Metal treatment – Process of modifying or maintaining internal physical... – With casting or solidifying from melt
Reexamination Certificate
2002-05-02
2004-08-24
Wyszomierski, George (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
With casting or solidifying from melt
C148S693000, C148S694000
Reexamination Certificate
active
06780259
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the production of aluminum alloy sheet for the automotive industry, particularly for body panel applications, having excellent bendability, together with good paint bake response and recyclability.
2. Description of the Prior Art
Various types of aluminum alloys have been developed and used in the production of automobiles, particularly as automobile body panels. The use of aluminum alloys for this purpose has the advantage of substantially reducing the weight of the automobiles. However, introduction of aluminum alloy panels creates its own set of needs. To be useful in automobile applications, an aluminum alloy sheet product must possess good forming characteristics in the as-received condition, so that it may be bent or shaped as desired without cracking, tearing or wrinkling. In particular, the panels must be able to withstand severe bending, as occurs during hemming operations, without cracking. Hemming is the common way of attaching outer closure sheets to underlying support panels and results in the edges of the sheet being bent nearly back on itself. In addition to this excellent bendability, the aluminum alloy panels, after painting and baking, must have sufficient strength to resist dents and withstand other impacts.
Aluminum alloys of the AA (Aluminum Association), 6000 series are widely used for automotive panel applications. It is well known that a lower T4 yield strength (YS), and reduced amount of Fe, will promote improved formability, particularly hemming performance. A lower yield strength can be achieved by reducing the solute content (Mg, Si, Cu) of the alloy, but this has traditionally resulted in a poor paint bake response, less than 200 MPa T8 (0% strain). This poor paint bake response can be countered by increasing the gauge, or by artificially aging the formed panels. However, both of these approaches increase the cost and are unattractive options. Furthermore, a reduced Fe content is not sustainable with the use of significant amounts of scrap in the form of recycled metal. This is because the scrap stream from stamping plants tends to be contaminated with some steel scrap that causes a rise in the Fe level.
Furthermore, the necessary material characteristics of outer and inner panels are sufficiently different that the natural trend is to specialize the alloys and process routes. For example, an AA5000 alloy may be used for inner panels and an AA6000 alloy for outer panels. However, to promote efficient recycling it is highly desirable to have the alloys used to construct both the inner and outer panel of a hood, deck lid, etc. to have a common or highly compatible chemistry. At the very least, the scrap stream must be capable of making one of the alloys, in this case the alloy for the inner panel.
In Uchida et al. U.S. Pat. No. 5,266,130 a process is described for manufacturing aluminum alloy panels for the automotive industry. Their alloy includes as essential components quite broad ranges of Si and Mg and may also include Mn, Fe, Cu, Ti, etc. The examples of the patent show a pre-aging treatment that incorporates a cooling rate of 4° C./min from 150° C. to 50° C.
In Jin et al. U.S. Pat. No. 5,616,189 a further process is described for producing aluminum sheet for the automotive industry. Again, alloys used contain Cu, Mg, Mn and Fe. The aluminum sheet produced from these alloys was subjected to a 5 hour pre-age treatment at 85° C. The disclosure furthermore states that the sheet can be coiled at 85° C. and allowed to cool slowly to ambient at a rate of less than 10° C./hr. The aluminum sheet used in this patent was a continuous cast (CC) sheet and sheet products produced by this route have been found to exhibit poor bendability.
It is an object of the present invention to provide an improved processing technique whereby an aluminum alloy sheet is formed which has excellent bendability.
It is a further object of the invention to provide an aluminum alloy sheet product having good paint bake response.
It is a still further object of the invention to provide an aluminum alloy sheet product which is capable of being recycled for use in the production of automotive body panels.
