A method for improving the strength of molybdenum and molybdenum alloy fusion welding welds

Abstract

The present invention discloses a method for improving the strength of the weld seam of molybdenum and molybdenum alloy fusion welding, comprising the following steps: 1) cleaning the surface of the to-be-welded area of ​​the workpiece to be welded; 2) pre-setting a niobium layer in the to-be-welded area; 3) placing the workpiece to be welded in an inert gas or vacuum environment to complete the welding of the workpiece to be welded; 4) after the welding is completed, the workpiece is continuously placed in the inert gas or vacuum environment until it is cooled to room temperature, so as to complete the microalloying of the weld seam of the workpiece fusion welding. The method can reduce the amount of molybdenum dioxide at the grain boundary of the weld seam, and can also provide a certain size and amount of second phase particles at the grain boundary, thereby significantly improving the strength of the weld seam of molybdenum and molybdenum alloy fusion welding.

Description

Technical Field

[0001] The invention belongs to the technical field of welding and relates to a method for improving the strength of a weld of molybdenum and molybdenum alloy fusion welding. Background Art

[0002] Molybdenum and molybdenum alloys have the advantages of stable chemical properties, high high temperature strength, and corrosion resistance, and are an ideal fuel cladding material. However, their weldability is poor and the joint strength is low. Under normal circumstances, the tensile strength of molybdenum and molybdenum alloy fusion welded joints can only reach about 30%-50% of the parent material. The essence of the reduction in joint tensile strength is that the oxygen element segregated at the molybdenum grain boundary in the weld combines with the molybdenum element to form a molybdenum dioxide phase, which leads to a reduction in the bonding strength of the molybdenum grain boundary. The above problems have seriously restricted the application of molybdenum and molybdenum alloys in the field of nuclear power. Summary of the invention

[0003] In order to solve the problems existing in the prior art, the purpose of the present invention is to provide a method for improving the strength of the weld of molybdenum and molybdenum alloy fusion welding, and the present invention can significantly improve the strength of the weld of molybdenum and molybdenum alloy fusion welding.

[0004] The technical solution adopted by the present invention is as follows:

[0005] A method for improving the strength of molybdenum and molybdenum alloy fusion welding seams comprises the following steps:

[0006] Pre-place a niobium layer in the area to be welded of the workpiece to be welded;

[0007] The workpiece to be welded with a niobium layer pre-placed in the welding area is welded in an inert gas or vacuum, and after welding is completed, it is further cooled to room temperature in the inert gas or vacuum environment, and the processing is completed.

[0008] Preferably, before pre-forming the niobium layer on the area to be welded of the workpiece to be welded, the surface of the area to be welded of the workpiece to be welded is cleaned, and the surface cleaning process includes: grinding the area to be welded with sandpaper, immersing the ground area to be welded in an acetone solution and performing ultrasonic cleaning for several times, and replacing new acetone solution each time until the surface of the area to be welded is clean.

[0009] Preferably, the material of the workpiece to be welded is molybdenum, molybdenum-rhenium alloy, molybdenum-tungsten alloy, titanium-zirconium-molybdenum alloy, or a molybdenum alloy with an alloying element content not exceeding 2wt% or a molybdenum alloy with a second phase doping amount not exceeding 2wt%.

[0010] Preferably, the weld of the workpiece to be welded is in the form of a butt weld or a lap weld, wherein the assembly gap of the butt weld is not greater than 0.05 mm; the lap width of the lap weld is not less than 0.5 mm, and the lap gap is not greater than 0.05 mm.

[0011] Preferably, the method of pre-depositing a niobium layer in the to-be-welded area of ​​the workpiece to be welded includes pre-depositing niobium foil, magnetron sputtering niobium plating, electroplating niobium, spraying niobium or laser cladding niobium.

[0012] Preferably, when the workpiece to be welded with a niobium layer pre-placed in the area to be welded is welded in an inert gas, the inert gas is argon or helium with a purity greater than 99.9%.

[0013] Preferably, when welding the workpiece with a niobium layer pre-placed in the welding area in a vacuum, the vacuum degree is not less than 10 -2 Pa.

[0014] Preferably, the final niobium content in the weld zone of the workpiece to be welded is controlled at 0.8wt% to 1.2wt%.

[0015] The present invention also provides a part, which is processed by the method for improving the strength of the weld of molybdenum and molybdenum alloy fusion welding as described above.

