Definition of Splash Defect: Splash in welding refers to the molten metal droplets ejected from the molten pool during the welding process. These droplets may fall on the surrounding working surface, causing roughness and unevenness on the surface, and may also cause loss of molten pool quality, resulting in dents, explosion points, and other defects on the weld surface that affect the mechanical properties of the weld.
Splash in welding refers to the molten metal droplets ejected from the molten pool during the welding process. These droplets may fall on the surrounding working surface, causing roughness and unevenness on the surface, and may also cause loss of molten pool quality, resulting in dents, explosion points, and other defects on the weld surface that affect the mechanical properties of the weld.
Splash classification:
Small splashes: Solidification droplets present at the edge of the weld seam and on the surface of the material, mainly affecting the appearance and having no impact on performance; Generally, the boundary for distinguishing is that the droplet is less than 20% of the weld seam fusion width;
Large splatter: There is quality loss, manifested as dents, explosion points, undercuts, etc. on the surface of the weld seam, which can lead to uneven stress and strain, affecting the performance of the weld seam. The main focus is on these types of defects.
Splash occurrence process:
Splash is manifested as the injection of molten metal in the molten pool in a direction roughly perpendicular to the welding liquid surface due to high acceleration. This can be clearly seen in the figure below, where the liquid column rises from the welding melt and decomposes into droplets, forming splashes.
Splash occurrence scene
Laser welding is divided into thermal conductivity and deep penetration welding.
Thermal conductivity welding has almost no occurrence of spatter: Thermal conductivity welding mainly involves the transfer of heat from the surface of the material to the interior, with almost no spatter generated during the process. The process does not involve severe metal evaporation or physical metallurgical reactions.
Deep penetration welding is the main scenario where splashing occurs: Deep penetration welding involves laser reaching directly into the material, transferring heat to the material through keyholes, and the process reaction is intense, making it the main scenario where splashing occurs.
As shown in the above figure, some scholars use high-speed photography combined with high-temperature transparent glass to observe the movement status of the keyhole during laser welding. It can be found that the laser basically hits the front wall of the keyhole, pushing the liquid to flow downwards, bypassing the keyhole and reaching the tail of the molten pool. The position where the laser is received inside the keyhole is not fixed, and the laser is in a Fresnel absorption state inside the keyhole. In fact, it is a state of multiple refractions and absorption, maintaining the existence of the molten pool liquid. The position of the laser refraction during each process changes with the angle of the keyhole wall, causing the keyhole to be in a twisting motion state. The laser irradiation position melts, evaporates, is subjected to force, and deforms, so the peristaltic vibration moves forward.
The comparison mentioned above uses high-temperature transparent glass, which is actually equivalent to a cross-sectional view of the molten pool. After all, the flow state of the molten pool is different from the real situation. Therefore, some scholars have used rapid freezing technology. During the welding process, the molten pool is rapidly frozen to obtain the instantaneous state inside the keyhole. It can be clearly seen that the laser is hitting the front wall of the keyhole, forming a step. The laser acts on this step groove, pushing the molten pool to flow downwards, filling the keyhole gap during the laser’s forward movement, and thus obtaining the approximate flow direction diagram of the flow inside the keyhole of the real molten pool. As shown in the right figure, the metal recoil pressure generated by laser ablation of liquid metal drives the liquid molten pool to bypass the front wall. The keyhole moves towards the tail of the molten pool, surging upwards like a fountain from the rear and impacting the surface of the tail molten pool. At the same time, due to the surface tension (the lower the surface tension temperature, the greater the impact), the liquid metal in the tail molten pool is pulled by the surface tension to move towards the edge of the molten pool, continuously solidifying. The liquid metal that can be solidified in the future circulates back down to the tail of the keyhole, and so on.
Schematic diagram of laser keyhole deep penetration welding: A: Welding direction; B: Laser beam; C: Keyhole; D: Metal vapor, plasma; E: Protective gas; F: Keyhole front wall (pre melting grinding); G: Horizontal flow of molten material through the keyhole path; H: Melt pool solidification interface; I: The downward flow path of the molten pool.
The interaction process between laser and material: The laser acts on the surface of the material, producing intense ablation. The material is first heated, melted, and evaporated. During the intense evaporation process, the metal vapor moves upwards to give the molten pool a downward recoil pressure, resulting in a keyhole. The laser enters the keyhole and undergoes multiple emission and absorption processes, resulting in a continuous supply of metal vapor maintaining the keyhole; The laser mainly acts on the front wall of the keyhole, and evaporation mainly occurs on the front wall of the keyhole. The recoil pressure pushes the liquid metal from the front wall of the keyhole to move around the keyhole towards the tail of the molten pool. The liquid moving at high speed around the keyhole will impact the molten pool upwards, forming raised waves. Then, driven by surface tension, it moves towards the edge and solidifies in such a cycle. Splash mainly occurs at the edge of the keyhole opening, and the liquid metal on the front wall will high-speed bypass the keyhole and impact the position of the rear wall molten pool.
Post time: Mar-29-2024
