GB2046784A

GB2046784A - Method for solution heat treatment of 6201 aluminum alloy

Info

Publication number: GB2046784A
Application number: GB7943706A
Authority: GB
Original assignee: Southwire Co LLC
Current assignee: Southwire Co LLC
Priority date: 1978-12-26
Filing date: 1979-12-19
Publication date: 1980-11-19
Also published as: BR7908465A; DE2951911A1; FR2445389B1; IT7951176A0; GB2046784B; NL7909310A; NO794091L; AU4656079A; FI65030C; IN153512B; ES481719A1; JPS55119141A; CA1133805A; AU531337B2; ZA796999B; IT1164818B; FR2445389A1; ATA815579A; SE7910553L; CH651323A5

Abstract

A method of continuously manufacturing a hot-formed heat-treatable aluminum base alloy product having a substantially extended shelf life comprises casting a molten aluminum base alloy metal, cooling the cast metal during casting at a rate at which inverse segregation will be substantially minimized, raising the temperature of the cast metal, prior to the initiation of the hot forming, to a temperature above the temperature at which the alloy metals will precipitate substantially, subsequently reducing the metal temperature from the solutionizing temperature of the metal to a temperature at which no substantial immediate precipitation occurs, within a time interval short enough to prevent any substantial precipitation, and so controlling the solution heat treatment time and temperature that the hot-formed product has controlled precipitation during natural aging.

Description

SPECIFICATION Method for solution heat treatment of 6201 aluminum alloy 6201 Aluminum Alloy is a high strength aluminum magnesium silicon alloy which in wire form and in the heat treated condition has a tensile strength of over 317.106 Pa (46,000 PSI), elongation greater than 3 percent, and an electrical conductivity greater than 52.5 percent IACS.In the past, 6201 Aluminum Alloy redraw rod and similar aluminum alloy redraw rods have been manufactured for commercial use by a plurality of separate steps which include DC casting an aluminum ingot reheating the ingot to about 370 to 455"C (700 to 850"F), hot rolling the cast ingot to redraw rod and solutionising the rod at a temperature of approximately 450"C (1000"F) and water quenching the rod.The rod is cold drawn to form wire, and the wire is artificially aged at temperatures between 120 and 230"C (250 and 450"F). This procedure is capable of producing wire having tensile strength and electrical and conductivity characteristics which are similar to or in excess of those for 6201 Aluminum.

While the foregoing procedure produces an acceptable product, such a batch process or noncontinuous casting process, is capable of producing only a limited amount of rod; that is, the given size billet will produce only a corresponding mass of rod, and the lengths of separately produced rod must be welded together to form longer lengths or rod. When the billet is reheated and rolled to form rod, it is customary to crop the leading end of the rod since it is of an inferior quality. Thus a substantial amount of waste is experienced in the former procedure.

An elongated rod which comprises several lengths of batch produced products welded together will include poor grain structure at the places where it is welded together, which affects tensile strength and conductivity. Furthermore, it is virtually impossible to create identical conditions in the reheating and rolling of different billets, and the lengths of rod welded together will usually have different grain characteristics.

In order to reheat the rod in this batch system, the rod must be carefully handled in order to achieve uniform heating and in order to produce a uniform product. For instance, the oven in which the rod is placed for solutionizing must create relatively even heat distribution in order that the rod be uniformly heated. Furthermore, the rod usually must be arranged so that there is enough circulation of air or gases in the oven between the coils to insure proper heat distribution. It is customary to place individual coils of rod on portable racks which space the coils from each other for this purpose; however, the racks occupy space in the oven and reduce the volume of rod which can be heated.While the purpose of reheating the rod is to solutionise the rod, it is desirable to keep the rod from reaching a temperature substantially higher than its solutionizing temperature since the overlapping portions of the rod in the coils in the rod tend to become tacked or welded together. This tacking together of the portions of the rod creates surface blemishes on the rod when pulled apart, and frequently the coils remain tacked together so that several coils of rod tend to pay out together. Thus even heat distribution within the solutionising oven is a practical necessity so that the rod can be rapidly and uniformly solutionised to hold the hazard of rod tacking to a minimum.

