Welding of Pure or Low Alloy Materials: Difficulties and Solutions

Pure metallic materials or low alloyed materials find many engineering applications and has many potential applications. For example aluminum alloys find extensive applications as thermally conducting fuel cladding materials in nuclear research reactors. Another important material is TZM (Titanium Zirconium Molybdenum); which contains more than 99.5% Mo. There are many applications where pure or nearly pure (or low alloy) materials need to be welded and welding of these materials is not so easy. So what are the problems and what could be potential solutions.
Difficulties:
1. High thermal conductivity:
Pure or nearly pure metallic materials exhibit high thermal conductivity. Therefore, the heat deposited at the joint line runs away quickly from the joint and thus entire component gets heated up at the cost of fusion of the material near the joint. Whether high energy density process like Laser Beam Welding or Electron Beam Welding will help? It helps to some extent but there are other problems.
2. High reflection:
The metallic materials which exhibit high thermal conductivity have high reflectivity as well and this acts as double whammy for laser beam welding of such materials – First, they will reflect most of the energy and then they will conduct away whatever was absorbed. But this is not all there is one major metallurgical issue.
3. Narrow liquidous:
Low alloy materials have narrow liquidous and therefore melt solidifies quickly leading to entrapment of gases / vapors and thus formation of porosity.
So what are the solutions?
Solutions:
1. Twin Spot Laser Beam Welding:
In this technique, the laser beam is focused at two spots – one leading and another trailing. Now many things can be varied to suit the purpose like power of the leading and the trailing beam and the distance between the two. The leading spots serves the purpose of melting the materials surface for promoting the absorption of the trailing beam and the trailing beam penetrates deep and prolongs the melt life and thus enables the release of the entrapped gas / vapor and thus minimizes occurrence of porosity.
2. Laser Arc Hybrid Welding:
Here the arc creates a shallow melt skin and the falls in this molten skin leading to deep penetration. Arc welding has longer melt life and thus occurrence of porosity is minimized. Additionally, arc welding has larger gap tolerance between the work pieces and therefore edge preparation is reduced to that extent.