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Microstructural characterization of nanostructured Al2O3-ZrO2 eutectic layer by laser rapid solidification method
摘要： In the present work, nanostructured surface layers of Al2O3-ZrO2 eutectic with a thickness of approximate 1000 μm and free of cracks and pores were produced on the surface of conventionally-sintered Al2O3-ZrO2 ceramic via the laser irradiation rapid solidification process. The molten pool geometry and microstructure were characterized by scanning electron microscopy and Raman spectroscopy. The geometrical evolution of molten pool in response to laser power and laser scanning velocity was established, where the top view of molten pool exhibits a circular shape at low velocities and gradually evolves into an oval-shaped surface at high velocities. Singular Al2O3-ZrO2 eutectic colonies with a size of 100-200 μm, which is formed around a spontaneously nucleated dendritic ZrO2 core, are found on the surface of laser-remelted layer. The eutectic colony has an interphase spacing of 190-280 nm. The variation of eutectic spacing with growth rate is essentially linear on the logarithmic scale as λ=KV-0.4 by binary regression analysis. Predicted by the Jackson-Hunt theory on eutectic solidification (JH theory), the eutectic spacing is consistent with the inverse-square-root dependence on growth rate with a proportionality constant of 3.32. The eutectic colonies consist of α-Al2O3, t-ZrO2 and m-ZrO2 phases, where α-Al2O3 and t-ZrO2 are the dominant phases and the m-ZrO2 phase increases with the decrease of laser scanning velocity.
关键词： Al2O3-ZrO2,Nanoeutectic layer,Laser remelting,Surface nanostructuring
更新于2025-11-28 14:24:20
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Effects of Power Modulation, Multipass Remelting and Zr Addition Upon Porosity Defects in Laser Seal Welding of End Plug to Thin-Walled Molybdenum Alloy
摘要： Aiming to solve the serious porosity defects in laser welded girth joints of thin-walled tube and end plug made of nano-sized Ce2O3 doped Mo alloy (NC-Mo), the influences of laser power modulation, multipass remelting and zirconium (Zr) addition on the number, size and distribution of porosity defects were experimentally studied. By utilizing X-ray computed tomography (XCT), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) analysis, transmission electron microscope (TEM) and Raman spectrum (RS), the porosity feature of welded joints achieved under various conditions was analyzed. The results showed that welding cycles had a significant influence on the porosity ratio of fusion zone (FZ) while the amplitude and frequency of laser power waveform slightly influenced the porosity. When the welding cycles increased from 2 to 8, the porosity ratio of FZ decreased from about 1.00% to about 0.48% and the maximum and average pore diameters reduced by about 53% and 27%, respectively. Adding minor Zr in molten pool can further reduce the RP of FZ to about 0.35%. Through analysis, it can be seen that the pores in FZ can be divided into irregularly-shaped keyhole-induced pores and spherical metallurgy-induced pores. The latter was generated possibly because some impurity elements (including O and H) pre-existing in base metal (BM). Increasing welding cycles can promote the gas in molten pool to float and outflow, thus significantly decreasing the porosity. Moreover, Zr added in molten pool can be preferentially reacted with O to generate ZrO2, which can inhibit the precipitation of volatile MoO2 to thus suppress the generation of metallurgy-induced pores.
关键词： Molybdenum alloy,Multipass remelting,Power-Modulated laser welding,Zirconium addition,Porosity
更新于2025-11-28 14:24:20
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Scratch resistance and damage mechanism of laser remelted thermally sprayed ceramic coating
摘要： Scratch resistance of plasma sprayed and laser remelted alumina and chromia coatings were assessed under constant and variable load. Failure load and cohesive strength of the coatings increased up to 65% and 94%, respectively upon laser remelting. The scratch wear resistance increased up to 88%. This is attributed to an improvement in hardness, elastic modulus and indentation fracture toughness of the remelted coatings. Upon laser remelting, lamellar microstructure of thermally sprayed coating was transformed into a columnar microstructure. A phase change from γ-alumina to α-alumina also occurred. Principal mechanisms of failure of the coatings were plastic deformation, tensile cracking, and spallation. In both types of coatings, a subsurface crack was found to grow in a direction parallel to the scratch. Propagation of this crack to the surface produced spallation of the coating. In the ball on disc test, the coatings were found to fail by splat delamination and radial cracking. The wear coefficients of the laser remelted chromia and alumina coating, respectively were 80% and 72% lower than those of the as-sprayed coatings.
