Abstract:
Forming a tailor-welded blank (TWB) having aluminum sheets includes various elements. For example, a hot-stamping methodology is applied to a welded blank for meeting load and stiffness requirements in tandem to weight efficiency. A TWB can result in improved material utilization, and applying the hot-stamping methodology can improve the formability of the TWB. After forming and subsequent processing, a peak-age heat treatment is utilized such that a lightweight, high strength part may be attained.

Description:
TECHNICAL FIELD 
       [0001]    This application relates to metal forming, and more specifically to hot stamping tailor-welded blanks. 
       BACKGROUND 
       [0002]    Automotive structural elements (e.g., panels, rails, pillars, etc.) are often made from mild steels. These parts sometimes include multiple regions or portions having different strength requirements. For instance, in a stamped door inner, a portion of the door inner that supports hinges might possess higher strength requirements than other portions of the door inner. As such, tailor-welded blanks or sheets can be used, which include two or more sheets of steel having different properties (e.g., thickness) that are welded together and that can be conventionally stamped into a part. Accordingly, the part can be engineered to meet loading and stiffness requirements with greater weight efficiency and cost savings. 
         [0003]    In an effort to decrease vehicle weight, aluminum-alloy body panels have been increasing in popularity, and utilizing tailor-welded aluminum blanks or sheets might further contribute to cost and weight savings. However, aluminum welds typically show reduced strength and formability when stamping compared to parent material. 
       SUMMARY 
       [0004]    An embodiment of the present invention is directed to forming a tailor-welded blank (TWB), which includes a first aluminum sheet welded to a second aluminum sheet. In one instance, the TWB is heated to at least a solidus temperature of the first sheet and is positioned in a die set. The die set is closed on the TWB to form the TWB into a part while simultaneously quenching the part. 
         [0005]    Another embodiment includes a method of forming a TWB. The TWB is fabricated by welding a first sheet of aluminum to a second sheet of aluminum. The formability of a weld created by the welding is improved by heating the TWB to at least a solidus temperature of the first sheet, positioning the TWB in a die set, and closing the die set on the TWB to form the TWB into a part while simultaneously quenching the part. 
         [0006]    In an additional embodiment, a method of improving formability of a weld that joins a first sheet of aluminum and a second sheet of aluminum includes various elements. For example, the first and second sheets that are welded together are heated to a solidus temperature of the first sheet and are positioned in a die set. The die set is closed on the sheets to form the sheets into a part while simultaneously quenching the part. 
         [0007]    Embodiments of the invention are defined by the claims below, not this summary. A high-level overview of various aspects of the invention is provided here to introduce a selection of concepts that are further described below in the detailed-description section. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in isolation to determine the scope of the claimed subject matter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    Illustrative embodiments of the present invention are described in detail below with reference to the attached drawing figures, which are incorporated herein by reference, wherein: 
           [0009]      FIG. 1  depicts an exemplary tailor-welded blank in accordance with an embodiment of the present invention; 
           [0010]      FIG. 2  depicts an exemplary system in accordance with an embodiment of the present invention; 
           [0011]      FIG. 3  depicts an exemplary staging apparatus in accordance with an embodiment of the present invention; and 
           [0012]      FIGS. 4 and 5  each depicts a respective flow diagram having steps that are carried out in accordance with embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    The subject matter of embodiments of the present invention is described with specificity herein to meet statutory requirements. But the description itself is not intended to necessarily limit the scope of claims. Rather, the claimed subject matter might be embodied in other ways to include different elements or combinations of elements similar to the ones described in this document, in conjunction with other present or future technologies. Terms should not be interpreted as implying any particular order among or between various steps herein disclosed except when the order of individual steps is explicitly stated. 
         [0014]    Referring to  FIG. 1 , a tailor-welded blank (TWB)  12  is depicted including a first sheet of aluminum  16  and a second sheet of aluminum  18  that are joined by a weld  17 . The TWB  12  might be created by fusion welding (e.g., laser, arc, etc.) or by friction stir welding, and typically a butt joint is utilized. For illustrative purposes,  FIG. 1  depicts a weld  17  that joins the first sheet  16  and the second sheet  18 . Although  FIG. 1  depicts TWB  12  having only the first sheet  16  and the second sheet, in other embodiments, the TWB  12  includes more than two different sheets and includes as many sheets as are desired to form a part. 
