Molybdenum and molybdenum alloy medium-thickness plate electron beam welding method

Abstract

The invention belongs to the technical field of metal welding, and particularly relates to an electron beam welding method for molybdenum and molybdenum alloy medium and thick plates. A vacuum electron beam welding machine is used for carrying out right-angle double-face tailor welding on corrosion-resistant molybdenum and molybdenum alloy medium-thickness plates with the thickness ranging from 5 mm to 50 mm in the chemical field. Through the fixing modes of assembling and designing the welding tool and the like, the problem of electron beam welding cracking caused by deformation is avoided. In addition, by controlling electron beam welding parameters, the high-temperature nitrogen oxidation risk in the molybdenum and molybdenum alloy plate welding process is reduced, and welding tip air holes are reduced. The implementation of the method can provide guidance for welding engineering application of the corrosion-resistant molybdenum plate in the field of chemical engineering.

Description

Technical Field

[0001] The invention belongs to the technical field of metal welding, and in particular relates to an electron beam welding method for molybdenum and molybdenum alloy medium-thick plates. Background Art

[0002] Molybdenum and its alloys are high-melting-point, high-strength, high-corrosion resistance, and excellent thermal conductivity. They are high-temperature, refractory metals with excellent overall properties and are widely used in defense, aerospace, electronics, energy, chemicals, metallurgy, and nuclear industries. Molybdenum alloys are typically manufactured using powder metallurgy, a technique that cannot meet the requirements for manufacturing large, complex structural components. Therefore, research into welding technologies for molybdenum and its alloys is crucial.

[0003] Molybdenum and its alloys are highly sensitive to gaseous impurities such as oxygen, nitrogen, and carbon. During welding, the primary challenges faced by molybdenum and its alloys are property changes caused by gaseous impurities and microstructural changes (grain growth and varying degrees of precipitation hardening) due to thermal cycling. Gaseous impurities (especially oxygen) easily form hardened phases and segregate at grain boundaries, increasing the tendency for weld cracking and even causing brittle fracture. At high temperatures, molybdenum and its alloys rapidly oxidize, forming an oxide film that hinders the welding process. Regarding molybdenum alloy welding process parameters, a low heat input is simply not sufficient to achieve a well-formed weld pool. However, excessive heat input can cause molybdenum grain growth, degrading the weld's mechanical properties.

[0004] To address the above-mentioned issues, vacuum electron beam welding technology has been used in existing technologies, which has improved the welding performance of molybdenum and molybdenum alloys to a certain extent. However, most of the current research on vacuum electron beam welding of molybdenum and molybdenum alloys focuses on thin-walled parts with a thickness range of 1mm to 3mm. For example, Pan et al. studied the welding process of 1.5mm thick molybdenum plates; Stutz and Zhang et al. each explored the vacuum degree, welding speed, workpiece microstructure, mechanical properties, and defect formation mechanism of electron beam welding of 2mm thick molybdenum alloy plates; Zhang Yongyun studied the microstructure and properties of 3mm thick molybdenum alloy electron beam welded joints; Cai Lihui et al. studied the electron beam welding characteristics of molybdenum, lanthanum, and yttrium alloys with a Mo-Ti-Zr filler layer, and used TZM alloy as the filler metal to solve the cracking problem of electron beam welding of molybdenum, lanthanum, and yttrium alloys.

[0005] However, as the thickness of molybdenum and molybdenum alloy sheet increases, the difficulty of welding also increases. This is because molybdenum and molybdenum alloys are hard and brittle materials. Molybdenum's brittleness is primarily due to two factors: the inherent brittleness of molybdenum and the accumulation of interstitial impurities at grain boundaries. For example, the accumulation of molybdenum oxide at grain boundaries can cause cracks. For thick-walled molybdenum alloy parts, increasing the heat input required during welding can increase the size of the molybdenum grains, reducing the mechanical properties of the weld. Furthermore, this can exacerbate the accumulation of molybdenum oxide at grain boundaries, leading to weld cracks and potentially brittle fracture. Therefore, preventing weld cracking in thick-walled molybdenum and molybdenum alloy parts remains a research challenge. Summary of the Invention

[0006] To address the aforementioned technical issues, the present invention provides a method for electron beam welding of medium-thick molybdenum and molybdenum alloy plates. This method utilizes a vacuum electron beam welder for right-angle, double-sided tailor-welding of corrosion-resistant molybdenum and molybdenum alloy plates (thickness range 5mm-50mm) for use in the chemical industry. By designing welding fixtures and modifying assembly methods, the molybdenum alloy plates are secured to prevent cracking caused by welding deformation. By controlling welding parameters and other measures, the risk of high-temperature nitrogen oxidation during the welding process is reduced, as well as weld tip porosity, ultimately improving weld quality. This method provides experimental guidance for the application of this type of molybdenum plate welding in engineering projects.

