A cadmium-free and low-silver solder

Abstract

A cadmium-free, low-silver solder is characterized by its composition, by mass percentage, of 4.0% to 5.0% Ag, 40.0% to 45.0% Zn, 11.0% to 13.0% Sn, 0.5% to 1.5% In, 0.5% to 1.5% Ga, 0.5% to 1.5% Ni, 0.01% to 0.05% nano-CeO2, 0.01% to 0.05% nano-Y2O3, and the balance being Cu. Silver plates, cathode copper, zinc ingots, tin ingots, metallic indium, metallic gallium, and metallic nickel are added in the appropriate proportions and smelted using a medium-frequency smelting process. Before casting, trace amounts of nano-CeO2 and nano-Y2O3 powders are added, thoroughly stirred, and then cast into an ingot. The desired solder wire is then obtained through extrusion and drawing. The solder has a solidus temperature of ≤735°C and a liquidus temperature of ≤780°C. When used with the commercially available FB102 flux, the brazing performance of the brazing filler metal is close to that of the commercially available BAg25CuZn filler metal. When brazing copper-stainless steel, copper-brass, brass-stainless steel, and Q235 steel-stainless steel, the shear strength of the brazed joints is superior to that of the commercially available BAg25CuZn filler metal.

Description

Technical Field

[0001] The invention belongs to the technical field of brazing materials of metal materials, and particularly relates to a cadmium-free and low-silver brazing filler metal. Background Art

[0002] While the BAg25CuZn solder recommended in the current national standard GB / T 10046-2018, "Silver Solder," has a melting temperature of 700°C at its solidus and 790°C at its liquidus, making it a highly popular and cost-effective silver solder for solder users, Ag is a precious metal and is expensive, and its 25% ± 1% Ag content does not meet the requirements for low-cost, high-quality manufacturing. Therefore, the development of a cadmium-free, low-silver silver solder with a lower silver content and comparable solder spreading properties and brazed joint strength to BAg25CuZn has been a hot topic and a challenge for solder R&D professionals.

[0003] Through literature searches, the applicant discovered that published Chinese patents contain a few examples of novel low-silver solder technologies. For example, document CN102416530B describes a cadmium-free, low-silver solder. Its chemical composition, by mass percentage, is 16.5% to 20.5% Ag, 36.5% to 42.5% Zn, 1.5% to 3.5% Sn, 0.01% to 0.6% Ni, 0.001% to 0.1% Si, 0.001% to 0.1% Al, 0.001% to 0.1% Ba, 0.001% to 0.1% Zr, and the balance is Cu. Its silver content is 16.5% to 20.5%, which is still relatively high.

[0004] Document CN112108790A reports a cadmium-free, low-silver solder and its preparation method. The cadmium-free, low-silver solder is composed of Ag, Cu, Zn, In, Sn, Ni, and the trace element R. The weight percentages of the components are: 13.5-20% Ag, 40-48% Cu, 1-3% In, 1-3% Sn, 0.1-1.0% Ni, and 0.001-0.2% trace element R, where the trace element R is composed of one or more of lanthanum, cerium, silicon, antimony, zirconium, and yttrium, with the balance being Zn. This invention's solder contains less than 20% silver and contains no cadmium. It has a low melting temperature and good processability, but the silver content is still high, exceeding 13.5%, and the solder's performance still has many deficiencies.

[0005] Document CN106077995A reports a cadmium-free, low-silver solder containing manganese and tin, and its preparation method, suitable for brazing stainless steel and copper alloys such as copper and brass. The invention comprises Ag, Cu, Sn, Mn, Zn, and the trace element R. The weight percentages of the components are: 13-19% Ag, 25.5-38% Zn, 35-40% Cu, 1.0-5.5% Sn, 7-14% Mn, 0.5-3.0% Ni, and 0.001-0.3% R. The solder of this invention contains less than 20% silver, contains no cadmium, has a low melting temperature, and exhibits excellent processability. This multi-component silver solder is used for brazing copper, brass, and stainless steel. This solder is similar to the solder reported in CN112108790A, but similarly suffers from several performance deficiencies.

