US3407062A - Method of impact extruding - Google Patents

Description

United States Patent 3,407,062 METHOD OF IMPACT EXTRUDING Russell E. Matthews and George S. Foerster, Midland,

Mich., assignors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware No Drawing. Filed Jan. 5, 1967, Ser. No. 607,369 12 Claims. (Cl. 75-214) ABSTRACT OF THE DISCLOSURE The improvement in method of producing impact extruded articles from a light metal alloy selected from the group consisting of magnesium-base alloys and aluminum-base alloys according to which there is provided light metal alloy in particulate form having at least one-third of the particles with a longest to shortest dimension ratio of at least about two and subjecting a charge of such particulate metal to an impact extrusion operation at a temperature of at least about 600 F. The particulate metal includes chips prepared from castings as well as flattened atomized pellets.

The invention relates to an improvement in a method of impact extruding and more particularly relates to a meth- 0d of making impact extrusions from pelletized magnesium or aluminum-base alloys.

For the purposes of the following description and the appended claims magnesium-base alloys or aluminumbase alloys are each understood to contain at least 70 percent by weight of the base metal.

'In the making of an impact extrusion, the work piece or blank is placed in a die cavity and forced to flow around a punch as the punch moves toward the bottom of the cavity. The top of a can formed in this manner is not heavily worked and heretofore has exhibited a tendency to crack if the extrusion blank is made from particulate metal. When cracks appear, they tend to propagate vertically towards the bottom of the can. The cracks can be removed by trimming off the top of the can but metal efficiency is markedly reduced by such trimming.

It is a principal object of the present invention to overcome the difliculties heretofore encountered in making impact extrusions using a die blank fabricated from particulate metal and to provide extruded cans and the like which are substantially free of the tendency to crack adjacent the top.

This and other objects and advantages of the present invention will be more clearly understood by those skilled in the art upon becoming familiar with the following description and the illustrative examples.

It has now been discovered that upon providing magnesium-base alloy or aluminum-base alloy in particulate form wherein at least about /3 of the particles have a ratio of the longest to shortest dimension of at least about 2, compressing a charge of such particulate metal into a compact and impact extruding the compact at an elevated temperature, the resulting impact extruded article is substantially free of any tendency to crack in the longitudinal direction of extrusion, e.g., from the top of the bottom of an extruded can.

Preferably, in the practice of the invention, at least about 50 percent of the metal particles employed will have a length/width dimension ratio of 2. More preferably, at least about 50 percent of the particles have a longest dimension to shortest dimension ratio of at least about 3. Even more preferably, substantially all of the particles have a longest to shortest dimension ratio of at least 5. The best form of particulate metal to be employed in the practice of the invention appears to be platelets.

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The particulate material used in the present invention may be made in several different ways. Particles may be made by mechanically reducing solidified cast light metal alloy, for example, an ingot. Mechanical reduction is accomplished by machining or chipping the cast metal into pieces, flakes or chips. Typically suitable chips are 0.25 inch long and 0.013 inch thick.

Particles of magnesium-base alloy or aluminum-base alloy are also obtained by jet atomizing or wheel atomizing, as well understood in the art. The resulting pellets usually have particle diameters passing about a No. 20 to a No. 200 sieve (U.S. Sieve Series). A charge of such pellets and a quantity of hardened tool steel balls, alumina cylinders, or similar inert, relatively dense objects ordinarily employed for ball mill grinding operations are placed in a suitable vessel such as a rotating ball mill or an oscillating paint shaker. When the vessel is rotated, tumbled, or oscillated, the movement of the steel balls or other heavy objects results in a peening or flattening of the pellets. Generally, the pellets are sufliciently flattened in about 15 to 30 minutes of motion of the vessel.

In each of several repetitive tests, a 50 gram charge of atomized pellets formed of the ASTM designated magnesium-base al'loy, ZKlO, was placed in an enclosed vessel along with steel balls each /13 in diameter. The vessel was attached to a paint shaker and shaken for either 5 or 15 minutes. At the end of the period of shaking, the contents of the vessel were separated from the steel balls with a coarse screen and graded by pouring successive portions onto an inclined table where rounder pieces rolled away from flatter pieces. Examination of the fractions obtained with the inclined grading table showed that pellets which had been peened about 5 minutes consisted of (1) about 34 percent of pellets which were slightly deformed, some being flattened on one side; (2) about 52 percent of pellets which were considerably more deformed, about A of the pellets being flat on both sides; and (3) about 14 percent of pellets which were generally flat platelets having longest length/shortest dimension ratios in the range of 5 to 10, typical particles having a longest dimension of 0,020" and a shortest dimension of 0.003.

Pellets which had been peened 15 minutes consisted of (1) about 12 percent of pellets which were slightly deformed, some being flattened on one side only; (2) about 54 percent of pellets which were more significantly deformed, approximately of the pellets being flat on both sides; and (3) about 34 percent of the pellets being generally flat platelets and having dimension ratios in the range of 2 to about 5, typical platelets being 0.003" thick and about 0.010" in greatest length.

