Understand the force of droplet transfer during steel pipe welding

Droplets are high-temperature liquid metal droplets formed after melting the welding rod. The process of transitioning to the welding base material is called droplet transition. In this process, the excessive force of the droplet plays a decisive role. This force originates from a variety of physical phenomena, including gravity, surface tension, electromagnetic contraction, and plasma flow forces. These forces act together on the droplet, affecting its behavior as it detaches from the electrode end and transfers to the workpiece.

Why do droplet transfers have these different forms mentioned above? This is due to the different external forces acting on the liquid metal droplets. When welding, take certain process measures. It is possible to change the force on the molten droplet so that the molten droplet can transition from the welding rod to the molten pool in the desired transition form.

1. Gravity of molten droplets

Any object will tend to sag due to its gravity. During flat welding, the gravity of the metal droplets promotes the transfer of the droplets. However, during vertical and overhead welding, the gravity of the molten droplets hinders the transition of the molten droplets to the molten pool and becomes a hindrance force.

Gravity is one of the most well-known forces in nature, and it drives the droplet downward toward the weld pool during the droplet transition process. However, the effect of gravity is usually weak and needs to be combined with other forces to achieve effective droplet transfer.

2. Surface tension

Liquid metal, like other liquids, has surface tension. That is, when there is no external force, the surface area of the liquid will be reduced as much as possible and shrink into a circle. For liquid metal, surface tension makes the molten metal become spherical.

During steel pipe welding, surface tension helps form a stable droplet and keeps it at the end of the electrode. However, excessive surface tension will make it difficult for the droplets to fall off, thus affecting the stability of the steel pipe welding process and the quality of the weld.

However, surface tension is beneficial to the transfer of molten droplets when welding in other positions such as overhead welding. First, under the action of surface tension, the molten pool metal hangs upside down on the weld and is not easy to drip;

Secondly, when the molten droplet at the end of the welding rod comes into contact with the molten pool metal, the molten droplet will be pulled into the molten pool due to the surface tension of the molten pool.

The greater the surface tension, the larger the droplet at the end of the welding core. The size of the surface tension is related to many factors. For example, the larger the diameter of the welding rod, the greater the surface tension of the droplet at the end of the welding rod;

The higher the temperature of liquid metal, the smaller its surface tension. Adding oxidizing gas (Ar—O2 Ar—CO2) to the protective gas can significantly reduce the surface tension of liquid metal, which is conducive to the formation of fine particle droplets and their transition to the molten pool.

3. Electromagnetic force (electromagnetic contraction force)

When welding, we can think of the charged welding wire and the liquid droplets at the end of the welding wire as being composed of many current-carrying conductors. Based on the above-mentioned electromagnetic effect principle, it is not difficult to understand that the welding wire and the molten droplet are also subject to radial contraction force from the surrounding to the center, so it is called electromagnetic compression force.

The electromagnetic shrinkage force is the force generated by the magnetic field generated by the welding current acting on the liquid metal. When electric current passes through the welding rod, it creates a magnetic field around the droplet, which in turn creates an inward compressive force on the droplet. This force promotes droplet detachment and helps control droplet size and transition frequency.

In this case, the droplet size is often larger. When such large molten droplets pass through the arc gap, an arc short circuit is often used, resulting in large splashes and arc burning that is very unstable. When the welding current is larger, the electromagnetic compression force is larger.

In comparison, the role of gravity is very small. The liquid droplets transition to the molten pool in smaller droplets mainly under the action of electromagnetic compression force, and the directionality is strong, whether in the flat welding position or the overhead welding position, the molten metal always transitions from the welding wire to the molten pool along the arc axis under the action of the magnetic field compression force.

During welding, the current density on the welding rod or wire is generally relatively large, so the electromagnetic force is a major force that promotes the transfer of droplets during the steel pipe welding process. When using a gas shield, controlling the droplet size by adjusting the density of the welding current is a major means of the process.

4. Extreme pressure (spot force)

The charged particles in the welding arc are mainly electrons and positive ions. Due to the action of the electric field, the electron wire moves to the anode, and the positive ions move to the cathode. These charged particles hit the bright spots on the two poles and are produced.

When direct current is connected, the pressure of positive ions hinders the transfer of droplets. What hinders the transfer of droplets during reverse connection is the pressure of electrons. Since positive ions have greater mass than electrons, the pressure of the positive ion flow is greater than the pressure of the electron flow.

Therefore, it is easy to produce fine particle transition in reverse connection, but not in forward connection. This is due to the different extreme pressures.

5. Gas blowing force (plasma flow force)

During manual arc welding, the melting of the electrode coating lags slightly behind the melting of the welding core, forming a short section of unmelted "trumpet"-shaped casing at the end of the coating.

There are a large number of gases generated by the decomposition of the coating gas-generating agent and CO gas generated by the oxidation of carbon elements in the welding core. These gases expand rapidly in volume due to heating to high temperatures and straighten along the direction of the unmelted casing. The (linear) and stable air flow rushes away and blows the droplets into the molten pool. Regardless of the spatial position of the weld, this airflow will be conducive to the transition of the droplets of metal.

In high-speed welding or high-standard industrial applications, the control of excessive droplet force is particularly important. For example, in an automated or robotic steel pipe welding process, any small change can cause a chain reaction that leads to welding defects. Therefore, an in-depth understanding of these forces not only helps to improve the level of welding technology but is also the basis for achieving efficient production.