Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C21/00—Alloys based on aluminium
  • C22C21/10—Alloys based on aluminium with zinc as the next major constituent

Definitions

• a specific preferred alloy composition consists essentially of 3.8-4.6% zinc; 1.0-2.0% magnesium; 0.20-0.7% manganese; 0.10-0.25% zirconium; not more than 0.4% iron; not more than 0.25% silicon; other elements, not more than 0.03% each and not more than 0.10% total; balance aluminum.
• This invention relates to the production of aluminumzinc-magnesium alloy articles which are solution heat treated and which may be subjected to substantial cold strain, as by cold working, after the solution heat treatment.
• the invention is directed to procedures for producing aluminum-zinc-magnesium alloy articles which are subjected to substantial cold working following solution heat treatment, and to alloy compositions having particular utility in such procedures, these procedures and compositions affording in the produced articles superior resistance to stress corrosion.
• Aluminum-zinc-magnesium alloys as herein contemplated are aluminum-based alloys wherein the principal alloying elements are zinc and magnesium (the major hardening constituent being a compound of magnesium and zinc), and which have a copper content of not more than a minor fraction of 1%.
• Various alloys of this type are known and have wide utility, for example in sheet, plate, extrusion and other wrought form.
• the aluminumzinc-magnesium alloys are heat treatable; especially as thus treated, such alloys are characterized by advantageously high mechanical strength.
• solution heat treatment workpieces of these alloys are commonly subjected to a so-called solution heat treatment, as well as to subsequent natural and/ or artificial age hardening steps for which the heat treatment is a necessary prerequisite, in order to improve the alloy strength and hardness.
• Conventional solution heat treatments include the successive steps of heating the workpiece to bring it to a temperature high enough to produce complete solid solution of the alloy constituents but too low to produce incipient fusion, soaking the workpiece by maintaining it at such temperature for a predetermined finite time to effect the desired complete solution of constituents, and quenching the workpiece at a relatively rapid rate in a suitable cooling medium, e.g. water.
• An object of the invention is to provide superior resistance to stress corrosion in wrought aluminum-zincmagnesium alloy articles which are subjected successively to solution heat treatment and substantial cold Workmg.
• the invention in one aspect broadly contemplates procedure for producing such articles, comprising in succession the steps of heating an aluminum-zinc-magnesium alloy workpiece to a temperature sufficiently high to elfect substantial solution of alloy constituents therein, cooling the workpiece from the last-mentioned temperature at a rate of between about 0.1 C. and about 20 C. per second at least during the cooling of the workpiece from about 350 C. to about 250 C., and subjecting the cooled workpiece to cold working of more than 5%
• percent cold working means percent cold reduction of the sheet. It will be understood, however, that the invention is not limited either to the treatment of sheet or to cold working by rolling, but embraces treatment of other articles and cold working operations other than rolling, e.g. stretching, bending and twisting.
• the cooling rate (at least between 350 C. and 250 C.) be not more than about 12 C. per second, and indeed the cooling step may very advantageously be performed by exposing the heated workpiece to still air, which effects cooling for most common thicknesses of sheet at a rate of between about 1 and about 6 C. per second.
• the alloy used contain about 0.20 to about 0.7% manganese, and about 0.10 to about 0.25% zirconium, inclusion of these two metals in the stated ranges being found to afford special improvement in stress corrosion resistance of the ultimately produced article.
• alloys having this preferred composition are employed, fully satisfactory stress corrosion resistance is obtained with cooling rates (i.e. between 350 and 250 C.) as high as and indeed even higher than the upper limit of 20 C. per second, whereas alloys lacking the combination of manganese and zirconium inclusions exhibit greatest resistance to stress corrosion when cooled (over the 350- 250 C. range) at rates provided by exposure to still air.
• the alloy used contain not more than 0.05% chromium. Presence of chromium in excess of this amount appears to promote exfoliation of the produced article and also increases the sensitivity of mechanical properties to cooling rate.
• the invention contemplates the provision of an aluminum-zinc-magnesium alloy especially suitable for use in the foregoing procedure and consisting essentially of about 3.8 to 4.6% zinc; about 1.0 to 3.0% magnesium; about 0.20 to 0.7% manganese; about 0.10 to 0.25% zirconium; not more than about 0.40% iron; not more than about 0.25% silicon; other elements, each not more than about 0.03%, total not more than 0.10%; balance aluminum.
• Articles fabricated of this alloy are found to exhibit a markedly superior resistance to stress corrosion when treated by the procedure described above.
