脉冲/连续双光束复合激光熔覆对316L不锈钢熔覆组织的影响

Effect of Pulsed/Continuous Double-Beam Hybrid Laser Cladding on Microstructure of 316L Stainless Steel

本团队以316L不锈钢为对象,研究了高频脉冲激光直接作用于液态熔池对熔覆层宏观形貌、显微组织和熔池流动行为的影响;采用脉冲/连续双光束复合激光熔覆技术制备了316L不锈钢涂层,采用光学显微镜观测了熔覆层的宏观形貌和显微组织结构,并进一步采用高速摄像机研究了熔池流动行为的变化。研究结果表明:在高频脉冲激光作用下,熔池表面产生了高频环状振动波,波速约为1.408 m/s,马兰戈尼对流显著增强,熔池表面后端熔体的流速由未施加高频脉冲激光时的0.308 m/s增至0.394 m/s,提高了约27.92%;熔池凝固的冷却速率显著提高,由未施加高频脉冲激光时的186.58 K/s增至475.76 K/s,提高了约154.99%;由于形核趋势的增加和温度梯度的降低,熔覆层组织得以显著细化,平均晶粒尺寸由未施加高频脉冲激光时的13.06μm减至6.32μm,减小了约51.61%。

Objective As a new type of additive manufacturing technology,laser cladding rapidly repairs and strengthens the surfaces of metal parts.However,owing to the characteristics of rapid heating and sudden cooling,the single laser cladding technology is prone to create defects such as pores,cracks,and coarse microstructures during actual processing.Moreover,achieving good improvements by changing the process parameters is often difficult.The introduction of composite energy fields and hybrid laser beams in laser cladding is an effective approach to improve cladding defects and refine microstructures.Herein,the effects of a high-frequency pulsed laser on the macroscopic morphology,primary dendrite spacing,microstructure,and molten pool flow behavior of a cladding layer are investigated by coupling a continuous laser with a high-frequency pulsed laser.The latter is directly introduced into the liquid molten pool,which provides a new idea for double-beam hybrid laser additive manufacturing.Methods Stainless steel(316L)was used in the present study.First,a 316L stainless steel cladding layer with good metallurgical bonding and microstructure was prepared on a 316L stainless steel substrate via the pulsed/continuous double-beam hybrid laser cladding technology,using a continuous laser as the primary heat source and a high-frequency pulsed laser as the high-frequency vibration source.Second,the macroscopic morphology and microstructure of the cladding layer were analyzed using a metallographic microscope,and the average primary dendrite spacing was measured to characterize the cooling rate of molten pool solidification.The average grain size was measured to verify the refining effect of the high-frequency pulsed laser acting directly on the liquid pool on the cladding structure.Finally,the molten pool morphology and flow behavior during the cladding process were observed with a highspeed camera,and the high-frequency shock vibration of the liquid molten pool,induced by the high-frequency pulse laser,was further studied.The influence mechanism of the high-frequency pulsed laser on the liquid molten pool was also analyzed.Results and Discussions In this study,pulsed/continuous double-beam hybrid laser cladding is realized by coupling a continuous laser with a high-frequency pulsed laser(Fig.3).Under the same laser power,the depth and area of the fusion zone increase significantly(Fig.5).The dendrite growth trend changes,whereas the porosity and other defects appear inhibited(Fig.6).This indicates that the cooling rate of the molten pool significantly increases because the primary dendrite spacing significantly decreases(Fig.8).The grain size is significantly refined with an increase in the nucleation trend and a decrease in the temperature gradient(Figs.9 and 10).In addition,the high-speed camera clearly captures that a high-frequency circular vibration wave is generated on the molten pool surface under the action of the high-frequency pulsed laser,the Marangoni convection is significantly enhanced,and the melt flow rate significantly increases(Figs.11 and 12).Conclusions In this study,the coupling effect of a continuous laser and high-frequency pulsed laser is successfully realized.The effects of the high-frequency pulsed laser on the macroscopic morphology,primary dendrite spacing,microstructure,and molten pool flow behavior are studied.Consequently,the effects of the high-frequency pulsed laser on the molten pool flow behavior,solidification process,and grain growth are revealed.The results reveal that high-frequency ring vibration waves with velocity of approximately 1.408 m/s are generated on the molten pool surface under the action of the high-frequency pulsed laser.The melt flow velocity at the back of the molten pool surface increases from 0.308 m/s to 0.394 m/s,an increase of approximately 27.92%.The primary dendrite spacing decreases from 18.32μm to 11.47μm,decreasing by approximately 37.39%.The decrease in the primary dendrite spacing reflects an increase in the cooling rate of the molten pool solidification.Based on the calculation results,the cooling rate of the molten pool solidification increases from 186.58 K/s to 475.76 K/s,an increase of approximately 154.99%.The average grain size decreases from 13.06μm to 6.32μm,demonstrating a decrease of approximately 51.61%,with an increasing nucleation trend and decreasing temperature gradient under the high-frequency pulsed laser.