WO2023117135A1

WO2023117135A1 - A method for manufacturing a die for press forming plate material

Info

Publication number: WO2023117135A1
Application number: PCT/EP2022/025581
Authority: WO
Inventors: Jaap TERPSTRA
Original assignee: Robert Bosch Gmbh
Priority date: 2021-12-24
Filing date: 2022-12-19
Publication date: 2023-06-29
Also published as: NL2030298B1

Abstract

The present invention relates to a method for manufacturing a die (20; 21) for press forming plate material (10) between an upper die (20) and a lower die (21) of a pressforming die pair (20, 21). The die (20; 21) is provided with a working surface (22) having a surface profile defined by relatively raised parts (24) and relatively recessed parts (23). According to the present invention the top surfaces (27) of the raised parts (24) of the die (20; 21) are laser hardened, such that locally a material hardness is higher than at the base (29) of these raised parts (24).

Description

A METHOD FOR MANUFACTURING A DIE FOR PRESS FORMING PLATE MATERIAL

The present invention relates to a method for manufacturing a die for press forming plate material between an upper die and a lower (counter) die of a pressforming die pair. In press forming the plate material is shaped perpendicular to the main plane thereof, which press forming is also known as embossing or 3D shaping in general. Typically during such press forming, one die of the die pair is moved relative to the other die of the pair that is fixed in place, which moveable die is commonly referred to as a punch. Press-formed, i.e. 3D-shaped plates are used in many applications, such as in construction, in heat exchange devices and in electrochemical devices, in particular as so-called fluid distribution plates in such devices that are also referred to as separator plates or bipolar plates.

Typically when press forming, a flat, relatively thin plate, i.e. a sheet material blank is used as a starting material, into which corrugations are pressed perpendicular to the main plane of the blank. In particular in case of the said fluid distribution plates, these corrugations constitute a so-called flow field for fluid, composed of channels located between relatively protruding islands, ridges, etc. that are provided on a relatively fine scale in the main plane of the plate (i.e. small corrugation pitch) and on a relatively large scale perpendicular thereto (i.e. large corrugation depth/height). That is to say that a width dimension of a single pit or channel of the plate corrugations is typically small compared to its depth or height dimension, while these dimensions are multiples of the plate thickness. The starting material can be a plastic, but in many of the said applications (sheet) metal is the preferred starting material.

An early example of the fluid distribution plate for an electrochemical cell is provided by the international (patent) application publication WO9516287 A1. In the electrochemical cell, the channels defined by the plate corrugations carry reactants and/or coolant along a so-called active area of the device to and from through-holes in the plate for the supply and discharge thereof. The channels of the plate corrugations allow fluid communication with an electrode or other porous layer, whereas the ridges thereof supports and fixates such porous layer in the electrochemical device. Moreover, the plate corrugations can also serve to contain a sealing or gasket between the plate and the porous layer or between two adjoining plates. In a typical manufacturing process of the separator plate, the said through-holes in the starting material are pierced through first. Then the said plate corrugations are created by press forming the fluid distribution plate between the dies of the said die pair. Finally, the outer circumference of the fluid distribution plate is cut, for example by means of laser cutting or blanking. In particular in case of so-called fine-blanking, the punch and counter punch that are applied in such well-known fine-blanking process step may serve as the presently considered die pair for 3D-shaping.

The working surface -being the surface that faces and at least in part engages the plate material- of at least one die of the die pair is thereto provided with a surface profile defined by protruding, i.e. relatively raised and relatively recessed parts of the respective die, corresponding to the desired 3D shape of the plate. In case of so-called rubber pad (press) forming only one die of the die pair is provided with such profiled working surface. However, commonly the working surfaces of both dies of the die pair are provided with respective surface profiles that are mutually complementary to one another, such that the raised parts of one die correspond with the recessed parts of the other die and vice-versa. More specifically, such complementary die pair can be designed as a so-called closed tooling or open tooling.

