GB2588795A

GB2588795A - Die for stamping sheet metal and method of manufacturing the same

Info

Publication number: GB2588795A
Application number: GB1916197.5A
Authority: GB
Inventors: Zheng Kailun; Ciaba Kolozsvari Zoltan; Shi Zhusheng; Anthony Dean Trevor; David Foster Alistair; Lin Jianguo
Original assignee: Individual
Current assignee: Individual
Priority date: 2019-11-07
Filing date: 2019-11-07
Publication date: 2021-05-12
Anticipated expiration: 2039-11-07
Also published as: GB2588795B; GB201916197D0

Abstract

A method of making a die 10 for hot stamping nonferrous metal alloy sheet by locating a cooling conduit 16, which has a profile in an axial direction that substantially conforms to its stamping surface 14, in a mould 30, pouring molten cast iron into the mould 30, solidifying and removing the die 10 in which the conduit 16 is embedded. The conduit 16 can be made of copper, brass or mild steel and have a part of its cross-section that conforms to the closest part of the stamping surface 14. Surface oxidation can be removed from the conduit 16. The conduit 16 can be filled with sand and/or located on a support in the mould 30. During casting coolant can be passed through the conduit 16. A thermocouple 46 can be provided in the mould 30 for incorporation in the die 10 (e.g. adjacent a cooling conduit inlet or outlet or between a cooling conduit and the stamping surface) whereby during hot stamping the die temperature can be monitored.

Description

Die for Stamping Sheet Metal and Method of Manufacturing the Same The present invention relates to a die for hot stamping a metal sheet, particularly a non-ferrous metal alloy sheet. The invention also relates to a method of manufacturing a die for hot stamping.

Many components used in a whole range of industries, such as the automotive and aerospace industries, are usually complex in shape. In order to reduce overall weight of the end product, which may be an automobile or aircraft for example, to improve fuel economy, handling of the vehicle, and load carrying capability, it is desirable for such components to be formed from light weight alloys, typically non-ferrous metal alloys such as aluminium or magnesium alloys (Al-or Mg-alloys).

The properties of such non-ferrous metal alloys make it difficult for components with complex shapes to be formed from sheets of such alloys, as they are less malleable than, for example, steel alloys.

One method of forming such components involves milling solid blocks of heat treated metal alloys. However, this is expensive to manufacture, particularly when Al-alloys or Mg-alloys are involved, and results in high percentage of wastage.

Metal alloy sheets can be shaped between a set of dies using normal force (die-closing force) at room temperature, known as cold metal stamping. However, such a method is not conducive to forming complex shapes in a large-scale manner due to limited ductility, springback and warping of the resultant sheets.

To address at least some of the above shortcomings, a series of smaller dies are used for forming subsections of the component, and these subsections are then welded together to form the complete component. However, this is time consuming and expensive to manufacture.

Another way of forming metal alloy sheets, including Al-alloy or Mg-alloy sheets, is by heating the sheets and then forming them into shape between a set of dies, known as hot metal stamping. This allows thinner sheets to be used and more complex shapes to be formed in one stroke using a single blank. After post-formed heat treatment, the resultant components have high tensile strength, are stress-resistant and do not spring back. One such method is disclosed in WO 2008/059242, which involves heating an Al-alloy sheet blank to its solution heat treatment temperature, transferring the sheet blank to stamping dies whilst the sheet blank is still hot, stamping the sheet blank by closing the dies into a shaped component, and quenching the shaped component in the closed dies. The shaped component can then be artificially aged through heat treatment to obtain a higher tensile strength.

In order to be successful, the forming of the shape needs to be carried out in the set of dies quickly and uniformly before the workpiece cools, and once formed, the workpiece needs to be cooled rapidly so as to lock the microstructure in the workpiece into the desired shapes.

Furthermore, during continuous production of such shaped components in large volume, the temperature tends to increase in the steel dies, which can affect subsequent cooling performance and efficiency of in-die quenching of the workpiece. This can lead to deterioration of microstructure and subsequent uniformity of the mechanical properties of the stamped component. In addition, such an increase in die temperature negatively affects the die lifespan and productivity (K. Zheng, J. Lee, W. Xiao, B. Wang, J. Lin, Experimental Investigations of the In-Die Quenching Efficiency and Die Surface Temperature of Hot Stamping Aluminium Alloys, Met. 8 (2018). doi:10.3390/met8040231).

Therefore, artificial cooling of the dies during continuous stamping is typically necessary, such as via internal cooling systems. The various methods of forming a die with a cooling system therein are summarised in H. Hoffmann, H. So, H. Steinbeiss, Design of hot stamping tools with cooling system, CIRP Ann. -Manuf. Technol. 56 (2007) 269-272. doi:10.1016/j.cirp.2007.05.062. These include 1) drilling cooling channels in subsections of the die and subsequently joining the subsections together to form a complete die; 2) dividing the die into upper and lower shells, sealingly attaching the shells to a base plate such that a cooling channel is provided between the upper and lower shells, and connecting the cooling channel to an inlet and an outlet on the base plate; and 3) casting the die with pre-embedded ceramic cores or mandrels.

