Welders can utilize multiple methods and techniques for virtually the same welding procedures. They can opt for Metal Inert gas (MIG), Gas Metal Arc Welding (GMAW), or Metal Cored Arc Welding. For non-positioned butt and fillet welds, however, the best welding technique is probably spray arc welding as it increases the efficiency of the welder.
What is Spray Arc Welding?
Spray arc welding is one of the processes used to transfer metal from the electrode or wire to the weld. Minute molten droplets of metal travel via the arc and on to the base metal or the joint being worked on.
Spray Transfer is ideal for use on thicker metals for butt or fillet joints. It is not suitable for positioned welding jobs as the flying metal droplets and gravity do not create an ideal situation for the welder or the welding job.
Spray arc welding is technically a spray of molten metal transferred over the arc, similar to water coming out of a garden hose with a restricted opening. Spray arc welding reduces welding splatter and produces a weld with higher finesse levels.
The method involves high amperage and voltage levels. The wire, when brought close to the base metal, produces a current before it touches the metal. The current heats the wire very quickly and melts it. The molten metal travels over the generated arc in the form of minute droplets, giving it the name spray arc welding.
Spray arc welding can produce high metal deposition rates when provided with the ideal combination of the shield gas, the metal, and the wire diameter, as well the contact to tip distance. Given the perfect combination of all, the process can produce very high transition currents. The process is also called axial spray.
The current levels used in this process need to be higher than the transition current. Only then will the metal transfer in the form of droplets and not just melt. The process requires using high current levels with sufficient voltage levels to ensure a spatter-free weld.
Spray arc welding is a very efficient process. Some of the significant benefits offered by this process include:
- High metal deposition rates
- Good metal fusion and penetration
- Excellent weld bead appearance
- Ability to use larger diameter electrode wires
- Very little spatter generation
Despite the presence of such extensive benefits, the spray arc transfer method does have substantial limitations.
The limitations of spray-arc transfer include, but are not limited to:
- It is only suitable for use on thick materials (about 1/8 in. (3 mm) and thicker)
- It is limited to flat and horizontal fillet weld positions
- It does not have open root capabilities
The large weld pool created in this technique limits the number of positions and types of joints it can be used on. It is not suitable for thinner sheets of metal as it utilizes high heat levels and can quickly burn through the layer. Some of the limitations of this process can be handled by using the pulse spray transfer technique instead.
What is Pulse Spray Transfer?
In pulse spray transfer, current levels are released in short pulses or intervals. Using short pulses instead of using a continuous current supply makes the weld pool more manageable, further increasing the efficiency of the process. It also allows for the spray transfer technique to be used in welds in more positions as compared to simple spray transfer.
This process uses lower background currents, which reduces how much heat is produced. It is ideal for operations on thicker metals that require controlled heat input and do not need a high travel speed or transition currents.
Besides the advantages, the spray transfer method already offers, the pulsing technique offers a few more.
- Better control over the weld pool
- Reduced heat input
- More manageable weld
- Further reduces spatter
- Smoother weld and better weld beads
With the right shield gas and the right filler metals, pulse spray transfer can even be used for positioned welds. The right shield gas is critical in the production of quality welds.
How to Select the Right Shielding Gas
Various factors can affect your shielding gas choice.
- Filler metal deposit rate and efficiency
- Spatter control
- Bead profile
- Post weld cleaning
- Bead penetration
- Weld positions
- Welding fume generations rates
- Welding process technicalities
The shield gas combination you choose will depend on the metal wire or electrode you’re using for the weld. As a rule of thumb, though, all shield gas combinations for spray arc welding use high levels of argon. At the very least, the mixture must contain a minimum of 80% Argon gas.
The shield gas directly affects the reactivity rates of the metals. A gas with higher reactivity can significantly increase the weld puddle fluidity. The speed at which the weld puddle increases may be too high for you to control, which could result in a disastrous situation.
Welding steel carbon requires a combination of argon and CO2. However, anything above 15% CO2 can decrease the transition current and metal deposition rates. At such low deposit levels, the process then turns into a globular transfer method and loses its efficiency.
Importance of the Shield Gas
The shield gas acts as a protective barrier between the arc and the atmosphere. The gases present in the air, mainly oxygen and nitrogen, can react with the molten metal and produce harmful fumes and bad welds. Besides ruining the weld, an inefficient shield gas can also cause other problems.
porosity refers to the contamination within the weld metal. It comes as a result of gas trapped within the metal, which then forms either round or elongated holes within the weld. Such a situation comes about as a result of either the absence of a shielding gas or its ineffectiveness.
Welding together old metal pieces requires extra pre-welding care. Old and rusted metals can also produce porous welds. Before you begin welding, appropriately prepare and clean the surface first.
A porous weld can sometimes be accepted if the level of porosity is too low, and the industry requirements are not too strict. Porosity is a severe defect, though, and, if found, can be the cause for rejection.
Low Metal Deposition Rate
Your choice of shield gas directly affects arc stability and production efficiency in weld procedures. For spray arc welding, in particular, a combination with high argon levels proves best. Argon acts as the ultimate protective barrier between the metals and the atmosphere. Because your choice of shield gas also directly influences the wire feed rate, the wrong gas can disrupt the entire procedure.
The shield gas also performs other functions besides ensuring a smooth weld.
- Formation of the arc plasma
- Stabilizing the arc on the metal surface
- It also guarantees the undisrupted transfer of metal droplets from the wire to the metal pool.
What is short-arc welding?
Short-arc welding is another metal transfer welding method. In this method, the weld or filler metal wire is literally short-circuited into melting. This technique is used to join shorter welds in thinner sheets. The heat generated by the short-lived short circuit is enough to melt the thin layer to form a weld.
Short arc or Short circuit welding is useful if you’re looking to join thin materials in any position and almost all types of joints. Metal is transferred from the wire to the base during each short circuit or when both come in contact.
What gas do you use for spray transfer?
A shield gas combination with high levels of argon is ideal for the spray transfer method. However, for carbon steel, a combination of argon and carbon dioxide proves more efficient. A different mixture may prove more suitable for different alloys as well, so there is no set gas for welding.
When choosing a shielding gas, make sure you check for:
- Gas reactivity with the weld metals
- Weld positions
- The desired weld penetration
- Desired bead profile
- Required metal filler rate
How do you perform spray transfer welding?
Spray transfer is officially known as the spatter free axial spray. It is a metal droplet transfer process that uses high voltage and high amperage. It works best on thick metal joints as it provides deep penetration. Because gravity plays a central role in the process, it is only suitable for vertical or horizontal joints. Ideally, it works best for butt and fillet welds as it produces smooth and sturdy joints.