US2378679A - Rolling magnesium alloys - Google Patents

Description

Patented June 19, 1945 UNITED STATES PATENT OFFICE ROLLING MAGNESIUM ALLOYS Gerhard Ansel, Midland, Mich, mam to The Dow Chemical Company, Midland, Mich a corporation of Michigan N Drawing. Application March '30, 1942,

Serial No. 486,167 I 7 Claims.

This invention relates to an improved method of cold rolling articles formed of magnesium andmagnesium-base alloys.

The rolling of magnesium and magnesiumbase alloy sheet has conventionally been carried out almost entirely at hot-working temperatures; cold rolling has been employed only to a limited extent and primarily for light finishing operations. It has recently been found, however,

that magnesium and magnesium alloys may advantageously beeold-rolled to effect reductions in thickness substantially greater than have previously been used, e. g. reductions of at least 20' per cent, and that the articles so produced'are characterized by a very desirable combination of high strength and high ductility. (Details for the process are set forth in a copending application Serial No. 349,254, filed August 1, 1940,

which has matured into Patent No. 2,294,648 of September 1, 1942, by the present applicant and John C. McDonald.)

While extensive cold rolling is highly advantageous in improving the physical properties of magnesium and magnesium-alloy sheet, application of the process has been somewhat restricted by thefact that the cold rolling must be carried out at relatively low rates of reduction per pass to avoid cracking the metal. As a result, rolling schedules are rather long, with consequent increase in cost of the finished product.

An object of the present invention is to avoid this disadvantage and to provide an improved method of cold rolling magnesium and magnesium-base alloy articles which permits the use of relatively high rates of reduction per. pass without danger of fracturing the metal. Another object is to provide a process by which cold rolling can be carried out at these higher rates of reduction to effect a greater total reduction in thickness than has heretofore been possible.

The invention is based on a discovery relative to an unusual efiect of surface friction on the rolling characteristics of'magnesium and its alloys. Thus, it has often been reported in the art that,' in the cold rolling of metals generally, greater rates of reduction and higher total reductions can be achieved without cracking the metal by lowering the friction between the roll surfaces and the metal being worked, as by pofishing the rolls to a mirror-like finish or by applying lubricant to the-sheet during rolling. It has now been found, however, that with magnesium and magnesium-base alloys an opposite effect is obtained, and that higher reductions per pass and greater total reductions can be achieved the metal passing through them.

' Utilizing these findings as basis, the present invention provides an improved process for cold rolling magnesium and magnesium-base alloy articles in which substantial reductions in thickness are effected without appreciable cracking of the metal, and involves the steps of passing the article repeatedly between rolls the working surfaces of which exhibit a frictional resistance relative to magnesium substantially greater than that of the polished steel rolls invariably used heretofore in rolling this metal.

Prior to cold rolling, according to the invention, the rolls to be used are first treated in any of a' variety of ways to raise the coefiicient of friction of the surface thereof to a value substantially greater than the coeflicient of friction between polished steel and magnesium. For instance, the rolls may be sandblasted carefully according to standard methods to produce a uniformly rough surface. Alternatively, the rolls may be dull-ground in known manner to provide a mat finish thereon. Other methods, such as etching orapplying abrasives to the sheet during rolling, may also be used.

Still another method of increasing the coefflcient of friction of conventional steel rolls is to coat the working surfaces thereof with a film consisting essentially of a magnesium-base alloy. To produce this coating, the rolls are first carefully cleaned, and are then coated by passing a magnesium-base alloy sheet heated to a temperature above 500 F. repeatedly betweenthe rolls without the use of a lubricant. Such treatment is continued until suflicient of the metal forming the magnesium-alloy sheet becomes transferred to the rolls by adhering thereto to form a coating of satisfactory thickness, e. g. 0.005 to 0.025 inch. This magnesium alloy surface on the rolls has particularly desirable frictional characteristics in the subsequent cold rolling.

In practicing the invention, the magnesiumbase alloy article to be cold-rolled is first formed to a thickness substantially above. the desired final thickness, e. g. at least 25 per cent above the latter, as by conventional hot rolling and annealing and is then cold-rolled, preferably without the use of a lubricant, to substantially final thickness, the rolls employed having a frictional resistance relative to magnesium substantially greater than that of polished steel rolls, as already explained. The resulting cold-rolled article may then, if desired, be annealed to develop maximum ductility.

The cold rolling step is preferably carried out at rates of reduction of from 1 to per cent per pass for a number of Passes sufllcient to effect a total reduction of at least per cent. Total reductions of at least per cent may advantageously be produced according to the invention with. many magnesium-base alloys, particularly when using magnesium-coated rolls.

While the process of the invention is, so far as known, broadly useful in cold rolling magnesium and magnesium-base alloys, it is especially applicable in rolling comparatively thin sheet and in the fabrication of magnesium-base alloys Example 3 room temperature on polished steel rolls at a containing a small proportion of manganese as the predominating alloying element.