It is further possible to utilise additive manufacturing technologies (K.W. Dalgarno, T.D.

Stewart, Manufacture of production injection mould tooling incorporating conformal cooling channels via indirect selective laser sintering, Proc. Inst. Mech. Eng. Part B J. Eng. Manuf. 215 (2001) 1323-1332. doi:10.1243/0954405011519042) for forming the cooling systems, whereby the lower portion of cooling ducts are machined or milled onto the die, and using a laser assisted deposition method, duct material is deposited on the milled grooves to form the upper portion of the cooling ducts.

However, each of the methods discussed have their own shortcomings. For example, drilling of cooling channels or provision of ceramic cores in the casting method is unsuitable for large dies or dies with a long longitudinal extent, and is also labour intensive and thus expensive.

Uniformity in cooling along the whole extent of the die is thus difficult or impossible to achieve. Methods which require machining and joining of individual die subsections means precise assembling and sealing of the parts, which add to manufacturing time and cost.

In particular, although the drilling method is a popular method in forming cooling systems in dies, it has many drawbacks. Firstly, due to the constraints in drilling depth and difficulties in drilling non-linear cooling channels, the die typically has to be divided into segments to allow linear cooling channels to be drilled into each. The segments then need to be assembled back together with sealant and to be machined. The assembling, sealing and machining of the segments require precision and machinery. This method is thus very labour intensive and costly, and the cooling channels in the resultant die tend to have poor conformity with the contour of the die's stamping surface.

In particular, additive manufacturing at present is time consuming, extremely expensive, inefficient, and difficult to be used for large-scaled dies, due to limitations in capabilities of the machinery. The accuracy of such a method and the quality of the die also cannot be guaranteed. Furthermore, dies typically used for hot metal forming, such as H13 steel, cannot be cast.

The present invention seeks to provide a solution to these problems.

According to a first aspect of the present invention, there is provided a method of casting a die for hot stamping a nonferrous metal alloy sheet, the die having a die body with a stamping surface thereon and at least one cooling conduit therethrough, the method comprising the steps: a] selecting a die material having a melting temperature (T.) between 1000°C and 1300°C for forming the die body; b] providing the at least one cooling conduit formed of metal which has a thermal conductivity the same as, substantially the same as, or higher than that of the die material, shaping the at least one cooling conduit such that the at least one cooling conduit has a profile in an axial direction which conforms or substantially conforms to a profile of a desired stamping surface of the die; c] providing a casting mould having a casting surface for casting the die, the casting surface having a stamping-surface forming portion for forming the stamping surface of the die; d] positioning at least a portion of the at least one cooling conduit in the casting mould; e] melting the die material; f] pouring molten die material into the casting mould; g] cooling the casting mould until the die material solidifies to form the die body, the said at least a portion of the at least one cooling conduit being embedded therein; and h] removing the casting mould and exposing an inlet and an outlet of the at least one cooling conduit to an exterior of the die body for allowing a cooling fluid to flow through the at least one cooling conduit.

The casting method as claimed enables a die to be manufactured which provides sufficient stamping load for stamping a workpiece made of nonferrous metal alloy sheet, such as Al-alloy or Mg-alloy sheet, which is typically lower than the stamping load required for stamping a workpiece made of boron steel. Compared to conventional die casting techniques for hot stamping, the claimed method allows a lighter and less expensive material to be used to produce a die, as well as production of more complex and curved shapes on the die's stamping surface. The resultant die provides higher and more uniform cooling capability across the stamping surface, thereby allowing quicker and more uniform cooling of the shaped component during stamping, which is especially important for shaped component made from a nonferrous metal alloy, such as aluminium alloy.

A die material having a melting temperature as claimed, such as gray cast iron or ductile cast iron, can be used for forming the die with a complex stamping surface, which provides a good balance of weight, ductility, and malleability. Such a die material is more advantageous than the conventional material used for hot stamping, typically steel such as hot work tool steel which is significantly more expensive, less ductile and less malleable.

A uniform and high quenching rate is important when hot stamping Al-alloy or Mg-alloy sheet, as it is required for achieving the desirable microstructure and specific post-stamped mechanical properties. For example, a quenching rate greater than 150°C/s and 50°C/s is required when stamping sheets made of 7075 aluminium alloy and 6082 aluminium alloy respectively, compared to around 40°C/s for sheets made of boron steel. The use and installation of the pre-shaped cooling conduit in the casting mould before casting allows the profile of the stamping surface of the die to be closely followed following casting, to improve cooling uniformity throughout the die. The claimed method also allows positioning of the cooling conduit close to the eventual stamping surface formed on the die to maximise quenching. Furthermore, the cooling conduit has a thermal conductivity no lower than that of the die material, which further improves quenching rate and uniformity on the stamping surface of the die during stamping of the workpiece, compared to conventional means of providing a cooling system such as ceramic core or drilling cooling channels. Such improved cooling capability allows complex stamping surface shapes to be provided on the die, which is not achievable on a die with cooling conduits made from ceramic cores or channels.

Preferably, the die material may be different to the metal forming the at least one cooling conduit, and/or the die material has a melting temperature higher than that of the at least one cooling conduit. The die material may have a melting temperature between 1100°C and 1250°C.