In a closed (press forming) tooling, the dies of the die pair are designed to engage the plate directly opposite one another, at least in an ultimate, i.e. fully closed position of the press-forming tooling including such die pair during the press forming of the plate. In this case, the material of the plate is compressed somewhat between the dies, such that it can plastically flow transversely thereof. Typically, the top and bottom sections of the corrugations (that typically remain essentially oriented in the main plane of the plate) are compressed in this way, such that some material thereof flows into wall sections thereof (that each extend between respective top and bottom sections at an angle relative to the main plane of the plate). This specific setup of press forming with the closed tooling can counteract a tearing of the plate material, i.e. can enable a favourably relatively sharp transition/sharp curvature between the wall sections and the top and bottom sections of the corrugations. On the other hand, the relatively high forces exerted by and between the dies of the closed tooling can result in relatively high tool wear.

The USA patent US 9,364,883 B2 describes a variant of the closed tooling. In this variant, the said wall sections of the corrugations are compressed between the dies of the die pair, rather than the top and bottom sections thereof. Hereby, potentially, the corrugation depth can be increased without tearing of the plate material. Furthermore, US 10,150,152 B2 describes a fully closed tooling, wherein both the top and bottom sections and the wall sections of the corrugations are compressed between the dies. Both these latter two variants, however, will be accompanied by even higher forces and more tool wear than the aforementioned, conventional closed tooling.

In an open (press forming) tooling, the dies of the die pair are designed to engage the plate only through the said raised parts of the respective working surfaces thereof, even in an ultimate, i.e. closest together position thereof in the press-forming tooling during the press forming of the plate. In this case, the plate is not compressed, but rather is bent and stretched by and around the raised parts of the dies. This specific setup of press forming can reduce tool wear. However, a known limitation of the open tooling is that the corners/transitions of the corrugations between the wall sections and the top and bottom sections thereof, cannot be as sharply curved when using the open tooling, as compared to the closed tooling. Moreover, since the top and bottom sections of the corrugations are engaged only on the (concave) inside thereof, i.e. are engaged by only one of the dies of the die pair, these will be more or less convexly/concavely shaped when using the open tooling. Nevertheless, also the open tooling is subjected to relatively high forces and is thus likewise prone to tool wear, in particular at the location of the raised parts of the dies thereof

The known press-forming die is typically made from carbon steel that is quench hardened to strengthen the die and thus to limit the wear thereof during use. Nevertheless, the die material must also possess a sufficient ductility to prevent the raised parts thereof from breaking-off and/or to equalise the internal stress levels in the die during use. Therefore, it is common practice in the art to additionally harden a surface layer of the die by means of a surface hardening heat treatment such as (carbo-)nitriding and/or by means of a hard surface coating, such as titanium nitride, whereby the core material of the die can remain comparatively soft and thus favourably ductile.

The present invention aims to improve these known methods for manufacturing the press-forming die. In particular, the invention aims to reduce the manufacturing cost of the die, while simultaneously improving the service life thereof.

According to the present invention, surface parts of the (working surface of the) die that engage the plate in press forming are hardened by means of laser hardening. Laser hardening is a relatively recent technology development. In laser hardening, a laser beam is moved along (i.e. tracks or scans) the surface of a workpiece, whereby at each instance in time the laser beam quickly heats up the workpiece in a small spot on its surface of 1 up to several millimetre diameter and over a limited depth of 0.1 up to 2 millimetre below its surface. When the surface temperature reaches around 1000 degrees Centigrade, a local, heat-induced austenitization is caused. As the laser beam moves on along the surface of the workpiece, the thus austenized part of the workpiece cools down quickly, a/o by heat conduction to the core material of the workpiece, such that a desired phase transition from austenite into supersaturated martensite is caused (so-called self-quenching). The particular microstructure obtained with laser hardening of fine grained martensite with some retained austenite was found to be very beneficial for the presently considered press-forming die, as it increased its service life by reducing wear.

