The direct fusion welding of tantalum and niobium easily causes solidification cracks during cooling. This cracking defect of bismuth often leads to welding failure. In addition, if the filler is not added with the filler material, that is, or the reasonable welding method and process parameters are adopted, it is still difficult to make the welding of the crucible succeed. This shows that the welding process is very difficult to achieve in the process. The main reason is: é“ direct melting welding, equivalent to the casting smelting process, easy to form a thick columnar crystal structure in the melting zone, combined with the brittleness of the bismuth material and the complex thermophysical properties, can not withstand the welding thermal stress and thermal deformation The role. During the welding process, due to the high temperature state, the metallurgical chemical reaction with the gaseous medium of the surrounding environment causes the tantalum weld to be contaminated again. These contaminants are stirred into the molten pool by welding and are present in the weld in the form of inclusions, making it even more difficult to weld. As early as the end of the 1950s, in the initial period of enamel welding, foreign countries used fusion welding without ruthenium filler material [1] . The welding method used was a relatively advanced vacuum electron beam welding and gas shielded welding at that time, and preheating measures were also implemented during the welding process. The results show that most of the welding experiments have not been successful with the direct fusion welding of the crucible without filling materials. Although the individual welding samples are not cracked, the control measures of the process are quite complicated. In the 1980s, foreign countries did not use filler materials when conducting spot welding tests on enamel, and as a result, the proportion of successful welding did not increase significantly. According to this situation, people try to use the filler material to weld the crucible. As long as the appropriate welding filler material is added, the probability of successful welding can be greatly increased by a reasonable welding method and a suitable process. The main reason for its success is that the filler material inhibits the crystal microcracks of the tantalum weld and prevents cracking of the tantalum weld. The following is an analysis and discussion on the basic selection principles and types of filler materials used in the welding and the interaction between the filler and the crucible in the welding process.
2 Principles of selection of filling materials
The use of any metal or alloy as a weld filler material is the key to successful brazing. As early as the 1960s and 1970s, process researchers engaged in tantalum welding conducted extensive research work on filler materials used in tantalum welding [2, 3, 4, 5] . At that time, the advanced EB (electron beam) welding and TIG (argon arc) welding technology were used for experimental verification. Later, after the development of laser technology became more mature, the laser welding research was carried out. Laser welding uses the results of electron beam welding and TIG welding in the use of filler materials. Through the summary and theoretical analysis of experimental techniques, the selection principle of bismuth welding filler materials is formed. The following three articles are summarized:
1) The filler material can wet the base metal well in a liquid state.
2) The filler material used cannot form brittle intermetallic compounds with niobium at high temperatures.
3) The melting point of the filler material is preferably lower than the melting point of the base material.
According to the above three basic principles, when selecting a brazing filler material, first consider some metals and alloys that form a eutectic alloy with bismuth, such as pure aluminum, Al-Si alloy, and the like.
3 Performance analysis of aluminum and Al-Si alloy filling materials
According to the theoretical and experimental studies of the binary alloy phase diagram [6] of bismuth, it is shown that a relatively good filler material should be able to form a eutectic alloy with a class of metal materials. It is best to avoid the use of materials that form intermetallic compounds with ruthenium. So far, the filler materials used for brazing of tantalum are only pure aluminum, Al-Si alloy, Al-12Si-1.5Mg alloy, pure Ag, Ag-Cu alloy, etc., but most of them are used. Aluminum alloy filling material.
