ALUMINIUM ALLOY MATERIAL

FIELD

The present disclosure relates to a process for preparing sheet metal alloy material for forming, for example sheet aluminium alloy material, and to sheet metal alloy material produced by the process. The present disclosure also relates to a method of forming a metal alloy material.

BACKGROUND

A sheet metal alloy, also known as a blank, may be processed to form alloy sheet components. One way in which this may be achieved is by the process of Solution Heat Treatment, forming and cold- die quenching (HFQ) as is described by the present inventors in the earlier applications WO

2010/032002 and WO 2008/059242, both of which are herein incorporated by reference.

In brief the HFQ process firstly comprises Solution Heat Treatment (SHT) of a blank. The blank is then rapidly transferred to a set of cold dies and the dies are immediately closed to form a shaped component. The shaped component is held in the cold dies during cooling.

'Solution Heat Treatment' (SHT) of a material usually involves heating a material to dissolve any alloying elements present as much as possible within a base metal matrix such that a solid solution is formed.

The HFQ process can be carried out using any condition of metal alloy material. For example, F, T4, T6, T7, O or any other condition of metal alloy material.

Blanks currently used for the HFQ process are conventionally processed blank material which, when heated in the first step of the HFQ process, can reach the desired state of solid solution in a short time. In such a conventional process, following homogenization heat treatment, a homogenized ingot is multi-pass hot rolled to a particular thickness. The resulting hot rolled plate is allowed to cool to room temperature and then multi-pass cold rolled until a specified thickness of sheet is attained. It may be necessary to interrupt the process to anneal the workpiece, depending on the type of alloy used, so that its thickness can be reduced further. SHT, quenching, stretching and cutting of the finished rolled sheet is carried out, in some cases followed by ageing of the material either artificially using heat treatment or naturally, for example, to obtain T4, T6, or T7 condition for aluminium alloys. This existing process is outlined in Figure 1.

While this method produces blanks suitable for use in the HFQ process, it has certain drawbacks. For example, the process, in particular the steps of SHT, quenching, stretching and cutting, is expensive, requiring expensive equipment and having large energy demands. Accordingly, blanks produced using this method are also expensive.

As an alternative, cheaper as-rolled condition and 0 condition (i.e. annealed) material may be used. However, these materials can require up to 15-60 minutes of Solution Heat Treatment in the HFQ process to achieve the required Solution Heat Treated condition. For the HFQ process to be industrially viable, particularly if automotive type cycle times of about 10 seconds are to be achieved, this time requirement requires either very large or expensive ovens, hence additional expense.

Accordingly, a process of preparing sheet metal alloy material for forming using the HFQ process is required which addresses the problems associated with existing methods.

SUM MARY

In a first aspect there is provided a process of preparing sheet metal alloy material for forming, the process comprising hot rolling a metal alloy material wherein the metal alloy material is at a temperature above that at which precipitates form in the material; then applying cooling means to cool the metal alloy material at a predetermined cooling rate; and cold rolling the metal alloy material.

It will be understood that precipitates form in the material when the temperature of material is below a particular temperature, referred to as the precipitate formation temperature. The precipitate formation temperature varies depending on the proportion of base metal and alloying elements present.

In the formation of alloys, the base metal and alloying elements may be heated to a certain temperature such that a homogenous phase is formed. At this point the alloying elements are said to be in a solid solution with the base metal. As the alloy is cooled it will reach a temperature at which the alloying elements are no longer soluble with the base metal and so will be forced to precipitate out of solution. The alloy then becomes heterogeneous comprising two phases.

Ensuring that the temperature of the material is above that at which precipitates form in the material in the hot rolling step prevents or reduces precipitate growth. In addition, applying cooling means to cool the material at a predetermined cooling rate enables the size of any precipitates formed in the material to be constrained. By cooling the material at a predetermined rate of cooling, phase transformations may be inhibited and hence precipitate formation and the size of any precipitates formed may be controlled. Hence, precipitate formation and growth in the material may be controlled and reduced. By controlling precipitate formation in this way, the mechanical properties of the material may also be controlled since the mechanical properties of the material are controlled by the size and distribution of precipitates.

