1、Welding TechniquesWelding Parameters and TechniquesTheir Effects On The Weld After having selected the wire and gas for a weld, operating conditions must be chosen. The four importantparameters are the welding current, wire electrode extension, welding voltage and arc travel speed. Theseparameters w
2、ill 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
3、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)
4、. 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 dia
5、meter 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 diffe
6、rent 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 t
7、he 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 resista
8、nce heating is known at PR heat where I = welding current and R = resistance. The greater the welding current, the greater the PR heating.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 e
9、nd of the wire electrode. Figure 7-2 schematically shows wire extension. It is in this area that PR preheating effect occurs.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
10、extent to which the welding current requirement can vary with tip-to-work distance. Basically, as the tip-to-work distance is increased, theamount of I2R heating increases and the welding current required to melt the wire is decreased. The converse is also true.Controlling tip-to-work distance is im
11、portant. 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 i
12、s 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 VOLTAGEAlthough discussed in Chapter 3, it
13、 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
14、 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 tr
15、avel 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
16、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 posit
17、ioned 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,
18、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 les
19、s 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 te
20、chnique 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 wel
21、ding should be performed only as shown. In eithercase, the arc must be kept on the puddles 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 manne
22、r in which each of these affect certain weldcharacteristics.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 relat
23、ed to welding current. An increase or decrease in the current will increase or decrease the weld penetration respectively.Weld Bead CharacteristicsHowever, 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.
24、 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 decreasewelding current and penetration. Of course, the converse is also true. In some applications, many operators have found it helpful to use this
25、 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 penetration and do not provide a good means of control. Figure 7-8 illu
26、strates 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.Effects of a
27、rc 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/
28、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 heatgenerated by the arc hasnt sufficient time to substantially melt the are
29、a 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 t
30、echnique 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 willincrease. For very thin materials or where low pe
31、netration is required, a forehand technique is generally used.DEPOSITION RATEThe 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 deposit
32、ion rate for any wire is calculated by the equation:Table 7-1 gives the inches of wire per pound (m/kg) for various wire electrodes in a variety of sizes. Deposition rate is synonymous with wire feed speed. Figure 7-11 gives deposition rate versus wire feed speed. The current to achieve a given depo
33、sition rate can also be varied by changing the tip-to-work distance. As Figure 7-12 shows, the wire feed speed can be increased with increasing tip-to-work distance to maintain a constant welding current. This results in a higher deposition rate than usually associated with a given current level. Lo
34、ng tip-to-work distances and high wire-feed speeds are used for high speed welding of thin materials, as the welding current can be kept low. Usually the forehand welding technique is employed. Increasing the deposition rate in this manner will also have an effect on weld penetration. Because more m
35、etal is beingdeposited at a given welding current, the penetration will be reduced. This results from a ”cushioning” of the arc force by the extra weld metal deposited.Weld Bead Appearance Two characteristics of the weld bead are the bead height and width, as shown in Figure 7-13. These characterist
36、ics are important to assure that the weld joint is properly filled, with a minimum of defects, particularly in multi-pass weldments. In this case, if the bead height is too great, it becomes very difficult to make subsequent weld passes that will have good fusion. The more peaked and narrow the weld
37、 bead, the greater the chance that poor fusion may occur. The weld bead characteristics may be altered via both size and shape.In order to change weld bead size, the lbs. (kg) of weld metal deposited per linear foot (m) of the weldment must be changed. Welding current and travel speed are the weldin
38、g parameters primarily used to control weld bead size. For instance, when the current is decreased, the weld bead will become smaller. The converse is also true. This relationship can be seen by referring to Figure 7-7. Weld bead size can also be changed by varying the arc travel speed. As seen in F
39、igure 7-9, bead size and travel speed are inversely related. A decrease in travel speed will result in an increase in the weld bead height and width. An increase in travel speed will result in a decrease in the weld bead height and width. Again, the pounds (kgs) of filler metal deposited in a linear
40、 foot (m) of weld are increased (or decreased).Both welding current and travel speed have little effect on weld bead shape. The bead width and height increase or decrease together.Arc voltage is used to control the shape of the weld bead. As can be seen inFigure 7-8, as the arc voltage (arc length)
41、increases, the bead height decreases and bead width increases. Here the overall size of the weld bead remains constant. Only the shape or contour of the bead is changed. By increasing the bead width, the bead height becomes flatter and the weld metal is said to ”wet” the base materials more efficien
42、tly. Fusion to the base plate is improved.Wire extension and the welding technique employed (backhand or forehand welding) also affects these characteristics, but only to a limited extent. When long extensions are used to increase deposition rates, bead height will increase to a greater extent than
43、bead width. Although larger, the weld bead becomes more peaked as shown in Figure 7-14. A backhand welding technique will also produce a high, narrow weld bead. Decreasing the lagging torch angle will decrease the bead height and increase the width. The forehand technique yields the flattest, widest
44、 weld bead.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 w
45、ill 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
46、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.Torch ManipulationsHORIZONTAL POSITION Recommended weaving patterns, torch positions and bead sequences are sho
47、wn 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.VERTICAL POSITION Recommended weaving patterns an
48、d 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 w
49、ith 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. Al
50、ways 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.OVERHEAD POSITION Recommended weaving patterns and torch positions for the overhead position are shown in Figure 7-18. Agai