3.1 physical and chemical properties and nuclear properties of pure aluminum filling materials
Pure aluminum is a low-density material, and the reserves of aluminum on the earth are quite large. The technology for manufacturing and smelting aluminum is currently being studied in depth. In fact, aluminum was serialized in the mid-20th century. Therefore, the use of aluminum as a solder filler material is very cheap. Aluminum is located in the third periodic IIIA element in the periodic table, with an atomic number of 13, an atomic weight of 26.98154, and a peripheral electronic configuration of the aluminum atom of 3S 2 3P 1 . The 13 electrons of aluminum are distributed in each layer orbit as 1S 2 2S 2 2P 6 3S 2 3P 1 . If two 3S electrons and one 3P electron are lost at the same time, a divalent aluminum ion (Al 2+ ) is generated. If one 3P electron is lost, a monovalent aluminum ion (Al + ) is generated. Low-cost aluminum ions are generally unstable at low temperatures. Aluminum is a face-centered cubic lattice metal with a lattice parameter of 4.04956×10 -10 m; when the volume is 999.6 mm 3 /mol atom, its density is 2.6987 g/cm 3 ; the specific strength of aluminum (tensile strength and The ratio of density - σ b / γ) is high. It has good thermal and electrical conductivity and its thermal conductivity is about 10 times that of stainless steel. The thermal conductivity of solid aluminum at room temperature is 2.35-2.237×10 -2 W/(mK); near the melting point, the thermal conductivity will be reduced to 2.1×10 -2 W/(mK); the thermal conductivity of liquid aluminum It is much smaller than solid aluminum, only 0.9×10 -2 W/(mK) near the melting point; at 1250K, it is increased to 1.0×10 -2 W/(mK). Aluminum has a strong ability to reflect light and heat and reflects 95% of the hot wire. Pure aluminum is not magnetic and does not create an additional magnetic field. Aluminum has a ductility of up to 25% and can be processed into a wire or sheet material by forging, extrusion and rolling. Aluminum has the ability to adsorb environmental moisture, and its high temperature melt has a strong hydrogen absorption capacity.
The heat of fusion and melting entropy of aluminum: at 933 K, the heat of fusion of aluminum is 10.71 ± 0.21 KJ / mol atom (or 396 J / g); the melting entropy is 11.5 J / (mol atom. K). The heat of evaporation of aluminum is 306 KJ/mol atom (or 113 J/g;); the evaporation entropy is 112 J/(mol atom.K).
Specific heat capacity: in the interval of 298-933K, the heat capacity of solid aluminum changes linearly with temperature
                        Cp=a+bt (1)
Where a = 4.94 and b = 2.96 x 10 -3 . The heat capacity of liquid aluminum is approximately 31.76 J/(mol.K). It increases as the temperature increases.
From the viewpoint of nuclear properties, the thermal neutron absorption cross section of aluminum is 0.22 target. When pure tantalum is used as the filling material to weld the crucible, the pure aluminum and the crucible melt and solidify and crystallize, and a eutectic reaction occurs, and the formed alloy is a binary eutectic alloy. However, in actual welding, there is a lot of segregation in the microstructure of the weld, depending on the amount of melting of the crucible and aluminum. Upon analysis, the weld has a eutectic composition or a side of the hypereutectic composition that deviates from the eutectic point. It was also found in the experiment that with pure aluminum as the filling material, the fluidity after melting at a high temperature is not as good as that of the Al-Si alloy, and the interstitial ability is worse than that of the Al-Si alloy.
3.2 Analysis of oxidation pollution of aluminum
At room temperature, aluminum has a significant tendency to oxidize. The oxidation reaction of the aluminum surface is actually weakened after 2 hours, and the thickness of the oxide film at this time is 2.5 to 5.0 nm. In the presence of moisture, the oxide film can be as thick as 10 nm. After 14 days, the thickness of the oxide film tends to be stable. Aluminum generally contains 0.002-0.02% by mass of gas, and a thin layer of oxide present on the surface can form oxide inclusions in the weld if it is not cleaned before welding. At room temperature, the aluminum surface forms a dense Al 2 O 3 oxide whose structure is amorphous. The thickness of the Al 2 O 3 oxide on the aluminum surface is 2-10 nm. As the temperature increases, the thickness of the oxide increases continuously. When the temperature is 500 ° C, the thickness of the oxide film increases to 30 nm; when the temperature reaches or approaches the melting point The thickness of the oxide can be increased to about 200 nm. The Al 2 O 3 oxide exhibits completely different properties from pure aluminum. As the temperature increases, the Al 2 O 3 oxide undergoes α, β, γ, and γ' phase transitions, and the 700-710 ° C transitions to γ-Al. 2 O 3 . When the temperature is higher than 900 ° C, the conversion to the α-Al 2 O 3 structure begins. Pure aluminum does not undergo a phase change from room temperature to melting point. Regardless of the chemical composition Al 2 O 3 and an oxide phase changes which produce, there is always a small amount of aluminum or some of the surface oxides, understand some surface properties Al 2 O 3 oxide, beryllium welding is significant. Aluminum has a strong interaction with oxygen and undergoes three different processes: (1) oxygen collides with fresh and clean aluminum surfaces (physical adsorption); (2) chemically forms a dissociated oxide film ( Chemical adsorption and chemical reaction); (3) The oxide film is thickened with time.
