US1946545A - Heat treatment of light alloys - Google Patents

Heat treatment of light alloys Download PDF

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US1946545A
US1946545A US427941A US42794130A US1946545A US 1946545 A US1946545 A US 1946545A US 427941 A US427941 A US 427941A US 42794130 A US42794130 A US 42794130A US 1946545 A US1946545 A US 1946545A
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aging
duralumin
corrosion
alloys
static pressure
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Pessel Leopold
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    • 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

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  • the object of this invention is to increase the resistance of heat treated light alloys against the intergranular embrittlement brought about by corrosion and to increase their elongation.
  • This invention relatesto the process of heat treating a certain class oflight alloys which are of such a nature that their tensile strength can be increased by heat treating; the latter will consist of the following stages: (1) heating, (2) cooling or quenching, (3) aging.
  • the process of aging may take place at room temperature, at
  • the period of aging must not be instant if measured at the lowest temperatures technically available. With otherwords, the period of aging must be long enough, at suitable temperatures, to be ascertained by customary physical tests at the beginning and at the practical completion of the aging period.
  • the alloys falling into this class are principally duralumin and similar alloys, which generally contain not less than 90 percent aluminum, the balance being composed of various elements, such as copper, magnesium, silicon, or others. There are, however, other light alloys showing the above mentioned heat treatment characteristics. These alloys, which generally contain not less than 75 per cent aluminum or magnesium or both elements together, also fall within the claims of this specification.
  • Duralumin which, among the alloys in question, shows the above described heat treating characteristics in a most pronounced manner, is very extensively used, the most important field of application being aircraft construction.
  • a great disadvantage of this alloy is its susceptibility to a form of corrosion known as intergranular embrittlement. Corrosion of this type will express itself in a decrease of the tensile strength, but also, and in a much more pro-- nounced manner, in a decrease of the elongation. This intergranular corrosion may take place while there is little indication of any changes on the surface and yet the material may become so embrlttled that failure in service may occur. Any method which decreases the susceptibility of duralumin towards intergranular embrittlement or which raises its elongation without affecting the tensile strength promises to be of usefulness to the aircraft industry.
  • Intergranular corrosion is characterized by a loosening of the bond between the metal grains.
  • duralumin it is believed that the hardening particles, especially those of the copper-alumium compound, have a tendency to gather along the crystal boundaries and that some form of electrolytic action tends to weaken the bond between the crystal grains. Any method that would diminish the number of particles along the grain boundaries or partly interfere with the migration of the particles from the interior of the grains towards the boundaries, would tend to produce a duralumin of superior resistance against this type of corrosion. It would also have the efiect of increasing the elongation of the material.
  • the alloy is subjected to prolonged static pressure during the period of aging. It may be that the increased rigidity of the compressed material at this stage of the heat treatment interferes with the movements of the particles, or that lines of strain are set up Within the grains which act as gathering points for the particles and diminish the number of those actually reaching the grain boundary. Whatever the theory of the causes of the phenomenon, I have found that duralumin, which has been kept under static pressure during a considerable part of its aging period, has shown an increased resistance against intergranular corrosion and an increased elongation. This method of improving corrosion resistance and elongation has-not been described before and is to be considered a new and original invention.
  • the object of doing this work on the material immediately after quenching is to avoid the detrimental effects which cold working of the aged material hasupon the ductility and corrosion resistance of the alloy.