[0006] The applicant and others jointly invented a cadmium-free, low-silver solder (publication number CN202211031512.5). Its composition, by mass percentage, is: 8.0% to 10.0% Ag, 38.0% to 42.0% Zn, 6.0% to 10.0% Sn, 1.5% to 2.5% In, 1.5% to 2.5% Ga, 1.5% to 2.5% Ni, 0.0001% to 0.0005% nano-HfC, 0.0001% to 0.0005% nano-ZrC, and the balance is Cu. The solder has a solidus temperature of ≤700°C and a liquidus temperature of ≤750°C. When used with the commercially available FB102 flux, the shear strength of brazed joints between copper and stainless steel, copper and brass, brass and stainless steel, and Q235 steel and stainless steel is superior to existing BAg25CuZn and BAg12CuZn(Si) fillers. However, the Ag content of these fillers ranges from 8.0% to 10.0%, making the material cost relatively high. The market is in urgent need of cadmium-free silver fillers with lower silver content and performance similar to BAg25CuZn.

[0007] It is well known that the key challenge in developing new cadmium-free silver solders lies in the fact that the solidus and liquidus temperatures of Ag-Cu-Zn solders reach their lowest point when the Ag content is 45%. Subsequently, as the Ag content increases, the solidus and liquidus temperatures of the silver solder increase, but the wetting and spreading properties of the silver solder and the solder joint strength do not decrease or even increase slightly. However, as the Ag content decreases, the wetting and spreading properties of the silver solder deteriorate, and the solder joint strength also decreases. Therefore, in addition to considering its cadmium-free nature, the development of new silver solders primarily focuses on reducing the content of the precious metal element Ag. However, reducing the Ag content inevitably leads to a decline in the silver solder's performance, particularly an increase in the solidus and liquidus temperatures, which deteriorates its wetting and spreading properties. Therefore, whether technical indicators such as the solidus and liquidus temperatures, the wetting and spreading properties of the solder, and the (tensile) shear strength of the brazed joint (generally compared with the "benchmark solder" in the industry) can achieve the performance of the "target product to be replaced" is the main evaluation indicator for measuring the success of the research and development of new cadmium-free silver solder.

[0008] Numerous research documents indicate that the conventional method for lowering the melting temperature (i.e., solidus and liquidus) of cadmium-free silver solder is to add at least one or more low-melting-point elements, such as Sn (melting point 231.9°C), In (melting point 156.6°C), Ga (melting point 29.76°C), Li (melting point 180.5°C), and Ni, which can lower the solder's melting point, to the main elements Ag, Cu, and Zn. Because In, Ga, and Li are rare elements, their annual global production is limited, and their prices are comparable to or higher than those of silver, they can only be added in small quantities. Sn, while relatively inexpensive compared to Ag and relatively abundant, can be added in large quantities. However, the formation of a hard and brittle intermetallic compound, Cu6Sn5, can make silver solder difficult to process. According to literature reports, except for BAg60CuSn solder, in which the Sn addition amount can reach 9.5% to 10.5%, the Sn addition amount in the Ag-Cu-Zn-Sn series of solders is generally 1.5% to 2.5%. Only in some individual types such as BAg56CuZnSn, the Sn addition amount is 4.5% to 5.5% (see GB / T 10046-2018 "Silver Solders", page 4, Table 1 (continued)).

[0009] Based on the market's great demand for "cadmium-free, low-silver solder" with a silver content of ≤5%, excellent wetting and spreading properties for common metals (such as copper, steel, stainless steel, etc.), high solder joint mechanical properties, and a solder liquidus temperature below 800°C (close to BAg25CuZn solder), the applicant has conducted extensive exploration and research on this and achieved beneficial results. This technical solution was invented in this context. Summary of the Invention

[0010] The task of the present invention is to provide a cadmium-free, low-silver solder with an Ag content in the range of 4.0% to 5.0% and a liquidus temperature of 800°C or less, which is suitable for brazing materials such as copper-brass (this application is mainly for H58 brass), copper-stainless steel (this application is mainly for 304 stainless steel), brass-stainless steel, and Q235 steel-stainless steel, so as to meet the needs of solder users for reducing costs, improving product quality, and enhancing product market competitiveness.

