HAVING A HEXAGONAL CLOSE PACKED CRYSTAL STRUCTURE.

This invention relates to a method and apparatus for fabricating articles from metals having a hexagonal close packed (HCP) crystal structure and in particular for making a magnesium alloy helmet shape (shell) by combining within a single press cycle a specialized hot deep draw (HDD) first step followed by a pneumatic coining second step.

Magnesium and magnesium-rich alloys (both of which are herein referred to simply as magnesium alloys) have high specific strength and stiffness and yet are significantly lighter than aluminum alloys. The durability and lightweight properties of magnesium alloys render the materials highly suitable for helmet construction, and indeed for the construction of a range of protective equipment, because magnesium alloys have the necessary strength and other obvious beneficial attributes for protecting the wearer. Recent alloy developments have further improved the strength and resistance to self-sustained flammability of certain magnesium alloys making magnesium alloy helmets an attractive and lighter alternative to the non- metallic military helmets currently in use.

It is well known however that the ability to cold deep draw magnesium alloys is very limited due to the inherent low ductility of such materials as a result of the HCP crystal structure of magnesium. This lack of ductility is because slip in metals having a HCP crystal structure is much more limited than in metals having body centered cubic (BCC) or face centered cubic (FCC) crystal structures. This arises through the lack of active slip systems that exist in HCP metals.

Improved ductility can be achieved at elevated temperatures in certain magnesium alloys by incorporating a combination of additional slip planes and twinning. A cost effective method however of forming a helmet-shaped article from a blank of wrought magnesium alloy is not known.

On the face of it, hot matched die forming offers a possible processing solution, with its potential to maintain a magnesium alloy blank at a

temperature sufficiently elevated to confer adequate ductility to the blank.

Such a method, however, has significant drawbacks. Hot matched die forming requires complementary dies to be pressed together to impart the required shape to a magnesium alloy sheet trapped between. The thickness and thickness variation associated with the forming of a typical helmet geometry from a magnesium alloy blank will demand very accurate and subtle tooling of the dies. Managing the temperature and the dimensions (as temperature changes will cause the dimensions of the dies to change) of matched dies at elevated temperature will also be challenging and it will be difficult to avoid 'jamming' as the two dies close together. There are also problems in controlling the thickness-defining gap between matched dies in which the magnesium sheet is trapped. In summary, forming a magnesium alloy helmet using a hot matched die process is likely to be prohibitively complex and expensive.

In a first aspect, the present invention provides a method of fabricating an article from a metal or alloy having a hexagonal close packed crystal structure, the method comprising a draw stage followed by a coining stage. The draw stage comprises the steps of: a) providing a blank of the metal or alloy having a hexagonal close packed crystal structure; b) mounting the blank in a deep draw press adjacent a draw ring; c) heating the environment of the blank to a predetermined

temperature; and d) moving a heated shaped plug at a predetermined speed onto the blank and through the draw ring so as to draw the heated blank into a drawing chamber as the plug advances through the draw ring.

The second, coining stage comprises the steps of: e) holding the plug stationary; f) clamping a flange of the drawn blank so as to form a seal between the blank and the drawing chamber; and g) passing a gas into the drawing chamber so as to increase the pressure therein, thereby applying a pneumatic force to the drawn blank to force the drawn blank to mate closely with the plug.

Use of such a two-stage process enables detailed features to be imparted to a blank, without the need to use matched dies. This accordingly avoids the host of difficulties associated therewith. The method of this invention is therefore primarily suited to processing magnesium and magnesium-rich alloys, which are particularly difficult to subject to a matched die process because of the elevated temperature required. The first hot draw stage gives the general shape to the blank. For example, the crown and sides of a helmet. The force of the blank against the plug in the draw process is however insufficient to impart detailed features to a blank material with a HCP crystal structure at practicable operating temperatures. Accordingly such secondary features, such as ear lobes and ridges of a helmet are added in a subsequent coining stage. Traditionally, coining uses matched dies but, for the reasons given above, it is not straightforward to use such dies at the elevated temperatures required to render HCP materials ductile. The present invention accordingly makes use of a so-called "pneumatic" coining process in which one part of the die is replaced by a high-pressure force. This force pushes the drawn blank against the plug with sufficient pressure to imprint detailed features, requiring less material movement than in the draw stage, to the blank.

