回字形扫描路径下高速激光熔覆数值模拟及实验研究

Numerical Simulation and Experimental Study of High-Speed Laser Cladding Under Circular Scanning Path

高速激光熔覆过程中的温度变化和扫描路径位置变换均对涂层组织与性能有较大影响。利用高速激光熔覆技术在27SiMn钢表面以回字形扫描路径制备了铁基TY-1涂层,对比分析了同一参数下回字形路径上的温度变化对涂层组织和性能的影响。利用ANSYS Workbench有限元分析软件模拟了高速激光熔覆过程,获得了回字形路径上不同位置处涂层的温度场分布规律,并通过微观组织和性能分析实验进一步分析了温度变化对涂层组织和性能的影响。结果表明:回字形扫描路径下由中心点向外的熔池最高温度分别为1890、1955、1998℃;不同位置处的涂层间温度相互影响,相较于回字形内部(距中心点1~19 mm)和中部(距中心点19~34 mm)处的涂层,外部(距中心点34~46 mm)涂层熔覆完成后在空气中冷却,没有后续熔覆道次对其施加温度上的影响,冷却速率相对较快,因而晶粒组织分布均匀,硬度和耐蚀性均较高。

Objective High-speed laser cladding has the characteristics of fast speed,high efficiency,good finish,a random cladding layer thickness,low heat input,low dilution rate,energy saving,and environmental friendliness.The binding strength,microstructure,and coating performance are strongly influenced by the scanning path,which is an important factor affecting the thermal field distribution during high-speed laser melting.Compared with unidirectional scanning,reciprocating scanning,and other scanning methods,the heat accumulation of the circular scanning method is greater,and the temperature of the adjacent passages is higher during cladding.This study analyzed the temperature change of the high-speed laser cladding process in a circular scanning path.The iron-based TY-1 coating was sintered on the surface of 27SiMn steel with a circular scanning path using high-speed laser cladding,and the influence of temperature changes from inside to outside on the grain growth,hardness and corrosion resistance of coatings in different regions under the same parameters was analyzed.Methods Monitoring the molten pool temperature in real-time is difficult during high-speed laser cladding.The influence of temperature changes on the material microstructure and performance was studied in the high-speed laser cladding process.In this study,the aforementioned relationship was clarified through simulations and experiments.A simulation clarified the temperature change of high-speed laser cladding.The microstructure,hardness,and electrochemical corrosion were studied by the experimental analysis of the cladding process,and the finite element analysis software ANSYS Workbench was adopted to simulate the high-speed laser melting process to obtain the thermal field distribution law of the coatings at different positions in the circular path.Then,some samples were prepared using the high-speed laser melting technique under conditions identical to the simulated conditions.Microscopic structures perpendicular and parallel to the laser scan direction were observed using an optical microscope.Subsequently,hardness and electrochemical corrosion experiments were conducted.The influence of temperature on the changes in the microscopic structures and performance of the cladding coating was analyzed.Results and Discussions The finite element simulation shows that the maximum temperature ranges from 1800 to 2065℃(Fig.2).Influenced by the temperature of the cladding layers,the coating temperature gradually increases from the inside to the outside.The insulation time of region A1 without preheating is longer than those of regions A2 and A3 with preheating.The preheating time of regions A2 and A3 gradually increases.In contrast,the insulation time gradually decreases(Fig.4).Owing to the thermal influence from adjacent coatings,the microstructure at the bottom of the coating is mostly columnar crystals(Figs.10 and 11),and the primary dendrite spacing of the columnar crystals ranges from 5 to 25μm(Fig.10).According to the fine crystal reinforcement theory,grain refinement increases the number of grain boundaries;therefore,the maximum average hardness of the coating section at region a3(the region corresponds to region A3 in the simulation)is 579 HV(Fig.12).The potential difference between the interface and core of the dendritic crystal is reduced because the coating is meticulous and uniform.Therefore,the maximum self-corrosion voltage of the coating at region a3 is-0.466 V,and the minimum self-corrosion current is 0.7943×10-6 A·cm-2(Table 5).The best coating performance is demonstrated at region a3.Conclusions The highest temperatures of the melting pool at regions A1,A2,and A3 were 1890,1955,and 1998℃,respectively,obtained by circular scanning path simulations using high-speed laser melting technology.The temperatures of the coatings at different positions were related.Influenced by the temperatures of the cladding layers,the coating temperature gradually increased from the inside to the outside;the insulation time of region A1 without preheating was longer than those of regions A2 and A3 with preheating.The preheating time of regions A2 and A3 gradually increased while the insulation time gradually decreased.The preheating and heat preservation of the coatings at regions A1,A2,and A3 reduced the temperature gradient and cooling rate.The temperature gradient and cooling rates reduced with increasing preheating and insulation time of the coatings at regions A1,A2,and A3.The slender columnar crystals at the bottom grew laterally and evolved to equiaxed and thick columnar crystals.The average hardness of the coating section at regions a1,a2,and a3 were 512,466,and 579 HV,respectively,and the hardness gradually increased along with the increase in cooling rate.When the cooling rate was high,the dimensions of the grains did not grow significantly.Thus,the microstructure remained small and compact and the plastic deformation resistance and coating hardness were high.The corrosion resistance voltages of the coatings at regions a1,a2,and a3 were-0.525,-0.514,and-0.466 V,respectively.The corrosion resistance increased with a decrease in the insulation time.Because the Cr element in the powder was partially consumed during heat preservation process,the corrosion resistance of the coating was reduced;thus,the coating at region A3 had relatively good corrosion resistance owing to the short insulation time.