Molded part and method for producing such a molded part

Abstract

Moulded part for producing a brazed joint from a material - with a first powder component, namely a solder powder based on metal powders or alloy powders with at least the three elements aluminium, zinc and silicon, - with a second powder component, namely a flux powder, - wherein the aforementioned powder components are bound in a binder, - wherein the binder has a melting or softening point or a melting or softening range which is, on the one hand, lower than the melting point of the solder powder and lower than the melting point of the flux powder, but, on the other hand, ensures that the binder is in the solid state at room temperature, so that the binder holds the solder powder particles and the flux powder particles together in the molded part at room temperature.

Description

[0001] The invention relates to a molded part for producing a brazed joint, in particular between two extruded aluminum alloy tubes. Furthermore, such a molded part can also be used for brazed joints with components of the same or different types.

[0002] European patent EP 0 816 009 B1 discloses the use of a ring made of brazing material, which is placed on the extruded pipe. The joint area is then coated with a flux, and the coating is dried. This extruded pipe with the brazing ring is inserted into the die-cast pipe so that the brazing ring is positioned in the overlap area. The brazing process is carried out in a nitrogen gas atmosphere. The additional process of coating with flux is disadvantageous.

[0003] An improved solder ring is described in WO 2007 / 140236 A1. This solder ring has a C-shaped cross-section, with an annular groove filled with a flux provided in the solder material on the inside of the ring. To ensure that this flux remains in the solder ring, the flux is bonded into the annular groove using a binding agent. Such a flux-coated solder ring has the advantage that additional coating with flux is not required during the soldering process. However, the disadvantage is the complex production of such a solder ring.

[0004] DE 10 2012 006 121 A1 describes an aluminum-silicon-based solder powder. The solder powder is applied – wet or dry – together with flux powder and, if necessary, a binder powder to the surface of the component to be soldered. The production of a preformed part from the solder material is not described here.

[0005] DE 198 59 735 A1 describes a process for coating surfaces with solder, flux, and binder powder. The goal here is to apply the coating as evenly as possible to achieve a good soldering result. Here, too, no preform is provided, which would make uniform powder application obsolete.

[0006] Finally, EP 0 563 750 A1 describes a preformed solder made of a copper-based solder. A solder material containing aluminum, zinc, and silicon is not described here.

[0007] The object of the present invention is to provide a molded part for producing a brazed joint, which can be easily manufactured and does not require a flux coating during the brazing process.

[0008] This object is achieved with a molded part for producing a brazed joint having the features of claim 1. Advantageous embodiments are specified in subclaims 2 to 9. Such a molded part can be produced by a method according to claims 10 to 15.

[0009] This molded part is made from a powder mixture. The molded part itself contains solder powder particles, flux powder particles, and a binder. A powder mixture is used for production that has three powder components: a solder powder as the first component, a flux powder as the second component, and a binder powder as the third component.

[0010] The solder powder contains powder particles based on metal powders or alloy powders containing at least the three elements aluminum, zinc, and silicon. This means that aluminum, zinc, and silicon can be present in elemental form or as an alloy in the original powder mixture and in the molded part. AlZnSi alloys or ZnAlSi alloys are preferred. AlZnSi alloys with 10 to 30 wt% zinc and 5 to 15 wt% silicon are particularly preferred. These alloys have a melting point of no more than 550°C, which is advantageous for the use of such a molded part for brazing an aluminum joint consisting of two extruded tubes made of aluminum alloys. The lower the melting point of the solder, the greater the variety of selectable alloys that can be used to produce the extruded tubes. AlZnSi alloys with a high zinc content have particularly low melting points.At low zinc contents, namely < 10 wt%, the melting point is not sufficiently reduced. With a zinc content of 30 wt%, the melting point can be reduced to 510°C. However, high zinc contents in the solder also negatively influence the wettability of the solder, so the zinc content should ideally not exceed 30 wt%. On the other hand, the wettability of the solder is improved by the addition of silicon. Furthermore, the melting point can be lowered by adding up to 4 wt% copper. However, such a solder can lead to corrosion problems when the formed part is used for a brazed joint between components made of less noble alloys.

[0011] To achieve a good brazing joint, the solder powder particles should not be too large. Good results have been achieved in the past with particle sizes of 45 µm or less.

[0012] The molded part further contains, as a second powder component, a flux powder based on alkali metal fluorides, which also have a melting and effective temperature below 550°C. In particular, a flux powder is selected that has a lower melting point than the solder. A further melting point reduction can be achieved by a proportion of cesium aluminum fluoride powder of more than 10 wt. If the total flux powder contains at least 40 wt.% cesium aluminum fluoride powder and the remaining maximum 60 wt.% flux powder is formed by a potassium aluminum fluoride powder, the melting point is at most 520°C and can advantageously be lowered to 450°C with increasing proportions of cesium aluminum fluoride.