As outlined above, this method is ideally suited for application to helmet fabrication from magnesium or magnesium-rich alloys. Obviously other HCP metallic materials exist, with similar fabrication problems arising from their low ductility. The hot deep draw process followed by pneumatic coining in accordance with the present invention may be equally beneficial to processing of these materials. Similarly other component shapes may be produced. To be of greatest benefit however, the final shape should comprise a basic, high- material-movement shape with more detailed, low-movement, features. A promising application is considered to be the fabrication of protective equipment generally from magnesium alloy material.

The draw ring is preferably mounted on a first clamping plate with a stripper plate being mounted on a second clamping plate, the blank being located between the first and second clamping plates. This arrangement enables a gap to be either maintained or closed between the plates. If maintained, it can be used to control back tension during the draw stage and, if closed, can effect the seal during the coining stage. The stripper plate can be usefully employed to retain the formed part (shell) as the plug is retracted through the draw ring, thereby separating the formed part from the plug.

The method may be followed by the steps of solution treating, quenching and then ageing the formed part, if the alloy material is one whose properties may be improved by such processing. Alternatively, the formed part may be quenched directly from the hot press, this quenching again being followed by a suitable ageing procedure.

In another aspect the present invention provides an apparatus for fabricating an article from a metal or alloy having a hexagonal close packed crystal structure, the apparatus comprising: a drawing chamber having a gas inlet; first and second clamping plates between which a heated blank may be located, a draw ring being mounted on the first clamping plate; and a heater for controlling the environmental temperature of the blank; a heated shaped plug suitable for insertion through the draw ring to draw the heated blank onto the plug and into the drawing chamber as the plug is advanced, wherein: the clamping plates are operable to clamp a flange of the drawn blank so as to form a seal between the blank and the drawing chamber; and the gas inlet is for passing pressurized gas into the drawing chamber such that the pressure therein is increased, thereby causing a pneumatic force to be applied to the drawn blank to force the drawn blank to mate closely with the plug. The apparatus may most usefully be used with a blank of magnesium or of a magnesium-rich alloy and to form a helmet-shaped article.

Figure 1 is a cross-section view of a magnesium alloy blank 101 mounted in a special purpose deep draw press 100 of the present invention, before the start of hot deep drawing HDD step, Figure 1 a being an enlarged view of a part of the press;

Figure 2 is a cross-section view of the magnesium alloy blank 101 mounted in the press 100 of the present invention during the HDD step;

Figure 3 is a cross-section view of the magnesium alloy blank 101 mounted in the press 100 of the present invention at the start of a pneumatic coining second step;

Figure 4 is a cross-section view of the magnesium alloy blank 101 formed into a shell prior to removal from the press 100.

With reference first to Figure 1 , there is shown a magnesium alloy blank 101 of circular shape mounted in a special purpose deep draw press 100. The magnesium alloy blank 101 is made of a wrought magnesium alloy such as, but not limited to, AZ231 B or WE43. The blank is lubricated and heated before being transferred to the press 100 in order to reduce the overall press cycle time. To date, blanks having a thickness of up to 0.25 in (6.35mm) have been found most suitable. The press 100 has an upper chamber 1 10, a lower chamber 120, a draw ring 131 and a stripper plate 132. The draw ring 131 and the stripper plate 132 are mounted between upper 133a and lower 133b removable modular clamping plates (see Figure 1 a).

The lower chamber 120 is a heated chamber in which is housed a helmet-shaped heated male tool (plug) 121. The plug 121 is mounted on a platform 122, which in turn is mounted on a piston 123 of a hydraulic ram 124. The hydraulic ram 124 is located outside of the lower chamber 120 and the lower chamber 120 is provided with an opening for the piston 123 to allow the piston 123 to move such that the plug 121 can be moved by the hydraulic ram 124 towards and away from the upper chamber 1 10. The piston 123 and ram

124 may be replaced by mechanical alternatives, such as an electrically-driven screw jack. The wall of the lower chamber 120 comprises an outer steel wall

