Welding variables and how they affect the fabrication process

Properly maintaining welding variables is critical to establishing high weld quality in any operation.

Properly maintaining welding variables is critical to establishing high weld quality in any operation. Many companies put welding procedures in place that define the recommended parameters to help create consistency among welders and parts. Understanding what each welding variable in a procedure is and what it does can go a long way in helping welders meet productivity goals and reduce downtime and rework costs.

For constant voltage (CV) or CV welding with solid or tubular wires, welders must consider key welding variables and their functions and understand how they affect the process.

Welding amperage refers to the amount and speed of electricity flowing in a circuit, which affects the heat available to melt the welding wire and the base material. It is directly correlated to wire feed speed (WFS): the speed and volume of filler metal going into the weld. When WFS increases, so does the welding amperage; when it decreases, so does the amperage. This correlation, in turn, affects weld penetration. Higher amperage settings yield greater joint penetration while lower amperage settings provide less.

Welding amperage has an inverse relationship with contact-tip-to-work distance (CTWD), which is the distance from the end of the contact tip to the base material. Some also use the term to indicate the arc length in combination with the stick-out or how far the wire extends from the contact tip when it is flush with the nozzle. If an operator increases stick-out, welding amperage will decrease and vice versa. Changes to CTWD also affect weld penetration: the closer the contact tip is to the base material, the greater the penetration.

In addition, welding amperage affects melt-off rate—or how much wire is being used—along with weld bead appearance and heat input. Amperage that’s too high, particularly when welding with metal-cored wire, can result in a dull, flaky weld. Amperage also directly increases or decreases heat input and combined with travel speed has the largest effect on heat input. Heat input is calculated by the following:

(60 x Amps x Volts)/(1,000 x Travel Speed in IPM) = KJ/in.

In addition to being directly correlated to amperage, WFS also affects welding transfer modes. Higher WFS and voltage moves the process into a globular mode where large droplets of wire transfer across the arc to the weld pool. Increasing WFS (and therefore, amperage) and voltage allows for the use of a spray transfer mode. This mode sprays small droplets of wire to the weld pool and is known for being a smooth, easy-to-use process that improves productivity. That is especially true when paired with a metal-cored wire. The American Welding Society (AWS) “Welding Handbook, Volume 1” provides the approximate current needed to transition from a globular to a spray transfer mode.

Increasing WFS also provides higher deposition rates: the amount of filler metal added to a weld joint in a given period of time.

Lower WFS and voltage keeps the process in the range for short-circuit welding, in which the wire touches the base material and shorts from the contact that transfers the metal. This short can occur up to 200 times per second. Overall, it is a slower process with lower deposition rates.

Voltage refers to the electrical pressure that causes amperage to flow within the welding circuit. It is directly responsible for adjusting arc length. Higher welding voltage equals a longer arc; however, it also effectively decreases stick-out, resulting in higher amperages. That’s why it’s important for welders to maintain a constant stick-out when welding with a CV power source. Welding voltage also is directly correlated to heat input, so higher settings mean more heat. Increasing voltage also produces a wider arc cone.

Welding variables relate to one another differently but ultimately work together to provide the desired weld performance.

Welding voltage affects the final weld in a variety of ways. If it is too high, the result will be a flatter bead and a concave weld profile. Voltage that is too high also can lead to undercut or a groove in the base material near the weld toe that isn’t filled with weld metal.

If welding voltage is too low, it can cause cold lap—a defect that occurs when the filler metal doesn’t fully fuse with the base material at the weld’s toes. Ropey or humped-up welds and excessive spatter can occur. For operators welding with long power cables, it’s important to know that voltage drops can occur at the point of welding despite the machine setting. For example, a power source may be set at 25 V but only be providing 23 V. This can also lead to cold lap.

Travel speed simply refers to how fast the arc moves along the weld joint, measured in inches per minute (IPM). In semiautomatic operations, many welding operators feel comfortable with an average of 10 to 12 IPM, but more experienced operators may weld in the 18 to 20 IPM range. Travel speeds tend to be faster when welding with metal-cored wire due to its construction and internal composite powders.

Because changes in travel speed affect heat input, it’s important to take care when welding heat-sensitive materials, like aluminum. Welding faster will reduce heat input and prevent issues like burnthrough. Multipass welding on thick materials may require slower travel speeds to fill each pass and support good grain refinement.

Travel speeds that are too slow can lead to too much heat, a wide weld bead and poor penetration, while traveling too fast creates a narrow weld with insufficient weld toe tie-in. Maintaining a steady pace for the given weld joint is important.

Shielding gas, whether argon or carbon dioxide (CO2)—the most commonly used — has an impact on weld characteristics and welding performance. One hundred percent CO2 shielding gas provides deep joint penetration on thicker material, but it does tend to have less arc stability and generate higher levels of spatter. Adding argon to CO2 helps create aesthetically pleasing welds with less spatter. Shielding gas mixture with high levels of argon creates welds with higher tensile and yield strengths but lower ductility. High levels of CO2 in the mixture improve ductility and crack resistance but decrease tensile and yield strengths.

Just as voltage and WFS affect welding transfer modes, so too does shielding gas.

For example, it’s possible to weld in short-circuit mode with solid and metal-cored wires using a blend of 75% argon and 25% CO2. Globular transfer welding with gas-shielded flux-cored wires requires 100% CO2, and at higher voltages, metal-cored wire can be paired with 80% argon and 20% CO2 for welding thicker materials in the spray transfer mode.

Welding variables relate to one another differently but ultimately work together to provide the desired weld performance. For example, when welding on ½-in. thick mild steel, like A36, approximately 250 amps is a good target and provides sufficient root fusion in most cases. Using 90% argon/10% CO2 gas mixture allows for welding in the spray transfer mode around 26 to 28 V and approximately 375 to 420 IPM WFS.

Maintaining proper variables helps to make the process cost-effective, keeps productivity goals in check, and produces sound welds.