Al 2 O 3 oxide has the following characteristics: (1) The protective property of Al 2 O 3 oxide is good, and in a certain oxidation stage, the further action of aluminum and gas can be prevented by virtue of this characteristic of oxide; (2) Chemical stability and high temperature stability are good. It is almost impossible to reduce aluminum from Al 2 O 3 oxide during welding; (3) High melting temperature, aluminum filler and tantalum material have already melted, Al 2 O 3 oxide Also in solid state; (4) Al 2 O 3 oxide has low solubility in liquid aluminum and solid aluminum, plasticity is lower than aluminum, and has higher hardness and brittleness; (5) the coefficient of linear expansion is only 1/3 of aluminum. When welding is heated, Al 2 O 3 oxide sometimes cracks; (6) Al 2 O 3 oxide has a relatively strong ability to adsorb water vapor.
Aluminum has a high solubility in hydrogen in liquid state. It has been reported [7] that the hydrogen content in aluminum alloy can account for more than 85%. If it is 0.034 ml/100 g Al in the solid state, the solubility in the liquid state is 0.65 ml/100 g Al. The difference between the two is 19.1 times. The hydrogen in aluminum is mainly derived from the reaction of aluminum liquid with water vapor. The ratio of gas partial pressure in liquid aluminum is: P H2 /P H2O = 7.3×10 14 , which indicates that even if P H20 is small, balanced P H2 can be reached. Very big. When the temperature of the aluminum liquid rises to 727 ° C, the aluminum liquid can react with water vapor under the conditions equivalent to dry air (P H2O = 2.59 × 10 -20 Pa). This means that either the rather dry environment or the walls of the drying vessel are wet to the aluminum liquid and also cause hydrogen absorption.
Al 2 O 3 oxide is often present in the weld in the form of inclusions under the action of welding agitation. Studies have shown that there is a symbiotic relationship between oxides in the aluminum liquid and gaseous hydrogen. Aluminum is easily contaminated with Al 2 O 3 oxides and gaseous hydrogen, so both are difficult to remove in aluminum. The oxide film on the surface of the liquid aluminum is dense against the aluminum liquid layer and has a protective effect on the aluminum liquid. However, the outer oxide film is loose, and there is a small pinhole of Φ5-10 nm in the oxide film, which is occupied by hydrogen, air and water vapor. Therefore, the alumina film usually contains at least 1% to 2% of water vapor. In this way, Al 2 O 3 oxide plays an important role in the formation of weld pores. Hydrogen attachment to oxide nucleation is mainly considered from the thermodynamic point of view. The behavior and interaction mechanism between oxide and gas at high temperature in aluminum must be analyzed from the characteristics and structure of the oxide. According to the form of oxides, they can be divided into three categories: 1) large oxides (>20μm) with uneven distribution. These oxides are extremely harmful, but easy to remove; 2) the size is 10-20μm. The oxide; 3) contains an oxide having a size of <10 μm. When these three types of oxides are welded, they are easily mixed into the molten pool by stirring force, which causes the weld to increase gas and oxide inclusions. (2) Reaction of aluminum with oxygen: 4Al+3O 2 →2Al 2 O 3 . Aluminum alloy is easily oxidized to form alumina in air and during welding. It is characterized by high melting point, very stable, moisture absorption and difficult to remove. It prevents the wetting of the crucible and creates pores in the crucible weld. Al 2 O 3 is a variant of α and β with a higher density than aluminum (3.9-4.0 g/cm 3 ) and a melting point of up to 2050. 2) Reacts with water: 2Al+6H 2 O→2Al(OH) 3 +3H 2 ↑, The molten aluminum reacts violently with the surrounding water vapor.