If desired, the pellets employed may also be flattened by passing the pellets between the rolls of a mill arranged for vertically downward feeding and the rolls being drawn together very tightly. Such method is further described in US. Patent No. 3,179,516.

In a practical way, the average longest dimension to shortest dimension ratio of flattened atomized pellets is indicated by the bulk density of a quantity of the flattened platelets. The lightest attainable bulk density on flattening pellets of these alloys appears to be about 45 percent of the theoretical or maximum attainable density. The theoretical density of magnesium is 1.74 grams per cubic centimeter. The theoretical density of aluminum is 2.70 grams per cubic centimeter. The theoretical density for the alloys of magnesium and of aluminum vary somewhat from these values. For the purposes of the invention the bulk density of flattened pellets must be no higher than about 60 percent of such theoretical density and more preferably is 55 percent or less of the theoretical density. Further, it appears that the longest dimension to shortest dimension ratios are greater than 10 for at least about half of the pellets when the bulk density is less than 60 percent of the theoretical density.

The particulate metal obtained as described hereinabove is compressed into a compact under the requisite conditions of pressure and temperature to form a rigid coherent compact having at least 75 percent of the theoretical density of the metal. In general, it is satisfactory to place a charge of the particles in a container having one end blocked off, e.g., the container of an extrusion press having the die opening blocked off, and to compress the metal under about 50,000 to 150,000 pounds per square inch pressure. Aluminum is ordinarily compressed at ambient room temperature, heating being avoided because of lubrication problems. Magnesium alloy pellets may be compressed cold or hot. In the latter case, temperatures of the order of 500 to 700 F. are employed. The compact is preheated sufliciently to permit impact extrusion to proceed Without cracking the work piece, generally to an elevated temperature above about 300 F. and typically at a temperature in the range of about 500 to 800 F. The preheated compact is then placed over the die opening and impact extruded at about the preheat temperature in a manner Well understood in the art.

In the event problems are met in forming a coherent compact by cold pressing, it may be found necessary to use freshly made machine chips or freshly flattened pellets. Also, it may be desirable to store the freshly peened particles or freshly made chips in a dry, cool place or in a substantially oxygen-free, dry enclosed zone. The pellets are preferably sufficiently worked to provide a bright new surface. Therefore, the peening of pellets is preferably carried out for a period of at last 15 minutes. In the case of magnesium, the problem of forming a compact usually may be overcome by simply compacting the magnesium at an elevated temperature above about 500 F.

If desired, the step of compacting prior to impact extruding may be avoided 'by simply placing a charge of the elongated or flattened metal particles of the invention directly in the die cavity, bringing the particles to an elevated temperature of at least about 600 F. and impact extruding the charge of particles as a rigid, substantially fully densified article.

More generally, the extruded article is made by placing a charge of the particulate metal in the die cavity and compressing the charge with the punch under the requisite conditions of temperature and pressure to make a coherent rigid mass. Such mass is a compact generally having a lower density than a fully densified compact or extrusion. Impact extrusion is then carried out as a separate step, using the same common die, if desired, or a different die, upon advancing an appropriate size of punch with sufficient clearance between punch and die for extrusion and under additional pressure, viz, sufficient pressure to bring about impact extrusion. The common die, e.g., may be used in a turntable arrangement wherein the turntable is rotated to advance the die from a larger to a smaller punch.

The extruded article obtained according to the practice of the invention is substantially not subject to longitudinal cracking adjacent the top of the article, and wherein cracking does occur at the top of the article, it does not propagate far from the top.

The following examples serve to illustrate the invention and do not limit the scope thereof.

EXAMPLES In each of a series of runs carried out in quadruplicate to illustrate the method of the invention, 50 gram quantities of jet atomized pellets formed of a magnesiumbase alloy having the A.S.T.M. designation ZKlO were, respectively, peened for various periods of time in a paint shaker having a volume of about 900 cubic centimeters and containing ninety 45" steel balls. In each case, the bulk density obtained on peening was determined by measuring the volume of the charge before and after peening. The volume measurement was made by observing the height of the charge when placed in a graduated cylinder and after tapping the graduated cylinder firmly. Bulk density was computed in terms of the density of the pellets as follows:

bulk density:

100 X l gght of unpeened charge) heim density Of as-atom1zed pGllGtS Atomized pellets of magnesium-base alloy are known to have about /3 of the theoretical density of the base metal. The percent of theoretical density was computed by multiplying the foregoing percentage by /3.