[0007] The present invention is specifically implemented through the following technical solutions.

[0008] The present invention provides a method for electron beam welding of molybdenum and molybdenum alloy medium and thick plates, comprising the following steps: A first plate and a second plate with a thickness of 5 mm to 50 mm are polished separately, and an edge of one end of the first plate to be welded is processed into a concave step, the step is used to place the end of the second plate to be welded, and the dimension of the step along the length direction of the first plate is equal to the thickness dimension of the second plate. A plurality of threaded holes are machined at the step, and the threaded holes pass through the step and extend into the second plate, and the threaded holes are used to threadably connect the first plate and the second plate; The processed first plate and the second plate are surface treated, and then the end of the second plate to be welded is placed on the step and threaded. The first plate and the second plate constitute a right-angle weldment, and the assembled right-angle weldment is placed on the welding platform. It should be noted that the present invention assembles it into a right-angle weldment so that it can be used to prepare corrosion-resistant and special-purpose containers.

[0009] After vacuuming, preheat the inner surface of the weld before electron beam welding the inner surface. After the vacuum atmosphere is maintained, cool down and take the product out of the furnace. In a vacuum environment, the outer surface of the weld is preheated, and then the outer surface is welded using an electron beam. After the vacuum atmosphere is maintained, the weld is cooled and taken out of the furnace.

[0010] Preferably, the depth of the step recess is 10% to 20% of the thickness of the first plate.

[0011] Preferably, the center of the threaded hole is located at 1 / 2 of the step along the length of the first plate, the length of the threaded hole extending into the second plate is more than 10 mm, and the distance between two adjacent threaded holes is greater than 100 mm.

[0012] Preferably, during thread assembly, the surfaces to be welded of the second plate and the first plate are fitted and aligned, and the maximum local gap is ≤0.05 mm; after the assembled right-angle weldment is placed on the welding platform, the horizontal and vertical straightness of the plate are both ≤0.2 mm.

[0013] Preferably, the inner surface of the weld is preheated twice, the first preheating beam current is 4mA~6mA, the focusing current is 550mA, the welding voltage is 65kV, and the preheating speed is 300mm / min; the second preheating beam current is 6mA~8mA, the focusing current is 600mA, the welding voltage is 70kV, and the preheating speed is 200mm / min.

[0014] Preferably, after preheating the workpiece's inner surface, electron beam welding should be performed directly without removing the workpiece from the furnace. The working distance between the welding gun and the surface to be welded is 400-450 mm, the welding beam current is 25-30 mA, the focusing current is 500 mA, the welding voltage is 90 kV, the welding speed is 600 mm / min, the electron beam scanning path is circular, and the frequency is 150-200 Hz. After welding, the workpiece should be maintained in a vacuum atmosphere for 15-20 minutes, and then cooled with air for at least 30 minutes before removal from the furnace.

[0015] Preferably, the outer surface of the weld is preheated twice, the first preheating beam current is 6mA~8mA, the focusing current is 550mA, the welding voltage is 70kV, and the preheating speed is 300mm / min; the second preheating beam current is 8mA~10mA, the focusing current is 550~600mA, the welding voltage is 75kV, and the preheating speed is 200mm / min.

[0016] Preferably, after preheating the workpiece's outer surface, electron beam welding is performed directly without removing the workpiece from the furnace. The working distance between the welding gun and the surface to be welded is 400-450 mm, the welding beam current is 30-35 mA, the focusing current is 500 mA, the welding voltage is 90 kV, the welding speed is 600 mm / min, the electron beam scanning path is circular, and the frequency is 150-200 Hz. After welding, the workpiece should be maintained in a vacuum atmosphere for at least 20 minutes and then cooled with air for at least 30 minutes before being removed from the furnace.

[0017] Preferably, vacuum is drawn before electron beam welding to ensure that the vacuum degree of the welding chamber is no greater than 5×10 -2 Pa, the vacuum degree of the electron beam gun chamber is not greater than 5×10 -3 Pa.

[0018] Preferably, after polishing, the surface roughness Ra of the surface to be welded is no greater than 3.2 μm. Surface treatment: Use anhydrous alcohol to remove surface oxides, oil stains, and impurities from the workpiece, arc starter plate, backing plate, and fixture. Remove visible defects such as burrs and cracks from the weld surface of the workpiece.