3.3 Properties, structure and hydrogen absorption characteristics of Al-12Si alloy filler
The use of Al-12Si alloy as a filling material for welding ruthenium can effectively suppress microcracks in the ruthenium weld and prevent ruthenium weld cracking. The melting point difference between the Al-Si alloy and the niobium is very large. During the solder cooling process, the Al-Si alloy is still in a liquid state when the liquid helium begins to solidify and nucleate. The liquid Al-Si alloy is used to fill the microcracks of the solidified tantalum. Therefore, the Al-Si alloy is a relatively successful filler material for tantalum welding. From the 1960s until now, Al-Si alloys have been used as filler materials for welding borings, no matter how the method of welding enamel changes. The Al-Si alloy has a high silicon content, which increases the fluidity in the liquid state, the heat shrinkage is smaller than that of aluminum, the airtightness of the weld is good, and the tendency of thermal cracking is small. Al-Si alloys have excellent physical properties, mechanical properties and processability after heat treatment under appropriate conditions. Compared with other aluminum alloys, its corrosion resistance is also better. In the soldering of tantalum, eutectic reaction occurs between aluminum and tantalum, niobium and silicon, and silicon and aluminum without the formation of intermetallic compounds. Considering the nuclear performance, the addition of filler Al-Si alloy has little effect on the nuclear properties. Because aluminum is a low-density material, the neutron absorption cross section is 0.22. The addition of silicon does not affect the overall nuclear performance of Al-Si alloy. Because the thermal neutron absorption cross section of silicon is smaller than aluminum, only 0.13 target. Therefore, the Al-Si alloy is a well-recognized filler material for welded niobium.
Si belongs to the face-centered cubic lattice, and although it belongs to the facet phase, the number of Jackson factors of the {111} close-packed surface is not high. The {111} plane of the Si crystal is a smooth interface, and the {100} and {111} planes are rough interfaces. In Al-Si alloys, there are differences in the growth behavior and composition exhibited by the solidification conditions and composition of the silicon. For the Al-Si alloy which has not been modified, the eutectic Si is in the form of a thick slab, and a small amount of twin crystals are present in the Si crystal. The flaky eutectic Si possesses two types of branches: 1) the large-angle branching behavior associated with the twinning behavior, at an angle of 70.5o with the {111} densely packed surface; 2) the thermal expansion coefficient due to the Si phase and the Al phase Differently, these behaviors also lead to the existence of small-angle branching, splitting, and parallel behavior of the two.
   In the early 1980s, based on the interface dynamics, the theory of facet-non-facet transformation was proposed. According to the theory, as the growth rate increases, Si has a small surface growth transition to non-small surface growth. The change in the appearance and size of Si is closely related to the eutectic subcooling during solidification. In the case of less subcooling, the Si phase grows in a faceted growth mode; when the degree of subcooling increases, Si grows in a non-small form of uniform growth. The modification of Al-Si alloy can change the morphology and size of Si, such as adding Na, Sr, Re and other elements in Al-Si alloy [8,9] , the eutectic temperature in the alloy (in the cooling curve) The eutectic platform is much lower than the unaltered, so that the eutectic supercooling is increased, and the eutectic Si is transformed from a coarse lath (or needle) into a fine fibrous shape, that is, the growth mode of the eutectic Si occurs. Changed.
However, Al-Si alloys for tantalum welding are required to be high, and elements such as Na, Sr, Re, etc. are not expected to exist because their presence may form a new source of corrosion in the weld, and the use of welded members. Will have an adverse effect. Therefore, other methods must be employed to improve the morphology and size of the eutectic Si in the Al-Si alloy of the filler material of the solder joint. The reaction of silicon with O 2 produces two different oxides of silicon: 1) 2Si + O 2 → 2SiO; 2) 2Si + O 2 → 2SiO 2 . The color of SiO is black or brownish black, which has also been encountered in the treatment of Al-Si alloys. The reaction of Si and O 2 is at 400 . Above C. The aluminum in the Al-Si alloy reacts with water: 2Al+6H 2 O→2Al(OH) 3 +3H 2 ↑, the molten aluminum reacts violently with the surrounding water vapor, and Si reacts with water to form SiO 2 and H 2 ↑. At high temperatures, Si also reacts with water vapor to produce H 2 ↑.
The macroscopic microstructure, grain size, impurity content, alloy homogenization and apparent quality of the Al-Si alloy have an important influence on the mechanical properties, corrosion resistance, performance and surface quality of the welded joint. Al-Si alloy is a typical eutectic alloy. The eutectic composition was Al+12.5% ​​Si (mass fraction, %) and the eutectic temperature was 577 °C. At this temperature, the alloy liquid alternately crystallizes α-Al and the second phase β-Si, and Al forms a eutectic with Si. α-Al is a solid solution of silicon in aluminum, and β-Si is a solid solution of aluminum in silicon. The amount of β-Si phase is small and brittle; the content of α-Al phase is high, but it is very soft. The α phase and the β phase formed by the eutectic transformation are a three-dimensional network structure and a dendritic structure. It can be seen from the phase diagram of the binary alloy of Al-Si alloy that the solubility of Si in aluminum is 1.65% at equilibrium temperature at eutectic temperature, and decreases to 0.05% when cooled to room temperature, while aluminum is in β-Si. Very low solubility. The solidified crystals form a supersaturated solid solution under non-equilibrium conditions, and the content of Si can reach about 3%. In fact, the Al-Si alloy can be understood as follows: the structure of the as-cast Al-Si alloy is a mixture of metallic aluminum (α phase) and single crystal silicon (β phase) by melting at room temperature.