The prior art batch process provides a substantial amount of time in which the aluminum can oxidize, as when the cast ingot cools or is being reheated, when the rod from the rolling mill cools or is being reheated for solutionizing, and when the solutionized rod from the reheating oven cools. The result is that the rod becomes substantially oxidized, which makes it relatively hard for redrawing purposes, and which causes a rod to have a relatively dull finish. Further, a highly oxidized and hard rod is more difficult to draw and the dies used for drawing deteriorate rapidly.Thus, the separate steps required in the prior art batch process for forming 6201 Aluminum Alloy rod are expensive in that separate handling of the rod is required between and during each step, the product must be handled in a careful manner, and extra equipment must be available and maintained to handle this product.

An improved method for continuously casting and rolling 6201 Aluminum Alloy was described in U.S. Patent No. 3,613,767. Briefly described, the invention'of U.S. Patent No.

3,613,767 comprised a method of continuously manufacturing aluminum base alloy rod, such as 6201 Aluminum Alloy rod, without the necessity of reheating the ingot or the rod during the process. The bar emerging from a continuous casting machine was passed through a rolling mill, a quench tube, and then cooled in a continuous process. The heat of the cast bar emerging from the continous casting machine was not dissipated and the bar temperature was maintained in the solutionizing temperature range of the metal as a rod was passed to the rolling mill.The rod was hot worked in the rolling mill and quenched immediately as it emerged from the rolling mill so that the time lapse from the point where the bar entered the rolling mill to where the rod was quenched to a temperature level below the crystallization temperature of the alloy metals was less than the time required for the alloy metals to precipitate to the grain boundaries of the metal. After the rod was quenched it was at a temperature below the temperature where immediate and substantial precipitation occurs. When the rod was subsequently cold drawn into wire it had an unusually high tensile strength and a relatively high electrical conductivity, and an unusually bright appearance. Thus, the major problems of separate handling between each of the steps in the prior art process were eliminated by the practice of the invention disclosed in U.S. Patent No. 3,613,767.However, the solution to the problem inherent in the prior art batch process for preparing 6201 Aluminum Alloy resulted in an aluminum alloy rod, which due to the heat loss between the casting wheel of the continuous casting machine and the point at which the bar entered the rolling mill, had large precipitates on the order of 2 mm (20,000 angstrom units) in size formed therein because of the relatively high temperature at which precipitation was occurring. Also the solution to the problems caused by batch preparation of 6201 Aluminum Alloy by the method of U.S. Patent No. 3,613,767 created an entirely new problem. In Column 5 beginning at line 38 of Patent No. 3,613,767 the following statement is found: "It has been found that the temperature and other conditions in the process can be varied within reasonable limits without detriment to the characteristics of the product.For instance, the temperature of the molten metal in the pouring pot and the metal bar extracted from the casting wheel appear to have no effect on the quality of the 6201 alloy rod as long as the temperature is not lowered below the solutionizing temperature".

While this statement may be true with respect to the alloy properties of 6201 it is incorrect with respect to the properties of the cast bar and the rod rolled from the cast bar. U.S. Patent No. 3,613,767 describes a method of continuously casting 6201 which requires that the cast bar exit the casting wheel at a temperature above the solutionizing temperature and remain above this temperature until the cast bar enters the rolling mill where hot working and quenching subsequently occur. In order to meet this requirement the cast bar of U.S. Patent No.

3,613,767 must exit the casting wheel at a temperature substantially above the solutionizing temperature of the alloy. To remove the cast bar from the casting wheel at the temperatures taught by U.S. Patent No. 3,613,767 the bar must be cooled in such a way that the bar does not become totally solid until it reaches a point on the casting wheel that molten metal cannot flow into, and fill, voids created in the bar by the shrinkage of the metal in the casting mold during solidification. If such voids are created on the exterior portions of the cast bar oxidation will occur within the void and when the bar is rolled oxide inclusions will be trapped within the resultant rod causing the rod to become brittle at the points where the oxide inclusions occur thereby significantly decreasing the drawability of the rod.If solidification shrinkage voids occur in the interior of the rod where oxidation cannot such voids will cause internal micro-cracking which significantly affects the elongation of the rod thereby directly affecting the post cold working characteristics of the rod.

It has been found that the solutionizing temperature of 6201 alloy varies according to the concentration of alloying elements present within the alloy in that the higher the concentration of alloying elements present the lower the solutionizing temperature range of the alloy.

