Patent Application
20150240339
This disclosure relates to tailored rolling of high strength aluminum alloys, such as 6xxx and 7xxx series alloys.
Vehicle body panels are known to be made from mild steels. One method of reducing weight in steel body panels is tailored rolling the material to create multiple thicknesses throughout the panel. Aluminum alloy body panels are also being developed to decrease vehicle weight. Body panels developed for the automotive and aerospace industries focus primarily on 5xxx and 6xxx series aluminum alloys, which are aluminum-magnesium and aluminum-magnesium-silicon alloys, respectively. 5xxx series aluminum alloys are generally shaped and processed by methods that are similar to methods used with mild steel sheets.
6xxx series aluminum alloys may require an age hardening treatment for some applications in order to have the required properties. Aluminum-zinc alloys of the 7xxx series, if age hardened, may achieve yield strengths similar to those of high strength steels. 7xxx series alloys may have a variety of tempers, that may be difficult to process and may require further heat treatment before the age hardening process. For example, a 7xxx material received with a T6 temper may be difficult to draw or stretch at room temperature. The age hardening process for 6xxx and 7xxx series alloys includes a continuous annealing and solution heat treatment (CASH) process that is limited to sheets having uniform thickness. Therefore, tailored rolling of high strength aluminum alloys (e.g., age hardening 6xxx and 7xxx series alloys) is not possible using current processes.
This disclosure is directed to solving the above problems, and other problems as summarized below.
According to one aspect of this disclosure, a method is provided for processing high strength aluminum sheet. The method comprises uncoiling a coil of O or F-temper 6xxx or 7xxxx series aluminum alloy sheet. The sheet is tailored rolled to form a tailored rolled sheet having at least two different thicknesses along its length. The tailored rolled sheet is cut into blanks to form a tailored rolled blank. The method may include re-coiling the tailored rolled sheet into a coil prior to the blanking step. The method may further include hot stamping the tailored rolled blank to form a component. The component then undergoes and age hardening.
The coil may be a 7xxx series aluminum alloy sheet and the age hardening step may include heat treating the component to a yield strength of at least 490 MPa. The coil may also be a 6xxx series aluminum alloy sheet and the age hardening step may include heat treating the component to a yield strength of at least 240 MPa. The tailored rolling step may include reducing a thickness of at least a region of the aluminum alloy sheet by up to 60%. The 6xxx or 7xxx series aluminum alloy sheet may have a thickness of 1 to 5 mm and the tailored rolled sheet may have a thickness of 0.5 to 5 mm.
The hot stamping step may include heating the tailored rolled blank to at least its solution temperature, positioning the tailored rolled blank in a die set and closing the die set on the tailored rolled blank to form the blank into a component and quench the component. The age hardening step may include heat treating the component to a T6 temper. The age hardening step may include a two-step heat treatment of the component, which may include a first heat treatment at 100 to 150° C. for 0.2 to 3 hours and a second heat treatment at 150 to 185° C. for 0.5 to 5 hours.
According to another aspect of this disclosure, a method of processing high strength aluminum sheet is provided that includes uncoiling a coil of O or F-temper 7xxxx series aluminum alloy sheet, tailored rolling the sheet to form a tailored rolled sheet having at least two different thicknesses along its length, and blanking the tailored rolled sheet to form a tailored rolled blank. The method further includes hot stamping the tailored rolled blank to form a component and subsequently age hardening the component to a yield strength of at least 490 MPa. The age hardening step may include heat treating the component to a T6 temper.
The tailored rolling step may include reducing a thickness of at least a region of the aluminum alloy sheet by up to 60%. The 7xxx series aluminum alloy sheet may have a thickness of 1 to 5 mm and the tailored rolled sheet may have a thickness of 0.5 to 5 mm. The 7xxx series aluminum alloy sheet may be a 7075 aluminum alloy. The hot stamping step may include heating the tailored rolled blank to at least its solution temperature, positioning the tailored rolled blank in a die set, and closing the die set on the tailored rolled blank to form the blank into a component and quench the component. The age hardening step may include a two-step heat treatment of the component, which may include a first heat treatment at 100 to 150° C. for 0.2 to 3 hours and a second heat treatment at 150 to 185° C. for 0.5 to 5 hours.
According to a further aspect of this disclosure, a method of processing high strength aluminum sheet is provided including tailored rolling a sheet of 7xxxx series aluminum alloy (e.g., 7075) to form a tailored rolled sheet having at least two different thicknesses along its length and blanking the tailored rolled sheet to form a tailored rolled blank. The tailored rolled blank is hot stamped to form a solution heat treated and quenched component that is then age hardened to a yield strength of at least 490 MPa. The age hardening may include a first heat treatment at 100 to 150° C. for 0.2 to 3 hours and a second heat treatment at 150 to 185° C. for 0.5 to 5 hours.