The die material may preferably be a gray cast iron or ductile cast iron, and/or the at least one cooling conduit is a copper, brass, or mild steel conduit. Thus, conventional cooling conduits, such as copper pipes, may be utilised, which are cost effective and easy to source.

In step b], the at least one cooling conduit may be shaped such that its cross-section is non-circular, and/or such that at least a portion of its cross-section may conform or substantially conform to the profile of the desired stamping surface of the die to which it is closest, in order to maximise exposure of cooling fluid flowing therein to the stamping surface, during use of the resultant die.

In step b], the profile of the at least one cooling conduit in the axial direction may be non-linear in order to conform or substantially conform to the profile of the stamping surface of the die.

In step d], the at least one cooling conduit may be proximate to but spaced apart from the stamping-surface forming portion of the casting mould, such that it does not impede the formation of the stamping surface in step f].

Before step d], the at least one cooling conduit may be surface treated to remove oxidation prior to introducing the at least one cooling conduit into the casting mould to encourage bonding between the at least one cooling conduit and the die material.

Prior to step f], the inlet and outlet of the at least one cooling conduit are blocked to prevent the molten die material from entering the at least one cooling conduit in step f]. For example, the at least one cooling conduit may be filled with packing material to maintain the shape of the at least one cooling conduit in step f], the packing material being removed in step h]. Such packing material may be sand, more preferably moulding sand. Alternatively, in step f] and/or step g] the inlet and outlet of the at least one cooling conduit may be connected to a cooling system, a cooling fluid flowing therethrough to prevent melting of the at least one cooling conduit. For these above two examples, at least, the at least one cooling conduit may have a melting temperature lower than that of the die material. Such a cooling conduit typically has a thermal conductivity significantly higher than that of the die material, which provides even better cooling capability to the stamping surface during hot stamping.

In step d] the at least a portion of the at least one cooling conduit may be supported on a cooling conduit support, to ensure the at least one cooling conduit is in its intended position during step f].

In step d], a temperature sensing means may be provided in the casting mould for monitoring the internal temperature of the stamping die during hot stamping. This negates installation of the temperature sensing means after the die is cast, which often requires drilling. Such drilling is difficult and risks damaging the die. Installation of the temperature sensing means before casting also provides freedom as to the positioning and number of the temperature sensing means not possible with post-cast drilling, such as location(s) between the at least one cooling conduit and the stamping surface.

The temperature sensing means may be located adjacent or proximate to at least one of the following: the inlet of the at least one cooling conduit, the outlet of the at least one cooling conduit, at least one location between the at least one cooling conduit and the stamping-surface forming portion of the casting surface. These locations are important when monitoring the performance of the die and its cooling capability.

The temperature sensing means may be at least one thermocouple. The at least one thermocouple may be at least one of: type K nickel alloy thermocouple, platinum alloy thermocouple, and tungsten alloy thermocouple.

Preferably, a portion of the at least one thermocouple may be insulated by an insulation layer to prevent damage to the at least one thermocouple during casting in step f] and/or hot stamping. The insulation layer preferably insulates the thermocouple along its entire length except its sensing end to prevent false reading of temperature, in use. The insulation layer may be made of ceramic.

In step h] after removal of the casting mould, the die may be cut to a required size and/or the stamping surface may be finished according to a required shape and surface finish requirement.

According to a second aspect of the present invention, there is provided a die for hot stamping a nonferrous metal alloy sheet, the die comprising: a die body made from a material having a melting temperature (Tm) between 1000°C and 1300°C, the die body having a stamping surface thereon for contacting the nonferrous metal alloy sheet during hot stamping; at least one cooling pipe formed of metal and extending through the die body, the at least one cooling pipe having a thermal conductivity the same as, substantially the same as or higher than that of the die body, and having an inlet and an outlet for allowing a cooling fluid to flow therethrough, wherein the at least one cooling pipe having a profile in an axial direction which conforms or substantially conforms to a profile of the stamping surface of the die body.

The material of the die body may be different to the metal which forms the at least one cooling pipe.

The die body may have a melting temperature between 1100°C and 1250°C. The die body may be made of a gray cast iron or ductile cast iron, and/or the at least one cooling pipe may be a copper, brass, or mild steel conduit.

The at least one cooling pipe may have a melting temperature lower than that of the die body.

A cross-section of the said at least one cooling pipe may be non-circular, and/or at least a portion of a cross-section of the said at least one cooling pipe may conform or substantially conform to the profile of the stamping surface of the die body to which it is closest. The profile of the at least one cooling pipe in the axial direction may be non-linear.

The at least one cooling pipe may be proximate to but spaced apart from the stamping surface of the die body.

A temperature sensing means may be located in the die body for monitoring the internal temperature of the stamping die during hot stamping. The temperature sensing means may be located adjacent or proximate to at least one of the following: the inlet of the at least one cooling pipe, the outlet of the at least one cooling pipe, a location between the at least one cooling pipe and the stamping surface of the die body. The temperature sensing means may be at least one thermocouple. The at least one thermocouple may be at least one of: type K nickel alloy thermocouple, platinum alloy thermocouple, and tungsten alloy thermocouple. The at least one thermocouple may be insulated by an insulation layer to prevent damage to the at least one thermocouple. The insulation layer may be made of ceramic.

