Patent Application
20220341015
This invention relates to methods for forming a component from an aluminium workpiece, and a component made by said methods.
The current trend, to achieve weight reduction in vehicles e.g. vehicles in the automotive, railway and aerospace industries, has led to an increase in attempts to form panels or components for said vehicles from aluminium, e.g. aluminium alloy sheet. Weight reduction in vehicles is of importance, particularly to reduce fuel consumption and minimise carbon dioxide air pollution. The use of lightweight components may also provide other advantages such as, in automotive applications, improved handling performance, and, in aerospace applications, allowing heavier loads to be carried. For these reasons, it is desirable to make components for such applications from lightweight alloys, such as aluminium alloys. When looking to reduce weight in vehicles it is important to, at the same time, maintain or even increase passenger safety standards, and therefore components and parts formed using new lightweight materials such as aluminium must be just as strong when formed as the materials being replaced.
Methods for forming components from lightweight alloys such as aluminium alloys are not as well researched or developed as methods for forming components from materials such as steels. Steels and light alloys such as aluminium alloys have very different microstructural evolution mechanisms and accordingly steel forming processes cannot generally be applied in the same way to effectively form components using light alloys. A difficulty encountered when forming aluminium alloy is its generally low formability (ductility) at room temperature (around 20 to 25 degrees centigrade), which restricts the complexity of shaped components which can be formed. The low formability (low ductility) of aluminium alloys at room temperatures has led to aluminium alloy components being made from milled solid blocks of treated aluminium alloy which leads to a high percentage of the alloy being wasted and hence high manufacturing costs.
WO 2008/059242 discloses a method of forming aluminium alloy sheet into components of complex shape. The method disclosed in WO 2008/059242 includes the following general steps:
SHT dissolves certain constituent alloying elements (precipitates) into a solid solution within the aluminium, and the cooling in step (iv) quenches the aluminium, preventing precipitation of the constituents and thermal distortion of the formed parts during rapid cooling. After SHT, the material is in a soft form suitable for forming. Whilst this method has certain advantages over earlier methods, it also has certain drawbacks. For example, the forming needs to be carried out before the sheet cools in order for the method to be successful. As the sheet tends to cool quickly (it is thin and has a low specific heat capacity and high thermal conductivity), the forming must be carried out very quickly. This is problematic in that the forming therefore requires a very quick press. Such presses are expensive and tool life is short due to high temperature and high friction. Also, it is difficult to form complex parts, since the sheet tends to cool before the complex part can be fully formed.
Similarly, it has been found that heating aluminium alloy significantly above room temperature, to an elevated temperature but below the SHT temperature, improves formability for certain alloys and helps to form parts with complex geometries. It has also been found that if such heating is combined with or carried out before a forming step (i.e. a hot forming process is carried out) prior to SHT and subsequent further forming, then it is easier to form a complex part. Aluminium alloys in certain temper states (e.g. T4 or T6 temper states) are preferred and are almost exclusively used in these types of processes involving preliminary hot forming followed by SHT since some different states such as O condition state (annealed alloys) subject to the same process result in a formed part with low mechanical properties.
O condition state aluminium alloys have relatively high formability (ductility) and are generally cheaper compared to other aluminium alloys typically used in hot forming/SHT processes (e.g. T4 and T6 temper aluminium alloys), but they have low strength at room temperatures and are not normally used in such processes since it's difficult to suitably reduce or avoid thermal distortion of formed parts thereof in the relatively long time required for SHT and then to achieve rapid cooling to enable good mechanical properties of the formed components.
The present disclosure aims to provide an alternative and/or improved method for forming shaped components from aluminium metals such as aluminium alloy sheets.
In a first aspect of the invention there is provided a method for forming a component to a target shape from an aluminium blank workpiece, the method comprising: (a) cold forming an aluminium blank workpiece between a set of dies, thereby producing a component fully or partially formed to a target shape; (b) solution heat treating the fully or partially formed component by heating to or above a solution heat treatment (SHT) temperature and substantially maintaining that temperature until SHT has been completed, thereby producing a solution heat treated fully or partially formed component; and (c) quenching the solution heat treated fully or partially formed component whilst held between a set of dies, wherein holding between the dies may provide additional forming at the same time as quenching, to produce a component fully formed to the target shape.
The method is for forming a component into a formed component of a target shape (a pre-determined shape). Cold forming, i.e. forming the aluminium workpiece at temperatures below its recrystallisation temperature, causes the aluminium alloy to strain harden, which both inhibits localised thinning during forming, enables greater deformation (greater shape complexity) to be obtained, increases static and dynamic strength of the formed component, and generally reduces likelihood of tears or cracks in the workpiece during the forming processes.
