Nickel-based brazing alloys and brazing processes

Abstract

Brazing alloy with a composition consisting of Fe a No Rest Cr b Mon c Cu d Si e B f P g with 10 < a ≤ 35 atom%; 5 ≤ b ≤ 18 atom%; 0.2 < c ≤ 3 atom%; 0.5 ≤ d ≤ 5 atom%; 4 ≤ e ≤ 15 atom%; 4 ≤ f ≤ 15 atom%; 0 ≤ g ≤ 6 atom%, balance Ni and incidental impurities, the brazing filler metal is provided in the form of an amorphous, ductile brazing filler metal foil.

Description

The invention relates to a brazing material based on nickel and to a method for brazing two or more parts.Brazing is a method of joining metallic or ceramic parts using a molten filler metal called brazing. Depending on the processing temperature of the solder, a distinction is made between soldering and brazing, wherein the processing temperature is typically 10° C. to 50° C. above the liquidus temperature of the solder. Soft solders are processed at temperatures below 450° C. and hard solders on the other hand at temperatures above 450° C. Hard solders are used in applications in which a high mechanical strength of the solder joint and / or a high mechanical strength at elevated operating temperatures is desired.Parts of stainless steel or Ni and Co alloys are often joined together with Ni-based brazing. The corrosion resistance of the soldered joints produced from the hard solder is a decisive criterion of use in some applications, in particular in stainless steel heat exchangers and related products. To increase the temperature range of use and to improve corrosion resistance, a nickel-based brazing filler alloy with a chromium content of 17 to 20 atomic % is known, for example, from EP 0 108 959.However, this increased chromium content has the disadvantage that the liquidus temperature and consequently the processing temperature is increased. This leads to undesirable coarse grain formation in the base material and a lowering of its mechanical strength, which is likewise not desired for many applications. Further, an increased chromium content of the brazing filler metal may result in the formation of Cr-B and Cr-Si brittle phases in the braze joint or in the base material, thereby compromising the mechanical strength of the joint.To reduce the chromium content and these problems, a nickel-based brazing material is known, for example, from WO 96 / 37335 A1, which has a molybdenum content of up to 5 atomic % and a lower chromium content of 9.5 to 16.5 atomic %.U.S. Pat. No. 5,183,636 discloses an iron-free brazing material which has components for diffusion prevention of iron from the base material into the brazing material and components for improving the corrosion resistance. For this purpose, this iron-free hard solder comprises copper, molybdenum, niobium and tantalum. This composition is intended to improve corrosion resistance, since the chromium content is retained by the niobium and tantalum additions and the soldered seam remains iron-free.However, these brazing materials have the disadvantage that the corrosion resistance of the brazing compound is not sufficient in aggressive media, such as acid-containing media. In addition, the brazing material from U.S. Pat. No. 5,183,636 is expensive due to its components.JP 60106691 A discloses a brazing filler metal consisting of, in atomic percent, 3≤Cu≤30, 0≤B≤15, 0≤Si≤20, balance Ni, and inevitable impurities.DE 39 29 222 A1 discloses nickel base solders for high-temperature solder joints which, in addition to nickel, comprise 5.5 to 15 atomic % of molybdenum, 0 to 12 atomic % of silicon, 0 to 19 atomic % of boron, 0 to 22 atomic % of phosphorus, 0 to 25 atomic % of chromium, iron and / or cobalt, 0 to 5 atomic % of manganese, copper, niobium, zirconium and / or titanium, wherein the total content of silicon, boron and phosphorus is 12 to 24 atomic %.EP 0051461 A1 discloses a homogeneous ductile brazing foil having at least 50 percent glass structure and a composition comprising as metal components nickel, at least one of the elements molybdenum, tungsten and cobalt and optionally one or both of the elements iron and chromium in a total amount of 76 to 84 atomic percent and as non-metal or semimetal component(s) one or more elements from the group consisting of boron, phosphorus, silicon and carbon in a total amount of 24 to 16 atomic percent and incidental impurities, wherein the atomic percentages of the composition comprise 0 to 6 percent iron, 0 to 21 percent chromium, 0 to 4 percent molybdenum, 0 to 5 percent tungsten, 