Laser welding under non-vacuum conditions, plus Al-S¡ The main defects of alloy filler materials in welds are weld pores and shrinkage cavities. It has long been known that pure aluminum absorbs hydrogen in the environment during heating during welding or casting. When cooled, the melt releases hydrogen to form hydrogen pores characterized by hydrogen, thereby affecting the quality of aluminum processing. This indicates that the pores formed by the welding of aluminum and aluminum alloy are mainly related to the hydrogen content.
   Literature [10] reported that Si can reduce the solubility of hydrogen in aluminum melt and inhibit the hydrogen absorption capacity of aluminum melt. Meng Qingge, Bian Xiufang and others measured and analyzed the hydrogen content of the self-made Al-Si alloy melt. The relative humidity of the environment at the time of measurement was about 55% (Shandong area). They measured the hydrogen content in the melt of Al-Si alloy at three different temperatures: (1) The hydrogen content of the liquid in the Al-Si alloy was measured; 2) The hydrogen content of the Al-Si alloy melt at a temperature of 10 ° C was measured; (3) The hydrogen content in the melt of the Al-Si alloy at a temperature of 100 ° C was measured. The results show that the curves of hydrogen content in the Al-Si alloy melts in the three temperature ranges have a similar relationship to the liquidus changes. The metallographic analysis and observation of the porosity of the hypoeutectic, eutectic and hypereutectic regions were carried out, and similar results were obtained: the porosity was the smallest near the eutectic point; in the hypoeutectic region The porosity of the hypereutectic region increases correspondingly.
   Hydrogen exists in the Al-Si alloy melt in three ways, namely atomic hydrogen, hydrogen in the compound, and hydrogen in the compound state. Since the material melt contains inclusion elements but the content thereof is relatively small, the hydrogen present in the form of the compound in the sample is relatively small and can be ignored. Hydrogen is mainly present in the Al alloy in the form of a gap solid solution. Meng Qingge and Bian Xiufang conducted X-ray diffraction analysis on the liquid structure of Al-Si alloy, and obtained the internal relationship between Si content and hydrogen content in Al-Si alloy melt. Conclusion: (1) The amount of hydrogen in the melt of aluminum alloy or the formation rate of pores is related to the atomic density of Al-Si alloy under different overheating conditions. As the Si content increases, the atomic density gradually increases, reaching a maximum near the eutectic region. Thereafter, as the Si content increases, the atomic density gradually decreases. The higher the atomic density, the smaller the hydrogen content. As the temperature increases, the atomic density decreases, resulting in an increase in hydrogen content. However, when the temperature is higher than 875 ° C, the rate of decrease in atomic density is slowed down. (2) In the alloy of a given composition, for the hydrogen content curve of different superheats, as the superheat degree increases, the curve of the hydrogen content will move up.
The laser welding or TIG welding of bismuth is carried out by adding an Al-Si alloy, and the protection condition is not in a vacuum state but in a gas-protected state. There are different numbers of air holes in the weld after welding, and there are also shrinkage holes in the root of the weld. Sometimes inclusions (especially non-metallic inclusions) are present. The presence of the above defects often leads to deterioration of the mechanical properties of the weld and reduces the airtightness and corrosion resistance of the weld.