• solution heat treated articles fabricated of the above-described alloy may be subjected to cold strain without developing stress the range of temperatures between 350 C. and 250 C.
• the method of the invention may be described as embodied in procedures for treating workpieces constituted of aluminum-zinc-magnesium alloys having the following composition: not more than about 0.35% silicon, not more than about 0.40% iron, not more than about 0.20% copper, not more than about 0.9% manganese, about 0.5 to about 3.0% magnesium, not more than about 0.25% chromium, about 3.6 to about 5.0% zinc, not more than about 0.15% titanium, not more than about 0.30% zircom'um, balance aluminum, with impurities present in amounts of not more than about 0.05 each and 0.15% total;
• An example of a known alloy of the foregoing type is the alloy designated by Aluminum Association No.
• X7004 also sometimes commercially known as 748 alloy
• 748 alloy which has the following composition: Si, not more than 0.25 Fe, not more than 0.40% Cu, not more than 0.20%; Mn, 0.40-0.9%; Mg, 1.02.0%; Cr, not more 4 than 0.25%; Zn, 1.0-4.6%; Ti, not more than 0.15%; impurities, not more than 0.05 each, not more than 0.15 total; balance aluminum.
• a workpiece fabricated of an alloy having the general composition set forth above is subjected to successive steps of solution heat treatment, cooling and cold working.
• the workpiece may be a strip of alloy sheet, although it will be understood that the invention broadly embraces treatment of other types of wrought articles as well.
• the solution heat treatment step is performed by heating the workpiece sufliciently to effect at least substantial solution of the alloy constituents therein, as will be understood by those skilled in the art. Specifically, in this heating step the workpiece is brought to a maximum temperature which is in a range between about 350 C. and about 550 C.
• the workpiece may be placed in or advanced through a suitable furnace for heating to effect such solution of constituents, being brought to a temperature of e.g. 465 C.
• the heating step may be performed incident to other operations on the workpiece, such as extrusion or hot rolling, i.e. the heating of the workpiece for these operations may serve to elfect solution of alloy constituents and thus may constitute the heating step of the present procedure, with out resort to a separate and special heat treatment of the workpiece.
• the heated workpiece is cooled from the solution-heat-treating step at least through the range of temperatures from about 350 C. to about 250 C; at a rate of between about 0.1 C. and about 20 C. per second.
• the cooling rate is at least about 05 C. per second; at lower rates some impairment of mechanical properties in the treated article is observed, but the tensile strength developed with cooling rates as low as 0.1 C. per second is still acceptable.
• This cooling may be carried out in any convenient manner, as with water sprays or jets of air or other gas. In many instances it is preferable that the cooling rate (at least through the range of temperatures above specified) be not more than about 12 C.
• the workpiece after the workpiece has been cooled it is cold worked to any desired extent in excess of 5%.
• the workpiece may be rolled to an intermediate gauge before solution heat treatment, and after cooling following the solution heat treatment, it may be further reduced in thickness in an amount of above 5%. It will be understood, however, that the invention also embraces application of cold strain by working operations other than rolling.
• the invention particularly contemplates procedures wherein the article under treatment is subjected to cold working of at least about after solution heat treatment (e.g.
• the increased resistance to stress corrosion afforded by the present method enables use of the product in many applications for which articles produced by conventional procedures, with conventional quenching after solution heat treatment, are not satisfactory owing to rapid stress corrosion failure.
• the advantages achieved in this regard are believed attributable to the very low quenching rate used, in the combination with solution heat treatment.
• the stress corrosion resistance of articles produced by the present method decreases as the cooling rate after solution heat treatment is increased above the upperlimit of 20 C. per second, although alloys having the preferred composition of the invention (described below) are less sensitive to variations in quenching rate above this limit than are other aluminum-zinc magnesium alloys.
• the solution heat treatment is fully effective to develop desired mechanical properties in the treated workpiece; i.e. no significant differences are found between the tensile and bend properties attained in a workpiece cooled at the low rate of this invention, and those developed in workpieces which are cooled after solution heat treatment by a conventional fast water quench.
• the workpiece may be naturally aged (i.e. stored at room temperature, without application of heat) for any desired period before and/or after the cold working step.
• No artificial aging is necessary prior to cold working in order to develop desired stress corrosion resistance, and it is convenient to perform the cold working operation without intervening artificial aging after the solution heat treatment.
• the workpiece may if desired be artificially aged (e.g. in conventional manner, by heating to and holding at a suitable elevated temperature as will be apparent to those skilled in the art) before and/or after the cold working step.
