1 Introduction
Direct melting and welding of crucibles and crucibles can easily cause solidification cracks during cooling. This cracking defect of niobium often leads to welding failure. In addition, if you do not add filler materials for the welding of crucibles, that is, take reasonable welding methods and process parameters, it is still difficult to make crucible welding success. This shows that the welding process is difficult to achieve in the process. The main reasons are: direct melting and welding, which is equivalent to the process of casting and smelting, easy to form a large columnar crystal structure in the melting zone, combined with the joint action of the brittleness and complex thermophysical properties of the crucible material, can not withstand the welding thermal stress and thermal deformation The role. During the welding process, due to the high temperature state of the crucible, a metallurgical chemical reaction occurs with the gaseous medium in the surrounding environment, so that the crucible weld seam is again contaminated. These contaminants are stirred into the molten pool by welding and are present in the weld as inclusions, making the flaws that are otherwise difficult to weld even worse. As early as the end of the 1950s, in the initial stage of the welding of helium, foreign countries used the fusion welding of helium without fillers [1]. The welding methods used were the more advanced vacuum electron beam welding and gas shield welding at the 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 melting and welding of niobium without filler material. Although there are occasional individual welding samples without cracking, the control measures for the process are quite complicated. In the 1980s, foreign countries did not use filler materials when carrying out spot welding tests with niobium using laser beams. As a result, the proportion of welding successes did not increase significantly. According to this situation, people try to use the filler material to weld the crucible. As long as the proper welding filler material is added, the welding success rate can be greatly increased by the use of reasonable welding methods and appropriate processes. The main reason for its success is that the filler material inhibits the crystallization microcracks in the beryllium weld and prevents cracking of the beryllium weld. The following will discuss and discuss the basic selection principles and types of filler materials used in welding, the interactions between the filler materials and the crucible in the welding process.
2 Selection principles of filling materials
What kind of metal or alloy is used as the welding filler material is the key to successful welding. As early as the 1960s and 1970s, researchers engaged in helium welding performed extensive research on the filler materials used in helium welding [2,3,4,5]. At that time, the use of the more advanced EB (electron beam) welding, TIG (argon arc) welding technology for experimental verification. Later, after the development of laser technology became more mature, the laser welding research of germanium was carried out. Laser welding In the use of filler materials, the research results of electron beam welding and TIG welding are cited. Through summarizing and theoretical analysis of experimental techniques, the principle of selection of helium welding filler materials has been formed. The following three are summarized:
1) The filler material wets the base material very well in the 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 the crucible welding filler material, first consider some metals and alloys that can form a eutectic alloy with germanium, such as pure aluminum, Al-Si alloy, and the like.
3 Performance Analysis of Aluminum and Al-Si Alloy Fillers
According to the theoretical and experimental studies of the binary alloy phase diagram of niobium [6], it is shown that a relatively good filling material should be able to form a eutectic alloy with a niobium alloy. It is better to avoid using a material that forms an intermetallic compound with ruthenium. So far, only a few kinds of fillers have been used for brazing brazing, such as pure aluminum, Al-Si alloy, Al-12Si-1.5Mg alloy, pure Ag, and Ag-Cu alloy. Aluminum alloy filler material.
