EP0804982A2 - Process for preparing metal foam parts - Google Patents

Abstract

Production of moulded parts from foamed metal, e.g. aluminium is claimed, in which a powder metallurgical preform compacted from a mix of metal powder and foaming agent is foamed by heating in a chamber (2) separate from the mould (8), the volume of the fully foamed starting material corresponds to the mould volume, and the foamed metal content of the chamber (2) is injected into the mould (8), where foaming is preferably continued with the remaining foaming capacity until the mould (8) is completely filled. Process equipment is also claimed.

Description

The invention relates to a process for the production of molded parts made of metal foam, for example aluminum foam, which is formed by powder metallurgy by foaming a mixture of gas-releasing propellant with metal powder as the starting material under heat, in particular compacted into a semi-finished product such as rods, pipes or granules. Furthermore, the invention also relates to a device for performing the method.

Light metal fittings can be designed as a solid casting, as a hollow body or as a metal foam body. While in the first-mentioned category the same and thin wall thicknesses and the avoidance of local accumulation of material have to be ensured, hollow bodies usually required expensive casting cores, which complicate the production. A modern alternative is achieved using metal foam casting. The cast skin forms a smooth outer surface of the cast product, the inside of which is loosely filled by a pore structure. Metal foam casting is suitable for many areas of application, which leads to a particularly light end product that offers good sound insulation and low thermal conductivity. The strength properties are surprisingly high. However, not every machine part can be manufactured using this casting technique.

A distinction is made between two fundamentally different processes for the formation of metal foam, namely the melt metallurgical and the powder metallurgical. DE 43 26 982 C1 describes a typical method and an apparatus for producing molded parts from metal foam formed by melt metallurgy. A melt, e.g. an aluminum melt is kept liquid in two communicating containers and in one of the two containers the metal melt is foamed by an agitator. The finished foam is pressed up into a mold by raising the level of the liquid aluminum in the latter container.

Powder-metallurgical metal foam is produced, for example, according to DE 41 01630 C2 from metal powder and a gas-releasing blowing agent. The material is hot compacted and subjected to a change in shape. This creates a semi-finished product made of firmly connected metal particles that enclose the blowing agent particles in a gas-tight manner. The semifinished product is foamed, for example in a heated steel mold, by the action of temperature, the metal foam gradually filling the mold. The disadvantage here is that the contour of the semi-finished product must correspond to the contour of the mold cavity, since otherwise there is no uniform foaming. If rod-shaped primary material is used, it must be cut to length and placed in the form. Cold welds can also form between the foamed rods.

A foam produced by melt metallurgy already goes into the state of collapse of the pores when it is pressed into the mold after its production. Heated molds are often used, but the temperature of the molds must not be too high, otherwise the metal foam will collapse to an increased extent. The pores also arise in foam production in an uncontrollable manner and in different sizes. In general, this process only allows the production of simple mold parts. Furthermore, an agitator is required in the melt metallurgical process, the positioning of the agitator in the melt being extremely problematic. The foam is generated by the stirrer only in the area of the stirrer, so that the foam already produced collapses during the production of the foam still required during the stirring time. In addition, the amount of foam required is not exactly adjustable, so that the same parts have no matching weight. The pores produced have no internal overpressure, so that they are compressed when pressed into the mold. This results in an inconsistent structure.

The invention aims to provide a method which enables the production of contoured, three-dimensional molded parts of high quality. Uniform pores and a uniform, homogeneous surface as well as the possibility of influencing the pore size and the pore density as well as the surface and its layer thickness are desired parameters in the manufacturing process. This goal is achieved in a method of the type described in the introduction in that the powder metallurgical starting material is heated Chamber foams outside the mold, that the volume of the powder metallurgical starting material introduced into the heatable chamber, in particular semi-finished product, corresponds essentially to the volume of a filling of the mold in its phase foamed with the total foaming capacity, and that the entire contents of the chamber as metal foam in the mold is pressed, in which foaming preferably continues with the remaining foaming capacity until the mold is completely filled.