  • the purpose is to perform forming or straightening operations while the material is relatively soft. It is well known that cold working of metals or alloys increases their hardness and tensile strength and decreases their ductility. It also tends to cause local strains which will be preferred centers of corrosion attack. It is just in order to avoid these effects that any mechanical operations that may be necessary are performed while the material is as soft as possible. Any kinetic pressure that may be applied is only incidental and is released as soon as the required shape or dimension is obtained.
  • the static pressure itself is one essential factor. Its effect upon the metal grain is only incidental and is overshadowed by the effect upon the precipitation, growth, migration and distribution of the hardening particles.
  • the efiect of kinetic pressure upon the metal grain would be to decrease elongation. With this method, however, the elongation may be increased.
  • the element of time is the second important factor and cannot be influenced by mechanical means, as the above mentioned changes of the particles require time, which can only be influenced by temperature changes. The time, during which the static pressure must be maintained to influence the distribution of the particles, must be long enough to permit a certain growth.
  • duralumin In duralumin the aging process makes very rapid progress immediately after quenching, but becomes slower and slower with the time, without reaching a well defined end point. To express this fact properly, one should speak of the completion of aging when the material is practically aged. In duralumin of average composition, aging at room temperature, this practical limit is reached after approximately five days. To be effective the static pressure must be applied before this state is reached. As a rule it will be desirable to apply the static pressure as soon as possible after quenching, before aging has progressed to any considerable extent. The amount of static pressure and the length of time during which it is to be exerted, will vary with the nature of the alloy, the temperature and the requirements regarding results. These factors cannot generally be specified.
  • the static pressure exceeds a certain limit a harmful effect upon the metal grain may be experienced.
  • a beneficial effect will be observed if the static pressure is at least 300 pounds per square inch and if the duration of the pressure period is not less than 10 minutes, provided the aging takes place at room temperature. If the material is aged at artificially raised temperatures, the pressure period may be less than 10 minutes, but in such a case it should be not less than 10 per cent of the aging period.
  • the static pressure may be transmitted to the surface of the material by means of solid, liquid or gaseous media. It may be convenient to use mechanical presses or to expose the material in suitable vessels to highly compressed liquids or gases. Arrangements may be made to maintain a specified temperature during the application of pressure.
  • the treatment of a commercial duralumin by this process and the results that have been obtained are described below. It is obvious, however, that an analogous treatment will give analogous results in that group of alloys, which, although of a different analysis, are commonly spoken of as alloys of the duralumin type. Furthermore, in view of the fact that the improvement described in this specification consists in infiuencing'the character and the distribution of precipitated hardening particles in an aluminum matrix, analogous.
  • results can be expected from hardening particles of a different chemical nature, as long as they have been dissolved in an aluminum matrix by a heating process, havebeen precipitated by a. quenching operation to give greater strength to the matrix, and do not exert their full effect immediately but only in the course of a measurable time, commonly known as aging period.
  • the material consisted of duralumin sheet, inch thick, and of the following approximate analysis: Cu 4.0%, Mg 0.5%, Si 0.35%, Mn 0.6%, Fe 0.5%, the balance being aluminum. It was in a fully annealed condition before heat treating.
  • the heat treatment consisted in heating the material to 950 F. in an electric furnace, holding it at this temperature for 20 minutes and quenching it in cold water. Immediately after quenching it was transferred to ice water, to retard aging, and kept there for approximately 30 minutes.
  • the material was then put under the press plates of a press (screw type) and a pressure estimated at approximately 600 pounds per square inch was applied. The material was kept under this pressure for 9 hours (at room temperature).

