Heat treatment to reduce distortion

Abstract

A method for processing an aluminum alloy component, comprising: Solution annealing of the component with a solution annealing (SHT) temperature of 500 °C to 535 °C; Quenching the component in a first medium at a first rate to approximately 290 °C; subsequently quenching the component in a second medium at a second rate which is lower than the first rate; and artificial aging of the component at an artificial aging temperature (AA) of 200 °C to 250 °C to a yield strength of at least 200 MPa.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to heat treatments, for example, for reducing or minimizing distortion in metals. STATE OF THE ART

[0002] One approach to reducing vehicle weight in automotive designs is aluminum-intensive vehicles (AIVs). AIVs have often been based on the self-supporting design of steel automotive architectures, which are assemblies of stamped sheet metal components. Automotive AIV design has primarily focused on 5XXX and 6XXX series aluminum sheet because of its ability to be formed and fabricated using processes consistent with those already used in automotive sheet metal manufacturing (e.g., sheet metal stamping, automated assembly, and painting processes). These alloys can have strengths equivalent to mild steel sheet generally used in steel vehicle platforms. 6XXX series aluminum alloys can experience enhanced mechanical strength properties when subjected to certain heat treatment processes.

[0003] For some applications, multiple components, such as sheet metal, may be joined together. One method for mechanically joining multiple components, such as 2T, 3T, and 4T stackups (e.g., 2, 3, or 4 sheets in a stack), may include the use of self-drilling rivets (SPRs). An SPR is a cold rivet joining process used to fasten two or more sheets of material by driving rivets through the top sheet(s), which may form a "button" on the bottom sheet. However, if the sheets lack sufficient joining strength or rivetability, defects in the sheets and / or rivet may occur. Examples of defects include radial cracking of the rivet button, cracking in the sidewall of the rivet button, a crack in the stack, or yielding of the rivet legs. In general, stronger materials tend to have lower joining strength.Therefore, joining processes such as SPRs can result in structural defects when several components with high strength are joined together. SUMMARY

[0004] In at least one embodiment, a method for processing an aluminum alloy component is provided. The method may include solution heat treating the component at a solution heat treatment (SHT) temperature of 500°C to 535°C, quenching the component in a liquid quench medium having a temperature of 75°C to 95°C, and artificially aging the component at an artificial aging (AA) temperature of 200°C to 250°C to a yield strength of at least 200 MPa.

[0005] The component may be a 6XXX series aluminum alloy. In one embodiment, the artificially aging step comprises artificially aging the component to an r / t ratio of less than 0.3. In another embodiment, the SHT temperature is 505°C to 530°C. The solution heat treating step may comprise heat treating the component for 2 to 4 hours. The liquid quench medium may have a temperature of 80°C to 90°C. In another embodiment, the liquid quench medium has a temperature of 82°C to 88°C. The liquid quench medium may be water. In one embodiment, the artificially aging step comprises heat treating the component for 2 to 8 hours. The method may further comprise joining the aluminum alloy component to a second component with a self-drilling rivet.

[0006] In at least one embodiment, a method for processing an aluminum alloy component is provided. The method may include solution heat treating the component at a solution heat treatment (SHT) temperature of 505°C to 530°C for 2 to 4 hours, quenching the component in a liquid quench medium having a temperature of 80°C to 90°C, and artificially aging the component to a yield strength of at least 200 MPa.

[0007] In one embodiment, the artificial aging step comprises heat treating the component at an artificial aging temperature of 200°C to 250°C for 2 to 8 hours. The component may be a 6XXX series aluminum alloy. In one embodiment, the artificial aging step comprises artificially aging the component to an r / t ratio of less than 0.3. The liquid quenching medium may be water. The method may further comprise joining the aluminum alloy component to a second component with a self-drilling rivet.

[0008] In at least one embodiment, a method of forming a vehicle structural component is provided. The method may comprise stamping a sheet of 6XXX series aluminum alloy in a progressive die to form a component having at least two non-coplanar surfaces, solution heat treating the component, quenching the component in a liquid quench medium having a temperature of 75°C to 95°C, and artificially aging the component to a yield strength of at least 200 MPa and an r / t ratio of no more than 0.3.

[0009] In one embodiment, the solution treatment step comprises solution treatment of the component at a solution treatment (SHT) temperature of 505°C to 530°C for 2 to 4 hours, and the artificial aging step comprises heat treating the component at an artificial aging (AA) temperature of 200°C to 250°C for 2 to 8 hours. The liquid quench medium may be water and may have a temperature of 82°C to 88°C. The method may further comprise joining the 6XXX series aluminum alloy component to a second component with a self-drilling rivet. BRIEF DESCRIPTION OF THE DRAWINGS Fig. Figure 1 is a schematic graph of strength versus artificial aging time showing several tempering stages of aluminum alloys, Fig. 2 is a photograph of a semi-guided wrap-bend tester that can be used to test the bending ability of an aluminum alloy, Fig. 3 is an example of a section tested using the wrap-bend tester of the Fig. 2 is tested, Fig. 4 is a cross-section of a stack of metal sheets to be joined according to an embodiment, Fig. 5 is a front perspective view of a side door latch reinforcement component that can be manufactured according to the disclosed methods, Fig. 6 is a rear perspective view of the side door latch reinforcement member of the Fig. 5, Fig. 7 is a perspective view of a vehicle underbody reinforcement component that can be manufactured according to the disclosed methods, Fig. 8 is another perspective view of the vehicle underbody reinforcement member of the Fig. 7, Fig. Figure 9 is a comparison of the dimensions before and after heat treatment of a plurality of components following a previous T7 heat treatment, Fig. 10 is a comparison of the dimensions before and after heat treatment of a plurality of components following a modified T7 heat treatment according to one embodiment, Fig. 11 is a table of solution heat treatment, quenching, and artificial aging parameters according to several embodiments, and the resulting properties for a plurality of components, Fig. 12 is a table of solution annealing parameters, including quenching temperature and resulting properties for a variety of components, Fig. 13 is a flow diagram of a method of forming or processing an air-quenching aluminum alloy according to one embodiment, and Fig. 14 is a comparison of the before and after heat treatment dimensions of a plurality of components following a modified T7 heat treatment according to one embodiment. DETAILED DESCRIPTION

[0010] As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in different and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Specific structural and functional details disclosed herein are therefore not to be interpreted as limiting, but merely as an illustrative basis for teaching one skilled in the art to variously employ the present invention.

