Weld seam formation quality

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In general welder training, manual arc welding is used as a basic skill, with the aim of enabling welders to master the selection of welding current, welding speed and wire feeding techniques, as well as the control of the welding arc. During the welding process, defects such as weld beads, burn-through, lack of penetration, undercuts, slag inclusions, and poor shape often occur. Analyzing the reasons for these defects, for welders who are new to the trade, it is mainly due to their inability to properly observe changes in the temperature of the molten pool during welding; they fail to control the temperature of the molten pool effectively, which leads to the aforementioned defects. By simplifying complex theories in this way, trainees can quickly understand and master welding techniques. Image: During fusion welding, under the action of the welding heat source, the portion of liquid metal with a certain geometric shape formed on the workpiece from the melted electrode metal and the locally melted base material is known as the weld pool. After cooling, it becomes a weld seam; therefore, the temperature of the molten pool directly affects the quality of welding. With a high pool temperature, a larger molten pool, and good fluidity of the molten iron, the fusion zone is more prone to fusion ； However, when it is too high, the molten iron tends to flow downward; the back side of the part that is welded on one side and shaped on the other side is prone to burn-through, weld beads may form, and shaping becomes difficult. Additionally, the plasticity of the joint decreases, making it prone to cracking under bending ； When the molten pool temperature is low, the molten pool is smaller, the molten iron appears darker, and its fluidity is poor; this easily leads to defects such as lack of penetration, lack of fusion, and slag inclusions. Therefore, effectively controlling the temperature of the molten pool is, especially for a beginner, an essential step to successfully enter this field and even become an excellent welder. The pool temperature is closely related to factors such as welding current, electrode diameter, welding technique, electrode angle, and arc combustion time. The following measures are taken to control the pool temperature by addressing these factors. 1. Welding current and electrode diameter: These two factors are important in welding, and they are also closely interconnected. During fusion welding, the current that flows back through the welded parts is called the welding current. The electrode diameter refers to the cross-sectional size of the filler metal rod. In simple terms, whether the welding rod can be properly melted is determined by the current flowing through it. If the current is too low, it is difficult to start an arc; the welding rod tends to stick to the workpiece, the fish-scale pattern formed is coarse, and the fusion on both sides is poor ； The current is too high, resulting in excessive spatter and smoke during welding; the electrode turns red, and the surface of the weld pool becomes very bright, which makes it easy to cause burn-through and undercutting ； The current level is appropriate, allowing for easy ignition and a stable arc; spatter is minimal, and a uniform cracking sound can be heard. The weld edges blend smoothly into the base material, with fine fish-scale patterns on the surface, and the slag can be easily removed. In terms of its applications, there are also complex relationships. 1.1 The welding current and electrode diameter should be selected based on the spatial position of the weld. In the vertical, horizontal, and overhead positions, the current required is relatively lower than that in flat welding; it is usually about 10% less than the current used for flat welding. Similarly, in the vertical, horizontal, and overhead positions, the diameter of the welding rod is usually smaller than that used for flat welding. For example, in flat welding to fill and cover gaps on plates larger than 12 mm thick, 5.0 mm diameter welding rods are often used; whereas in the vertical, horizontal, and overhead positions, welding rods with a diameter of 5.0 mm are hardly ever used. 1.2 Select the welding current and electrode diameter based on the welding layer of the weld. For butt welding of 12mm thick plates, a 3.2mm thick electrode should be used for the root pass, with a welding current of 90–110A; for the fill and cover passes, a 4.0mm thick electrode can be used, with a welding current of 160–175A. Therefore, selecting the welding current and electrode diameter appropriately is essential for easily controlling the molten pool temperature, which forms the basis for a well-formed weld. The welding current is too low, resulting in a low temperature in the weld pool; this causes instability in the arc and may also prevent full penetration of the workpiece. If the welding current is too high and the temperature of the molten pool is too high, it can cause severe splashing or flowing of the molten metal, and may even burn through the workpiece, resulting in weld beads. Below is a list showing the relationship between welding current and electrode diameter. You can make a reasonable choice based on your own experience or preferences. There’s no need to use the same parameters as others; as long as the settings seem appropriate to you and result in good weld formation, that’s fine. 2. Welding rod movement: The welding rod is fed in the direction of the weld pool along its axis; once the rod melts, it is necessary to maintain a constant arc length. Therefore, the speed at which the welding rod is fed toward the weld pool must be equal to the speed at which it