Production of material in this manner is particularly suited for producing material for forming using the HFQ process previously described. Since the precipitate formation in the sheet metal alloy material is reduced, only a short SHT time is required to dissolve any precipitates present.

Accordingly the SHT step of the HFQ process is shorter, reducing the energy and time required and hence the cost associated with the HFQ process.

Further, the sheet metal alloy material produced by this method may be used directly in the HFQ process without additional processing. Accordingly, for example the post-rolling steps of SHT, quenching, stretching and cutting associated with known methods are not required and the sheet metal alloy may be used in the HFQ process in an as-rolled condition. Consequently the equipment required to produce the sheet metal alloy and the associated cost, together with the energy and time required by the process, is reduced. Hence sheet metal alloy for HFQ processing can be produced more cheaply.

For the reasons outlined above, the process disclosed herein facilitates viable industrialization of the HFQ process for mass production of components.

Metal alloy material in any condition may be used as a starting material in the process. To reduce the SHT time, T4 or T6 temper material may be used. The metal alloy material may be hot rolled at a temperature below the temperature at which the material begins to melt and above that at which precipitates form in the material. Both the precipitate formation temperature and the temperature at which the material begins to melt vary depending on the proportion of base metal and alloying elements present. Together, the precipitate formation temperature and the temperature at which the material begins to melt define a temperature range between which the metal alloy material is in a solid solution. Accordingly, during the hot rolling step, the material may be at a temperature above that at which precipitates form and below that at which the material begins to melt.

The precipitate formation temperature may be between 20 and 450 °C. For example, for AA6082, the temperature below which precipitates form is approximately 420 °C.

The temperature at which the metal alloy material begins to melt may be between 560 and 620°C. For example, for AA6082, the temperature at which the metal alloy material begins to melt is approximately 595 °C.

Optionally the metal alloy material is an aluminium alloy. For example the metal alloy material may be that known as AA6082. Other suitable types of metal alloy material which may be used include, amongst others, magnesium alloys. Optionally the metal alloy material is an aluminium-magnesium alloy.

Optionally the temperature of the metal alloy material is maintained above that at which precipitates form in the material throughout all or part of the hot rolling step. Optionally the material is above the temperature at which precipitates form in the material on a final hot rolling pass.

Optionally the material is hot rolled using a rolling mill. The rolling mill may comprise a plurality of rollers, for example, 2, 3, 4, 5, 6, 7, 8 or any other suitable number of rollers may be used.

Alternatively, the rolling mill may comprise a single roller which may, for example, be used together with a plate for hot rolling the material. In some embodiments the rolling mill may comprise a one or two high roll system.

Heated rollers may be used to hot roll the metal alloy material. Optionally the rollers are heated to a predetermined temperature, for example between the precipitate formation temperature and the temperature at which the metal alloy material begins to melt, for example between 400 °C and 600 °C, between 400°C and 450°C, for example approximately 425°C. The heated rollers may be heated throughout all or part of the hot rolling step. Optionally the heated rollers are heated using, for example, electric cartridges or flames. Heating of the heated rollers may be carried out prior to hot rolling the material. Provided that the temperature of the heated rollers is high enough, heating during rolling may not be required. Alternatively or in addition, the heated rollers may be heated via heat transfer from the hot metal alloy material being hot rolled.

Alternatively, cold rollers may be used. In such cases the rollers remain at room temperature.

Optionally heated rollers used to hot roll the metal alloy material may be used to help keep the temperature of the material above that at which precipitates form in the material. For example, the heated rollers may be heated such that the material is heated, wholly or in part, by the heated rollers to a temperature above that at which precipitates form in the material.

Optionally the heated rollers may be heated such that heat lost from the material when brought into contact with the rollers is reduced.

Optionally the surfaces of the rollers used for hot rolling comprise a material having a low heat transfer coefficient, for example a ceramic material, so to reduce heat loss from the material during the time in which the metal alloy material and rollers are in contact. This helps keep the temperature of the material above that at which precipitates form in the material.

Optionally the material is hot rolled by passing the material through the rolling mill a single time. Alternatively, the material may be passed through the rolling mill multiple times, for example up to 30 passes, optionally 10 to 15 passes. The number of passes through the rolling mill may depend on the metal alloy material being hot rolled.