[0011] Aluminum alloys are generally identified by a four-digit number, with the first digit typically identifying the main alloying element. Additional digits, represented by the letter "x" in the series designation, define the exact aluminum alloy. For example, the main alloying element of the 5XXX series is magnesium, and for the 6XXX series, it is magnesium and silicon. Aluminum alloys of the 5XXX and 6XXX series, which are aluminum-magnesium and aluminum-magnesium-silicon alloys, respectively. The 5XXX and 6XXX series aluminum alloys can be formed and processed using processes consistent with those for structural steel sheet. The 7XXX series, which generally has great strength, has aluminum and zinc as its main alloying elements.

[0012] Examples of specific 6XXX series alloys may include 6061, which has a composition including 0.4-0.8% silicon, up to 0.7% iron, 0.15-0.40% copper, up to 0.15% manganese, 0.8-1.2% magnesium, 0.04-0.35% chromium, up to 0.25% zinc, up to 0.15% titanium, and other elements up to 0.05% each (0.15% total), with all percentages by weight, the remainder being aluminum. Numerous automotive components may include aluminum 6061, such as brackets, body panels, fasteners, and others. Another specific example of a 6XXX series alloy may be 6111, which may have a composition comprising 0.5-1% magnesium, 0.6-1.1% silicon, 0.5-0.9% copper, 0.1-0.45% manganese, up to 0.4% iron, up to 0.15% zinc, up to 0.1% chromium, up to 0.1% titanium, and other elements up to 0.05% each (0.15% total), with all percentages being by weight, the remainder being aluminum.Numerous automotive components may incorporate aluminum 6111, such as body panels, stanchions, and others. Components utilizing aluminum 6111 may require higher yield strengths than those utilizing aluminum 6061. Other specific alloys of the 6XXX series are known in the art, such as 6009, 6010, 6016, 6022, 6053, 6063, 6082, 6262, 6463, and others.

[0013] The 6XXX and 7XXX series aluminum alloys can be age-hardened (precipitation-hardened) to increase their strength and / or toughness. Age-hardening by solution heat treatment (SHT or solutionizing) is preceded by quenching the aluminum alloy metal. Generally, solution heat treatment involves heating the alloy at least above its solvus temperature and holding it at the elevated temperature until the alloy forms a homogeneous solid solution, or a solid phase and a liquid phase. The temperature at which the alloy is maintained during solution heat treatment is known as the solution temperature. This solution temperature can be the temperature at which a substance mixes easily. Miscibility is the property of materials to mix in all proportions and form a homogeneous solution. Miscibility can be present in all phases: solid, liquid, and gas.

[0014] Following solution treatment, the quenching step is performed, in which the alloy is rapidly cooled below the solvus temperature to form a supersaturated solid solution. Due to the rapid cooling, the atoms in the alloy do not have time to disperse over distances long enough to form two or more phases within the alloy. The alloy is therefore in a non-equilibrium state. Quenching can be accomplished by immersing the alloy in a quenching medium, such as water or oil, or by other means of applying the quenching medium (e.g., spraying). Quenching can also be accomplished by contacting the alloy with a cooled surface, such as a water-cooled plate or die. The quenching rate can be any suitable rate to form a supersaturated solution in the quenched alloy.The quenching rate can be set within a specific temperature range, for example, from 400 °C to 290 °C. Quenching can be carried out until the alloy is at a sufficiently cool temperature such that the alloy remains in a supersaturated state (e.g., where diffusion is significantly slowed), such as about 290 °C. The alloy can then be air-cooled or otherwise cooled at a rate slower than the quenching rate until a desired temperature is reached. Alternatively, quenching can be carried out to a lower temperature, such as below 100 °C, or down to about room temperature.

[0015] Age hardening involves heating and holding the alloy at an elevated temperature where two or more phases are in equilibrium. The supersaturated alloy forms fine, scattered precipitates throughout as a result of diffusion within the alloy. The precipitates begin as clusters of atoms that 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 increase in size, they become precipitates, which strengthen the alloy by preventing dislocation movement. Because the precipitates are very finely distributed within the alloy, dislocations cannot move easily and must either go around the precipitates or cut through them to propagate.

[0016] Five basic treatment condition designations can be used for aluminum alloys: F—as produced, O—as annealed, H—work hardened, T—heat treated, and W—quenched (between solution heat treatment and artificial or natural aging). The raw material in the "as received" condition for the disclosed solutionizing and age hardening processes may originally have any of the above treatment condition designations. The treatment condition designation may be followed by a single- or double-digit number for further description. An aluminum alloy with a T6 treatment condition designation may be an alloy that has been solution heat treated and artificially aged, but has not been cold worked after solution heat treatment (or such that cold work would not be evident in the material properties).T6 may represent the point of peak aging yield strength along the yield strength versus time and temperature profile for the material. A 6XXX series aluminum alloy that has a T6 temper may have a yield strength of at least 240 MPa. For example, 6061 with a T6 temper may have a yield strength of about 250 MPa, and 6111 with a T6 temper may have a yield strength of about 300 MPa. A T7 temper may indicate that solution heat treatment has occurred and that the material has been artificially aged (overaged) beyond the peak aging yield strength along the yield strength versus time and temperature profile. A material with T7 temper may have a lower yield strength than a material with T6 temper, but the T7 temper may improve other properties, such as increased toughness compared to the T6 temper.A T8 temper is similar to a T7 temper in that it has been aged beyond its peak yield strength (for example, T6). However, a material with a T8 temper is artificially aged after the material has been cold worked. For example, sheet made of 6111 alloy can be stamped in a T4 temper and then age-hardened to T8, forming a T8 temper.