In contrast to known powder metallurgical processes, metal foam is produced outside the mold by thermal foaming of the quantity of the semi-finished product predetermined for the mold. Unlike a melt metallurgical process, this metal foam can be pressed into the mold during its formation. The final phase of foam formation then takes place there. This means that even remote areas or difficult-to-reach contours or undercuts are reliably filled. An early collapse of the pores is avoided. The density of the molded part can be adjusted via the degree of filling of the gas-heated chamber, for example, with semi-finished or starting material or via the chamber volume. The time at which the metal foam is transferred from the chamber into the mold is also a further criterion. This preselects the residual foam capacity that will take effect in the mold. In the case of simple shapes, foam formation in the mold can be dispensed with at most. According to a development of the method, it is advantageous if the chamber with the foam-forming starting material is rotated with respect to the casting mold in the manner of a rotary drum furnace and, if necessary, tilted into the casting mold for emptying. As a result, the mold is filled by the inherent pressure of the foam material. It is particularly advantageous if the metal foam in the chamber is pressed into the mold by a piston. The piston speed and the pressure form further criteria for the appearance of the molded part, both with regard to its surface and the pore shape and density. The resulting metal foam can also be introduced by the metal foam being melted by a non-metal foam For example, a molten salt, to which a pressure is exerted and on which the powder-metallurgical metal foam floats, is lifted and pressed into the mold. For this purpose, the non-metallic melt is introduced into the chamber or pressed into it. This lifts the metal foam directly into the casting mold, either directly or via a floating piston plate. It is advantageous if the melt carrying the metal foam is specifically heavier than the mother metal of the foam and the melting point is lower (eg zinc or tin and aluminum). In contrast to the knowledge available hitherto in metal foam casting processes, it was recognized that excellent results are achieved if the metal foam formed by powder metallurgy is pressed into a non-metallic casting mold, for example a sand mold. In contrast to a steel mold, the unheated sand mold does not immediately remove the heat of the metal foam introduced during the filling process, so that the foam phase is retained until the remote moldings are also filled. Added to this is the supportive effect of the residual foam that occurs in the mold. The foam thus gets into the mold in its active phase and contributes to a significant improvement in quality.

In order to ensure a uniform pore or cell structure of the metal foam formed by powder metallurgy, uniform heating of the powder metallurgy starting material, that is to say of the semi-finished product, is necessary. Large temperature gradients in the chamber should be avoided and the foam should be formed as quickly as possible by rapid heating of the chamber in the edge region of the chamber as well as inside. This is achieved by introducing a tubular or tube-shaped semi-finished product into the chamber. It is expedient if this is provided with as little play as possible to the inner chamber wall, that is to say in thermal connection to the wall in the chamber. The heating can be electrical, for example inductive. Eddy currents and the skin effect of inductive heating must be taken into account. The inductive heating in conjunction with a tubular semi-finished product leads to the best foam quality. An independent transfer of the foam from the chamber into the mold is achieved at the right time in that the semi-finished tube is pressed against the nozzle plate with a defined and adjustable force at least in the final phase of the heating process by the piston, so that the injection process is initiated in the mold as soon as the semi-finished product reaches the melting point and thus foams. To reduce oxide layers, it is expedient if the heating or preheating of the semi-finished product is carried out under protective gas and preferably if the chamber is flushed with protective gas.

The mass of the metal foam produced in the chamber and available for a molded part is limited. In order to be able to manufacture large and spatially extensive metal foam parts, it is advantageous if the powder-metallurgical foam of several chambers, which are connected in parallel, is pressed into the cavity of one or more molds at the same time or by means of a control over several gates. A series of casting modules are thus combined in one system, which fill the shape of the foam part to be cast over several cuts. The modules are generally controlled synchronously so that they feed into the mold at the same time. Depending on the shape, it can also be advantageous to control the individual modules in a staggered manner, so that the density profile in the foam part can be influenced, for example. A device for performing the method is characterized in that the chamber is surrounded by a jacket for externally heated, non-metallic foam melt for heating the chamber. The melt used to heat the chamber is heated in a separate furnace. In this way, a large temperature volume can be fed uniformly to the chamber for the powder metallurgical semi-finished product. The device can be automated in that one or more chambers are arranged on a slide or carousel and can be moved or rotated from a loading or cleaning position into the heating position for the loaded semifinished product with respect to a connection to a mold.