Optionally, during hot rolling of the material, the material is hot rolled to a predetermined thickness, for example, to a thickness below 20 mm, for example between approximately 3 and 10 mm. Hot rolling the material to a predetermined thickness for subsequent cold rolling reduces the amount of cold rolling required.

Following hot rolling of the metal alloy material, cooling means are applied to cool the material at a predetermined cooling rate. Cooling of the material may be carried out until a predetermined target temperature is reached. Optionally, the target temperature is standard temperature at standard pressure. Optionally, any suitable target temperature may be used, for example, the target temperature may be in the range 20 to 200 °C, for example below 100 °C. Optionally the target temperature may depend on the metal alloy used. Cooling of the material may be carried out for a predetermined period of time, for example, the predetermined period of time may be a function of the temperature of the material on a final hot rolling pass.

During cooling of the material, the predetermined cooling rate may depend on the metal alloy material concerned. For example, the predetermined cooling rate may be in the range from 8 °C per second to 200 °C per second, for example 50 to 150°C per second, 75 to 125°C per second, 85 to 115 °C per second, for example >12 °C per second. In the case of AA6082 the predetermined cooling rate is >12°C per second. The predetermined cooling rate may be such that precipitate formation and/or the size of precipitates formed is controlled, for example, precipitate formation may be reduced or prevented entirely. This may be achieved by inhibiting phase transformations in the material. Optionally the predetermined cooling rate is such that the maximum size of precipitates formed is constrained, for example, the maximum size of precipitates formed may be <200nm, for example <50nm.

Optionally, the cooling rate is sufficiently rapid during the quenching sensitive temperature range of a particular alloy. At certain quenching sensitive temperature ranges, for example approximately between 250°C and 350°C depending on the alloy, precipitates may form and grow rapidly. Ensuring a sufficiently rapid cooling rate in these ranges reduces or eliminates precipitate formation.

Optionally applying cooling means to cool the material may comprise applying cooling means for cooling the material by conduction, convection, a combination of conduction and convection, and/or any other suitable means.

Optionally the application of cooling means comprises bringing the metal alloy material into contact with a thermally conductive material, for example tool steel, such that heat is transferred away from the material via conduction. For example, the hot rolled metal alloy material may be placed on or in between thermally conductive material, for example, cooling plates. Optionally the application of cooling means comprises cold rolling the metal alloy material such that heat is transferred away from the material via conduction when the material is brought into contact with the rolls.

The surface of the thermally conductive material may comprise a high heat transfer material, for example cold rollers may be coated with a high heat transfer coating layer, such that heat is more efficiently transferred away from the material. For example, a high heat transfer coating layer may comprise a ceramics material.

Optionally, the application of cooling means comprises employing a cold working method to cool the metal alloy material. For example, immediately following hot rolling, the hot rolled material can be rolled using cold rollers such that the temperature of the material may be quickly reduced.

Optionally the application of cooling means comprises applying spray water or spray of another coolant to the material such that the material is cooled further. Cold working the metal alloy material prior to water cooling reduces the residual stress generated by from subsequent water cooling. Optionally application of cooling means to cool the material comprises exposing the material to a coolant. Suitable coolants comprise any fluid having heat transfer characteristics such that heat is transferred away from the hot rolled material when the material is brought into contact with the coolant, facilitating cooling of the hot rolled material at the predetermined rate. The material may be exposed to a liquid coolant, for example water; a gas coolant, for example air; and/or a mist or spray coolant. Optionally the material may be exposed to a controlled stream of coolant, for example, the stream of coolant may be controlled via adjustment of the pressure and/or amount of coolant applied.

Optionally application of cooling means to cool the material comprises exposing the material to an assembly of air blades. For example, an air blade may comprise a blade having holes through which pressurized air may pass through.

Optionally application of cooling means to the hot rolled material may comprise transferring the material to a temperature controlled chamber, for example, an enclosed temperature controlled chamber.

Other suitable cooling methods may be used as will be apparent to those skilled in the art.

Any combination of cooling methods may be used to cool the hot rolled material.

