US3222778A

US3222778A - Process for retaining the ductility of metal

Info

Publication number: US3222778A
Application number: US166930A
Authority: US
Inventors: Noble N Ida
Original assignee: Martin Marietta Corp
Current assignee: Lockheed Martin Corp
Priority date: 1962-01-17
Filing date: 1962-01-17
Publication date: 1965-12-14
Anticipated expiration: 1982-12-14

Description

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United States Patent Ofifice 3,222,778 Patented Dec. 14, 1965 3,222,778 PROCESS FOR RETAINING THE DUCTILITY F METAL Noble N. Ida, Boulder, (1010., assignor to Martin-Marietta lCorporation, Baltimore, Md., a corporation of Maryand No Drawing. Filed Jan. 17, 1962, Ser. No. 166,930 3 Claims. (Cl. 29-528) This invention relates to metals having improved ductility characteristics and the method of making the same. More particularly, this invention relates to metal products having at least a portion of the surface thereof treated to produce a high degree of ductility without a corresponding decrease in ultimate tensile or yield strengths and to the method of producing same. The present invention is particularly useful for the fabrication of metal parts by forming operations.

In the past, metal parts have generally been formed while in the soft, annealed or solution treated condition. Subsequent to the forming operation, the part is heat treated. In many instances, however, parts formed and heat treated by the known procedures must undergo subsequent sizing -or stretcher levelling operations to achieve the strength and dimensional conformity that 'is needed prior to use of the part. Basically, the aforementioned forming process of the prior art has been developed because the ductility of a metal while in the soft or annealed state is substantially lost when the metal is heat treated. That is, a metal that has been hardened by heat treating is often difiicult if not impossible to form due to the increased brittleness thereof. It is believed that the loss of ductility that accompanies the tempering of a metal is, to a considerable extent, a direct result of an impure film that forms on the surface of the metal with an oxide of the metal quite frequently being the predominant constituent of this film.

Accordingly, the present invention is a product and process whereby the film on the surface of a heat treated metal is at least temporarily removed so as to significantly increase the ductility of the metal without sacrifice of the ultimate tensile or yield strength thereof. By use bf the present invention, the base metal can be heat treated and subsequently formed at room temperature thus removing the necessity of sizing or re-working the part after it has been formed. To be more specific, the present invention contemplates the product and process wherein the film and particularly the oxide that tends to coat a base metal during and after heat treating can be prevented from forming or can be removed after formation thereof until a protective flowable and deformable sealant is adhered to the surface of the metal. To this end, it is contemplated that the relatively pure (i.e. film free) surface of the metal will be maintained in an inert atmosphere until the sealant is adhered thereto. The surface area of the metal that is protected by the sealant will generally demonstrate a marked increase in ductility and, therefore, will be particularly useful for the forming of a part from the metal.

To better understand the detailed explanation of the present invention as presented hereinafter, consider the following theory of the structure and occurrences within a metal. The molecules and/or atoms of a metal tend to bond together in a generally crystalline lattice in .three dimensions. .The crystal formations within this lattice tend to assume a regular configuration such that the application of a tensile force to the metal will cause the lattices to slip over each other much like a deck of It is this slipping phenomena that is believed to impart ductile strength to the metal while retaining the yield strength thereof.

Unfortunately, the crystals of a metal do not always form into regular configurations. In fact, the crystals quite often are imperfect due to the lack of an atom or molecule necessary to form a regular and stable crystal. However, the atoms and molecules will bond into an imperfect crystal notwithstanding the absence of one or more atoms or molecules, and this state of affairs will be referred to hereinafter as a dislocation. These disclocations tend to attract towards each other and towards the surface of the metal whenever a stressing force is applied thereto. This results because the imperfect crystal will attract sufficient atoms or molecules from its neighbor to form a more perfect or regular crystal when stress forces are applied while the neighbor will do the same to its neighbor and so on. Thus, the dislocations will tend to travel towards the location of least force which, of course, is either another dislocation or the surface of the metal or both.