[0017] The relative strengths and toughnesses of 6XXX series aluminum alloys as a function of aging time are shown in Fig. 1. As discussed above, T6 represents peak aging and the highest yield strength, while T7 represents overaging and reduced (but still improved) yield strength. The T8 temper is not shown on the graph, but is similar to T7 in that it has a lower yield strength than T6 and is located to the right of the T6 peak age. The T4 temper is shown to the left of peak aging and may have similar properties to T7 / 8 (e.g., reduced strength and increased toughness compared to T6), but represents underaging rather than overaging. Underaging to a T4 temper may replace age hardening to T7 or T8 tempers in the present disclosure, but underaging may be more difficult to control and repeat. Overaging may therefore be a robust and consistent process compared to underaging.

[0018] Aluminum alloys with T7 and T8 tempers (e.g., 6XXX and 7XXX) generally have increased flexural toughness compared to the T6 temper. One method of measuring toughness can be determining the failure type a component exhibits after deformation. When a sheet or section of material is bent to failure, the failure may be transgranular or intergranular. Transgranular failure, or failure across or through the grains of the alloy, may indicate higher toughness than intergranular failure, in which failure occurs along grain boundaries (e.g., between grains). Intergranular failure can occur when the grain boundaries are brittle or weak, which may be due to the alloy composition, the type of heat treatment, or other factors (or a combination thereof).

[0019] The T7 and T8 alloys disclosed herein may exhibit transgranular failure instead of intergranular failure during bending due to their increased toughness (for example, compared to T6).

[0020] While the flexural strength of T7 and T8 tempers may be greater than that of a T6 temper, a 6XXX series aluminum with a T7 or T8 temper may have a lower yield strength than a T6 temper due to overaging. However, 6XXX series alloys hardened according to the disclosed embodiments may maintain a yield strength of at least 200 MPa. Certain alloys (e.g., 6061) age-hardened to a T7 or T8 temper (e.g., by using the hardening treatments described above), for example, may have a yield strength of at least 200, 210, 220, 230, 240 MPa, or higher. Some alloys (for example 6111) may have higher yield strengths following age hardening heat treatment (such as that described above), for example at least 250, 260, 270, 280, 290 MPa or higher.

[0021] The ability of an aluminum alloy component or member to be joined to other components or members can be described as its "joinability." One method of joining components together is riveting. Conventional rivets have a head and a cylindrical body, with the body being inserted into an opening in the components to be joined and then deformed to form a second head. Self-drilling rivets (SPRs) are another form of rivet that does not require pre-formed holes in the components to be joined. SPRs generally have a hardened, semi-tubular body that is inserted into the upper component(s) to be joined, but does not penetrate all the way through the lower component. A lower die is placed beneath the base component, causing the SPR to expand and form a ring-shaped knob on the base component.

[0022] Additionally, it has been discovered that joinability (e.g., the ability to be riveted) can be correlated with and / or predicted by bendability measurements. Bendability, as used in the present disclosure, can be quantified as an "r / t ratio," which is the ratio of the bend radius (r) to the sheet thickness (t). The smaller the r / t ratio, the more bendable the sheet. An example of a piece of equipment used to measure bendability is shown in Fig. 2. The equipment shown is a semi-guided wrap-bend tester that complies with standards such as ASTM E290 and Ford Laboratory Test Method (FLTM) B114-02. The bendability can be defined and measured according to FLTM BB 114-02, and the r / t ratio can be calculated based on a prescribed bend rate. In at least one embodiment, the r / t ratio to failure can be calculated based on a bend rate of about 5 or more, with a crack propagating completely across the width of the bent specimen. The r / t failure calculation can be viewed as a normalized, relative mechanical assessment of the toughness of an aluminum alloy. An example of a coupon tested using the wrap-bend tester is shown in Fig. 3. In general, it has been discovered that 6XXX series aluminum alloys (e.g., 6061 and 6111), which have a bendability r / t ratio of about 0.3 or less, can be joined using SPRs without the defects mentioned above (e.g., stack or button cracking). Certain alloys may be joinable at higher r / t ratios; for example, 6111 may be joinable at r / t ratios up to about 0.4. Joinability may be possible at r / t ratios above 0.4, however, the riveting process may not be robust at higher r / t ratios, which can lead to unacceptable failure rates.

[0023] To be used in certain vehicle applications, aluminum alloys (for example, the 6XXX series) must be able to be joined to other metal components. With reference to Fig. 4, a stack 10 of elements or layers is shown. The stack 10 may have a top element 12 and a bottom element 14. In addition, additional intermediate elements / intermediate layers 16 (not shown) may be located between the top element 12 and the bottom element 14. In one embodiment, the stack 10 has up to four layers: a top layer 12, a bottom layer 14, and one or two intermediate layers 16. At least one of the layers may be a 6XXX series aluminum alloy, which may have a T7 or T8 temper. In at least one embodiment, the bottom layer 14 is a 6XXX series aluminum alloy, which may have a T7 or T8 temper and the properties described above. The top layer 12, the bottom layer 14, and the intermediate layer(s) 16 may be made of the same material (for example, a 6XXX alloy with a T7 or T8 temper).However, one or more of the layers may be formed from different materials, such as other aluminum alloys or steels. The stack 10 may have a total thickness of up to 6, 8, 10, or 12 mm. Each layer may have a thickness of 0.5 to 5 mm or any subrange therein, such as 0.8 to 4 mm, 1 to 3.5 mm, or others. In one embodiment, the bottom layer 14 may be thicker than any of the other layers (but it is not necessarily thicker). For example, the bottom layer 14 may have a thickness of 1.5 to 4 mm or any subrange therein.

