The requirement for precision welding of small diameter, thin walled tubes made from stainless steel has escalated over the years due to the need for miniaturization, ultra clean welds and disposable hypodermic needles.
The use of very fine welding energy beams has been the principle solution to the problem but there is some confusion regarding the relative merits of the use of Laser Beam Welding (LBW) or Electron Beam Welding (EBW). Contrary to popular belief the heat input using either technology is similar; there is not "more heat" generated by one or the other if the correct equipment and the right parameters are used. So how do you choose?
There are many technical factors involved, but frequently either process will do the job. Here are some of the technical considerations to be considered.
1. Physical Size:
Normally small tubes are associated with small parts, so the requirement to perform welding in a vacuum chamber with EBW is not a limitation. With LBW the weld area is usually shrouded with Argon gas via a nozzle.
2. Cleanliness:
For both processes the components must be clean and free from any particles, oxides, or any organic contamination. If not the weld metal can be ejected when the beam hits it and there is a possibility of the weld cracking, porosity in the weld, or an uneven surface.
3. Fit up:
Normally, with any thin material there needs to be an intimate contact between the thin and thick materials. Most attachment of capillary tubes is made to a heavier component, but occasionally one capillary is joined to another. The reason for the components to be in contact is that the surface tension of the molten material will tend to result in the tube material and the parent molten weld pools remaining separated and not bridging the gap. Any increase in weld power would result in the thin walled tube melting back away from the desired joint. However, there is at least one technique which can be used to bridge very small gaps. The beam is directed onto the parent metal and a molten zone of sufficient width is created to "wash over" onto the capillary tube to form a sound joint.
4. Joint Preparation
In addition to cleanliness and good contact there are a few other tips which will aid in high integrity welds.
When welding capillary tubes it is preferable to weld on the end of the tube, which is normally flush or slightly protruding from the parent. This technique avoids the possibility of the beam penetrating the wall of the tube and is used extensively in the manufacture of mass flow devices which pass gas through a capillary 'U' tube. (See Figure 1)
The welding of a tube where it passes through a parent is also possible, but requires even closer control of fit and beam position and power. One method for joint preparation is to form the parent into a tapered knife edge (Fig. 2) and use the "roll over" technique to avoid putting the beam directly onto the thin walled tube.
5. Laser vs. Electron Beam
The laser weld is performed in air with an argon shielding gas. This technique is widely used, but has a couple of disadvantages compared to using an electron beam. First, the shielding gas is difficult to route to the underside of the weld, and second the volatile elements in the material are not vaporized as they would be with electron beam welding, which is performed in a high vacuum of around 10-4 Torr. The EB process is therefore "cleaner" than the laser weld, a very important factor when the device is to be used in the manufacture of semi conductors where contamination must be eliminated or reduced to very low levels.