The tailored rolling step may include reducing a thickness of at least a region of the aluminum alloy sheet by up to 60%. The hot stamping step may include heating the tailored rolled blank to at least its solution temperature and positioning the tailored rolled blank in a die set. The die set is then closed on the tailored rolled blank to form the blank into a component and quench the component.
The above aspects of this disclosure and other aspects will be described below in greater detail with reference to the attached drawings.
The illustrated embodiments are disclosed with reference to the drawings. However, it is to be understood that the disclosed embodiments are intended to be merely examples that may be embodied in various and alternative forms. The figures are not necessarily to scale and some features may be exaggerated or minimized to show details of particular components. The specific structural and functional details disclosed are not to be interpreted as limiting, but as a representative basis for teaching one skilled in the art how to practice the disclosed concepts.
Aluminum alloys are generally identified by a four-digit number, with the first digit generally identifying the major alloying element. The major alloying element in 7xxx series aluminum is zinc while the major alloying element of 5xxx series is magnesium and for 6xxx series is magnesium and silicon. Additional numbers represented by the letter “x” in the series designation define the exact aluminum alloy. For example, a 7075 aluminum alloy may be used that has a composition of 5.1-6.1% zinc, 2.1-2.9% magnesium, 1.2-2.0% copper, and less than half a percent of silicon, iron, manganese, titanium, chromium, and other metals.
The 7xxx series and certain 6xxx series alloys require an age hardening process (also known as precipitation hardening) to achieve a high yield strength (YS). For example, age hardening can create a YS of over 400 MPa for a 7xxx series alloy. Heat treating 6xxx series alloys (e.g., 6061 and 6111) and the 7xxx series alloys (e.g., 7075) requires a solutionizing treatment, a quench, and a subsequent age hardening process. The solution heat treatment process may distort a part originally stamped from a 6xxx or 7xxx alloy.
In 7xxx series alloys the major alloying elements are added to introduce specific properties such as strength and toughness through precipitation hardening. The minor alloying elements indirectly affect properties as grain refiners/pinners. The major alloying elements in 7xxx series are Zn, Mg, and Cu, which have solid solubility for solution heat treatment. The major alloying elements in 6xxx series are Mg, Si, and Cu, which have solid solubility for solution heat treatment. These major alloying elements support strength, toughness, and ductility. The minor alloys elements have low solid solubility, and thus support grain refinement during solution heat treatment and quench.
Age hardening is preceded by a solution heat treatment (or solutionizing) and quench of the aluminum alloy material. Solution heat treatment generally includes heating the alloy to at least above its solvus temperature and maintaining it at the elevated temperature until the alloy forms a homogeneous solid solution or a single solid phase. The temperature that the alloy is held during solutionizing is known as the solution temperature. For example, the solution temperature for a 7xxx series aluminum alloy may be approximately 460° C. to 490° C. and the solution heat treatment may last from about 5 to 45 minutes. For a 6xxx series aluminum alloy, the solution temperature may be approximately 510° C. to 580° C. and the solution treatment may last from about 1 minute to two hours. The solution temperature and/or time is determined based upon the composition of a given aluminum alloy. The solution temperature may be the temperature at which a substance is readily miscible. Miscibility is the property of materials to mix in all proportions, forming a homogeneous solution. Miscibility may be possible in all phases; solid, liquid and gas.
Following the solution treatment, a quenching step is performed in which the alloy is rapidly cooled to below the solvus temperature to form a supersaturated solid solution. Due to the rapid cooling, the atoms in the alloy do not have sufficient time to diffuse long enough distances to form two or more phases in the alloy. The alloy is therefore in a non-equilibrium state. The quench rate may be any suitable rate to form a supersaturated solution in the quenched alloy. The quench rate may be determined in a certain temperature range, for example from 400° C. to 290° C. In at least one embodiment, the quench rate is at least 100° C./sec. The quench may be performed until the alloy is at a cool enough temperature that the alloy stays in a supersaturated state (e.g., diffusion is significantly slowed), such as about 290° C. The alloy may then be air cooled or otherwise cooled at a rate slower than the quench rate until a desired temperature is reached. Alternatively, the quench may be performed to a lower temperature, such as below 100° C. or down to about room temperature.