[0019] Compared with the prior art, the present invention has the following beneficial effects: The present invention is directed to the welding of medium-thick molybdenum and molybdenum alloy plates with a thickness of 5mm to 50mm. By assembling and designing fixing methods such as welding tools, the problem of electron beam welding cracking due to deformation is avoided. Specifically, the first plate and the second plate are polished separately, and the edge of one end of the first plate to be welded is processed into a concave step. The step is used to place one end of the second plate to be welded. The size of the step along the length direction of the first plate is equal to the thickness of the second plate. A plurality of threaded holes are machined at the step. The threaded holes pass through the step and extend into the second plate. The threaded holes are used to thread the first plate and the second plate together. The processed first plate and the second plate are surface treated, and then the end of the second plate to be welded is placed on the step and threadedly assembled. The first plate and the second plate form a right-angle weldment. The right-angle weldment is conducive to the preparation of subsequent chemical industry containers. The right-angle weldment is then double-sided welded using an electron beam. By controlling the electron beam welding parameters, the risk of high-temperature nitrogen oxidation during the welding process of molybdenum and molybdenum alloy plates is reduced, and the pores at the welding tip are reduced. The implementation of the present invention can provide guidance for the welding engineering application of corrosion-resistant molybdenum plates in the chemical industry. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 The right-angle assembly cross-section of molybdenum and molybdenum alloy plates to be welded.

[0021] Figure 2 Schematic diagram of the step and threaded hole structure.

[0022] Figure 3 This is an enlarged view of point A in the figure.

[0023] Figure 4 This is a schematic diagram of the structure of the molybdenum and molybdenum alloy plates to be welded after assembly.

[0024] Figure 5 This is a physical picture of the molybdenum and molybdenum alloy plates to be welded after processing.

[0025] Figure 6 This is a physical picture of the molybdenum and molybdenum alloy plates to be welded after assembly.

[0026] Figure 7 This is a physical picture of the threaded connection positions of molybdenum and molybdenum alloy plates to be welded.

[0027] Figure 8 This is the X-ray flaw detection result after welding of Example 1.

[0028] Figure 9 The weld of the welding sample in Example 1 was cross-sectioned to obtain the microstructure morphology of the weld cross section, where (a) is a physical image of the weld cross section and (b) is a metallographic photograph of the cross section.

[0029] Figure 10 The weld seam of the weld specimen from Comparative Example 1 was cross-sectioned to obtain the microstructures of the inner and outer surfaces of the weld seam. (a) is a photo of the weld seam cross section, (b) is a metallographic photograph of the inner surface of the cross section, and (c) is a metallographic photograph of the outer surface of the cross section. DETAILED DESCRIPTION

[0030] To enable those skilled in the art to better understand and implement the technical solutions of the present invention, the present invention is further described below with reference to specific examples and accompanying drawings. However, the examples are not intended to limit the present invention. The experimental methods and detection methods described in the following examples are conventional methods unless otherwise specified; the reagents and materials described are commercially available unless otherwise specified.

[0031] The present invention provides an electron beam welding process for welding 5-50 mm thick molybdenum and molybdenum alloy plates. The process mainly includes raw material preparation and processing, surface treatment, welding assembly, vacuuming, preheating before welding, electron beam welding, cooling and heat treatment. Specific process requirements include: (1) Raw material preparation and processing: Prepare two molybdenum and molybdenum alloy plates to be welded, with a thickness range of 5mm~50mm. In order to clearly distinguish the two plates, they are named the first plate 1 and the second plate 2 respectively. Figures 1 to 4 As shown, the two plates are first milled flat and mechanically polished so that the surface roughness Ra of the surface to be welded is not greater than 3.2μm; the edge to be welded of the first plate 1 is processed into a concave step 11, which is used to place one end of the second plate 2 to be welded. The concave depth of the step 11 is 10% to 20% of the thickness of the first plate 1, and the height of the step 11 (that is, the dimension of the step 11 along the length of the first plate 1) is equal to the thickness of the second plate 2; a threaded hole 12 is machined at the step 11, which passes through the step 11 and extends into the second plate 2. The threaded hole 12 is used to threadedly connect the first plate 1 and the second plate 2. The center of the threaded hole 12 is located at 1 / 2 of the step height (that is, the center of the threaded hole 12 is located at 1 / 2 of the step 11 along the length of the first plate 1). The specifications are M2.0×0.4mm to M22.0×2.5mm. The depth of the threaded hole 12 passes through the step 11 and extends more than 10mm into the interior of the second plate 2. The spacing between the threaded holes 12 is greater than 100 mm and is evenly distributed on the step 11. Figure 5 shown.

[0032] (2) Surface treatment: Use anhydrous alcohol to remove surface oxides, oil stains, impurities, etc. on the workpiece, arc starting plate, backing plate, and fixture. Remove burrs, cracks, and other visible defects on the welding surface of the workpiece.