The invention will now be more particularly described, by way of example only, with reference to the accompanying drawings, in which: Figure la shows a schematic cross-sectional view of an embodiment of a cooling conduit being shaped before being introduced into a casting mould, the cooling conduit having a profile which conforms to a profile of a stamping-surface forming portion of the casting mould following shaping; Figure lb shows a schematic cross-sectional view of an embodiment of the casting mould having an interior space, with the cooling conduit of Figure la positioned inside the interior space in accordance with a method of the first aspect of the invention; Figure lc shows a schematic cross-sectional view of the casting mould of Figure lb, whereby the interior space is filled with molten die material for forming a die body of a first embodiment of a die, in accordance with a second aspect of the invention; Figure ld shows a schematic cross-sectional view of the die of Figure lc, with the casting mould removed, in accordance with the method of first aspect of the invention; Figure le shows a schematic cross-sectional view of the die of Figure ld in accordance with the second aspect of the invention, having undergone surface machining to remove excess material and to expose an inlet and an outlet of the cooling conduit, in accordance with the method of first aspect of the invention; Figure 2a shows a first optical microscopic representation of a section of the die of Figure le; Figure 2b shows a second optical microscopic representation of a section of the die of Figure le; Figure 3 shows a perspective cross-sectional view of a second embodiment of a die in accordance with the second aspect of the invention; Figure 4 shows a perspective transparent view of a third embodiment of the a in accordance with the second aspect of the invention; Figure 5a show a schematic cross-sectional view of another embodiment of a cooling conduit, which may be used in a die in accordance with the second aspect of the present invention; Figure 5b show a schematic cross-sectional view of another embodiment of a cooling conduit, which may be used in a die in accordance with the second aspect of the present invention; Figure 5c show a schematic cross-sectional view of another embodiment of a cooling conduit, which may be used in a die in accordance with the second aspect of the present invention; Figure 5d show a schematic cross-sectional view of another embodiment of a cooling conduit, which may be used in a die in accordance with the second aspect of the present invention; Figure 6 shows a plot of die stamping surface temperature during hot stamping against different locations on the stamping surface, with different material cooling conduits, based on finite element method (FEM) simulations; and Figure 7 shows a plot of temperature difference during hot stamping between points A and C on the stamping surface of the die against number of hot stamping cycles, with cooling conduits having different cross-sectional shapes, based on finite element method (FEM) simulations.

In order to at least mitigate the problems outlined above, it is proposed here to provide a method of casting a die for hot forming, stamping or forging a nonferrous metal alloy sheet, for example an aluminium alloy sheet such as 7075 aluminium alloy or 6082 aluminium alloy sheet, or a magnesium alloy. Figures la to le illustrate an embodiment of such a method, in accordance with a first aspect of the invention, of casting a first embodiment of a die, indicated generally as 10, which comprises a die body 12 having a stamping surface 14 thereon for contacting a nonferrous metal alloy sheet during hot forming, stamping or forging, and a cooling conduit or pipe 16 extending through the die body 12. The cooling conduit 16 has an inlet 18 and an outlet 20 for a cooling fluid to flow therethrough. As illustrated in Figure le, the stamping surface 14 of the die body 12 in this embodiment has a non-linear and convex portion 22.

The method includes selecting a die material having a melting temperature (Tm) between 1000 °C and 130000 for forming the die body 12. In this embodiment, D6510 iron is chosen as the die material. However, other suitable die material may be used, such as gray cast iron, which has a melting temperature of around 1127 °C to 1204 °C or ductile cast iron, which has a melting temperature of around 1149 °C, such as those listed in NAAMS Index for Stamping Dies: Cast Materials.

The cooling conduit 16 is then provided, which is formed of a metal which has a thermal conductivity preferably higher than that of the die material. In this embodiment, mild steel is used, which has a melting temperature higher than that of the die material. Thus, the cooling conduit 16 will not melt and will maintain its shape when molten die material is poured into the casting mould during casting.

However, it will be appreciated that other suitable material may be used for the cooling conduit which may have a lower melting temperature than that of the die material, such as copper, or brass. Gray or ductile cast iron as a die material has a thermal conductivity of around 29.5 to 32 W/(m*K) at around 2000; whereas copper has a thermal conductivity of around 401 W/(m*K) at around 20°C; brass has a thermal conductivity of around 109 to 125 W/(m*K) at around 20 °C; and mild steel has a thermal conductivity of around 56 to 67 W/(m*K) at 20 °C. Copper, brass, and mild steel have melting temperatures of around 1084 °C, 930 °C, and 1350 °C -1530°C respectively.

It will be appreciated that the cooling conduit may also be formed from a material the same or similar to the die material. The thermal conductivity and/or the melting temperature of the cooling conduit and the die material may be the same or substantially the same.