The solution heat treatment of 6xxx and 7xxx series sheet is conventionally performed using a continuous annealing and solution heat treatment (CASH) furnace. The quenching is generally done by immersing the sheet in a quenching medium, such as water or oil, or otherwise applying the quenching medium (e.g., spraying). Furnaces that perform the CASH process are configured and setup to treat a sheet having a uniform thickness. Settings such as temperature, speed of the conveyor, length of conveyor, treatment time, and others are tailored to the thickness of the sheet being treated in order to ensure that the correct solution heat treatment is achieved in the sheet. Similarly, the quenching process is tailored to the sheet thickness in order to ensure that the correct quench is achieved in the sheet (e.g., quenching medium, quench time, quench rate, and others).
A solution treated and quenched 7xxx series aluminum alloy must be age hardened (or precipitation hardened) to achieve a YS of at least 400 MPa or more, while an age hardened 6xx series alloy may achieve a YS of at least 200 MPa. Age hardening includes heating and maintaining the alloy at an elevated temperature at which there are two or more phases at equilibrium. The supersaturated alloy forms fine, dispersed precipitates throughout as a result of diffusion within the alloy. The precipitates begin as clusters of atoms, which then grow to form GP zones, which are on the order of a few nanometers in size and are generally crystallographically coherent with the surrounding metal matrix. As the GP zones grow in size, they become precipitates, which strengthen the alloy by impeding dislocation movement. Since the precipitates are very finely dispersed within the alloy, dislocations cannot move easily and must either go around or cut through the precipitates in order to propagate.
Five basic temper designations may be used for aluminum alloys which are; F—as fabricated, O—annealed, H—strain hardened, T—thermally treated, and W—as quenched (between solution heat treatment and artificial or natural aging). The as-received raw material for the disclosed solutionizing and age hardening processes may initially have any of the above temper designations. The temper designation may be followed by a single or double digit number for further delineation. An aluminum alloy with a T6 temper designation is an alloy that has been solution heat treated and artificially aged, but not cold worked after the solution heat treatment. T6 may represent the point of peak age yield strength along the yield strength vs. time and temperature profile for the material. A T7x temper may designate that a solution heat treatment has occurred, and that the material was artificially aged beyond the peak age yield strength (over-aged) along the yield strength vs. time and temperature profile. A T7x temper material may have a lower yield strength than a T6 temper material, but the T7x temper generally provides increased corrosion performance compared to the T6 temper. In one embodiment, a 7xxx series aluminum alloy part with a T6 temper is formed with a YS of 490 MPa or greater (e.g., at least 500 MPa). In another embodiment, a T7x temper is formed, such as a T73 or T76 temper. A T7x temper material may have a YS of at least 435 MPa. A 6xxx series aluminum alloy may be age hardened to a T6 temper having a YS of 240 MPa or greater (e.g., at least 260 MPa) or a T7 temper having a YS of at least 200 MPa. For example, 6061 at a T6 temper may have a yield strength of about 275 MPa and 6111 at a T6 temper may have a yield strength of about 300 MPa.
In the automotive industry, replacing steel vehicle components with aluminum components may allow for a reduction in weight of the components and, therefore, the vehicle as a whole. Another approach to reducing weight in steel vehicle components is to tailor the thickness of the components in multiple regions such that the steel component has relatively thick regions (e.g., high load regions) and relatively thin regions (e.g., low load regions). Tailoring the thickness of steel components may allow the thickness to be reduced in portions of the component that were previously thicker than they needed to be. Tailored thickness of steel components may be accomplished using a process called tailored rolling.
Referring to
The sheet 12 may be cut into blanks 24 downstream of the rollers 14 and 16. As shown in
The tailored rolling of aluminum sheet has previously been limited to materials that may only require annealing prior to a subsequent forming process. For example, 5xxx series aluminum alloys may be batch annealed after tailored rolling and then stamped. However, as discussed above, 6xxx and 7xxx series aluminum alloys require a solution heat treatment and quench before they can be processed into high strength components. In industries that require high volume throughputs at acceptable costs, such as the automotive industry, solution heat treatment for age hardening aluminum alloys is carried out using continuous annealing and solution heat treatment furnaces (e.g., CASH) and subsequent quenching. This approach cannot be used with tailored rolling, however, because CASH cannot accommodate the change in sheet thickness that is present in a tailored rolled blank (TRB). The furnaces used in the CASH process are configured to run at a certain temperature, time, and/or speed for a predetermined sheet thickness. Therefore, the CASH process may not be able to accommodate the changes in sheet thickness that are generated during the TRB process. Portions of the TRB that are thicker compared to the furnace's target settings may receive insufficient heat treatment and portions that are thinner may receive too much heat treatment. Either over or under heat treating may result in unacceptable microstructure or properties in the sheet. Therefore, it is not possible using conventional methods to perform tailored rolling on high strength aluminum alloys (e.g., 6xxx and 7xxx series) with high volume throughput. In addition, coils of 6xxx and 7xxx series aluminum are typically purchased with at least some age hardening already performed (e.g., a T4 or T6 temper). Age hardened coils cannot be used with the tailored rolling process.