[0033] (3) Welding assembly: Weld the two molybdenum and molybdenum alloy plates processed in step (1) into Figure 3 As shown in the figure, the end of the second plate 2 to be welded is placed on the step 11, and the surfaces to be welded are closely fitted and aligned, with the maximum local gap ≤ 0.05mm; and the bolt 3 is threaded into the threaded hole 12, and the first plate 1 and the second plate 2 are threadedly connected and fixed to obtain a right-angle weldment, as shown in the actual picture. Figure 6 and Figure 7 Fix the assembled workpieces to be welded on the welding platform so that the horizontal and vertical straightness of the right-angle welded parts are both ≤0.2mm.

[0034] (4) Vacuuming: Vacuum before electron beam welding to ensure that the vacuum degree of the welding chamber is no more than 5×10 -2 Pa, the vacuum degree of the electron beam gun chamber is not greater than 5×10 -3 Pa.

[0035] (5) Preheating the inner surface of the weld before welding: Preheat the inner surface of the right-angle weld twice. The first preheating beam current is 4mA~6mA, the focusing current is 550mA, the welding voltage is 65kV, and the preheating speed is 300mm / min; the second preheating beam current is 6mA~8mA, the focusing current is 600mA, the welding voltage is 70kV, and the preheating speed is 200mm / min.

[0036] (6) Electron beam welding of inner surface: After the inner surface of the workpiece is preheated, electron beam welding is performed directly without removing the workpiece from the furnace. The working distance between the welding gun and the surface to be welded is 400mm~450mm, the welding beam current is 25mA~30mA, the focusing current is 500mA, the welding voltage is 90kV, and the welding speed is 600mm / min. The electron beam scanning trajectory is circular, and the frequency is 150Hz~200Hz.

[0037] (7) After the workpiece is welded, it should be kept in a vacuum atmosphere for 15 to 20 minutes. After completion, the workpiece should be cooled by gas for more than 30 minutes before being taken out of the furnace.

[0038] It should be noted that there are two main reasons for the brittleness of molybdenum and molybdenum alloys: one is their inherent brittleness, and the other is the enrichment of interstitial impurities in the grain boundaries. In order to overcome this problem and improve the welding performance of 5-50mm thick molybdenum and molybdenum alloys, the present invention assembles and fixes the plates to be welded into right-angle weldments. Right-angle weldments are beneficial for the subsequent preparation of containers in the chemical industry and avoid cracking due to welding deformation. Preheating is then performed before welding to reduce the temperature difference between the weld and the substrate during welding, thereby reducing the tendency to crack due to excessive temperature difference. During the preheating process, a two-pass step-by-step preheating method is used, which is the preferred preheating method obtained after testing. The present invention uses parameters with a large aspect ratio for welding, which increases the welding line energy and shortens the solidification time of the weld pool to reduce the grain size. In addition, the use of a lower defocusing current + welding pool stirring method (the electron beam scanning trajectory is circular, and the scanning frequency is 150HZ~200HZ) is conducive to the smooth discharge of the tip gas generated during the welding process, reducing the brittleness of the oxide while reducing the weld tip porosity. Following the same principle, the outer surface of the weldment is preheated and welded, as shown in the following steps.

[0039] (8) Preheating the outer surface before welding: In a vacuum environment, preheat the outer surface of the right-angle welded parts twice. The first preheating beam current is 6mA~8mA, the focusing current is 550mA, the welding voltage is 70kV, and the preheating speed is 300mm / min; the second preheating beam current is 8mA~10mA, the focusing current is 550mA~600mA, the welding voltage is 75kV, and the preheating speed is 200mm / min.

[0040] (9) Electron beam welding of outer surface: After the outer surface of the workpiece is preheated, electron beam welding is performed directly without removing the workpiece from the furnace. The working distance between the welding gun and the surface to be welded is 400mm~450mm, the welding beam current is 30mA~35mA, the focusing current is 500mA, the welding voltage is 90kV, and the welding speed is 600mm / min. The electron beam scanning trajectory is circular, and the frequency is 150Hz~200Hz.

[0041] (10) After welding, the workpiece should be kept in a vacuum atmosphere for more than 20 minutes, and then filled with air and cooled for more than 30 minutes before being taken out of the furnace.

[0042] The present invention is described in detail below through the following specific embodiments: The electron beam welding machine used in the present invention is THDW-30, and the welding power is 30KW.