[0016] The present invention has the following beneficial effects:

[0017] In the specific implementation process of the method for improving the strength of molybdenum and molybdenum alloy fusion welding, the four elements of oxygen, carbon, niobium and molybdenum interact with each other during the welding thermal cycle. When a certain amount of Nb is added to the joint weld zone, the niobium element will enter the molten pool along with the melting of the parent material; when the molten pool temperature decreases, the high-temperature phase Nb2C (3500°C) will first precipitate in the liquid phase; the molten pool temperature is further reduced, and the molybdenum matrix begins to solidify accompanied by the precipitation of NbO2 and MoO2; the temperature continues to decrease, and NbO2 combines with excess oxygen to generate Nb2O5. Assuming that the oxygen content in the weld is constant, the generation of niobium oxide first reduces the content of free O, and secondly, reduces the amount of MoO2. The presence of Nb2C in the weld can also play a role in strengthening the grain boundary as a second phase. In summary, the method of niobium microalloying can significantly improve the strength of molybdenum and molybdenum alloy fusion welding joints. According to experiments, the maximum tensile strength of the fusion welded joints of molybdenum and molybdenum alloys at room temperature reached more than 62.7% of the base material after adopting the method provided by the present invention, and obvious Nb2O5 and Nb2C phase precipitation was observed on the grain boundary surface of the weld tensile fracture, while the distribution of MoO2 was significantly reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] FIG. 1( a ) is a schematic diagram of the dimensions of a molybdenum tube in Example 1 of the present invention;

[0019] FIG1( b ) is a schematic diagram of the dimensions of the end plug in Example 1 of the present invention;

[0020] FIG. 2( a ) is a front view of a niobium foil added in Example 1 of the present invention; FIG. 2( b ) is a longitudinal cross-sectional view of a niobium foil added in Example 1 of the present invention;

[0021] Figure 3 1 is the tensile stress-displacement curve of the laser welded joint obtained without alloying and with niobium alloying in Example 1 of the present invention;

[0022] FIG4(a1) is a first macroscopic photograph of the fracture position of the molybdenum alloy laser welded joint obtained without alloying; FIG4(a2) is a second macroscopic photograph of the fracture position of the molybdenum alloy laser welded joint obtained without alloying; FIG4(a3) is a third macroscopic photograph of the fracture position of the molybdenum alloy laser welded joint obtained without alloying;

[0023] Figure 4(b1) is a first macroscopic photograph of the fracture position of the molybdenum alloy laser welded head obtained by niobium alloying in Example 1 of the present invention; Figure 4(b2) is a second macroscopic photograph of the fracture position of the molybdenum alloy laser welded head obtained by niobium alloying in Example 1 of the present invention; Figure 4(b3) is a third macroscopic photograph of the fracture position of the molybdenum alloy laser welded head obtained by niobium alloying in Example 1 of the present invention. DETAILED DESCRIPTION

[0024] The present invention is further described in detail below with reference to the accompanying drawings and embodiments:

[0025] The purpose of the present invention is to solve the problem of low weld strength caused by reduced grain boundary bonding strength during fusion welding of molybdenum and molybdenum alloys, and provide a method for improving the weld strength of fusion welding of molybdenum and molybdenum alloys: niobium element microalloying. The method can reduce the amount of molybdenum dioxide at the grain boundary of the weld, and can also provide a certain size and amount of second phase particles at the grain boundary, and the strength of the resulting weld is relatively high. Reducing the free oxygen content in the weld, reducing the amount of molybdenum dioxide or using the second phase to strengthen the grain boundary are all effective methods for improving the strength of the weld.

[0026] Specifically, the method for improving the strength of the weld of molybdenum and molybdenum alloy fusion welding of the present invention comprises the following steps:

[0027] 1) Clean the surface of the area to be welded of the workpiece to be welded: First, use sandpaper to grind the area to be welded, then soak the grinded area to be welded in a glass beaker filled with acetone solution, and place the beaker in an ultrasonic cleaning machine for ultrasonic cleaning 3 to 5 times, each cleaning lasting 10 to 15 minutes, and replace new acetone solution each time.

[0028] 2) Pre-depositing a niobium layer in the area to be welded: Specifically, the method may be direct pre-depositing of niobium foil, magnetron sputtering niobium plating, electroplating niobium, spraying niobium or laser cladding.

[0029] 3) The workpiece to be welded is placed in an inert gas or vacuum environment, and one of the welding methods selected from laser welding, laser arc hybrid welding, electron beam welding, plasma beam welding, and argon arc welding is used to weld the workpiece to be welded. When the workpiece to be welded is placed in an inert gas for welding, the inert gas used is argon or helium with a purity greater than 99.9%; when the workpiece to be welded is placed in a vacuum environment for welding, the vacuum degree of the vacuum chamber is required to be no less than 10 -2 Pa.

[0030] 4) After welding is completed, the workpiece is placed in an inert gas or vacuum environment until it cools to room temperature to complete the micro-alloying of the workpiece fusion weld.

[0031] In the method for improving the strength of the weld seam of molybdenum and molybdenum alloy fusion welding of the present invention, the material of the workpiece to be welded can be molybdenum, molybdenum-rhenium alloy, molybdenum-tungsten alloy, titanium-zirconium-molybdenum alloy, and molybdenum alloy with an alloying element content not exceeding 2wt% or a molybdenum alloy with a second phase doping amount not exceeding 2wt%. The weld seam of the workpiece to be welded can be a butt weld or a lap weld, wherein the assembly gap of the butt weld is less than 0.05 mm; the lap width of the lap weld is not less than 0.5 mm, and the lap gap is less than 0.05 mm. The final niobium content in the weld seam area of ​​the workpiece to be welded is controlled to be 0.8wt% to 1.2wt%.