[0011] A comparative analysis and comparison of the currently valid versions of the EU RoHS2.0 directive and China's GB / T 10046-2018 "Silver Solder" standard found that among the silver solders that do not contain cadmium (i.e., Cd ≤ 0.010%) and whose main components are Ag, Cu, and Zn, the solidus temperature of BAg5CuZn(Si) solder is 820°C and the liquidus temperature is 870°C; the solidus temperature of BAg25CuZn solder is 700°C and the liquidus temperature is 790°C. As the Ag content decreases, such as in BAg20CuZn(Si) and BAg12CuZn(Si) solders, their liquidus temperatures increase to 810°C and 830°C, respectively. This can easily cause "overburning" and softening of the copper and brass base materials for brazing of copper and brass structures or workpieces (the melting point of copper is 1085°C, and the melting point of H58 brass is approximately 934-967°C). It can be seen that if technical means are used to reduce the liquidus temperature of AgCuZn solder with a silver content of ≤5% to below 800°C or even lower, the economic benefits and practical value will be very significant.

[0012] Task of the present invention is accomplished by:

[0013] A cadmium-free low-silver solder is characterized by comprising the following proportions by mass percentage: 4.0% to 5.0% of Ag, 40.0% to 45.0% of Zn, 11.0% to 13.0% of Sn, 0.5% to 1.5% of In, 0.5% to 1.5% of Ga, 0.5% to 1.5% of Ni, 0.01% to 0.05% of nano-CeO2, 0.01% to 0.05% of nano-Y2O3, and the balance being Cu.

[0014] Silver plates, cathode copper, zinc ingots, tin ingots, indium metal, gallium metal, and nickel metal are added in a proportioned, medium-frequency smelting process. Before casting, trace amounts of nano-CeO2 and nano-Y2O3 powders are added, thoroughly stirred, and then cast into an ingot. The desired solder wire is then obtained through extrusion and drawing. The solder has a solidus temperature of ≤735°C and a liquidus temperature of ≤780°C.

[0015] The silver plate used in the present invention has a purity of 99.99%, the cathode copper, zinc ingot, tin ingot, metal nickel, metal indium, and metal gallium have a purity of 99.9%, and the nano-CeO2 and nano-Y2O3 powders are commercially available products with a particle size of 100 to 200 nm and a purity of ≥99%.

[0016] The newly invented brazing filler metal has a solidus temperature of ≤735°C and a liquidus temperature of ≤780°C. When used with the commercially available FB102 flux, the shear strength of the brazed joints surpasses that of existing BAg25CuZn filler metal when brazing copper to stainless steel, copper to brass, brass to stainless steel, and Q235 steel to stainless steel. This brazing filler metal can meet the brazing needs of various structures, including copper to brass, copper to stainless steel, brass to stainless steel, and Q235 steel to stainless steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 The spreading performance and solder joint mechanical properties of the newly invented cadmium-free and low-silver solder on several typical materials are compared with the test data of BAg25CuZn solder. DETAILED DESCRIPTION

[0018] Compared with previous studies, the technical solution provided by this application creatively solves the following key technical problems:

[0019] 1) The researchers discovered that adding trace amounts of nano-CeO2 and nano-Y2O3 powders to a cadmium-free, low-silver solder with a silver content of 4.0% to 5.0% significantly refines the solder and the solder joint grains. This results in excellent wetting and spreading properties on materials such as copper, brass, Q235 steel, and 304 stainless steel, while significantly improving solder joint strength. The experiments revealed that the optimal particle size for commercially available nano-CeO2 and nano-Y2O3 is 100 to 200 nm, and the addition amount (mass percentage) is controlled between 0.01% and 0.05%, respectively. Excessive addition of CeO2 and Y2O3 can lead to poor spreading properties of the new solder and excessive solder strength, which can affect processing properties (such as extrusion and drawing).

[0020] Studies have shown that due to the large atomic radius of metallic rare earth elements, they cannot form a solid solution with the AgCuZn alloy. When added, metallic rare earth elements are typically "free" at the grain boundaries. When added in large quantities (or during segregation), the rare earth elements will form a rare earth phase with the parent metal. However, added rare earth oxides exist completely in a "free" state, and when added in large quantities, they may even exist as "inclusions." Since CeO2 has a melting point of 2397°C (some literature gives a melting point of 2600°C) and Y2O3 has a melting point of 2410°C, CeO2 and Y2O3 do not undergo a reduction reaction at the solder smelting temperature (silver solder smelting temperature is generally between 1000°C and 1300°C), and can remain stably present in the solder matrix. The technical solution provided by the present application is to combine mechanical stirring with the electromagnetic stirring force of the medium frequency furnace itself, so that CeO2 and Y2O3 nanoparticles with a particle size of 100 to 200 nm can be evenly dispersed and distributed in the grain boundaries of the newly invented solder grains. The average diameter of the grains is usually in the range of 0.015 to 0.25 mm, that is, 15,000 to 250,000 nm, thereby maximally refining the grains of the new solder and improving the strength of the new solder and the solder seam.

