Abstract
The film removal mechanism of FB3-F silver brazing flux consisting of H3BO3, K2B4O7, and KF with the mass ratio of 7:10:3 was studied. Results show that K2B4O7 or KF cannot individually remove the oxide film on Q235 steel at 700 °C, and KF even accelerates the oxidation rate of steel surface at high temperature. H3BO3 can remove the oxide film at 700 °C, but the product possesses an obvious amorphous structure feature and poor fluidity. Furthermore, H3BO3 can react with KF at 700 °C, and the reaction product can remove the oxide film. Similarly, K2B4O7 can react with KF, but the product is hard. The mixture solder consisting of H3BO3, K2B4O7, and KF can react with the oxide film on surface of Q235 steel plate, and the reaction product presents obvious amorphous feature. The addition of KF transforms the nonreactive H3BO3-K2B4O7 binary system into the reactive KF-H3BO3-K2B4O7 ternary system at 700 °C. KF shows no corrosivity and promotes the removal of oxide film of steel plates in this ternary system.
Science Press
Furnace brazing has been irreplaceably used in aerospace, refrigeration, electronics, and other fields due to properties of uniform heating and low-cost in mass produc
However, KF-containing flux has the risk of significant increase in hygroscopicity, resulting in the fact that the FB3-F flux normally becomes agglomerate during storag
Research on film removal mechanism is a key point for the modification of silver flux. However, a lot of studies focused on the low-temperature reaction mechanism of silver brazing flux. Chen et a
In this research, the melting characteristics of FB3-F brazing flux were investigated, and the film removal mechanism of FB3-F silver brazing flux at 700 °C was studied.
FB3-F flux was composed of K2B4O7, H3BO3, and KF according to the mass ratio of 10:7:3. A three-dimensional powder mixer was used to prepare FB3-F flux. Rotation speed was 20 r/min and the mixing time was 2 h.
Q235 steel plates with the size of 40 mm×40 mm×2 mm were prepared as base material. The chemical composition of Q235 steel is listed in
A box-type resistance furnace (SX2-5-13C) was used to analyze the melting characteristic of FB3-F flux. The test temperature started at 850 °C and then decreased by 10 °C every 5 min until the specimen was completely melted.
X-ray diffraction (XRD, Smart lab 9K) equipped with Cu Kα radiation was used to analyze the film removal mechanism of FB3-F brazing flux.
The surface morphologies of Q235 steel plates after film removal at different temperatures are shown in
Fig.1 Surface morphologies of Q235 steel plate after film removal at 800 °C (a) and 700 °C (b) at 1:1 scale
The XRD pattern of the surface oxide of Q235 steel at 700 °C is shown in Fig.3. There are Fe3O4, Fe2O3, and FeO in the product, which is consistent with the results of Xie et a
Fig.
| 2H3BO3→B2O3+3H2O | (1) |
| MO+B2O3→MO·B2O3 | (2) |
Fig.4 Surface morphologies of Q235 steel plate after reaction with different flux at 700 °C: (a) K2B4O7, (b) KF, and (c) H3BO3 at 1:1 scale
Fig.6 Surface morphologies of Q235 steel plate after reaction with different mixed flux at 700 °C: (a) K2B4O7+H3BO3, (b) KF+H3BO3, and (c) KF+K2B4O7 at 1:1 scale
According to the XRD pattern in Fig.8, Fe2F5, B2O3, and KF exist in the product. F element replaces O element in the oxide, resulting in the formation of Fe2F5. However, KF cannot react with the oxide film individually. Hence, the F element is in the reaction product of KF and H3BO3. The reaction formulae are as follows:
| H3BO3+KF→H3BO3(KF)n | (3) |
| H3BO3(KF)n+FexOy→Fe2F5+FexOy·B2O3 | (4) |
The condition is 0<n<1 for the above formulae. When x=1, 2, 3, y=1, 3, 4, respectively. Thus FexOy denotes the FeO, Fe2O3, and Fe3O4.
However, the peaks of KF and B2O3 in the product present crystal and amorphous features, respectively, suggesting the surplus KF after reaction with the mixture of this mass ratio.
| K2B4O7→B2O3+2KBO2 | (5) |
| MO+B2O3+2KBO2→(KBO2)2·M(BO2)2 | (6) |
Fe2F5 can be observed in the product, indicating that F element in the formation of KF and K2B4O7 replaces the O element in the oxide. The related reactions are as follows:
| K2B4O7+KF→K2B4O7(KF)n | (7) |
| K2B4O7(KF)n+FexOy→Fe2F5+FeO(BO2)2 | (8) |
The condition is 0<n<1 for the above formulae. When x=1, 2, 3, y=1, 3, 4, respectively.
FB3-F silver brazing flux shows good performance of removing oxide film at 700 °C, as shown in
Therefore, the KF or mixture of H3BO3 and K2B4O7 cannot remove oxide film. Although the mixture of KF and H3BO3 and the mixture of KF and K2B4O7 can remove the oxide film, their reaction products present poor fluidity. The mixture of K2B4O7, H3BO3, and KF with the mass ratio of 10:7:3 can remove the oxide film. Therefore, KF plays an important role in the process of film removal. The addition of KF can transform nonreactive H3BO3-K2B4O7 binary system into reactive KF-H3BO3-K2B4O7 ternary system at 700 °C. Meanwhile, in this ternary system, KF shows no corrosivity to steel plates and it promotes the film removal process.
1) Individual KF or K2B4O7 cannot remove the oxide film at 700 °C. KF can accelerate the oxidation rate of steel surface at high temperature. H3BO3 can remove the oxide film at 700 °C, but the reaction product has poor fluidity.
2) The mixture of KF and H3BO3 (mass ratio of 3:7) and the mixture of KF and K2B4O7 (mass ratio of 3:10) can remove the oxide film on the Q235 steel surface at 700 °C. But their reaction products present poor fluidity
3) The addition of KF can transform the nonreactive H3BO3-K2B4O7 binary system into reactive KF-H3BO3-K2B4O7 ternary system at 700 °C. KF shows no corrosivity and promotes the removal of oxide film of steel plates in this ternary system.
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