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I. Definition of lack of fusion: Lack of fusion refers to the areas where the weld metal and the base material, or between different portions of the weld metal, have not been fully melted and bonded together. Cause: The welding current is too low, resulting in insufficient heat to fully melt the base material and filler metal. The welding speed is too fast, resulting in a short residence time of the molten pool, which does not allow for sufficient melting and fusion. An improper groove design, such as an excessively small angle or a narrow gap, limits the fluidity of the molten pool and the quality of welding. Impurities such as oil and rust on the surface of the welded parts prevent the metals from fusing together. Improper operation, such as an incorrect angle of the welding rod or insufficient swinging amplitude. Hazard: The strength of the unfused areas is significantly lower than that of normal welds, making them prone to cracking under stress. Poor fusion can reduce the sealability of the weld, potentially leading to leaks. Unfused defects can become stress concentration points, reducing the fatigue life of the structure. II. Definition of lack of penetration: Lack of penetration refers to the condition in welding where the root of the joint is not fully melted through. Cause: The welding current is too low, resulting in insufficient arc penetration. An excessively small groove angle and a large root edge make it difficult to achieve full penetration at the root. An excessively large electrode diameter hinders the formation of a good molten pool at the root. Improper welding positions, such as vertical or overhead welding, cause the molten pool metal to easily drop due to gravity, resulting in lack of penetration at the root. The welding technique is not proficient, the wire feeding method is incorrect, and the arc stays at the root of the groove for too short a time. Hazard: The load-bearing area at the lack of weld penetration decreases, resulting in reduced strength and an increased tendency to fracture under stress. Incomplete penetration can affect the density of the weld, potentially leading to leaks. Underwelding defects can also become stress concentration points, reducing the reliability and service life of the structure. Improvement measures: 1. Preventive measures against incomplete welding – Design the groove properly to ensure that the groove angle is appropriate; generally, the groove angle should be determined based on the thickness of the base material and the welding method used. A larger groove angle can increase the welding penetration and reduce the risk of under-welding. Control the size of the root gap; it should not be too large, as this can hinder the penetration of the welding pool. Generally, the thickness of the root margin should be within a reasonable range so that the welding arc can fully melt the base metal. The weldment must be thoroughly cleaned of contaminants such as oil, rust, and oxide scale from its surface before welding. These impurities affect the stability of the welding arc and the fluidity of the molten pool, thereby leading to incomplete welding. Cleaning can be carried out using mechanical methods (such as sandblasting, grinding) or chemical methods (such as acid washing, alkali washing). The interior of the welding groove also needs to be carefully cleaned to ensure there are no residual impurities or oxides. Tools such as wire brushes and grinding wheels can be used for cleaning. Select appropriate welding parameters to adjust the welding current level; too low a current will result in insufficient welding penetration, making incomplete welding more likely. The appropriate welding current should be selected based on factors such as the thickness of the base material, the type of groove, and the welding position. Generally, as the thickness of the base material increases, the welding current should also increase accordingly. Control the welding voltage and welding speed; too high a welding voltage or too fast a welding speed can also affect the welding penetration depth. The welding voltage and welding speed should be adjusted according to the welding current and the properties of the base material, so that they are compatible with each other, ensuring that the welding pool has sufficient time and heat to melt the base material. Ensure that welding procedures are followed properly, so that the welding arc remains aligned with the root of the groove and does not deviate from the center of the groove. During welding, the welder should maintain a stable welding posture and technique, and control the length and angle of the arc. When performing multi-layer, multi-pass welding, it is important to control the welding thickness of each layer as well as the welding sequence. After each layer is welded, the slag and oxides on the surface of the weld bead should be carefully removed before proceeding to welding the next layer. Avoid repeated welding at the same location to prevent welding overheating and coarse grains. Appropriate welding processes should be employed for thick plate welding; methods such as multi-pass welding, double-sided welding, and groove welding can be used to increase the weld depth and reduce the risk of under-welding. Measures such as preheating and post-heating can reduce the cooling rate of welding joints, improve their microstructure and properties, and minimize the occurrence of underpenetration. The preheating temperature and post-heating temperature should be determined based on factors such as the material composition of the base metal, its thickness, and the welding method used. II. Preventive measures against lack of fusion: Increasing the welding current and voltage – Raising the welding current and voltage can increase the heat input during welding, thereby raising the temperature of the weld pool and enhancing its fluidity, which in turn facilitates the melting and bonding of the base material. However, care must be taken not to increase the current and voltage excessively, to avoid defects such as overheating, porosity, and slag inclusions during welding. Adjusting the welding speed – reducing it – can increase the welding heat input, giving the weld pool enough time to melt the base material and the filler metal, thereby reducing the risk of incomplete fusion. However, the welding speed must not be too slow, otherwise it will lead to low welding efficiency and increased welding deformation. Ensure the weldment surface is clean. Before welding, the surface of the weldment must be thoroughly cleaned to remove impurities such as oil, rust, and oxide scale. These impurities can hinder the fusion between the welding pool and the base material, leading to unfused defects. Cleaning can be carried out using methods such as mechanical cleaning, chemical cleaning, or flame cleaning. When controlling the welding angle and position, it is necessary to maintain the correct welding angle and position so that the welding arc can fully illuminate the weld joint, ensuring that the base material and filler metal melt and bond adequately. For different welding positions, such as flat welding, vertical welding, horizontal welding, and overhead welding, corresponding welding techniques and procedures must be employed. Appropriate welding processes should be employed; for materials or weld joints that are difficult to fuse, special welding methods such as TIG welding, plasma arc welding, laser welding, etc., can be used. These welding processes feature high energy density, a small heat-affected zone, and good fusion properties, which can effectively reduce the occurrence of unfused defects. When using multi-layer, multi-pass welding, it is important to control the welding quality of each layer in order to avoid defects such as slag inclusions and pores, which could affect the welding fusion of the subsequent layers. After each layer is welded, the surface of the weld bead must be cleaned to remove slag and oxides before proceeding with welding the next layer. Comprehensive consideration and control are required in aspects such as groove design, weldment preparation, welding parameter selection, welding procedure specifications, and welding technology. Welders must possess excellent welding skills and experience, and operate strictly in accordance with the welding procedure specifications to ensure welding quality.