         [0015]    The TWB might be used for various purposes, and in one embodiment, the TWB  12  is stamped into a part for a motor vehicle. For instance, the TWB might be stamped into a A/B pillar, a rocker, a roof rail, a cross-car beam, a door inner, and the like. In one embodiment, it is desirable for the part into which the TWB  12  is stamped to have portions or regions with different characteristics. For instance, it might be desirable for a front portion of a door inner that supports door hinging to be stronger than other regions of the door inner. As such, in one embodiment, the first sheet  16  and the second sheet  18  are both aluminum and are dissimilar in one or more respects. 
         [0016]    The first sheet  16  and second sheet  18  might be different in various respects to optimize the part characteristics. For example, the first sheet  16  and the second sheet  18  might have different thicknesses, alloys, or a combination thereof. 
         [0017]    In one embodiment the first sheet  16  includes a first thickness that is in a range of about 0.8 mm to about 4.0 mm, and the second sheet  18  includes a second thickness that is different than the first thickness and is also in the range of about 0.8 mm to about 4.0 mm. 
         [0018]    In another embodiment, the first sheet  16  and the second sheet  18  might have different properties (e.g., alloy composition or series). For instance, in one embodiment, the first sheet  16  and second sheet  18  are any combination of the alloys 2xxx, 5xxx, 6xxx, and 7xxxx. That is, aluminum alloys are typically identified by a four-digit number, the first digit of which generally identifies the major alloying element. For example, 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. In one embodiment, 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. In addition, the first and second sheets might have a variety of tempers, such as F, W, O, T4x, T6x, T7x, and T8x. 
         [0019]    The TWB  12  might be obtained from various sources. For instance, in one embodiment, the TWB  12  is obtained as a pre-made blank. Alternatively, a tailor-welded coil might be obtained and the TWB  12  is cut from the coil. In another embodiment, the TWB  12  is fabricated in a coil and the TWB is blanked from the tailor-welded coil. For example, a welding apparatus  14  might apply a weld (e.g., laser, arc, or the like) to two or more sheets of aluminum to fabricate a tailor-welded coil. 
         [0020]    An embodiment of the present invention is directed to improving a formability of the weld  17  that joins the first aluminum sheet  16  and the second aluminum sheet  18 . That is, absent the present invention, the weld  17  includes characteristics that reduce formability. As such, embodiments hereof address some manufacturing limitations associated with reduced formability of welded joints of aluminum-alloy tailor-welded blanks. 
         [0021]    Referring to  FIG. 2 , a system  10  for forming a tailor-welded blank  12  (TWB) is shown. The system  10  of  FIG. 2  includes a heating apparatus  20 , a transfer apparatus  22 , and a die set  24 . The various components of the system  10  function together to fabricate a tailor-welded blank that is constructed of one or more aluminum alloys and is formable into a desired part. 
         [0022]    In an embodiment of the present invention, the heating apparatus  20  heats the TWB  12 . The heating apparatus  20  might includes various types, such as an industrial furnace or oven capable of producing internal temperatures high enough to heat TWB  12 , which are placed in the heating apparatus  20 , to a predetermined temperature. In one embodiment, the heating apparatus  20  heats the TWB  12  to a solution or solidus temperature of at least one of the first sheet  16  or second sheet  18  of the TWB  12 . In a further embodiment, the heating apparatus  20  does not heat the TWB  12  past its liquidus (melting) temperature. 
         [0023]    The solution temperature might vary depending on the series of the first sheet  16  and the second sheet  18 . For instance, the solution temperature for a 7xxx series aluminum alloy is typically about 460° C. to about 490° C. The solution temperature is usually 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. 
         [0024]    The solidus temperature may be the locus of temperatures on a curve on a phase diagram below which a given substance is completely solid. The solidus temperature quantifies the temperature at which melting of a substance may begin, but not the temperature at which the substance is melted completely. With some materials there may be a phase existing between the solidus and liquidus temperatures wherein the substance consists of solid and liquid phases simultaneously. The closer the material is to the solidus temperature, the more the material is in a solid phase, and the closer the material is to the liquidus temperature, the more the material is in a liquid phase. As such, the TWB  12  may be heated to at least the solidus temperature of one of the aluminum sheets but less than the liquidus temperature, thereby providing a TWB  12  that is substantially solid to facilitate handling and transport yet more readily formable due to its near liquid or partial liquid phase. 