0 to 20 percent cobalt, 0 to 19 percent boron, 0 to 12 percent silicon, 0 to 22 percent phosphorus, 0 to 3 percent carbon and the balance substantially nickel and incidental impurities.JP 63079931 A discloses an amorphous nickel alloy having a composition consisting of 10-40% Ta and Cr, wherein ≤ 20% Ta, 15-23% in total of P, B, wherein ≤ 7% B, and / or Si, balance Ni.It is therefore an object to provide a nickel-based brazing material with improved corrosion resistance, which is also cost-effective.According to the present invention, there is provided a brazing filler metal having a composition consisting essentially of Fe a Ni Rest Cr b Mo c Cu d Si e B f P g wherein 10<a≤35 atomic %; 5≤b≤18 atomic %; 0.2<c≤3 atomic %; 0.5≤d≤5 atomic %; 4≤e≤15 atomic %; 4≤f≤15 atomic %; 0≤g≤6 atomic %, balance Ni, and incidental impurities. The brazing material is provided in the form of an amorphous, ductile brazing material foil.This brazing filler metal preferably has a combination of a molybdenum addition of 0.2 to 1.5 at % and a copper addition of 0.5 to 3 at % for improving corrosion resistance. Surprisingly, it has been found that this hard solder exhibits good corrosion resistance without the expensive additions of tantalum and niobium. Furthermore, this good corrosion resistance is also maintained at an iron content of up to 50 atomic %. This leads to a further reduction in the raw material costs, since iron is less expensive than nickel.The brazing material according to the invention has been found to be suitable for applications in highly aggressive media, such as, for example, in heat exchangers for internal combustion engines and exhaust gas recirculation coolers. In these applications, the solder compound produced is located in reducing or oxidizing acidic media which may also contain sulfate and / or nitrate and / or chloride ions. Soldered joints produced with the brazing alloy according to the invention also have good corrosion resistance in these highly aggressive media. Further applications of the brazing material according to the invention are the joining of two or more parts of stainless steel heat exchangers for industrial applications and heat exchangers in the automotive and commercial vehicle sector, in which aggressive media are produced.The good corrosion resistance of the hard solder according to the invention is achieved with a moderate chromium content of 5 to 18 atomic %, so that the disadvantages of the high-chromium alloys are avoided. The combination of the Mo and Cu additive, in contrast to an increase in the chromium content, does not lead to an undesired increase in the liquidus temperature and thus in the processing temperature of the solder. This chromium content ensures that the massive formation of Cr-B and Cr-Si brittle phases in the soldered seam and in the base material is avoided. Good corrosion resistance is provided despite the low chromium content by the Mo and Cu additions.The brazing alloy according to the invention has a liquidus temperature of less than 1200° C. This is desirable since the maximum soldering temperature for many industrial soldering processes, in particular for joining stainless steel base materials, is limited to approximately 1200° C. As a rule, the aim is to achieve a soldering temperature which is as low as possible, since, starting from a temperature of 1000° C., undesired coarse grain formation of the base material occurs. This undesirable coarse grain formation leads to a reduction in the mechanical strength of the base material, which is critical for some industrial applications, such as, for example, in heat exchangers. This problem is significantly reduced by the brazing material according to the invention.Thus, the brazing material can be reliably used for industrial applications whose maximum brazing temperature is limited to 1200° C. A reliable brazing joint is provided.In further embodiments, the brazing alloy has an Si content of 7≤e≤12 atomic % and / or a B content of 5≤f≤13 atomic % and / or a Cr content of 5≤b≤14 atomic %.The elements boron, silicon and phosphorus are metalloids and glass forming elements and allow the brazing alloy to be fabricated as an amorphous, ductile foil. A higher content of these elements leads to a reduction in the melting or liquidus temperature. If the content of the glass-forming elements is too low, on the one hand, the films solidify in crystalline form and the films are very brittle. On the other