[0024] It has been found that the use of SPRs with 6XXX series aluminum alloys with a T6 temper may not be feasible. Numerous joint defects can occur when using SPRs on a stack containing a 6061 alloy with a T6 temper as the bottom layer 14. Cracking can occur within the stack 10. The bottom layer 14 can crack and separate at least partially around the edge of the knob. The knob itself can crack, for example, in the sidewall. Radial cracking of the knob can also occur. Additionally, the legs of the SPR can yield.

[0025] However, it has been discovered that 6XXX series alloys with a T7 or T8 temper have increased joinability, for example, with SPRs. Without adhering to any particular theory, it is believed that the increased flexural toughness of T7 / T8 temper alloys compared to T6 temper alloys can improve joinability. Stack cracking can be avoided, as can button cracks (both sidewall and radial). In addition, the rivet (for example, an SPR) can remain in close contact with the 6XXX aluminum alloy after riveting. In other words, the rivet can remain in essentially continuous contact with the 6XXX aluminum alloy along the portion of the rivet surface that is embedded in the 6XXX alloy. For example, there may be no cracks in the 6XXX alloy or the SPR, and / or no gaps between the 6XXX alloy and the SPR surface.

[0026] To achieve a T6 temper in a 6XXX series alloy, solution heat treatment and quenching are performed as described above, followed by an age-hardening heat treatment. The standard age-hardening heat treatment to achieve a T6 temper in a 6XXX series alloy may be at a temperature of approximately 160°C to 180°C for 6 to 18 hours (generally, if the temperature is near the top of the range, the time goes to the bottom of the range, and vice versa). However, there is no industry standard for tempering a 6XXX series alloy to a T7 or T8 temper (for example, no ASTM standard or military specification). A T7 temper was previously described in common-owner, co-pending US 2015 / 0 125 713 A1, the disclosure of which is incorporated herein by reference in its entirety.Additionally, an air-cooled T5 coating was previously described in common-owner, co-pending US 2016 / 0 168 676 A1, the disclosure of which is incorporated herein by reference in its entirety.

[0027] In general, faster quenching of an age-hardened aluminum alloy can result in a finished component that has a higher yield strength but lower toughness or ductility compared to a component of the same alloy that was cooled more slowly. It has been found that faster quenching can also cause increased distortion in the component after the artificial aging heat treatment. However, it has been shown that increased cooling temperatures result in lower yield strengths, which can make components unsuitable for certain applications. It has been discovered that for the 6XXX series alloys, a certain threshold of distortion, yield strength, and ductility (an indication of toughness) can be achieved by liquid quenching with a liquid medium that has a certain range of temperatures above room / ambient temperature.In at least one embodiment, a 6XXX series component having a yield strength of at least 200 MPa, a bendability of r / t ≤ 0.3, and reduced distortion can be realized by using a liquid quench and a modified solution heat treatment and artificial aging system.

[0028] Liquid quench temperatures are generally at or near room / ambient temperature, such as approximately 20°C to 26°C. These quench temperatures can provide rapid cooling (for example, hundreds of °C / second), resulting in high yield strengths. However, this rapid cooling can lead to high levels of distortion in the finished parts, particularly those with more complex geometries. The resulting amount of distortion at any given location on the part may, for example, be equal to or greater than 0.5, 0.7, 1.0, or 1.5 millimeters from the target geometry surface of the part. Distortion may be more severe or problematic in larger and / or more complex components.For example, components that have multiple joining surfaces may experience distortion significant enough to cause misalignment or cause one or more components in a system to be outside acceptable tolerances.

[0029] The problem can be even more serious when a component has multiple non-coplanar joining surfaces. Examples of two components having multiple non-coplanar joining surfaces are shown in the Fig. 5 to 8. A side door latch reinforcement component 20 is shown in the Fig. 5 and Fig. 6. The side door latch reinforcement component 20 has a plurality of mating surfaces 22, 24, and 26 that are not coplanar. If one or more of the mating surfaces 22, 24, or 26 are distorted beyond a certain acceptable tolerance or threshold, the other mating surfaces may be misaligned or out of specification. Misalignment can lead to numerous problems, such as water / wind noise, visual misalignment, and problems with latching and / or sealing the door. A vehicle underbody reinforcement 30 is shown in the Fig. 7 and Fig. 8. The vehicle underbody reinforcement 30 has a plurality of mating surfaces 32, 34, 36, and 38. If one or more of the mating surfaces 32, 34, 36, or 38 are distorted beyond a certain acceptable tolerance or threshold, the other mating surfaces may be misaligned or out of specification. Distortion tolerances for mating surfaces may vary depending on the application, but in at least one embodiment, a distortion tolerance for a mating surface may not exceed ±1.5 mm, for example, less than or equal to ±1.0 mm, ±0.7 mm, or ±0.5 mm. Components manufactured using the disclosed processes may have a variety of mating surfaces that are within the distortion tolerance.

