Weldability of various materials

The possibility to join metal parts by fusion welding depends first of all on their metallurgical properties. This problematic is extremely wide, therefore, in this paper, we shall restrict our interest to only some specific questions, more important for our needs. Apart from those classical, commonly used materials, also some exotic ones, applied mainly in new branches of research and industry are also needed. This needs accelerated in the second half of 20. century the development of new methods of welding, applying laser or electron beam. The welding by electron beam in vacuum proved to be the most suitable, if not the only one applicable method for welding some materials.

The definition of the word weldability is neither easy nor ambiguous, as it depends on too many conditions. In this paper we shall consider the material as „weldable“ if the fusion zone is without cavities, cracks, and if mechanical or other properties will not much differ from the basic material. As weldable may be considered also such materials when the welds are not perfect, but still meet the given demands.

The materials can be classified from the point of view of required conditions and demands as:

• well weldable (without special precautions)
• satisfactorily weldable (acceptable results obtained when special precautions are taken),
• conditionally weldable (by restricted demands).

Of course, such classification is nothing but convention, which is limited only to this paper. More closely is this question elaborated in many books or magazines (see for example 1).

Alloyed steels and other materials

In our Institute of scientific instruments, electron beam welding is applied to join components of vacuum or cryogenic instruments, made mainly of stainless steel. We have made very good experience with welding of the ČSN 17 246 , –48 (X69CrNiTi1810) stainless steel, which we classify as very good weldable. Worse experience we have made with the austenitic stainless steel ČSN 17 240 , which, as consequence of higher carbon contents (about 0,25 %), by rapid cooling of the weld tends to crack. Such a weld leaks and can not be used for vacuum parts.

The problem of welding various types of steel far exceeds the scope of this paper. The more detailed information is to be looked for in specialized literature. The best way how to solve a specific problem is, in any case, to make and evaluate test welds.

Refractory metals (W, Mo, Ta, Zr)

The high power concentration in electron beam makes possible to heat any material rapidly to its melting temperature, no matter how high it is. Hence, the tungsten with the highest melting point, 3.683 K, and all other refractory metals can be melted easily. The first condition for their weldability is so fulfilled. Other conditions are their metallurgical properties, and possibly their change caused by remelting and subsequent cooling. Some such changes can not be prevented and are not reversal. One material with such properties is, e.g. tungsten, which, being treated at high temperature becomes very brittle. No such negative consequences have we observed by welding other refractory metals, like e.g. tantalum or zirconium.

Non-ferrous metals

In modern technologies more and more often, we need to join parts made of non-ferrous materials. In most cases, the electron beam welding is the best method, if not the only one applicable. In this paper we shall present some interesting examples from our own practice.

Aluminium and its alloys

The thin layer of oxide, which makes the welding of aluminium in air rather difficult, is by electron beam welding in high vacuum no obstacle. Regardless of the high thermal conductivity of aluminium, deep and narrow welds can be achieved in this material, if suitable parameters are chosen: high power concentration and higher welding speed. By welding thin-walled aluminium parts must be kept in mind, that in consequence of their small thermal capacity and high thermal conductivity, wide region in the vicinity of the weld can be warmed to a temperature close to, or even reaching the melting temperature of aluminium. This can happen without being noticed by the operator, as the hot aluminium at the melting point does not emit visible light. This could be prevented by high welding speed or application of cooling jigs.

There are many alloys of aluminium with other metals, used in engineering, like Mg, Cu, Si and Zn. As we have not our own experience with such metals, the kind reader is referred to other literature.

Copper and its alloys

As a consequence of high thermal conductivity, the fused copper is cooled rather rapidly, so preventing the possible impurities to leave the metal while it is liquid. This does not happen if the impurities like P, O, or C do not exceed 5 ppm.

The alloys with Sn (bronzes) are good weldable, even with some other materials, e.g. steel or stainless steal. Alloys of copper with Zn (brass) can't be welded in vacuum, because the high vapour pressure of zinc causes intensive spraying of the melted copper. Alloys of copper with nickel (called monel) are also easily weldable by electron beam in vacuum.

Titanium and its alloys

Titanium, more and more used in modern technologies, at temperatures higher than about 250°C strongly reacts with gases like oxide, hydrogen or nitrogen, that's why welding at atmospheric pressure is almost impossible. In high vacuum (pressure < 10^-3mbar) its welding by electron beam makes no trouble. The same is true about most of its alloys.

Nickel and its alloys

Pure nickel is very rarely used in engineering constructions, but if needed, it can be electron beam welded without any troubles. The same is true about its alloys, e.g. with copper, known as monel. This material is composed primarily of nickel (up to 67%) and copper, with some iron and other trace elements. By electron beam welding it behaves similarly as copper. In vacuum can be electron beam welded without any substantial problems.

1. 1. Schultz, H.  Electron Beam Welding. Cambridge: Abington Publishing, 1994.