The method according to the invention is explained in the following in principle using exemplary embodiments.
Fig. 1 shows an oven with a chamber and a mold before the start of foaming, FIG. 2 shows the arrangement according to FIG. 1 after transfer of the metal foam into the casting mold, FIG. 3 shows an alternative embodiment of the arrangement, FIG. 4 shows another alternative analogous to FIG. 1. FIG. 5 shows another embodiment, Fig. 6 is a multiple execution. 7 shows an embodiment with a heating variant and FIG. 8 shows an embodiment with displaceable chambers.

In a furnace 1 or a heating device with gas firing or inductive heating there is a chamber 2 for receiving a powder metallurgical starting material 3. It is a compacted semi-finished product, for example wire pieces or pipe pieces made of metal powder and a propellant, which, when exposed to the appropriate temperature, form a metal foam form. With the chamber 2 is a mold 4 via a nozzle 5 in the manner of a perforated diaphragm for gate adjustment for the casting in connection. A piston 6 is guided in the chamber 2.

By increasing the temperature in furnace 1 to approximately 500 to 600 ° C., an aluminum foam is formed in chamber 2 from the semifinished product, for example aluminum wire pieces, produced for example according to EP 460 392 A1, which is completely and completely transferred into casting mold 4 with the aid of piston 6 will (Fig. 2). The chamber 2 is emptied and can then be filled again with semifinished product as the starting material for the foam formation, the filling being precisely matched to the volume of the cast body.

Depending on the selected time of transfer from the chamber 2, the foam formation continues with the aid of the piston in the mold 4. The point in time when pressure is exerted on the resulting foam or the extent of the foam capacity still present in the mold, together with the volume of the semi-finished product used, its consistency and the temperature profile during foam formation and cooling, is an essential parameter for the structure of the foam part. As soon as the foaming capacity is exhausted and the foam formation in the mold is complete, the mold is removed from the oven 1 for cooling. This prevents the foam pores from collapsing as a result of excessive heat input. The casting 9 can be demolded and the chamber 2 in furnace 1 with a new shape. A steel mold can also be used repeatedly after a cleaning cycle. In this connection, reference is made to FIG. 4. 4 shows a chamber 2 which has an inductive heating 7. A furnace 1, which accommodates the entire arrangement, is not available here. The mold 8 is unheated. A sand mold is advantageously used.

The foaming takes place in FIG. 4 in the chamber 2 analogously to FIG. 1. The foam is pressed by the piston 6 into the casting mold 8 (sand mold). In the contact between the metal foam and the wall of the mold, namely the sand, only a small amount of heat is removed, so that the metal foam retains its viscosity and reaches the last corners of the mold. The continued foam formation in the mold supports this effect. In this way, very complicated castings with narrow ribs, undercuts or the like can also be produced. Due to the excellent heat conduction of the casting mold, the steel molds normally used in metal foam casting technology lead to a sudden heat removal as soon as the metal foam gets into the casting mold, which leads to an at least superficial loss of viscosity and thus to a much poorer distribution behavior of the metal foam in the casting mold. The steel molds therefore had to be additionally heated in certain critical areas in order to maintain the viscosity of the casting mass locally. This resulted in internal stress states, different pore structures and collapse of the structure at temperatures that were not precisely matched. The unheated sand mold 8 shown in FIG. 4 solves the problems. Any non-metallic form, including a ceramic or plaster form, can be used with the advantages mentioned.

