US3333990A - Aluminum base alloy forgings - Google Patents

Description

United States Patent 3,333,990 ALUMINUM BASE ALLOY FORGINGS Melvin H. Brown, Leechburg, and Bernard W. Lifka,

New Kcnsington, Pa., assignors to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania N0 Drawing. Filed Feb. 5, 1965, Ser. No. 430,765

1 Claim. (Cl. 148-325) ABSTRACT OF THE DISCLOSURE Forged members of an aluminum base alloy containing 3 to 6% copper, 0.8 to 3% magnesium and 0.3 to 1% manganese normally exhibit marked susceptibility to stress corrosion cracking where the stress is applied across a short transverse section. Such forgings can be rendered substantially immune to this stress corrosion effect if the magnesium content is related to the copper and manganese contents in accordance with the following relation:

Mg min.=0.32 Cu where Mn does not exceed 0.5%, Mg min.=0.2+0.32 Cu-0.4 Mn where Mn is greater than 0.5%.

This invention relates to aluminum base alloy forged members having improved resistance to stress corrosion.

For some time aluminum base alloy members containing copper, magnesium and manganese have found considerable acceptance for various structural purposes. One such alloy known in the art contains nominally 4.5% copper, 1.5% magnesium and 0.6% manganese and carries the Aluminum Association designation of 2024 alloy. In the past this alloy has mainly been used in plate form which exhibits outstanding tear resistance and toughness, a very good strength to weight ratio and good resistance to general and stress corrosion especially in thin sections which are not stressed in the short transverse direction. Recently it has been determined that this type alloy has considerable potential in forged members in that such members exhibit, in general, the same highly desirable general mechanical properties as exhibited by the plate form. One further advantage in providing these forged members is that they are often used in conjunction with plate members in fabricating a structure and it is often desirable that all the members in a given structure have the same, or at least highly similar, alloy composition. However, forged members of the described type alloy tend to exhibit a marked susceptibility to stress corrosion type failures when the stress is applied in a short transverse direction. Results of accelerated alternate im'rnersion tests conducted in 3.5% aqueous sodium chloride solution revealed stress corrosion failures at stress levels as low as 35% of the yield stress and lower and that such could occur in as short a time as three days. These results are considered to be a reliable indication of the performance to be expected of these forged members in that they exhibit a degree of susceptibility to stress corrosion which would make their use highly objectionable for many applications. As explained in more detail hereinafter, almost all forgings exhibit some short transverse directional properties and hence present this problem.

It has been discovered that certain highly critical composition limits over and above those currently recognized in the art will impart substantial immunity to short transverse stress corrosion in forged members of the described alloy type, such that in accelerated alternate immersion stress corrosion tests, at stress levels of 50% and 75% of the yield strength the forgings have exhibited substantially no failures for periods of at least 30 days. This is considered to be a reliable indication of substantial im- "ice munity to stress corrosion in these members under ordinary service conditions.

Accordingly it is the primary object of the invention to provide forged members of the alloy'type described which possess substantial immunity to short transverse stress corrosion.

Other objects will, in part, be obvious and will, in part, appear hereinafter.

In referring to forgings, the invention contemplates any forged member including both die and hand forgings. For example, an ingot or other body of suitable size may be formed into an intricate shape by die forging wherein a body cast in a special shape or even a flat or round, etc., body is subjected to one or more compressions between a series of blocker and finish dies which gradually approach the final configuration. Another example is a member formed by progressively compressing an ingot between two relatively fiat die faces in a forging press. The products of such operations are generally referred to as hand forgings in the art and include a variety of standard and special shapes. Standard shapes include multi-faced rounds, rectangles, biscuits and other simple shapes Well known in the forging practice. By suitable manipulation of the member within the forging press or hammer more complex hand forging shapes may be formed such as stepped, wedged, I, multiple raised face and various other useful configurations. The forged member may be subsequently rolled, extruded, spun, bent, sawed, punched, machined, or otherwise processed as desired to facilitate its ultimate or end configuration and use and reference to forged members is intended to include such members subjected to additional fabrication steps. Of course, these operations often precede thermal treatment if desired. For instance, slab type hand forgings are often hot rolled into plate and then solution heat treated, quenched and artificially aged. Also, a hand forged member may, if desired, be used as stock for a die forging operation.

