US2076568A - Free cutting alloys - Google Patents

Description

Patented Apr. 13, 1937 UNITED STATES PATENT OFFICE 2,076,568 FREE CUTTING ALLOYS Louis W. Kempf and Walter A. Dean,

Cleveland,

Ohio, assignors to Aluminum Company of America, Pennsylvania No Drawing. Application Serial No.

Claims.

range the alloy may be mechanically deformed by the well known commercial processes such as rolling, forging, drawing, or extrusion. containing more than about 6 per cent of are generally used Throughout the entire properties by thermal The addition of zinc to the foregoing aluminum-copper alloys in amounts of 0.1 to-3 per cent serves to increase the tensile strength without substantially affecting the ductility or corrosion resistance. The presence of zinc also tends to increase the hardness of the alloy. Our 5 invention, which is described hereinbelow, is directed to improving the machining quality of such aluminum-copper-zinc alloy.

There are, however, some applications wherein aluminum-copper-zinc alloys as hereinabove 3o disclosed might be conveniently and profitably used except for an inherent disadvantage which militates against their use in the production of certain articles requiring exacting machining operations. Mechanical cutting operations such as drilling, shaping, or lathe-cutting are successfully carried out only by using certain precautions which increase the cost of production and perhaps favor the choice of another metal or alloy which can be machined more readily but which is not so desirable in other respects, as for example, in physical properties. When alloys are diflicult to machine this disadvantage becomes evident, in many cases, through rapid wear of the cutting tool edge, so that frequent tool re-sharpening is required. Despite continual lubrication the machined surface is rough and irregular, and the chip has a tendency to form a continuous curl or spiral which often fouls the tool or the moving parts of the machine. It is immediately apparent that there is need for an alloy of good working characteristics and satisfactory physical properties, yet possessing such favorable machining properties that the complete machining operations may be performed economi- Pittsburgh,

Pa., a corporation of December 28, 1935, 56,542

cally and successfully, and may be productive of a pleasing surface appearance.

Accordingly an object provision of an aluminum base alloy containing economically machined.

Our invention resides in the discovery that the foregoing object is effected by the addition of two or more of the elements lead, tin, thallium, cadmium or bismuth. The aluminum base alloys to which these elements are added in the proportions specified below, are known as free cutting or free machining alloys because they can be machined more rapidly than similar alloys without these elements and yet have as good or a In recognition of this effect we term lead, tin, thallium, cadmiumand bismuth free machining elements. We have further discovered that the simultaneous presence of two or more of these elements is productive of an improvement in free machining characteristics which is considerably greater than that caused by the presence of the same single free machining element. addition of 0.5 per cent of lead of tin to an aluminum base alloy containing about 5 per cent of copper and 2.5 per cent of zinc effects a greater improvement in machining quality than does the addition of 1.0 per cent of either lead or tin singly.

These five elements we believe are unique with respect to their effect on the machining characteristics of aluminum-copper-zinc alloys. It is a fortunate circumstance, therefore, that they are also of relatively low melting point, a fact which makes possible their addition to molten aluminum in the pure state, without the intervention of so-called rich alloys. As a matter of fact, we have observed that of all the metals whose melting point is lower than about 327 C., the melting point of lead, the five elements we have selected are the only ones which are commercially suitable and that impart free cutting characteristics, but do not have undesirable efiects on the fundamental physical properties of the base alloy.

The total amount of free machining elements should not be less than about 0.05 per cent since For example, the and 0.5 per cent total amount of a below this amount there is scarcely any advantageous eflect. We have determined that a maximum limit of about 6 per cent total of two or more of the free machining elements is sufllcient for satisfactory commercial results, since although the free machining effect persists beyond this amount, certain of the other physical properties may be unfavorably affected.

Aluminum-copper-zinc alloys containing two or more of the free machining elements lead, tin, thallium, cadmium and bismuth may be machined more rapidly, with less tool wear, less tool sharpening, better quality of chip and better machined surface than the same base alloys without the free machining additions, and in fact better than the same base alloys containing an equivalent total amount of a single free machining element.