In case of corrugated plate having a plate thickness between 0.05 and 0.5 mm, having a corrugation pitch between 1 and 10 mm and having a corrugation depth that exceeds the plate thickness by a factor of 2 to 8, the corresponding die for press forming such corrugated plate is ideally suited for being manufactured in accordance with the present invention. Namely in this case, a width dimension of the raised parts and of the recessed parts of such die correspond favourably closely to a laser spot diameter in laser hardening.

Preferably according to the present invention, only the said engaging surface parts of the die working surface are laser hardened, whereas remaining, non-engaging surface parts thereof are not. Thus, in case of the said open tooling, favourably only the (tops of the) raised parts of the die are laser hardened in accordance with the invention. Further in case of the said open tooling, a semi-finished die, whereof the working surface is not yet profiled, is preferably laser hardened along the entirety of such unfinished working surface thereof. Only thereafter, the said raised and recess parts are formed in such working surface by milling the (relatively) recessed parts. In this case, the laser hardened surface layer remains only at the (relatively) raised parts.

Preferably according to the present invention and regardless of whether or not the (bottoms of the) recessed parts of the die are laser hardened or not, a base of the raised parts is not subjected to laser hardening, such that locally the material hardness of the raised parts remains at the same level as that of the core material of the die.

Preferably according to the present invention, the surface hardness of the laser hardened surface parts exceeds 60-62 HRC and more preferably amounts to 64 to 65 HRC, whereas the core hardness of the die (that is realised by quench-hardening followed by tempering prior to laser hardening) preferably amounts to between 50 and 58 HRC, more preferably amounts to between 52 and 55 HRC.

Further according to the present invention, the laser hardened surface parts are preferably post-treated to remove unwanted shape distortions and/or surface imperfections. Suitable post-treatments are (laser) polishing and laser ablation, which processes do not breach the laser hardened surface layer. Since laser hardening provides minimal distortion of the press-forming die, the extend and/or cost post- treatments can be favourably

In the following, the press forming die and the method for manufacturing it in accordance with to the present invention are explained further and in more detail by way of example embodiments and with reference to the drawings, whereof:

Figure 1 schematically depicts a typical example of a known fluid distribution plate defining fluid channels on both sides thereof by virtue of being provided with corrugations;

Figure 2 is a schematically drawn, cross-sectional view of a part of a die pair of the known closed press-forming tooling prior to press forming the corrugated plate, illustrating the surface profile of the working surface of the dies thereof;

Figure 3 provides the same cross-sectional view of the known die pair as in figure 2 however in a fully closed position of such closed press-forming tooling;

Figure 4 provides a cross-sectional view of a part of a die pair of the known open press-forming tooling;

Figure 5 schematically illustrates the basic setup of a laser surface hardening process in relation to the die of the known die pair.

Figure 6 provides a schematic cross-sectional view of a part of the die manufactured in accordance with the present invention; and

Figure 7 diagrammatically summarizes the method according to the present invention for manufacturing the die for press forming plate material.

In figure 1 , a generic example of fluid distribution plate 1 is illustrated, such as is used as a separator in a stack of electrochemical cells. The fluid distribution plate 1 has flat outer rim 2 and a central, so-called active area 3 that is 3D-shaped. The outer rim 2 of the plate 1 is provided with circular holes 4 for accommodating bolts that hold the stack together and with oblong holes 5 for the supply and discharge of (fluid) reactants and/or coolant to and from the said active area 3 of the plate 1. In the shown example of the fluid distribution plate 1, the active area 3 is composed of multiple channels and multiple ridges oriented in parallel. Moreover, the channels on one main side of the plate 1 represent the ridges on the other main side thereof and vice-versa.

The 3D-shape of the active area 3 of the fluid distribution plate 1, i.e. the concavely shaped channels 6 and the convexly-shaped ridges 7 thereof is typically created by press forming plate material 10 between an upper die 20 and a lower die 21 of a die pair, which dies 20, 21 are thereto moved together with the plate material 10 placed therebetween, as schematically illustrated in figures 2 and 3.