Each charge of peened pellets was cold compacted under a pressure of 25 tons in a 1.175 inch diameter container of an extrusion press having the die opening blocked off. Each compact, in turn, was placed in the die cavity of an impact extrusion press maintined at a temperature of about 700 F. After two minutes of preheating in the die cavity, the compact was impact extruded into a cylindrical can 1.187 inches in diameter and about 2.5 inches in length with a side-wall thickness of about 0.062 inch and a bottom wall thickness of about 0.10 inch. Each impact extrusion was examined visually for longitudinal cracking adjacent the top of the can. The peening times, the bulk densities obtained, and the results of the visual examination of the impact extrusion are summarized in the following table, bulk densities being averaged for the quadruplicate tests.

Run Peening time, Bulk density, Results on No. minutes percent of impact theoretical extrusion 2. 5 60 All 4 cracked. 5 64 2 0i 4 cracked. 5 60 3 of 4 cracked.

10 60 2 of 4 cracked.

10 54 None cracked.

15 54 1 of 4 cracked.

20 47 None cracked.

The results of runs number 5 to 10 in the table demonstrate the importance of employing an adequate proportion of the sufficiently elongated or flattened pellets, in the charge to be compacted and impact extruded, as indicated by the bulk density determination.

In an additional run made by way of comparison, the as-atomized pellets without being peened, were employed as a control. In this control study, no coherent compact was formed on cold compressing.

As an additional example of the method of the invention, 50 grams of jet atomized pellets formed of aluminum alloy consisting of about 2 percent by weight of silicon and the balance aluminum were peened for 30 minutes in a paint shaker having a volume of about 900 cubic centimeters and containing ninety inch diameter steel balls. The so-trcated charge of pellets exhibited a bulk density of about 50 percent of the theoretical density for aluminum. The charge of peened pellets was cold pressed into a compact having a density about percent of theoretical for aluminum. The compact was preheated for 2 minutes by placing it in the dies of an extrusion press maintained at 500 F. Then the compact was impact extruded into a cylindrical can. Visual examination of the cylindrical can showed no longitudinal cracking adjacent the top thereof.

The method of the invention having been thus fully ticles from particulate metal formed of a light metal alloy selected from the group consisting of magnesium-base alloys and aluminum-base alloys, by subjecting a charge of such particulate metal to an impact extrusion operation at an elevated temperature thereby to form a rigid, substantially fully densified impact extruded article, the improvement which comprises: employing as the charge a particulate metal of which at least /a of the particles have a longest to shortest dimension ratio of at least about 2.

2. The method as in claim 1 in which the particulate light metal alloy is provided by mechanically reducing solidified cast light metal alloy to chips.

3. The method as in claim 1 in which the particulate light metal alloy is provided by mechanically flattening atomized pellets of light metal alloy.

4. In the method of producing impact extruded ar ticles from particulate metal formed of a light metal alloy selected from the group consisting of magnesium-base alloys and aluminum-base alloys by compressing a charge of such particulate metal under the requisite conditions of temperature and pressure to form a rigid coherent compact and subjecting the resulting compact to an impact extrusion operation at an elevated temperature, the improvement which comprises, employing as the charge a particulate metal of which at least 50 percent of the particles have longest to shortest dimension ratios of at least about 2.

5. The method as in claim 4 in which the particulate light metal alloy is provided by mechanically reducing solidified cast light metal alloy to chips.

6. The method as in claim 4 in which the particulate light metal alloy is provided by mechanically flattening atomized pellets of light metal alloy.

7. The method as in claim 6 in which the mechanically flattened atomized pellets have the form of platelets.

8. The method as in claim 4 in which substantially all of the particulate particles have longest to shortest dimension ratios of at least about 5.

9. The method as in claim 4 in which at least about /2 of the particulate particles have longest to shortest dimension ratios of at least about 3.

10. The method as in claim 4 in which the particulate metal is an aluminum-base alloy, and the compressing of said metal into a compact is carried out at substantially ambient room temperature.

11. The method as in claim 4 in which the particulate metal is a magnesium-base alloy and the compressing of said metal into a compact is carried out at a temperature in the range of from about 500 to about 700 F.

12. The method as in claim 4 in which the steps of first compressing the particulate metal into a compact and subsequent impact extruding the resulting compact are performed sequentially in a common die.

W lletere nces Cited UNITED STATES PATENTS 2,842,440 7/1958 Nacht -2 14 X 3,126,096 3/ 1964 Gerard. 3,160,502 12/1964 Quartullo. 3,178,280 3/1965 McGee 75-211 X 3,189,988 6/1965 Crane 29420.5 3,219,490 11/ 1965 Foerster 75--226 X 3,276,867 10/ 1966 Brite 75-226 X FOREIGN PATENTS 464,727 3/ 1937 Great Britain.

542,268 6/ 1957 Canada.

686,673 5/ 1964 Canada.

706,486 3/ 1954 Great Britain.

768,204 2/1957 Great Britain.

797,577 7/1958 Great Britain.

CARL D. QUARFORTH, Primary Examiner.

A. J. STEINER, Assistant Examiner.