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Surface metamorphic layer and surface mechanical properties during water jet cutting machine processing

Post time:2019-09-11    By:EAAK    Clicks:
It is very different from the water jet cutting machine.During the processing of the electric spark, under the instantaneous high temperature of the spark discharge and the rapid cooling of the working fluid, the chemical composition and microstructure of the surface layer of the material have changed greatly to form a layer. There are usually metamorphic layers of residual stress and microcracks. For melting and gasifying materials, the metamorphic layer can be divided into a molten solidified layer and a heat affected layer.
The molten solidified layer is located at the uppermost layer on the surface of the workpiece, and it is melted by the instantaneous high temperature when it is discharged, and is retained, and is solidified by rapid cooling of the working fluid. For carbon steel, the molten layer appears white on the metallographic photograph, so it is also called white layer. It is completely different from the base metal. It is a dendritic quenched cast structure, and the bond with the inner layer is not very strong. . It consists of martensite, finely retained austenite and some carbides.
The thickness of the molten solidified layer becomes thicker as the pulse energy increases, approximately 1-2 times Rmax, but generally does not exceed 0.1 mm.
The heat affected layer is between the molten solidified layer and the matrix. The metal material of the heat-affected layer is not melted, but is affected by the high temperature, which changes the metallographic structure of the material, and there is no obvious boundary between it and the matrix material. Due to the difference in temperature field distribution and cooling rate, for quenched steel, the heat-affected layer includes a re-quenching zone, a high-temperature tempering zone and a low-temperature tempering zone: for unhardened steel, the heat-affected zone is mainly a quenching zone. The thickness of the heat affected layer of the hardened steel is larger than that of the untempered steel.
The part of the heat-affected layer close to the molten solidified layer is cooled by the high temperature and rapidly cooled to form a quenching zone. The thickness is related to the specific conditions, generally 2-3 times the maximum microscopic unevenness value, thus also affecting the water cutting machine. Process surface integrity. For hardened steel, the part adjacent to the quenching layer is affected by the near phase transition temperature and the slower cooling to form a high temperature, low temperature tempering zone, and the thickness of the tempering zone is about 3-4 times of the maximum microscopic unevenness.
The metallographic structure of the heat-affected layer of different metal materials is different, and the heat-affected layer of the heat-resistant alloy is not much different from the matrix.
Microcracks generally occur only in the molten layer, and it is only possible to extend to the heat-affected layer when the pulse energy is large (when roughing). The effect of pulse energy on microcracks is very obvious. The larger the energy, the wider and deeper the microcracks. When the pulse energy is small (for example, when the surface roughness is better than Ra1.25), microcracks generally do not occur. The sensitivity of different workpiece materials to cracks is also different. Hard and brittle materials are prone to cracks, and the effect of pre-heat treatment of the workpiece on cracks is also obvious. The quenching material is more prone to cracking than the material that is tempered or annealed after quenching, because the quenching material is brittle and the original internal stress is also large.
The formation of the surface metamorphic layer results in a change in the mechanical properties of the EDM surface, usually an increase in hardness and wear resistance, and a decrease in fatigue resistance.
As one of the characteristics of water jet cutting, that is, to improve surface integrity, it is necessary to reduce the thickness of the surface metamorphic layer. When the surface roughness is in the range of Ra0.32-0.08 μm, the thickness of the surface deterioration layer is 3-5 μm, and the fatigue resistance of the EDM surface will be similar to that of the precision machined surface. The high energy narrow pulse width EDM process is an effective method to solve surface integrity and improve processing efficiency.

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