Therefore given the range of concentrations acceptable within 6201 alloy the solutionizing temperature may vary from about 455"C (850"F) to about 615"C (1140"F). Accordingly U.S.

Patent No. 3,613,767 does not provide an acceptable method for continuously producing 6201 alloy rod having alloying element concentrations in the range which cause the alloy to solutionize at temperatures in the upper portion of the 6201 alloy solutionizing temperature range. Accordingly, there still remains significant improvements to be made in a process for continuously casting heat treatable aluminum alloy rod from aluminum alloys such as 6201.

For the purpose of clarity, heat treatable aluminum alloys as used in this specification shall means those aluminum alloys which contain alloying elements which have a high solid solubility in aluminum at high temperatures and low solid solubility in aluminum when cooled to room temperature. These alloys harden by precipitation of a second phase during heat treatment and the alloying elements are kept in solution by rapid quenching from high temperatures.

For the purpose of clarity, wrought aluminum alloys as used in this specification shall mean those aluminum alloys which contain alloying elements which have low solid solubility in aluminum at high temperatures as well as at low temperatures. These alloys normally harden by work hardening which is a hardening mechanism which operates during cold working of the alloy.

Thus, it is an object of this invention to provide an improved method for producing aluminum alloy products from heat treatable aluminum alloys.

Another object of this invention is to provide a method of continuously manufacturing a heat treatable aluminum alloy rod without the necessity of reheating an ingot or rod to produce a product having a high tensile strength and high conductivity characteristics.

Another object of this invention is to provide an improved 6201 aluminum produce and a method for forming such a product without the formation of large precipitated intermetallic particles in the grain structure.

Another object of this invention is to provide an economical and expedient method for manufacturing 6201 aluminum alloy rod.

Still another object of this invention is to provide a method for continuously manufacturing a heat treatable 6201 aluminum alloy rod from 6201 aluminum alloys having solutionizing temperatures within the range of from about 455"C (850"F) to about 580"C (1080"F).

Yet another object of this invention is to provide a method for continuously casting and rolling a heat treatable 6201 aluminum alloy rod wherein the cast bar is not subject to solidification shrinkage.

Another object of this invention is to provide an improved 6201 heat treatable aluminum alloy product with a more uniform heat treatment along its entire length. Other objects, features and advantages of the present invention will become apparent upon reading the following specifications when taken in conjunction with the accompanying drawings.

Figure 1 is a schematic side elevational view of a casting machine, rolling mill, quenching tube, and coiler utilized in the procedure herein set forth.

Figure 2 is a ternary diagram which graphically represents the solubility of magnesium, silicon and the intermetallic compound magnesium, silicon and the intermetallic compound magnesium silicide in aluminim at various temperatures.

Figure 3 is a graphical representation of the effect of heat treating 6201 aluminium alloy by the present invention compared to prior art method of preparing 6201 aluminum alloy.

Referring now in particular to the drawings, in which like numerals indicate like parts throughout the several views, Fig. 1 shows a casting machine 10, a heater 11, a rolling mill 12, a quench tube assembly 13, and coiler 14. In summary, the process of the present invention comprises pouring molten metal from a furnace (not shown) into a casting wheel 1 0a of casting machine 10.The molten metal is cooled and solidified in casting wheel 1 0a and extracted as a solid bar 1 5 at a temperature below 505"C (940"F) and is guided toward and through heater 11 wherein the solid bar 1 5 is continuously heated until the temperature of the bar is within the range of from about 455"C (850"F) to about 580"C (1080"F). The heated bar 1 5 is then guided toward and through rolling mill 12.The product is lengthened and reduced in its crosssectional area within rolling mill 12, and emerges as a wrought rod 1 7. Rod 1 7 is passed through quench tube assejbly 1 3 which includes first stage quench tube 18, pinch rollers 19, second stage quench tube 20, pinch rollers 21, and rod conduit 22. The rod emerges from rod conduit 22 and is formed into coils by coiler 14. Pump 23 receives the quenching liquid from sump 24 and pressurizes first stage quench tube 1 8. The quenching liquid is passed through quench tube 1 8 in a direction of flow which is along the path of travel of rod 1 7 and is passed through a conduit system to cooling tower 26, where it is cooled and recirculated back to sump 24.Pump 27 receives quenching liquid from sump 28 and pressurizes second stage quench tube 20. The quenching liquid of the second stage quench tube is passed through quench tube 20 in a counterflow relationship with the respect of the movement of the rod 17, and is passed through a conduit system to cooling tower 31 where it is cooled and recirculated back to sump 28. Thus, the quenching liquids are maintained at controlled temperatures during the quenching process.

