US3661657A - Method for making aluminum sheet - Google Patents

Abstract

A method for forming aluminum sheet material containing 0.5-3.8 percent magnesium, having little or no propensity for forming ripples when subsequently plastically stretched during fabrication said method comprising hot rolling, intermediate annealing to effect substantially complete recrystallization and subsequently cold rolling to final gauge thickness and the product formed thereby.

Description

D United States Patent 51 3,661,657 Wong 51 May 9, 1972 [54] METHOD FOR MAKING ALUMINUM 3,359,085 12/1967 Anderson ..148/11.5 A SHEET 3,502,448 3/1970 Anderson et al... ..148/32 3,560,269 2/1971 Anderson et al 148/1 1.5 A [72] Inventor: Winston A. Wong, Livermore, Calif. [73] Assignee: Kaiser Aluminum & Chemical Corpora- 'f 'f Rutledge on, Oakland Calif: Assistant E.\ammerW. W. Stallard Attorney-James E. Toomey, Paul E. Calrow, Harold L. Jen- [22] Filed: Dec. 7, 1970 kins and Edward J. Lynch [21] App]. No.: 95,930 ABSTRACT [52] U 5 Cl 148/32 145ml 5A A method for forming aluminum sheet material containing [51] 1/04C22c zi/oo 0.5-3.8 percent magnesium, having little or no propensity for 58 1 Field 1 48/1 A 159 32 forming ripples when subsequently plastically stretched during fabrication said method comprising hot rolling, intermediate 56] References Cited annealing to effect substantially complete recrystallization UNITED STATES PATENTS 2/1966 Anderson 148/1 1.5 A

and subsequently cold rolling to final gauge thickness and the product formed thereby.

3 Claims, No Drawings BACKGROUND OF THE INVENTION This invention relates to a process for producing aluminum sheet material free of rippled surface markings and the like when the sheet material is plastically stretched during subsequent fabrication procedures. Fabrication procedures which employ stretching as a prime mode of deformation include stretch forming and drawing operations. These types of fabricating procedures are frequently used for aluminum materials in the production of automotive products such as wheel openings, door and window trim, hub caps and the like. ln these procedures, the aluminum material is frequently heated to a temperature between about 400 and 550 F. to increase the formability of the aluminum materials.

The surface finish of aluminum sheet material is particularly important in automotive applications. However, it has been found that aluminum sheet material exhibits a rippled surface after stretch forming and the like which detract from its appearance. This is particularly noticeable after the shapes are anodized. The ripples are defined herein as ridges extending as parallel surface markings in a direction transverse to the rolling direction of the aluminum sheet material. Apparently, the ripple formation occurs only when the principal direction of deformation during fabrication is the same as the rolling direction.

,In the past, automotive trim stock has been prepared by hot rolling the aluminum material to an intermediate gauge thickness (e.g. 0.1 in.,) cooling to room temperature and cold rolling to the desired final thickness, normally about 0.03 in. Prior to hot rolling the material is given the normal treatments such as homogenizing, scalping, and an initial breakdown. When the automotive trim stock is stretch fabricated into the desired shape, it has been necessary to buff the shapes with a fine abrasive material to remove the rippled appearance. Usually the shapes are complex which make it difficult to buff adequately to remove the ripples. Most, if not all, automotive trim applications for aluminum and aluminum alloys require that the automotive trim be bright dipped in a solution of hydrofluoric acid and nitric acid and then clear anodized in a 15 percent sulfuric acid electrolyte to form an oxide coating having a nominal thickness of 0.3 mil. The finishing steps further accentuate the rippled appearance of the surface.

SUMMARY OF THE INVENTION The present invention relates to a method for making aluminum sheet material which does not have the propensity to form ripples when subsequently stretched during fabricating procedures. The alloy composition of the sheet material consists essentially of about 0.5-3.8 percent magnesium and the balance aluminum, incidental elements and impurities. Preferably the alloy contains from about 2.03.6 percent magnesium.

In the process of the present invention, after the alloy is subjected to the usual pretreatment such as homogenizing, scalping and initial breakdown, the material is hot rolled, intermediate annealed to effect substantially complete recrystallization and subsequently cold rolled to efi'ect a reduction of thickness between about 50 and 90 percent of the hot rolled thickness.

