Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
  • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
  • C22F1/04—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of aluminium or alloys based thereon
  • C22F1/053—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of aluminium or alloys based thereon of alloys with zinc as the next major constituent
• • 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

• the aluminum forging industry has for some time made wide use of an alloy containing 5.1 to 6.1% zinc, 2.1 to 2.9% magnesium, 1.2 to 2% copper, 0.18 to 0.35% chromium with the following limits on impurities: 0.3% Mn, 0.2% Ti, 0.4% Si and 0.5% Fe.
• This alloy carries the Aluminum Association designation 7075 and is noted for its comparatively high strength and other useful properties among which is very good resistance to stress corrosion cracking when specially aged for such.
• the special aging treatment referred to is that set out in US Pat. No. 3,198,676 which specifies a two-step aging treatment to improve resistance to stress corrosion cracking while lowering the strength properties somewhat. While forgings of this alloy and other 7000 type alloys are highly useful in the aircraft and space industries because of their strength, their usefulness would be further enhanced by a significant strength improvement which is the purpose of this invention.
• the aluminum alloy is cast, preferably continuously cast, into an ingot which preferably has a maximum cross section dimension of l7 inches.
• the aluminum alloy according to a preferred embodiment consists essentially of 5.6 to 7% zinc, 2.4 to 2.75% magnesium, .1 .4' to 1.9% copper, 0.18 to 0.35% chromium, with the balance being essentially aluminum together with other elements or impurities with the following maxima applying thereto: 0.1% Mn, 0.1% Ti, 0.12% Si and 0.15% Fe.
• This alloy is preferred since it consistently develops outstanding properties when produced as forgings under the improved practice.
• the invention contemplates other alloys since it is believed that many of the benefits of the present invention can be achieved with these other alloys. Accordingly, the invention broadly contemplates a composition consisting essentially of 4.8 to 8.5% zinc, 1.7 to 3.5% magnesium, 0.8 'to 2.5% copper, the balance being aluminum and incidental elements and impurities together with at least one grain controlling element selected from the group consisting of 0.1 to 0.75% manganese, 0.05 to 0.4% chromium, 0.05 to 0.3% zirconium, 0.05 to 0.3% vanadium and 0.05 to 0.3% molybdenum. Silicon should be limited to 0.12% and iron to 0.15% as impurities.
• the invention contemplates up to 3% copper although as copper exceeds 2.5% the extent of the improved properties diminishes.
• the gas content of the molten metal within the mold is limited to a maximum of 0.15 ml. per 100 grams of melt. Bubbling dry nitrogen through the melt is helpful in this respect as are other known procedures which are applied to the extent required to reduce the melt gas content to the prescribed level. It is desired that the resulting ingot be substantially free from porosity as revealed by a dye-penetrant examination of the ingot cross section and this is the purpose of carefully controlling the molten metal gas content.
• the dye-penetrant examination referred to is of the type described in Military specification MIL-l-6866B under Inspection Type 11, Method A.
• the chill rate through the solidification temperature zone is maintained at a level of at least 2F per second and preferably at least 10F per second. Maintaining this chill rate through the solidification temperature zone, the zone between the liquidus and solidus temperature, usually requires a most drastic cooling of the metal.
• the aforementioned chill rates result in the ingot having a desirable dendritic cell size of 0.0030 inch maximum. The faster chill rates are preferred and may reduce the dendritic cell size still further to a maximum of 0.0020 or, better yet, 0.0010 inch.
• the ingot preferably does not have a cross sectional dimension greater than 17 inches. Still better results are effected where the ingot cross section does not exceed 9 inches in any dimension. For instance, a circular ingot having a diameter of 9 inches is very useful.
• the ingot is homogenized by heating to a temperature of at least 860F. It is preferred that it be heated to 860 to 880F for 24 or 48 hours or more to properly homogenize its internal structure. While 24 to 48 hours or more represents an increase over the time normally employed, 8 hours, it is preferred as a precaution to assure the desired results.
• the ingot after any necessary scalping or other pro cedures is worked into forging stock, preferably by extrusion, at a temperature of at least 750F, preferably at least 790F, which temperature is maintained throughout the extrusion process. That is, the ingot is not merely heated to a desired minimum temperature prior to working at an uncontrolled and often lower temperature, but rather care is taken to be absolutely sure that the minimum temperature throughout the working cycle is 750 or preferably 790F. This temperature level is somewhat above those normally maintained, about 600F in the case of the extrusion process, but is essential to achieve the desired improvement. Where extrusion is employed it is preferred that the portion of the extrusion first emerging from the extrusion press be removed and discarded.