As illustrated in Figure la, the cooling conduit 16 has a cooling-conduit body 24 and the inlet 18 and outlet 20 are provided at either ends thereof. The unshaped cooling conduit 16 is shown in the upper portion of Figure la, and the shaped cooling conduit 16 is shown in the lower portion of Figure la. The cooling conduit 16 is pre-shaped or pre-bent such that it has a profile in an axial direction which conforms or substantially conforms to a profile of the stamping surface 14 of the die 10. Thus, in this embodiment, the cooling conduit 16 has a convex portion 26 which corresponds with the convex portion 22 of the stamping surface 14 of the die body 12, i.e. the cooling conduit 16 has a non-linear profile. The cooling conduit 16 may be pre-shaped in a number of conventional ways, such as by ram-type bending, roll bending, compression bending, or rotary draw bending. Alternatively or additionally, short lengths of conduits may be cut and welded together. In this embodiment, the inlet 18 and the outlet 20 of the cooling conduit 16 are temporarily blocked by stoppers 28 to prevent the die material from entering therein during casting. Alternatively or additionally, the cooling conduit 16 may be filled with packing material, such as sand, more preferably moulding sand, especially if the cooling conduit 16 has a lower melting temperature than that of the die material, which also helps to maintain the shape of the cooling conduit 16 during casting.

Then, as illustrated in Figure lb, a casting mould 30 having a casting surface 32 accessible from an exterior of the casting mould 30 is provided for casting the die 10. In this embodiment, a two-part sand casting is used, which includes a casting flask 33 having an upper flask portion 34a, known as a cope, and a lower flask portion 34b, known as a drag. The casting flask 33 is typically made from wood or metal and the casting mould 30 includes a sand mould in the casting flask 33, having an upper sand mould portion 36a and a lower sand mould portion 36b which correspond with the upper flask portion 34a and the lower flask portion 34h respectively.

The sand mould defines the casting surface 32 for casting the die 10, in particular the casting surface 32 has a stamping-surface forming portion 38 for forming the stamping surface 14 of the die 10. In turn, the casting surface 32 defines an interior casting space or void 40 for receiving the die material. In this embodiment, the casting surface 32 is in fluid communication with an exterior of the casting mould 30 by a channel or sprue 42, thus allowing access to the casting surface 32 from the outside.

The casting mould 30 is first opened to allow the cooling conduit 16 to be placed therein. The casting mould 30 includes a cooling-conduit support, in the form of two cooling-conduit support frames 44 in this embodiment, for supporting the cooling conduit 16 at a desired position in the casting mould 30. It will be appreciated that the cooling-conduit support may be provided by other means, for example the cooling-conduit support may be part of the sand mould.

As illustrated in Figure lb, the cooling conduit 16 is proximate to but spaced apart from the stamping-surface forming portion 38 of the casting surface 32, such that the cooling conduit 16 will serve the purpose of cooling the stamping surface 14, but not so close that it negatively affects the stamping or forming performance of the resultant die 10. Preferably, the distance between the stamping surface 14 and the surface of the cooling conduit 16 closest to the stamping surface 14 is 5mm or greater. The distance between the cooling conduit 16 and the stamping-surface forming portion 38 may vary depending on the dimension of the resultant die 10, the die and/or cooling conduit 16 material, the size of the cooling conduit 16, the thickness of the cooling-conduit body 24, and the amount of cooling effect required. The shape of the cooling conduit 16 conforms or substantially conforms to the contour of the stamping-surface forming portion 38 of the casting surface 32.

In this embodiment, temperature sensing means, in the form of a plurality of thermocouples 46, is provided in the casting mould 30 for monitoring the internal temperature of the stamping die 10 during hot stamping. A sensor end 48 of each thermocouple 46 is provided in the interior casting space 40 of the casting mould 30 and each thermocouple 46 extends through the casting surface 32 and into the sand mould such that, after casting, the sensor end 48 is embedded within the die 10 and a connector end (not shown) opposing the sensor end 48 is outside the die 10 and can be connected to a multimeter to read the temperature sensed by the sensor end 48. A portion of each thermocouple 46, such as the longitudinal extent of each thermocouple 46 apart from its sensor end 48, is preferably insulated with an insulation layer to protect each thermocouple 46 during casting and/or hot stamping. The insulation layer is preferably made of ceramic, or other material suitable to be used for insulation under high temperature. In view of the material used, such insulation layers further provide support for positioning the sensor ends 48 of the thermocouples 46 in the desired location in the interior casting space 40. The thermocouples 46 may be at least one of: type K nickel alloy thermocouple, platinum alloy thermocouple, and tungsten alloy thermocouple. It will be appreciated that only one thermocouple may be provided. The thermocouples may be of the same type or of different types.

The temperature sensing means is positioned strategically in the interior casting space 40 to monitor temperature at locations of interest, and in this embodiment, it is positioned to monitor temperature adjacent the inlet 18 and outlet 20 of the cooling conduit 16, and locations along a lateral extent of the die 10 and between the cooling conduit 16 and the stamping surface 14. It will be appreciated that the temperature sensing means may be provided in more or fewer locations, and/or different locations.

