Galvanized steel is huge in the automobile industry. The protective layer of zinc oxide on its surface makes it corrosion resistant and protects the car’s structural integrity from
rust, providing higher safety standards. Galvanized steel also provides higher strength and durability, even at thinner gauges.
“Galvanized steel for the automotive industry provides a lower cost than non-ferrous metals like stainless or aluminum and its corrosion resistance protects parts against the environment, increasing their life,” says Jose Abal-Lopez, global product manager, non- and low-alloyed solid wires, ESAB Welding & Cutting Products.
While galvanized delivers numerous advantages, there also issues that have to be addressed when welding it. The zinc oxide coating on the galvanized steel creates welding challenges, including porosity, spatter and burnthrough. These issues result in rework and rejected parts, increasing cycle times and adding cost. To complicate matters, different galvanizing methods create different coating thicknesses, increasing variability.
Fortunately, the right welding wire can prevent many of the adverse effects caused by welding galvanized steel. To further increase the quality of welds on galvanized, there are various tips and techniques of which to be aware.
To ensure a sound weld, the heat of the welding arc must burn through the zinc oxide coating, establish a weld pool and sufficiently penetrate the base material. However, given that typical automotive components measure only a few millimeters thick, applying more heat increases the potential for burning through the base material.
Weld porosity proves to be one of the greatest challenges. With faster travel speeds, the weld pool tends to freeze faster. That is a problem because zinc vaporizes (boiling at 1,664 degree F) at a much lower temperature than steel melts (2,498 degrees F). This can lead to gas pockets becoming trapped as the weld pool solidifies before the zinc vapor can escape, creating porosity issues that leads to scrapping of parts entirely. These weld defects can also lead to mechanical failures.
Zinc vapor interaction with the weld pool also contributes to excess spatter. Spatter adheres to the part, requiring post-weld cleanup, and also can adhere to tooling, leading to damaged guns, torches and tips as well as sensors and clamps on robotic systems. Spatter also causes cosmetic and painting issues.
However, proper part design, welding procedures and techniques, as well as advancements in welding equipment and consumables all offer automotive manufacturers new ways to address these challenges and improve their overall galvanized steel welding operations.
Because zinc vapor is the root cause of welding problems when working with galvanized steel, welding procedures must be developed to allow the vapor to escape. Some techniques include using a steeper-than-normal torch angle, welding in a horizontal position, running slower travel speeds and using gas mixtures with a higher argon content instead of the standard 75/25 argon/CO2 mixture for short-circuit MIG welding. Using an anti-spatter spray on the weldments eases the spatter removal process.
If the welding power source is capable, using a pulsed spray transfer welding process manages the deposition of the wire across the arc at controlled intervals in the welding cycle. This process can allow time for the zinc vapor to escape and reduce porosity and spatter. In addition, pulsed MIG provides the ability to control heat input (kilojoule per inch), making it less likely to burn through the base material.
As for consumables, the standard low-cost option is an ER70S-6 solid wire. However, this wire causes the most problems with porosity and spatter (as well as convex bead shape issues), so it ends up costing more in the long run. Metal-cored wire formulations can address spatter and porosity, but can cost three times as much as solid wire.
One of the best options – one that balances cost and performance – is an ER70S-6 wire developed specifically for MIG welding galvanized steel with the short-circuit and pulsed spray transfer welding processes.
“Through tightly controlled micro-alloying elements in its formulation, OK AristoRod 38 Zn solid wire controls surface tension in the weld pool in a manner that allows the zinc vapor to escape,” Abal-Lopez says. “The AristoRod 38 Zn performs similarly to metal-cored wire as far as reducing or controlling porosity, and sometimes better in terms of burnthrough risk. It also costs less than a metal-cored wire while addressing the root cause of porosity and spatter associated with standard ER70S-6 wire.”
In addition to the tighter control of the main alloying elements that comprise the OK AristoRod 38 Zn wire, its specification consists of 14 additional elements.
“Microalloying and otherwise controlling wire composition and surface finish are the key to fine tuning solid wire performance, whether that’s to address zinc vapor issues or
issues related to spatter, porosity or silica islands,” Abal-Lopez says. “People sometimes buy welding wire based on the cost per weight because they assume all wires in a particular category perform similarly. Unfortunately, those who buy purely on cost miss out on the benefits of reduced spatter and less cleaning, fewer weld rejects and other benefits that reduce the overall cost of the part.”
Even with using an optimized solid wire, companies must consider every variable in their welding procedures.
“It has to be a complete approach, and the best approach is to work with experts who understand every aspect of the filler metal, process and welding application,” Abal-Lopez concludes.