US1751468A - Method of forming articles from heat-treatable aluminum-base alloys - Google Patents

Description

Patented Mar. 25, 1930 UNITED STATES PATENT OFFICE ROBERT S. ARCHER, OF CLEVELAND, OHIO, ASSIGNOR TO ALUMINUM COMPANY OF AMERICA, OF PITTSBURGH, PENNSYLVANIA, A CORPORATION OF PENNSYLVANIA METHOD OF FORMING- ARTICLES FROM HEAT-TREATABLEJALUMINUM-BASE ALLOYS No Drawing.

The invention relates to the forming of wrought articles made from aluminum base alloys, certain of whose physical properties may be substantially increased through the application of a solution heat treatment, and particularly it has to do with a method of work-forming those alloys which renders them capable of such improvement without the application of any solution heat treatment subsequent to working.

As used herein the term work-forming designates generally those mechanical processes which produce an article by reducing or altering the section of a metallic blank, billet or other shape, such as forging, rolling, etc.

The invention is particularly applicable to those aluminum base alloys which contain,

in addition to aluminum, one or more constituents capable of forming a solid solution with aluminum, and whose solubility in solid aluminum decreases with falling temperature. These alloys chiefly contain copper, magnesium, manganese, silicon, and zinc, in varying combinations and proportions. Specific examples of them are as follows: (A) Copper 3 to 5 per cent, magnesium 0.5 per cent, manganese 0.6 per cent; (B) copper 4 to 5.5 per cent, manganese and silicon about 0.75 per cent each; (C) magnesium about 0.5 per cent, silicon about 1 per cent; the

balance in'each case being aluminum except for the customary impurities.

It is well known that certain physical properties of aluminum alloys of the class indicated can be improved very materially by subjecting the alloys to a heat treatment at a characteristic elevated temperature, for the purpose of bringing into solution as far as possible the undissolved solid-solution forming constituents, and cooling quickly from that temperature. By this treatment tensile strength, hardness and usually ductility are improved as compared with the values for the alloys just after hot working or annealing, and ductility is very markedly improved as compared with that of cold worked material. When the alloy thus heat treated is subsequently aged either at room temperature or at temperatures up to about 200 0.,

Application filed December 24, 1927. Serial No. 242,581.

tensile strength and hardness may be further improved, while ductility is little affected by room temperature aging or in the first stages of aging at higher temperatures, but is usually reduced by aging at elevated temperatures long enough to develop maximum hardness. The changes brought about by aging are believed to ,be due to preci itation of one or more constituents in the orm of very minute particles.

Large amounts of these alloys are used for the production of articles by work-forming relatively heavy stock. The practice heretofore has been to work-form stock heated at a suitable work-forming temperature, which is usually considerably below the heat treating temperature, and then to reheat it at the solution heat treatment temperature, and quench and age it to increase its physical'properties. The alloys of the class referred to cannot usually be work-formed successfully at temperatures as high as that required for the solution heat treatment, and heat treatment at the work-forming temperature usually is neither complete enough nor rapid enough to make the alloys capable of developing the increased physical properties to the desired extent. This heat treatment after work-forming has been hitherto considered indispensible, because the full increase in physical properties could not be'developed by cooling and aging the article directly after forming.

This heat treatment subsequent to forming is, however, undesirable, because it involves extra time and expense, because of the possibility of increase in grain size which is apt to occur upon heating the formed article above the recrystallization temperature, and also because distortion may frequently result from the heating step, the rapid quench after heat treatment, or from other causes. A further disadvantage of this subsequent heat treatment lies in the possibility of fusing the lowmelting constituents of the alloy. If the alloy is thus burned before forming it will in general be detected by the manner in which the alloy works; but if is occurs during heat treatment after forming, defective articles may be produced and may escape detection.

