Welding Toolbar Download Page

Welding Directory Image

Index >> Physics of Welding >> Forces Affecting Metal Transfer

Forces Affecting Metal Transfer

Forces Affecting Metal Transfer - It is generally agreed upon that Lorentz force plays an important role in the process of metal transfer. Considerable research work has been taken up in the field and different persons have put forward their views. Ludwig suggested that force aiding metal transfer arises from the pressure gradient within the globule. Needham was of the opinion that plasma jets aid drop detachment. AA. Wells put forward that pressure drop across the electrode and arc plasma interface helps metal transfer; and Amson and Salter suggested that metal transfer was affected by aerodynamic drag. There are mainly two types of forces affecting the drop transfer: one which aid metal transfer and others which retard metal transfer. In general, various forces involved in the phenomena are given below:
(1) Surface Tension This force is associated with the formation of free surface of the drop. It is a retarding force which tries to keep the drop in its position. The force of surface tension acting on the drop when it is just to detach is given by πdo K where d is the electrode diameter, σ is surface tension and K is a function dependent upon electrode diameter and capillarity constant of electrode material. Normally the value of K varies from 0.6 to 1.0. The force of surface tension ranges between 400 to 800 dynes for electrodes from 1.5 to 3 mm diameter. At higher temperatures the surface tension is lowered. (2) Viscosity of the Liquid Metal It is also a retaining force. (3) The High Velocity Gas Jets striking the job and getting back may retard the movement of the metal drop tending to fall down in the molten pool.
	(4) Gravity acts as a detaching force when welding in a flat position, and is a retarding force when welding overhead. The force of gravity is equal to V pg where V is the volume of the globule, p is density of globule material and g is the acceleration due to gravity. Gravitational force is almost negligible on\mall diameter droplets. (5) Lorentz force Lorentz force is the result of interaction of the arc current with its self-induced magnetic field. Lorentz force is an electromagnetic force which exercises pinch effect on the globule, aids in neck formation and drop detachment. The self-induced magnetic field of the arc plasma results in plasma streaming which carries the detached drop to the workpiece. The pinch forces vary from 250 to 1000 dynes for electrodes 1.5-3 mm diameter.
Surface tension and viscosity of the liquid metal help droplet to grow in size, whereas electromagnetic forces constrict (i.e., neck) the molten end of the electrode to such an extent that the material at the thin neck gets easily atomized and the drop separates from the electrode. Besides other forces, the drop transfer rate also depends upon, arc current, arc length, type of polarity, electrode material and electrode extension (i.e., distance between the electrode tip and the point at which current is fed to it). The drop transfer rate increases, with DCRP, with the increase in arc current and electrode extension and with the decrease in arc length. Effect of arc current and voltage on drop transfer rate Flux covered 4 mm mild steel electrodes were used to lay beads on plate on an automatic welding set. Fig shows that as the current increases, drop transfer rate increases for all values of arc voltage.
This is because increasing the current increases the electrode burn off rate and thus the number of drops being transferred. Figure also shows that at constant current, drop transfer rate decreases as arc voltage increases; it may be because of increased heat losses from longer arcs. Type and rate of drop transfer has an appreciable effect on weld bead geometry. As the drop transfer rate increases penetration and reinforcement heights both increase whereas bead width reduces.

Home | Site map | Submit Article | Directory | Search

Language >> English | Français | Español | Deutsch | Italiano | Portugues | Japanese | Korean | Sim-Chinese