It has been found that cold forming in step (a) in combination with steps (b) and (c) advantageously allows a wider range of aluminium alloys to be formed effectively. The method of the first aspect may effectively be used with aluminium of alloys of O condition state, due to the strain hardening resulting from the cold forming. Cold forming is less complex compared to forming at high temperatures, requires less energy, and accordingly provides a cost saving over warm/hot forming processes in terms of energy use. Dies used for cold forming are easier to control compared to hot dies due to the elimination of heating which may impart defects on the component such as distortion or poor surface quality. Cold forming may use lower forming speeds compared to hot forming, and therefore, the life span of tools used in cold die processes is understood to be longer than those for hot forming processes.
SHT is completed in step (b) when sufficient alloying elements are in solid solution so that after the quenching in step (c), and optional ageing, a precipitation-hardened temper of the required strength is formed. The time necessary for SHT completion depends on the material being formed and the temperature it's being exposed to but can be determined experimentally using known techniques.
The quenching of the solution heat treated fully or partially formed component in step (c) means that quenching may be carried out on the solution heat treated formed component after step (b). Rapidly cooling to quench the fully or partially formed component by holding it in-between the set of dies (a closed set of dies) in step (c) advantageously minimises any distortion in the formed component after the quenching step.
It has been found that the combination of steps set out in the method in accordance with the first aspect enables the design of a complex target shape with little springback, in a material which has a metallurgical structure containing no, or few, precipitates.
The low ductility of aluminium alloy at cold forming temperatures may lead to an incomplete shaped component being formed in step (a). Depending on the target shape of the formed component, the cold forming step (a) may or may not result in a component which matches the target component shape. If the target shape is not achieved in step (a) it may advantageously go on to be achieved in step (c). Accordingly, the component formed in step (a) may be fully formed to the target shape of the formed component or partially formed in a shape part-way towards the target shape of the formed component. The first forming step (a) advantageously reduces the burden on the forming in step (c) if present. Advantageously, if the target shape is fully achieved in the forming of step (a) but springback is exhibited in the workpiece (possibly as a result of SHT step (b)) then the secondary forming step may be used in quenching step (c) to remove the springback effects.
In step (a), the aluminium workpiece may be cold formed at a temperature between 0 and 100 degrees centigrade, preferably between 15 and 50 degrees centigrade and most preferably between 15 and 30 degrees centigrade.
The temperature of the cold forming step (a) may be controlled by the set of dies, and/or temperature treatment of the aluminium blank workpiece in a step prior to cold forming in step (a).
The component formed in step (a) may be formed to from 20% to 100% of the target formed component shape, for example from 20% to 60% of the target formed component shape, from 30% to 70%, or from 80% to 100% of the target formed component shape. Partial forming may be used advantageously to avoid any failures that might result from tight bends of the workpiece in the initial pressing, before SHT treatment.
The set of dies i.e. the set of dies in step (a) and/or the dies in step (c) may be provided with a grip comprising one or more protrusions configured to grip the workpiece during forming when the workpiece is positioned between the dies, and wherein steps (a) and/or (c) of the method further comprise using the grip to grip the workpiece during forming. It has been found that using the grip and protrusions advantageously controls the flow of the aluminium blank workpiece into the dies during forming and prevents wrinkling in the formed part whilst reducing the likelihood of tearing or cracking the part being formed. The one or more protrusions may be referred to as drawbeads. The method may further comprise providing the grip with depressions which are configured so that, when the grip is gripping the workpiece, the depressions receive the protrusions and grip the workpiece between the protrusions.
The dies used in step (a) may be a first set of dies and the set of dies used in step (c) may be a second set of dies (i.e. different tools may be used for the dies in step (a) and in step (c)). Alternatively, the dies used in step (a) and in step (c) may be the same set of dies (i.e. the same tool).
If the dies in steps (a) and (c) are the same set of dies, the drawbeads (if present) may be removable or adjustable so that they are more prevalent during the first forming step (a) and less prevalent during step (c).
The aluminium blank workpiece used in step (a) may be an aluminium sheet workpiece. The aluminium blank workpiece used in step (a) may be annealed or in any temper condition states such as, but not limited to temper conditions T3, T4 to T6, T7, O, F or W. The aluminium blank workpiece used in step (a) may be fully or partially annealed or may be in the T4 or T6 temper condition state. The aluminium blank workpiece used in step (a) may be fully annealed. Preferably the aluminium blank workpiece used in step (a) is not fully strain hardened.