An object of the invention is to provide a method of work-forming articles from this type of heat treatable aluminum base alloys which renders them capable of having certain of their physical properties improved without the use of the solution heat treatment hitherto necessary subsequent to the forming operation.

Another object of the invention is to provide a method of work-forming heat treatable aluminum base alloys which eliminates the subsequent solution heat treatment heretofore required, and which also substantially eliminates grain growth and burning in the formed article.

I have discovered that no subsequent solution heat treatment of articles made by workforming alloys of the type herein contemplated is necessary to develop the increased physical properties if the alloy stock to be formed is first heated at a suitable solution heat treatment temperature, its temperature then lowered to a suitable forming temperature, and the stock then formed and the formed article immediately cooled quickly enough to retain substantially all of the dissolved constituent in solution, audit is upon this discovery that my invention is predicated. VVork-formed articles made in this manner, and without again heating'to the elevated solution heat treatment temperature, are then aged at room temperature or at a suitable elevated temperature.

In the practice of the invention forming stock composed of alloys of the types described is heated to approximately the characteristic solution heat treatment temperature, that is to a temperature at which-material solution of the excess solid-solution forming constituent will occur. The temperature to which the alloy is heated will depend upon the particular alloy used, and, in the case of alloys A, B and C, it varies from about 480 to 530 C. The forming stock is heated long enough to permit substantially maximum solution of the soluble constituent at the temperature used; and this will of course depend somewhat upon its. mass, composition, and structure.

After being heated for a sufficient period of time the alloy is cooled partially. because as previously mentioned these alloys usually cannot be formed successfully at temperatures as high as those used for heat treatment. This cooling may be performed in air or in water, and while the proper forming temperature varies somewhat according to the alloy used and the forming process employed, it may be said in general to lie between about 400 to 450 G. Since the work-forming tools, such as forging dies or rolls, become heated in these hot working processes, and because heat is developed in the metal during working, the alloy is maintained in a heated condition during the forming operation.

Immediately upon completion of the forming operation the article is removed from the die and cooled rapidly in water, air or other suitable medium.

The articles may then be aged to increase their tensile strength, ductility and hardness. In the ease of alloy A, aging takes place at room temperature, while alloys B and C may be aged at elevated temperatures such as about 150C.

As a specific example of the practice of the invention, forging stock comprising 1% inch square bar of an aluminum base alloy containing about 4: to 5.5 per cent of copper, and about 0.75 per cent each of manganese and silicon was heated at about 510 C.. removed from the furnace, cooled in still air to 435 0., and thereupon forged into an automobile connecting rod. Upon completion of the forging operation, the rod was trimmed and quenched in water, after which it was aged for 30 hours at about 150 C. At the end of this time the average Brinell hardness of the rod was 116, as compared with 82 in the forged condition.

In the example just given it will be observed that the stock was heated within the characteristic solution heat treatment range, and cooled before forging. This cooling was carried only to the point where the alloy could be forged successfully, and as the results show, was not sufficient to prevent the attainment of the desired properties. It has not been found practicable to forge this alloy at temperatures much in excess of that given, and the preliminary high temperature heating is necessary to secure the desired ultimate results. Another connecting rod made by heating the stock at the usual forging temperature of about 435 C. and forging, followed by quenching, failed to show high hardness or any marked increase in hardness after artificial aging, the hardness values as forged and after aging in the same manner as above being 72 and 77, respectively.

A'sample of the same stock heated to 515 C. was removed and cooled in an air blast to 415 C., and forged, trimmed and quenched as before, the total time from removal from the furnace to quenching the rod being'122 seconds. The Brinell hardness of the rod as forged was 82, and after aging 30 hours at alptlmt 150 C. the hardness had increased to l Test bars cut from connecting rods made in a similar manner showed after aging a tensile strength of 56,000 to 58,000 pounds per square inch, with an elongation of 20 to 30 per cent in two inches. Test bars cut from rods made by heating to the forging temperature and forging, followed by quenching, did not show high strength, either before or after aging.

