Although discussed in Chapter 3, it should be re-emphasized that the voltage setting directly controls the arc length. In addition, a certain range is required to maintain arc stability at any given welding current level. ARC TRAVEL SPEED The arc travel speed is the linear rate that the arc moves along the workpiece. This parameter is usually expressed as inches or meters per minute. Three general statements can be made regard ing the arc travel speed: 1) As the material thickness increases, the travel speed must be lowered. 2) For a given material thickness and joint design, as the welding current is increased, so is the arc travel speed. The converse is also true. 3) Higher welding speeds are attainable by using the forehand welding technique. WELDING TECHNIQUES The first general welding technique that affects weld characteristics is torch position. This refers to the manner in which the torch is held with respect to the weld joint. The position is usually described from two directions – the angle relative to the length of the weld and the angle relative to the plates as illustrated in 7-4 and 7-5 respectively. Both backhand and forehand welding techniques are shown in 7-4. The backhand method means the torch is positioned so that the wire is feeding opposite to the direction of arc travel. Filler metal is being fed into the weld metal previously deposited. For the forehand method, the torch is angled so that the electrode wire is fed in the same direction as arc travel. Now the filler metal is being deposited, for the most part, directly on the workpiece. It should be noted that a change in welding direction is not required to facilitate forehand or backhand welding, only a reversal in the longitudinal torch positioning. Generally, operators find that the backhand technique yields a more stable arc and less spatter on the workpiece.
The angle relative to the plate for the fillet weld shown in Figure 7-5 is usually 45 deg. However, for a beveled butt joint, this angle may only be a few degrees from the vertical to allow for proper wetting of the weld metal to the side wall. The second general welding technique that should be considered is that of arc travel direction when the welding must be performed in the vertical position. As Figure 7-6 illustrates, there are two methods with which this welding can be done – vertical up and vertical down. Here the torch positioning is extremely important and welding should be performed only as shown. In either case, the arc must be kept on the puddle’s leading edge so as to insure complete weld penetration. This completes a definition of the factors which make up the controllable welding parameters and techniques. We shall now turn our attention to the manner in which each of these affect certain weld characteristics. Figure 7-4 - Longitudinal Torch Positions Figure 7-5 – Transverse Torch Positions
Figure 7-6 - Welding in the Vertical Position Up Travel and Down Travel
Weld Bead Characteristics PENETRATION Weld penetration is the distance that the fusion line extends below the surface of the material being welded. Welding current is of primary importance to penetration. As Figure 7-7 illustrates, weld penetration is directly related to welding current. An increase or decrease in the current will increase or decrease the weld penetration respectively. However, we have seen that welding current can be varied without changing the wire feed speed; namely, through the variation of the tip-to-work distance. The effect of tip-to-work distance on weld penetration is opposite in nature to that of welding current. An increase in the tip-to-work distance will decrease welding current and penetration. Of course, the converse is also true. In some applications, many operators have found it helpful to use this property to control penetration. Changing the tip-to-work distance while welding prevents burnthrough when there are discontinuities in material thicknesses or joint gap.
The remaining factors have comparatively little effect on pene- tration and do not provide a good means of control. Figure 7-8 illustrates the effect of welding voltage. In this example, penetration is greatest at 24 volts and decreases as the voltage is either increased or decreased. Twenty-four volts is the optimum voltage for the amperage used and yields the most stable arc. Arc instability decreases penetration.
TOP Figure 7-7 - Effect of Welding Current on Weld Penetration Carbon Steel-Short Arc C-25 Shielding
BOTTOM Figure 7-8 – Effect of Welding Voltage on Weld Penetration Aluminum-Spray Arc-Argon Shielding
Effects of arc travel speed are similar to that of welding voltage – penetration is a maximum at a certain value and decreases as the arc travel speed is varied. Figure 7-9 shows that at 12 inches per minute (30.5 cm/min) travel speed, penetration is at a maximum. At either 7 ipm (17.8 cm/min) or 17 ipm (43.2 cm/min) it is decreased. With the lower speeds, too much metal is deposited in an area and the molten weld tends to roll in front of the arc and ”cushions” the base plate. This prevents further penetration. At high speeds, the heat generated by the arc hasn’t sufficient time to substantially melt the area of base material. Torch position has a slightly greater effect than does welding voltage or arc travel speed. The effect of changing the longitudinal torch angle, or switching from a forehand to backhand welding technique is shown in Figure 7-10. It can be seen that generally the forehand welding technique yields shallower penetration than does the backhand technique. Maximum weld penetration is achieved with a torch angle of 25 deg. and the backhand welding technique. However, beyond this degree of torch angle, arc instability and spatter will increase. For very thin materials or where low penetration is required, a forehand technique is generally used.
Figure 7-9 - Effect of Welding Travel Speed on Weld Penetration Aluminum-Spray Arc-Argon Shielding
Figure 7-10 - Effect of Longitudinal Torch Position on Weld Penetration
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