[0043] Example 1 (1) Prepare two pure molybdenum plates of the same specifications with a thickness of 5 mm, a length of 200 mm, and a width of 200 mm. First, mill the plates flat and mechanically polish them. The surface roughness Ra of the surface to be welded is tested to be 3.2 μm. The edge to be welded on one of the plates is machined into a concave step with a depth of 1 mm and a height of 5 mm. A threaded hole is machined at the step. The center of the threaded hole is located 2.5 mm from the bottom. The specification is M2.0 × 0.4 mm. The hole depth penetrates the base plate and extends 10 mm into the interior of the plate to be welded. Two threaded holes are machined on the bottom surface with a center-to-center spacing of 100 mm.

[0044] (2) Surface treatment: Use anhydrous alcohol to remove surface oxides, oil stains, impurities, etc. on the workpiece, arc starting plate, backing plate, and fixture. Remove burrs, cracks, and other visible defects on the welding surface of the workpiece.

[0045] (3) Welding assembly: Thread two molybdenum and molybdenum alloy plates together. The surfaces to be welded should be tightly fitted and aligned, with a maximum local gap of 0.05 mm to obtain a right-angle welded part. Fix the assembled workpiece to be welded on the welding platform so that the horizontal straightness of the plate is 0.2 mm and the vertical straightness is 0.15 mm.

[0046] (4) Vacuuming: Vacuuming before electron beam welding, the vacuum degree of the welding chamber is 5×10 -2 Pa, the vacuum degree of the electron beam gun chamber is 5×10 -3 Pa.

[0047] (5) Preheating the inner surface of the weld before welding: Preheat the inner surface of the right-angle weld obtained in step (3) twice. The first preheating beam current is 4 mA, the focusing current is 550 mA, the welding voltage is 65 kV, and the preheating speed is 300 mm / min; the second preheating beam current is 6 mA, the focusing current is 600 mA, the welding voltage is 70 kV, and the preheating speed is 200 mm / min.

[0048] (6) Electron beam welding of inner surface: After the inner surface of the workpiece is preheated, electron beam welding is carried out directly without taking it out of the furnace. The working distance between the welding gun and the surface to be welded is 400 mm, the welding beam current is 25 mA, the focusing current is 500 mA, the welding voltage is 90 KV, the welding speed is 600 mm / min, the electron beam scanning trajectory is circular, and the frequency is 200 Hz.

[0049] (7) After the workpiece is welded, it is kept in a vacuum atmosphere for 15 minutes. After completion, it is filled with air and cooled for 30 minutes before being taken out of the furnace.

[0050] (8) Preheating the outer surface before welding: Under vacuum environment, preheat the outer surface of the right-angle welded parts twice. The first preheating beam current is 6mA, the focusing current is 550mA, the welding voltage is 70kV, and the preheating speed is 300mm / min; the second preheating beam current is 8mA, the focusing current is 550mA, the welding voltage is 75kV, and the preheating speed is 200mm / min.

[0051] (9) Electron beam welding of outer surface: After the outer surface of the workpiece is preheated, electron beam welding is performed directly without leaving the furnace. The working distance between the welding gun and the surface to be welded is 400 mm, the welding beam current is 30 mA, the focusing current is 500 mA, the welding voltage is 90 kV, and the welding speed is 600 mm / min. The electron beam scanning trajectory is circular, and the frequency is 150 Hz.

[0052] (10) After the workpiece is welded, it should be kept in a vacuum atmosphere for 20 minutes. After completion, it should be filled with air and cooled for 30 minutes before being taken out of the furnace.

[0053] The X-ray inspection results after welding are as follows: Figure 8 As shown, there are no defects (black dots are the locations of threaded holes). Take the test weld sample and cross-section the weld to observe the microstructure of the cross-section. Figure 9 Among them, (a) refers to the actual picture of the cross-section of the weld, and (b) refers to the metallographic photograph of the cross-section. It can be seen that there is no cracking in the weld of the workpiece.

[0054] Example 2 (1) Raw material preparation and processing: Prepare two molybdenum alloy plates of the same specifications with a thickness of 20 mm, a length of 300 mm, and a width of 300 mm. First, mill the plates flat and mechanically polish them to a surface roughness Ra of 2.6 μm on the surface to be welded. The edge of one of the plates to be welded is machined into a concave step with a depth of 3 mm and a height of 20 mm. Machine a threaded hole at the step with the center of the threaded hole located at 1 / 2 the height of the step. The specification is M10×1.5 mm. The hole depth penetrates the base plate and extends 10 mm into the interior of the plate to be welded. Machine two threaded holes on the bottom surface with a center-to-center spacing of 100 mm.

[0055] (2) Surface treatment: Use anhydrous alcohol to remove surface oxides, oil stains, impurities, etc. on the workpiece, arc starting plate, backing plate, and fixture. Remove burrs, cracks, and other visible defects on the welding surface of the workpiece.