Since aluminum base alloys containing from 3 to 12 per cent of copper have a wide variety of applications we list several alloys each of which may be said to be preferred for a particular purpose. As an alloy for mechanical deformation we suggest an aluminum base alloy containing per cent of copper, 2.5 per cent of zinc and a total of 1 per cent of free cutting constituents, for example, 0.5 per cent of bismuth and 0.5 per cent of cadmium, the balance being aluminum. For an alloy with excellent casting characteristics to be used in the unworked condition we suggest an alloy containing per cent of copper, 2.5 per cent of zinc and a total of 3 per cent of free machining elements, the balance being aluminum.

For certain purposes, notably the improvement of tensile strength, hardness and grain structure, the alloys as hereinabove disclosed may be improved by the addition of one or more of the group of elements composed of .molybdenum, vanadium, titanium, tungsten, zirconium, and chromium. From 0.05 to'l per cent of any one of these elements may be used alone, but if more than one is employed the total amount should not exceed about 2 per cent.

It is characteristic of the five elements lead, tin, thallium, cadmium and bismuth that they form with aluminum a series of alloys of limited liquid solubility. We have reason to believe that the free machining elements are the only elements which exhibit this characteristic, with the possible exception of several metals which are not promise as Within the range disclosed and claimed machining elements may be added without un- We suspect that this characteristic feature of the disclosed elements may be one of the significant factors which contribute to their free machining effect. We believe that this effect is further strengthened by distributing the free machining constituent relatively homogeneously throughout the solid matrix, since these free machining constituents are practically insoluble in the solid aluminum base.

The free machining alloys which have been described hereinabove may be subjected to the thermal treatments well known in the art to improve their strength and hardness. We have found that a solution heat treatment and subsequent aging does not impair the free machining quality of the alloys and in many instances the treatment even tends to improve this property. For many purposes a relatively high strength and hardness are necessary to the successful performance of the machined article and hence the alloy must be heat treated. This treatment is generally applied prior to the machining operation.

As hereinabove indicated the free machining elements, by reason of their low melting point, may be added to the molten aluminum alloy in pure metallic form. However, since some dimculty may be encountered in introducing them in the higher percentages of our disclosed range we prefer to use the method which is more fully described in U. S. Patent No. 1,959,029, issued March 15, 1934. Briefly it involves heating the melt to a somewhat higher temperature than is customary, and vigorously stirring it in excess of a critical period of time.

The term aluminum as used herein and in the appended claims embraces the usual impurities found in aluminum ingot of commercial grade or picked up in the course of the ordinary handling operations incident to melting practice.

We claim:

1'. An aluminum base alloy consisting of about 5 per cent copper, 2.5 per cent zinc, 0.5 per cent lead and 0.5 per cent tin, the balance being aluminum.

2. An aluminum base alloy consisting of about 5 per cent copper, 2.5 per cent zinc, 0.5 per cent bismuth and 0.5 per cent cadmium, the balance being aluminum.

3. A free cutting alloy containing from 3 to 12 per cent of copper, from 0.1 to 3 per cent of zinc and a total of from 0.05 to 6 per cent of at least two of the elements from the following metals, lead, tin, thallium, cadmium, and bismuth, to improve its machining properties, the balance being substantially aluminum.

4. A free cutting alloy containing from 3 to 12 per cent of copper, from 0.1 to 3 per cent of zinc, from 0.05 to 2 per cent of hardening metal from the group composed of molybdenum, vanadium, titanium, tungsten, zirconium and chromium, and a total of from 0.05 to 6 per cent of at least two of the elements from the following metals, lead, tin, thallium, cadmium, and bismuth to improve its machining properties, the balance being substantially aluminum.

5. An aluminum base alloy consisting of about 5 per cent copper, 2.5 per cent zinc, 0.5 per cent bismuth, and 0.5 per cent lead, the balance being aluminum.