125 provided with a thermal shield 126. The wall has a reinforced portion 127 around the opening for the piston 123 that acts as a thermal barrier. The thermal shield 126 of the lower chamber 120 is designed to be capable of withstanding temperatures over 600°C and to limit the temperature of the outer steel surface 125 of the lower chamber 1 10 such that the outer steel surface 125 does not exceed 200°C. To achieve these thermal properties, the thermal barrier 126 designed using a suitable refractory material such as an

aluminosilicate having a typical thickness of 2" (5.1 cm). The lower chamber 120 is provided with heating elements 128. It is thus designed to create and maintain the thermal environment of the moving plug 121. The upper chamber 1 10 is, in use, both heated and pressurized: it is constructed to provide a thermal barrier and to contain the applied gas pressure used in the method of this invention. The upper chamber 1 10 is lined with a layer of thermal insulation 1 16 similar to the thermal shield 126 of the lower chamber 120. The layer of insulation 1 16 is designed to contain temperatures over 600°C within the upper chamber 1 10 and to limit the temperature of the outer steel surface 1 15 of the upper chamber 1 10 such that it does not exceed 200°C. The upper chamber is also designed to withstand internal pressures of over 500psi (3.5 MPa). An inlet 1 17 is provided to allow pressurized air or another suitable gas (for example N ₂ ) to enter the upper chamber 1 10. The upper chamber is also provided with heating elements 1 18. A control panel 1 19 is provided on the outside of the upper chamber 1 10 for controlling the temperatures of the upper chamber 1 10 and lower chamber 120.

The draw ring 131 and stripper plate 132 are housed by the pair of removable modular clamping plates 133a, 133b between the upper 1 10 and lower 120 chambers. The lower plate 133b is attached to the lower chamber 120 at an upper edge. The draw ring 131 and upper plate 133a are mounted on the pistons 134 of hydraulic rams 135. The hydraulic rams 135 are used to control the size of the gap between the upper plate 133a and lower plate 133b. As shown in Figure 1 , the magnesium alloy blank 101 is mounted between the upper 133a and lower 133b removable modular clamping plates. The gap between these plates has to be maintained at an appropriate thickness for the blank 101 during the hot draw operation and to be closed, as will be described later in more detail, during the coining step.

The draw ring 131 is connected to the upper plate 133a, which in turn is connected to the upper chamber 1 10 through a ring of 'hard' insulation 140 of a suitable refractory material such as a compacted calcium silicate. The insulation 140 acts as a thermal barrier to prevent the outer steel surface 1 15 of the upper chamber 1 10 from exceeding 200°C. Similarly, the stripper plate 132 is connected to the lower plate 133b, which in turn is connected to the lower chamber 120 via another ring of 'hard' insulation 140.

Figure 2 shows the plug 121 at the start of the draw step, as it is being raised to engage with the blank 101. The plug 121 advances towards and into the heated magnesium alloy flat circular blank 101 at a prescribed speed. The sheet is drawn about the plug 121 into the desired shape with limited thinning. The thickness of the shell of the finished article is consequently close to the initial starting gauge of the blank 101. The plug 121 is maintained to a temperature in the range of 400°C to 500°C and preferably between 480°C to 500°C by the heating elements 128 during this step. The plug 121 is made from a material that is capable of withstanding the temperature and drawing forces created by this process. For example, a grade of ductile iron that is resistant to oxidation (scaling) at the operating temperatures involved. Control of the blank forming temperature (i.e. the temperature of the blank 101 in the press), the draw speed (i.e. the speed at which the plug 121 advances through the draw ring 131 ) and the size of the gap between upper 133a and lower 133b clamping plates all affect the process result in some way. For improved temperature control, the special purpose press 100 is provided with heating elements 1 18, 128, 129, and controls 1 19. Element 1 18, 128 control and maintain the temperature within the chambers 120, 1 10. Additional elements 129 in the clamping plates essentially compensate for heat losses in the critical vicinity of the blank 101. Too low a temperature will result in catastrophic failure. To date, best results have been obtained by marinating the blank 101 at temperatures in the range 400°C to 500°C and preferably between 480°C and 500°C.