[0056] (3) Welding assembly: Thread two molybdenum and molybdenum alloy plates together. The surfaces to be welded should be tightly fitted and aligned, with a maximum local gap of 0.04 mm to obtain a right-angle welded part. Fix the assembled workpiece to be welded on the welding platform so that the plate is straight to 0.1 mm in the horizontal direction and 0.1 mm in the vertical direction.

[0057] (4) Vacuuming: Vacuum before electron beam welding, the vacuum degree of the welding chamber is 3×10 -2 Pa, the vacuum degree of the electron beam gun chamber is 4×10 -3 Pa.

[0058] (5) Preheating the inner surface of the weld before welding: Preheat the inner surface of the right-angle weld twice. The first preheating beam current is 5mA, the focusing current is 550mA, the welding voltage is 65kV, and the preheating speed is 300mm / min; the second preheating beam current is 7mA, the focusing current is 600mA, the welding voltage is 70kV, and the preheating speed is 200mm / min.

[0059] (6) Electron beam welding of inner surface: After the inner surface of the workpiece is preheated, electron beam welding is carried out directly without taking it out of the furnace. The working distance between the welding gun and the surface to be welded is 400 mm, the welding beam current is 27 mA, the focusing current is 500 mA, the welding voltage is 90 KV, the welding speed is 600 mm / min, the electron beam scanning trajectory is circular, and the frequency is 200 Hz.

[0060] (7) After the workpiece is welded, it is kept in a vacuum atmosphere for 20 minutes. After completion, it is filled with air and cooled for 30 minutes before being taken out of the furnace.

[0061] (8) Preheating the outer surface before welding: Under vacuum environment, preheat the outer surface of the right-angle welded parts twice. The first preheating beam current is 7mA, the focusing current is 550mA, the welding voltage is 70kV, and the preheating speed is 300mm / min; the second preheating beam current is 9mA, the focusing current is 570mA, the welding voltage is 75kV, and the preheating speed is 200mm / min.

[0062] (9) Electron beam welding of outer surface: After the outer surface of the workpiece is preheated, electron beam welding is carried out directly without taking it out of the furnace. The working distance between the welding gun and the surface to be welded is 400 mm, the welding beam current is 33 mA, the focusing current is 500 mA, the welding voltage is 90 KV, the welding speed is 600 mm / min, the electron beam scanning trajectory is circular, and the frequency is 150 Hz.

[0063] (10) After the workpiece is welded, it is kept in a vacuum atmosphere for 20 minutes. After completion, it is cooled by air for 40 minutes and then taken out of the furnace. The flaw detection results show that there are no defects in the weld.

[0064] Example 3 (1) Prepare two molybdenum alloy plates of the same specifications with a thickness of 50 mm, a length of 500 mm, and a width of 500 mm. Mill the plates flat and mechanically polish them. The surface roughness Ra of the surface to be welded is 2.1 μm. The edge to be welded on one of the plates is machined into a concave step with a depth of 10 mm and a height of 50 mm. Machine a threaded hole at the step. The center of the threaded hole is located 25 mm from the bottom. The specification is M22.0 × 2.5 mm. The hole depth penetrates the base plate and extends 12 mm into the interior of the plate to be welded. Four threaded holes are machined on the bottom surface with a center spacing of 100 mm.

[0065] (2) Surface treatment: Use anhydrous alcohol to remove surface oxides, oil stains, impurities, etc. on the workpiece, arc starting plate, backing plate, and fixture. Remove burrs, cracks, and other visible defects on the welding surface of the workpiece.

[0066] (3) Welding assembly: Thread two molybdenum and molybdenum alloy plates together. The surfaces to be welded should be tightly fitted and aligned, with a maximum local gap of 0.04 mm to obtain a right-angle welded part. Fix the assembled workpiece to be welded on the welding platform so that the plate is straight to 0.1 mm in the horizontal direction and 0.1 mm in the vertical direction.

[0067] (4) Vacuuming: Vacuuming before electron beam welding, the vacuum degree of the welding chamber is 2×10 -2 Pa, the vacuum degree of the electron beam gun chamber is 4×10 -3 Pa.

[0068] (5) Preheating the inner surface of the weld before welding: Preheat the inner surface of the right-angle weld twice. The first preheating beam current is 6mA, the focusing current is 550mA, the welding voltage is 65kV, and the preheating speed is 300mm / min; the second preheating beam current is 8mA, the focusing current is 600mA, the welding voltage is 70kV, and the preheating speed is 200mm / min.