Fig. 3 shows an alternative to FIGS. 1 and 2. Almost the entire arrangement, consisting of the chamber 2, the nozzle 10 and the mold 11 is rotatably arranged over one or two separate heating devices 12, 13 which are separately adjustable or can be switched on and off. A drive 14 with a bearing 15 faces the piston rod 16, which is designed as a bearing on the other side. The process proceeds as described for FIGS. 1 and 2. The rotation homogenizes the powder-metallurgical foam formation in the chamber 2 and also in the casting mold 11. The latter can remain unheated in the sense of the explanations for FIG. 4 as a non-metallic casting mold. It is also possible to arrange only the chamber 2 or only the mold 11 in a rotatable manner.

5, which represents a further development of the embodiment according to FIG. 4, a tubular semi-finished product 3 'is provided in the chamber 2 as powder metallurgical starting material on a cutting disc 20, for example made of titanium or ceramic. The tubular semifinished product 3 'is heated uniformly by inductive heating 21, so that the foam is also formed very uniformly and homogeneously. The foam as the content of the chamber 2 floats according to FIG. 5 - with the interposition of the cutting disc 20 on a "liquid piston", which is formed by a zinc, tin or lead melt. For this purpose, the tub 22 is kept at the melting temperature (heating not shown). A piston 23 presses the melt down, as a result of which the cutting disc 20 is raised and the foam content of the chamber 2 is pressed into the mold 8. As mentioned, depending on the time of transfer, the residual foam formation can take place there. The high heating-up speed and the heating in the semi-finished product itself, which can be attributed to inductive heating, contribute to the reduction of oxide formation. 5 with the melt in the tub 22, oxide residues on the wall of the chamber 2 are removed.

FIG. 6 shows a multiple application of modules according to FIG. 4 or 5. One or — as shown in FIG. 6, two molds 24, 25 (possibly also more) are fed by a plurality of heated chambers 2 ′ with metal foam. The individual chambers 2 'can be inductively heated and feed the metal foam formed into the mold or molds synchronously or with a time delay via a control.

The foam formation in the chamber or chambers according to FIGS. 1 to 6 can also take place according to FIG. 7 in that the chambers 2 or the chambers are or are surrounded by a jacket made of metal foam-foreign melt 26 for heating. The melt 26 is heated in an oven 27. It follows due to the large heat potential of the melt, the powder metallurgical semi-finished product is in an ideal heating state, which has a positive influence on the evenness of the foaming and the foaming time. In order to be able to heat the primary material (semi-finished product) as quickly as possible at the lowest possible temperatures, the shape of the semi-finished product is particularly important. In particular, care should be taken to ensure that any air gap between the wall of the chamber 2 and the semifinished product 3, 3 'is avoided, since this worsens the heat transfer due to the insulating effect of the air and at the same time represents a space for foam formation, which would likewise have an adverse effect as an insulating layer . The device according to FIG. 7 can of course also be equipped with a floating piston according to FIG. 5.

Another variant of the invention consists in pressing the semi-finished tube 3 ′ inserted into the chamber 2 against the nozzle plate 5 with a defined force by the piston 6. As soon as the semifinished product reaches the melting point and begins to foam, it loses its strength and the piston 6 can press the prepared foam into the mold. It can be expedient to change the force on the piston and thus the speed at which the foam shoots into the mold 8 as soon as the piston 6 starts to move. This measure results in a very simple control of the system, which is able to compensate for any variations, for example in the preheating temperature of the semi-finished product, in its heating-up speed or in the melting point of the semi-finished alloy, since only a corresponding viscosity of the foam is reached at the time of injection. Melt is decisive and all other parameters are disregarded. With this measure, very uniform foam formation is achieved in a very simple manner. It is also much easier to convert the system to another foam casting, since the otherwise very complex optimization of the production parameters is considerably simplified and changeover times can be drastically reduced.

FIG. 8 shows a further alternative with a plurality of chambers 2 ′, 2 ″, which here have electrical heaters 27 ′, 27 ″, for example corresponding to FIG. 7, and from a melt 26 ', 26 "are encased for heat transfer. The chambers 2, 2' are together with their heaters 27 ', 27" in the device according to FIG. 8 about a central axis 28 from a cleaning and optionally loading position for the tubular semi-finished product 3 "into one In order to inject the foam formed by powder metallurgy, the rotatable part also moves in the axial direction, so that the chamber 2 "directly adjoins the (divided) mold 4. A piston 6 'presses the metal foam into the interior of the mold 4.