The aluminium blank workpiece used may be an aluminium alloy i.e. a formable aluminium alloy. The aluminium blank workpiece used may be a heat-treatable aluminium alloy or a non-heat-treatable aluminium alloy, but preferably a heat-treatable aluminium alloy. If the aluminium blank workpiece is a non-heat-treatable aluminium alloy then the solution heat treating step (b) is a heating step involving heating the fully or partially formed component to below a metallurgically stable temperature so that the microstructure remains stable and step (c) comprises quenching the heated fully or partially formed component from step (b).
The aluminium blank workpiece may be an AA2XXX, AA6XXX, or AA7XXX series alloy. The aluminium alloy may alternatively be an AASXXX series alloy.
Preferably, the aluminium blank workpiece used is annealed aluminium alloy sheet (e.g. AA6082 aluminium alloy) in an O condition state.
The solution heat treating step (b) may comprise heating the fully or partially formed component to a temperature within the range from 450 to 600 degrees centigrade.
In step (c), the solution heat treated fully or partially formed component is preferably rapidly transferred to the set of dies once solution heat treatment is completed, preferably being transferred to the dies within from 1 to 20 seconds, more preferably from 3 to 10 seconds. This timeframe (a rapid timeframe) is sufficient to prevent any adverse cooling effects, such as evolution of precipitates, occurring to the solution heat treated formed component before quenching in step (c) is carried out.
In step (c), the set of dies may be maintained at a temperature of between 0 to 250 degrees centigrade such as between 15 and 60 degrees centigrade or between 20 and 25 degrees centigrade. Preferably, the set of dies in step (c) is maintained at a temperature below 150 degrees centigrade.
The temperature of the set of dies in step (c) will increase as heat energy is transferred from the solution heat treated formed component being held in-between. Maintaining the temperature of the set of dies between 0 to 250 degrees centigrade and preferably below 150 degrees centigrade allows the dies to provide sufficient cooling, so as to achieve a quenched microstructure suitable for precipitation ageing.
In step (c), the solution heat treated formed component may be subjected to localised cooling (cooling specific parts of the component), to ensure consistency of the temperature and therefore a substantially homogenous metal crystal structure within the part. The set of dies may be configured to provide localised cooling.
In step (c), the quenching may be carried out at a temperature of below an artificial ageing temperature of the material if the material is a heat-treatable aluminium alloy. Alternatively, in step (c) the quenching may be carried out at a temperature of below a metallurgically stable temperature of the material if the material is a non-heat-treatable aluminium alloy, thereby ensuring the alloy microstructure remains stable.
In step (c), the quenching may be carried out at a rate of from 15 degrees centigrade per second and above to 200 degrees centigrade per second and above, e.g. the temperature of the solution heat treated fully or partially formed component may be cooled at a rate of from 16 degrees centigrade and above, 20 degrees centigrade and above, 30 degrees centigrade per second and above, etc. Quenching as described herein advantageously ensures that the proper metallurgical microstructure is achieved in the final product and avoids detrimental effects associated with low quench rate, such as low strength or poor corrosion performance.
Step (c) of the method may further comprise providing the formed component with insulation configured to protect said component from any adverse cooling effects during quenching.
The method may further comprise a further step (d) of artificially aging the formed component to obtain improved mechanical properties.
The method may further comprise using a lubricant to lubricate the interface between the set of dies and the workpiece in step (a) and/or the set of dies and the component in step (c).
Lubrication may be used to advantageously reduce interface friction between dies and workpiece to improve formability.
In a second aspect of the invention there is provided a component formed by a method in accordance with the first aspect described herein.
The above and other aspects of the invention will now be described, by way of example only, with reference to the accompanying drawing, in which
A detailed description of an example method of the first aspect shall now be given in relation to forming a component in a target shape from a blank of a fully annealed aluminium alloy sheet (hereafter referred to as “the workpiece”) in the O condition state, making reference to
The workpiece is initially received by a first die tool. The workpiece may be fed into the first die tool from a roll of annealed aluminium alloy sheet whereby the blank is formed by cutting the sheet to appropriate dimensions upon receipt thereof in the die tool, or may have been pre-cut to desired dimensions.