[0013] In addition, the molded part contains a binder that holds the solder powder particles and the flux powder particles together in the molded part. To produce the molded part, a binder powder is added to the mixture of solder powder and flux powder. The binder powder can be a granulate made from known binders, preferably from a thermoplastic material, for example polyethylene, polypropylene, or polymethyl methacrylate. Various polycarbonates can also exhibit properties similar to these thermoplastic materials. Materials selected as binder powders are those that have a low melting or softening point, orhave a low melting or softening range, ensuring that on the one hand the binder powder can melt or soften below the melting point of the solder powder and the flux powder, but on the other hand is in a solid state at room temperature, so that the molded part retains its desired shape at room temperature. The binder powder in the original powder mixture thus enables the powder mixture to be shaped into a powder blank without the solder powder or flux powder being melted, since the molten binder alone creates a flowable mass that allows easy shaping into a desired powder blank during the production of the molded part. Furthermore, the thermoplastic properties of the binder enable further forming of the powder blank.After a cooling process, during which the binder solidifies, and optionally one or more further processing steps, including, for example, cutting operations, the desired final contour of the molded part is achieved. Handling and transporting the molded part at room temperature is made possible by the solidified binder, which holds the solder powder and flux powder particles together in the molded part.

[0014] The molded part is preferably an open, slotted ring, as is known from the prior art. Such a ring is pushed onto the pipe to be soldered, for example, an extruded aluminum pipe, by slightly bending the slotted ring. Once pushed on, this ring clamps firmly onto the pipe end. Such a pipe end can be inserted into an aluminum pipe and soldered without an additional coating process. During this soldering process, thermal debinding occurs.

[0015] The molded part can also have a different shape for other applications, such as a straight rod or a rod section, an arc, an open or closed profile shape, or a closed ring. The molded part contains all three components resulting from the original three powder components. These three components are preferably evenly distributed throughout the molded part. For special applications, the flux powder can also be concentrated on one side of the molded part.

[0016] A preformed part for soldering, for example a ring, is produced from a powder mixture containing the three powder components: solder powder, flux powder, and binder powder. The production process is not limited to the aforementioned solder powder, flux powder, and binder powder. Other powder components, such as a metal solder, a suitable flux, and a binder, can also be used. However, the binder should have a melting or softening point below the melting points of the solder and flux. A powder mixture is produced, preferably containing 65 to 80 vol% solder powder and 20 to 35 vol% flux powder, and then 5 to 10 vol% binder powder is added to this binary powder mixture, i.e. the resulting powder mixture contains 90 to 95 vol% of the binary powder mixture.The powder mixture with the three powder components is thoroughly mixed, and a powder blank is created from the powder mixture, which is then processed into an open or closed ring through final processing. The powder blank is created from the powder mixture using heat, which melts or softens the binder powder contained in the powder mixture to produce a flowable and moldable mass. After the powder blank has been formed, the binder is solidified through a cooling process, so that the solder powder and the flux powder are held together by the solidified binder in the powder blank.

[0017] Such a powder blank can be produced by injection molding, also known as metal injection molding (MIM) or powder injection molding (PIM). For this purpose, the powder mixture is heated to a temperature at which the binder powder melts or at least softens enough for the original powder mixture to form a flowable mass that can be injected into the injection mold. The injection mold consists of two mold halves, each containing an annular recess. When the two molds are closed, an annular cavity is created that can be reached via an injection channel. The flowable mass is injected into this injection channel until the cavity in the injection mold is filled. The mold is then cooled so that the binder solidifies again and the annular powder blank is removed.In a final processing step, the sprue is removed from the ring-shaped powder blank, which still has a sprue point. The advantage of this injection molding process is that, using an injection mold with several ring-shaped cavities, several ring-shaped powder blanks can be produced simultaneously.

[0018] Another method involves feeding the powder mixture into an extrusion device. In the extrusion device, the powder mixture is heated until the binder powder melts or softens and can be pressed through the extrusion die with low pressure. In this process, powder blanks in various shapes can be produced, for example an endless longitudinal wire, but also a spiral wire or a slotted tube. Such a longitudinal wire must then be formed into a ring. For this purpose, appropriate sections can be cut off from the longitudinal wire and, in a subsequent bending process, the ends of each section can be formed into a ring. Another possibility is to bend the still-warm longitudinal wire after it has left the extrusion device and, after bending, to cut off appropriate ring-shaped sections.

[0019] To obtain a spiral wire in an extrusion step, an extrusion die is used, which preferably has a spiral guide channel. Furthermore, it is also possible to force a wire emerging from the extrusion die into a spiral shape immediately behind the extrusion die by means of a deflection device. If such an endless spiral wire is obtained, a plurality of open rings can be simultaneously obtained by making a longitudinal cut on one side of the spiral.

[0020] If a slotted tube is formed by extrusion, ie an open tubular profile, the desired open rings can be separated from the slotted tube by a cutting process in the cross-sectional direction of the tube.