SUMMARY OF THE INVENTION
In accordance with one embodiment of this invention, an aluminum alloy sheet of improved bendability is obtained by utilizing an alloy of the AA6000 series, with carefully selected Mg and Si contents and, with an increased manganese content and a specific pre-age treatment. The alloy used in accordance with this invention is one containing in percentages by weight 0.50-0.75% Mg, 0.7-0.85% Si, 0.1-0.3% Fe and 0.15-0.35% Mn. According to an alternative embodiment, the alloy may also contain 0.2-0.4% Cu.
The procedure used for the production of the sheet product is the T4 process with pre-aging, i.e. T4P. The pre-aging treatment is the last step in the procedure.
The target physical properties for the sheet products of this invention are as follows:
T4P, YS
90-120 MPa
T4P UTS
>200 MPa
T4P E1
>28% ASTM, >30% (Using JIS Specimen)
BEND, r
min
/t
<0.5
T8 (0% strain), YS
>210 MPa
T8 (2% strain), YS
>250 MPa
In the above, T4P indicates a process where the alloy has been solution heat treated, pre-aged and naturally aged for at least 48 hours. UTS indicates tensile strength, YS indicates yield strength and E1 indicates total elongation. BEND represents the bend radius to sheet thickness ratio and is determined according to the ASTM 290C standard wrap bend test method. T8 (0% or 2% strain) represents the YS after a simulated paint bake of either 0% or 2% strain and 30 min at 177° C.
For Cu-free alloys the functional relationships are revealed which allow the T4P strengths to be related to alloy composition, and the paint bake strength to the T4P strength
The T4P yield strength is given by:
T4P YS (MPa)=130(Mgwt %)+80(Siwt %)−32
where the T4P is obtained by a simulated pre-age of 85° C. for 8 hrs.
The T8 (0% strain) yield strength is given by:
T8 (MPa)=0.9(T4P)+134
Using these relationships the following alloys will meet the T4P/TB (0%) requirements:
T4P 90 MPa, T8 215 MPa+(0.5 wt % Mg-0.7 wt % Si)
T4P 110 MPa, T8 233 MPa+(0.6 wt % Mg-0.8 wt % Si)
T4P 120 MPa, T8 242 MPa+(0.75 wt % Mg-0.7 wt % Si)
and this gives the nominal composition range for the alloys of the invention of Al-0.5 to 0.75 wt % Mg-0.7 to 0.8 wt % Si.
For Cu containing alloys, the functional relationships are not so straightforward and depend on the Mg and Si content. A Cu content of about 0.2-0.4 wt % is desirable for enhanced paint bake performance.
For reasons of grain size control, it is preferable to have at least 0.2 wt % Mn. Mn also provides some strengthening to the alloy. Fe should be kept to the lowest practical limit, not less than 0.1 wt %, or more than 0.3 wt % to avoid forming difficulties.
For the outer panel the Fe level in the alloy will tend toward the minimum for improved hemming. On the other hand, the Fe level in the alloy for inner panel applications will tend towards the maximum level as the amount of recycled material increases.
The alloy used in accordance with this invention is cast by semi-continuous casting, e.g. direct chill (DC) casting. The ingots are homogenized and hot rolled to reroll gauge, then cold rolled and solution heat treated. The heat treated strip is then cooled by quenching to a temperature of about 60-120° C. and coiled. This quench is preferably to a temperature of about 70-100C., with a range of 80-90° C. being particularly preferred. The coil is then allowed to slowly cool to room temperature at a rate of less than about 10° C./hr, preferably less than 5° C./hr. It is particularly preferred to have a very slow cooling rate of less than 3° C./hr,
The homogenizing is typically at a temperature of more than 550° C. for more than 5 hours and the reroll exit gauge is typically about 2.54-6.3 mm at an exit temperature of about 300-380° C. The cold roll is normally to about 1.0 mm gauge and the solution heat treatme
Bull Michael Jackson
Lloyd David James
Alcan International Limited
Cooper & Dunham LLP
Morillo Janelle
Wyszomierski George
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