[0030] Aluminum alloys, for example, the 6XXX series aluminum alloys such as 6061-O, may experience distortion when heat treated with previous solution heat treatment and age hardening systems. The distortion may be greater if the component has been cold worked, such as by progressive stamping. With reference to Fig. Figure 9 shows before and after heat treatment measurements for a T7 heat treatment, with light cells representing an within-spec measurement and shaded cells representing an out-of-spec measurement. The y-axis represents test specimens 1 through 15, and the x-axis represents ten different test locations on each test specimen. The ± units are in mm for the specified location (for example, -0.05 is 0.5 mm less than the specification). The T7 heat treatment performed on these specimens included a solution heat treatment (SHT) at approximately 529 °C for 3 hours and quenching using liquid water at approximately 54 °C. The artificial aging process involved a temperature of 230 °C for 6 hours. The resulting components had an average yield strength of 235 MPa and a bending strength ratio (r / t ratio) of less than 0.30, which is quite good. However, as shown in Fig. As shown in Figure 9, a significant number of samples had multiple out-of-specification locations, which may be unacceptable in certain applications.

[0031] In an attempt to improve the distortion values, a different heat treatment process was performed with a reduced solution heat treatment temperature of approximately 503 °C and an increased quench temperature of approximately 88 °C. The SHT time and the AA temperature and time were kept the same. The distortion results obtained in Fig. The results shown in Figure 10 indicate that distortion was largely reduced, and all 15 specimens were within specification at all locations. However, the modified heat treatment resulted in a significant loss of yield strength, averaging 124 MPa. While the modified heat treatment consequently improved the distortion of the parts compared to the original heat treatment, the yield strength properties were reduced to a level that may be unacceptable in certain applications (for example, certain automotive structural components).

[0032] It has been discovered that by varying the solution heat treatment, quenching, and artificial aging heat treatment parameters (e.g., temperature and time), age-hardened 6XXX series aluminum alloy components can have high yield strength (e.g., at least 200 MPa), good bendability (e.g., r / t ratio less than 0.3), and low distortion levels. It has been found that each parameter can have a significant impact on these properties, and that adjustments to one parameter may require adjustments to the others to maintain the above properties.

[0033] It has been found that lower SHT temperatures can generally reduce or minimize distortion. However, adjusting the SHT temperature alone may not provide sufficient distortion reduction for certain part compositions and / or geometries. Furthermore, it has been found that increasing the temperature of the quench medium, for example, a liquid quench medium (e.g., water), to a certain range can provide improved distortion characteristics while also providing higher yield strength values. However, adjusting both the SHT temperature and the quench medium temperature may require additional changes to the SHT and AA temperatures and / or times to achieve mechanical properties similar to those of the preceding heat treatments (e.g., YS of at least 200 MPa and r / t ratio below 0.3).

[0034] As described above, it has been found that a lower solution heat treatment temperature can reduce or minimize distortion (other parameters remaining constant). In at least one embodiment, the SHT can be performed at a temperature between the solvus temperature and 540°C, or in any subrange therein. In one embodiment, the SHT temperature can be between 500°C and 535°C. In another embodiment, the SHT temperature can be between 505°C and 530°C. In another embodiment, the SHT temperature can be between 505°C and 530°C. Non-limiting examples of SHT temperatures can include about 508°C, about 519°C, or about 530°C. Maintaining furnaces at exact temperatures can be difficult, and therefore the term "about" can have a tolerance of ± 5°C.In one embodiment, the SHT may be performed at a constant or substantially constant temperature within the above ranges (e.g., ±5°C). The SHT time may vary depending on the SHT temperature. Generally, a higher SHT temperature may allow for a shorter SHT time, and vice versa. In at least one embodiment, the SHT time may be between 0.5 and 5 hours, or any subrange therein. The SHT time may be, for example, 1 to 5 hours, 2 to 5 hours, 1 to 4 hours, or 2 to 4 hours. Non-limiting examples of SHT times may include about 2, 3, or 4 hours, where "about" generally means ±15 minutes.It has been found that the above SHT temperatures and times can allow for higher strength components that are flexible and slightly distorted when combined with the quench medium and artificial aging process described below.

[0035] It has been discovered that a liquid quench medium, such as water, having a temperature within a certain range can allow a 6XXX series aluminum alloy component to have great strength and flexural capability while reducing distortion (particularly for complex parts). In at least one embodiment, the liquid quench medium can have a temperature of 75°C to 95°C, or some subrange therein. For example, the liquid quench medium can have a temperature of 80°C to 90°C or 82°C to 88°C. In one embodiment, the liquid quench medium can have a temperature of about 85°C, where "about" can be ± 3°C. Typical liquid quench medium temperatures are at room temperature (e.g., 20°C to 26°C) or slightly above (e.g., up to about 55°C). Lower quench temperatures result in faster cooling rates, which generally result in higher yield strengths.A room temperature quench medium is also easier and more cost-effective to maintain. Low quench temperatures are therefore typically preferred. However, a certain liquid medium temperature range has been discovered that still provides great strength and flexural capability after artificial aging (e.g., ≥ 200 MPa) while also reducing the amount of distortion that occurs in components from AA heat treatment. In one embodiment, the quench rate may be maintained at about 80 to 100 °C / s. The quench rate may, for example, be maintained at 80 to 90 °C / s or about 85 °C / s (e.g., ± 5 °C / s). The quench rate may be maintained during at least a portion of the cooling process, such as from when the component is about 475 °C to about 290 °C.To achieve the high strength and flexural capability using the elevated temperature of the liquid quenching medium, the parameters of SHT and AA temperature and time may need to be adjusted from conventional parameters and, similar to the temperature of the quenching medium, must be within certain specific ranges.

