The extension length of the welding wire refers to the distance from the end of the conductive tip to the end of the welding wire (Figure Extension length of welding wire), also known as dry elongation. Keeping the extension length of welding wire constant is one of the basic conditions to ensure the stability of welding process. This is because the current density of CO2 gas shielded welding is higher, the larger the extension length of the welding wire, the stronger the preheating effect of the welding wire; And vice versa. The strength of preheating will also affect the welding parameters and welding quality.
1-welding stick ；2-Contact tip；3-parent metal；Ls-Extension length of welding wire；L—The distance of Contact tip to parent metal
When the wire feeding speed is constant, if the extension length of the welding wire increases, due to the strong preheating effect, welding wire melting fast, high arc voltage, so that the welding current is reduced, droplet and molten pool temperature is reduced, will cause insufficient heat, easy to cause incomplete welding, fusion and other defects. On the contrary, if the extension length of welding wire decreases, the temperature of droplet and pool will increase, which may cause the loss of molten iron in the pool during all-position welding. The size of the small preheating effect is also related to wire resistivity, welding current and wire diameter. The allowable length of the welding wire varies with the diameter and material of the welding wire. For actual operation, see Table 《Allowable extension length of wire》
The extended length of welding wire is too small, which hinders the observation of welding arc and affects operation; It is also easy to clamp the welding wire due to the overheating of the conductive mouth, or even burn the conductive mouth, and destroy the normal process of welding. When the length of the welding wire is too large, the position of the arc changes greatly due to the swing of the welding wire end, and the protection effect becomes poor, so that the welding seam is not well formed, and it is easy to produce welding defects. Figure 《Influence of wire extension length on weld forming》 shows the influence of welding wire extension length on weld forming.
When the length of the welding wire is small, the resistance preheating effect is small, the arc power is large, the melting depth is large, and the splash is less; When the extension length is large, the resistance has a strong preheating effect on the welding wire, and the arc power is small, the melting depth is shallow, and the spatter is much. The extension length of the welding wire is not an independent welding parameter. Usually, when the welder determines the height of the nozzle according to the welding current and protection gas flow, the extension length of the welding wire is also determined.
CO2 gas shielded welding usually uses DC reverse welding (reverse polarity): the weldment is connected to the cathode, the wire is connected to the anode. The welding process is stable, the splash is small, the melting depth is large DC is connected (positive polarity), the welding piece is connected with the anode, the welding wire is connected with the cathode, at the same time the welding current, the welding wire melts fast (the melting speed is 1.6 times of the reverse polarity), the melting depth is shallow, the residual is high, the dilution rate is small, but the splash is large. Therefore, according to these welding characteristics, the positive polarity is mainly used for surfacing welding, cast iron repair welding and high current high speed CO2 gas shielded welding.
The flow of CO2 gas should be selected according to the protection effect of the welding zone. Joint form, welding current, arc voltage, welding speed and operating conditions all affect the flow rate. Too large or too small flow will affect the protection effect, easy to produce welding defects. Usually when the wire welding, the flow is 5~15L/min; When welding thick wire, it is about 20L/min to correct the wrong concept of “the greater the flow of protective gas, the better the protection effect”. The protection effect is not that the bigger the flow, the better. When the protective gas flow exceeds the critical value, the protective gas ejected from the nozzle will change from laminar flow to plain flow, which will involve the air in the protection area, reduce the protection effect, make the porosity in the weld and increase the burning loss of alloy elements.
During the welding process, the included Angle between the welding torch axis and the weld axis is called the welding torch tilt Angle, referred to as the welding torch tilt Angle. The Angle of welding torch is a factor that can not be ignored. When the Angle of the welding gun is between 80° and 110°, it has no obvious influence on the welding process and weld forming whether it is tilted forward or backward. However, if the inclination Angle is too large (e.g., rake Angle A >115°), the melting width and depth will be increased and the spatter will be increased. Figure 《Influence of welding gun Angle on weld forming》 shows the influence of welding torch Angle on weld forming.
As can be seen from Figure《Influence of welding gun Angle on weld forming》, when the welding torch and the weldment have a backward dip Angle (the arc always points to the welded part), the weld is narrow, the residual is large, the fusion depth is large, and the weld is not well formed. When the welding torch and welding parts into rake Angle (arc always pointing to the welding part), the weld width, height is small, the depth of the weld is shallow, the weld shape is good. Generally, welders are used to hold the welding gun with their right hand. When using the left welding method (welding from right to left), the welding gun adopts the dip Angle, which can not only get better weld forming, but also can clearly observe and control the molten pool. Therefore, when CO2 gas shielded welding, the left welding method is usually used.
In the vertical cross section of the weld, the intersection of the welding torch axis and the weld surface is called the arc centering position, as shown in Figure 《The arc is centered》. In the cross section of the weld, the included Angle B of the welding torch axis and the weld surface and the arc centering position determine the distribution ratio of arc power on both sides of the groove. When the arc alignment position is in the center of the groove, if B<90°, the heat of A side is more; If B=90°, the heat on both sides of A and B is equal; If B>90 degrees, there is more heat on the B side. In order to ensure good fusion on both sides of the groove, appropriate arc alignment position and B must be selected. The arc centering position is the swinging center of the arc. The number of weld passes, centering position and swing size should be selected according to the width of the groove at the welding position.
The distance between the lower surface of the nozzle and the surface of the weld pool is called the height of the nozzle, which is an important factor affecting the protection effect, production efficiency and operation. The greater the height of the nozzle, the more convenient to observe the pool, the greater the scope of protection, the greater the extension length of the welding wire, the greater the preheating effect of the welding current on the welding wire, the faster the melting of the welding wire, the greater the swing of the end of the welding wire, the greater the disturbance of the protection air flow, so the greater the flow of the protection gas; The smaller the nozzle height is, the smaller the protective gas flow is and the shorter the welding wire extension length is. The height of the nozzle is usually selected according to the size of the welding current according to Figure 《The relationship between the distance between nozzle and weldment and welding current》