[0021] 2) Invented a method to suppress Cu6Sn5 in "high Sn" silver solder.

[0022] The industry believes that when the Sn content in silver solder exceeds 8%, the formation of the brittle intermetallic compound Cu6Sn5 makes extrusion and drawing of the silver solder difficult. By optimizing and selecting the addition amounts of the main component elements (Ag, Cu, Zn), the alloying element Sn, trace alloying elements (In, Ga, Ni), and nano-CeO2 and Y2O3 powders, this applicant has found that the addition of trace nano-CeO2 and Y2O3 powders can effectively suppress the formation of the hard and brittle intermetallic compound Cu6Sn5. Through observation of the microstructure of the new solder ingot, it was found that nano-CeO2 and Y2O3 were evenly dispersed at the grain boundaries of the AgCuZnSn alloy. The high-melting-point nano-CeO2 and Y2O3 acted as nucleation particles, greatly refining the grains of the AgCuZnSn alloy and effectively inhibiting the formation of Cu6Sn5 in the new solder. Even when the Sn addition reached 13%, it could still be smoothly extruded into a wire with a diameter of 2 mm.

[0023] At the same time, by utilizing the "synergistic effect" of low-melting-point elements Sn, In, and Ga, the solidus temperature of the newly invented solder is reduced to ≤735°C, and the liquidus temperature is reduced to ≤780°C. The wetting and spreading properties of the new solder and the mechanical properties of the brazing seam are significantly improved, thereby effectively avoiding the risk of "overburning" and softening of copper and brass (H58 brass) base materials during the brazing process. Its brazing performance can be guaranteed, which is conducive to the promotion and application of the newly invented cadmium-free and low-silver solder.

[0024] Experiments have found that the melting points of nano-CeO2 and Y2O3 are both above 2000°C. At the silver solder smelting temperature, they do not undergo a reduction reaction, nor can they form a "solid solution" or "intermetallic compound" with the AgCuZnSn-In-Ga-Ni alloy. Instead, they are dispersed at the grain boundaries of the AgCuZnSn-In-Ga-Ni alloy. Due to the "interfacial activity" of rare earth oxides, during the casting and solidification process of the new solder after smelting, the nano-CeO2 and Y2O3 act as "nucleation points" for crystallization, thereby refining the cast structure of the new solder. This ensures that even with a Sn content as high as 11.0% to 13.0%, the new solder still has good plasticity, allowing the ingot to be smoothly extruded into wire with a diameter of 2mm, and subsequent drawing processing is extremely excellent.

[0025] During brazing, the nano-CeO2 and Y2O3 in the new filler metal act as a surfactant, promoting the wetting and spreading of the silver filler metal on the base material. Compared to BAg25CuZn filler metal, the filler metal's spreading area is significantly increased by over 10% on copper, H58 brass, Q235 steel, and 304 stainless steel. The brazing joint tensile strength (σb) and shear strength (τ) are both increased by over 20% to 30%, reaching and exceeding the performance of BAg25CuZn filler metal. Figure 1 .

[0026] A specific embodiment of a cadmium-free, low-silver solder of the present invention that embodies the above-mentioned technical effects is as follows.

[0027] Example 1:

[0028] A cadmium-free low-silver solder is characterized by comprising, by mass percentage, 4.0% Ag, 45.0% Zn, 13.0% Sn, 0.5% In, 1.5% Ga, 0.5% Ni, 0.01% nano-CeO2, 0.05% nano-Y2O3, and the balance being Cu.

