For the steel maker, Laser Welding is caracterized by the absence of filler metal. It implies that the weld metal is chemically homogeneous to the base metal. Particularly it means that the amount of impurities depends only on the steel making. For many applications, and specially for high strength (or hardness) steels, Laser Welding is one of the best possibilities to obtain really matching welds (with Electron Beam Welding and Gas Tungsten Arc Welding).
As to assess this assumption, Laser Welding has been used to join high hardness steel plates. Conducted in industrial conditions, the operation has proven to be safe when using high quality steel. No defects such as cracks or porosities have appeared even for thicknesses up to 10 mm and for non optimized conditions.
The main properties to check in the welded or affected zones of such materials are hardness and toughness. Concerning hardness, it is clear that the weld metal and the heat affected zone are always harder than the base material. Only a short zone tempered by the welding thermal cycle is softer than the base material. Compared to the usual Gas Metal Arc Welding, the soft zone is 5 to 8 times thinner. This is essentially due to the use of austenitic or low carbon ferritic wires when GMA Welding.
Due to the high purity of the base material, the toughness of the welds can appear as surprisingly high in regard of the obtained hardnesses. The prediction of critical defects shows that for instance undercuts of 10% of the thickness are acceptable for 500Hb weld metals.
For the materials subjected to ballistic requirements, the tests have shown that the welded zones are at least as resistant as the base material when such levels of toughness combined to the high hardness are obtained.
Based on these excellent results, applications begin to appear. For instance, on armoured vehicles, Laser Welding and Cutting can be done with very low distortions (one of the technological features of Laser Welding) without any specific tooling for the previous adjustment and stiffening of the different parts. Working in all positions Laser Robots avoid also any tooling for positionning of pieces during welding. The same Laser system can also be used for cutting after welding on finished assemblies. This introduces a new flexibility in this type of fabrication, particularly when associated with “off line” programmed robots, but not only. This is the accurate solution for prototype and short series or “just in time” fabrication of various welded products.