Welding Parameters and Techniques-part1

Their Effects On The Weld
After having selected the wire and gas for a weld, operating conditions must be chosen. The four important parameters are the welding current, wire electrode extension, welding voltage and arc travel speed. These parameters will affect the weld characteristics to a great extent. Because these factors can be varied over a large range, they are considered the primary adjustments in any welding operation. Their values should be recorded for every different type of weld to permit reproducibility.

WELDING CURRENT
The welding current is the electrical amperage in the power system as the weld is being made. It is usually read from the power source meter, but a separate ammeter is often used. In the mig process, welding current is directly related to wire- feed speed (if the wire extension beyond the guide tip is constant). As the wire-feed speed is varied, the welding current will vary in the same direction. In other words, an increase (or decrease) in the wire-feed speed will cause an increase (or decrease) of the current. Figure 7-1 shows the typical wire-feed speed vs. welding current relationship for various diameter E70S-3 wires. This relationship is commonly called the ”burn-off’ characteristic. The graph also shows that when the diameter of the wire electrode is increased (or decreased) at any wire-feed speed, the welding current is higher (or lower). Each type of wire (steel, aluminum, etc.) has a different burn-off characteristic. One important fact that should be noticed in Figure 7-1 is the shape of each burn-off curve. In the lower current range for each wire size, the curve is nearly linear. In other words, for every addition to the current, there is a proportional (and constant) increase in the melt off. However, at higher welding currents, particularly with small diameter wires, the burn-off curve becomes non-linear. In this region, higher welding currents cause larger increases in the burn-off. This is due to resistance heating of the wire extension beyond the guide tube. This resistance heating is known at PR heat where I = welding current and R = resistance. The greater the welding current, the greater the PR heating


Figure 7- 1 – Burn-Off Characteristics

Welding Parameters and Techniques-part2

WIRE ELECTRODE EXTENSION
Wire extension or ”stick-out” is the distance between the last point of electrical contact, usually the end of the contact tip, and the end of the wire electrode. Figure 7-2 schematically shows wire extension. It is in this area that PR preheating effect occurs.


Figure 7-2 - Tip-To-Work Distance

The contact tip-to-work distance, because of its effect on the wire extension, affects the welding current required to melt the wire at a given feed speed. Fig. 7-3 shows the extent to which the welding current requirement can vary with tip-to-work distance. Basically, as the tip-to-work distance is increased, the amount of I2R heating increases and the welding current required to melt the wire is decreased. The converse is also true.


Figure 7-3 – Effect of Tip-To-Work Distance on Welding Current

Controlling tip-to-work distance is important. Long extensions result in excess weld metal being deposited with low arc heat. This can cause poor bead shape and low penetration. In addition, as the tip-to-work distance increases, the arc becomes less stable. For short arc welding 3/8 in. (9.6mm) tip-to-work distance is recommended. It is very important that the wire extension be kept as constant as possible during the welding operation. In view of the substantial effect on the welding operation, it is always wise to record not only current and voltage, but also the wire-feed speed.

Welding Parameters and Techniques-part3

WELDING VOLTAGE
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

Welding Parameters and Techniques-part4

DEPOSITION RATE
The deposition rate describes how much usable weld metal will be deposited in one hour of actual arc-on time. Because the mig process is very efficient, only a very small amount of weld metal is lost as spatter. The deposition rate for any wire is calculated by the equation:

Torch Manipulations No discussion of welding techniques would be complete without some reference to the methods of torch manipulation. The recommendations which follow are only to serve as a guide to be used during welder training. As the individual welders become more proficient with the Mig process, they will adapt their torch manipulations to best suit the job at hand. FLAT POSITION Recommended weaving patterns, torch positions and bead sequence are shown in Figure 7-15. For the single-pass, butted joint, a slight back-stepping motion is used. Gapped root passes are made with a small, back-and-forth weave pattern. For fill and cover passes, the same weave, with an adjustment for the desired width, is used, with care taken to pause at the sidewalls to obtain adquate fill in these areas.


Figure 7-15 - Torch Manipulations

HORIZONTAL POSITION Recommended weaving patterns, torch positions and bead sequences are shown in Figure 7-16. For fillet welds, a circular motion is recommended. For butt weld root passes and fill passes, an in-line, back-and- forth motion is used with width adjustments as required. A slight pause is used at the tie-in to the previous bead.


Figure 7-16 - Torch Manipulations

VERTICAL POSITION Recommended weaving patterns and torch positions for vertical up and vertical down are shown in Figure 7- 17. With vertical up, for a square edge preparation an in-line, back-and-forth weave is used. For a bevelled, multipass joint a ”U” pattern is used for the root. The fill and cover passes are made using a side-to-side weave with a backstep at the walls. The length of the backstep is on the order of a wire diameter. For a vertical up fillet a ”Christmas Tree” pattern is used with pauses at the side walls. For vertical down an inverted ”U” pattern is used, pausing at the side walls for the root, fill, and cover passes. Always take care in vertical down welding to keep the arc on the leading edge of the puddle. Preventing the molten metal from running ahead of the arc will improve weld soundness.


Figure 7-17 -Torch Manipulations

OVERHEAD POSITION Recommended weaving patterns and torch positions for the overhead position are shown in Figure 7-18. Again, a back-and- forth weave is used with pauses at the plate sidewalls. This applies to root, fill, and cover passes.


Figure 7-18 Recommended weaving patterns and torch positions for the overhead position