[0069] (6) Electron beam welding of inner surface: After the inner surface of the workpiece is preheated, electron beam welding is carried out directly without taking it out of the furnace. The working distance between the welding gun and the surface to be welded is 450mm, the welding beam current is 30mA, the focusing current is 500mA, the welding voltage is 90KV, the welding speed is 600mm / min, the electron beam scanning trajectory is circular, and the frequency is 200HZ.

[0070] (7) After the workpiece is welded, it is kept in a vacuum atmosphere for 20 minutes. After completion, it is filled with air and cooled for 40 minutes before being taken out of the furnace.

[0071] (8) Preheating the outer surface before welding: Under vacuum environment, preheat the outer surface of the right-angle welded parts twice. The first preheating beam current is 8mA, the focusing current is 550mA, the welding voltage is 70kV, and the preheating speed is 300mm / min; the second preheating beam current is 10mA, the focusing current is 600mA, the welding voltage is 75kV, and the preheating speed is 200mm / min.

[0072] (9) Electron beam welding of outer surface: After the outer surface of the workpiece is preheated, electron beam welding is performed directly without leaving the furnace. The working distance between the welding gun and the surface to be welded is 450mm, the welding beam current is 35mA, the focusing current is 500mA, the welding voltage is 90kV, and the welding speed is 600mm / min. The electron beam scanning trajectory is circular and the frequency is 150Hz.

[0073] (10) After the workpiece is welded, it should be kept in a vacuum atmosphere for 20 minutes. After completion, it should be cooled by air for 40 minutes before being taken out of the furnace. The flaw detection results show that there are no defects in the weld.

[0074] Comparative Example 1 When the tooling of the present invention is not used for fixing, the specific welding process is as follows: (1) Prepare two pure molybdenum plates of the same specifications with a thickness of 5 mm, a length of 200 mm, and a width of 200 mm. First, mill the plates flat and perform mechanical polishing. The surface roughness Ra of the surface to be welded is tested to be 3.2 μm.

[0075] (2) Surface treatment: Use anhydrous alcohol to remove surface oxides, oil stains, impurities, etc. on the workpiece, arc starting plate, backing plate, and fixture. Remove burrs, cracks, and other visible defects on the welding surface of the workpiece.

[0076] (3) Welding assembly: Assemble two molybdenum and molybdenum alloy plates at right angles (do not use the step and threaded fixing assembly method in Example 1, and simply fit the two plates into a right angle). The surfaces to be welded should be tightly fitted and aligned, with a maximum local gap of 0.05 mm. Fix the assembled workpieces to be welded on the welding platform so that the horizontal straightness of the plates is 0.2 mm and the vertical straightness is 0.15 mm.

[0077] (4) Vacuuming: Vacuuming before electron beam welding, the vacuum degree of the welding chamber is 5×10 -2 Pa, the vacuum degree of the electron beam gun chamber is 5×10 -3 Pa.

[0078] (5) Electron beam welding inner surface: the working distance between the welding gun and the surface to be welded is 400 mm, the welding beam current is 25 mA, the focusing current is 500 mA, the welding voltage is 90 KV, and the welding speed is 600 mm / min.

[0079] (6) After welding, the workpiece should be kept in a vacuum atmosphere for 15 minutes, and then filled with air and cooled for 30 minutes before being taken out of the furnace.

[0080] (7) Electron beam welding of outer surface: After the outer surface of the workpiece is preheated, electron beam welding is carried out directly without taking it out of the furnace. The working distance between the welding gun and the surface to be welded is 400 mm, the welding beam current is 30 mA, the focusing current is 500 mA, the welding voltage is 90 KV, and the welding speed is 600 mm / min.

[0081] (8) After the workpiece is welded, it is kept in a vacuum atmosphere for 20 minutes. After completion, it is filled with air and cooled for 30 minutes before being taken out of the furnace.

[0082] The welding results are as follows: take the test welding sample and cross-cut the weld, such as Figure 10 As shown in (a), (a) is a cross-section of the weld. Observe the microstructure of the cross-section of the weld. Figure 10 As shown in (b) and (c), (b) is a metallographic photograph of the inner surface of the cross section, and (c) is a metallographic photograph of the outer surface of the cross section. It can be seen that there are obvious cracks on both the inner and outer surfaces of the workpiece.

[0083] It can be seen that the conventional electron beam welding process in comparative example 1 cannot achieve the welding of thicker molybdenum plates, while the method of the present invention can achieve high-quality welding of 5mm~50mm thick molybdenum and molybdenum alloy plates. The present invention fixes the molybdenum alloy plates by designing welding tooling and changing the assembly method to avoid cracking due to welding deformation; by controlling welding parameters and other measures, the risk of high-temperature nitrogen oxidation during the welding process of the molybdenum alloy plates is reduced, the porosity at the welding tip is reduced, and the purpose of improving the welding quality is achieved.