As indicated at the outset of this description, the susceptibility to stress corrosion observed in forgings of the described type alloy is encountered in the short transverse direction. When referring to the short transverse direction with respect to forgings, such is intended to mean a direction as revealed by the grain structure and not necessarily the shortest dimension of the member. In an elongated fiat slab type of forging, this direction most often coincides with the thickness, as is the case with plate or sheet, and it is therefore easily recognized. The short transverse direction can vary widely when the forged shape deviates from this straight forward configuration as where the cross section of the forging is relatively symmetrical or Where the forging has a relatively complex shape. Further, the short transverse direction in one portion of a forging can vary considerably from that in another portion as described in more detail hereinafter. However, the short transverse direction can be readily determined by macro examination of the grains and by testing various properties including mechanical and stress corrosion properties, and hence can be ascertained for a given portion of a forging by those skilled in the forging art.

An interesting situation arises with a hand forging of relatively symmetrical cross section, say 4 inches square, which may be viewed at first glance as having no short transverse direction, at least if such is predicted upon an analogy to plate. However, when the grain structure is examined, it reveals that while the extent of the short transverse structure is not as pronounced as that observed in highly worked fiat members, the grains do exhibit a generally short transverse type structure the extent of which is dependent on the amount of work imparted to the member. Also, the grain structure generally lacks any long transverse characteristic and certainly has a longitudinal characteristic. In such a case, for purposes of this description, the body is considered to exhibit substantially short transverse grain structure in any orientation transverse to its axis although to a lesser degree than that observed in fiat shapes. This is verified by macro examination and by measuring mechanical properties and stress corrosion effects in various elongated forgings having relatively symmetrical cross sections. These test indicate that cros sectional pecimens taken at various orientations all exhibit properties closely approximating those generally associated with the short transverse direction in flat members.

Even relatively flat slab type forgings sometimes defy complete analogy to plate regarding short transverse grain orientation. As is generally known in the art, a fiat slab having a cross section of say 4 inches by 6 inches can, by appropriate forging sequence, exhibit short transverse grain structure across either the 4 inch or 6 inch dimension or both. Again, these effects can be verified by macro examination and tests measuring mechanical and stress corrosion properties. Further, a fiat forging will often exhibit a pronounced short transverse grain structure across its thickness and also, across its width, a partially short transverse characteristic in its grain structure in lieu of the expected pronounced long tranverse structure. Again, this effect can be verified by macro examination together with mechanical and stress corrosion tests.

A further complication arises in the case of a more completely worked and shaped forging which can exhibit varying short transverse directions at different portions thereof. While this effect is generally insignificant in simple hand forging shapes, it can be very pronounced in more complex hand forged shapes and especially in die forgings where metal is often caused to move in different directions and at varying rates thereby giving rise to short transverse grain structures of varying extent and direction throughout the forging. Basically the short transverse grain structure will prevail transversely to a region of high metal movement and will generally coincide with the region where the grains become elongated and are compressed as they fiow under pressure. For instance, a bar is made into a valve body by die forging between two contoured die surfaces. When the bar is compressed between the opposing dies, some of the metal is extruded or caused to flow as flash into the gutters disposed at the parting line of the dies and perhaps into a gutter exit gate. This metal movement or flow orients the grains such that a short transverse structure prevails in a direction across this fiash line which coincides with the direction in which the grains were compressed as they moved through the flash, or gutter, area. Again, this effect is verified by macro examination and by mechanical and stress corrosion tests where the structure generally exhibits severe susceptibility to stress corrosion cracking. If the valve body has a pronounced neck or other protrusions, similar zones of short transverse grain structure, sometimes of varying degree and direction, will also be observed. While the alloy type described is frequently forged into larger structural shapes, the valve body illustration is set forth to amplify the general description concerning short transverse orientation in more complex forged shapes. One additional factor influencing metal movement is the shape of the forging stock prior to forging. Of course, differently shaped stock used to form a given forged shape will flow in varying directions and to varying degrees. Hence, different grain patterns in like forged shapes are often traced back to the forging cycle as affected by the stock employed.

