As the automotive industry evolves, the need for thinner and lighter weight components are needed to meet the ever-challenging CAFÉ standards to improve gas milage. An unforeseen issue arises as the materials today are thinner than the materials in the past, which require some sort of protection to prevent premature failures from rust out. Most of the under-belt materials are now zinc coated and are painted after processing to aid corrosion life. The GMAW welding process presents a unique challenge before the painting process, as the weld deposit leaves behind surface silicates. These silicates will not bond to the paint, resulting in potential corrosion initiation points. In the past with thicker materials that were not zinc coated, an acid descale process could be performed. This acid descale process would remove the surface silicates from the weld before the painting process. With zinc coated materials you can no longer use the same acids to remove the surface silicates as it would also remove the zinc.
Per the request of several global automotive OEMs, GMAW wire manufacturers created a new type of wire that has very low silicon, resulting in a change to the surface of the welds. This change results in a surface oxide not a surface silicate. These oxides have been proven to coat, resulting in improved corrosion resistance verified in numerous industry studies, from more than one GMAW wire manufacturer. A side benefit from the changed chemistry is an improvement in the weld quality when welding on zinc coated materials as it pertains to porosity. Studies have shown that with the reduced silicon in the wire, the wires become less resistive, requiring more arc energy. This increased arc energy and the resulting change in surface tension of the liquid weld metal pushes zinc gases out at the arc cone. With traditional GMAW wires the gases try to escape behind the arc in the solidification zone. Studies have shown 3 times increase in zinc outgassing with low silicate wires, resulting in an average reduction of porosity by 40%.
These new wires were initially introduced to the market in 2020 under a general AWS classification such as ER70S-G or ISO G42 3 M20Z which both had very open chemistry ranges. In 2023 a new AWS classification was balloted and approved for these new low silicate wires, ER70S-9 under the A5.18 specification. The reason for the presentation is to get the word out about this new classification and the benefits from using these new GMAW wires to aid in paint adhesion, improving corrosion life and improving the weld quality on zinc coated materials