In figures 2 and 3, the dies 20, 21 of the die pair and the plate material 10 are illustrated in a cross-section of a relevant part thereof. The working surface 22 -being the surface that faces and at least in part engages the plate material 10 in press forming- of both dies 20, 21 is provided with a surface profile that is defined by relatively recessed parts 23 of the dies 20, 21, located between relatively raised parts 24 thereof. The working surface 22 of the dies 20, 21 are mutually complementary in the sense that the raised parts 24 of one of the dies 20, 21 can fit into the recessed parts 23 of the respective other one die.

When the dies 20, 21 of the die pair are moved towards each other the plate material 10 is permanently deformed, i.e. is plastically bent and stretched around the said raised (surface) parts 24 of the working surfaces 22 thereof. In figure 3 the dies 20, 21 of the die pair are illustrated in their ultimate position of press-forming, wherein the corrugations of the active area 3 of the fluid distribution plate 1 is completely formed. In such ultimate position of the dies 20, 21, a mutual separation S remaining there between is about equal to or marginally smaller than a thickness T of the plate material 10 indicated in figure 2 (so-called closed tooling). In this case, a top or bottom section 25 of the corrugations (that typically remain essentially oriented in the main plane of the press-formed plate 1) are compressed between the dies 20, 21 , such that some material is forced to flow into wall sections 26 of the corrugations extending between the said top or bottom sections 25 thereof.

Alternatively, the said mutual separation S remaining between the dies 20, 21 of the die pair in the said ultimate position of press-forming thereof, can be larger than the thickness T of the plate material 10 (so-called open tooling), as is illustrated in figure 4. In such open (press forming) tooling, the dies 20, 21 of the die pair are designed to engage the plate material 10 only through the said raised parts 24 of the respective working surfaces 22 thereof. In this case, the plate material 10 is not compressed, but rather is bent and stretched by and around the raised parts 24 of the dies 20, 21.

The known press-forming dies 20, 21 are made from carbon steel that is quench hardened to increase die strength. Nevertheless, both in the closed tooling and in the open tooling, the dies 20, 21 and in particular the raised parts 24 thereof are subjected to relatively high forces during use and are thus prone to wear. According to the present invention, the wear resistance of the dies 20, 21 is conveniently and cost- effectively improved by additionally hardening specifically the raised parts 24 of the (working surface 22 of the) dies 20, 21 by means of laser hardening. At the same time, other, non-engaging surface parts of the dies 20, 21 , such as in particular a base 29 of the raised parts 2, are preferably not subjected to laser hardening in order to locally retain ductility as much as possible. A basic setup of such laser (surface) hardening process is schematically illustrated in figure 5. In laser hardening, a (diode) laser 30 emits a laser beam (indicated in figure 5 by the dashed lines) that irradiates a laser spot (indicated in figure 5 by the dotted circle) on a to-be-hardened surface part 27 of the die 20; 21 , in particular a top surface 27 of a respective raised part 24 thereof, and that is moved along such surface part 27. An inert shielding gas is supplied from a nozzle 31 to the laser spot and its surroundings, to prevent oxidation of the die 20; 21.

At each instance in time, the laser beam quickly and locally heats up the die 20; 21 on the top surface 27 respective raised part 24 of the die 20; 21 to well above the austenitization temperature of the carbon steel in question. As the laser beam moves on along the said top surface 27, a die part that was previously irradiated and heated by the laser beam rapidly cools down, such that locally the steel self-quenches and a hard, supersaturated martensite microstructure is created.

Ideally and as illustrated in figure 5, the diameter of the laser spot on the die 20; 21 that is instantaneously irradiated by the laser beam essentially corresponds to the width of the top surface 27, i.e. of the respective raised part 24, such that the laser 30, or at least the laser beam, needs to move in the long direction of the top surface 27 only, i.e. only along the length of the respective raised part 24.