It is this short transverse grain structure, almost always observed in forge-d members, which exhibits the impaired resistance to stress corrosion thereby rendering forged members of a described type alloy unsatisfactory for severe service requirements. Hence, the invention will benefit any member which exhibits an internal structure having a substantially short transverse grain configuration as the result of a forging operation. By a substantially short transverse grain configuration is meant that this characteristic is of sufficient extent as to exhibit the susceptibility to stress corrosion associated with short transverse grain structures produced by forging. If significant metal working operations other than forging are performed in fabricating the member, its internal structure will be influenced by both the forging and the other operations and the invention applies to such in that the internal structure is effected substantially by forging.

From a composition standpoint the alloy used in making forged members in accordance with this invention consist essentially, on a weight basis, of aluminum, 3 to 5% copper, 1 to 2% magnesium and 0.3 to 1% manganese together with incidental impurities. In a broader sense, forged members made of an alloy consisting essentially of aluminum, 3 to 6% copper, 0.8 to 3% magnesium and 0.3 to 1% manganese are benefited by the practice of the invention. Generally speaking the impurity limits associated with the described type alloy preferably apply to the practice of the invention. Thus the following maxi-mum impurity limits are generally fol lowed: silicon 0.5%, iron 0.5%, chromium 0.1%, and Zinc 0.25%. A preferred composition is aluminum, 4.15 to 4.60% copper, 1.45 to 1.65% magnesium, 0.5 to 0.65% manganese, together with the following maximum limits on impurities: silicon, 0.15%, iron 0.25%, chromium 0.10%, zinc 0.20%, and nickel 0.05%. To these compositions there may be added up to 0.05% titanium and 0.002% boron for grain refining purposes. A further composition limit, over and above that practiced in the prior art, which is required by the practice of the invention is a highly critical relation between the principal alloying constituents, copper and magnesium. Broadly speaking the minimum magnesium content is equal to 0.32 times the copper content except that where the manganese content exceeds 0.5% this minimum may be reduced slightly as explained further hereinafter. Preferably the magnesium content is at least 0.2% above this minimum. Within the broader range (minimum magnesiumi=0.32 copper) forged members of the described alloy will exhibit substantial immunity to stress corrosion under stresses up to or slightly exceeding 50% of the yield strength, for example, a stress level of about 30 K s.i. based on a nominal yield strength of 60 K s.i. for thermally treated 2024 aluminum alloy. The additional 0.2% magnesium in accordance with the preferred practice of the invention raises the permissible stress level to up to or slightly exceeding of the yield strength, a stress level of about 45 K s.i. for 2024 aluminum alloy. These conclusions are based on accelerated alternate immersion tests in a 3.5% sodium chloride aqueous solution.

An important modification to the basic magnesium and copper relationship occurs where manganese exceeds 0.5%. For this higher manganese content, the minimum magnesium necessary to maintain substantial immunity to stress corrosion is slightly lessened. Accordingly the following equations govern the minimum magnesium content in accordance with the invention where stress levels do not exceed 50% of the yield strength by a substantial amount:

(1) Min. Mg=0.32 Cu where Mn 0.5% (2) Min. Mg=0.2+0.32 Cu0.4 Mn where Mn 0.5%

Under the preferred practice of the invention where the applied stress can equal and even slightly exceed 75 of the yield strength, the equations become:

(3) Min. Mg=0.2+0.32 Cu where Mn 0.5% (4) Min. Mg:0.4+0.32 Cu0.4 Mn where Mn 0.5%

The alloy forging stock used in accordance with the invention is generally continuously ease ingot of suitable size to be used in the forging operations. Of course, ingot stock is generally scalped to remove serious surface defects and cropped to remove end defects as is the general practice in the forging art. However, the use of ingot stock is not a limitation on the invention which contemplates the use of any alloy body adapted to the particular forging operations contemplated. For instance, in making relatively small hand and die forgings small rolled rounds and the like can often be employed. Generally the ingot or other body is subjected to a thermal treatment which homogenizes its internal structure and relieves internal stresses prior to being forged. This treatment may generally be accomplished by a prolonged exposure, for example over 10 hours, at a temperature exceeding 850 F., although a 20 to 25 hour soak at a temperature of 900 to 925 F. is more often used. However, even longer exposure times of up to 50 hours and longer are often employed. The duration of this treatment is expressed functionally as being s-uflicient to effect substantial solution of the soluble alloy constituents. The forging operations are generally performed at elevated, or hot working, temperatures of about 600 to 900 F. although more often between 700 and 900 F. as is the general practice in the art.