The sheet material formed by the process of the present in vention is characterized by tensile strength between about 29,000 and 53,000 psi, an elongation of between I and percent and further characterized by the absence of transgranular microscopic shear bands at or near the surface thereof which occur transverse to the rolling direction at an angle between about 30 and 45 from the surface when viewing a longitudinal cross-section of the sheet material. By X-ray analysis it has been found that the microscopic structure of the product of this invention contains between about 5 and 30 percent cube texture component, the remaining being the normal rolling texture.

DETAILED DESCRIPTION OF THE INVENTION The present invention is directed to a process for making a thin sheet of an aluminum-magnesium alloy consisting essentially of from about 0.5 to 3.8 percent magnesium and the balance aluminum, incidental elements and impurities. Preferably the magnesium level is maintained between 2.0 and 3.6 percent. The impurities and incidental elements should not exceed the limits of up to 0.10 percent silicon, up to 0.15 percent iron, up to 0.10 percent copper, up to 0.30 percent manganese, and other elements up to 0.05 percent each, up to 0.15 percent total.

In accordance with this invention, after the usual pretreatment such as homogenizing, scalping and initial breakdown, the above-described alloy is hot rolled at an initial temperature between 800 1,000 P. to an intermediate gauge thickness. The intermediate gauge thickness is then annealed to effect complete recrystallization by heating to a temperature between about 575 and 950 F. for a period of time between about 2 and 24 hours. Preferably the grain size is controlled to ASTM 7 or finer. Recrystallizing to obtain a grain I size of ASTM or finer is difficult to control industrially above a temperature of about 800 F. The intermediate gauge sheet is then cold rolled to effect a reduction in thickness between about 50 and percent of the intermediate gauge thickness. Cold rolling is usually conducted at less than 250 F. Preferably, the hot rolling is conducted in such a manner and at such a temperature to effect recrystallization during the hot rolling operation to avoid a subsequent recrystallization annealing step. Normally the hot rolling temperature is maintained above 600 F preferably over 700 F. With multistand mils it is usually required to slow down the mill speed to provide sufficient time to allow for recrystallization.

It has also been found that the recrystallization anneal can be used between the early cold rolling passes. However, at least half of the reduction during cold rolling should occur after the recrystallization anneal to avoid orange peel. Orange peel is a surface roughening in the form of a grain pattern where the surface of the sheet has a large grain structure and is stretched beyond its elastic limit.

As used herein the expression intermediate anneal includes substantially complete recrystallization during hot rolling, before cold rolling or during the initial phase of the cold rolling operations subject to limitations set forth above.

It is believed that the ripple surface, which forms when the sheet material is stretched during fabrication, is due to the presence of strong transgranular shear bands which are generated during the hot rolling and cold rolling steps normally employed in producing automotive trim sheet material. The microscopic shear bands occur transverse to the rolling direction at an angle between about 3045 from the surface of the sheet when viewing a longitudinal cross-section. It has been found that the shear bands must extend to or near the surface of the sheet before the rippled appearance develops appreciably during the stretch fabrication.

In accordance with the present invention it has been found that the shear bands which are initially developed during hot rolling can be eliminated or minimized by efiecting a substantially complete recrystallization during an intermediate anneal. Preferably at least half of the reduction during cold rolling must occur after recrystallization, otherwise theresultant sheet products exhibit severe orange peel and substantially reduced strength which severely limits the usefulness of the sheet material for automotive applications. To minimize the effect of orange peel the recrystallization should be controlled in such a manner so as to obtain a grain size of ASTM 7 or finer. The relatively high strength is necessary in these applications to prevent any loss in dent resistance which is a primary requirement for automotive applications.

In the process described above, the alloy is normally hot rolled to a gauge thickness of between about 0.08 and 0.20 in. and cold rolled to a gauge thickness between about 0.02 and 0.10 in. The described thermal and rolling practices of the present invention provide for a sheet material having a tensile strength between about 29,000 and 53,000 psi, an elongation between about I and 5 percent in 2 in. A microscopic examination of the resultant product shows that it is characterized by the absence of transgranular shear bands and further characterized by significant amounts of cube texture components which are determined by X-ray analysis. Preferably the cube texture components amount to between 5 and 30 percent of the total texture. The rest of the structure is primarily the normal rolling texture.

