Electron beam welding
Weldability of materials
Materials can be classified according to conditions and requirements as:
- well weldable (good results without special measures)
- conditionally weldable (acceptable results with certain measures)
- conditionally usable (under limited property requirements)
Weldability, i.e. the properties of welds and the conditions for achieving them, is dealt with in detail in a large number of publications in journals or even books. A practically interesting and useful overview of the subject is given in the book by H. Schultz .
Alloy materials and steels
At the Institute of Instrumentation Technology, electron beam welding is used for welding components of cryogenic and vacuum instruments made mainly of stainless steel. Very good experiences have been made with welding of components made of steel ČSN 17 246, 17 248. According to our experience we can evaluate it as very good weldable.
The experience with welding parts made of CSN 17 242 is worse. Apparently, due to its higher carbon content (0.25%), it has the properties of a hardenable material, which becomes brittle during rapid cooling and cracks due to internal stress. Such a weld is not tight and is unusable for vacuum apparatus.
The problems of welding different types of steel are completely beyond the scope of this manual. If necessary, it is necessary to seek instruction in the literature or to find out all that is necessary by your own experiments.
In modern technology, the need to join components made of non-ferrous metals is becoming more and more frequent. Generally speaking, electron beam welding under vacuum is usually the best technology for this purpose, sometimes even the only one.
Even this subject is beyond the scope of this manual, so we will only briefly mention some interesting examples from our own practice.
High melting point metals (W, Mo, Ta, Zr)
The physical nature of electron beam heating is due to its ability to heat any material to a temperature limited only by its physical properties. Any material can therefore be heated to the temperature required for welding. In this respect, welding tungsten, with a maximum melting temperature of 3683 K, is no more difficult than for any other material. Their “weldability” is determined by their metallurgical properties and the change in properties relevant for the purpose as a result of the welding process. For example, changes in structure may be such that they cannot be influenced either by the welding conditions or by additional procedures.
These adverse effects of remelting or even just heating above a certain temperature and are more commonly irreversible are well known for tungsten and molybdenum. Although they can be welded relatively easily by electron beam, they are unusable for some applications because of the brittleness of the joint. We have welded other metals such as tantalum and zirconium without significant problems.
Aluminium and its alloys
The oxide layer that makes welding aluminium in an atmosphere difficult does not cause any problems when welding with an electron beam under vacuum. Despite the considerable thermal conductivity of aluminium, it is possible to achieve relatively close welds in this material, provided that the welding parameters are chosen appropriately, i.e. high power density and higher welding speed.
When welding thin-walled components, it must be taken into account that their low heat capacity and high thermal conductivity will result in an increase in temperature during welding, which is not visually apparent and can reach the melting temperature unobserved to a large extent.
There are a large number of aluminium alloys with other metals such as Mg, Cu, Si and Zn. We have no experience with welding these alloys, so we refer interested parties to the literature.
Copper and its alloys
Due to the high thermal conductivity of copper, the molten material solidifies very quickly behind the advancing weld, so that any gaseous components that may be present are not able to escape and cause porosity in the weld. Copper with impurities such as P, O, C content less than 5 ppm is well weldable.
The group of copper alloys we call bronze are generally well weldable. The same applies to the alloy with nickel, called monel.
Alloys with zinc (brass) cannot be welded in a vacuum because of the high vapour pressure of zinc, which causes spattering of molten copper by rapid evaporation.
Titanium and its alloys
Titanium, which is increasingly used in modern technology, reacts strongly with gases such as O, N, H at temperatures above about 250 °C, making welding in an atmosphere virtually impossible. In a high vacuum (pressure < 10-3 mbar) in an electron beam welder its welding does not cause any difficulties. The same is true for most of its alloys with other metals.
Nickel and its alloys
Pure nickel is probably not used much in the manufacture of components, as we have only rarely encountered the need to weld it. We have not encountered any problems in welding it.
The nickel-copper alloy, called monel, in our experience resembles copper more than nickel in its welding behaviour. In this case, too, we have not encountered any difficulties in electron beam welding.
Schultz, H. Electron Beam Welding. Cambridge: Abington Publishing, 1994.