melts. If the speed at which the welding rod is fed is lower than the speed at which it melts, the length of the arc will gradually increase, leading to arc interruption ； If the welding rod is fed too quickly, the arc length shortens rapidly, and the end of the rod comes into contact with the workpiece, causing a short circuit that also leads to the extinction of the arc. Crescent-shaped welding rod movement: The end of the welding rod makes a crescent-shaped left-right oscillation in the direction of welding; the movement in the middle should be rapid, with brief pauses on both sides. This method can effectively control the weld pool temperature. Since the weld pool is relatively shallow, undercut on both the front and back sides should be prevented. The crescent-shaped welding motion is one of the main welding motions in continuous arc welding for single-side welding with double-sided formation. Zigzag welding method: The end of the welding rod is moved forward in a zigzag pattern, stopping briefly on both sides to prevent undercutting. This method is easy to operate and widely applied. Suitable for welding the various layers of weld beads in butt welds in the flat, vertical, and overhead welding positions. The temperature of the crescent-shaped cutting path is higher than that of the zigzag-shaped cutting path in the molten pool. For the 12mm flat welding root pass, a zigzag welding pattern is used, and the amplitude of the movement as well as the pauses on both sides of the groove help to control the temperature of the molten pool effectively. This results in weld pores of roughly uniform size, reduces the likelihood of weld bulges and burn-through at the root of the groove, and improves the quality of the weld penetration. As a result, single-sided welding with double-sided formation in flat plate butt welding is no longer a challenging task. 　　 3. Electrode angle and feed position: During welding, the angle of the electrode should change as the welding position changes, in order to maintain an appropriate temperature in the molten pools on both sides of the root edge. Too high a temperature can cause burn-through, while too low a temperature can lead to incomplete welding and failure to fuse. When the angle between the electrode and the welding direction is 90 degrees, the arc becomes concentrated, resulting in a high weld pool temperature ； If the angle between them decreases, the arc disperses, and the temperature of the molten pool is lower. For example, in the case of a 12mm flat welding root pass, if the electrode angle is 50-70 degrees, this reduces the temperature of the molten pool, thereby preventing the formation of weld bumps or protrusions on the back side. For example, when changing the welding electrode for the root pass in vertical welding of 12mm thick plates, we used a welding electrode angle of 90-95 degrees while moving the electrode, which allowed the temperature of the molten pool to rise rapidly; as a result, the penetration hole could be formed smoothly, the back side of the joint achieved a smoother shape, and the problem of indentation within the joint was effectively controlled. If the electrode feeding position is insufficient, it will result in incomplete welding or slag inclusions. Since the arc is more dispersed at this time, the melting temperature of the root edge of the base material is insufficient, resulting in incomplete fusion of the bottom part of the base material ； To fully melt the metal, the melting time must be increased. If the molten pool cannot hold the molten iron, it will flow downward, resulting in undercuts or weld beads. Additionally, multiple layers of molten pools can lead to slag inclusion. The correct method is to insert the welding rod at a 75-degree angle into the root gap, align it with the base metal of the gap and move it back and forth; each movement should take about 1 second. This results in the formation of the first molten pool, after which the process proceeds to form the next molten pool. At this time, the melting time of each molten pool is short and their weight is low, so they are less likely to sag; weld beads do not form, and the shallow grooves also facilitate welding on the surface. The subsequent molten pool moves forward in a overlapping manner, covering 2/3 of the previous one; each molten pool is relatively thin, and the latter acts as a post-heating mechanism for the former, ensuring that the gases within the molten pool have sufficient time to escape and thus preventing the formation of pores. 4. Arc combustion time: 57×3.5. In practical teaching for horizontal and vertical welding of pipes, the arc interruption method is used for welding. At the start of welding, when the temperature of the base material is low, if the welding rod is not positioned at the edge of the groove, the molten metal will contract back quickly, resulting in undercutting. The weld shape will also be tall and narrow, failing to achieve a smooth transition; moreover, surface lack of fusion is likely to occur. Based on an analysis of the molten pool shape, if it takes on a drooping form, the weld quality will definitely be poor, and weld beads may form. Therefore, the starting point of the overhead welding must be thoroughly preheated; the angle between the welding rod and the pipe should be 75 degrees. After the arc is ignited, it should be stretched out to carry out preheating. Only after the first drop of molten iron from the tip of the welding rod falls should the welding rod be moved forward. At this point, the temperature of the molten pool should be such that its size is equal to the groove width plus about 1 millimeter, so that the base material can melt fully and form droplets that constitute the weld.