The application of cooling means described above may be followed by another cooling method if necessary. For example, a further conduction or convection method may be applied. Optionally the metal alloy material may be rolled, for example during hot rolling or cooling of the material, using rollers comprising a textured surface, for example, a micro-scale textured surface, such that the rolled material is provided with a textured surface. This serves to increase the cooling area to facilitate cooling of the sheet metal alloy material.

In some circumstances, residual stress may arise from cooling the material during application of cooling means. Such residual stress may be reduced by the step of cold rolling the material due to plastic deformation of the metal alloy material.

Optionally application of cooling means to cool the material is carried out, wholly or in part, by the step of cold rolling the material. This reduces the equipment required by the process.

Optionally cold rolling of the metal alloy material is carried out using a rolling mill. The rolling mill may comprise a plurality of rollers, for example, 2, 3, 4, 5, 6, 7, 8 or any other suitable number of rollers may be used. Alternatively, the rolling mill may comprise a single roller which may, for example, be used together with a plate for cold rolling the material.

Optionally cold rolling of the metal alloy material is carried out by passing the material through the rolling mill a single time. Alternatively, the material may be passed through the rolling mill multiple times, for example up to 30 passes, optionally 10 to 15 passes or 2 to 8 passes.

Optionally cold rolling of the metal alloy material is carried out until a predetermined sheet thickness is obtained. For example, the predetermined sheet thickness may be a thickness suitable for subsequent application of the HFQ process. The use of cold rolling enables an accurate sheet thickness to be obtained. The predetermined sheet thickness may depend on the subsequent use of the metal alloy material.

In some cases annealing may be required during cold rolling. The process of annealing softens the material for further cold rolling such that a desired thickness can be achieved. If annealing is required, the annealing temperature may be above the precipitation formation temperature, for example above 420 °C for AA6082. The cooling rate may be above a critical cooling rate such that the formation of precipitates is avoided, e.g. the critical cooling rate may be between 10 to 80 °C/second, 30 to 60 °C/second, for example >12 °C/second. If the cooling rate is not sufficiently high, annealing can result in the formation of large precipitates in the material which take a long time to dissolve in an SHT process.

Hot rolling the material to a sufficiently small predetermined thickness may reduce or avoid the need for annealing during subsequent cold rolling of the material whilst still achieving a desired resulting sheet thickness.

Optionally, the metal alloy material is cold rolled until a target shape of sheet is obtained. The target shape may depend on the subsequent use of the metal alloy material.

In a second aspect there is provided a sheet metal alloy material obtained by the process outlined above.

Following the process outlined above, the resulting sheet metal alloy material may be applied directly to the HFQ process.

In a further aspect there is provided a process of forming metal alloy material, the process comprising hot rolling a metal alloy material wherein the metal alloy material is at a temperature above that at which precipitates form in the material; then applying cooling means to cool the metal alloy material at a predetermined cooling rate; cold rolling the metal alloy material; heating the metal alloy material to its Solution Heat Treatment temperature and maintaining the material at that temperature until Solution Heat Treatment of the material is complete; transferring the metal alloy material to a set of cold dies such that heat loss from the material is minimized; closing the cold dies to form the metal alloy material into a shaped component; and holding the formed component in the closed dies during cooling of the formed component.

The Solution Heat Treatment temperature may be in the range from 450-600°C, for example 500 to 550°C.

Solution Heat Treatment may be carried out, for example, between 1 second and 30 minutes, between 3 seconds and 3 minutes, 5 seconds and 5 minutes, between 10 seconds and 1 minute, for example 20 seconds. Optionally, the metal alloy material is transferred to the set of cold dies such that forming is initiated within 10 seconds of completion of the Solution Heat Treatment. For example less than 5 seconds, for example less than 3 seconds.

Optionally, closure of the cold dies occurs in less than 2 seconds, for example less than 0.15 seconds, less than 0.1 second, or less than 0.05 seconds.

Optionally, the formed component is cooled to below 200°C in less than 10 seconds. Optionally the cold dies are maintained at a temperature of 150°C or lower.

Optionally the formed component is held in the closed dies for a duration between 1 to 30 seconds, for example 2 to 20 seconds, 5 to 10 seconds, 3 tolO seconds, or 8 to 12 seconds, for example less than 1 minute, e.g. 3 seconds.