Furthermore, it can be seen that these dislocations or collections of dislocations (dislocation densities) will be attracted to each other once they have reached the surface of the metal thereby creating stress concentrations at a point or series of points. The presence of these stress concentrations would not be a major problem if they are not inhibited from being relieved beyond the surface of the metal. However, the presence of some films on the metal surface and particularly the presence of an oxide coat will prevent the relieving of these stress concentrations and in fact will cause the stress concentrations to build up until micro-cracks and cleavage failures begin to appear at the metal surface. It is believed that what has happened is that the surface film or oxide coat has effectively locked the outer grains and lattices from slipping so that the next few layers must withstand a greater tensile stress. A chain reaction has thus been started which could course through the entire body of the metal until a total failure occurs.

It should be born in mind that the dislocations mentioned hereinbefore are not actually voids or holes in the metal as such but may more properly be described as units or measures of energy relative to the energy of a regular crystal structure. Further, the relationship between the surface film and the metal itself can be considered in terms of a pressure relationship, P, which can be expressed as pressure, load, force, mass, energy or the like in accordance with the folowing general equation:

where F is the free energy in the metal base, F is the free energy of the surface film on the metal, T is temperature and AS is the change in system entropy or the amount of available energy in the whole system.

If the surface film is an oxide coating, for instance, then F will be much greater than F and in effect, a barrier will be created at the interface between the oxide and the metal so that stress energy will be prevented from flowing into the oxide. In fact, the only force flow that would be allowed under this condition would be from the oxide to the metal. By removing the oxide and replacing it with a paraffin coat, for example, then the pressure relationship will be reversed so that F will be greater than F and stress energy will tend to flow from the metal into the paraffin. This situation can be analogized somewhat to a series circuit having two batteries, A and B, each connected to cause current flow in said circuit in an opposite direction to the other battery. Battery A is analogous to the free energy of the surface film, F battery B to the free energy of the base metal, F and the current flow in the circuit to the tendency of stress forces to flow across the interface. If battery A produces a larger voltage than battery B, then current flow in the circuit will be allowed in a direction against the potential or voltage of battery B. However, if the potential of battery B is fixed, but the potential of battery A can be reduced below that of battery B, then the current flow will reverse and will tend to flow through the circuit in the direction against battery A.

In accordance with this invention, it has been discovcred that by removing the surface film and oxide coating from the metal and adhering a flowable and deformable sealant to the pure metal surface, the sealant will absorb the stress energy from the dislocation density concentrations at the metal surface and will allow more normal slippage of the grains and lattices near the surface of the metal. By this means, the ductility of the metal is considerably increased with practically no sacrifice of tensile or yield strengths. Aliphatic hydrocarbons or long chain fatty acids have been found to be quite satisfactory for use with this invention as sealants and paraffin has been found to demonstrate particularly desirable characteristics. However, it should be understood that any sealant can be utilized as long as it will absorb the stress forces from the metal and can be retained on the purified surface of the metal long enough to provide a deformable or fiowable barrier therefor against oxidation and the like until the part has been actually formed or the increased ductility of the metal has been retained for a desired length of time. Obviously the sealant should be able to adhere to the metal at a temperature below the heat treat temperature of the metal.

The present invention has been advantageously utilized with gratifying results for forming of parts from 2014- T6 aluminum alloy and beryllium. Accordingly, the following detailed description will be particularly directed towards the use of this invention with respect to 2014- T6 aluminum alloy and beryllium although it is to be understood that the present invention is in no way limited to those metals. For instance, encouraging possibilities have been indicated for this invention with respect to tungsten, molybdendum, aluminum and other aluminum alloys, and in fact practically any metal that, as a result of heat treating or for other reasons, tends to form a hard film on the surface thereof in such a manner that the relieving of stresses resulting from localized concentrations of dislocation densities will be prevented.

The initial condition to be realized for the metal is that at least a portion of the surface area to undergo dimensional change should be relatively pure or free from oxide and/or foreign film material. 2014-T6 aluminum alloy and berylluim, this has been accomplished by three initial steps. The first initial step is to degrease the metal and exposure in trichloroethylene in accordance with MIL-T-7003 specifications has been found satisfactory for this purpose. tial step is to alkaline clean the metal in order to remove oil stains, markings and the like. This alkaline cleaning step is basically to prepare the metal so that deoxidation in accordance with the following step can be uniformly accomplished. Immersion in Turco Aviation cleaner for 10 minutes was found to be satisfactory for this purpose. The third and final initial step is to deoxidize the metal. For this purpose, immersion in Turco #4461 deoxidizer solution (sometimes known as Smut-go) for minutes was satisfactory. It should be noted that there are many Ways of accomplishing the foregoing initial steps such as through acid-base systems or milling or grinding in an inert atmosphere and so on.