In addition, the draw speed of the plug 121 needs to be carefully controlled. Too fast a draw speed will increase local thinning of the blank 101 as it is pulled through the draw ring 131 and onto the plug 121. The plug speed may be varied as the draw stage progresses. This provides the optimum forming conditions for certain shape elements of the final product. That is, to achieve a relatively uniform thickness distribution without localized thinning, splitting or flange wrinkling. To date, best results have been obtained using draw speeds in the range 50mm/min to 300mm/min and preferably between 150mm/min and 200mm/min. As the blank 101 is drawn onto the plug 121 , the gap between the upper plate 133a (upon which the draw ring 131 is mounted) and the lower plate 133b (upon which the stripper plate 132 is mounted) of the pair of removable modular clamping plates 133a, 133b is controlled by the hydraulic rams 135. The gap between the upper plate 133a and the lower plate 133b determines the sliding conditions of the blank 101 and the draw ring 131. The selected gap size is based on the thickness of the blank in use and in order to control back tension. As the plug 121 advances, the force applied to the blank 101 by the plug acts in conjunction with the sliding conditions and the resistance of the blank 101 to sliding to create a back tension. Thus, the gap between the upper plate 133a and the lower plate 133b and the speed of advance of the draw ring 131 (as well as the choice of lubricants used on the blank 101 ) can be used to control the back tension. If the back tension is too low the blank 101 will buckle radially as it is drawn through the draw ring 131 by the advancing plug 121. Conversely, if the back tension is too high, the blank will be excessively stretched as it is drawn through the draw ring 131 by the advancing plug 121 , leading to excessive or localized thinning or even to premature failure. Therefore, the back tension should be no more than that which results from largest gap between the upper plate 133a and the lower plate 133b and that still substantially prevents radial buckling of the blank 101 as it is drawn through the draw ring 131 by the advancing plug 121

The lubrication of the blank 101 reduces friction between the heated blank 101 and the surface of the plug 121 and the draw ring 131 during processing in the press 100. However, the particular geometry of a helmet, specifically the convex curved upper portion of the helmet (the crown) creates conditions that can promote local thinning of the helmet during the draw step. As stated above, the conditions for promoting local thinning are created by the back tension that results as the blank 101 is drawn through the draw ring 131 by the advancing plug 121. This tension is initially reacted at the contact point (area) at the 'crown' of the helmet and can result in localized thinning. To counter this and to further limit and control thinning during the first hot deep draw step, lubricants having different coefficients of friction are applied to the blank 101 in a prescribed way before the blank 101 is heated or mounted in the press 100. Reduced 'crown' thinning is accomplished by applying a high coefficient of friction liquid suspension such as, for example, aqueous solutions of magnesium hydroxide (milk of magnesia) to the central region of the blank 101 and a low coefficient of friction lubricant such as, for example, colloidal graphite to the rest of the blank 101 (typically both sides). The central region is that which will initially contact the center and 'crown' of the advancing helmet shaped plug 121. Use of a high coefficient of friction suspension limits localized thinning via 'sticktion'. The lower coefficient of friction lubricant used on the remainder of the blank promotes easy slippage of the blank 101 as it transitions from horizontal to the near vertical side wall of the helmet as it is hot drawn through the draw ring 131 by the advancing helmet shaped plug 121. Of course, multiple high coefficient of friction liquid suspensions having differing coefficients of friction and multiple low coefficient of friction lubricants having differing coefficients of friction, or mixtures of high coefficient of friction liquid suspensions and low coefficient of friction lubricants can be used to achieve a desired gradient of friction across the surface of the blank 101.

Figure 3 shows the press 100 and blank 101 at the end of the draw step and start of the coining step. At the end of the draw step, the blank 101 has been drawn over the plug 121. This first step is key to producing the basic helmet shape quickly and with limited thinning. As the draw step is completed, the plug 121 is held stationary by the hydraulic ram 124 and piston 123. A flange of the blank 101 remains in the gap between the upper plate 133a and the lower plate 133b. At the start of the coining step, the hydraulic rams 135 advance so as to close the gap and so trap the flange of the blank 101 between the upper plate 133a and the lower plate 133b. The pressure applied by the hydraulic rams 135 is increased until a gas-tight seal is formed between the flange of the blank 101 and the removable modular clamping plates 133a, 133b. A gas-tight seal is thereby also formed between the upper chamber 1 10 and the blank 101 . Air, or another suitable gas such as N ₂ , is pumped through the inlet 1 17 into the upper chamber 1 10 and so increases the pressure therein. The pressure differential between the upper chamber 1 10 and lower chamber 120 results in a pneumatic force being applied evenly to the side of the blank 101 facing away from the plug 121. The blank 101 is maintained at a temperature in the range 400°C to 500°C and preferably between 480°C to 500°C and the pneumatic force urges and plastically deforms the blank 101 (which both stretches and compresses) into intimate contact with the plug 121 , thereby imparting detailed features of the finished helmet (such as ear lobes, ridges etc.) to the blank 101. The pneumatic forces can be several hundred psi (and preferably over 500psi (3.5 MPa)) and so the hydraulic clamping force applied by the hydraulic cylinders 135 must be sufficient to stop the pneumatic force from separating the removable modular clamping plates 133a, 133b. If the plates separate, the seal would not be maintained, allowing gas to leak from the chamber 1 10 and the pressure to drop. This would prevent the coining stage from completing.

