Welded joints | Forces in welding arc
Updated: Apr 20, 2021
When feeling lazy and want to be motivated, you need a guiding force. And if you are doing welding, you need to understand the welding force.
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By the law of conservation, we know that energy can neither be created nor be destroyed. When we perform any activity there is force (energy) needed for it. Similarly, for welding, we need force as well as energy. Energy? We have seen earlier that elevated temperatures are needed for a welding operation. To achieve this high temperature we need an energy source. What about force? In the process of welding, there are multiple forces involved when the arc is created. The electric arc also introduces an electromagnetic field with that we also see the forces due to charge impact. And naturally, we have gravitational force. It's always there.
An electric arc is developed by a steady current produces high temperature to accommodate fusion.
Arc Forces and Their significance on Welding
All the forces acting in the arc zone are termed arc forces. The molten deposition of the weld material, which is melted from the filler electrode is affected by these forces. The effect can be either facilitating to the metal transfer and weld metal disposition efficiency or can be resisting.
The metal transfer is basically detachment and movement of molten metal drops from the tip of the electrode to the weld pool in the workpiece and is of great importance because of two reasons:
Flight duration of molten metal drop in arc region affects the quality of weld metal and efficiency of element transfer.
arc forces affect the deposition efficiency.
Let's take a look at the different forces within an arc.
1. Gravity Force
This is due to the gravitational force acting on molten metal drop hanging at the tip of the electrode. Gravitational force depends on the volume of the drop and density of the metal. In the case of the down hand welding or under arm welding, gravitational force helps in detachment/transfer of molten metal drop from electrode tip. This force is dependent on the material properties of filler material such as density, mass, and volume.
The opposite of down hand welding, in overhead welding the gravitational force assists the detachment of the molten metal.
2. Surface Tension Force
This force is experienced by a drop of the liquid metal hanging at the tip of the electrode due to surface tension acting on the drop. The magnitude of the surface tension force is influenced by the size of the droplet, electrode diameter, and surface tension coefficient. As per the property of surface tension, this force tends to resist the detachment of molten metal drop from electrode tip and usually acts against gravitational force. In the case of vertical and overhead welding positions, a high surface tension force helps in placing the molten weld metal at the required position more effectively by reducing the tendency of falling down of molten weld metal. Accordingly, flux/electrode composition for odd position welding purposes must be designed to have viscous and high surface tension weld metal/slag.
3. Force Due to Impact of Charge Carriers
As per polarity, the charged particles (ions & electrons) move towards an anode or cathode and eventually collide with them. Force generated due to the impact of charged particles on the molten metal drop hanging at the tip of the electrode tends to obstruct the detachment.
Force due to impact of charged particles (Fm)
Where m is the mass of charged particles, V is the velocity and t is the time.
4. Force Due to Metal Vapors
Molten metal evaporating from the bottom of the drop and weld pool moves in the upward direction. Forces generated due to the upward movement of metal vapors act against the molten metal drop hanging at the tip of the electrode. Thus, this force tends to obstruct the detachment of the droplet.
5. Force Due to Gas Eruption
Welding involves the melting and fusion of metals. There is an endothermic reaction that results in residual gasses. Gases present in molten metal such as oxygen, hydrogen, etc. may react with some of the elements (such as carbon) present in molten metal drop and form gaseous molecules (carbon dioxide). The growth of these gases in molten metal drop as a function of time ultimately leads to the bursting of metal drops which in turn increases the spattering and reduces the control over handling of molten weld metal.
6. Force Due to Electro-Magnetic Field
The flow of current through the arc gap develops the electromagnetic field. Interaction of this electromagnetic field with that of charge carriers produces a force that tends to pinch the drop hanging at the tip of the electrode also called pinch force. The pinch force reduces the cross-section for molten metal drop near the tip of the electrode and thus helps in detachment of the droplet from the electrode tip A component of pinch force acting in the downward direction is generally held responsible for detachment of droplet and is given by:
Along with these forces acting within the weld arc, there is another force induced due to the deflection of the arc. It's known as arc blow.
Arc blow is basically a deflection of a welding arc from its intended path i.e. axis of the electrode.
Deflection of arc during welding reduces the control over the handling of molten metal by making it difficult to apply the molten metal at the right place. A severe arc blow increases the spattering which in turn decreases the deposition efficiency of the welding process. According to the direction of deflection of the arc with respect to welding direction, an arc blow may be termed as a forward or backward arc blow. Deflection of arc ahead of the weld pool in direction of the welding is called forward arc blow and that in the reverse direction is called backward arc blow.
Gravitational force assists the detachment of molten metal hanging from the tip of the electrode.
Gases present in molten metal such as oxygen, hydrogen, etc. may react with some of the elements present in molten metal drop and form gaseous molecules.
These gasses can burst and will cause metal spatter. This will tamper the weld efficiency.
Deflection of arc ahead of the weld pool in direction of the welding is called forward arc blow and that in the reverse direction is called backward arc blow.
These forces affect the quality of the weld and the strength of the joint. To know more about welding and types of joints please check our previous posts. And to keep learning, stay tuned with us.