To approach the useful strength potential of the described heat treatable alloys, they are solution heat treated and artificially aged in accordance with general methods known in the art. Thus, the forged member is solution heat treated at a temperature of 850 to 950 F. for a hold time of about 1 to 12 hours or longer, the hold time, of course, being to some extent a function of the size of the member. The temperature hold time is described functionally as of sufficient duration to effect substantial solution of the soluble alloy constituents. The forging is then quenched to retain the solution effects of this treatment; the means used in quenching are generally determined by the size of the forging, simple water immersion being adequate for a forging of relatively small size and spray quenching being more appropriate for a thicker member in order to rapidly cool its entire thickness. The forging is then artificially aged at a ternperature generally ranging from 300 to 400 F, and a hold time of about 6 to 20 hours although narrower ranges of 360 to 390 F. and 10 to 13 hours are more often employed. If desired, the quenched forging can be slightly cold worked prior to artificial aging. The cold working is generally accomplished by compressing, or sometimes possibly stretching, the member at substantially room temperature and serves to mechanically relieve internal stresses and impart-s other benefits known in the art. By substantially room temperature is generally meant around 60 to 100 F. for hand forgings and sometimes slightly higher for die forgings in that the dies are often heated to about 100 to 150 F. prior to the cold reduction. This step is, of course, more feasible with hand forgings because of their relatively simple shapes as opposed to die forgings which often have intricate shapes which would be excessively disturbed by any cold working. Further, in a complex die forging achieving a specific degree of metal working uniformly throughout is often quite diflicult. Of course, when a die forging is'to be cold worked such is usually performed by compressing such in finish dies. Hence, the forgings are preferably compressed in a suitable forging operation performed at room temperature to effect a permanent reduction in thickness of up to e.g. 2% to 5%, or stretched to effect a permanent stretch of up to 3%, e.g. 1 /2 to 3%, where such is feasible. It appears that this substantially room temperature working, in addition to imparting stress relief and the other generally recognized benefits, also can exert an influence on resistance to stress corrosion in that hand forgings in this temper often exhibit slightly better resistance to stress corrosion than do die forgings which are more often not so treated-Hence hand forgings represent a preferred embodiment of the invention because of the consistently good results experienced therewith. Our invention is illustrated in the following examples:

Example 1 A group of ingots were prepared from alloys having compositions within the preferred range composed of aluminum, 4.15 to 4.60% copper, 1.45 to 1.65% magnesium, 0.5 to 0.65% manganese, with the following maximum impurity limits: silicon 0.15%, iron 0.25%, chromium 0.1%, zinc 0.2% and nickel 0.05%. The ingots also contained up to 0.05% titanium and up to 0.002% boron for grain refining purposes. The ingots were continuously cast in the shape of cylinders, which, after scalping, were about 19 inches in diameter and were cut to lengths of about 54 inches. Each ingot section was homogenized by a 20 to 25 hour soak at a temperature of about 900 to 925 F. While still hot, the ingot sections were transferred to a forging press, and compressed in an axial direction at a temperature of from about 850 to 900 F. to form a biscuit about 24 inches high and 24 inches in diameter. The biscuit was squared on the press to form two parallel sides and further drawn down into a hand forging having the shape of an elongated slab about 20 inches wide, inches long and 4 inches thick. The slab was solution heat treated at 900 F. for 20 hours, spray quenched, compressed at room temperature to effect a 3% permanent reduction, and then artificially aged at 370 to 385 F. for 10 to 13 hours. Short transverse tensile specimens taken from these slabs were subjected to accelerated stress corrosion tests by alternate immersion at different stress levels in a 3.5% aqueous sodium chloride solution. All the forgings had a magnesium content above the minimum requirements of this invention and, they exhibited substantial immunity to stress corrosion at stress level of 50% of the yield stress in that the specimens survived the accelerated tests for at least 30 days. Further, the specimens whose minimum magnesium content satisfied the requirements of Equations 3 and 4 Were found to exhibit this same immunity at a stress level of 75% of the alloy yield stress.