Once the cooling conduit 16 and the temperature sensing means are in position, the casting mould 30 is closed. The die material is molten and introduced into the casting mould 30 via the channel 42 using conventional techniques, as illustrated in Figure lc whereby the temperature sensing means is omitted for clarity. In this embodiment, one of the inlet 18 and outlet 20 of the cooling conduit 16 is spaced away from the casting surface 32 to prevent obstruction to the flow of molten die material during casting. However, it will be appreciated that both ends of the cooling conduit 16 may abut or substantially abut the casting surface 32 as long as the channel 42 of the casting mould 30 is unobstructed.

The casting mould 30 with the die material therein is then cooled until the die material solidifies to form the die body 12, with the cooling conduit 16 embedded therein.

As illustrated in Figure id whereby the temperature sensing means is omitted for clarity, the casting mould 30 is then removed, revealing a die block 50 having the die 10 and excess die material. The excess die material is removed by machining, and stoppers 28 and/or moulding sand inside the cooling conduit 16 are removed, such that only the die 10 is left and the inlet 18 and outlet 20 of the cooling conduit 16 are in fluid communication to an exterior of the die 10 for allowing the cooling fluid to flow therethrough, as illustrated in Figure le whereby the temperature sensing means is omitted for clarity. It will be appreciated that the inlet 18 and outlet 20 may be flush with sides of the die 10, rather than extending or protruding outwardly from the sides.

The resultant die 10 can then be used with a corresponding second die, preferably formed from the same method as described above and which has a concave portion which corresponds to the convex portion 22 of the stamping surface 14 of the die 10, for hot stamping the nonferrous metal alloy sheet. In use, the non-ferrous metal alloy sheet is heated, for example to its solution heat treatment temperature, and is then transferred to between the dies whilst the sheet is still hot. The sheet or workpiece is then stamped by closing the dies to form a component shaped according to the stamping surfaces of the dies, and the shaped component is quenched in the closed dies by providing a flow of cooling fluid, such as water and gas, through the cooling conduits of the dies. The cooling fluid may be supplied by an external pumping and control system and cooling fluid station, and the cooling fluid may be at room or a lower temperature.

Referring to Figures 2a and 2b, two microscopic representations of different magnifications of a section of the first embodiment of the die 10 are illustrated to demonstrate microscopically an interface bonding between the die body 12 and the cooling conduit 16. It can be seen that there is good contact at the interface without any voids and leaks.

It is therefore possible to provide a method of casting a die 10 for hot stamping a nonferrous metal alloy sheet which uses a die material that is cheaper, lighter, requires less energy and labour during casting, is easier to cast due to improved malleability, and is easier to form a complex stamping surface 14 due to improved ductility compared to conventional casting techniques. The die material used is sufficient to withstand the stamping impact associated with hot stamping the nonferrous metal alloy sheet, thus does not negatively impact on the lifespan of the die 10.

The above described method is particularly suited for producing a dies 10 with a large stamping surface 14 or a complex shape, as the cooling conduit 16 is able to be pre-shaped and pre-installed before casting to conform to the profile of the stamping surface 14 across the entire or substantially the entire longitudinal and/or lateral extent of the die 10. It is also possible to position the cooling conduit 16 closer to the stamping surface 14 than compared to drilling cooling channels post cast. This allows much improved cooling capabilities and uniformity during stamping. There is no longer a need to drill holes or to use ceramic cores as means to provide cooling to the die 10, which is restricted as to its shape and depth. Using mild steel or copper conduits as cooling conduits 16 is also comparatively inexpensive compared to the labour involved in drilling holes or forming ceramic cores.

Furthermore, the material of the cooling conduit 16 having a thermal conductivity the same, substantially the same, or higher than that of the die material also improves cooling capability. The advantage of this is illustrated in Figure 6, which demonstrates the temperature across the stamping surface utilising cooling conduits formed from different materials during hot stamping, based on FEM simulations. The die surface temperature is lower in a die with a cooling conduit that is higher in thermal conductivity, such as copper or mild steel conduit, than that of the die material, than in a die with a cooling conduit that is lower in thermal conductivity, such as stainless steel conduit, than that of the die material.

Referring to Figures 3 and 4, there is provided second and third embodiments of a die in accordance with the present invention. Features on the second and third embodiments which are similar to features of the first embodiment have similar reference numerals with "100" and "200" added respectively.

In Figure 3, the stamping surface 114 of the die 110 is wavy, having a double curvature and the cooling conduit 116 is correspondingly wavy in order to conform or substantially conform to the profile of the stamping surface 114. In Figure 4, the stamping surface 214 of the die 210 has a step or drop 252 along its lateral extent and is wavy along its longitudinal extent, and multiple cooling conduits 216a, 216b, 216c are provided, each being shaped such that each conforms to a profile at least along the longitudinal extent of a corresponding portion of the stamping surface 214. The cooling conduits 216a, 216b, 216c may have the same shape and size or they may vary in shapes and sizes, as illustrated in Figures 4, and 5a to 5d. One or more cooling conduit may be adapted such that at least a portion of its cross-section which is closest to the stamping surface of the die conforms or substantially conforms to the profile or the contour of the stamping surface. For example, the cooling conduit 216c having a rectangular cross-section in its radial extent may be provided to maximise the cooling surface area closest to the stamping surface 214. In particular, the rectangular cross-section has an L-shaped portion 218 which conforms or substantially conforms to the profile of the stamping surface 214 to which it is located closest, which is also L-shaped.