3.1 Physical and chemical properties and nuclear properties of pure aluminum fillers
Pure aluminum is a kind of low-density material. The reserves of aluminum on the earth are quite large. The technology for manufacturing and smelting aluminum has been studied in depth at present. In fact, aluminum was serialized in the middle of the 20th century. Therefore, the use of aluminum as a filler material for helium is very cheap. Aluminum in the periodic table is located in the third period of the IIIA group element, with an atomic number of 13, atomic weight of 26.98154, and the external electron configuration of the aluminum atom is 3S23P1. The 13 electrons of aluminum are distributed on the tracks of each layer as 1S22S22P63S23P1. If two 3S electrons and one 3P electron are lost at the same time, divalent aluminum ions (Al2+) are generated. If one 3P electron is lost, monovalent aluminum ions (Al+) are generated. Low-valent aluminum ions are usually 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 mm3/mol atom, the density is 2.6987 g/cm3; the specific strength of aluminum (the ratio of tensile strength to density) -σb/γ) is high. Thermal and electrical conductivity is good, and its thermal conductivity is about 10 times that of stainless steel. The thermal conductivity of solid aluminum at room temperature is 2.35-2.237×10-2W/(mK); near the melting point, the thermal conductivity is reduced to 2.1×10-2W/(mK); the thermal conductivity of liquid aluminum is higher than that of solid Aluminum is much smaller, only 0.9x10-2W/(mK) near the melting point, and increases to 1.0x10-2W/(mK) at 1250K. Aluminum has a strong reflection of light and heat, reflecting 95% of the hotline. Pure aluminum is not magnetic and does not generate additional magnetic fields. The ductility of aluminum can reach 25% and can be processed into welding wire or sheet material by forging, extruding and rolling. Aluminum has the ability to adsorb ambient moisture, and its high-temperature melt has a strong ability to absorb hydrogen.
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 vaporization 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 range of 298-933K, the heat capacity of solid aluminum changes linearly with temperature
Cp=a+bt (1)
In the formula, a=4.94, b=2.96×10-3. The heat capacity of liquid aluminum is approximately 31.76 J/(mol.K). As the temperature increases, it increases.
From the nuclear performance point of view, the thermal neutron absorption cross section of aluminum is 0.22 target. When pure aluminum is used as the filler material to weld the crucible, the pure aluminum and the crucible melt, solidify and crystallize, and a eutectic reaction occurs. The alloy formed is a binary eutectic alloy. However, in actual welding, there are many segregations in the microstructure of the weld, depending on the melting amount of niobium and aluminum. Upon analysis, there is a eutectic component on the weld or a side of the hypereutectic component deviating from the eutectic point. In the experiment, it was also found that with pure aluminum as the filler material, its fluidity after high-temperature melting is not as good as that of Al-Si alloy, and the gap filling ability is inferior to Al-Si alloy.
3.2 Analysis of aluminum oxide pollution
At room temperature, there is a clear tendency for oxidation of aluminum. The oxidation reaction on the aluminum surface will actually be significantly reduced after 2 h. At this time, the thickness of the oxide film is 2.5-5.0 nm. In the presence of moisture, the thickness of the oxide film can reach 10 nm. After 14 days, the thickness of the oxide film tends to be stable. Aluminum generally contains 0.002 to 0.02 mass% of gas, and a thin layer of oxide exists on the surface. If the cleaning is not clean before welding, these oxides can form oxide inclusions in the weld. At room temperature, a dense Al2O3 oxide forms on the surface of aluminum and its structure is amorphous. The thickness of Al2O3 oxide on the surface of aluminum is 2-10 nm. With the increase of temperature, the thickness of the oxide is increasing. When the temperature is 500 °C, the thickness of the oxide film increases to 30 nm; when the temperature reaches or close to the melting point, the oxide The thickness can be increased to about 200nm. Al2O3 oxides show completely different properties from pure aluminum. With increasing temperature, Al2O3 oxides produce alpha, beta, gamma, and gamma prime phase transitions, and 700-710 degrees Celsius change to gamma-Al2O3. When the temperature is higher than 900°C, it starts to change to α-Al2O3 structure. Pure aluminum does not undergo phase change from room temperature to melting point. Regardless of the chemical composition and phase change of the Al2O3 oxide, there are always some or a few oxides present on the aluminum surface. It is understood that some of the surface properties of the Al2O3 oxide are significant for the welding of niobium. Aluminum and oxygen have a strong ability to interact and undergo three different processes: (1) Oxygen impinges on fresh fresh aluminum surfaces (physical adsorption); (2) Chemically generates a layer of dissociated oxide film ( Chemical adsorption and chemical reaction); (3) Thickening of oxide film with time.