• the discarded portion of the extrusion in some cases preferably should amount to at least 20% of its total length so that the remaining portion, constituting a maximum of of the total extrusion length, is employed as the forging stock in the preferred practice.
• extrusion is a preferred working operation to produce the forging stock the invention contemplates other operations as well.
• a forging or rolling procedure can be employed especially where a flatter or more planar forged product is desired, although some decrease in properties may result over the preferred practice employing extrusion. Compensation for such reduced properties may be achieved for forgings in the stress corrosion resistant temper by adjustments to aging conditions. For example, when using hand forged stock less aging is required to develop the required resistance to stress corrosion cracking than when using extruded forging stock.
• deformation ratio must be at least 8:1 which, in the case of extruding, rolling and other simplecases, means that the cross sectional area of the metal normal. to the principal direction of working before working is at least 8 times that of the worked stock measured normal to its major axis.
• a deformation ratio of 8:1 means that the length of the dimension or axis which is lengthened or extended the most during the working operation is at least 8 times that dimension or axis prior to working. It becomes immediately apparent that an extrusion ratio of at least 8:.1 agrees 'with a deformation ratio of at least 8:1 and the latter is considered as encompassing the former.
• a scalped ingot approximately 20by 20 inches in cross section and about 40 inches long is stood upright in a forging press and squeezed to reduce its 40 inch dimension.
• the resulting biscuit is about 20 inches high and about 30 inches across.
• This biscuit is then drawn out in a hand forging operation to produce a slab about 160 inches in length, 30 inches in width and a little over 3 inches in thickness.
• the 160 inch length in comparison with the original starting dimension of 20 inches represents a deformation ratio of 8: 1. It should be noted that the decrease in thickness here was from 40 inches down to a little over 3 inches but this does not represent the deformation ratio which is based on the dimension which has been increased to the greatest extent.
• a scalped ingot approximately 20 by 20 by 40 inches is rolled along the 40 inch dimension to provide a plate 20 by 2 by 400 inches.
• the deformation ratio is :1 based on the ratio of the final length, 400 inches, versus the starting length of 40 inches.
• the deformation ratio does correspond to a reduction ratio but, it will be appreciated, such is not always the case and the deformation ratio aspect is a moreaccurate concept in delineating the practices contemplated .by the invention in a broader or more comprehensive sense where operations such as forging are contemplated to prepare the forging stock.
• the working operation employed to produce the forging stock must be carried out under conditions which maintain a metal temperature of at least 750F, preferably at least 790F, substantially throughout the entire working operation.
• the wrought forging stock Prior to forging, the wrought forging stock is heated to a temperature of at least 900F, preferably 925 to 970F or even more, for a period of at least 2 hours, preferably at least 4 or 6 or 8 hours.
• This temperature is well above the solidus temperature of the high strength 7000 type alloys, typically 890F for 7075 alloy, and accordingly care must be taken to avoid incipient melting.
• a maximum heating rate of 150F per hour from 870F to the hold temperature provides satisfac- -tory results. It is worth mentioning thatno high temperature soak is normally employed with 7075 and other 7000 typev alloy forging stock.
• the forging stock which can be segmented in desired lengths, is forged at temperatures of at least 750F, preferably at least 790F.
• the temperature of the forging stock and the forging tools are all maintained so as to provide actual metal working temperatures at the required level.
• 7075 and similar alloys are forged under conditions where some metal temperature may drop well below 600F during the metal working stages of the forging operations. This must be avoided in practicing the invention and care must be taken to positively assure maintenance of the required minimum metal working temperature.
• the basic characteristics of the forging operation i.e., pressing between opposing surfaces, are known and need not be elaborated upon here.
• the invention is especially suited to the production of die forgings for two reasons.
• the metal when the metal is confined by a die cavity there is less tendency for the metal to crack during working at the relatively high temperatures required by the improved process than when making simple shapes designated hand forgings between flat dies which do not confine the metal on all surfaces.
• metal working temperatures are more easily controlled during the single stroke forging operations used for most die forgings than during the multiple stroke operations used in making hand forgings.