The cooled component may be artificially aged by heated the component to an artificial aging temperature and maintaining the component at that temperature such that precipitate hardening may occur. For example, the artificial aging temperature may be between 150-250°C. For example, the aging time may be between 5 to 40 hours, or may be of the order of minutes, for example 20 minutes.

In a further aspect there is provided a formed component obtained by the process outlined above.

It will be appreciated that each of the features described above may apply to each aspect described. All possible combinations are not listed in detail here for the sake of brevity.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments are described below by way of example only and with reference to the accompanying drawings in which:

Figure 1 is a process diagram illustrating a known production process for producing aluminium alloy sheets; Figure 2 illustrates an alloy in which large precipitates have formed;

Figure 3 illustrates an alloy in which precipitate formation has been controlled;

Figure 4 is a process diagram illustrating a process for preparing sheet metal alloy material for forming;

Figure 5 is a graph illustrating the change in temperature over time of a metal alloy material to which the process of Figure 4 followed by HFQ and artificial aging is applied; and

Figure 6 is a binary phase diagram for aluminium-magnesium alloys.

SPECIFIC DESCRIPTION OF CERTAIN EXAMPLE EMBODIMENTS

With reference to Figure 1, an existing process for the preparation of sheet aluminium alloy material is shown. As outlined above, the alloy material is initially heat treated such that the alloy material is in a solid solution state prior to hot rolling. The homogenized ingot is then multi-pass hot rolled 2 to a particular thickness. The hot rolled plate is cooled by air cooling to room temperature and then multi-pass cold rolled 6 until a specified thickness of sheet is obtained. It may be necessary to interrupt the cold rolling 6 step in order to anneal 8 the workpiece, depending on the type of alloy used, so that its thickness can be reduced further. Following these steps, the material may be referred to as being in an 'as-rolled' condition 10. As a result of the steps carried out in this process, and particularly where annealing has been carried out, large precipitates will be present in the as- rolled metal alloy material, as shown in Figure 2.

To control the precipitate size, the as-rolled metal alloy material then undergoes SHT 12, quenching 14, stretching and cutting 16, in some cases followed by an aging process, either artificial 18 or natural 20, for example, to obtain T4, T6, or T7 condition for aluminium alloys. These post-rolling processes act to reduce the precipitate size in the metal alloy material such that smaller precipitates are present as shown in Figure 3. Such material is then suitable for use in the HFQ process since a short SHT time will be required.

As outlined above, this process is costly in terms of energy, equipment and time required.

Consequently, the resulting sheet material is also expensive. With reference to Figure 4, a process of preparing sheet aluminium-magnesium alloy material for forming which aims to address the problems associated with existing processes and which is a first example embodiment will now be described. In overview, in a first step, following initial heat treatment such that metal alloy material is in a solid solution state prior to hot rolling, a

homogenized aluminium-magnesium alloy material is multi-pass hot rolled 42. In a second step the hot rolled material is cooled at a predetermined cooling rate 44, following which the material is multi-pass cold rolled in a third step 46 to produce sheet material. The aluminium alloy known as 'AA6082' is used.

During the first step, the alloy material is passed through a set of heated rollers multiple times until a predetermined thickness of material is achieved. Alternatively, rollers comprising a low heat conductivity material may be used instead of heated rollers. Typically the alloy material is passed through the rollers between 10 and 15 times. The predetermined thickness is between 3mm and 10mm and is a thickness suitable for further cold rolling at the third step. By hot rolling the material to a particular thickness prior to cold rolling the need for additional annealing of the material in the cold rolling step to achieve the required thickness of the resulting sheet is reduced and so the formation of large precipitates typically associated with annealing is avoided.

By the use of heated rollers or rollers comprising a low heat conductivity material, heat lost from the alloy material is reduced such that the temperature of the alloy material is maintained between a temperature at which the metal alloy material begins to melt Τ _α and a temperature at which precipitates form in the material T _C2 throughout the hot rolling step, as shown in Figure 5. For AA6082, Tci is approximately 595°C and Τ _α is approximately 420°C.

