Patent ID: 12220758

DETAILED DESCRIPTION

FIG.1schematically illustrates a soldering process for joining a rare earth magnet10with a substrate20, which is made of stainless steel in the examples described here. In the same way, two rare earth magnets or one rare earth magnet can be soldered to a soft magnetic substrate (e.g. cobalt iron). The substrate materials play a subordinate role in the soldering process, but the substrate usually has to be tinned. That is, in the present example, the surfaces to be joined of the rare earth magnet10and the substrate20are tinned (tin coatings11and21). A tinned nanofoil30is arranged between the components10and20. As already mentioned at the beginning, the production of a well-adhering tin layer on a stainless steel substrate using conventional methods is problematic. However, before the pretreatment of the stainless steel substrate20for producing the tin coating21is discussed in more detail, the soldering process using nanofoil will first be explained.

The nanofoil is a multilayer system which, for example, has a multiplicity of alternately arranged aluminum and nickel layers. Multi-layer systems made of other material combinations are also possible, e.g. Aluminum/titanium or nickel/silicon. A nanofoil can contain several thousand alternating layers of e.g. Have aluminum and nickel, wherein the individual layers can have thicknesses in the range from 25 nm to 90 nm. The nanofoil as a whole can have a thickness in the range of 10-100 μm. If the layers are sufficiently thin and the reaction products of the layers have a negative enthalpy of binding (e.g. with −59.2 kJ per mole for aluminum/titanium), such multilayer systems can use a relatively low energy input (e.g. by applying an electrical voltage VS, seeFIG.1) a self-propagating exothermic reaction is triggered, whereby the nanofoil is heated and a diffusion process is started, by which the solder connection is formed. The solder becomes at least partially liquid when the solidus temperature is exceeded, as a result of which a material connection is produced due to diffusion processes between the solder and the surfaces of the parts to be joined.

During the soldering process, pressure (seeFIG.1, force F) is exerted on the joining zone via a device (not shown inFIG.1). The pressure (joining pressure) during soldering can e.g. are in the range of 0.1 and 0.3 MPa. The amount of heat released during soldering depends on the area of the joint and the thickness of the reactive foil. The achievable energy density is approx. 1000-1250 J/g and (locally in the joining zone) temperatures in the range of 130-1500 degrees Celsius can arise. The heat is generated directly in the joining zone, the amount of heat remaining small enough that the components10and20are not significantly heated. In this way, a thermal influence on the magnetic (or other) properties (in particular a reduction in the remanent magnetization) of a magnetic component can be avoided. As already mentioned at the beginning, suitable nanofilms are e.g. from Indium Corp., Utica, NY, under the name NanoFoil®. The associated joining process is called NanoBond®. Fluxes, which are necessary for other soldering processes, are not required.

For a sufficiently strong solder joint, the adhesion of the tin layers11and21on the underlying surfaces of the rare earth magnet10and the substrate20is of crucial importance. As already mentioned, it is not so easy with conventional methods to produce a sufficiently firmly adhering tin coating on a stainless steel surface. The exemplary embodiments described below relate to a method for pretreating the substrate20(in particular a substrate made of stainless steel), a tin layer being deposited on the substrate20and adhering firmly to the substrate. Stainless steel is understood here as a stainless steel (see DIN EN 10088-2). In the exemplary embodiments described here, for example, a stainless austenitic steel, in particular a chromium-nickel-molybdenum steel, can be used as the stainless steel. Steel with the material designation X2CrNiMoN17-13-3 (material number 1.4429 according to DIN EN 10027-2) was used in the experiments carried out. However, other stainless steels can also be used.