At this point, the surface of the metal is relatively pure and care must be exercised that the deoxidized surface is not exposed to any film or oxide forming material. Thus, it is preferable that the cleaned and deoxidized surface of the metal be retained in an inert atmosphere until the step of adhering a resilient sealant thereto has been per- For heat treated The next iniformed as described hereinafter. A nitrogen (N blanket has been successfully employed for this purpose.

While the metal is still in the inert atmosphere, a relatively resilient sealant is adhered to the now pure surface area. The purpose of the sealant is first to absorb the stress energy from the localized dislocation concentrations at the metal surface as was described hereinbefore and secondly to provide a barrier whereby non-resilient films and oxide coatings are prevented from being formed. If the temper of the metal to be formed is to be maintained, then obviously the sealant must be adhered to the metal at a temperature below the heat treat temperature of the metal.

For practical purpose, it is better if the sealant can be cured into a solidified mass on the metal, but the invention is not limited to solid sealants. The important points are that it must provide a barrier and a stress relieving medium as described. To this end, the sealant preferably should have a slow diffusion rate of atoms therein and have suificient mass density so that atom migration therethrough will be relatively slow. Plastic materials and, as was mentioned before, most long chain fatty acids or fats of aliphatic hydrocarbons are satisfactory as sealants for use in conjunction with this invention and in fact parafiin has been found to be highly satisfactory.

As soon as an appropriate sealant has been adhered to the relatively pure metal surface, the metal can be removed from the inert atmosphere and fabricated into a part by forming operations. Thereafter the sealant can be removed from the part or allowed to remain thereon as may be desired. If paratfin is used, it can be quite easily removed by using methyl-ethyl-ketone.

By use of this invention, parts can be formed into shapes and configurations that were previously impossible or difiicult to attain especially from base metals that have been heat treated. The present invention provides substantially increased ductility without sacrificing the other properties and characteristics of the metal. The increased ducility realized through use of the present invention has been markedly demonstrated by a series of controlled comparative tests, some of which will be described hereinafter.

Tensile teszs at liquid hydrogen temperature In one test, several specimens were constructed from the same stock of 2014 aluminum alloy in the T6 temper. The specimens were uniform in configuration being .048 inch in cross-sectional area and 2 inches in gage lengt In the following table, specimens 1, 2 and 3 were untreated while specimens 4, 5 and 6 were all processed and paraffin coated in accordance with the present invention as described hereinbefore. The specimens were exposed to a liquid hydrogen environment and tensile tested to fracture failure in accordance with standard test procedures while so exposed.

Specimen Elonga- Ultimate Yield Yield Modulus Number tio Load, lbs. Load, Strength, 10 p.s.i.

lbs. p.s.i.

All specimens demonstrated an ultimate strength of 100,000 p.s.i. As can be seen in the foregoing table, the

specimens treated in accordance with this invention had an increased ductility averaging over as compared to the untreated specimens with virtually no change in strengthproperties.

Bend tests In another series of tests at room temperature 2014-T6 aluminum alloy specimens in 6 inch lengths were bent until fracture. The maximum bend angle achieved with the untreated specimens was 120 whereas the specimens that Were treated could be bent to a maximum angle of 145 illustrating a bend increase of approximately 20%. Similar room temperature bend tests were performed upon beryllium specimens in 4 inch lengths. In this test, the maximum deflection to failure for the untreated specimens was 0.011 inch whereas the treated specimens could be deflected to 0.489 inch. This is truly a significant and somewhat startling result for a metal having extremely interesting properties at the present time.

Bulge tests Room temperature bulge or cupping tests Were performed upon both treated and untreated 2014T6 aluminum alloy specimens. These specimens were 5 inch square blanks of 0.050 inch thickness and a 1.25 inch diameter cup was used. The untreated specimens cupped to a 1.00 inch deflection and fractured in all cases. In contrast, the specimens coated with paraffin in accordance with this invention cupped to the full 1.25 inch deflection with no fractures occurring whatsoever. This test graphically demonstrated the superior flowing characteristics of the metal treated in accordance with this invention as compared to the untreated specimens of the same material and configuration.