The horizontal arrangement of the chambers 2 ', 2 "is advantageous because the piston 6' does not have to stay in the heated area during the foaming and is therefore subject to only a low temperature load. The device can have one, two or more chambers 2 ', 2" which occupy two or more positions (e.g. separate cleaning, loading with semi-finished products, heating and injecting). A turret or carousel construction with several stations, but also a linear displacement back and forth by means of a slide construction can be provided.

Claims (14)

A process for the production of molded parts made of metal foam, for example aluminum foam, which is formed by powder metallurgy by foaming a mixture of gas-releasing propellant with metal powder as the starting material which is compacted into a semi-finished product, such as rods, pipes or granules, characterized in that the foaming in a heatable chamber outside of a mold, the volume of the powder metallurgy starting material, in particular semi-finished product, introduced into the heatable chamber essentially corresponds in its phase foamed with the entire foaming capacity to the volume of a filling of the mold and that the entire content of the chamber is powder metallurgy Metal foam is pressed into the mold, in which foaming preferably continues with the remaining foaming capacity until the mold is completely filled. Method according to claim 1, characterized in that the density of the molded part to be produced can be adjusted via the degree of filling of the chamber with starting material or via the chamber volume. Method according to claims 1 or 2, characterized in that the chamber with the foam-forming starting material is rotated relative to the casting mold in the manner of a rotary drum furnace and pressed into the casting mold for emptying, optionally tilted. Method according to one of claims 1 to 3, characterized in that the metal foam in the chamber is pressed into the mold by a piston, the time within the course of the method, the extent of the remaining foam capacity and thus determining the structure of the casting, being preselected. Method according to one of claims 1 to 3, characterized in that the metal foam through a melt, in particular a metal foam foreign, for example through a salt melt, to which a pressure is exerted and on which the Powder-metallurgical metal foam, possibly with the interposition of a small plate, floats, is lifted and pressed into the mold. Method according to one of claims 1 to 5, characterized in that the metal foam is pressed into a casting mold made of non-metallic material, for example into a sand mold, ceramic mold, plaster mold or the like. Method according to one of claims 1 to 6, characterized in that the metal foam is pressed through a nozzle between the chamber and the cavity of the mold. Method according to one of claims 1 to 7, characterized in that a tubular or tube-shaped semi-finished product is introduced into the chamber. A method according to claim 8, characterized in that the tubular or tubular support-shaped semifinished product is introduced into the chamber with as little play as possible to the chamber inner wall and the chamber is preferably heated electrically, in particular inductively. Method according to one of claims 8 or 9, characterized in that the semi-finished tube is pressed against the nozzle plate by the piston with a defined and adjustable force at least in the final phase of the heating process, so that the injection process is initiated in the mold as soon as the semi-finished product has reached the melting point reached and thus foams. Method according to one of claims 1 to 10, characterized in that for the reduction of oxide layers the heating or preheating of the semi-finished product is carried out under protective gas and preferably that the chamber is flushed with protective gas. Method according to one of claims 1 to 11, characterized in that the powder-metallurgical foam of a plurality of chambers which are connected in parallel is pressed into the cavity of one or more molds at the same time or by means of a control at different times via a plurality of gates. Device for carrying out the method according to one of claims 1 to 12, characterized in that the Chamber (2, 2 ') is surrounded by a jacket for externally heated, non-metallic foam melt (26) for heating the chamber (2,2'). (Fig. 7) Device for carrying out the method according to one of claims 1 to 13, characterized in that one or more chambers (2, 2 ') are arranged on a carriage or carousel and from a loading or cleaning position to the heating position for the loaded semi-finished product opposite one Connection to a mold can be moved or rotated. (Fig. 8)

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