| AM50 magnesium alloy is widely used in aerospace, transportation, automotive and other industrial fields because of its low density, high strength, high thermal conductivity,high dimensional, high damping characteristics and so on. Due to the poor surface strength,hardness and wear resistance, the application of magnesium alloy in engineering is seriously hindered. Therefore, improving the surface properties of magnesium alloy has become one of the key problems in the manufacture of important components. Surface treatment technology can effectively improve the mechanical properties of magnesium alloy by improving the microstructure of the magnesium alloy surface layer. Laser shock processing(LSP) is a new surface treatment technique and widely used in the surface modification of metals. The key beneficial characteristic after LSP treatment is the presence of compressive residual stresses beneath the treated surface of metallic materials,mechanically produced by high magnitude shock waves induced by a high-energy laser pulse. To achieve the aim of anti-fatigue manufacturing of key parts with a long life is not only the single way for China to move from a great mechanical manufacturing country to a powerful mechanical manufacturing country, but also our country’s strategic measures and urgent demand. The program of action of “China manufacturing 2025” released in2015 explicitly classified the advanced laser manufacturing as the frontier technology in manufacturing industry. Based on the above present situation, the stress field and tensile properties of die casting magnesium alloy subjected to LSP were studied in this paper.Some main conclusions and innovative results are shown as follows:(1) The influence of different LSP paths on the residual stress field of AM50 magnesium alloy specimens and the difference of the residual stress between double-sided simultaneous LSP and double-sided non-simultaneous LSP had been studied by using finite element software ABAQUS. After the establishment of a three-dimension analysis model, the distribution of residual stress along thickness direction and on the surface in different models was analyzed and compared, and the best method of LSP was obtained.Results showed that under the same laser parameters, although LSP could induce high compressive residual stresses, it also generated undesirable tensile residual stresses in the mid-thickness of the treated specimens. In the middle part of the specimen along thethickness, there was not only residual tensile stress, but also compressive stress after the treatment of double-sided simultaneous LSP. The surface residual stress distribution induced by the LSP path perpendicular to tensile force direction was more uniform, less volatile, and the average residual stress was larger, which had a better strengthening results. In addition, although the maximum residual compressive stress on the upper surface induced by double-sided non-simultaneous LSP was larger, the residual compressive stress on both surfaces induced by double-sided simultaneous LSP was symmetrical in the depth direction with a better uniformity. In summary, the optimal LSP method of strengthening AM50 magnesium alloy specimens should be double-sided simultaneous LSP with the LSP path perpendicular to tensile force direction.(2) The AM50 magnesium alloy specimens with different LSP coverage area were treated by high energy nanosecond laser beam. The relationship between LSP coverage area and tensile crack initiation location was obtained by studying the influence of LSP coverage area on tensile stress-strain curves and analyzing the fracture morphology.The stress-strain curves of specimens under different LSP coverage area were compared and analyzed. The influence mechanism of LSP coverage area on the tensile properties of specimen was studied, and the optimal LSP coverage area was obtained. Results showed that, massive LSP impacts could cause an evident improvement in UTS and YS of tensile specimen, and these values increased with the increasing coverage area. However, tensile specimen plasticity(including elongation rate and RA) initially increased and subsequently decreased with the increasing coverage area for both cases. A brittle-ductile transition occured when LSPed length increased from 0 mm(LSP-0) to 40 mm(LSP-40),which was attributed to the crack initiation location with different LSPed coverage areas.(3) The surface microstructures of magnesium alloy before and after LSP were characterized and compared from a microscopic perspective, and the influence of LSP coverage area on tensile properties of AM50 magnesium alloy specimens was obtained.The surface grain refinement process of AM50 magnesium alloy subjected to LSP was obtained by observing the microstructure in the LSP area. The change of tensile properties of specimens was explained by studying the effect of microstructure on tensile properties.Results showed that, The grain refinement process in the top surface layer of AM50 magnesium alloy caused by massive LSP impacts was schematically presented as follows:(i) deformation twin formation with a 5 μm spacing in the coarse grain with an average size of 50 μm after the mechanical effect of ultra-high laser shock wave;(ii) divided T-Mlamellae by deformation twins with a direction; and(iii) the formation of randomly refined grains with an average size of 5 μm. With the increase of LSP coverage area, the region of grain refinement also gradually expanded, and the crack initiation location transferred,resulting in the change of tensile properties of AM50 magnesium alloy with the various LSP coverage area.Laser shock processing, as a novel material surface strengthening technology, can not only induce a high magnitude of residual compressive stress, but also obviously refine the grain in the surface layer of materials. Due to the mutual influence of residual compressive stress and grain refinement, the tensile properties of AM50 magnesium alloy is significantly improved, and the crack initiation location transfers with the variety of LSP coverage area. |
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