[0036] In at least one embodiment, the artificial aging temperature may be between 200°C and 250°C, or any subrange therein. The AA temperature may, for example, be between 200°C and 245°C, 215°C and 245°C, 225°C and 245°C, or 230°C and 245°C. Non-limiting examples of AA temperatures may include about 200°C, about 215°C, about 230°C, or about 245°C. Maintaining ovens at exact temperatures can be difficult, and therefore the term "about" may have a tolerance of ± 5°C. In one embodiment, the AA process may be carried out at a constant or substantially constant temperature within the above ranges (for example, ± 5°C). In general, lower AA temperatures relative to lower SHT temperatures may result in higher yield strength, but may also result in lower flexural strength.The AA time may vary depending on the AA temperature and may generally be shorter for higher temperatures (and vice versa). In at least one embodiment, the AA time may be between 0.5 and 10 hours, or any subrange therein. For example, the AA time may be 1 to 9 or 2 to 8 hours. Non-limiting examples of AA times may include about 2, 3, 4, 5, 6, 7, or 8 hours, where "about" may generally mean ± 15 minutes.

[0037] In at least one embodiment, the components heat-treated and processed according to the disclosed methods may be sheet metal components. Sheet metal components may have a thickness of 0.5 to 5 mm or any subrange therein, such as 0.8 to 4 mm, 1 to 3.5 millimeters, or others. Sheet metal may be in large format sheets wound on a coil and unwound to be cut and formed. In embodiments where a sheet stack is formed and joined, for example, using self-drilling rivets, there may be 2 or more sheets, such as 2 to 10 sheets or any subrange therein (for example, 2, 3, 4, 5, or more sheets). The stack 10 may have a total thickness of up to 6, 8, 10, or 12 mm. The processing may include a forming process prior to the heat-treating steps.The forming process may include a stamping process in which the component is stamped and / or formed. In one embodiment, the forming process may include progressive stamping using a progressive die. Progressive stamping generally involves multiple sheet metal stamping operations using more than one die or die station. Progressive stamping allows the formation of complex components, such as those having multiple non-coplanar joining surfaces. The process may also be applied to unformed components, such as sheet metal from a coil. Additionally, the process may begin with a component that has previously been formed, for example, using progressive stamping or other forming processes. The heat treatment of sheet metal may differ from that of castings.For example, castings are not affected by the quenching rate to the same extent as sheet metal stampings due to the higher surface-to-volume ratio of sheet metal. Castings therefore typically require a longer soak time for homogenization. Additionally, castings tend to solution heat treat at higher temperatures, for example, approximately 30°C higher than sheet metal.

[0038] With reference to Fig. Figure 11 shows a table showing 18 runs of 6061 aluminum alloy coupons undergoing various SHT and AA treatments with an elevated quenching temperature. SHT temperatures of 508 °C, 519 °C, and 530 °C were tested with a common SHT time of 4 hours. The quenching temperature was maintained at a constant 88 °C for each run, and the quenching medium was liquid water. AA temperatures of 200 °C, 215 °C, 230 °C, and 245 °C were tested, while the AA time varied between 2, 4, 6, and 8 hours. The average yield strength was tested for each run, as was the bendability ratio to determine if it achieved a target r / t ratio of less than 0.3 (15 specimens per run, 5 for YS and 10 for bendability).

[0039] As shown in the table, all runs resulted in an average yield strength of at least 200 MPa. This is in stark contrast to the samples tested with reference to Fig. 10 were measured, which had an average yield strength of 124 MPa despite the same elevated quench temperature. The data thus demonstrate that high yield strengths can be achieved even at an elevated quench temperature using certain combinations of SHT temperature / time and AA temperature / time. The runs with the lowest AA temperature in each set of SHT temperatures (for example, runs 1-2, 7-8, and 13-14) had substantially higher yield strengths than others in the same set, but did not meet the target r / t < 0.3. Consequently, even within ranges discovered to provide high strength and low distortion, certain combinations do not provide all of the desired properties.This further demonstrates that there is a complex relationship between SHT temperature / time, quenching temperature, and AA temperature / time, and that changes in any one parameter have a strong impact on the final properties.

[0040] For comparison, Fig. Figure 12 shows a table of experimental data from 10 runs of aluminum alloy 6061 components subjected to T7 heat treatment at various SHT and quenching temperatures. The data show mean yield strength and Percent Inspection Points That Satisfy Tolerance (PIST) difference values. As shown, it is very difficult to achieve satisfactory values for all three properties (e.g., yield strength ≥ 200 MPa, r / t < 0.3, and PIST difference low or equal to zero).

[0041] With reference to Fig. 13, a method or process 100 is shown for forming an aluminum alloy component having high strength, good bendability, and low distortion, such as a T7 temper. At step 102, an optional forming process may be performed. The forming process may include a stamping process in which the component is stamped and / or formed. In one embodiment, the forming process may include progressive stamping using a progressive die (described above). While the process 100 is shown with the forming step 102, the process is also applicable to unformed components, such as sheet metal from a coil. Additionally, the process 100 may begin with a component that has previously been formed, for example, using progressive stamping or other forming processes.

[0042] At step 104, a solution heat treatment (SHT) is performed on an aluminum alloy component, such as an age-hardenable 6XXX series aluminum alloy component (e.g., 6061 or 6111). The component may have been formed at step 102 or may be an as-received component. The alloy may have any of the basic treatment condition designations described above, for example, an O-temper (annealed) or an F-temper (as manufactured). In at least one embodiment, the SHT may be performed at a temperature between the solvus temperature and 540°C, or in some subrange therein. In one embodiment, the SHT temperature may be between 500°C and 535°C. In another embodiment, the SHT temperature may be between 505°C and 530°C. In another embodiment, the SHT temperature may be between 508 °C and 530 °C.In another embodiment, the SHT temperature may be between 505°C and 515°C. Non-limiting examples of SHT temperatures may include about 508°C, about 519°C, or about 530°C (where "about" may be ± 3°C). The SHT temperature depends on the solvus temperature of each alloy. A solution heat treatment temperature significantly above the solvus temperature may result in the onset of melting. A SHT temperature significantly below the solvus temperature may result in insufficient dissolution of solid solutions. Both conditions can be detrimental to the mechanical properties of heat-treatable aluminum alloys. Additionally, relatively high SHT temperatures may lead to increased distortion in a finished part following an artificial aging heat treatment. The SHT time may vary depending on the SHT temperature. In general, a higher SHT temperature may allow a shorter SHT time and vice versa.In at least one embodiment, the SHT time may be between 0.5 and 5 hours, or any subrange therein. For example, the SHT time may be 1 to 5 hours, 2 to 5 hours, 1 to 4 hours, or 2 to 4 hours. Non-limiting examples of SHT times may include about 2, 3, 3.5, or 4 hours. The SHT may be performed using any suitable heating equipment, such as a furnace or oven, which may be stationary or continuous.