Other cooling conduit cross-sectional shapes may also be envisaged, such as a square shaped cross-section conduit 216d as illustrated in Figure 5a, circular shaped cross-section conduit 216a as illustrated in Figure 5b, diamond shaped cross-section conduit 216e as illustrated in Figure 5c, and irregular shaped cross-section conduit 216f as illustrated in Figure 5d. It will be appreciated that other shapes not already illustrated may also be suitable, depending on the contour of the stamping surface. Forming of conduits with different cross-section shapes may be achieved by a manual or mechanical process, such as hydroforming and/or tube pressing.

Figure 7 demonstrates the temperature difference across points A and C during hot stamping on a substantially planar stamping surface over an increasing number of hot stamping cycles, with cooling conduits having different cross-sections, based on FEM simulations. The best uniformity in temperature is achieved by the cooling conduit with a rectangular cross-section, as it delivers the greatest amount of cooling fluid adjacent the planar stamping surface in a given volume. The diagonal or diamond cross-section conduit has the steepest difference in temperature, as it provides the least amount of cooling fluid to the stamping surface in the same or similar volume. The circular cross-section conduit performs substantially better than the diamond cross-section conduit, but not as good as that of the rectangular cross-section conduit. Therefore, it will be appreciated that the closer the match is between the cross-sectional shape of the cooling conduit and the contour of the stamping surface the cooling conduit is located under, the better the cooling capability and uniformity.

It will be appreciated that it may be possible to provide a network of multiple cooling conduits in the die, as illustrated in Figure 4, and they may have the same or different lengths, widths, cross-section shapes and/or materials. They may be pre-shaped, pre-bent, and/or welded together. The number of cooling conduits used may depend on the dimensions of the stamping surface, which may be determined through finite element method (FEM) simulations. The distance between adjacent cooling conduits may preferably be 2mm or greater.

The die material may be the same as that of the cooling conduit, or may be different.

The melting temperature of the die material may more particularly be between 1100°C and 1250°C, and more preferably 1125°C and 1250°C. The die material and/or the cooling conduit may preferably have a thermal conductivity of 30 W/(m-K) or higher. The cooling conduit, such as a copper cooling conduit, may have a melting temperature lower than that of the die material, such as cast iron.

It will be appreciated that the stamping surface of the die and the corresponding cooling conduit need not have a convex portion. They may have a profile that is linear or non-linear in their respective axial directions.

The cooling conduit may be surface treated to remove oxidation on its outer surface and/or to provide an optimal surface roughness prior to its introduction into the casting mould to encourage robust bonding between the cooling conduit and the die material, such as by at least one of etching, polishing, shot blasting, sand blasting, surface coating, grinding, and micro-texturing.

It will be appreciated that it may be possible to provide a casting mould where the inlet and outlet of the cooling conduit abut the casting surface, such that the die material is prevented from entering the cooling conduit, thus negating the need to provide stoppers or packing material in the cooling conduit, during casting.

Alternatively, the cooling conduit may be long enough such that the inlet and outlet extend out of the casting mould before casting. This has the added advantage of allowing the cooling conduit to be connected to a cooling system to allow a cooling fluid to flow therethrough during casting, which helps to maintain the shape of the cooling conduit, as well as further prevents melting of the cooling conduit if it has a lower melting temperature than that of the die material.

It will be appreciated that although a casting flask and a sand mould are used to form the casting mould, one or both of these may be dispensed with. The sand mould may also be formed from a material other than sand. Furthermore, the casting flask may be formed by left and right flask portions, or it may be formed from more than two or just one part. Instead of providing a channel or sprue for introducing the molten die material into the casting surface, it may be possible to simply provide an opening on the casting mould. As yet another alternative, molten die material may be introduced into the casting mould at or near its base, by gravity casting.

It will be appreciated that the temperature sensing means may be dispensed with, or a temperature sensing means other than a thermocouple may be provided.

It will be appreciated that each cooling conduit may be formed from a unitary piece of metal, thus reducing the risk of any leakage.

It will be appreciated that although the die formed by the method in accordance with the first aspect of the invention and the die in accordance with the second aspect of the invention are predominantly used for hot stamping nonferrous metal alloy sheets, it will be appreciated that they may also be suitable for hot stamping steel sheets.

The words 'comprises/comprising' and the words 'having/including' when used herein with reference to the present invention are used to specify the presence of stated features, integers, steps or components, but do not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination.

The embodiments described above are provided by way of examples only, and various other modifications will be apparent to persons skilled in the field without departing from the scope of the invention as defined herein.