hand, if the content of the glass forming elements is too high, the films are brittle and cannot be processed any longer for industrial processes.Furthermore, the content of the metalloids is chosen such that the soldered seam produced from the hard solder foil has suitable mechanical properties. A high B content leads to precipitation of B hard phases in the soldered seam and in the base material, which lead to a deterioration of the mechanical properties of the soldered composite. In this case, boron reacts with chromium, which likewise leads to a marked reduction in corrosion resistance. A higher Si content also leads to the formation of undesirable Si hard phases in the braze joint, which also causes a deterioration in the strength of the braze joint.The brazing filler metal has an Fe content of 10<a≤35 at%. It has been found that the good corrosion resistance of the brazing alloy according to the invention is also maintained in the case of brazing alloys having an iron content of up to 50 atomic %. The raw material cost of the brazing materials having an increased iron content is reduced because the nickel content is partially replaced by iron.The brazing material according to one of the preceding exemplary embodiments is provided in the form of an amorphous, ductile brazing material foil. The hard solder according to the invention can be produced as an amorphous, ductile foil, for example, by means of rapid solidification processes.In one embodiment, the braze foil is up to 50% amorphous, preferably at least 80%.The braze foils of the present invention can be made in thicker strip thicknesses and larger widths than ductile foils. The brazing alloys according to the invention are thus excellently suitable for casting with thicknesses of more than 20 μm, preferably of 20 μm ≤ D ≤ 40 μm and with widths of more than 20 mm or of 20 mm ≤ B ≤ 200 mm, which is possible to a very limited extent in the case of the brazing alloys based on nickel known from the prior art.In one embodiment, a heat exchanger is provided that includes at least one braze seam made with a braze according to any one of claims 1 to 10.This braze joint is made from a braze having this composition that has been made in the form of an amorphous ductile braze foil. The heat exchanger may include at least one braze seam made of an amorphous ductile braze foil according to any of the preceding embodiments. The braze joint made with an amorphous ductile braze foil has a thickness of at least 20 μm.The amorphous ductile braze foil braze seam differs from a crystalline powder braze seam by the size of the B and Si hard phases.A method for the material-bonding joining of two or more parts is provided, which comprises the following steps. An amorphous, ductile brazing foil according to one of the preceding embodiments is introduced between two or more metal parts to be joined. The parts to be joined have a higher melting temperature than the brazing foil and can consist of stainless steel, a Ni or a Co alloy. The solder bond is heated to a temperature above the liquidus temperature of the brazing foil and cooled to form a brazing bond between the parts to be joined.The parts to be joined are preferably parts of a heat exchanger or exhaust gas recirculation cooler or components of a fuel cell. These products require a reliable brazing composite which is completely dense, corrosion-resistant at higher operating temperatures, mechanically stable and consequently reliable. The brazing foils of the invention provide such a connection.The braze foils of the invention can be used to make one or more braze joints in an article. The brazed article may be used as a heat exchanger, an exhaust gas recirculation cooler, or as a component of a fuel cell. The brazed article is, in one embodiment, for use in a reducing or oxidizing acidic medium or for use in a reducing and, in another embodiment, in an oxidizing acidic medium further comprising sulfate and / or nitrate and / or chloride ions or for use in a reducing or oxidizing acidic medium of an internal combustion engine.The brazing alloys according to the invention are manufactured in one embodiment of the method as amorphous, homogeneous and ductile brazing foils by means of rapid solidification. In this case, a molten metal is provided from a melt consisting of Fe a- Ni Rest Cr b Mo c Cu d Si e B f P g where 10<a≤35 at.%, 5≤b≤18 at.%, 0.2<c≤3 at.%, 0.5≤d≤5 at.%; 4≤e≤15 at.%, 4≤f≤15 at.%, 0≤g≤6 at.