         [0025]    The transfer mechanism  22  is configured to move and position the TWB  12 , which is heated to the solidus temperature of at least one of the sheets. In at least one embodiment, the transfer mechanism  22  is a manipulator, such as a robot. The transfer mechanism  22  might be configured to quickly transfer the TWB  12  from the heating apparatus  20  to the die set  24  to reduce the opportunity for heat loss from the TWB  12 . For example, the system  10  and transfer mechanism  22  may be configured such that the temperature of the TWB  12  does not decrease to or below the critical quench temperature of at least one of the aluminum sheets  16  and  18 . 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 415° C. 
         [0026]    The die set  24  is provided to form the TWB  12  into a part having a predetermined shape. In at least one embodiment, the die set  24  includes a first die  26 , a second die  28 , at least one actuator  30 , and a staging apparatus  32 . 
         [0027]    The first and/or second dies  26 ,  28  are configured to form the TWB  12  into the part having a predetermined shape. An actuator  30  actuates the first die  26  and/or the second die  28  toward or away from each other and provide force to form the TWB  12 . The actuator  30  may be of any suitable type, such as hydraulic, pneumatic, mechanical, electromechanical, or combinations thereof. The die set  24  and actuator  30  combination may also be referred to as a machine press, stamping press, or quenching press. 
         [0028]    A staging apparatus  32  may be provided for positioning the TWB  12  between and spaced apart from the first and second dies  26 ,  28 . As such, the staging apparatus  32  may inhibit conductive heat transfer between the TWB  12  and the die set  24 , thereby helping to maintain the TWB  12  at or above the critical quench temperature. The staging apparatus  32  may receive the TWB  12  from the transfer mechanism  22  and may release the TWB  12  as the first die  26  and/or the second die  28  are closed and engage the TWB  12 . In addition, the system  10  may be configured such that little heat is lost from the TWB  12  between removal from the heating apparatus  20  and closing of the die set  24 . In at least one embodiment, the temperature of the TWB  12  may decrease by less than 10° C.; however, the TWB  12  could experience a greater temperature loss, such as up to 75° C., such as when the TWB  12  is heated to 490° C. and the critical quench temperature is 415° C. 
         [0029]    The die set  24  may include piping  34  that facilitates cooling of the first and/or second dies  26 ,  28  and quenching of the part formed from the TWB  12 . The piping  34  may be voids or channels formed into the die set  24 , or any combination of externally connected piping and channels. The piping  34  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  24  to a desired temperature. The heat transfer medium may be any fluid medium capable of cooling the die set  24  to a predetermined temperature range, such as from 1° C. to 30° C. The die set  24  may be cooled in a manner that inhibits formation of condensation on one or more surfaces of the die set  24 . In a mass production setting, the temperature of the die set  24  may be cooled to the predetermined temperature range before forming and quenching a TWB  12  to remove heat that may have been transferred from a TWB  12  to the die set  24  during forming of a previous part. 
         [0030]    In one embodiment, forming the heated TWB  12  into a part occurs 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  24  to provide dimensional stability and facilitate the room temperature material handling of the part during subsequent processing. 
         [0031]    The system  10  may be designed to operate continuously with a number of TWB  12  being heated in series or parallel by one or more heating apparatuses  20  and then transferred to at least one die set  24  for forming and quenching. At least one die set may become hotter than 30° C. during, or after, the forming of the TWB  12  and/or simultaneous quenching of the part, and as such more than one die set  24  may be used to provide faster production speeds. 
         [0032]    The part may be removed from the die set  24  by the transfer mechanism  22 , 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. 
         [0033]    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 temper designation may be followed by a single or double digit number for further delineation. An aluminum alloy with a T6 temper designation may be an alloy which has been solution heat treated and artificially aged, but not cold worked after the solution heat treatment (or such that cold working would not be discernable in the material properties). 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 versus time and temperature profile. A T7x temper material may have a lower yield strength than a T6 temper material, but this may be done to increase corrosion performance. 
