Patent Application
20050000609
The present invention relates to aluminum alloys, and more particularly relates to aluminum sheet products in which alloy compositions and processing methods are controlled in order to produce improved crash resistance properties.
The use of aluminum sheet in automotive applications has generally been limited to Aluminum Association 6xxx alloys (Al—Mg—Si) for outer panels and 5xxx alloys (Al—Mg) for inner panels and structural members. In order to maximize the weight savings potential of aluminum, it is desirable to replace relatively low strength 5xxx alloys in the structure with higher strength 6xxx alloys. However, a shortcoming of existing 6xxx auto body sheet (ABS) alloys is their ability to absorb energy during crash situations. This is generally termed crashworthiness.
Autobody sheet requires a combination of good forming properties along with good strength after paint baking operations. The forming properties require good stretch forming and good bending. This traditionally has been achieved with rapid water quenching from solution heat treat temperatures. However, rapid water quenching often results in distortion, surface irregularities and water staining that are unacceptable for outer auto body applications. Air quenching offers many advantages over water quenching with respect to eliminating quench distortion problems, but air quenching can lead to poor bending performance.
The present invention has been developed in view of the foregoing and to address other deficiencies of the prior art.
The present invention controls alloy compositions and quench rates to produce aluminum alloy sheet products exhibiting good as-processed formability and shape, and good crashworthiness and strength in the artificially aged condition.
An aspect of the invention is to provide a 6xxx alloy with a desired combination of strength and crashworthiness.
Another aspect of the present invention is to provide a heat treated and slow quenched aluminum alloy sheet comprising from about 0.5 to about 0.7 wt. % Si, from about 0.5 to about 0.7 wt. % Mg, from about 0.1 to about 0.3 wt. % Mn, and the balance Al and incidental impurities.
A further aspect of the present invention is to provide a method of treating an aluminum alloy sheet, the method comprising providing a heat treated aluminum alloy sheet comprising Si, Mg, Mn, and the balance aluminum and incidental impurities, and slow quenching the heat treated aluminum sheet at a rate of less than about 200° F./second.
These and other aspects of the present invention will be more apparent from the following description.
a and 4b are graphs of tensile properties versus paint bake time at 185° C. for two different alloys.
a-5c are computer generated illustrations taken from different views of a sample crash box made of alloy 6060 sheet without a slow quench.
a-6c are computer generated illustrations taken from different views of a sample crash box made of alloy 6xxA sheet with a slow quench.
a-7c are computer generated illustrations taken from different views of a sample crash box made of alloy 6060 sheet with a slow quench.
a-8c are computer generated illustrations taken from different views of a sample crash box made of alloy 6xxA sheet without a slow quench.
The present invention provides aluminum alloy sheet products having favorable crash resistant properties. As used herein, the term “sheet” refers to aluminum alloy products having thicknesses from 0.2 to 6.3 mm. For auto body sheet products, thicknesses of from 0.7 to 3.5 mm are preferred. The aluminum alloy sheet products exhibit favorable crash resistance or crashworthiness properties. For the purpose of this invention, crashworthiness is defined as the ability of a material to absorb energy by plastic deformation without appreciable cracking. The crashworthiness of the sheet products can be quantified by critical fracture strain (CFS).
A preferred process path includes the following steps: casting of an aluminum alloy ingot by conventional or continuous methods; hot rolling; intermediate annealing; cold rolling; solution heat treating; and slow quenching, e.g., air quench or minimum distortion water quench. The steps of solution heat treating and slow quenching preferably occur on a continuous heat treater or temper line. After slow quenching, the sheet may optionally be reheated and coil cooled. The optional cooling step may be performed as an off-line batch process. The steps of solution heat treating and slow quenching, in addition to an optional reheating step, are schematically illustrated in
In the solution heat treatment step, the aluminum alloy sheet may be run through a continuous heat treater to substantially dissolve soluble phases formed during upstream processing. This process typically involves furnace temperatures of 800 to 1,100° F. at speeds from 20 to 150 feet per minute. The temperature and dwell time in the furnace may be adjusted based upon alloy composition and gauge.
In the slow quenching step, upon exit from the furnace zone of the continuous heat treater, the sheet is quenched at a controlled rate to retain the solute in solid solution. This can be accomplished, for example, with air or minimum distortion water. An aspect of this invention is the use of relatively slow quench rates that minimize sheet distortion while still developing favorable physical properties. As used herein, the term “slow quench” means quenching at a rate of less than about 200° F./second, preferably less than about 100° F./second. Quench rates for air type processes preferably range from about 20 to about 100° F./second, more preferably from about 40 to about 70° F./second. Water quench rates preferably range from 50 to 1,000° F./sec, more preferably from 100 to 200° F./second.