Claims

Claims 1. A method of casting a die for hot stamping a nonferrous metal alloy sheet, the die having a die body with a stamping surface thereon and at least one cooling conduit therethrough, the method comprising the steps: a] selecting a die material having a melting temperature (Tni) between 1000°C and 1300°C for forming the die body; b] providing the at least one cooling conduit formed of metal which has a thermal conductivity the same as, substantially the same as, or higher than that of the die material, shaping the at least one cooling conduit such that the at least one cooling conduit has a profile in an axial direction which conforms or substantially conforms to a profile of a desired stamping surface of the die; c] providing a casting mould having a casting surface for casting the die, the casting surface having a stamping-surface forming portion for forming the stamping surface of the die; d] positioning at least a portion of the at least one cooling conduit in the casting mould; e] melting the die material; f] pouring molten die material into the casting mould; g] cooling the casting mould until the die material solidifies to form the die body, the said at least a portion of the at least one cooling conduit being embedded therein; and h] removing the casting mould and exposing an inlet and an outlet of the at least one cooling conduit to an exterior of the die body for allowing a cooling fluid to flow through the at least one cooling conduit.
2. A method as claimed in claim 1, wherein the die material is different to the metal forming the at least one cooling conduit, and/or the die material has a melting temperature higher than that of the at least one cooling conduit.
3. A method as claimed in claim 1 or claim 2, wherein the die material is a gray cast iron or ductile cast iron, and/or the at least one cooling conduit is a copper, brass, or mild steel conduit.
4. A method as claimed in any one of the preceding claims, wherein in step b] the at least one cooling conduit is shaped such that its cross-section is non-circular, and/or such that at least a portion of its cross-section conforms or substantially conforms to the profile of the desired stamping surface of the die to which it is closest.
5. A method as claimed in any one of the preceding claims, wherein in step d] the at least one cooling conduit is proximate to but spaced apart from the stamping-surface forming portion of the casting mould.
6. A method as claimed in any one of the preceding claims, wherein before step d] the at least one cooling conduit is surface treated to remove oxidation.
7. A method as claimed in any one of the preceding claims, wherein prior to step f] the inlet and outlet of the at least one cooling conduit are blocked to prevent the molten die material from entering the at least one cooling conduit in step f].
8. A method as claimed in claim 7, wherein the at least one cooling conduit is filled with packing material to maintain the shape of the at least one cooling conduit in step f], the packing material being removed in step h].
9. A method as claimed in any one of claims 1 to 7, wherein in step f] and/or step g] the inlet and outlet of the at least one cooling conduit are connected to a cooling system, a cooling fluid flowing therethrough to prevent melting of the at least one cooling conduit.
10. A method as claimed in any one of the preceding claims, wherein in step d] the at least a portion of the at least one cooling conduit is supported on a cooling conduit support.
11. A method as claimed in any one of the preceding claims, wherein in step d], a temperature sensing means is provided in the casting mould for monitoring the internal temperature of the stamping die during hot stamping.
12. A method as claimed in claim 11, wherein the temperature sensing means is at least one thermocouple.
13. A method as claimed in claim 12, wherein the at least one thermocouple is at least one of: type K nickel alloy thermocouple, platinum alloy thermocouple, and tungsten alloy thermocouple.
14. A method as claimed in claim 12 or claim 13, wherein a portion of the at least one thermocouple is insulated by an insulation layer to prevent damage to the at least one thermocouple during casting in step f] and/or hot stamping.
15. A method as claimed in any one of the preceding claims, wherein in step h] after removal of the casting mould, the die is cut to a required size and/or the stamping surface is finished according to a required shape and surface finish requirement.
16. A die for hot stamping a nonferrous metal alloy sheet, the die comprising: a die body made from a material having a melting temperature (Tm) between 1000°C and 1300°C, the die body having a stamping surface thereon for contacting the nonferrous metal alloy sheet during hot stamping; at least one cooling pipe formed of metal and extending through the die body, the at least one cooling pipe having a thermal conductivity the same as, substantially the same as or higher than that of the die body, and having an inlet and an outlet for allowing a cooling fluid to flow therethrough, wherein the at least one cooling pipe having a profile in an axial direction which conforms or substantially conforms to a profile of the stamping surface of the die body.
17. A die as claimed in claim 16, wherein the material of the die body is different to the metal which forms the at least one cooling pipe.
18. A die as claimed in claim 16 or claim 17, wherein the die body is made of a gray cast iron or ductile cast iron, and/or the at least one cooling pipe is a copper, brass, or mild steel conduit.
19. A die as claimed in any one of claims 16 to 18, wherein the at least one cooling pipe has a melting temperature lower than that of the die body.
20. A die as claimed in any one of claims 16 to 19, wherein a cross-section of the said at least one cooling pipe is non-circular, and/or at least a portion of a cross-section of the said at least one cooling pipe conforms or substantially conforms to the profile of the stamping surface of the die body to which it is closest.
21. A die as claimed in any one of claims 16 to 20, wherein the at least one cooling pipe is proximate to but spaced apart from the stamping surface of the die body.
22. A die as claimed in any one of claims 16 to 21, further comprises a temperature sensing means located in the die body for monitoring the internal temperature of the stamping die during hot stamping.
23. A die as claimed in claim 22, wherein the temperature sensing means is at least one thermocouple.
24. A die as claimed in claim 23, wherein the at least one thermocouple is at least one of: type K nickel alloy thermocouple, platinum alloy thermocouple, and tungsten alloy thermocouple.
25. A die as claimed in claim 23 or claim 24, wherein the at least one thermocouple is insulated by an insulation layer to prevent damage to the at least one thermocouple.