The following examples will serve further to illustrate the invention, but are not tobe strued as limiting its scope:

Example 1 The working surfaces of a pair of polished steel rolls 16.5 inches in diameter and 66 inches wide were sandblasted to increase markedly the surface frictional resistance thereof relative to magnesium.

Using these rolls, a. 0.085 inch thick sheet of Dowmetal M (a magnesium-base alloy containing 1.5 per cent manganese, balance magnesium),

which had been formed by previous hot rolling and annealing, was rolled at'room temperature by passing it repeatedly through the rolls at an average reduction per pass of about 5.0'per cent until the sheet had been reduced to a thickness Example 2 The working surfaces of a pair of polished steel rolls 12 inches in diameter and 12 inches wide were coated with a film consisting essentially of magnesium by repeatedly passing a magnesium sheet 0.250 inch thick heated at a temperature of 500 F. between the rolls until a coating of magnesium .010 inch thick adhered to the rolls.

Using these rolls, a 0.035 inch thick annealed sheet of Dowmetal M wasrolled at room temperature by passing it repeatedly through the rolls at an average reduction per pairs of about 5.0 per cent until the sheet had been reduced in thickness to 0.014 inch, a total reduction in thickness of per cent. The resulting sheet was entirely free of cracks.

For purposes of comparison, a test, not in accordance with invention, was carried out in-which a 0.035 inch thick Dowmetal M annealed sheet was passed repeatedly at'room temperature between polished steel rolls (no coating of any character being present on the roll surface).

Withthese rolls, even though a rate of reduction of only 2.4 per cent per pass was used, failure of the sheet by cracking occurred after a total reduction of 34 per cent.

reduction per pass of 3.5 per cent, serious crackingpf the sheet occurred after a total reduction of 41 per cent. In another test in which polished steel rolls and an oil lubricant were used during the rolling, and in which the rate of reduction was 3.9 .per cent per pass, cracking of the wmetal M sheet occurred after a 24 per cent total reduction had been reached.

From the foregoing it will be appreciated that, in the process of the invention, substantially greater total reductions in thickness are accomplished than has heretofore been obtained in cold working magnesium alloys; in consequence, the rolled product, after annealing, exhibits strength and ductility superior to those of ordinary cold-rol'led magnesium-alloy sheet. In addition, greater rates of reduction are achieved during the rolling according to the invention, thereby making possible substantial decreases in the total rolling time and the amount of handling of the sheet required to produce a given article.

Other modes of applying the principle of the invention may be employed instead of those ex plained, change being made as regards the details disclosed, provided the step or steps recited in any of the following claims is used.

Iclaim: I v

1. In a process of cold rolling articles formed of magnesium and magnesium-base alloys, the improved method of effecting substantial reductions in thickness without cracking of the metal which comprises passing the article repeatedly between rolls the working surfaces of which exhibit a frictional resistance relative to-magnesium substantially greater than that of polished steel rolls.

2. In a process-of cold rolling magnesium-base alloy sheet, the improved method of effecting a reductionin'thickness substantially in excess of 20 per cent without appreciable cracking of the metal which comprises passing the sheet repeatedly .between rolls the working surfaces of which exhibit a frictional resistance relative to magnesium. substantially greater than that of polished steel rolls; I

3. In a process of cold rolling magnesium-base alloy sheet, the. improved method of efiecting a reduction in thickness substantially in excess of 20 per cent without appreciable cracking of the metal which comprises passing the sheet repeatedly between dull ground steelrolls.

4. In a process of cold rolling magnesium-base alloy sheet, the-improved method of effecting a reduction in thickness substantially in excess of 20 per cent withoutappreciabl'e cracking of the metal which comprises passing the sheet repeatedly between sandblasted steel rolls. I

.5. In a process of cold rolling magnesium-base alloy sheet, the improved method of effecting a reduction in thickness substantially in excess of 20 per cent without appreciable cracking of the metal which comprises passing the sheet repeatedly between rolls the working surfaces of which are provided with a coating consisting essentially of a magnesium-base alloy. 2

6. In a process of producing a high strength ductile magnesium-base alloy sheet wherein a magnesium-base alloy is formed to a thickness at least 25 per cent above the desired final thickness and is then cold-rolled substantially to final thickness, the improved method of efiecting such cold rolling which comprises passing the sheet through rolls the working surfaces or which exhibit a frictional resistance relative to magnesil0 metallic magnesium.

um substantially greater than that of polished steel rolls, a I

7. In a process wherein articles of magnesium and magnesium-base alloys are cold-rolled to effect substantial v reductions in thickness, the

' method of minimizing cracking of the metal during such cold rolling which comprises coating the working surfaces of the .rolls prior to the cold rolling step with a film consisting essentially of GERHARD ANSEL.