Various methods are described in the literature for the adhesive galvanic coating of stainless steel surfaces, e.g. Pickling in a hot (approx. 70° Celsius) sulfuric acid solution with 20% to 50% (mass percent) sulfuric acid, cathodic treatment in sulfuric acid or hydrochloric acid, activation in an iron or nickel attack bath (also called nickel strike) with subsequent electroplating. All of these methods, in conjunction with galvanic tinning, lead to firm, adhesive layers, but the layer composite dissolves during the subsequent soldering with nanofoil and the tin layer loses its adhesion to the stainless steel substrate. With the exemplary embodiments described here, stainless steel substrates with firmly adhering galvanic tinning can be produced, the strength of which is also given after the soldering process with nanofilm. Experiments have shown that the magnet-stainless steel system has a strength of more than 15 N/mm2. The magnetic-stainless steel composite components were tested in a shear test to determine the strength

FIG.2schematically illustrates the different process steps. Accordingly, the surface of the stainless steel substrate20to be coated (and possibly cleaned) is first sandblasted (see diagram (a) fromFIG.2). For sandblasting, for example, corundum particles (blasting corundum) at a blasting pressure of 1-10 bar (100-1000 kPa), e.g. 8 bar can be used. The surface can then (optionally) be blown off with oil-free compressed air. The blasting corundum can have a mixed grain size with particle sizes between 250 and 500 μm. Various suitable sandblasting techniques are known. The most common method is also known as compressed air jets. Other techniques include blast wheel blasting, in which the particles are accelerated by a paddle wheel, and vacuum suction blasting.

In a next step, the substrate20is immersed in an acid bath31for a period of, for example, 2-3 minutes (see diagram (b) fromFIG.2). The acid bath31can be formed by an aqueous solution with 1-20% sulfuric acid, 1-20% nitric acid and 1-15% hydrogen fluoride (hydrofluoric acid). The rest at 100% is water. The percentages are percentages by mass. In a specific embodiment, the acid bath contains 50 g of concentrated sulfuric acid, 100 g of nitric acid (53% solution) and 75 g of hydrofluoric acid (40% solution, corresponds to 30 g of hydrogen fluoride) per liter of demineralized water. The duration of treatment can be about 2-3 minutes. According to the exemplary embodiments described here, the acid bath31can be tempered. For example, the acid bath31has a temperature between 40 and 95 degrees Celsius, in particular around 70 degrees Celsius.

The acid bath31removes oxides which form a passivation layer on the surface of the substrate20and thus activates the surface (surface activation). Stainless steel generally forms a passivation layer made of chromium oxide on the surface, which is removed in the acid bath31. The relevant surfaces of the substrate20are then rinsed in dilute hydrochloric acid32(see diagram (c) fromFIG.2). In one embodiment, the substrate is rinsed twice in hydrochloric acid for 20 to 60 seconds each (e.g. 30 seconds). The hydrochloric acid32can have a concentration of at least 5%. In some embodiments, the concentration is in the range of 8-12% (mass percent). The steps described above can improve the adhesion of the subsequently deposited coatings.

Without intermediate rinsing with water and without prior drying (i.e. “wet in wet”), the substrate20is then coated using a so-called nickel strike process. In experiments, e.g. uses a nickel strike bath33which contains a solution of demineralized water (also known as fully demineralized water or demineralized water), nickel (II) chloride (e.g. in the form of nickel (II) chloride hexahydrate, NiCl2·6 H2O) and hydrochloric acid (36 percent by mass). For every 1000 ml of water 240 g nickel (II) chloride hexahydrate and 125 g 36% hydrochloric acid. The galvanization in the nickel strike bath33can e.g. at a current density of 2-10 amperes per dm2 for approx. 2 minutes (first stage). The current density can then be reduced to approx. 1-2 amperes per dm2 for a further 2 minutes (second stage). The current densities and the duration of treatment can e.g. depending on the specific composition of the nickel strike electrolyte33may also be different. The nickel strike method outlined in diagram (d) fromFIG.2is known per se and suitable nickel strike electrolytes are commercially available and are described in the relevant specialist literature. The nickel-plated substrate20can then be rinsed with demineralized water (for about 30 seconds, not shown inFIG.2). The nickel coating21′ produced on the substrate serves as an adhesion promoter layer for the subsequent tin coating. Even if the entire surface of the stainless steel substrate20is coated in the example fromFIG.2, it may be sufficient to coat only the joining surface of the stainless steel substrate20, i.e. the surface that will be soldered on later. The layer thickness of the nickel coating21′ is comparatively thin, e.g. smaller than 1 μm. In the exemplary embodiments described here, the thickness of the nickel coating21′ is clearly below 500 nm.