• the invention is also applicable to hand forgings in which case the soaking step at 925F or more can be employed either on the starting material when using wrought stock or after forging when forging directly from ingot.
• the soaking step at 925F or more can be employed either on the starting material when using wrought stock or after forging when forging directly from ingot.
• properties will be reduced under the preferred practice where extrusion or the like is employed to produce the forging stock prior to soaking.
• hand forgings is meant compressing between opposing surfaces, normallysubstantially flat surfaces, which do not confine metal movement transverse to the direction of forging.
• metal working temperatures below 790F, down to 750F are permissible in both working the material into forging stock and in the forging operation itself, a major portion, preferably the predominant portion, of the work should be performed at metal temperatures of 790F or more.
• a billet can be introduced to an extrusion press at a temperature of 750F, or perhaps even a little less. If the extrusion is effected rapidly enough, sufficient energy is imparted to the metal to very rapidly raise its temperature significantly over 790F.
• the forgings may be solution heat treated, preferably by heating to a temperature of at least 900F and preferably at'least 925F or 940F or more up to 970 or 980F, for a time sufficient to place substantially all the soluble alloy constituents in solid solution. A time of 4 hours, preferably 6 or 8 hours, is usually adequate. This heat treatment temperature is considerably above the 880F level normally employed with 7075 alloy forgings but is deemed necessary in practicing the invention to achieve the best results.
• the forgings are quenched, preferably in a fluid media maintained at a temperature of less than F which is somewhat below the level of to F normally used for high strength 7000 alloy forgings.
• the improved method contemplates two separate thermal exposures of sustained duration, at least two hours and preferably at least 4 or 6 or 8 hours, at a temperature of at least 900F, preferably at least 925F, up to 950F or even 970F or more.
• the exposures referred to are the soaking of the wrought forging stock after the 8:1 deformation working operation and the solution heat treatment.
• the use of these two separate extended thermal exposures is a preferred practice to help assure the best possible properties. However to some extent one exposure, especially the first, can be traded off against the other.
• the soaking period of the wrought forging stock before the final forging operation can be reduced, even to below 2 hours, or still further, even eliminated, provided the solution heat treatment is of sufficient temperature and duration.
• the invention contemplates at least one sustained or extended exposure to a temperature of at least 900F, but preferably at least 925, or better 940F or more.
• a sustained exposure is meant one of substantial duration, suitably at least 2 hours and preferably at least 4 or 6 or at least 8 hours.
• the forgings are artificially aged to develop the desired strength and other properties. Where strength is the prime consideration, the forgings are aged to a T6 type temper.
• One preferred practice within the invention achieves the T6 temper by heating to a temperature within the range of 215 to 250F for a minimum period of 50 hours and preferably for a minimum period of 70 hours, for instance, a period of 70 to 75 hours. This is a substantial departure. from the aging treatment normally employed for the T6 type temper, 24 hours at Y 250F, but is desired to achieve good results.
• the forgings may be treated according to the procedure of US. Pat. No. 3,198,676 in order to achieve very high resistance to stress corrosion cracking with some sacrifice in strength.
• the forgings after quenching are subjected to a first aging treatment at a temperature within the range of 215 to 250F for a minimum period of 6 hours followed by a second aging treatment manner.
• Table l compares the tensile properties of the improved high strength forgings (H. S.) with those of ordinary 7075 alloy forgings.
• H. S. high strength forgings
• an ingot was cast in accordance with the improvement and the ingot was homogenized at a temperature of about 900F.
• the ingot was scalped and cut to provide extrusion stock which was extruded at an extrusion ratio of about 10:] while maintaining a metal temperature of over 800F.
• the extrusion was then soaked for about 6 hours at 950F and then forged at just over 800F.
• the forgings were solution heat treated at approximately 925F, quenched and artificially aged to the T6 type temper.
• the standard 7075 forgings were produced according to standard forging practices. lncluded are tensile strength (T. 5.), yield strength (Y. S.) and percent elongation in 2 inches for specimens taken both in the longitudinal direction (the direction of maximum metal flow and longest grain dimension) and for specimens taken in a direction transverse to the longitudinal direction.
• the comparison includes both the higher strength (T6 type) temper and the highly stress corrosion cracking resistant (T73 type) temper.