关键词： radial crack,splat delamination,laser remelting,chromia,scratch wear
更新于2025-11-21 11:18:25
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Investigation of arc dynamics during vacuum arc remelting of a Ti64 alloy using a photodiode based instrumentation
摘要： Understanding the arc dynamics during VAR is important for controlling the final quality of the ingot, as the spatial and temporal distribution of the arc dictates the electric current and energy inputs at the ingot top, which in turn affect the hydrodynamics of the metal in the molten pool and the solidification conditions of the ingot. The arc dynamics during vacuum arc remelting was quantitatively investigated based on the analysis of the light emitted by the arc and reflected by the molten pool using photodiodes placed inside a full-scale furnace. The measurements allowed us to determine the location of the arc luminosity centroid and to follow its displacement as a function of time. They were made during the melt of a Ti64 alloy with an imposed alternated electromagnetic stirring of high intensity. The arc dynamics was found to be most of the time periodic, asymmetrical, and strongly correlated to the evolution of the stirring magnetic field. The arc centroid exhibited three regular displacement patterns throughout the melt. A common characteristic to all three patterns was the relative stability of the centroid during stirring plateaus and its rapid motion during stirring ramps. An evolution towards a much more centered distribution of the centroid was observed with the arc power decrease during the hot-topping phase. Possible implications of the observed arc dynamics on the final quality of the VAR ingot are discussed.
关键词： Electric arc,Photodiodes,Titanium alloy,Vacuum arc remelting,Dynamics
更新于2025-09-23 15:21:21
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Effect of Static Magnetic Field on the Evolution of Residual Stress and Microstructure of Laser Remelted Inconel 718 Superalloy
摘要： As a typical additive manufacturing technique, direct energy deposition is restricted from further application due to the presence of residual stress and the structural deformation. Thus, minimizing the residual stress plays a crucial role in additive manufacturing. In this work, a transverse static magnetic field is introduced in the laser remelting of Inconel 718 superalloy to investigate the effects on residual stress and microstructural change. The x-ray diffraction technique was used to examine the residual stress variation. Optical microscope and scanning electron microscope were applied to observe the microstructure evolution. It was found that the compressive residual stress of the remelted region was notably reduced from 392.50 to 315.45 MPa under the effect of the magnetic field of 0.55 T. Furthermore, it was observed that the average dendrite spacing was reduced by about 32% under the magnetic field. During the laser remelting process, the imposed electromagnetic force minimized the flow field within the molten pool, inhibiting the heat transfer and minimizing the cooling rate. These directly reduced the residual stresses. Based on research findings, the magnetic field can be a potential method to eliminate the residual stress in laser additive manufacturing components.
关键词： laser direct deposition,static magnetic field,Inconel 718 superalloy,laser remelting,residual stress,thermoelectric magnetic force
更新于2025-09-23 15:21:01
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The roles of stress in the thermal shock failure of YSZ TBCs before and after laser remelting
摘要： In order to reveal the roles of stress in the thermal shock failure of thermal barrier coatings (TBCs), yttria-stabilised zirconia (YSZ) TBCs were prepared via atmospheric plasma spraying and then remelted by a laser remelting process. Subsequently, thermal cycling tests of the as-sprayed YSZ TBCs and remelted YSZ TBCs were carried out at 1100 °C. The thermal shock failure mechanism of the two TBCs was investigated comparatively from the perspective of stress, i.e., phase transformation stress of ceramic coating, growth stress and thermal mismatch stress generated by thermally grown oxide (TGO). The results showed that, compared with the as-sprayed YSZ TBCs, there was no m-ZrO2 in the remelted YSZ TBCs, which avoided the ceramic coating phase transformation stress produced by the transformation between m-ZrO2 and t-ZrO2 during thermal cycling. Therefore, the thermal shock failure mode of the as-sprayed YSZ TBCs was severe buckling driving delamination, while that of the remelted YSZ TBCs was slight edge delamination. In addition, the TGO growth stress and the thermal mismatch stress between the TGO and the bonding coating were reduced by the laser remelting treatment, leading to the spallation tendency of YSZ TBCs was decreased. Consequently, the remelted YSZ TBCs exhibited superior thermal shock resistance to the as-sprayed YSZ TBCs. Therefore, the phase transformation stress, growth stress and thermal mismatch stress determined the thermal shock failure mode and thermal shock life of these two YSZ TBCs.