With reference to the binary phase diagram of Figure 6, the temperature at which the metal alloy material begins to melt T _a is indicated by line AB and the precipitate formation temperature Τ _α is indicated by line BC. Depending on the proportions of aluminium and magnesium present in the alloy material used, the temperature of the alloy material is maintained within the region, a, defined by the lines AB and BC and the vertical axis, throughout the hot rolling step; the region, a, corresponding to the solid solution phase.

By ensuring that the temperature of the material in the hot rolling step is maintained above Τ _α , formation and growth of precipitates, for example β phase, is prevented or reduced. In the second step 44 of the process, the hot rolled alloy material is cooled to room temperature, as shown in Figure 5. The material is cooled at a predetermined cooling rate. By controlling the cooling rate, formation and growth of precipitates can be controlled, as described above. The cooling rate will vary depending on the alloy used and is selected to ensure that precipitate size is constrained, for example to <200nm. The predetermined cooling rate typically is between 8°C per second and 200°C per second, depending on the material used. In the case of AA6082, the predetermined cooling rate is >12°C per second.

Controlled cooling of the hot rolled material is carried out using conduction cooling methods, convection cooling methods, or a combination of both. Any of the cooling methods previously described may be applied. For example, the alloy material is exposed to a controlled stream of water so to cool the material at the desired cooling rate. The pressure and amount of water applied to the material may be varied so to achieve the desired cooling rate.

By virtue of the temperature control in the first step 42 (hot rolling) and the controlled cooling in the second step 44, the precipitate formation and size in the metal alloy material is reduced such that smaller precipitates are present as shown in Figure 3.

Following cooling of the hot rolled material, the alloy material is multi-pass cold rolled at the third step 46 which acts to remove some or all of the residual stress brought about by cooling 44 the material. Typically the alloy material is passed through the cold rollers between 10 and 15 times. The material is cold rolled until a desired shape and thickness of sheet aluminium-magnesium alloy material is obtained. The particular shape and thickness will depend on the eventual use of the alloy material. A rolling mill comprising cold rollers is used to achieve this.

The sheet aluminium alloy material produced by this process may then be directly used in the HFQ process since a short SHT time will be required.

A second embodiment will now be described with reference to Figure 5. In overview, the second embodiment comprises the steps described in relation to the first embodiment followed by the HFQ process. As shown in Figure 5, following the cold rolling step as described in the first embodiment, the cold rolled sheet material is heated 48 to its Solution Heat Treatment temperature. In the case of AA6082, the Solution Heat Treatment temperature is 525°C. The material is maintained at this temperature for a required duration such that any alloying elements present are dissolved as much as necessary in the base metal matrix such that a solid solution is formed. For AA6082, such a duration is between 1 second and 30 minutes, for example 20 seconds. As described previously, since the steps described in relation to the first embodiment result in reduced precipitate formation in the material, the time required for Solution Heat Treatment of the material is accordingly shorter than if a material having larger precipitates were to be used. This reduces the energy required and hence cost associated with the HFQ process.

Once the Solution Heat Treatment is complete, the material is rapidly transferred to a set of cold dies, comprising an upper and lower die, and placed on the lower die. Typically the material is transferred to the cold dies in less than 5 seconds to reduce heat loss from the material to the environment. The upper and lower dies are then brought together to form the desired component.

Following formation of the component, the material is cold die quenched 50 by holding the material in the closed dies until the material has cooled to a desired temperature. The upper and lower dies are maintained at a temperature below 150°C to ensure that the material is efficiently quenched. The high quenching rates which can be achieved by this process are such that the formation and growth of precipitates can be controlled, hence enabling the material to be maintained in a supersaturated solid state in the quenched state.

Once cooled, artificial aging 52 is then carried out on the formed component to increase the strength of the component. In the case of AA6082, this is carried out for 9 hours at 190°C and may be combined with a baking process.

It will be understood that the above description is of specific embodiments by way of example only and that many modifications and alterations will be within the skilled person's reach and are intended to be covered by the scope of the appendent claims. For example, the process may be applied to other suitable metal alloy materials, for example, magnesium alloys. In addition, alternative or additional cooling methods may be used as previously described, for example, cold working or texturing the material to facilitate cooling.