%, remainder Ni and incidental impurities. This melt is injected through a casting nozzle onto at least one rapidly rotating casting wheel or casting drum and cooled at a cooling rate of more than 10 5 °C / sec. The cast strip is then typically stripped from the casting wheel at a temperature between 100° C. and 300° C. and wound directly into a so-called coil or onto a coil former to indicate an amorphous, ductile brazing foil.In a further method, amorphous brazing foils are used for the material bonding of two or more parts, wherein the following steps are carried out:providing a melt consisting of Fe a Ni Rest Cr b Mo c- Cu d Si e B f P g with 10<a≤35 atomic %; 5≤b≤18 atomic %; 0.2<c≤3 atomic %; 0.5≤d≤5 atomic %; 4≤e≤15 atomic %; 4≤f≤15 atomic %; 0≤g≤6 atomic %, remainder Ni and incidental impurities.producing an amorphous brazing foil by rapidly solidifying the melt on a moving cooling surface at a cooling rate of more than about 10 5 °C / sec;forming a solder bond by introducing the brazing material foil between the metal parts to be joined;heating the solder bond to a temperature above the liquidus temperature of the brazing foil;cooling the solder bond to form a connection between the metal parts to be joined.The material-to-material joining described in this way represents brazing using a nickel brazing material according to the invention, with which satisfactory brazing material connections can be achieved without joining defects.The liquidus temperature of the brazing materials according to the invention is less than 1200° C. ° C. With the brazing method according to the invention, in particular metal parts made of stainless steel and / or nickel and / or Co alloys can be joined together in a material-bonded manner. Typically, parts are considered which are installed to form heat exchangers or related products and exhaust gas recirculation coolers.The invention will be described in detail below with reference to Examples and Comparative Examples. FIG. 1 shows the mass loss in a corrosion test of stainless steel samples with soldered joints produced from a first base composition with Mo and / or Cu additions, FIG. 2 shows the mass loss in a corrosion test of stainless steel samples with brazing joints produced from a second base composition with different Mo additions, FIG. 3 shows the mass loss in a corrosion test of stainless steel samples with soldered joints produced from a second base composition with different Cu additions, and FIG. 4 shows the mass loss in a corrosion test of stainless steel samples with solder joints with different iron contents.At least partially amorphous brazing nickel-iron based foils having different compositions were produced by rapid solidification. The corrosion resistance of braze joints with Cu additions, Mo additions and a combination of Mo and Cu additions was compared to molybdenum-free and copper-free braze foils.In a first exemplary embodiment not according to the invention, the corrosion resistance of a combination of Mo and Cu additions was investigated with respect to a Mo addition alone and a Cu addition alone in the case of a first basic composition. At least partially amorphous brazing foils of different compositions were thereby produced by means of rapid solidification technology. The compositions of the film are summarized in Table 1.In this first noninventive embodiment, the brazing foils have a basic composition of a Cr content of 12.3 atomic %, an Fe content of 3.7 atomic %, an Si content of 7.9 atomic %, and a B content of 12.8 atomic %, balance nickel. Further films having a Cr content of 12.3 atomic %, an Fe content of 3.7 atomic %, an Si content of 7.9 atomic % and a B content of 12.8 atomic % were produced with additions of copper and / or molybdenum, remainder nickel.A brazing foil has a copper addition of 2 at %, a second foil has a Mo addition of 1 at %, and a third foil has a molybdenum addition of 1 at % and a copper addition of 2 at %.Samples of stainless steel (316L, 1.4404) in which a base plate with two pipe sections is joined were soldered to these different foils at 1200° C. in a vacuum. The brazed parts were disposed in a corrosion medium with pH<2 and SO 42-, NO 3- and Cl - ions at 70°C. The mass loss of the various samples is shown in FIG. 1 after a storage time of 720 hours.It can be seen from FIG. 1 that a Cu additive alone and a Mo additive alone provide only a moderate improvement in corrosion resistance compared to a Mo- and