关键词： Stress,Laser remelting,TGO,Thermal shock failure mechanism,TBCs
更新于2025-09-23 15:19:57
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Selective Laser Melting and Remelting of Pure Tungsten
摘要： The processing of pure tungsten encounters a substantial challenge due to its high melting point and intrinsic brittleness. Selective laser melting (SLM) technique is gaining popularity and offers an excellent processing approach for refractory metals. Herein, dense pure tungsten specimens are produced by optimizing SLM processing parameters. The mechanical property of the SLM-produced tungsten with an ultimate compressive strength of about 1200 MPa, which is obviously superior to that reported in other literature, is achieved. The increased laser energy input is instrumental in raising density and surface roughness of tungsten specimens. Interestingly, additional remelting of processed layers during SLM improves the surface quality and the microstructure and achieves the highest relative density (98.4% (cid:1) 0.5%). After laser remelting, the surface roughness is reduced by 28% and a large number of ?ne grains are obtained. The ?ow of ?uids caused by remelting plays a decisive role in the formation of ?ne grains and the defect level. Therefore, these ?ndings offer a new insight into SLM of pure tungsten.
关键词： microstructure,tungsten,selective laser melting,mechanical properties,laser remelting
更新于2025-09-23 15:19:57
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Bionic Repair of Thermal Fatigue Cracks in Ductile Iron by Laser Melting with Different Laser Parameters
摘要： Nodular iron brake discs typically fail due to serious thermal fatigue cracking, and the presence of graphite complicates the repair of crack defects in ductile iron. This study presents a novel method for remanufacturing ductile iron brake discs based on coupled bionics to repair thermal fatigue cracks discontinuously using bio-inspired crack blocking units fabricated by laser remelting at various laser energy inputs. Then, the ultimate tensile force and thermal fatigue crack resistance of the obtained units were tested. The microhardness, microstructure, and phases of the units were characterized using a digital microhardness meter, optical microscopy, scanning electron microscopy, and X-ray di?raction. It was found that the units without defects positively impacted both the thermal fatigue resistance and tensile strength. The unit fabricated at a laser energy of 165.6+19?15 J/mm2 had su?cient depth to fully close the crack, and exhibited superior anti-cracking and tensile properties. When the unit distance is 3 mm, the sample has excellent thermal fatigue resistance. In addition, the anti-crack mechanism of the units was analysed.
关键词： bionic crack blocked unit,repair discontinuously,thermal fatigue crack,ductile iron,laser remelting
更新于2025-09-23 15:19:57
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Laser surface re-melting of additive manufactured samples with a line focused beam
摘要： The laser polishing mechanism is based on the fusion of a micro layer of material due to the action of the high temperature laser beam. Laser polishing, also known as laser surface remelting, involves melting a thin layer of the substrate, with surface tension causing the material to flow from peaks to valleys. In laser polishing, the material is not removed; rather it is relocated as a molten pool. The laser beam with sufficient energy density is first allowed to quickly fuse a layer on the substrate surface of microscopic thickness. Accordingly, the peaks of the asperities on the substrate surface are fused. Owing to the fluidity of this molten metal, it easily flows into the adjacent valleys of the asperities on the surface. The same molten layer is then allowed to re-solidify so that solidified layer remains adhered to the substrate surface. This, in turn, diminishes the peaks and valleys of the asperities, which ultimately results in a smoother surface. Since surface roughness is one major limitation of additive manufacturing components, so this article makes an attempt to address the roughness of such components. The capability of laser polishing (or laser surface remelting) in reducing surface roughness is explored for various beam power and scan speed. The better set of parameters for this purpose are presented. A final surface roughness in the range of 1.0–1.1 lm is obtained for three different set of parameters. 85% reduction in surface roughness due to laser polishing is also observed.
关键词： Cylindrical lens,Laser polishing,Surface roughness,Additive manufacturing,Laser surface remelting
更新于2025-09-23 15:19:57
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The effect of temperature distribution and cooling rate on microstructure and microhardness of laser re-melted and laser-borided carbon steels with various carbon concentrations
摘要： Laser heat treatment was arranged as a laser remelting and laser boriding. Materials with different carbon concentration were used. Three zones characterized the microstructure after laser treatment: re-melted zone (MZ), heat-affected zone (HAZ) and substrate material. The differences in carbon content and physical properties of used materials influenced the obtained microstructure, temperature distribution and hardness. In case of laser re-melted specimens, the limited martensite transformation proceeded only in C20, C45 and C90 steels. As a result of laser boriding the microstructure of MZ contained additionally iron borides. The specimens with higher dilution ratio value was characterized by higher hardness of re-melted zone. The aim of this study was to indicate the possibility of predicting the microstructure and depths of MZ and HAZ with the usage of Ashby and Esterling model. The influence of carbon concentration in steel on the thickness, dilution ratio and hardness of laser-borided layers was determined.
关键词： microhardness,laser remelting,temperature distribution,laser boriding,cooling rate,microstructure
更新于2025-09-19 17:13:59