US2076570A - Free cutting alloys - Google Patents

Description

30 tainable.

Patented Apr. 13, 1937' UNITED STATES PATENT OFFICE FREE ALLOYS Louis w. Kemp! and hi0, assignors Pennsylvania Walter A. man, to Al America, littsburgh, Pa,

Cleveland, ompany of a corporation of uminum C No Drawing. Application December 28, 1935, Serial No. 56,544

5 Claims. 25-141) It is immediately apparent that there is The invention relates to aluminum base alloys and is particularly concerned with aluminum base alloys containing copper, magnesium and zinc.

Aluminum base alloys containing between 3 5 per cent and 12 per cent of copper have been widely used heretofore. The copper imparts 15 Throughout the entire range of 3 to 12 per cent When alloys are difiicult 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 55 fouls the tool or the moving parts of the machine.

need for an alloy of good working characteristics and satisfactory physical properties, yet

possessingsuch favorable machining properties that the complete machining operations may be performed may be productive of a ance.

Accordingly an object of provision of an aluminum from about 3 per cent economic-ally and successfully, and

pleasing surface appearour invention is the base alloy containing to about 12 percent of copper, from about 0.1 to 1 per cent of magnesium, and from about 0.1 to about 3 per cent of zinc which may chined.

be readily Our invention resides in the foregoing object is of two or more of the lium, cadmium or alloys to which these proportions specified and, economically mathe discovery that efl'ected by the addition elements lead, tin, thalbismuth. The aluminum'base elements are added in below, are known as free the 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 better finished surface. After an extended series of investigations we have discovered that these five metals when added to aluminum base alloys, form a class of alloying elements by reason of their favorable efiect upon the machining properties of these-alloys. effect we term lead, tin, bismuth "free machining further discovered that ence of two or more of these tive of an improvement of magnesium and 1.75 per 1 greater improvement in does the addition of 1.0 per cent of either lead 46 or tin singly.

These flve elements, we believe, are unique with In recognition of this thallium, cadmium and elements. We have I the simultaneous preselements is producin free machining charcent of zinc eflects a machining quality than respect to their eflect on the machining characteristics alloys.

aluminum-copper-magnesium-zinc It is a fortunate circumstance, therefore,

that they are also of relatively low melting point, 50 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 haveobserved that of all the metals whose melting p olnt 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 undesir- 5 able effects 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 below this amount there is scarcely any advantageous efiect. We have determined that a maximum limit of about 6 per cent total of two or i more of the free machining elements is sufiicient for satisfactory commercial results, since although the free machining effect persists beyond this amount, certain of the other physical prop;

erties may be unfavorably affected.

Aluminum-copper-magnesium-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 may be said to be preferred for a particular purpose. As an alloy for mechanical deformation we suggest an aluminum base alloy containing 5 per cent of copper, 0.5 per cent of magnesium, 1.75 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 10 per cent of copper, 0.5 per cent of magnesium, 2 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 strulcture, the alloys as hereinabove disclosed maybe 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 1' 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 regarded as having any commercial 1 promise as additions to aluminum base alloys. Within the range disclosed and claimed however the free machining elements may be added without unusual diificulty. 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 each of which 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 difficulty 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 commercialgrade 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, 0.5 per cent magnesium, 1.75 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, 0.5 per cent magnesium, 1.75 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, from 0.1 to l per cent magnesium, 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 0.1 to 1 per cent of magnesium, from 0.05 to 2 per cent of hardening metal from the group composed of molybdenum, titanium, tungsten, zirconium and chromium, and a total of from 0.05 to 6per 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, 0.5 per cent magnesium, 1.75 per cent zinc, 0.5 per cent bismuth, and 0.5 per cent lead, the balance being aluminum.

LOUIS w. KEMPF. WALTER A. DEAN.

vanadium,