Al2O3 oxides have the following characteristics: (1) The Al2O3 oxides have good protection properties. In certain oxidation stages, this property of oxides can be used to prevent the further action of aluminum and gases; (2) chemical stability and high temperature stability With good properties, it is almost impossible to reduce aluminum from Al2O3 oxides during welding; (3) high melting temperature, long-term melting of Al-filled and crucible materials, and Al2O3 oxides in solid state; (4) Al2O3 oxides in liquid state Aluminum and solid aluminum have low solubility, lower plasticity than aluminum, and have higher hardness and brittleness; (5) the coefficient of linear expansion is only 1/3 of that of aluminum. During welding and heating, Al2O3 oxide sometimes cracks; (6) ) Al2O3 oxide has a stronger ability to adsorb water vapor.
Aluminum has high solubility in hydrogen in the liquid state. It has been reported [7] that the hydrogen content in aluminum alloys can account for more than 85%. If it is 0.034 ml/100 g Al in the solid state, the solubility in the liquid is 0.65 ml/100 g Al. The difference between them is 19.1 times. The hydrogen in aluminum is mainly derived from the reaction of aluminum liquid with water vapor, and the ratio of partial pressure of gas in liquid aluminum is: PH2/PH2O=7.3×1014, indicating that even if PH20 is small, the equilibrium PH2 can reach large. When the liquid temperature of the molten aluminum rises to 727°C, the aluminum liquid can react with water vapor in the dry air condition (PH2O=2.59×10-20 Pa). This means that either the very dry environment or the walls of the dry containers are all wet with respect to the liquid aluminum and will also make it hydrogen-absorbing.
Al2O3 oxide exists in the weld in the form of inclusions under the action of welding stirring force. Studies have shown that there is a symbiotic relationship between oxides in gaseous aluminum and gas hydrogen. Aluminum is easily contaminated by Al2O3 oxides and gas hydrogen, so both are difficult to remove in the aluminum bath. The layer of the oxide film on the liquid aluminum surface close to the aluminum liquid is dense and has a protective effect on the aluminum liquid. However, the oxide film on the outside is loose, and there are small pinholes of Φ5-10 nm in the oxide film, which are occupied by hydrogen, air, and water vapor. Therefore, the aluminum oxide film usually contains at least 1%-2% of water vapor. In this way, Al2O3 oxide plays an important role in the formation of welded pores. The dependence of hydrogen on the nucleation of oxides is mainly considered in terms of thermodynamics. The behavior and interaction mechanism between aluminum oxides and gases at high temperatures must be analyzed starting from the characteristics and structure of the oxides. According to the morphology of the oxides can be divided into three categories: 1) The uneven distribution of large oxides (> 20μm), these oxides are extremely hazardous, but easy to remove; 2) The resulting size is 10-20μm Oxide; 3) Contains oxides with 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 will increase the gas and oxide inclusions in the weld. (2) Reaction of aluminum and oxygen: 4Al+3O2→2Al2O3. Aluminum alloys are easily oxidized to alumina in air and during welding. They are characterized by high melting point, very stable, and can absorb moisture and are not easy to remove. Inhibiting the wetting of the crucible can generate pores in the crucible weld. Al2O3 is an alpha and beta variant with a higher density than aluminum (3.9-4.0 g/cm3) and a melting point of up to 2050. 2) Reaction with water: 2Al+6H2O→2Al(OH)3+3H2↑, molten aluminum reacts with surrounding water vapor severe.
