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At present, the surface shape of the laser deep-weld weld is generally considered to be due to the distribution of the surface tension in the molten pool, the difference between the internal pressure of the molten pool and the atmospheric pressure. The shape of the weld surface satisfies the following relationship:
▽2ε=-δp
Where у──the surface tension coefficient;
ε──The vertical displacement of each point on the surface of the molten pool;
δp—The difference between the liquid pressure and the external pressure.
The deformation of the workpiece has a significant influence on the surface formation of the weld. When the laser is just irradiated on the surface of an isotropic flat sheet, the temperature of the laser-applied region on the upper surface of the workpiece rises sharply, and the material in the irradiated area is cooled by the surrounding heat. The barrier of the matrix, thereby forming compressive stress in the region, and causing slight downward warpage of the sheet; when the laser continues to act, the working surface temperature continues to rise, the compressive stress increases, and at the same time, the material yields. The limit decreases with the increase of temperature. At a certain stage, the yield limit of the material is smaller than the compressive stress value around it. This part of the material undergoes yield deformation and forms a pile; under the continuous action of the laser, some materials in the irradiated area begin to melt. Forming the molten pool, the yield strength will be zero. Under the extrusion of the surrounding medium, the molten pool level rises and forms a curved surface under the surface tension. At the same time, the compressive stress in the irradiated area is partially released, and the degree of warpage of the sheet is reduced; when the laser stops, the liquid in the molten pool is rapidly cooled and solidified under the cooling action of the gas above it and the surrounding medium, and the surface after solidification The liquid surface shape before solidification is maintained, thereby forming a "protrusion" of the weld surface. Then the temperature of the surrounding material also decreases. Due to the permanent deformation of the weld and its vicinity, the points near the weld on the surface of the workpiece can not fully recover its position before the laser action, thus forming a new residual pull on the upper part of the workpiece. The stress, and ultimately the workpiece, produces a degree of upward warpage.
When the process parameters such as laser power and scanning speed are different, the energy injected into the workpiece per unit time is also different, and the size of the molten pool and the range of the heat affected zone will also change, which will inevitably affect the magnitude and distribution of thermal stress, which in turn leads to The degree of protrusion on the surface of the weld. This provides the possibility to control the shape of the weld surface by selecting suitable process parameters.
Surface forming in laser welding
In practical applications, laser deep-melting weld seam surfaces tend to form protrusions of a certain height (or collapse at a certain depth). However, in some specific application environments, the post-weld surface requirements are quite high, and no visible unevenness is allowed. Therefore, it is necessary to study the influencing factors of the surface shape of the laser weld, that is, the cause of the surface protrusion and the corresponding elimination method.