More in particular, preferably all surface parts of the working surface 22 of the dies 20, 21 that engage, i.e. that contact the plate material 10 in press forming are laser hardened. Thus, in case of the said closed tooling, not only the top surface 27 of the raised parts 24 of the die 20; 21 are laser hardened, but advantageously also the bottom surface 28 of the recessed parts 23 thereof. This latter embodiment of the present invention is illustrated in figure 6 by way of a cross-section of (a part of) the resulting press-forming die 20; 21. In figure 6, the cross-hatched areas schematically indicate the laser hardened areas of the closed tooling die 20; 21.

As illustrated in figure 7, the preferred manufacturing method according to the present invention includes the quench-hardening and tempering of a semi-finished die 20; 21. Following such quench-hardening/tempering of the semi-finished die 20; 21 , the parts 27, 28 of the working surface 22 thereof is laser hardened, at least locally in the areas thereof that are intended to engage the plate material 10 in press forming. Following such laser hardening, at least the laser hardened surface parts 27, 28 of the die 20; 21 are polished to remove unwanted shape distortions and/or surface imperfections. Moreover, a process step of milling the semi-finished die 20; 21 to shape the working surface 22 thereof can be included in the manufacturing method according to the present invention either in-between laser hardening and polishing (in particular in case of a die 20; 21 for open tooling) or in-between quench- hardening/tempering and laser hardening, as preferred for a die 20; 21 for a closed tooling.

The present invention, in addition to the entirety of the preceding description and all details of the accompanying figures, also concerns and includes all the features of the appended set of claims. Bracketed references in the claims do not limit the scope thereof, but are merely provided as non-binding examples of the respective features. The claimed features can be applied separately in a given product or a given process, as the case may be, but it is also possible to apply any combination of two or more of such features therein.

The invention(s) represented in the present disclosure is (are) not limited to the embodiments and/or the examples that are explicitly mentioned herein, but also encompasses amendments, modifications and practical applications thereof that lie within reach of the person skilled in the relevant art.

Claims

9 CLAIMS

1. A method for manufacturing a die (20; 21) made from carbon steel and intended for the press forming of plate metal (10), which die (20; 21) is provided with a profiled working surface (22) with relatively raised parts (24) between relatively recessed parts (23) thereof, characterized in that a top surface (27) of the raised parts (24) of the working surface (22) of the die (20; 21) is laser hardened.

2. The method for manufacturing a die (20; 21) according to claim 1, characterized in that a diameter of a laser beam applied in the laser hardening therein corresponds to a width of the said top surface (27).

3. The method for manufacturing a die (20; 21) according to claim 1 or 2, characterized in that a bottom surface (28) of the recessed parts (23) of the working surface (22) of the die (20; 21) is laser hardened therein.

4. The method for manufacturing a die (20; 21) according to claim 1 or 2, characterized in that a bottom surface (28) of the recessed parts (23) of the working surface (22) of the die (20; 21) is not laser hardened therein.

5. The method for manufacturing a die (20; 21) according to claim 3 or 4, characterized in that a base (29) of the raised parts (24) of the working surface (22) of the die (20; 21) is not laser hardened therein.

6. The method for manufacturing a die (20; 21) according to a preceding claim, characterized in that, the laser hardened parts (27; 28) of the die (20; 21) have a surface hardness of more than 60 HRC and preferably have a surface hardness in the range from 62 to 65 HRC.

7. The method for manufacturing a die (20; 21) according to a preceding claim, characterized in that, prior to the laser-hardening thereof, the die (20; 21) is quench hardened and subsequently tempered as a whole.

8. The method for manufacturing a die (20; 21) according to claim 7, characterized in that, subsequently to the quench-hardening and tempering thereof, but still prior to the laser-hardening thereof, the recessed parts (23) of working surface (22) thereof are milled.

9. The method for manufacturing a die (20; 21) according to claim 7 or 8, characterized in that, subsequently to the quench-hardening and tempering thereof, the die (20; 21) has a core hardness in the range from 50 to 58 HRC and preferably in the range from 52 to 55 HRC.

10. The method for manufacturing a die (20; 21) according to a preceding claim, characterized in that, subsequently to the laser-hardening thereof, the said top surface (27) is post-treated, in particular is polished.