• the zinc content for the improved forgings described in Table l was between 5.6 and 6% which is a preferred range from the standpoint of ductility in the T6 type temper.
• the composition of the improved forgings was otherwise also in accordance with the preferred alloy set forth in the early part of this description.
• the figures in Table I need little elaboration.
• the strength improvements for the most part substantially exceed 10% particularly in the longitudinal direction.
• the forgings in this temper exhibit an electrical conductivity of 38 to 42% of the International Annealed Copper Standard (IACS) which is a characteristic indication of the T73 type conditions.
• IACS International Annealed Copper Standard
• the above times at 350F are illustrative in that the optimum time can vary from one forged shape to another and should be determined for a particular shape either empirically or from experience with similar shapes. This time can vary from 6 to ll hours for die forgings and from 3 a to 7 hours for hand forgings.
• Forgings produced in accordance with the improved method exhibit marked strength improvements over ordinary 7075 forgings. Furthermore, this improvement is realized in a highly repeatable and consistent firmed that the improved process will develop the properties shown in the table for the H. S. forgings. Statistical analysis of test data showed confidence that 99% of test values actually exceed the values in Table l for H. S. forgings.
• an alloy containing about 5.9% Zn, 2.5% Mg, 1.6% Cu, 0.12% Zr, balance aluminum was formed into die forgings according to two different procedures.
• the first procedure embodying the present invention included casting the ingot in accordance with the improvement to provide extrusion stock, which was homogenized, extruded, soaked and forged in accordance with the improvement.
• the second included standard practices. Table ll below compares the minimum tensile properties of the forgings produced in accordance with the present improvement vs. those pro prised by standard methods. Minimum tensile properties are shown since such offers one critical aspect of comparison useful to commercial suppliers and purchasers of forgings in that the guaranteed properties are associated with the minimum rather than the normal or average properties. Hence, a comparison of minimum properties is of very substantial importance in weighing the merit of a fabrication procedure.
• the first procedure embodying the present invention included casting the ingot in accordance with the improvement to provide extrusion stock, which was homogenized, extruded, soaked and forged in accordance with
• table-below compares the longitudinal properties of the two forgings.
• an ingot having a composition consisting essentially of 4.8 to 8.5% zinc, L7 to 3.5% magnesium, 0.8 to 2.5% copper, at least one grain controlling element selected from the group consisting of 0.1 to 0.75% manganese, 0.05 to 0.4% chromium, 0.05 to 0.3% zirconium, 0.05 to 0.3% vanadium, 0.05 to 0.3% molybdenum, the balance being substantially aluminum, said ingot being cast while maintaining in the molten metal phase within the casting mold a maximum gas content of 0.15 ml/l00 g of melt to achieve substantial freedom from porosity, the metal solidification being controlled such that the chill rate throughout the solidification temperature zone is a minimum of 2F per second,
• casting-an ingot having a composition consisting essentially of 5.6 to 7% zinc, 2.4 to 2.75% magnesium, 1.4 to 1.9% copper, 0.18 to 0.35% chromium, the balance being aluminum with the following maxima on impurities: 0.1% manganese, 0.1% titanium, 0.12% silicon and 0.15% iron, said casting being accomplished while maintaining in the molten metal phase within the mold a maximum gas content of 0.15 ml/100 g of melt, the metal solidification being controlled such that the chill rate through the solidification temperature zone is a minimum of 2F per second, to produce an ingot exhibiting substantial freedom from any porosity as revealed by a dye-penetrant examination of its cross section,
• said forging is artificially aged by heating to a temperature of 215 to 250F and holding at said temperature for a minimum time of 6 hours and thereafter heating to a temperature within they range of 340 to 360F and holding at said temperature for a duration which imparts the effects ofa 7 A to 9 k hour exposure at a temperature of exactly 350F so as to impart to said forging an electrical conductivity of-38 to 42% lACS, said forging being characterized by high resistance to stress corrosion cracking and by a minimum strength level of 76,000 psi tensile and 66,000 psi yield strength and a minimum elongation of 7% in the longitudinal direction.
• balance being substantially aluminum, said working including at least some forging,

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Cited By (31)

Citations (1)

• 1972
  • 1972-10-24 US US00300153A patent/US3791876A/en not_active Expired - Lifetime
• 1973
  • 1973-03-21 IT IT48938/73A patent/IT979937B/it active

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