Aluminum sheet material containing magnesium greater than 3.8 percent, which has been cold rolled large amounts, e.g. 70-95 percent, is extremely hard and brittle. When this sheet is subject to fabrication in which the primary mode of deformation isby stretching, the sheet has a tendency to crack or break, and, moreover, this tendency is not eliminated by heating the material to an elevated temperature such as between 400-550 F. Further, the resultant sheet material containing more than 3.8 percent magnesium and the normal impurity levels, when bright dipped and anodized in a percent sulfuric acid electrolyte, does not have the surface characteristics required for automotive trim applications in that the specular reflectivity and image clarity is severely reduced. Moreover, there is some evidence that sheet material containing more than 3.8 percent magnesium has the tendency to form shear bands when subjected to large amounts of cold rolling, for example, 80-95 percent. Sheet material containing less than 0.5 percent magnesium does not have the required strength and dent resistance for automotive applications.

The following is an example of the embodiment of the present invention in comparison with the prior methods of producing auto trim material. The example is intended to be illustrative and it has been selected as a typical example to demonstrate the invention rather than to limit it.

A 5252 aluminum alloy ingot having a composition set forth below Ti Al 0.009 Balance.

Si Fe Cu Mn Mg Zn was homogenized, scalped and preheated in a normal manner to slightly above hot rolling temperatures. The ingot was then passed through a breakdown mill to reduce the thickness of the ingot to about 1 inch. The 1 inch plate was hot rolled to a thickness of 0.102 inch with a mill entry temperature of 875 F. and a mill exit temperature of 540 F. Half of the hot rolled sheet was subjected to a recrystallization anneal at a temperature of 650 F. for about 2 hours and then separate samples of the sheet were cold rolled to effect a thickness reduction of 70, 80 and 90 percent respectively. The remaining half of the hot rolled sheet was divided into separate samples and the separate samples were cold rolled to effect a reduction in thickness of 70, 80 and 90 percent respectively. The samples ture of approximately 450 that had been annealed to effect recrystallization exhibited no transgranular shear bands upon microscopic examination, and, when the samples were subsequently stretch formed at a temperature of approximately 450 F., exhibited no rippled formation on the surface thereof. The samples having no intermediate anneal after hot rolling, exhibited rather severe shear bands and severely rippled when stretch formed at a tempera- F. The annealed samples subjected to the intermediate anneal showed a slight diminution of reflectivity and strength when compared to the unannealed samples, but the differences where well within acceptable commercial limits. The tensile properties of both the annealed and unannealed samples are set forth in the following table:

[Properties in the rolling direction] Ks. in

As indicated in the above example, the product of the present invention does not form ripples when subjected to stretching during fabricating procedures and, furthermore, has the strength and reflectivity requirements for automotive trim applications. Moreover, it should be noted that the product of the the present invention has no tendency to give an orange peel effect when subjected to a fabrication procedure in which the primary mode of deformation is stretching.

What is claimed is:

1. An aluminum-magnesium sheet suitable for trim stock which has been cold rolled between about 50 and percent characterized by improved reflectivity and surface finish, by a tensile strength between about 29,000 and 53,000 psi, an elongation between about i and 5 percent and by the absence of microscopic transgranular shear bands transverse to the rolling direction extending proximate to the surface of said sheet at an angle between about 30 and 45 to said surface, said sheet having a composition consisting essentially of 0.5 to 3.8 percent magnesium and the balance aluminum, impurities, and incidental elements, said impurities and incidental elements not exceeding the limits of up to 0.10 percent silicon, up to 0.15 percent iron, up to 0.10 percent copper, up to 0.30 percent manganese, and other elements up to 0.05 percent each up to 0.15 percent total.

2. The aluminum-magnesium sheet of claim 1 wherein said sheet has a composition containing 2.0 to 3.6 percent magnesium.

3. The aluminum-magnesium sheet of claim 1 further characterized by a micro structure containing between about 5 and 30 percent cube texture component.

Claims (2)

1. 2. The aluminum-magnesium sheet of claim 1 wherein said sheet has a composition containing 2.0 to 3.6 percent magnesium.
2. 3. The aluminum-magnesium sheet of claim 1 further characterized by a micro structure containing between about 5 and 30 percent cube texture component.