Description

Patented Feb. 13, 1934.
UNITED STATES HEAT TREATMENT OF LIGHT ALLOYS Leopold Pessel, Philadelphia, Pa.
No Drawing.
2 Claims.
The object of this invention is to increase the resistance of heat treated light alloys against the intergranular embrittlement brought about by corrosion and to increase their elongation. I
have found that this can be accomplished by subjecting these alloys to considerable and prolonged static pressure after quenching but before the aging is completed, i. e. during the process of aging.
This invention relatesto the process of heat treating a certain class oflight alloys which are of such a nature that their tensile strength can be increased by heat treating; the latter will consist of the following stages: (1) heating, (2) cooling or quenching, (3) aging. The process of aging may take place at room temperature, at
higher or at lower temperatures, it may require a short or a long period of time for its completion, but the period of aging must not be instant if measured at the lowest temperatures technically available. With otherwords, the period of aging must be long enough, at suitable temperatures, to be ascertained by customary physical tests at the beginning and at the practical completion of the aging period.
The alloys falling into this class are principally duralumin and similar alloys, which generally contain not less than 90 percent aluminum, the balance being composed of various elements, such as copper, magnesium, silicon, or others. There are, however, other light alloys showing the above mentioned heat treatment characteristics. These alloys, which generally contain not less than 75 per cent aluminum or magnesium or both elements together, also fall within the claims of this specification.
Duralumin, which, among the alloys in question, shows the above described heat treating characteristics in a most pronounced manner, is very extensively used, the most important field of application being aircraft construction. A great disadvantage of this alloyis its susceptibility to a form of corrosion known as intergranular embrittlement. Corrosion of this type will express itself in a decrease of the tensile strength, but also, and in a much more pro-- nounced manner, in a decrease of the elongation. This intergranular corrosion may take place while there is little indication of any changes on the surface and yet the material may become so embrlttled that failure in service may occur. Any method which decreases the susceptibility of duralumin towards intergranular embrittlement or which raises its elongation without affecting the tensile strength promises to be of usefulness to the aircraft industry.
While the main efforts to prevent intergranular corrosion have been spent in the direction of applying protective coatings on the surface of the duralumin, they have not succeeded in render- Application February 12, 3930 Serial No. 427,941
ing this material safe from the point of view of corrosion resistance. An important step was the discovery that duralumin quenched in cold water and aged at room" temperature has a better resistance towards intergranular corrosion than material that has been quenched in hot water or aged at elevated temperatures. This has been, so far, the only method of decreasing the susceptibility of duralumin towards this type of corrosion by treatment of the alloy itself.
Intergranular corrosion is characterized by a loosening of the bond between the metal grains. In the case of duralumin it is believed that the hardening particles, especially those of the copper-alumium compound, have a tendency to gather along the crystal boundaries and that some form of electrolytic action tends to weaken the bond between the crystal grains. Any method that would diminish the number of particles along the grain boundaries or partly interfere with the migration of the particles from the interior of the grains towards the boundaries, would tend to produce a duralumin of superior resistance against this type of corrosion. It would also have the efiect of increasing the elongation of the material.
By the method described in this specification the alloy is subjected to prolonged static pressure during the period of aging. It may be that the increased rigidity of the compressed material at this stage of the heat treatment interferes with the movements of the particles, or that lines of strain are set up Within the grains which act as gathering points for the particles and diminish the number of those actually reaching the grain boundary. Whatever the theory of the causes of the phenomenon, I have found that duralumin, which has been kept under static pressure during a considerable part of its aging period, has shown an increased resistance against intergranular corrosion and an increased elongation. This method of improving corrosion resistance and elongation has-not been described before and is to be considered a new and original invention.
It is true that sometimes forming operations have been and are being performed on quenched duralumin before aging, such as pressing, bending, rolling etc., which may also involve a temporary application of kinetic pressure.
The object of doing this work on the material immediately after quenching (preferably within one hour), is to avoid the detrimental effects which cold working of the aged material hasupon the ductility and corrosion resistance of the alloy. The purpose is to perform forming or straightening operations while the material is relatively soft. It is well known that cold working of metals or alloys increases their hardness and tensile strength and decreases their ductility. It also tends to cause local strains which will be preferred centers of corrosion attack. It is just in order to avoid these effects that any mechanical operations that may be necessary are performed while the material is as soft as possible. Any kinetic pressure that may be applied is only incidental and is released as soon as the required shape or dimension is obtained.
In the method claimed by this specification the static pressure itself is one essential factor. Its effect upon the metal grain is only incidental and is overshadowed by the effect upon the precipitation, growth, migration and distribution of the hardening particles. The efiect of kinetic pressure upon the metal grain would be to decrease elongation. With this method, however, the elongation may be increased. The element of time is the second important factor and cannot be influenced by mechanical means, as the above mentioned changes of the particles require time, which can only be influenced by temperature changes. The time, during which the static pressure must be maintained to influence the distribution of the particles, must be long enough to permit a certain growth.