As a standard of comparison, a similar slab containing aluminum, 4.6% copper, 1.4% magnesium and 0.5% manganese was formed by the same method as set forth earlier in this example. Short transverse specimens taken across the thickness of this slab were found to exhibit marked susceptibility to stress corrosion in that almost all the specimens failed during the accelerated stress corrosion tests and at stress levels of 50% of the short transverse yield strength and lower.

At this point it is worthwhile to note that a c0nsiderable quantity of flat or slab type hand forgings have been produced using the preferred composition limits set forth at the outset of this example and that these particular members have proved consistently superior in resistance to stress corrosion over other similar members of the described type alloy without the critical composition control set forth herein and hence such slabs having a thickness generally ranging from 1 inch to 6 inches represent a preferred embodiment of the invention. These slabs may be machined, bent, or otherwise further processed although such other fabrication steps most often precede thermal treatment. Further, forged slabs of this alloy composition have been hot rolled into plate having improved mechanical properties over similar plate hot rolled from ingot stock without a preliminary forging operation. This forged and rolled plate exhibited the same immunity to stress corrosion as did the forged slab. Hence this forged and rolled plate likewise represents a preferred embodiment of the invention.

Example 2 An ingot about 6 inches in diameter and about 55 inches long, and containing aluminum, 4.3% copper, 1.5% magnesium and 0.5% manganese, is flattened across a diameter in a forging press to form a slab hand forging about 4 inches thick. This member is then used as stock in a die forging operation to form an aircraft wing span about 48 inches long, about 18 inches wide at one end, tapering slightly to about 14 inches at the other end, and having a considerable number of webs and reinforcing flanges and ribs. The thickness of this span varies from about 1 inch at its thinnest, or web, portion to about 5 inches at some of the rib regions.

The die forging sequence includes heating the slab to about 850 F. and compressing between blocker dies which roughly approximate the final shape. The flash extruded at the parting line between the dies is removed and the piece reheated to 850 F. and pressed between another pair of blocker dies more closely approximating the details of the final configuration. Again, the flash is removed and the piece reheated to 850 F. and compressed between a pair of first finish dies which very closely approximate the details of the final configuration. The piece is then solution heat treated at a temperature of about 900 to 925 F. for about five hours hold time and rapidly quenched. The piece is then compressed in a pair of final dies at substantially room temperature to eflect a permanent deformation of about 2% thus forming the final shape. This span has a number of ribs, flanges, webs, bosses and similar variations in section which exhibit different degrees of short transverse structure depending on the extent and severity of metal movement involved in forming such. Short transverse specimens are taken from these various points and subjected to accelerated stress corrosion tests. The results of these tests indicate substantial immunity to stress corrosion in that almost all the specimens survive the tests for at least thirty days and at stress levels of 50% of the short transverse yield strength. Further, where the alloy composition of the die forged span is modified to include 1.65% magnesium thus satisfying the requirements of Equation 3, specimens are found to exhibit the same substantial immunity to stress corrosion at stress levels equal to 75% of the short transverse yield strength.

The invention has been described with particular reference to presently preferred embodiment; however, the invention is not limited to such. Various minor modifications will suggest themselves to those skilled in the art and it is intended to cover in the appended claim all such modifications as fall within the true spirit and scope of the invention.

What is claimed is:

An improved forged aluminum base alloy member having an internal structure substantially produced by a forging operation, solution heat treating, quenching and artificial aging, the forging operation imparting to at least a portion of the member a grain structure having a substantially short transverse characteristic thereby establishing a short transverse direction for said portion, the member being composed of an alloy consisting essentially of aluminum, 3 to 6% copper, 0.8 to 3% magnesium, and 0.3 to 1% manganese, the minimum magnesium content being governed by the relation:

Mg min.:0.32 Cu where Mn does not exceed 0.5%, Mg min.:0.2+0.32 Cu0.4 Mn where Mn is greater than the member exhibiting substantial immunity to stress corrosion in the short transverse direction at a stress of of the yield level in the short transverse direction.

References Cited UNITED STATES PATENTS 2,522,575 9/1950 Hall et al 14832.5 2,671,559 3/1954 Rosenkranz 148-427 X 3,265,493 8/1966 Foerster 14832.5 X

DAVID L. RECK, Primary Examiner.

CHARLES N. LOVELL, Examiner.