         [0034]    Referring to  FIG. 3 , an embodiment of a staging apparatus  32  is shown in more detail. One or more staging apparatuses  32  may be provided with the die set  24 . For example, a staging apparatus  32  may be provided proximate to a corner or side of a die in one or more embodiments. A staging apparatus  32  may be positioned or configured so as not to interfere with actuation or closing of the die set  24 . Moreover, the staging apparatus  32  may help insulate or may be provided with materials that inhibit heat transfer from the TWB  12  to a die. The staging apparatus  32  may include a base  40 , a support member  42 , a finger  44 , and an actuator  46 . 
         [0035]    The base  40  may be disposed on the die set  24  and may facilitate mounting of the staging apparatus  32 . The support member  42  may extend from and may be fixedly disposed on the base  40 . The support member  42  may include a slot  50 . The slot  50  may be configured to receive and accommodate rotation of the finger  44 . 
         [0036]    The finger  44  may be pivotally disposed on the support member  42 . For example, a pivot pin may rotatably couple the finger  44  to the support member  42  in one or more embodiments. The finger  44  may rotate between a first position and a second position. In the first position, the finger  44  may extend away from the support member  42  and may support the TWB  12 . The finger  44  may rotate with respect to the support member  42  and toward or into the slot  50  to a second position (as indicated by the arrows in  FIG. 3 ) to permit the TWB  12  to disengage the staging apparatus  32  and drop onto a die, such as the second die  28 . 
         [0037]    The actuator  46  may be placed in proximity of the staging apparatus and may be used to provide position control of finger  44 . For example, in some embodiments the actuator  46  may be an electric motor connected to the pivot pin which rotates the finger  44  from the first position to the second position when power is applied, and a spring  52  may return the finger  44  from the second position to the first position when power is removed. The actuator  46  may be controlled by an automated control system, or by an operator. The actuator  46  may also be a servomechanism utilizing electricity, hydraulics, pneumatics, magnetic, or mechanical principles, or any combination, to provide position control of the finger  44 . 
         [0038]    Referring to  FIG. 4 , a flow diagram depicts a series of steps that, when executed, provide a method  410  of forming a tailor-welded blank. In describing the steps of  FIG. 4 , reference will also be made to  FIGS. 2 and 3 . 
         [0039]    Step  412  includes fabricating the tailor-welded blank by welding a first sheet of aluminum to a second sheet of aluminum. For example, fusion welding or friction stir welding might be used to join the first sheet  16  to the second sheet  18  with the weld  17 . 
         [0040]    The method  410  also includes various steps for improving a formability of a weld (e.g., weld  17 ) created by the welding. Step  414  includes heating the tailor-welded blank to at least a solidus temperature of the first sheet. For instance, the heating apparatus  20  might be used to heat the TWB  12  to the solidus temperature of at least one of the sheets  16  and  18 . At step  416 , the TWB is positioned in the die set. For example, the robot  22  might be used to move the TWB  12  from the heating apparatus  20  to the dies set  24 . 
         [0041]    Step  418  includes closing the die set on the tailor-welded blank to form the tailor-welded blank into a part while simultaneously quenching the part. For example, the die set  24  is closed on the TWB  12  while the cooling coils  34  are used to reduce a temperature of the part. 
         [0042]    Referring now to  FIG. 5 , another flow diagram is depicted showing a series of steps that are similar to method  410 , but that are more detailed. Similar to the method  410 , the method  510  is for processing or forming an aluminum-alloy TWB. 
         [0043]    At  512 , the method  510  includes providing an aluminum-alloy tailor-welded coil. The aluminum-alloy tailor-welded coil might be fabricated by welding a first sheet to a second sheet, or might be obtained in a pre-made coil. 
         [0044]    At  514 , the coil might be lubricated to facilitate blanking, if blanking is necessary. For instance, lubrication may aid blank formation, reduce heat generation at the edges of the blank, and facilitate blank removal. However, if lubrication is deemed not necessary, or a pre-made blank is obtained, then step  514  might be omitted. 
         [0045]    Step  516  includes blanking the coil or otherwise cutting the coil into pieces to provide smaller workpieces, also referred to herein as tailor-welded blanks (TWB). The TWB are transferred  518  to a heating apparatus. 
         [0046]    Step  520  includes heating the TWB to a desired temperature, such as with the heating apparatus  20 . The TWB might be heated to at least either its solution or solidus temperature of one of the sheets as previously discussed. The step of heating the TWB may occur between 1 to 45 minutes, and still remain commercially viable. 
