Effect of welding process parameters on weld shape

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Welding process parameters refer to the collective term for various physical quantities selected during welding to ensure weld quality. The welding process parameters vary depending on the welding method; for shielded metal arc welding, these parameters mainly include electrode diameter, welding current, arc voltage, welding speed, and preheating temperature. Among the welding process parameters, welding current, arc voltage, and welding speed are the three most critical. Only by properly combining welding current, voltage, and welding speed can a qualified weld be formed. During fusion welding, the ratio φ = B/H, where B is the weld width and H is the calculated weld thickness in the cross-section of a single weld pass, is known as the weld shape factor. Since the microstructure and properties of welds and heat-affected zones are generally weaker than those of the base material, it is desirable to have a smaller formation factor when designing welded joints. However, the smaller the weld formation factor, the narrower and deeper the weld becomes. Such welds are prone to defects such as pores, inclusions, and cracks, and welding them is difficult; therefore, considering all factors, the weld formation factor should be maintained at a certain value. The characteristics of weld formation mainly include penetration depth, weld width, and bead height. It should be noted that weld width and weld thickness are different concepts from welding penetration depth and welding weld width; welding penetration depth refers to the distance between the deepest point where the base material is melted and the surface of the base material, while welding weld width is the width over which the base material is melted due to the heat from the arc. The main factors affecting weld formation are welding current, arc voltage, and welding speed. 1. Effect of welding current on weld formation: When the welding current increases (with all other conditions unchanged), the penetration and bead height of the weld increase, while the bead width remains largely unchanged (or increases slightly). This is because: as the current increases, both the arc force on the workpiece and the heat input increase; the position of the heat source moves downward, resulting in an increased penetration depth. The penetration depth is nearly proportional to the welding current. As the current increases, the amount of wire that melts increases almost proportionally; since the weld width remains nearly constant, the bead height increases. As the current increases, the arc column diameter grows, but the depth to which the arc penetrates into the workpiece increases, limiting the range of movement of the arc spot; as a result, the weld width remains almost unchanged. 2. Influence of arc voltage on weld formation: As the arc voltage increases, the arc power rises, resulting in increased heat input to the workpiece. At the same time, the arc length increases and its distribution radius expands; as a consequence, the penetration depth decreases slightly while the weld width increases. The remaining height decreases because the weld width increases, while the amount of wire that melts slightly decreases. 3. Influence of welding speed on weld formation: As the welding speed increases, the energy decreases, resulting in reduced penetration and width, as well as a lower weld bead height. Because the amount of weld metal deposited per unit length of weld is inversely proportional to the welding speed, while the weld width is approximately inversely proportional to the square root of the welding speed. It can be seen that current affects the penetration depth, while voltage affects the weld width; the current should be set at a level that ensures thorough melting without causing burn-through, and the voltage should be set at a level that minimizes spattering. By fixing one of these parameters, it is possible to adjust the other one. Figure 1 shows the relationship between welding current (I), arc voltage (U), and welding speed (V) on the one hand, and weld penetration (S), weld width (c), and weld bead height (h) on the other hand. (a) The relationship between I and S, c, h; (b) The relationship between U and S, c, h; (c) The relationship between V and S, c, h. S – weld penetration, c – weld width, h – weld bead height, I – welding current, U – arc voltage, V – welding speed. The magnitude of the welding current has a significant impact on both the quality of welding and welding productivity. If the welding current is too low, the arc becomes unstable, the penetration depth is reduced, which can lead to defects such as incomplete welding and slag inclusions, in addition to low productivity ； If the current is too high, defects such as undercutting and burn-through can occur in the weld, while spatter is also generated. Therefore, the welding current must be selected appropriately; generally, it can be determined using empirical formulas based on the diameter of the electrode, with further adjustments made according to factors such as the location of the weld, joint type, welding layer, and thickness of the workpiece. The arc voltage is determined by the arc length; the longer the arc, the higher the arc voltage ； If the arc is short, the arc voltage is low. During welding, the arc should not be too long; otherwise, the arc will burn unstably, increasing metal spatter. Moreover, the intrusion of air can cause pores in the weld. Therefore, during welding, efforts should be made to use a short arc, with the arc length generally required to not exceed the diameter of the electrode. The welding speed is directly related to the productivity of welding. To achieve the highest welding speed, a larger electrode diameter and welding current should be used while ensuring quality; at the same time, the welding speed should be adjusted appropriately according to specific conditions, in order to maintain consistent weld height and width.