[0032] Example 1

[0033] This embodiment takes the laser butt welding of a thin-walled molybdenum alloy tube and an end plug strengthened by rhenium oxide with a rhenium content of 0.25 wt % as an example, and the specific dimensions are shown in FIG. 1( a ) and FIG. 1( b ). Before welding, the molybdenum tube and end plug were surface cleaned: first, 200#, 400#, 600#, 1000#, and 1500# sandpaper were used for grinding in sequence; then, the polished molybdenum tube and end plug were placed in a glass beaker filled with acetone, and then the beaker was placed in an ultrasonic cleaner for ultrasonic cleaning; secondly, the niobium foil used for alloying was placed in a glass beaker filled with acetone solution, and the beaker was placed in an ultrasonic cleaner for cleaning. The specific dimensions of the niobium foil ring used for alloying are shown in Figure 2(a) and Figure 2(b); thirdly, the cleaned molybdenum end plug with a stepped end was inserted into the molybdenum tube, and the niobium foil ring was prefabricated at the docking position of the molybdenum tube-end plug to ensure that the three were tightly assembled; finally, the assembled molybdenum tube-end plug was placed in a high-purity argon protective atmosphere, the laser power was 1.2kW, the defocus amount was +2mm, the welding speed was 0.5m / min, 1 circle was welded, and the joint after welding was cooled to room temperature in the protective atmosphere and then taken out.

[0034] According to the above steps, a laser welded molybdenum tube-end plug without niobium alloying was welded as a control. The tensile test was carried out on the laser welded joints without alloying and with niobium alloying. The tensile results are shown in Figure 3As shown in the figure: the maximum tensile strength of the laser welded joint without alloy is 168MPa, and the maximum tensile strength of the laser welded joint with niobium alloy is 424MPa. The maximum tensile strength of the laser welded joint with niobium alloy is 2.5 times that of the joint without alloy. The fracture position of the laser welded joint of the molybdenum tube and the end plug without alloy (the part between the two bright solid lines drawn from top to bottom in the figure) is mostly located in the center of the weld, such as Figure 4(a1) to Figure 4(a3) As shown in the figure, the fracture position of the molybdenum tube-end plug laser welded joint obtained by niobium alloying (the part between the two bright solid lines drawn from top to bottom in the figure) is mostly located near the fusion line and heat affected zone of the weld, such as Figure 4(b1) to Figure 4(b3) The above results show that the present invention can significantly improve the strength of the molybdenum tube-end plug laser welded joint.

Claims

1. A method for improving the strength of molybdenum and molybdenum alloy fusion welding welds, characterized in that: The process is as follows: Pre-setting a niobium layer on the area to be welded of the workpiece to be welded; before pre-setting the niobium layer on the area to be welded of the workpiece to be welded, the surface of the area to be welded of the workpiece to be welded is cleaned, and the surface cleaning process includes: grinding the area to be welded with sandpaper, immersing the ground area to be welded in an acetone solution and performing ultrasonic cleaning for several times, and replacing a new acetone solution each time until the surface of the area to be welded is cleaned; The workpiece to be welded is provided with a niobium layer in advance in the welding area and is welded in an inert gas or vacuum. After the welding is completed, the workpiece is cooled to room temperature in the inert gas or vacuum environment to complete the processing; the workpiece to be welded is welded by one of the welding methods selected from laser welding, laser arc hybrid welding, electron beam welding, plasma beam welding and argon arc welding; When welding a workpiece with a niobium layer pre-placed in the welding area in an inert gas, the inert gas is argon or helium with a purity greater than 99.9%; When welding the workpiece with a niobium layer in the welding area, the vacuum degree shall not be less than 10 -2 Pa; The final niobium content in the weld zone of the workpiece to be welded is controlled at 0.8wt%~1.2wt%; The material of the workpiece to be welded is molybdenum, molybdenum-rhenium alloy, molybdenum-tungsten alloy, titanium-zirconium-molybdenum alloy, or a molybdenum alloy with an alloying element content not exceeding 2wt% or a molybdenum alloy with a second phase doping amount not exceeding 2wt%; The method for pre-setting a niobium layer in the to-be-welded area of ​​the workpiece to be welded includes pre-setting a niobium foil, magnetron sputtering niobium plating, electroplating niobium, spraying niobium or laser cladding niobium.

2. A method for improving the strength of molybdenum and molybdenum alloy fusion welding according to claim 1, characterized in that: The weld of the workpiece to be welded is in the form of a butt weld or a lap weld, wherein the assembly gap of the butt weld is not greater than 0.05 mm; the lap width of the lap weld is not less than 0.5 mm, and the lap gap is not greater than 0.05 mm.

3. A part, characterized in that: The part is manufactured by adopting the method for improving the strength of the weld of molybdenum and molybdenum alloy fusion welding as described in any one of claims 1-2.

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