Welding coated steel sheets

In the automotive industry, but also in household appliances and other sectors that use steel sheet, a major trend in recent decades has been the use of pre-coated sheet to prevent corrosion, thus ensuring good control and uniformity of the coating thicknesses applied.

The most common method of deposition is to immerse the sheet metal in a molten bath of the metal to be deposited (dip galvanizing). The deposit generally consists of almost pure zinc (galvanized), which can be enriched with iron (10%) by heat treatment (galvannealed). The bath may also contain aluminum in varying proportions (less than 0.5% for galvanized, 5% for Galfan, 55% for Aluzinc), or even consist essentially of aluminum (Alupur or Alusi, which contains 10% silicon).

The more costly electroplating process is still used for pure zinc, nickel-alloyed zinc and tin in food packaging.

The thickness of the deposited metal coating is typically of the order of 10 µm, and in all cases between a few micrometers and a few tens of micrometers.

In some cases, this metallic coating may be supplemented by a chemical surface conversion treatment (mainly phosphating or chromating) designed to enhance corrosion protection, and facilitate sheet shaping or paint adhesion. These treatments modify the surface over a very small thickness (1 to 2 µm for phosphating, much less for chromating).

Lastly, organic coatings are sometimes applied in addition to the previous layers, either in thin layers (ROM or Organic Thin Coatings, less than 5 µm thick), or in thicker layers (primers before painting, 10 to 20 µm thick), or even in multi-layers up to full lacquering (pre-painted sheet metal, with an organic coating thickness of several tens of micrometers).

It turns out that the presence of these coating layers on sheet metal complicates processing, and in particular welding. The main difficulty, common to all welding processes, is linked to the early melting of zinc (419°C) and its vaporization (906°C) well before the melting of steel (1,536°C). Similarly, any organic coating decomposes between 300 and 500°C. These gaseous, liquid or solid coating residues in the weld zone are the source of various forms of pollution of the molten bath or welding tools, to a greater or lesser extent damaging the quality of the weld and the smooth running of the process.

This article reviews the main thin sheet welding processes (resistance welding, arc welding, laser welding), the particularities involved in the presence of a coating, and the industrial solutions proposed to overcome the difficulties encountered. Among these, the issue of welding fumes from coatings concerns all processes. In the presence of a metallic...