It is noted that in contrast to a traditional "coining" process, matched dies are not used. A lower die is provided by the plug 121 , but the pneumatic pressure generated in the chamber effectively plays the part of the upper die and presses the blank into the shape of the plug. This dispenses with the need for matched dies and associated problems such as the need to control the match over a temperature range, avoiding jamming and the need for high- accuracy machining referred to above.

Figure 4 shows the plug 121 retracted and the formed helmet prior to removal from the press 100. As the plug 121 retracts into the lower chamber 120, the edge of the formed helmet 101 is caught on the stripper plate 132 attached to the lower 133b clamping plate. This causes the helmet 101 to be removed from the plug 121.

Although the two-stage hot deep draw and pneumatic coining steps are central to the helmet-forming process, this design of chamber can also be used for further processing of the formed helmet, if required. Ideally the two- stage process is carried out not only at a temperature suitable for the drawing and coining operations but also one that is an appropriate 'solution' temperature for the shell material. That is, the temperature of the lower chamber is in the range 450°C to 520°C. After the helmet shell has undergone solution heat treatment, it is then quickly removed from the press 100 and directly quenched (rapidly cooled) into a aqueous solution.

Quenching is then followed by an appropriate ageing cycle. For certain magnesium alloys it is possible to achieve improved mechanical properties by this post-forming solution heat treatment followed by an ageing cycle. A helmet shell fabricated from such an alloy, which is then processed in this manner will exhibit improved mechanical properties in comparison with a helmet shell that has been hot formed and then slow air cooled. Once the helmet shell has been formed and cooled, it is then trimmed to its finished dimensions before having a liner (typically non-metallic composite) attached by appropriate means (bonding etc.). Alternatively, the un-trimmed helmet shell may itself be utilized as a mold into which a suitable non-metallic liner, such as a composite reinforcing material, can be shaped, consolidated and bonded \n-situ to create a composite assembly, combining the liner to the helmet shell, before final trimming.

It will be clear to one skilled in the art that many modifications of the embodiment of the invention described herein can be implemented without departing from the spirit of the invention. For example, the embodiment described has been directed specifically towards fabrication of a magnesium alloy helmet. The method and apparatus can however be used to process aluminum and aluminum alloys, which are cheaper materials but without the specific strength and stiffness of magnesium alloys. This makes them less attractive for military purposes but does not preclude their utility in other applications. Clearly the specific operating conditions quoted herein may need to be adapted in order to process effectively different materials. The two-stage process can also be used for other non-helmet shapes although typically the greatest benefit would be to quasi-circular shaped metal components with secondary features that are not producible by deep drawing alone. The secondary features are therefore imparted by a subsequent die-forming operation.

The operating temperature range quoted herein is primarily set to impart enough ductility to the magnesium alloy to allow it to draw. A secondary consideration if the formed part is to be directly quenched as it is removed from the press is to ensure that the solution temperature is sufficient to realize the beneficial mechanical properties post quenching and ageing. Clearly, if different metallic alloys are used, temperature requirements, and indeed drawing conditions and coining pressure, would all be dictated by the properties of the material to be formed.

In theory, it is also possible to adjust the environmental temperature of the chamber between the deep draw, coining and (optional) solution processing steps. However this is undesirable as the thermal mass of the plug and other components within the chamber means that any temperature changes would take time to have effect and so slow the whole process down. The primary function of the heating elements 1 18, 128, 129 is therefore to create and maintain a stable temperature environment.

In other embodiments, the inlet 1 17 may be used to extract or inject gas before or during the drawing stage of the forming operation. Such a pressure variation may, under some conditions, benefit the drawing process, although normally this stage will be carried out under atmospheric pressure.