[0084] Obviously, those skilled in the art may make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if such changes and modifications of the present invention fall within the scope of the claims and their equivalents, such changes and modifications are intended to be included.

Claims

1. A method for electron beam welding of molybdenum and molybdenum alloy medium and thick plates, characterized in that: The following steps are involved: A first plate and a second plate with a thickness of 5 mm to 50 mm are polished separately, and an edge of one end of the first plate to be welded is processed into a concave step, the step is used to place the end of the second plate to be welded, and the dimension of the step along the length direction of the first plate is equal to the thickness dimension of the second plate. A plurality of threaded holes are machined at the step, and the threaded holes pass through the step and extend into the second plate, and the threaded holes are used to threadably connect the first plate and the second plate; The processed first plate and the second plate are subjected to surface treatment, and then the end of the second plate to be welded is placed on the step and screwed together. The first plate and the second plate form a right-angle weldment, and the assembled right-angle weldment is placed on the welding platform; In a vacuum environment, the inner surface of the weld is preheated, and then the inner surface is welded using an electron beam. After the vacuum atmosphere is maintained, the weld is cooled and taken out of the furnace. In a vacuum environment, the outer surface of the weld is preheated, and then the outer surface is welded using an electron beam. After the vacuum atmosphere is maintained, the weld is cooled and taken out of the furnace.

2. The electron beam welding method for molybdenum and molybdenum alloy medium and thick plates according to claim 1, characterized in that: The depth of the step recess is 10% to 20% of the thickness of the first plate.

3. The electron beam welding method for molybdenum and molybdenum alloy medium and thick plates according to claim 1, characterized in that: The center of the threaded hole is located at 1 / 2 of the length of the step along the length of the first plate, and the length of the threaded hole extending into the second plate is at least 10 mm.

4. The electron beam welding method for molybdenum and molybdenum alloy medium and thick plates according to claim 1, characterized in that: During thread assembly, the surfaces to be welded of the second plate and the first plate are fitted and aligned, and the maximum local gap is ≤0.05mm; after the assembled right-angle weldment is placed on the welding platform, the straightness of the plate in the horizontal and vertical directions is ≤0.2mm.

5. The electron beam welding method for molybdenum and molybdenum alloy medium and thick plates according to claim 1, characterized in that: The inner surface of the weld is preheated twice. The first preheating beam current is 4mA~6mA, the focusing current is 550mA, the welding voltage is 65kV, and the preheating speed is 300mm / min; the second preheating beam current is 6mA~8mA, the focusing current is 600mA, the welding voltage is 70kV, and the preheating speed is 200mm / min.

6. The electron beam welding method for molybdenum and molybdenum alloy medium and thick plates according to claim 1, characterized in that: After the inner surface is preheated, electron beam welding is carried out directly without leaving the furnace. The working distance between the welding gun and the surface to be welded is 400mm~450mm, the welding beam current is 25mA~30mA, the focusing current is 500mA, the welding voltage is 90KV, the welding speed is 600mm / min, the electron beam scanning track is circular, and the frequency is 150HZ~200HZ.

7. The electron beam welding method for molybdenum and molybdenum alloy medium and thick plates according to claim 1, characterized in that: The outer surface of the weld is preheated twice. The first preheating beam current is 6mA~8mA, the focusing current is 550mA, the welding voltage is 70kV, and the preheating speed is 300mm / min; the second preheating beam current is 8mA~10mA, the focusing current is 550mA~600mA, the welding voltage is 75kV, and the preheating speed is 200mm / min.

8. The electron beam welding method for molybdenum and molybdenum alloy medium and thick plates according to claim 1, characterized in that: After the outer surface is preheated, electron beam welding is carried out directly without leaving the furnace. The working distance between the welding gun and the surface to be welded is 400mm~450mm, the welding beam current is 30mA~35mA, the focusing current is 500mA, the welding voltage is 90KV, the welding speed is 600mm / min, the electron beam scanning track is circular, and the frequency is 150HZ~200HZ.

9. The electron beam welding method for molybdenum and molybdenum alloy medium and thick plates according to claim 1, characterized in that: The vacuum degree of the welding chamber is not greater than 5×10 -2 Pa, the vacuum degree of the electron beam gun chamber is not greater than 5×10 -3 Pa.

10. The electron beam welding method for molybdenum and molybdenum alloy medium and thick plates according to claim 1, characterized in that: After polishing, the surface roughness Ra of the surface to be welded is not greater than 3.2 μm.

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