The first die tool may be a forming die tool of the conventional type for cold forming such as that shown generally in
The die tool comprises one or more grip sets shown generally at items 106, configured to grip the workpiece during the forming process. The grip sets 106 help to control flow of the metal during forming and prevent or at least minimise wrinkling of the workpiece 112 during forming. The grip sets 106 come in two parts 106a and 106b which are positioned around the edges of the plates 102 and 104, and above and below the workpiece 112. When brought together, parts 106a and 106b are configured so that they clamp and grip the workpiece 112. Grip sets 106 may comprise a drawbead set which assist the grip sets 106 in gripping the workpiece 112 during forming. A protrusion part 108 of the drawbead set is configured to protrude from the grip so that when clamping the workpiece (as shown in
The first step in the method is a cold forming step shown generally at item (1) in
The intermediate component is removed from the first die tool and, in a second step (shown generally at item (2) of
Once SHT is completed, in a third step shown generally at item (3) in
In this example, the second die tool comprises punch and die plates of the same shape as the first die tool, although they may differ in other embodiments. If the component was fully formed to the target shape in the first cold forming step then there is no additional forming when the dies close around the component, but the closed dies act to prevent any distortion that may occur from the quenching step. If the component was fully formed in the first cold forming step but becomes distorted before the second forming step, e.g. during SHT, then the second forming step may recover the component back to the target shape.
The solution heat treated intermediate component is transferred to the second die tool as quickly as possible as soon as heating is no longer being applied thereto, e.g. within 1-10 seconds. The forming speed in the third step is in the range of 50 millimetres per second to 500 millimetres per second. The pressure for holding the set of dies is in the range of 1 megapascal to 30 megapascals. The holding time (i.e. the time the workpiece is held between the closed die tool at the required pressure) is in the range of 2 seconds to 15 seconds.
The temperature of the punch and cavity plate parts of the second die tool are at room temperature when the solution heat treated intermediate component is placed therebetween. Heat energy will transfer to the second die tool from the solution heat treated intermediate component when between the closed second die tool. The second die tool is configured so that the temperature of the parts contacting the intermediate component stays below 150 degrees centigrade. Liquid cooling may be employed by the second die tool to control the temperature thereof. The solution heat treated intermediate component is rapidly cooled as it is held between the closed second die tool, preferably at a rate of over 15 degrees centigrade per second, in order to prevent the formation of coarse phase. In other words, the component being formed inside the second die tool is quenched. Alternatively, the solution heat treated intermediate component may be rapidly cooled after the second step (after SHT and before entering the second die tool). Accurate control of the cooling rate is not necessary so long as it is above 15 degrees centigrade per second. Factors to consider which may affect cooling rate are blank size, blank thickness, holding pressure etc. The second die tool may comprise a grip in accordance with that described above in relation to the grip of the first die tool.
The particular quenching rate should preferably be selected based on the particular alloy being used and the final mechanical and corrosion requirements of that component. For example, a 2xxx or 7xxx alloys such as 2024 may require fast quenching of >50 degrees centigrade per second or even >200 degrees centigrade per second in order to achieve good corrosion resistance. Alternatively, a less sensitive alloy such as 7020 may be quenched slower to allow lower quench forces or less accurate or simpler tooling to be used. A lubricant may be applied to the first and/or second die tool in order to reduce interface friction between dies and workpiece to improve formability. The lubricant for the first die tool may be any lubricant suitable for use in cold forming processes, and for the second die tool may be any lubricant suitable for use at both the elevated temperatures the workpiece will be at initially when received by the second die tool and the rapid cooling temperatures of the quenching process.
An additional artificial ageing step (not shown in
The shape of the first and second die tools (i.e. the shape of the punch plate and cavity (die) plate) may be substantially identical to each other or may be different in their shape. Differences in shape may be used to promote partial forming in the first cold forming step, e.g. the shape of the first die tool may be 20-100% towards the shape of the target shape, to produce a partially formed intermediate formed component and the shape of the second die tool may match the target shape (i.e. 100% of the target shape). If the first and second die tools are the same then partial forming in the first forming step may be achieved by varying forming variables such as forming force and time. The second die tool may be shaped to “over form” the solution heat treated intermediate formed component to account for any distortion effects expected to occur after forming. In this way, it is possible to use the same die tool or different die tools to produce an intermediate formed component which is 20-100% towards the shape of the target shape for the formed component in the first (cold) forming step, and to achieve a formed component which is 100% of the target shape for the formed component in the second forming step. The way in which the degree of shaping (the percentage towards the target shape) in the first and/or second die tools is determined is not limited, but may include measuring dimensions, e.g. using a 3D scanner, and then comparing the measured values with the corresponding values of the target shape.
The method disclosed herein is a notable departure from the conventional methods of forming aluminium alloy. Those skilled in the art will appreciate that the presently disclosed method teaches by way of example and not by limitation. Therefore, the matter contained in the above description or shown in the accompanying drawing should be interpreted as illustrative and not in a limiting sense. The following claims are intended to cover all generic and specific features described herein, as well as all statements of scope of the method, which, as a matter of language, might be said to fall there between.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1913810.6 | Sep 2019 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2020/076893 | 9/25/2020 | WO |
