During CO2 welding, molten metal particles and slag often fly around.
The form of spatter during CO2 welding is shown in the figure.
It can be seen that there are roughly three types, one is the splash caused by the short circuit; the other is the splash caused by the gas evolution; the third is the splash caused by the free transition.
A small metal bridge is formed in the later stage of the droplet short circuit, and the splash caused by the electric explosion is also called normal short circuit splash. This kind of spatter is very small at low current, and the amount of spatter is also small, as shown in Figure a.
When the current is large, an instantaneous short circuit often occurs, that is, a small liquid metal bridge is formed in the early stage of the short circuit, which often causes a large electric explosion splash, as shown in Figure b.
When the current is large, the loop inductance is small, and once the short circuit occurs, it is easy to cause strong splashing of molten droplets and molten pools, as shown in Figure c.
Under the conditions of high current, thick welding wire and low voltage, it is often submerged arc welding. Once a short circuit occurs, the molten steel in the molten pool is often flushed out to form splashes, as shown in Figure d.
Due to the arc striking or wire feeding too fast, the welding wire and the molten pool will be short-circuited. At this time, the welding wire may burst in sections, causing spatter, as shown in Figure e.
Caused by metallurgical factors during welding, the molten pool and droplets are filled with CO2 (or CO) gas. Due to excessive internal pressure, the gas escapes or explodes, often accompanied by splashes, as shown in Figures f and g.
In the free transition, due to the contraction of the CO2 arc, the arc concentrates on the bottom of the droplet and causes the droplet to deviate from the axis of the welding wire, so that when the droplet falls off, it will fly away in a rotating form or the thin neck between the droplet and the welding wire will pass through. A large current explodes, resulting in the splashing form in Figures h and k.
Causes and Reduction Measures of CO2 Welding Spatter
Spatter is one of the main process problems in CO2 welding. There are two main ways to generate spatter, one is the spatter caused by the electric explosion of the short-circuit bridge; the other is the spatter caused by metallurgical factors.
The former Soviet Union scholar Binchuk found that when a large current is passed through the short-circuit bridge, the short-circuit bridge will overheat and explode, resulting in a splash. Its energy is accumulated in the 100~150us time before the explosion.
This kind of electric explosion splash, in normal short-circuit (short-circuit time > 2ms), the short-circuit bridge occurs between the welding wire and the molten droplet (as shown in Figure a). When the bridge is destroyed, a large amount of liquid is pushed to the molten pool, and only a small amount of The fine droplets become spatters.
Usually, the spatter is small when the short-circuit current peak value is small; on the contrary, the spatter is larger when the value is large. In the case of an instantaneous short-circuit (short-circuit time < 2ms), the short-circuit bridge occurs between the molten droplet and the molten pool (as shown in Figure b). The splash of large particles, which is easy to adhere to the surface of the workpiece, is difficult to remove, and even damages the surface finish of the workpiece.
Obviously, the way to reduce the electric explosion splash is to avoid instantaneous short circuit first, that is, to reduce the current in the early stage of the short circuit (such as suppressing the rising speed of the short circuit current). Second, reduce the peak current of normal short circuit. It is often to reduce the short-circuit current rising speed, and to rapidly reduce the short-circuit current in the later stage of the short circuit, and rely on the metal surface tension to break the small bridge, then a spatter-free transition will be achieved.
Another type of spatter, caused by gas escape or even explosion, is often related to the characteristics of the welding metallurgical process. The reduction measure is to use deoxidized welding wire, which should contain sufficient silicon and manganese elements. When the requirements are high, welding wires containing aluminum and titanium can also be used. They suppress the formation of CO gas.
In addition, attention should be paid to the cleaning of the welding wire and the surface of the workpiece, and to derusting and degreasing.