Cu-free hard solder connection. The brazing alloy which contains an Mo additive and a Cu additive has the lowest mass loss and thus the best corrosion properties. A combination of Mo and Cu additions provides a brazing foil having improved corrosion resistance.In a second exemplary embodiment, the influence of a combination of Mo and Cu additives on the corrosion resistance of a second base composition was investigated. In this second embodiment, a brazing alloy having a combination of Mo and Cu additions with copper-free brazing alloys was compared with increasing Mo content.At least partially amorphous brazing foils were thereby produced using rapid solidification technology. In this second embodiment, the brazing foils have a basic composition of a Cr content of 11 at%, a Ni content of 35 at%, a Si content of 11.5 at%, and a B content of 7 at%, balance iron. Copper-free foils of a Cr content of 11 at%, a Ni content of 35 at%, a Si content of 11.5 at% and a B content of 7 at% with molybdenum additions of 0.5, 1 and 1.5 at%, balance iron were prepared. A film of a Cr content of 11 at%, a Ni content of 35 at%, a Si content of 11.5 at%, and a B content of 7 at% with a copper addition of 2 at% and a Mo addition of 1 at%, balance iron was prepared. These compositions are summarized in Table 2. The second base composition thus has a significantly higher iron content than that of the first base composition.Samples of stainless steel were prepared as in the first embodiment and the corrosion resistance was measured as described in the first experiment. In the second embodiment, the samples were discharged for 864 hours, and thereafter the mass loss was measured.It can be seen from FIG. 2 that the alloy comprising a combination of an Mo and a Cu additive has the lowest mass loss and thus the best corrosion resistance. By varying only the Mo content, the corrosion resistance of the alloy with Mo and Cu was not obtained.In a third embodiment, the influence of a combination of Mo and various Cu additives on the corrosion resistance of a third base composition was investigated. At least partially amorphous brazing foils were thereby produced using rapid solidification technology. In this third embodiment, the brazing foils have a basic composition of a Cr content of 11 at%, a Ni content of 35 at%, a Si content of 11.5 at%, and a B content of 7 at%, balance iron. A Cu-free film of a Cr content of 11 at%, a Ni content of 35 at%, a Si content of 11.5 at%, and a B content of 7 at% with an Mo addition of 1 at%, balance iron was prepared. Molybdenum-free films of a Cr content of 11 at%, a Ni content of 35 at%, a Si content of 11.5 at% and a B content of 7 at% with a Cu addition of 1 at% and a copper addition of 2 at% each with 1 at% of Mo, balance iron were prepared. These compositions are summarized in Table 3.Samples of stainless steel were prepared as described in the first test and the corrosion resistance was measured as described in the first and second tests. FIG. 3 shows the loss of mass of the latter after a storage time of 720 hours.It can be seen from FIG. 3 that the corrosion resistance of brazing alloys with Mo and Cu additions is significantly better than in the alloys which exclusively comprise Mo.In a fourth exemplary embodiment, the corrosion resistance of at least partially amorphous brazing alloy foils was investigated with a combination of 1 at % Mo and 1 at % Cu and increasing iron content.At least partially amorphous brazing foils were thereby produced using rapid solidification technology. At least partially amorphous films having Fe contents of 0, 10, 20, 30, 40, 50, 60 and 70 atomic % and each having Cr contents of 11 atomic %, Si contents of 9 atomic %, B contents of 9 atomic %, Mo contents of 1 atomic % and Cu contents of 2 atomic %, balance nickel were prepared. These compositions are summarized in Table 4.FIG. 4 shows that the corrosion resistance of the foils each comprising Mo and Cu remains almost the same up to an Fe content of 50 at%. This has the advantage that the Ni content can be replaced by iron up to an Fe content of 50 atomic %, without the corrosion resistance being substantially impaired. Thus, the raw material cost can be reduced. Table 1 Compositions of the brazing sheet of the first noninventive embodiment Table 1 Compositions of the brazing sheet of the first noninventive embodimentRemainder: Remainder:3,712,3007, 912, 8Remainder: Remainder:3,712,3027, 912, 8Remainder: Remainder:3,712,3107, 912, 8Remainder: Remainder:3,712,3127, 912, 8Table 2 Compositions of the brazing sheet of the second embodimentTable 2 Compositions of the brazing sheet of the second embodiment35Remainder: Remainder:110,5011,5735Remainder: Remainder:111011,5735Remainder: Remainder:111,5011,5735Remainder: Remainder:111211,57Table 3 Compositions of the brazing sheet of the third embodimentTable 3 Compositions of the brazing sheet of the third embodiment35Remainder: Remainder:111011,5735Remainder: Remainder:111111,5735Remainder: Remainder:111211,57Table 4 Compositions of brazing sheet of the fourth embodimentTable 4 Compositions of brazing sheet of the fourth embodiment690*11119959101111994920111199393011119929401111991050111199960111199069111199* According to the invention

Claims

A brazing alloy having a composition consisting of Fe a Ni Rest Cr b Mo c Cu d Si e B f P g with 10 < a ≤ 35 atomic %; 5 ≤ b ≤ 18 atomic %; 0.2 < c ≤ 3 atomic %; 0.5 ≤ d ≤ 5 atomic %; 4 ≤ e ≤ 15 atomic %; 4 ≤ f ≤ 15 atomic %; 0 ≤ g ≤ 6 atomic %, balance Ni and incidental impurities, the brazing alloy being provided in the form of an amorphous, ductile brazing alloy foil.Brazing material according to claim 1, characterised byan Si content of 7 ≤ e ≤ 12 atomic %.Brazing material according to claim 1 or 2, characterised bya Cr content of 5 ≤ b ≤ 14 atomic %.Brazing material according to one of claims 1 to 3, characterised bya B content of 5 ≤ f ≤ 13 atomic %.Brazing material according to any one of claims 1 to 4, characterised bya liquidus temperature of less than 1200°C.Brazing material according to one of Claims 1 to 5, characterized bythe brazing material foil being at least 80% amorphous.Brazing material according to any one of claims 1 to 6, characterised in that the brazing foil has a thickness D of more than 20 μm.Brazing material according to claim 7, characterised in that the brazing material foil has a thickness D of 20 μm ≤ D ≤ 40 μm.Brazing material according to one of Claims 1 to 8, characterized in that the brazing material foil has a width B of 20 mm ≤ B ≤ 200 mm.Brazing material according to claim 9, characterised in that the brazing material foil has a width B of 40 mm ≤ B ≤ 200 mm.Method for the material-bonding joining of two or more parts, comprising the following steps: - introducing a brazing material according to one of Claims 1 to 10 between two or more parts to be joined, wherein the parts to be joined have a higher melting temperature than the brazing material; - heating the brazing material composite to a temperature above the liquidus temperature of the brazing material; - cooling the brazing material composite to form a brazing material connection between the parts to be joined.Method for the material-bonding joining of two or more metal parts according to Claim 11, characterized in that the parts to be joined are parts of a heat exchanger or of an exhaust gas recirculation cooler or of a fuel cell.A method for producing an amorphous ductile brazing foil, comprising the following steps: - providing a melt consisting of Fe a Ni Rest Cr b Mo c Cu d Si e B f P g with 10 < a ≤ 35 atomic %; 5 ≤ b ≤ 18 atomic %; 0.2 < c ≤ 3 atomic %; 0.5 ≤ d ≤ 5 atomic %; 4 ≤ e ≤ 15 atomic %; 4 ≤ f ≤ 15 atomic %; 0≤g≤6 atomic %, balance Ni and incidental impurities, - Produce an amorphous ductile brazing foil by rapidly solidifying the melt on a moving cooling surface at a cooling rate of greater than about 10<5> °C / sec.A brazed article characterized in that at least one braze seam is made of a braze according to any one of claims 1 to 10.The brazed article according to claim 14, characterized in that the brazed article is a heat exchanger, exhaust gas recirculation cooler or component of a fuel cell.A brazed article according to claim 14, for use in a reducing or oxidizing acidic medium.The brazed article of claim 14, for use in a reducing or oxidizing acidic medium further comprising sulfate and / or nitrate and / or chloride ions.The brazed article of claim 14, for use in a reducing or oxidizing acidic medium of an internal combustion engine.Use of a hard solder according to one of Claims 1 to 10 for the material-bonding joining of two or more parts made of stainless steel or a Ni alloy or a Co alloy.Use according to Claim 19, characterized in that the hard solder is used for producing a heat exchanger or an exhaust gas recirculation cooler or components of a fuel cell.

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