A novel method for hot stamping age hardening aluminum alloys was recently developed and described in commonly assigned U.S. Pat. No. 8,496,764, the disclosure of which is incorporated by reference in its entirety. FIG. 1 of U.S. Pat. No. 8,496,764 is reproduced as
The heating apparatus 54 is provided to heat the blank 52. The heating apparatus 54 may be an industrial furnace or oven capable of producing internal temperatures high enough to heat blanks 52 placed in the heating apparatus 54 to a predetermined temperature, such as a solution, solvus, or solidus temperature of the blank 52. The heating apparatus 54 does not heat the blank 52 past its liquidus (melting) temperature. The blank 52 may be heated to at least its solvus or solidus temperature but less than its liquidus temperature, to provide a blank 52 that is substantially solid to facilitate handling and transport but that is more readily formable due to its near liquid or partial liquid phase.
The transfer mechanism 56 is configured to move and position the blank 52. The illustrated transfer mechanism 56 may be a manipulator, such as a robot. The transfer mechanism 56 may be configured to quickly transfer the blank 52 from the heating apparatus 54 to the die set 58 to reduce heat loss from the blank 52. For example, the system 50 and transfer mechanism 56 may be configured such that the temperature of the blank 52 does not decrease to or below its critical quench temperature. The critical quench temperature is the temperature at which quenching must begin to achieve a proper quench of the material. For example, the critical quench temperature for most 7xxx series aluminum alloys is approximately 400° C.
A die set 58 is provided to form the blank 52 into a part having a predetermined shape. The die set 58 may include a first die 60, a second die 62, at least one actuator 64, and a staging apparatus 66. The first and/or second dies 60, 62 are configured to form the blank 52 into the part having a predetermined shape. An actuator 64 may move the first die 60 and/or the second die 62 toward or away from each other and provide force to form the blank 52. The actuator 64 may be of any suitable type, such as hydraulic, pneumatic, mechanical, electromechanical, or combinations thereof. The die set 58 and actuator 64 combination may also be referred to as a machine press, stamping press, or quenching press.
A staging apparatus 66 is provided for positioning the blank 52 between and spaced apart from the first and second dies 60, 62. As such, the staging apparatus 66 may inhibit conductive heat transfer between the blank 52 and the die set 58, thereby helping to maintain the blank 52 at or above its critical quench temperature. The staging apparatus 66 receives the blank 52 from the transfer mechanism 56 and releases the blank 52 as the first die 60 and/or the second die 62 are closed and engage the blank 52. In addition, the system 50 may be configured to minimize heat loss from the blank 52 between removal from the heating apparatus 54 and closing of the die set 58. The temperature of the blank 52 may decrease by less than 10° C. However, the blank 52 could experience a greater temperature loss, for example, the blank 52 could lose up to a 75° C. assuming that the blank 52 is heated to 490° C. and the critical quench temperature is 415° C.
The die set 58 may include piping 68 that facilitates cooling of the first and/or second dies 60, 62 and quenching of the part formed from the blank 52. The piping 68 may include voids or channels formed in the die set 58, and may include a combination of externally connected piping and channels. The piping 68 may be connected to a cooling source and may receive a heat transfer medium, such as a fluid, from the cooling source for cooling the die set 58 to a desired temperature. The heat transfer medium may be any fluid medium capable of cooling the die set 58 to a predetermined temperature range, such as from 1° C. to 30° C. The die set 58 may be cooled in a manner that inhibits formation of condensation on one or more surfaces of the die set 58. In a mass production setting, the temperature of the die set 58 may be cooled to the predetermined temperature range before forming and quenching a blank 52 to remove heat that may have been transferred from a blank 52 to the die set 58 during forming of a previous part.
Forming the heated blank 52 into a part may occur simultaneously with quenching of the part. The quench rate affects the final temper strength and corrosion performance of the material. In some embodiments, the quench rate for the aluminum alloy, as it passes from 400° C. to 290° C., may be equal to or greater than 150° C./second. The part may be further cooled to a final temperature from 200° C. to 25° C. before removal of the part from the die set 58 to provide dimensional stability during subsequent processing.