         [0047]    At step  522 , a die set  24  is cooled to a predetermined temperature as previously described. Cooling of the die set may occur simultaneously with one or more of the previous steps. Step  524  includes transferring the TWB to the die set. For instance, the TWB  12  may be transferred to the staging apparatus  32  with the transfer mechanism  22  such that the TWB  12  is spaced apart from the forming surfaces of the die set  24  as previously discussed. In at least one embodiment, the transfer mechanism  22  may transfer the TWB  12  from the heating apparatus  20  to one die set  24  in 30 seconds or less. 
         [0048]    Step  526  includes positioning the TWB  12  in the die set  24 , such as by actuating the staging apparatus  26  from the first position to the second position to release the TWB  12  onto a die, such as the second die  28 . At step  528 , the die set  24  is closed to form the TWB  12  into a part. In at least one embodiment, the closing of the die set  24  occurs before the TWB  12  cools past a critical quench temperature as previously discussed. In at least one embodiment, the rate of closure of the first and second dies  26  and  28  may be at least 50 millimeters per second to provide “quick contact” between the surfaces of the TWB  12  and the die set  24  and allow for effective conductive heat transfer between the TWB  12  and the die set  24  during quenching. 
         [0049]    Step  530  includes forming and quenching the TWB into a part having a predetermined shape. Quenching occurs simultaneously with forming the TWB  12  as previously discussed. Quenching occurs between a specified temperature cooling range or until the temperature of the part decreases below a predetermined temperature. A temperature sensor may be used to detect the temperature of the part or the die is held for a predetermined period of time, i.e. the hold time. The predetermined hold time may be determined by experimentation or by numerical approximation. 
         [0050]    At step  532 , the die set  24  is held in a closed position, and in one embodiment, the die set  24  is held in the closed position until sufficient heat transfer is complete. In at least one additional embodiment, the die set  24  remains closed on the part for approximately 3 to 60 seconds to remove the remaining heat from the part to be ready for subsequent processing. In addition, the part may be cooled to a temperature that facilitates material handling. 
         [0051]    Step  534  includes opening the die set  24  to facilitate removal of the part, and at step  536 , the part is removed from the die set  24 . Manual or automated material handling techniques may be employed to remove the part as previously discussed. Cooling of the die set  24  may continue during part removal in one or more embodiments. 
         [0052]    At step  538 , additional manufacturing steps may be performed on the part. For instance, additional material may be removed from the part using any suitable process, such as cutting or drilling. In addition, additional forming steps may be taken, such as bending or flanging the part to provide a configuration that may not be provided with the die set  24 . Such steps may be performed within a predetermined period of time, such as within 24 hours, since the part may become too brittle after that time period to allow for the additional manufacturing. 
         [0053]    At  540 , the part may be aged. Aging of the part may consist of naturally aging and/or artificially aging to achieve a high strength temper such as T6 or T7x. There are numerous aging schedules provided by ASM or MIL standards. One aging schedule that works with this method is to naturally age the part at room temperature for 24 hours followed by artificial aging the part at 120° C. for 24 hours. 
         [0054]    The above system and methods may produce a high-strength aluminum-alloy part that is comprised of sheets having different characteristics to balance desired strength and energy-absorbing characteristics with cost and other efficiencies. In addition, the part can have similar characteristics to that of high strength and ultra-high strength steels of similar geometry. High strength aluminum parts may be lighter than parts made from steel of similar geometry. Furthermore, the system and methods in this application produce high strength aluminum alloy parts at a high volume, high quality, and low cost consistent with conventional automotive metal forming. Thus a part made following the teachings of this application may replace a steel structural part with an aluminum alloy structural part without sacrificing safety while reducing overall vehicle weight. In a vehicular application, a lighter automotive part, such as a body structure component including but not limited to a rocker panel, roof rail, bumper structure, or A, B or C-pillar, may reduce vehicle weight and may result in reduced fuel consumption and energy conservation. 
         [0055]    Many different arrangements of the various components depicted, as well as components not shown, are possible without departing from the scope of the claims below. Embodiments of our technology have been described with the intent to be illustrative rather than restrictive. Alternative embodiments will become apparent to readers of this disclosure after and because of reading it. Alternative means of implementing the aforementioned can be completed without departing from the scope of the claims below. Certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations and are contemplated within the scope of the claims.