• the alloy employed consist essentially of about 3.8 to 4.6% zinc, about 1.0 to 2.0% magnesium, about 0.20 to 0.7% manganese, about 0.10 to 0.25% zirconium, not more than about 0.40% iron, not more than about 0.25% silicon, other elements, each not more than about 0.03%, total not more than 0.10%, balance aluminum.
• the alloy having this composition which constitutes a further and particular feature of the invention, exhibits especially high resistance to stress corrosion (when solution heat treated, cooled and cold worked in accordance with the method of the invention) and has a correspondingly reduced sensitivity to quenching rate, i.e. so that very superior stress corrosion resistance is achieved even when the alloy is cooled after solution heat treatment at a rate as high as 20 C. per second, and indeed at rates as high as 80 C. per second or even higher. This result is believed attributable to the inclusion of manganese and zirconium in the preferred composition.
• Absence of chromium from the composition is also advantageous since the presence of greater amounts of chromium (e.g. above 0.05%) tends to promote exfoliation and consequent failure of the alloy article under corrosion-producing conditions while at the same time increasing the sensitivity of the mechanical properties of the alloy to cooling rate after solution heat treatment.
• workpieces fabricated of an alloy having the composition just set forth may be subjected to cold strain (e.g. by cold working) after solution heat treatment without developing significant stress corrosion susceptibility when cooled from solution heat treatment (at least through the range of temperatures from 350 C. to 250 C.) at rates as high as C. per second or even higher. That is to say, the alloy composition of the invention provides increased stress corrosion resistance in solution heat treated and subsequently cold strained articles, and this improvement is enhanced by cooling through the 350- 250 C. range at a rate of not more than about 80 C. per second and preferably not more than about 20 C. per second.
• EXAMPLE I A plurality of aluminum-zinc-magnesium alloy sheets of various gauges were produced by rolling an alloy having the following approximate composition: 4.4% zinc, 1.8% magnesium, 0.71% manganese, 0.26% iron, 0.15% silicon, 0.03% copper, 0.018% titanium, balance aluminum. These sheets were solution heat treated by soaking for one hour at 465 C. One group of the sheets was cooled after solution heat treatment by quenching in cold (10 C.) water, and the remaining group of sheets was cooled from solution heat treating temperature by exposure to still air. The approximate cooling rates for the various sheet thicknesses in the two cooling media used, over the range of temperatures from 450 C. down to 250 C., were as follows:
• each bent strip was exposed to an aqueous solution of 1 normal NaCl+0.2 normal H the solution was monitored daily to maintain the available oxygen at a constant level. Specifically, each strip was alternately exposed to the solution for 16 hours and air dried for 8 hours (on working days but not on weekends), and stress corrosion failure of the strips was determined by daily visual inspection. Strips which survived 100 days of such exposure without stress corrosion failure were considered insensitive to stress corrosion and were removed from test. Other corrosion effects such as pitting and exfoliation become noticeable at this time.
• T3 refers to the temper of samples cold worked after solution heat treatment and natural aging
• T4 designates the temper of samples naturally aged for 35 days but not subjected to cold reduction
• T6 designates the temper of samples artificially 0
• No stress OOZ'HZiSlOII failures in ays Sheets of 0.064 inch gauge were produced by rolling direct cast ingots of each alloy. A plurality of sheets of each alloy were solution heat treated. Twosheets of each alloy were air cooled after solution heat treatment ata rate of about 1.9 C.
• Procedure according to claim 1 wherein the step of cooling the workpiece is performed by exposing the workpiece to air.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Forging (AREA)
• Metal Rolling (AREA)
• Heat Treatment Of Sheet Steel (AREA)
• Shaping Metal By Deep-Drawing, Or The Like (AREA)
• Heat Treatment Of Steel (AREA)

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• 1967
  • 1967-11-17 US US683782A patent/US3580747A/en not_active Expired - Lifetime
• 1968
  • 1968-10-30 GB GB51542/68A patent/GB1244740A/en not_active Expired
  • 1968-11-14 DE DE19681808910 patent/DE1808910B2/de active Pending
  • 1968-11-14 BE BE723888D patent/BE723888A/xx unknown
  • 1968-11-14 FR FR1591792D patent/FR1591792A/fr not_active Expired
  • 1968-11-15 SE SE15523/68A patent/SE342849B/xx unknown
  • 1968-11-15 ES ES360306A patent/ES360306A1/es not_active Expired
  • 1968-11-15 NL NL6816317A patent/NL6816317A/xx unknown
  • 1968-11-15 CH CH1712168A patent/CH515333A/fr not_active IP Right Cessation

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