In more detail, the molten metal process through the apparatus is a heat treatable aluminum alloy. If the product to be formed is to be 6201 aluminum alloy, the ranges of silicon and magnesium contents are from about 0.50 to about 0.90 percent, and from about 0.60 to about 0.90 percent, respectively. The range of silicon and magnesium alloys can vary in this metal beyond the range of 6201 alloy to 0.3 to about 1.2 percent and to 0.3 to about 1.2 percent, respectively, if desired.The metal in its molten state is poured through a fibreglass screen into a holding pot maintained at a temperature above 650"C (1200"F), usually at about 687"C (1270"F). From a holding pot, the metal is poured into casting wheel 10a where it is cooled and soidified into a cast bar 1 5 at a rate at which inverse segregation will be substantially minimized, for example at a rate of from about 13.3"C (24"F) per second when casting a 21.3 sq. cm (3.3 square inch) bar at a rate of 9m (30 feet) per minute to about 18"C (32"F) per second when casting a bar of equal cross-section at a casting rate of 1 2m (40 feet) per minute and approximately 28"C (50"F) per second when casting a bdr of equal cross section at a casting rate of 1 5 m (50 feet) per minute. The cast bar is stripped from casting wheel 1 0a at a temperature of from about 370"C (700"F) to about 505"C (940"F) and passed to and through heater 11 wherein the temperature of the cast bar is increased to a point at which the alloying elements are solutionized.Heater 11 continuously supplies energy to the rod thereby increasing the temperature of rod 1 5 to from about 455"C, (850"F) to about 580"C (1080"F), usually to from about 510"C (950"F) to about 550"C (1020"F) and depending upon the alloy composition to from about 550"C (1020"F) to about 580"C (1080"F). As the cast bar exits heater 11 it is guided toward and through rolling mill 12, the bar is hot formed and coated with a soluble oil concentration maintained at about 40 percent and at a temperature below 93"C (200"F) usually at about 70"C (160"F). Rolling mill 1 2 includes a plurality of roll stands which compress the cast bar alternatively from top to bottom and side to side, which functions to lengthen the cast bar and reduce the cross-sectional area of the cast bar, so that the cast bar is progressively formed into redraw rod 1 7. The volume of the soluble oil concentration in rolling mill 1 2 is maintained at a level of about two-thirds the volume in a typical continuous casting system for EC rod.The temperature and volume of the coolant applied to the rod in the rolling mill are adjustable so that when the rod 1 7 emerges from rolling mill 12, the temperature of the rod is at a level so that the rod is still within its hot forming temperature range, which is usually above 340"C (650 F), so that the alloy metals have not precipitated from th aluminum. The low volume of coolant applied to the rod in the rolling mill requires a higher concentration of lubricant, approximately 40 percent solution as compared to approximately 10 percent for an EC rod system, and the flow is adjusted so that approximately equal flow of coolant is maintained at each roll stand.

Fig. 2 is a Ternary diagram which graphically represents the solubility of magnesium, silicon and magnesium silicide in aluminum at various temperatures ranging from 440"C or 825"F to 535"C or 995"F.

Straight line 40 represents the increase in solubility of magnesium silicon and magnesium silicide in the 6201 alloy system as temperature increases to approximately 535"C (995"F).

Point 42 on straight line 40 represents the amount of magnesium, silicon and magnesium silicide which is in solution in a continuously cast rod of 6201 aluminum alloy when the rod has been treated by the prior art method of heat treating continuously cast 6201 alloy. Point 43 represents the amount of magnesium, magnesium silicide and silicon which is retained in the 6201 alloy system in solution when a continuously cast rod of 6201 aluminum alloy is heat treated by the present invention. As can be seen from the diagram of Fig. 2, there is a 1 62 percent increase in the amount of magnesium silicide in solution in the 6201 alloy system when the alloy is continuously cast and rolled into a rod and is heat treated according to the present invention during the continuous caasting and rolling operation.

