Dual shield welding, aka flux core arc welding (FCAW), is a process where an arc is used between a consumable electrode and the weld pool created from the metal shields. The pool is shielding by a flux contained within the tubular electrode with an additional shielding from an externally supplied gas.
The method is called dual shielding because two layers of shielding are provided to the weld during the process.
In this blog, we examine how dual shield welding is carried out and why many welding professionals prefer to use this method for their projects.
Dual shield welding
Dual shield welding uses both a slag system and an external shielding gas to protect the arc from the environment. The wire used in the process is often described as a “double shielded” electrode.
The wire adds the desired level of alloy elements to the puddle created by the melting metal. It affects the structural properties of the weld.
The core metal of the wire also produces a slag, similar to the self-shielded wires, which coats the weld.
Additional protection is added by a shielding gas that covers the arc to prevent contamination from the atmosphere. Two types of shielding gases are used in the process.
The first is 100% carbon dioxide (CO2). The second is a mixture of 75 – 85% argon (Ar) and 15 – 25% Carbon Dioxide (CO2).
The use of shielding gas greatly improves the operator appeal and usability of these wires.
Most types of shielding wires have a small droplet arc that transfers in a smooth, spraying arc. The recommended polarity is DC+ for all types of wires.
Dual Shield welding is generally preferred for indoors, shop welding as the arc offers smoother arc characteristics. This method of shield welding can be used outside, but require extra precautionary measures to keep the wind from blowing away the shielding gases.
If the shielding gases are dissipated, porosity will appear in the final weld.
Application of dual shield welding
Dual shield welding is used in welding applications similar to manual metal arc or MIG welding. The process is generally controlled by hand but can be mechanized depending on the nature of the welding job.
Many auto and machine production factories use dual shield welding in their assembly lines.
Cored, tubular electrodes are available for use in dual shield welding in a wide variety of compositions. They can be more useful than solid wires because of how easy it is to introduce alloying elements in powder form.
Gas shielded flux cored wires can easily meet the mechanical property requirements in a wide range of welding applications. In comparison, the mechanical properties you can achieve with self-shielded cored wires are quite limited, with maximum weld metal strengths of 700 N.
Dual shielded welds are also suitable for thicker metals or in cases where you have to apply out-of-position welds. They leave a molten slag that solidifies quicker than the weld pool.
This helps create an outer “shelf” to hold the molten pool inside which cools to form a stronger joint. It is ideal when you are welding overhead or vertically up.
Advantages of dual shield welding
For certain types of welding projects, dual shield welding is the ideal choice. Let’s consider some of the advantages of dual shield welding that make it popular among welders.
The dual shield welding method offers much better productivity and cost savings for businesses. Generally, labor and overhead become the most expensive activities for management in a welding operation.
They take up about 60 to 80 percent of the total cost of business.
Welding with dual shield FCAW electrodes allows welding businesses to complete jobs quicker, with higher deposits and deeper penetrations. It gives an immediate means of cost reduction without spending an exorbitant amount of money on equipment or labor.
Experts believe that businesses using dual shield welding in their operations can save up to 60% of the total cost of depositing one lb. (0.45 kg) of weld metal when compared to self-shielded welding.
High deposition rates
The dual shield welding process allows you to make a high deposit on a weld because of the relatively high current density. In most cases, a higher deposit means greater structural strength.
The ratio of current (in amperes) on a cross-sectional area of an electrode is taken as its current density. In FCAW, a very thin metal sheath provides the primary current path for the arc.
Since the resistance heating used for welding is concentrated in a very tiny area, the flux-cored electrode reaches its melting point very quickly.
This heats up metal in seconds and leads to very high deposit rates on the puddle.
Another benefit is that the dual shield electrodes’ small cross-sectional current path makes it easy for the arc stream to have a more columnar impact rather than a wider surface area. This allows the current to penetrate deep inside the metal.
A deeper penetration allows you to increase the effective throat of a fillet joint. If the effective throat of the fillet is increased due to deeper penetration, the strength of the joint does not rely much on the exterior size of the weld but on the melded metal itself.
With this technique, it is possible to reduce the leg dimensions and decrease the fillet size to as small as 1/16″ (1.6 mm). This can reduce the total weld metal required by as much as 50 to 60 percent, which is great for a smoother but strong finish.
In comparison, a fillet type weld that you make with a self-shielded stick electrode has a shallow root penetration.
Note here: The deepest penetration with dual shielded welding occurs when you use straight CO2 gas for shielding. It is more expensive but highly effective at shielding the arc.
Dual shielding allows you better control on the penetration and deposit on the weld. This gives you better control over the design of the meld.
You can decrease the angle or root opening of a joint with dual shielding electrodes more effectively because they have a smaller diameter and deeper penetration.
The tighter joint design can also reduce the volume of weld metal required to fill the joint. The deeper penetration that you get with FCAW also lowers chances of mistakes with out-of-position work that is common with stick welding where you have low current with short circuit transfer.
Since dual shielded electrodes operate at a higher welding current in out-of-position work, they allow better sidewall fusion and increase weld integrity while minimizing expensive reworks on a project.
Rapid learning and little skill required
Dual shield welding with flux-cored electrodes does not require a lot of skills or hundreds of hours of training. It is quite easy to train even new welders on how to make all kinds of welds with FCAW electrodes than it is with other types of welding processes.
In dual shielded welds, the outer surface of the slag created during the weld holds the molten puddle inside in place. This allows greater control and smoother finish without creating porosity.
FCAW allows you to produce high-quality welds in a period.
Welding tips for dual shield welding
Here are some tips that will help you improve your welds with dual shielding technique:
- Be mindful of cleaning off slag after each pass.
- For backhand welds, drag the gun instead of pushing forward.
- For overhead welds try to maintain a fast travel speed.
- Generally, MIG welding is electrode positive and welders switching to FCAW will need to ensure that electrode polarity is negative before they start.
- If possible, try not to use smooth wire drive rolls. Use knurled drive rolls instead.
- If you are looking to make vertical welds you can angle either up or down. For thinner metals, it is better to weld down vertically. For metals thicker than 1/4″ weld up. Also, make sure to turn parameters down 10 to 15% on the machine.
- Only put 1/2″ to 3/4″ of wire stick out.
- Weld in a side to side motion to avoid undercut with the gun.
- When making flat welds weld at 90 degrees and 10 degrees back. For T Joints weld at 45 degrees. For Lap joints, weld at 60 degrees to 70 degrees in a single straight weld.
- When working on horizontal angles, point the gun upwards at around 10 degrees. Turn welding parameters on the machine down about 10 to 15%.
- You will mostly use dual shielded welding indoors or in confined spaces. Make sure there is plenty of ventilation in the workshop to avoid health hazards.