Without prior drying (wet in wet), the substrate20is then placed in a tin bath34(tin electrolyte) and tin-plated. For this purpose, the substrate20(with a nickel coating31′) can be immersed in the tin bath without current for about 10-120 seconds. A strongly acidic electrolyte is used as the tin electrolyte (e.g. with a pH value of less than 1). Other methods are usually less suitable. For example, a bright tin bath which is commercially available, for example, from Dr.-Ing. Max Schlötter GmbH & Co. KG, Geislingen, Germany, is available under the name SLOTOTIN 30-1. In some embodiments, the dive time is 20-40 seconds (de-energized). The galvanic coating then takes place at currents of approximately 0.5-1.5 amperes per dm2 (for example 1-1.3 A/dm2) until a layer thickness dT of approximately 10-30 μm has been achieved. In some exemplary embodiments, the layer thickness dT is in the range of 12-15 μm. The tin coating21adheres with sufficient strength even after the subsequent soldering process. Since, as mentioned, the thickness of the nickel coating21′ is usually significantly smaller than 1 μm, the total thickness of the layers21′ and21is essentially determined by the layer thickness dT of the tin coating21.

FIG.3is a flowchart illustrating the joining process for connecting a rare earth magnet to a stainless steel substrate by means of soldering, which has previously been tinned as described above. In the first step S1, a tin-plated rare earth magnet (for example a neodymium-iron-boron magnet with galvanically produced tin-plating with a thickness of approximately 15 μm) is provided. In a second step S2, the rare earth magnet is arranged on the substrate, the substrate being a stainless steel substrate coated according to the method described above (seeFIG.2). A tinned, reactive foil is arranged between the joining surface of the rare earth magnet and a corresponding joining surface of the stainless steel substrate20, for example the NanoFoil® mentioned above. The third step S3relates to the actual soldering process. An exothermic reaction is triggered in the reactive film (e.g. by applying an electrical voltage). The resulting heat sets in motion a diffusion process by which the rare earth magnet is firmly bonded to the tinned stainless steel substrate.

With the help of the method described above (seeFIG.2) for pretreating the stainless steel substrate20it is achieved that the tin coating adheres sufficiently firmly to the stainless steel surface even after the soldering process. As mentioned, strengths of more than 15 N/mm2 could be achieved in the shear test. With the above-described soldering process using reactive nanofoil, of course, not only rare earth magnets, but also other metallic components can be soldered onto a (appropriately pretreated) stainless steel substrate.

The sandblasting is crucial for the strength of the solder joint created later. Without sandblasting, an adhesion fracture between the stainless steel substrate20and the tin layer21was observed in the shear test on the soldered composite stainless steel-substrate-rare earth magnet with a shear stress of less than 1 MPa. However, the tin coating21itself (without subsequent soldering) is sufficiently strong even without sandblasting, which could be shown in an adhesive/shear test. The same applies to the hydrochloric acid rinse of the stainless steel substrate20. In a control experiment, this hydrochloric acid rinse was replaced by a rinse with demineralized water, which also had the consequence that the strength of the composite was not sufficiently high in the shear test after soldering. In this case, too, an adhesion break between the stainless steel substrate20and the tin layer21could be observed. In the case of stainless steel substrates, which were pretreated according to the procedure described here before soldering, no adhesion break could be observed in the shear test on the soldered bond, but rather a cohesive break in the tin layer. The cohesive break occurred at a shear stress of more than 15 MPa.