An example of the improved properties result from the increased amount of magnesium silicide in solution in the alloy matrix prior to aging and precipitation follows. A cast bar was continuously cast using the prior art method for heat treating continuously cast 6201 aluminum alloy and the following results were obtained. Ultimate tensile strength of the final wire was 315, 106Pa (45,700 PSI) with an elongation of 8.3 percent and a conductivity of 52.5 percent IACS. After establishing the above properties as a base line, the bar temperature at a point between the casting machine and the entry of the bar into the rolling mill was raised from 480"C (900"F) to 550'C (1020"F) by the method of the present invention.The bar was then rolled into a rod and made into wire and the physical properties of the wire made from the bar treated according to the present invention were as follows: Ultimate tensile strength-350. 1 06Pa (50,800 PSI) Elongation 7.9% Conductivity 52.5% Fig. 3 is a graphical representation of properties resulting from prior art heat treatment of continuously cast 6201 aluminum alloy rod and 6201 alloy rod continuously cast and heat treated according to the present invention wherein curve 50 depicts the relationship between the conductivity and ultimate tensile strength of wire fabricated from 6201 aluminum alloy rod processed by prior art techniques and curve 52 depicts the relationship between the conductivity and ultimate tensile strength of wire fabricated from 6201 aluminum alloy rod processed by the method of the present invention.

While this invention has been described in detail with particular reference to preferred embodiments thereof, it will be understood that variations and modifications can be effective within the spirit and scope of the invention as described hereinbefore and as defined in the appended claims.

Claims

1. A method of manufacturing a heat treatable hot formed aluminum base alloy product having a substantially extending shelf like comprising: casting a molten aluminum base alloy metal within a continuous casting mold, cooling said molten aluminum base alloy metal during casting to a temperature below 505"C (940"F), at a rate at which inverse segregation will be substantially minimized, to form a cast bar; continuously removing said cast bar from said continuous casting mold and heating said cast bar, prior to the initiation of the step of hot forming the cast bar, to a temperature above the temperature at which the alloying metals would substantially precipitate; initiating the step of hot forming the cast bar while the cast bar is at a temperature within the hot forming temperature range of the metal and which is a solutionizing temperature of the metal, continuing the hot forming process while maintaining the temperature of the cast bar within the hot forming temperature range, reducing the temperature of the bar after the step of hot forming the cast bar, and controlling the temperature of the bar during the hot forming step and the temperature reducing step so as to reduce the temperature of the bar rom the solutionizing temperature to a temperature at which no substantial immediate precipitation occurs within the time interval before which substantial precipitation ocurs and controlling the solution heat treatment temperature of the bar within the solutionizing temperature range and controlling the time the bar is within the solutionizing temperature range such that said product has controlled precipitation during natural aging.

2. A method of manufacturing an aluminum base alloy rod having a substantially extended shelf life containing from about 0.3 to about 1.2 weight percent silicon, about 0.3 to about 1.2 weight percent magnesium and the remainder essentially aluminum comprising the steps of: (a) pouring a molten aluminum base alloy containing from about 0.3 to about 1.2 weight percent silicon, about 0.3 to about 1.2 weight percent magnesium and the remainder essentially aluminum into the casting groove of a continuous casting wheel at a temperature above the melting point of the aluminum base alloy; (b) cooling the molten aluminum base alloy in the casting groove at a rate at which inverse segregation will be substantially minimized to form a cast bar; (c) removing the cast bar from the casting groove at a temperature below 505"C (940"F).

(d) passing the cast bar through a heater and raising the temperature of the cast bar to a temperature above the temperature at which the alloying metals would substantially precipitate; (e) continuously hot forming the cast aluminum base metal to form a rod at a temperature above the temperature at which the alloying metals precipitate; (f) continuously quenching the rod to a temperature level below the temperature at which immediate substantial precipitation of alloying metals occur, and completing the cooling of the cast metal from the beginning of the hot forming step to the end of the quenching step within a time interval before which substantial precipitation of the alloying metals occur; and (g) controlling during steps (c), (d), (e) and (f) the solution heat treatment temperature of the bar within the solutionizing temperature range and controlling during said steps the time the bar is within the solutionizing temperature range such that said product has controlled precipitation during natural aging.

