

No references cited.

Certificate of Correction Patent No. 2,477,508 July 26, 1949 EDWARD J. VARGO It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as fellows:

Co1umn"35\,1ine 13, for the syllables and hyphen speciiearead specification; 9, line 25, for againg read aging; line 54, for the Words manganese-base read magnesium-base;

and that the said Letters Patent should be read With these corrections therein that the same may conform to the record of the ease in the Patent Oice.

Signed and sealed this 20th day of December, A. D, 1949.

THOMAS F. MURPHY,

Assistant Uommssz'oner of Patents.