As any kinetic pressure that may have been exerted upon duralumin incidental with its mechanical working before aging has always been released after the required shape or dimension was obtained, any change of such kinetic pressure into static pressure and prolongation of the latter over a period of time reasonably suflicient to advance aging, would fall within the claims of this specification.
In duralumin the aging process makes very rapid progress immediately after quenching, but becomes slower and slower with the time, without reaching a well defined end point. To express this fact properly, one should speak of the completion of aging when the material is practically aged. In duralumin of average composition, aging at room temperature, this practical limit is reached after approximately five days. To be effective the static pressure must be applied before this state is reached. As a rule it will be desirable to apply the static pressure as soon as possible after quenching, before aging has progressed to any considerable extent. The amount of static pressure and the length of time during which it is to be exerted, will vary with the nature of the alloy, the temperature and the requirements regarding results. These factors cannot generally be specified. It is well to remember that if the static pressure exceeds a certain limit a harmful effect upon the metal grain may be experienced. For practical purposes it may be assumed that a beneficial effect will be observed if the static pressure is at least 300 pounds per square inch and if the duration of the pressure period is not less than 10 minutes, provided the aging takes place at room temperature. If the material is aged at artificially raised temperatures, the pressure period may be less than 10 minutes, but in such a case it should be not less than 10 per cent of the aging period.
The static pressure may be transmitted to the surface of the material by means of solid, liquid or gaseous media. It may be convenient to use mechanical presses or to expose the material in suitable vessels to highly compressed liquids or gases. Arrangements may be made to maintain a specified temperature during the application of pressure. The treatment of a commercial duralumin by this process and the results that have been obtained are described below. It is obvious, however, that an analogous treatment will give analogous results in that group of alloys, which, although of a different analysis, are commonly spoken of as alloys of the duralumin type. Furthermore, in view of the fact that the improvement described in this specification consists in infiuencing'the character and the distribution of precipitated hardening particles in an aluminum matrix, analogous. results can be expected from hardening particles of a different chemical nature, as long as they have been dissolved in an aluminum matrix by a heating process, havebeen precipitated by a. quenching operation to give greater strength to the matrix, and do not exert their full effect immediately but only in the course of a measurable time, commonly known as aging period.
The material consisted of duralumin sheet, inch thick, and of the following approximate analysis: Cu 4.0%, Mg 0.5%, Si 0.35%, Mn 0.6%, Fe 0.5%, the balance being aluminum. It was in a fully annealed condition before heat treating. The heat treatment consisted in heating the material to 950 F. in an electric furnace, holding it at this temperature for 20 minutes and quenching it in cold water. Immediately after quenching it was transferred to ice water, to retard aging, and kept there for approximately 30 minutes. The material was then put under the press plates of a press (screw type) and a pressure estimated at approximately 600 pounds per square inch was applied. The material was kept under this pressure for 9 hours (at room temperature). It was then removed from the press and permitted to age completely at room temperature (requiring approximately one week) Some of the duralumin specimens treated by this process were exposed to a salt spray corrosion test lasting 30 days, together with other material that had the same composition and history, but had been aged without application or static pressure. The following figures, which are average values obtained in a number of determinations, show the relative physical qualities before and after exposure to corrosion, of duralumin treated according to this method and treated in the heretofore customary manner.
Tensile Strength (lbs/sq. inch) Before corrosion Alter corrosion Aged with static pressure Aged without static pressure Elongation per cent I in 2 inches Alter corrosion Before corrosion Aged with static pressure Aged without static pressure I claim:
1. The method of increasing the elongation and resistance against intergranular corrosion embrittlement of duralumin and alloys commonly known as alloys of the duralumin type, which are of such a nature that their strength can be increased by a heat treating process including a period of aging, which comprises subjecting these alloys, during the period of aging and for not less than nine hours, to a static pressure which will not result in any essential change of the shape or dimension of the material and which shall be not less than 600 pounds per square inch.
2. The method of increasing the elongation and resistance against inter-granular corrosion embrittlement of duralumin and alloys commonly known as alloys of the duralumin type,
which are of such a nature that their strength can be increased by a heat treating process ineluding a periodof aging, which comprises aging the material while subjecting it to a static pres sure which will not result in any essential change of the shape or dimension of the material and which shall be not less than 600 pounds per square inch.
LEOPOLD PESSEL.
US427941A 1930-02-12 1930-02-12 Heat treatment of light alloys Expired - Lifetime US1946545A (en)

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Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3073294A (en) * 1959-07-02 1963-01-15 Eaton Mfg Co Aluminum valve
US3168607A (en) * 1960-12-28 1965-02-02 Greene Ben Methods of heat treating articles
US3496624A (en) * 1966-10-25 1970-02-24 Aluminum Co Of America Castings
US4045254A (en) * 1974-12-30 1977-08-30 Mitsubishi Jukogyo Kabushiki Kaisha Method for toughening treatment of metallic material

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3073294A (en) * 1959-07-02 1963-01-15 Eaton Mfg Co Aluminum valve
US3168607A (en) * 1960-12-28 1965-02-02 Greene Ben Methods of heat treating articles
US3496624A (en) * 1966-10-25 1970-02-24 Aluminum Co Of America Castings
US4045254A (en) * 1974-12-30 1977-08-30 Mitsubishi Jukogyo Kabushiki Kaisha Method for toughening treatment of metallic material
US4140553A (en) * 1974-12-30 1979-02-20 Mitsubishi Jukogyo Kabushiki Kaisha Method for toughening treatment of metallic material

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