3.3 Properties, Structure and Hydrogen Absorption Properties of Al-12Si Alloy Filler
The use of Al-12Si alloy as filler material for welding crucibles can effectively suppress microcracks in the beryllium welds and prevent crucible weld cracking. The melting point difference between the Al-Si alloy and niobium is very large. During the welding cooling process, the Al-Si alloy is still in the liquid state when the liquid niobium begins to solidify and crystallize. Liquid Al-Si alloys fill the microcracking of solidification crucibles. Therefore, Al-Si alloys are relatively successful fillers in crucible welding. From the beginning of the 1960s until now, Al-Si alloys have always been a kind of filler material used in many welding crucibles, regardless of the method of welding crucibles. The silicon content in Al-Si alloys is very high, which increases the fluidity in the liquid state, the thermal shrinkage is smaller than that of aluminum, the hermeticity of the weld seam is good, and the tendency of hot cracking is small. Al-Si alloys have excellent physical properties, mechanical properties and processing properties after heat treatment under suitable conditions. Compared with other aluminum alloys, its corrosion resistance is also better. In the crucible welding, eutectic reaction occurs between aluminum and germanium, germanium and silicon, and between silicon and aluminum, and no intermetallic compound is generated. Considering the nuclear performance, the Al-Si alloy with filler material has less influence on the nuclear properties because aluminum is a low-density material and the neutron absorption cross-section is 0.22. The addition of silicon does not affect the overall nuclear properties of Al-Si alloys. Because silicon has a smaller thermal neutron absorption cross section than aluminum, it has only 0.13 target. Therefore, Al-Si alloys are well-known filler materials for welded crucibles.
Si belongs to the face-centered cubic lattice. Although it belongs to the facet phase, the Jackson factor of its {111} close-packed face is not high. The {111} plane of the Si crystal is a smooth interface, and the two planes {100} and {111} are rough interfaces. In Al-Si alloys, the difference in solidification conditions and the growth behavior exhibited by the composition varies with silicon. For the Al-Si alloy without modification, the eutectic Si is in the form of a thick slab, and there is a small amount of twin crystals in the Si crystal. Sheet-like eutectic Si possesses two types of branching: 1) large-angle branching behavior related to twin behavior, and an angle of 70.5o with a {111} close-packed surface; 2) thermal expansion coefficient due to Si phase and Al phase Differently, these behaviors also lead to the existence of parallelism, branching, splitting, and the parallelism of the two.
In the early 1980s, according to the view of interface dynamics, a facet-non-facet transformation theory was proposed. The theory is that as the growth rate increases, Si has a small-surface growth to a non-facet growth transition. The change of Si appearance and size is closely related to the degree of eutectic undercooling during solidification. When the degree of undercooling is small, the Si phase grows in a faceted side growth mode. When the degree of undercooling increases, Si grows in a uniform growth mode without facets. Modifications to Al-Si alloys can change the morphology and size of Si, such as the addition of Na, Sr, Re, and other elements to Al-Si alloys [8, 9]. The eutectic temperature in the alloy (in the cooling curve) The eutectic platform is much lower than that of undeformed ones, which increases the degree of eutectic undercooling, and the eutectic Si transforms from coarse slats (or needles) to fine fibrous forms, ie the growth of eutectic Si takes place. Changed.
However, Al-Si alloys for brazing welding are more demanding and it is not desirable to have elements such as Na, Sr, Re, etc., because their presence may create new sources of corrosion in welds and the use of welded components. Will have adverse effects. Therefore, other methods must be used to improve the morphology and size of eutectic Si in Al-Si alloy as filler material for the welded crucible. The reaction of silicon with O2 produces two different structural silicon oxides: 1) 2Si+O2→2SiO; 2) 2Si+O2→2SiO2. The color of SiO is black or brown-black, which has also been encountered in the processing of Al-Si alloys. The reaction of Si and O2 is at 400. C above. The aluminum in Al-Si alloy reacts with water: 2Al+6H2O→2Al(OH)3+3H2↑. The molten aluminum reacts violently with the surrounding water vapor, and Si reacts with water to form SiO2 and H2↑. At high temperatures, Si also interacts with water vapor to produce H2 helium.