Abstract : Electron beam welding has its unique advantages and is widely used. In order to obtain a high quality weld, there are a number of issues to be aware of. Through the discussion and analysis of practical examples, this paper discusses some process factors that affect the quality of electron beam welding, such as weld structure design, tooling die, welding parameters, electron beam spot position, preheating and annealing, filling materials and electron beam tracking. Welding, etc.
Keywords : electron beam welding, quality, process factor
1 Introduction
Electron beam welding (EBW) technology is an important part of High Energy Density Beam (HEDB) processing technology, and has advantages compared with laser welding [1]. In addition to the characteristics of high input energy density, small heating area, fast welding speed, narrow weld heat affected zone and small deformation of the workpiece, electron beam welding also has deep electron beam penetration, large weld width ratio, and electron beam. Convenient control and weld seams in a vacuum environment are not contaminated [2]. Electron beam welding is suitable for precision welding, penetration and depth (large thickness workpiece) welding, high efficiency welding and special welding. As one of the hot processing methods, electron beam welding technology has been relatively mature, but it has a good development momentum. The research contents of mechanism, automation degree, quality monitoring and application field are still improving.
We have been conducting electron beam welding processes and applications since the 1960s. The materials involved are high melting point metals, high elastic alloys, Kovar alloys, stainless steels, high strength steels, non-ferrous metals and their alloys, and non-metallic materials such as ceramics. The workpieces involved are of various structures, mainly small and medium-sized precision parts. Over the past 30 years, it has achieved many scientific and technological achievements. This paper attempts to discuss several process factors affecting the quality of electron beam welding from the welding examples of some specific workpieces (mainly based on the application examples of this unit).
2. Weld structure and fit clearance
In the welding practice, there will be a variety of workpieces, the weld structure is also different, but generally can be divided into: butt weld, end weld, fillet weld (including penetration weld), or divided into straight lines Welds, loop welds, curved welds, spot welds, as well as equal-section welds and variable-section welds. In order to achieve the best welding effect, the design of the weld structure and the fit clearance is very important. It is necessary to consider the role of the workpiece (component) in the whole machine, and must meet the weldability of the material to be welded and the specific welding process requirements. Therefore, before the welding is carried out, the weld structure of the welded parts should be discussed with the engineering designer, or the reasonable structure and gap size should be determined through the process test.
3. Tooling mold
In order to place the workpiece to be welded in the welding machine, the tooling die (clamp) directly affects the implementation effect of the welding. In a certain sense, the correct design of the mold is half of the success of the welding work.
(1) Clamping action. The correct application of the fixture is related to the precision of the welding. A reasonable fixture must ensure the correct assembly of the workpiece and the accessibility of the electron beam. For easily deformable workpieces, the pressure should be moderate during clamping (or tightening) and can be cushioned with a suitable spring.
(2) Heat dissipation. For workpieces that are easily broken or heat-sensitive, the heat dissipation of the fixture cannot be ignored, as shown in Figure 1.
Photoelectric device, in which glass tube shell and metal tube shell, fiber screen
Glass and window frame trays have been sealed or bonded, the last one
The seal is welded by electron beam. The first two welds are subject to no thermal shock, so there must be a suitable clamp for electron beam welding to help dissipate heat. Pay attention to the design and material selection: the fixture and the workpiece should be well matched, and the contact surface should be large; the material should be made of materials with good heat conduction, such as pure copper.
(3) Closing effect. Some workpieces are hollow type, such as hollow spheres, which can only be made by welding. The welding precision of the electron beam is high, and the appropriate hemming mold is used to weld the two hemispheres in the welding chamber, and satisfactory results can be obtained, such as the special mold shown in Fig. 2. This welding process can not only obtain the vacuum state of the inner cavity; but also ensure that the weld sputtering problem does not occur during welding (if the air is disturbed in advance, the inner cavity air is not easily extracted, and the weld pool will be welded when welding) Sputtering occurs).
4. Welding parameters
Selecting the corresponding process parameters according to the material, size and structure of the workpiece to be welded is the main content of the welding work.
(1) The influence of welding power. The welding power of the electron beam means:
P=U·I
In the formula, P-power (w), U-voltage (kV), and I-beam (mA) directly affect the penetration of the weld. As the welding power increases, the weld penetration increases linearly, as shown in Fig. 3. Shown.
From the accelerating voltage level, the high-voltage welding machine (such as 150kV) has a stronger electron beam penetration capability. Compared with the low-medium-pressure welding machine, the welding penetration will be larger at the same power; however, there is also a view that the welding is melting. Deep depends on the performance of the electron gun.
(2) The influence of welding line energy. Welding line energy means:
E=P/S
E-line energy (J/mm), P-power (w), S-welding speed (mm/s)
The input size of the welding line energy plays a large role in the formation of the weld, such as the best aspect ratio of the weld. In addition, the workpiece is less deformed during rapid welding; slow welding prevents cracks in workpieces such as high-strength steel.
High carbon steel will crack after welding due to its structural changes (such as the formation of martensite for a long time, its expansion force and cooling shrinkage imbalance). According to reports [3], the carbon content (C) of steel is less than 0.35%, it is safe, no crack will occur during welding; when the amount of C increases, in order to avoid cracks, corresponding measures need to be taken, one of which can be welded The speed is reduced to slow down the cooling rate, at which point a good weld is easily obtained. If the amount of C is high, other methods such as preheating, annealing or filler wire welding are required.
(3) The role of critical welding parameters. When we tested the welding process of thin parts and high-precision workpieces, we found that the welding parameters were very strict. If it was too large or too small, it would lead to failure. This parameter is called critical welding parameter. As shown in Fig. 4, the strain sensor is welded [4], and the substrate (thickness 0.15 mm) and the thin tube (φ1 mm, thickness 0.1-0.15 mm) need to be welded to the outside. This is a difficult process problem. We use semi-penetration welding to reduce the deformation of the substrate. The welding parameters at this time should not be too large, otherwise the full penetration welding will cause the sputtering of the MgO powder inside the thin tube in addition to the deformation increase; if the parameters are too small, the welding strength is too low or the welding is not completed. In this case of welding, the process parameters are very critical.
5. Electron beam focal spot and bombardment site
A major feature of electron beam welding is that the weld is narrow and the penetration depth is large. This is because the electronic focal spot can be focused very finely, and the general focus diameter is about 0.5-1 mm (depending on the power of the electron beam).
(1) Conventional welding. When electron beam welding of a workpiece, the electron beam focal spot is generally adjusted to an optimum state, that is, the beam spot is focused on the surface of the workpiece (for a workpiece having a large thickness, the focus position is controlled to the inside of the workpiece). The part of the electron beam bombardment is on the contact seam (weld seam) of the workpiece.
(2) The beam spot is slightly offset from the weld on one side of the weld. For some cases or workpieces, the success of the welding, the bombardment of the electron beam plays a key role. For workpieces with uneven thickness, the electron beam bombardment should be biased to one side of the thick workpiece; for workpieces with different melting points, the electron beam bombardment should be biased to the side of the workpiece with a higher melting point. Some special workpieces, depending on the specific situation, such as the welding of porous tungsten and molybdenum, although the melting point of tungsten is higher, in order to obtain a smooth weld, the electron beam bombardment is biased to the molybdenum material, so that the molybdenum parts are partially melted. The back flow is attached to the porous tungsten surface and inside (surface) [5].
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