In GMA welding of AHSS, slag particles are observed to form on the weld surface resulting in poor corrosion resistance and reduced strength.
Current methods to decrease the formation of slag islands are costly as they attempt 1) to better protect the weld pool by enriching the Argon content in the shielding gas and 2) to mechanically clean the weld surface after GMAW.
In this study, the amount of slag was reduced by adjusting the chemical composition of welding consumables. Silicon was controlled to minimize silica/silicate formation and S addition affected the flow of slag particles in the weld pool to regions of easier removal.
2. Experimental Procedures
In this study, three welding consumables were investigated. Wire A was an existing wire that meets AWS A5.9 ER70-3 specification. Wire B with a reduced Si content was developed to minimize silica/silicate formation and an increased S content to affect the flow pattern of the molten pool. Wire C had its composition adjusted to form Mn3O4 and SiO2 slags.
Gas metal arc welding was performed using a Fronius CMT welding machine and base metal was GA steel (2.3t). Lap joints were produced using 240A, 24V and 80cm/min. A high-speed camera was used to observe the slag flow behavior in the molten pool.
The fractions of slag coverage and weld morphologies were observed on welds conducted on actual automobile body parts using wires A and B. RT (Radiography Test) was conducted to determine whether weld metals on Gr.780MPa galvanized steel exhibited porosity defects. Tensile and impact testing were also performed.
Finally, paintability and corrosion resistance of the experimental welds were also examined following industry established procedures.
3. Results and Discussion
1) The flow of slag behavior
In the case of conventional welding wire, slags distributed to the center and toe of the weld bead. When observed with high speed camera, slags moved towards the back of the weld pool along the edge and solidified as large particles in the center and as small particles along the toe. This slag movement behavior is affected by the weld pool flow pattern, which moves from low surface tension (high temperature) region to high surface tension (low temperature) region.
In the case of wire B, slag was observed only in the crater. Two large slag particles formed during welding and moved along the welding direction. These two large particles eventually agglomerated into one large particle and solidified in the crater after the welding was finished. The presence of surface-active elements like sulfur changed the weld pool flow behavior as described by Marangoni flow. The slag moved from the weld periphery towards the center of the weld pool.
Unlike wires A and B, wire C showed less slag on the molten pool, because low Si produced less silica/silicate.
2) Slag fraction
In Ar-20%CO2 shielding gas, the slag fraction of the wire A weld was 11.8%, and the slag fraction of the wire B weld was 5.3%, i.e. with a reduction greater than 50%. The slag of the wire B agglomerated and solidified into a large particle on the weld bead that could be removed easily.
In the case of the conventional wire (A), the slags on the bead did not paint well compared to those of wire B, which showed no unpainted area because no slag formed. Unpainted areas will not have good corrosion resistance and will shorten the life of automotive parts.
4) Corrosion resistance Testing (CCT)
Commercially available automobile parts were welded using the wires A and C and painted for CCT testing. After 47 days, the automobile part using the wire C had less corrosion than that using the wire A.
5) Mechanical properties
The tensile and yield strength of welds made with wires B and C are higher than those made using wire A, and the elongation of wires B and C welds was slightly lower than those of wire A. However, they all satisfied requirements set by AWS A5.18 specification. In the case of the impact test, welds of all wires showed greater than 27J, which was the minimum requirement set for testing at -30 degrees Celsius.
6) Welding high strength galvanized steel
When applied to Gr.780MPa galvanized steel, tensile strength was greater than 700MPa with the fracture located at the HAZ. Also, no porosity was visible in the RT films.
The use of the two experimental welding consumables reduced the amount of slags, which improved the corrosion resistance and the mechanical properties of the welds. Also, surface porosity in galvanized Zn coated steel welds was reduced.
Keywords: welding consumables for automotive welding, advanced high strength steel, galvanized steel, low slag formation, weld pool flow pattern, corrosion resistance