The system 50 may be designed to operate continuously with a number of blanks 52 being heated in series or parallel by one or more heating apparatuses 54 and then transferred to at least one die set 58 for forming and quenching. At least one die set may become hotter than 30° C. during, or after, the forming of the blank 52 and/or simultaneous quenching of the part, and as such more than one die set 58 may be used to provide faster production speeds.
The part may be removed from the die set 58 by the transfer mechanism 56, another transferring device, or by hand. The part then progresses on to subsequent processing which may include flanging, trimming, and a natural and/or artificial aging to bring the aluminum alloy part to a high strength temper such as T6 or T7x.
Referring to
At 104, the coil may be unrolled according to methods known in the art. At 106, the unrolled sheet may be tailored rolled, for example, using an apparatus described with respect to
At 108, the tailored rolled sheet may either be rolled back into a coil or it may be cut into blanks. At 110, tailored rolled blanks (TRBs), either received directly from the tailored rolling process or later cut from a coil, are stamped. The stamping operation may be a hot stamping process, for example, using an apparatus described with respect to
At 112, the TRBs are age hardened in order to increase their strength. The TRBs may be age hardened to one of the well-known tempering designations, such as T4, T6, or T7x. For example, the TRBs may be formed of a 7xxx series alloy and age hardened to a T6 temper. The industry established standard age hardening heat treatments for 7xxx alloys comprises holding the alloy at a temperature of about 110-130° C. for over 20 hours, generally about 24 hours. For example, the standard age hardening heat treatment for 7075 aluminum is 115-126° C. for 24 hours to achieve a T6 temper. Suitable age hardening heat treatments for the other known tempering designations are also known in the art. Alternatively, a novel two-step age hardening treatment for 7xxx series alloys may be performed, which is described in commonly assigned, co-pending U.S. application Ser. No. 14/055,476, the disclosure of which is incorporated by reference in its entirety. The two-step age hardening treatment may include, for example, a first heat treatment step at 100 to 150° C. for about 0.2 to 3 hours and a second heat treatment step at 150 to 185° C. for about 0.5 to 5 hours.
The standard age hardening heat treatment to achieve a T6 temper in a 6xxx alloy may be at a temperature of about 160° C. to 180° C. for 8 to 18 hours (generally, if the temperature is near the top of the range then the time is towards the bottom of the range, and vice versa). However, there is no industry standard for tempering a 6xxx alloy to a T7 or T8 temper (a T8 temper is artificially aging after the material has been cold worked). As an alternative to the standard age hardening heat treatment for 6xxx series alloys, a novel age hardening treatment may be performed to form a T7 or T8 temper 6xxx series aluminum alloy, which is described in commonly assigned, co-pending U.S. application Ser. No. 14/189,050, the disclosure of which is incorporated by reference in its entirety. The age hardening treatment may include, for example, heat treating the alloy at a temperature of 215° C. to 245° C. for 15 minutes to 8 hours.
According to the methods disclosed above, tailored rolling of age hardening 6xxx and 7xxx series aluminum alloys may be performed. Previously, the solution heat treatment (e.g., CASH) and quenching required for age hardening of these alloys prevented the use of tailored rolling. O or F-temper coils of 6xxx and 7xxx series alloys may be directly utilized in the disclosed methods, which is highly unusual. The aluminum hot stamping process allows the solution treatment and quenching steps to be performed quickly and effectively after the tailored rolling process. With steel TRBs, processing is essentially complete following a stamping process that forms martensite. Age hardening aluminum TRBs requires the further step of age hardening after hot stamping. Age hardening may be performed by aging for a standard time period (e.g., 24 hours), however, the standard age hardening process may not provide adequate volume throughput for high-volume industries (e.g., automotive). The novel two-step age hardening process allows for greatly increased throughput by reducing the age hardening time to less than about 8 hours.
By tailored rolling high-strength aluminum alloys, even greater reduction in weight may be possible in vehicle components compared to just replacing steel components with aluminum. Many vehicle components may benefit from the inclusion of tailored rolled high-strength aluminum, for example, rocker panels, roof rails, bumper structures, A, B, or C pillars, and others. In addition to weight, the function, cost, and/or complexity of the high-strength aluminum components can be improved.
While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the disclosure. The words used in the specification are words of description rather than limitation. Changes may be made to the illustrated embodiments without departing from the spirit and scope of the disclosure as claimed. The features of the illustrated embodiments may be combined to form further embodiments of the disclosed concepts.