[0021] In one embodiment, a solder powder made of an AlZn16.2Si9.3 alloy and, in another embodiment, an AlZn28.7Si7.6 alloy with particle sizes of < 45 µm were blended with a flux powder with an average particle size of 20 µm, consisting of 50% potassium aluminum fluoride and 50% cesium aluminum fluoride, together with binder granules made of polypropylene carbonate. The powder mixture contains 70 vol% solder powder, 20 vol% flux powder, and 10 vol% binder powder. The well-mixed powder mixture is then heated to temperatures of 320°C, melting the binder powder to form a flowable mass that can be easily injected into the preheated injection mold, which has 10 annular cavities formed by the two injection mold halves. After the flowable mass has been poured in and these annular cavities have been filled, a cooling process takes place during which the binder can solidify.The 10 ring-shaped powder blanks can be removed from the mold, and the gates are separated in a final processing step. This results in 10 open rings with an inner diameter of 9.3 mm and a ring thickness of 2 mm. The rings made of both brazing materials resulted in good brazed joints.

Claims

[1] Moulded part for producing a brazed joint from a material - with a first powder component, namely a solder powder based on metal powders or alloy powders with at least the three elements aluminium, zinc and silicon, - with a second powder component, namely a flux powder, - wherein the aforementioned powder components are bound in a binder, - wherein the binder has a melting or softening point or a melting or softening range which is, on the one hand, lower than the melting point of the solder powder and lower than the melting point of the flux powder, but on the other hand ensures that the binder is in the solid state at room temperatures, so that the binder holds the solder powder particles and the flux powder particles together in the molded part at room temperatures. [2] Moulded part according to claim 1, characterized bythat the molded part contains a solder powder based on an AlZnSi alloy or a ZnAlSi alloy, preferably a solder powder based on an AlZnSi alloy with 10 to 30 wt% zinc and 5 to 15 wt% silicon. [3] Moulded part according to claim 2, characterized by that the molded part contains a solder powder based on an AlZnSi alloy or a ZnAlSi alloy with up to 4 wt% copper (Cu). [4] Moulded part according to one of claims 1 to 3, characterized by that solder powder only contains powder particles with a particle size of 45 µm or less. [5] Moulded part according to one of claims 1 to 4, characterized by that the molded part contains a flux powder based on alkali metal fluoride powders with an effective temperature below 550°C, preferably with a proportion of more than 10 wt% CsAlF4 powder and at most 90 wt% KAlF4 powder. [6] Moulded part according to one of claims 1 to 5, characterized bythat the molded part contains a binder made of a thermoplastic material such as polyethylene (PE), polypropylene (PP) or polymethyl methacrylate (PMM) or of a material based on polycarbonates. [7] Moulded part according to one of claims 1 to 6, characterized by that the molded part is in the form of an open, slotted ring, which can be easily pushed onto a pipe to be soldered and can be held to this pipe by means of its clamping force. [8] Moulding according to one of claims 1 to 7, characterized by that the solder powder particles and the flux powder particles are evenly distributed over the cross-section of the ring. [9] Moulded part according to one of claims 1 to 7, characterized by that the solder powder is evenly distributed over the cross-section of the ring and that the flux powder is concentrated in the area of ​​the inside of the ring. [10] Method for producing a molded part for soldered joints, characterized by the following procedural steps: - Production of a binary powder mixture of 65 to 80 vol% solder powder and 20 to 35 vol% flux powder and - Addition of 5 to 10 vol% of binder powder to the binary powder mixture, wherein the binder powder has a melting or softening point or a melting or softening range which is lower than the melting point of the solder powder and the flux powder, but the binder powder is in the solid state at room temperature, - producing a powder blank from the powder mixture, whereby the binder powder is melted or softened by applying heat to form the powder blank and then, after forming, cooling takes place to solidify the binder, - Finishing the powder blank into an open, slotted ring, whereby the solidified binder holds the solder powder particles and the flux powder particles together in the ring. [11] Method according to claim 10, characterized by that the powder blank is produced from the powder mixture by injection molding, whereby first the binder powder in the powder mixture is melted into a flowable mass by application of heat and this flowable mass is injected into an injection mold, whereby preferably several ring-like powder blanks are produced simultaneously and then, after a cooling process to solidify the binder in the powder blank, the ring-like powder blanks are removed from the mold and finally, in a finishing step, the sprue points are removed from the ring-like powder blanks to form the open, slotted ring. [12] Method according to claim 10, characterized bythat the powder blank is produced from the powder mixture by extrusion, wherein the binder powder melts or softens in the extrusion device under the application of heat and a desired powder blank in the form of a longitudinal wire, a spiral wire or a slotted tube is pressed out of the extrusion tool. [13] Method according to claim 12, characterized by that the still warm endless longitudinal wire is bent into ring-shaped structures, with a section of the longitudinal wire being cut off before or after bending. [14] Method according to claim 12, characterized by that the extrusion tool has a spiral guide channel which causes a deformation into a spiral wire, this spiral wire is cooled and a plurality of open rings are produced from the spiral by a subsequent longitudinal cut. [15] Method according to claim 12, characterized bythat the slotted tube obtained by extrusion is cooled and an open ring is obtained from the slotted tube by a respective cutting process in the cross-sectional direction of the slotted tube.