4 thickness of the filling material
The thickness of the filler material has a great influence on the quality of the weld. Zhang Youshou et al. [20] used a 0.2-1.0mm thick Al-Si alloy filling material to conduct laser beam welding experiments, and obtained three results: (1) Adding Al- according to the beam spot diameter of the high energy beam The thickness of the Si alloy sheet. After the laser beam, electron beam and microbeam plasma welding are focused, the diameter of the beam spot is very small, usually only a few tenths of a millimeter or even finer. If the filling material is thick, the beam spot can only be irradiated onto the filling material, only heating and melting. The filler material, while the base metal itself melts very little, often results in poor solder joints or no connection. (2) For laser welding, the reflectivity of the laser to the Al-Si alloy and the tantalum material is different. At the time of welding, at least two-thirds of the Gaussian peak of the laser energy should be irradiated onto the Al-Si alloy filler sheet. The rest of the laser energy is irradiated onto the base metal so that a reasonable distribution of the laser energy can be formed during welding, so that the quality of the welded joint can be ensured. (3) Control and select the thickness of the filling material according to the composition of the filling material in the é“ weld. First, it must be confirmed that the thickness of the filler material added does not cause the weld of the crucible to crack. On this basis, the thickness of the filling material is determined. It has been reported that the tantalum weld does not crack when the average content of the Al-Si alloy in the tantalum weld is greater than 20%. The optimum thickness of the filler material used for the electron beam and laser beam welding of the crucible determined by the test should be 0.3-0.4 mm. When the thickness of the Al-Si alloy sheet is less than 0.3 mm, the average aluminum content of the filler in the weld is lowered, and the tendency to suppress cracking of the weld is small. When the thickness of the filling material is greater than 0.8 mm, the beam spot of the high energy beam density welding is only irradiated onto the filling material, and only the portion of the filling material is heated and melted, while the base material of the crucible is relatively melted relatively little, and it is difficult to form a good connection or cause Not fused or even desoldered. Gas-shielded tungsten arc welding is adopted. Due to the large heat input of the weld, the depth and width of the weld are relatively small, the penetration depth of the weld is relatively shallow, and the amount of the base metal is more melted, so the thickness of the filler material can be appropriately Add some. Zhang Youshou et al. used a different thickness of Al-Si alloy filler material such as 0.4-1.0mm in the tungsten arc welding of tantalum to prevent cracking of the tantalum.
   In the Al-Si alloy, when the content of silicon is ≤ 5%, the fluidity of the alloy is not so good. When the silicon content is in the range of 5% to 15%, the fluidity also increases as the silicon content increases. When the silicon content reaches 15% of the hypereutectic composition, the fluidity is the best. When the content of silicon exceeds 15%, the fluidity is rather reduced. The reason for the decrease in fluidity is: (1) The latent heat of fusion of Si is much larger than the latent heat of melting of the base metal Al, so that the fluidity of the alloy liquid becomes better as the Si content increases. (2) The fluidity of liquid metal can be expressed by the length of flow under a certain condition. The maximum value of the length is not in the range of the eutectic composition (12% Si content), but is shifted to the right side of the hypereutectic composition (Si content is 15%), because the Al-Si alloy is quenched in the quench. Under the condition, the eutectic point is shifted to the side of the hypereutectic.
5 method of adding filler material
From the point of view of the welding process of the crucible, there are some restrictions on the method of adding the filling material. The wire feeding mechanism or the powder feeding type of the filling material which is often used in the welding industry is rarely used. In addition, since the filler electrode is not welded at the present time, the weldment cannot be opened into a V-groove when welding.
5.1 Clip-on join.
When machining the jaws, leave a margin for the thickness of the filler. In the processing of the welded parts of the crucible, the Al-Si alloy sheets are also processed by machining. When processing, several pieces of 0.4mm thick Al-Si alloy can be overlapped and installed in a specially designed special fixture, so that multiple pieces of filling material can be processed at one time, and Al-Si alloy deformation can be prevented. . Before the welding, the processed filling material is cleaned and degassed, and then assembled and welded. The filler material is sandwiched and the quality of the weld is good. The content of the Al-Si alloy in the weld pool is relatively uniform.
5.2 Feed-in Â
é“ After the welding, the quality is detected, and the resulting stomata defects need to be repaired. When the welding is performed, the Al-Si alloy powder material can be added by the powder feeding mechanism . The filling material is directly fed into the air holes, and then the powder material is irradiated with a laser beam to melt and seal the air holes. Filling the filler with the powder material, the weld is not well formed, the uniformity of the weld composition is poor, the wetting effect of the powder material and the original weld (the weld formed by the melting of bismuth aluminum silicon) is poor, and the surface ratio of the powder material is poor. Big easy oxidation and other issues. However, since the air hole is a local position or a minute area in the entire weld, there is a pile of the filling material at the local position during the repair welding, and repairing by mechanical grinding after the repair welding is allowed. In addition, it is also possible to use a less corrosive flux to remove the oxide film on the weld bead, the crucible and the solder surface, improve the wettability of the filler to the crucible, and improve the brazing quality.
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