In another actual test, automobile horn resonator discs about inch thick were made from an alloy of the type referred to cooled during 104 seconds in still air to 415 C., when it was forged and quenched. The 'Brinell hardness value after five days at room temperature was 68, and after aging twenty-four hours at 150 C. it rose to 100.

Thefollowing example illustrates the aplicability of my process to work-forming by rolling. A inch slab of an alloy of type A, previously referred to, was heated at about 510 (1, removed from the furnace and cooled in air to about 425 (1., when it was reduced by one pass through the rolls to a thickness of 0.135 inch, and then immediately quenched in water. After aging about two weeks at room temperature the tensile strength of this material was 57,900 pounds per square inch, with an elongation in 2 inches of 18.2 per cent. The value of the process is shown by the fact that when heat treated in the usual manner after rolling, this alloy normally gives a tensile strength of from about 55,000 to 63,000 pounds per square inch with an elongation of about 20 per cent.

Where high ductility or plasticity is required, the work-forming operation should in general not be performed at too low a temperature, as shown by the following example. A 4 inch slab of alloy A was heated at about 525 (3., removed from the furnace and cooled to about 400 0., and rolled in two passes to 0.089 inch thickness. The first pass cooled the metal considerably, so that the second pass involved cold working. This material after aging two weeks at room temperature showed a tensile strength of 57,150 pounds per square inch, and an elongation of 13.7 per cent. Here it will be observed that the tensile strength is well within the range normally developed by heat treatment after rolling, and substantially that of the preceding example, but the elongation is considerably less than that obtained by completing the rolling at a higher temperature, so that the ductility of this material will be less than that produced in the foregoing example.

Thus by selecting preheating temperatures appropriate to the particular alloy, and by partially cooling and forming the thus heated alloy and cooling the article thus formed, it is unnecessary to reheat the article to secure the properties characteristic of the subsequent heat treatment hitherto employed, thereby saving the time and cost involved.

According to the provisions'of the patent statutes, I have explained the principle and mode of operation of my invention, and have given specific examples of how it may be practiced. However, I desire to have it understood that, Within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

I claim: 7

1. The method of forming an article capable of developing increased tensile strength and hardness from an aluminum base alloy containing a solid-solution forming constituent in' excessof the amount soluble at normal temperature, comprising heating the alloy to cause increased solution of the soluble constituent, cooling the alloy to a suitable workforming temperature, work-forming the partially cooled alloy, and cooling the formed article rapidly enough to retain the major portion of the dissolved constituent in solution.

2. The method of forming an article capable of developing increased tensile strength and hardness from an aluminum base allo containing copper in excess of the amount so uble at normal temperature, comprising heating the alloy to cause increased solution of the copper, cooling the alloy to a suitable workforming temperature, working-forming the partially cooled alloy, and cooling the formed article rapidly enough to retain the major portion of the dissolved copper in solution.

3. The method of forming an article capable of developing increased tensile strength and hardness from an aluminum base alloy containing from about 3 to 5.5 per cent of copper, comprising heating the alloy to cause increased solution of the copper, cooling the alloy to a suitable work-forming temperature, work-forming the partially cooled al loy, and cooling the formed article rapidly enough to retain the major portion of the dissolved copper in solution.

4. The method of forming an article capable of developing increased tensile strength and hardness rom an aluminum base alloy containing from about 3 to 5.5 per cent of copper, and about 0.75 per cent each of manganese and silicon, comprising heating the alloy to about 515 C., removing the alloy from the furnace and cooling it to about 440 0., work-forming the alloy and quenching the formedarticle, to retain the major portion of the dissolved copper in solution.

, 5. The method of forming an article capable of developing increased tensile strength and hardness from an aluminum base alloy contalning a solid-solution forming constituent in excess of the amount soluble at normal temperature, comprising heating the alloy ROBERT S. ARCHER.