[0043] At step 106, a quenching process is performed following the SHT. The time interval between the end of the SHT and the start of the quenching process may be referred to herein as the quench delay. In at least one embodiment, the quench delay may be 30 seconds or less, for example, up to 20 seconds or up to 15 seconds. The quenching process may comprise liquid quenching, wherein the component is exposed to a liquid medium (e.g., water or oil) that has a temperature lower than the component. The liquid may be heated (e.g., above room temperature). In at least one embodiment, the liquid quench medium may have a temperature of 75°C to 95°C or any subrange therein. For example, the liquid quench medium may have a temperature of 80°C to 90°C or 82°C to 88°C.In one embodiment, the liquid quench medium may have a temperature of about 85°C, where "about" may be ± 3°C. In one embodiment, the liquid medium may be water.

[0044] It has been discovered that for 6XXX series aluminum alloys, a certain threshold level of yield strength, ductility (an indication of toughness), and reduced distortion can be achieved by liquid quenching within a specific quenching temperature range. In at least one embodiment, a 6XXX series component having a yield strength of at least 200 MPa and a ductility of r / t ≤ 0.3 and low distortion (e.g., less than ± 1.0, 0.7, or 0.5 mm) can be achieved using an elevated temperature liquid quench. These properties may allow components to be used as structural members in certain applications, such as vehicles (e.g., Al-intensive trucks). Quenching temperatures outside these ranges may result in components that are 1) strong but not tough, 2) tough but weak, or 3) strong and tough but too distorted.

[0045] In at least one embodiment, the component(s) may be quenched through the entire cooling temperature range using the elevated temperature liquid medium, such as from the SHT temperature to the natural aging temperature or the onset of the artificial aging temperature. The process 100 may include only liquid quenching and no other type of quenching, such as air. In one embodiment, the quenching step 106 may include quenching at the temperatures described above at least a certain temperature range, such as from the temperature of the component after the SHT (and any quench delay) to a low threshold temperature at which the quenching process is substantially complete.For example, quenching using heated water can be performed at least from the time the components are at approximately 475°C (e.g., after SHT and any quench delay) until they are at approximately 290°C. Once the component(s) have reached a certain temperature, such as approximately 290°C, they can continue to be quenched, but at a lower rate (e.g., by using air or a different fluid temperature / medium).

[0046] At step 108, the component may be naturally aged. Natural aging generally involves allowing a component to rest at or near room temperature for a specified period of time. Following a quench, natural aging may cause precipitation hardening to begin, albeit at a very slow rate. In the context of large-batch or continuous manufacturing, natural aging may occur as a result of production scheduling and different batch sizes for different processes. The SHT and quench processes may have smaller batch sizes than a subsequent artificial aging process. The first batches of components to be solution treated may be set aside until the remaining batches are complete so that they can all be artificially aged in one large batch. While the batches await artificial aging, they age naturally.Because such components may wait longer than others before being artificially aged, the amount of natural aging for each component may vary depending on the batch it is in, the batch size, or other factors. In at least one embodiment, the component(s) may be naturally aged for up to 24 hours. However, some components may be naturally aged within a shorter time, such as 4, 8, 12, 16, or 20 hours, and some components may not be naturally aged at all (for example, a final batch may be artificially aged immediately after a THT and quenching). Natural aging for more than 24 hours is also possible, but such a relatively long aging process may not be favorable for higher-volume manufacturing processes or those where high levels of consistency between batches are very important.

[0047] At step 110, the component is artificially aged to precipitation harden the component. As described above, the standard age-hardening heat treatment to achieve a T6 temper for a 6XXX alloy may be at a temperature of approximately 160°C to 180°C for 8 to 18 hours. However, the standard heat treatment is based on an alloy that is conventionally quenched (for example, using a low-temperature liquid quench). It has been discovered that a significantly shorter artificial aging heat treatment can be used to produce a 6XXX series aluminum alloy with liquid quench, high strength, high flexion, and low distortion. In at least one embodiment, the artificial aging temperature may be between 200°C and 250°C, or some subrange therein.For example, the AA temperature may be between 200°C and 245°C, 215°C and 245°C, 220°C and 245°C, 220°C and 235°C, 220°C and 230°C. Non-limiting examples of AA temperatures may include about 200°C, about 215°C, about 220°C, about 220°C, 230°C, or about 245°C (where "about" may be ± 3°C). The AA time may vary depending on the AA temperature and may generally be shorter for higher temperatures (and vice versa). In at least one embodiment, the AA time may be between 0.5 and 10 hours, or some subrange therein. The AA time may be, for example, 1 to 9 or 2 to 8 hours. Non-limiting examples of AA times may include 2, 3, 4, 5, 5.5, 6, 7, or 8 hours.