Explosive forming tests In one test, 5 grains of Powertol 7 dynamite were used to explosively form 2014-T6 specimens into a 2.8 inch diameter steel die. Untreated specimens of 0.032 inch thickness could only be formed to a maximum deflection of 0.020 inch whereas paraflin coated specimens of the same 0.032 inch thickness could be formed to a maximum deflection of 0.62 inch.

In another explosive forming test, 50 grains of Powertol 7 dynamite were used to explosively force specimens into a steel die of 6 inch diameter. Uncoated 2014-T6 specimens of 0.032 inch thickness could be deflected to a maximum of only 0.41 inch into the die Whereas the treated specimens of 0.032 inch thickness attained a maximum deflection of 1.50 inches. The explosive forming tests were all performed at room temperature.

Since all of the foregoing tests for coated and uncoated specimens were carried out under identical conditions, it is readily apparent that significantly greater formability can be accomplished by advantageously utilizing base metals that have been treated and coated in accordance with the present invention particularly when paraffin is used as the sealant.

Although the product and process of the present invention has been described herein with particularity, the invention is not intended to be limited thereto. In fact, many variations of this invention will be obvious to one having normal skill in the art without departing from the spirit of this invention. For instance, it is apparent that a base metal that has been freed of any surface film and/ or oxide coating could be retained in an inert atmosphere and formed while so retained. In addition, a base metal could conceivably be forged or heat treated and formed with the entire process being carried out in an inert atmosphere. Furthermore, the product and/or process can be used for other than forming operations. The present invention can be advantageously utilized wherever it is desirable to increase the ductility of a base metal and, among other advantages, could be used to increase the fatigue life of the metal. The relative simplicity of the invention makes it readily adaptable to existing shop practices.

What I claim as my invention is:

t. An improved process for forming parts from metal by retaining the ductility of the metal comprising the steps of degreasing said metal, alkaline cleaning said metal, removing the brittle films comprising reaction products with said metal, coating said metal With a flowable sealant of aliphatic hydrocarbon that will adhere to said metal at a lower temperature than the heat treating temperature of said metal, maintaining said metal in an inert atmosphere after deoxidizing thereof until said sealant is adhered thereto, and deforming said sealed metal into a part.

2. An improved process for forming parts from metal by retaining the ductility of the metal, as set forth in claim 1, wherein said metal is an aluminum alloy.

3. An improved process for forming parts for metal by retaining the ductility of the metal, as set forth in claim I, wherein said metal is beryllium.

References Cited by the Examiner UNITED STATES PATENTS 1,044,656 11/1912 Howell 117-135 1,098,368 6/1914 Caffall 117-135 1,399,270 12/1921 Nusbaum 117-135 XR 1,466,380 8/1923 Nusbaum.

2,070,918 2/1937 Peterson 117-135 2,294,717 9/1942 Carney 117-135 2,850,999 9/1958 Kaplan et al 29-528 XR 2,851,372 9/1958 Kaplan et al.

2,871,140 1/1959 Goss 117-49 3,015,576 1/1962 Henderixson et al. 117-49 3,154,426 10/1964 Kohnken 29-528 XR 3,154,849 11/1964 Dolch 29-528 OTHER REFERENCES Metal Finishing Surface Preparation of Metals, vol. 58, No.6, June 1960, pp. 53-58.

WHITMORE A. WILTZ, Primary Examiner.

Claims

1. AN IMPROVED PROCESS FOR FORMING PARTS FROM METAL BY RETAINING THE DUCTILITY OF THE METAL COMPRISING THE STEPS OF DEGREASING SAID METAL, ALKALINE CLEANING SAID METAL, REMOVING THE BRITTLE FILMS COMPRISING REACTION PRODUCTS WITH SAID METAL, COATING SAID METAL WITH A FLOWABLE SEALANT OF ALIPHATIC HYDROCARBON THAT WILL ADHERE TO SAID METAL AT A LOWER TEMPERATURE THAN THE HEAT TREATING TEMPERATURE OF SAID METAL, MAINTAINING SAID METAL IN AN INERT ATMOSPHERE AFTER DEOXIDIZING THEREOF UNTIL SAID SEALANT IS ADHERED THERETO, AND DEFORMING SAID SEALED METAL INTO A PART.