Spray arc welding is a high-energy method of metal transfer used in Gas Metal Arc Welding (GMAW), often referred to as MIG welding. It differs significantly from the common short-circuit transfer mode used for thinner materials. Spray arc is defined by the continuous, non-contact transfer of molten metal from the wire electrode to the weld pool. The metal is propelled across the arc as a stream of fine, uniform droplets, resulting in a smooth, low-spatter weld, which makes the process highly efficient for joining thicker sections of metal.
How the Spray Arc Transfer Works
The mechanism of spray arc transfer relies on a continuous arc that never extinguishes or shorts out against the weld pool. Achieving this requires a high current and voltage combination that pushes the process past a specific threshold known as the transition current. Once this threshold is crossed, the physics of the arc fundamentally change, allowing for the unique transfer mode.
The high current generates a powerful electromagnetic force, often called the pinch force, which acts on the molten tip of the electrode wire. This force is the dominant factor in detaching the metal, overcoming gravity and surface tension. The electromagnetic force constricts the molten metal, pinching off tiny droplets that are smaller than the diameter of the wire itself. These fine droplets are then axially propelled across the arc gap at a rate of several hundred per second, creating a stable, focused stream.
This continuous, high-velocity transfer of fine droplets gives the process its name and produces a smooth, steady sound, often described as a distinct hiss. Because the arc is continuous and the droplets are small, they never bridge the gap to the weld pool. This mechanism is the primary reason for the minimal spatter produced, resulting in a highly stable arc and a very clean weld bead profile.
The High Power and Gas Requirements
Achieving true spray arc transfer requires specific equipment capabilities that often exceed the capacity of common home or hobbyist MIG welders. The process demands a high-output machine capable of delivering the necessary current and voltage to surpass the transition current. For mild steel, this typically means the machine must be able to sustain a minimum of 24 volts and often 200 or more amperes, depending on the wire size. Many smaller 120-volt or 200-amp class welders simply cannot reach the required voltage and amperage combination to initiate and maintain the spray transfer mode.
The shielding gas composition is equally specific and is a major factor in enabling the spray transfer. The process requires an argon-rich mixture, generally a minimum of 80% argon with the balance being an active gas like carbon dioxide or oxygen. Common hobbyist gas, such as 75% argon and 25% carbon dioxide (C25), will not effectively support a true spray transfer because the higher percentage of carbon dioxide suppresses the electromagnetic pinch force. Gas mixtures like 90% argon/10% carbon dioxide or 98% argon/2% oxygen are far more effective at stabilizing the arc and promoting the fine droplet transfer.
To handle the high current density, the process typically utilizes larger diameter welding wire, such as 0.040 inches (1.0 mm) and up. The combination of high voltage, high amperage, argon-rich gas, and larger wire diameter are necessary inputs. Without these specific inputs, the transfer mode will revert to the less efficient globular transfer, which is prone to spatter.
Deep Penetration and High Speed Welding
The high-energy nature of spray arc welding translates directly into significant benefits for specific applications, primarily involving thicker materials. The continuous, high-heat arc provides excellent fusion and deep penetration into the base metal. This makes it an ideal choice for structural welds on materials that are typically 1/4 inch (6.4 mm) thick and greater, where deep root fusion is necessary for strength.
The continuous transfer of fine droplets allows for a high deposition rate, meaning filler metal can be laid down quickly. This significantly increases welding speed and productivity, making the process highly valued in industrial and fabrication settings. The resulting weld bead is characterized by a smooth profile and minimal spatter, which reduces the time needed for post-weld cleaning.
Why It Is Not for Thin Metal or Out-of-Position
While spray arc welding offers high speed and deep penetration, the process is limited by its intense heat input and the fluidity of the weld pool. The high heat makes it unsuitable for welding thin materials, as the intense energy quickly causes burn-through or excessive distortion. For materials thinner than approximately 1/8 inch, the heat input is too high to manage effectively.
The other major limitation is the positional restriction of the process. The large, highly fluid weld pool created by the high heat is difficult to control against gravity. For this reason, spray arc welding is restricted primarily to flat and horizontal welding positions. Attempting to weld in vertical or overhead positions will cause the molten metal to sag or run out of the joint, unlike short-circuit transfer, which uses a small, fast-freezing weld pool suitable for all positions.