[0048] The AA process can result in components having a yield strength of, for example, at least 200 MPa, at least 210 MPa, or at least 220 MPa. In addition to the increased yield strength, the components can also have good flexural strength and toughness, as demonstrated by low r / t ratios. In one embodiment, the components can have an r / t ratio of less than 0.4, for example, 0.3 or less, or 0.27 or less. Components produced using process 100 can therefore have yield strengths of at least 200 MPa and r / t ratios of 0.3 or less, while also having low distortion levels.Distortion tolerances for components may vary depending on the application, but in at least one embodiment, a distortion tolerance for a component at at least one or more locations (e.g., a joining surface) may not be more than ± 1.5 mm, for example, less than or equal to ± 1.0 mm, ± 0.7 mm, or ± 0.5 mm. Components manufactured using the disclosed processes may have a plurality of test locations (e.g., joining surfaces) that are within the distortion tolerance. These properties make the components suitable for a wide range of applications, including some where 6XXX series aluminum alloys could not previously be used. For example, the components may be used as structural components in vehicles (e.g., aluminum-intensive cars and trucks).These components can be formed from thick aluminum sheet (for example, 2 to 4 mm) and can have complex shapes, such as those having multiple non-coplanar joining surfaces.

[0049] At step 112, the component may be joined to another component, for example, by riveting. In one embodiment, the component may be joined by one or more self-drilling rivets (SPRs). The component may be a sheet or stack of sheets, such as stack 10, or may be a formed component, such as components 20 and 30. The component joined to the heat-treated component of process 100 may be a sheet, stack of sheets, or formed component, and may have undergone a similar / same process 100 or a different (or no) process.

[0050] With reference to Fig. Figure 14 shows pre- and post-heat treatment measurements for samples treated according to the disclosed modified T7 heat treatment, with light cells representing an within-spec measurement and shaded cells representing an out-of-spec measurement. The y-axis represents 22 test samples, and the x-axis represents ten different test locations on each test sample. The ± units are in mm for the specified location (for example, -0.05 is 0.5 mm less than the specification). The modified T7 heat treatment performed on these samples included a solution heat treatment (SHT) at approximately 508°C for 3.5 hours and a quench using liquid water at approximately 88°C. The artificial aging process involved a temperature of 220°C for 5.5 hours.The resulting components had an average yield strength of 226 MPa (standard deviation of 2 MPa) and a bending strength ratio (r / t ratio) of less than 0.30, which is generally sufficient for many structural applications where joining capability (e.g., riveting capability) is also important. As shown in . Fig. As shown in Figure 14, after the heat treatments, only a single location for a single part was out of tolerance (Part 13, first column) for a PIST of 99.55%. These data confirm that the applicant has discovered a relatively narrow set of SHT, quench, and AA temperatures and times that provides a combination of high strength (e.g., at least 200 MPa), good bendability (e.g., r / t < 0.3), and low or minimal distortion (e.g., PIST of at least 95, 98, 99, or 99.5 percent).

[0051] Although exemplary embodiments have been described above, it is not intended that these embodiments describe all possible forms of the invention. The words used in the specification are words of description rather than limitation, and it is to be understood that various changes may be made without departing from the spirit and scope of the invention. In addition, the features of the various embodiments may be combined to form further embodiments of the invention.

Claims

[1] A method of processing an aluminum alloy component, comprising: Solution annealing of the component with a solution annealing (SHT) temperature of 500 °C to 535 °C; Quenching the component in a first medium at a first rate to approximately 290 °C; subsequently quenching the component in a second medium at a second rate which is lower than the first rate; and artificial aging of the component at an artificial aging temperature (AA) of 200 °C to 250 °C to a yield strength of at least 200 MPa. [2] The method of claim 1, wherein the component is a 6XXX series aluminum alloy comprising 0.4 to 0.7 weight percent silicon and 0.7 to 1.2 weight percent magnesium. [3] The method of claim 1, wherein the step of artificially aging comprises artificially aging the component to an r / t ratio of less than 0.

3. [4] The method of claim 1, wherein the SHT temperature is between 505 °C and 530 °C. [5] The method of claim 1, wherein the solution heat treatment step comprises heat treating the component for 2 to 4 hours. [6] The method of claim 1, wherein the first medium has a temperature of 75°C to 95°C. [7] The method of claim 1, wherein the first medium has a temperature of 82°C to 88°C. [8] The method of claim 1, wherein the step of artificially aging comprises heat treating the component for 3 hours. [9] The method of claim 1, further comprising joining the aluminum alloy component to a second component with a self-drilling rivet. [10] A method of processing an aluminum alloy component, comprising: Solution annealing of the component with a solution annealing (SHT) temperature of 505 °C to 530 °C for 2 to 4 hours; in response to cooling the component during a quench delay to a temperature of 475°C, quenching the component in a liquid quench medium having a temperature of 75°C to 95°C; and Artificial aging of the component for 2 to 4 hours to a yield strength of at least 200 MPa. [11] The method of claim 10, wherein the artificial aging step comprises heat treating the component at an artificial aging temperature (AA) of 200°C to 250°C for 3 hours. [12] The method of claim 10, wherein the step of artificially aging comprises artificially aging the component to an r / t ratio of less than 0.

3. [13] A method of forming a vehicle structural component comprising: Punching of a sheet of an aluminum alloy of the 6XXX series; Solution annealing of the component; quenching the component in a liquid quenching medium at a rate of 80 °C / s to 100 °C / s; subsequently quenching the component in an air quenching medium at a rate which is less than 80 °C / s; and Artificial aging of the component for 2 to 4 hours to a yield strength of at least 200 MPa. [14] The method of claim 13, wherein the solution treatment step comprises solution treatment of the component at a solution treatment (SHT) temperature of 505°C to 530°C for 2 to 4 hours, and the artificial aging step comprises heat treating the component at an artificial aging (AA) temperature of 200°C to 250°C for 3 hours. [15] The method of claim 13, wherein the liquid quenching medium is water and has a temperature of 82°C to 88°C. [16] The method of claim 13, further comprising joining the stamped 6XXX series aluminum alloy component to a second component with a self-drilling rivet.

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