In the optional reheating step, a heating unit may follow the quench unit and any coil handling equipment, preferably just ahead of the coiling equipment on the exit end of the line. The heating unit raises the temperature of the sheet such that an elevated coiling temperature can be achieved. A preferred range of coiling temperatures is from about 130 to about 190° F. In the coil cooling step, the warm coil is allowed to cool slowly, typically as a 5,000 to 50,000 lb. mass of metal. This typically results in cooling rates of from about 0.1 to about 5° F./hour.
In accordance with an embodiment of the present invention, the composition of the aluminum alloy sheet is controlled in order to provide favorable crash resistance properties. The Si and Mg levels are controlled in order to provide high strengths. The Mn level is sufficient to control the grain size of the sheet, particularly during heat treating. Suitable alloys include 6xxx alloys such as 6009, 6060, 6063 and 6005. Typical, preferred and more preferred alloy composition ranges are listed in Table 1.
A particularly preferred Al—Mg—Si—Mn alloy is listed in Table 2. Table 2 lists the preferred 6xxA alloy compositions and a 6060 alloy composition in wt. percentages, with the balance comprising aluminum and incidental impurities.
An advantage of the present invention is the improvement in the crashworthiness of the aluminum alloy sheet product, which may be measured by critical fracture strain (CFS) and axial crush tests. Using the typical engineering stress-strain output from a standard r&n tension test, a critical fracture strain can be determined:
CFS=−1n(1et,eng)
in which et,eng represents the total engineering thinning strain. The total engineering thinning strain is a function of em, σm and σf:
εt,eng=f(em, σm, σf)
where em is the engineering strain at the maximum load; σm is the engineering stress at the maximum load; and σf is the engineering stress at the fracture load.
The following engineering assumptions are made in the development of the CFS: strains in the thickness and width directions are the same before the maximum load (Pmax); the true stress after Pmax is a constant; and the width strain is constant after Pmax. The total thinning strain at fracture may therefore be determined. In accordance with the present invention, a minimum CFS crashworthiness value of about 15 is preferred, with a value of at least 18 being more preferred.
A typical property comparison for alloys is shown in Table 3.
Twelve lots of materials 2.0 mm thick were fabricated. Details of the fabrication are given in Table 4. Prior to hot rolling, each of the cast samples was scalped and preheated at 590° C. for 8 hours followed by 560° C. for 9 hours. The main variables were alloy composition, use of a slow spray quench at an approximate cooling rate of 150° F./second following hot rolling, and the line speed of the continuous heat treat furnace (CHT). The compositions of the two 6xxA and 6060 alloys studied are shown above in Table 2.
The sheet was evaluated in the as received T4 temper and also after a simulative paint bake treatment at 180° C. (365° F.).
The tensile properties of the sheet in the T4 temper are presented in Table 5. There was a slight tendency for the T4 yield strength to decrease with increasing CHT line speed, which is probably indicative of incomplete dissolution of Mg2Si at the faster line speed. Minor variations in other T4 properties were found.
Guided bend tests using T4 sheet pre-strained 10% show that the slow quench is beneficial to bending of both alloys. Both alloys fabricated using the slow quench withstood the maximum sharp bend. Downflange and hemming tests illustrate that both alloys are flat hem capable.
The sheet r&n tensile properties after the paint bake were measured using 2 inch gage length specimens. Table 6 lists the r&n tensile data.
a is a graph of Rm, Rp0.2 and A values versus paint bake time at 185° C. for the 6060 sample listed in Table 6 which was subjected to the slow quench and a CHT speed of 11 meters/minute.
Crash boxes were assembled having a rectangular cross section measuring 63 mm by 133 mm. Welds or rivets may be used at approximately 1 inch on center with the first and last weld approximately ½ inch from the end. The number of spot welds or rivets specified were 20 per flange. An adhesive sold under the designation Betamate 1494 by Gurit Essex is a one-component toughened epoxy that is applied warm along the side seams of the crash boxes, followed by riveting. A pneumatic heated cartridge gun is used to dispense the adhesive at approximately 40 to 50° C. (104 to 122° F.). The metal components to be joined were also heated to approximately the same temperature to assist in application of the adhesive and improve flow and wettability. The adhesive was applied to warm metal on the flanges just prior to spot welding or riveting. Rivets were installed at the same locations specified for welding. End caps are then welded in place. After assembly, the boxes were paint baked. The paint baked boxes were tested in axial crush. The crush loads and energy absorbed at displacements of 100, 150, and 200 mm is given in Table 7.
Computer generated illustrations of the crushed appearance of the boxes are shown in
The performance of the materials met the goals of a sheet alloy product for use in crash critical applications. The paint baked sheet had yield strengths of about 235 MPa, total elongation of 15% and good static crush performance. The T4 properties indicate acceptable formability.
Whereas particular embodiments of this invention have been described above for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details of the present invention may be made without departing from the invention as defined in the appended claims.
This application claims priority to Provisional Application Ser. No. 60/436,123, filed Dec. 23, 2002.
| Number | Date | Country | |
|---|---|---|---|
| 60436123 | Dec 2002 | US |