[0029] The cadmium-free, low-silver solder obtained using the above composition ratio has a solidus temperature ≤735°C and a liquidus temperature ≤780°C (both taking into account measurement errors). It has excellent wetting and spreading properties on copper, H58 brass, Q235 steel, and 304 stainless steel. Using flame brazing with FB102 flux, the braze joint strength for the following base material combinations is shown in the brackets: copper-H58 brass (σ b =255±10MPa, τ=250±10MPa), copper-304 stainless steel (σ b =340±10MPa, τ=345±10MPa), H58 brass-304 stainless steel (σ b=350±10MPa, τ=345±10MPa), Q235 steel-304 stainless steel (σ b =380±10MPa, τ=435±10MPa).

[0030] Example 2:

[0031] A cadmium-free low-silver solder is characterized by comprising, by mass percentage, 5.0% Ag, 40.0% Zn, 11.0% Sn, 1.5% In, 0.5% Ga, 1.5% Ni, 0.05% nano-CeO2, 0.01% nano-Y2O3, and the balance being Cu.

[0032] The cadmium-free, low-silver solder obtained using the above composition ratio has a solidus temperature ≤735°C and a liquidus temperature ≤780°C (both taking into account measurement errors). It has excellent wetting and spreading properties on copper, H58 brass, Q235 steel, and 304 stainless steel. Using flame brazing with FB102 flux, the braze joint strength for the following base material combinations is shown in the brackets: copper-H58 brass (σ b =255±10MPa, τ=250±10MPa), copper-304 stainless steel (σ b =340±10MPa, τ=345±10MPa), H58 brass-304 stainless steel (σ b =350±10MPa, τ=345±10MPa), Q235 steel-304 stainless steel (σ b =380±10MPa, τ=435±10MPa).

[0033] Example 3:

[0034] A cadmium-free low-silver solder is characterized by comprising, by mass percentage, 4.5% Ag, 42.0% Zn, 12.5% ​​Sn, 1.0% In, 1.0% Ga, 1.0% Ni, 0.02% nano-CeO2, 0.03% nano-Y2O3, and the balance being Cu.

[0035] The cadmium-free, low-silver solder obtained using the above composition ratio has a solidus temperature ≤735°C and a liquidus temperature ≤780°C (both taking into account measurement errors). It has excellent wetting and spreading properties on copper, H58 brass, Q235 steel, and 304 stainless steel. Using flame brazing with FB102 flux, the braze joint strength for the following base material combinations is shown in the brackets: copper-H58 brass (σ b =255±10MPa, τ=250±10MPa), copper-304 stainless steel (σ b=340±10MPa, τ=345±10MPa), H58 brass-304 stainless steel (σ b =350±10MPa, τ=345±10MPa), Q235 steel-304 stainless steel (σ b =380±10MPa, τ=435±10MPa).

[0036] Example 4:

[0037] A cadmium-free low-silver solder is characterized by comprising, by mass percentage, 4.6% of Ag, 43.0% of Zn, 12.0% of Sn, 0.8% of In, 1.2% of Ga, 1.1% of Ni, 0.03% of nano-CeO2, 0.02% of nano-Y2O3, and the balance being Cu.

[0038] The cadmium-free, low-silver solder obtained using the above composition ratio has a solidus temperature ≤735°C and a liquidus temperature ≤780°C (both taking into account measurement errors). It has excellent wetting and spreading properties on copper, H58 brass, Q235 steel, and 304 stainless steel. Using flame brazing with FB102 flux, the braze joint strength for the following base material combinations is shown in the brackets: copper-H58 brass (σ b =255±10MPa, τ=250±10MPa), copper-304 stainless steel (σ b =340±10MPa, τ=345±10MPa), H58 brass-304 stainless steel (σ b =350±10MPa, τ=345±10MPa), Q235 steel-304 stainless steel (σ b =380±10MPa, τ=435±10MPa).

[0039] The technical advantages and creativity of the present invention are Figure 1 The solder spreading performance (spreading area, mm) of Examples 1 to 4 and commercially available BAg25CuZn solder 2 ) and the comparison data of brazing seam strength, the advancement of this technical solution can be more clearly seen.

Claims

1. A cadmium-free low-silver solder, characterized in that: The proportions by mass percentage are: 4.0% to 5.0% Ag, 40.0% to 45.0% Zn, 11.0% to 13.0% Sn, 0.5% to 1.5% In, 0.5% to 1.5% Ga, 0.5% to 1.5% Ni, 0.01% to 0.05% nano-CeO2, 0.01% to 0.05% nano-Y2O3, and the balance is Cu.

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