Weldability Improvement Using Coated Electrodes for RSW of HDG Steel

The increased use of zinc coatings on steels has led to a decrease in their weldability. Weld current and time need to be increased in order to achieve sound welds on these materials compared to uncoated steels, and electrode tip life suffers greatly due to rapid alloying and degradation. In this work, typical uncoated Class II electrodes were tested along with a TiC metal matrix composite (MMC) coated electrode. Tests were conducted to study the weldability and process of nugget formation for both electrodes on HDG (hot dipped galvanized) HSLA (high strength low alloys) steels. Current and time ranges were constructed for both types of electrodes by varying either the weld current or weld time while holding all other parameters constant. Analysis of weld microstructures was conducted on cross-sectioned welds using SEM (scanning electron microscopy). Using the coated electrodes reduced weld current and times needed to form MWS (minimum weld size) on the coated steels. Current and time ranges forming satisfactory nuggets were also increased, improving the weldability of the steels. A notable cooling effect was found at the EW (electrode workpiece) interface of the steel sheet and was confirmed by software modeling to be due to rapid cooling through the electrode. The coated electrode weld did not exhibit the cooling artifact, which was believed to be due to the thermal effects of the TiC MMC coating. The lower weld current and time needed to form welds of MWS when using the coated electrodes as well as the increase in the usable current and time range indicate an increase in the weldability of the coated electrodes when welding HDG material. This notable improvement was caused by the presence of the TiC MMC electrode coating increasing the interfacial resistance as well as acting as a thermal barrier preventing the rapid extraction of heat from the faying interface through the electrodes leading to larger weld sizes at identical weld parameters than the uncoated electrodes.