3. The method claim 2 wherein the molten aluminum base alloy is cooled to a temperature of from about 370"C (700"F) to about 505"C (940"F) in the casting groove.

4. The method of claim 2 wherein the temperature of the cast bar after passing through the heater is from about 455"C (850"F) to about 580"C (1080"F).

5. The method of claim 2 wherein the temperature of the cast bar entering the hot forming step is from about 455"C (850"F) to about 580"C (1080"F).

6. The method of claim 3 wherein the step of continuously hot forming the cast metal to form a rod at a temperature above the temperature level at which the alloying metals precipitate comprises controlling the temperature of the bar during hot rolling the aluminum alloy bar by applying a soluble oil to the bar as it is rolled, said soluble oil being at a temperature of less than 93"C (200"F).

7. The method of claim 3 wherein the step of continuously quenching the rod to a temperature below the temperature level at which immediate substantial precipitation of the alloying metals occur, and completing the cooling of the alloyed aluminum metal from the beginning of the hot forming step to the end of the quenching step within the time interval before which may substantial precipitation of the alloying metals occur comprises: quenching the hot rolled rod immediately after it exits the hot rolling mill to a temperature is less than 205"C (400"F), the time interval between entrance into the hot rolling mill and the completion of the quench to a temperature of less than 205"C (400"F) being between 4 and 30 seconds.

8. The method of claim 2 wherein the temperature of the cast bar after passing through the heater is from about 455"C (850"F) to about 510"C (950"F).

9. The method of claim 2 wherein the temperature of the cast bar after passing through the heater is from about 510"C (950"F) to about 550"C (1020"F).

1 0. The method of claim 2 wherein the temperature of the cast bar after passing through the heater is from about 550"C (1020"F) to about 580"C (1080"F).

11. The method of claim 2 wherein the temperature of the cast bar entering the hot forming step is from about 455"C (850"F) to about 510"C (950 F).

1 2. The method of claim 2 wherein the temperature of the cast bar entering the hot forming step is from about 510"C (950"F) to about 550"C (1020"F).

1 3. The method of claim 2 wherein the temperature of the cast bar entering the hot forming step is from about 550"C (1020'F) to about 580"C (1080'F).

14. The method according to claim 2 further including the step of drawing the rod to wire, said wire having a minimum tensile strength of 350. 106 Pa (50,800 p.s.i.), a minimum elongation of 7.9% and a minumum electrical conductivity of 52.5% I.A.C.S.

1 5. The new use of the known state of the art casting device used in continuous casting of the type comprising the steps of: (a) pouring a molten aluminum base alloy into the casting groove of a continous casting wheel at a temperature above the melting point of the aluminum base alloy; (b) cooling the molten aluminum base alloy in the casting groove to form a cast bar; (c) removing the cast bar from the casting groove; (d) continuously hot forming the cast aluminum base metal to form a rod; and (e) continuously forming said rod into coils; wherein the new process is characterized in that: (f) in step (a) the aluminum alloy contains from about 0.3 to about 1.2 weight percent silicon, about 0.3 to about 1.2 weight percent magnesium and the remainder essentially aluminum; step (b) is performed at a rate at which inverse segregation will be substantially minimized; said step (c) is performed at a temperature below 505"C (940 F); the cast bar prior to hot forming is passed through a heater and the temperature thereof raised above the temperature at which the alloying metals would substantially precipitate; step (d) is performed at a temperature above which the alloying metals precipitate; prior to step (e) said rod is continuously quenched to a temperature level below the temperature at which immediate substantial precipitation of alloying metals occur, the cooling of the cast metal from the beginning of the hot forming step to the end of the quenching step is completed within a time interval before which substantial precipitation of the alloying metals occur; and the solution heat treatment temperature of the bar is controlled during step (c) through (d) within the solutionizing temperature range and during said steps the time the bar is within the solutionizing temperature range is controlled such that said final product has controlled precipitation during natural aging.

1 6. The method of claim 2 wherein the aluminum alloy contains from about 0.5 to about 0.9 weight percent silicon and from about 0.6 to about 0.9 weight percent magnesium.

1 7. The method of claim 15 wherein the aluminum base alloy contains from about 0.5 to about 0.9 weight percent silicon and from about 0.6 to about 0.9 weight percent magnesium.