厚板高功率激光切割重铸层微观组织研究

Microstructure of Recast Layer During High Power Laser Cutting of Thick Plates

本文研究了18 mm厚316L不锈钢激光切割重铸层宏微观成形特征,基于扫描电镜和电子背散射衍射仪,分析了重铸层表面和截面晶粒的形态和尺寸。结果表明:重铸层表面的Fe元素少量蒸发,从上部到下部重铸层表面由紊流向层流转变,厚度逐渐增大,组织呈现为分层的针状晶形态;对于晶体取向,切缝顶部晶粒的外延生长比例低于底部,前者生长方向的随机分布由熔体紊流引起,后者的外延生长由熔体层流引起;母材中的γ相为等轴晶形态,δ相呈带状分布;重铸层中γ相的晶粒形态不规则,粗化至母材的2倍左右,而δ相则弥散分布,细化至母材中δ相尺寸的1/6~1/2;组织转变的原因是激光切割过程中极大的温度梯度大幅缩短了δ相形成温度的持续时间,同时,熔体扰动促使δ相弥散分布。

Objective Owing to the excellent strength,plasticity,and corrosion resistance,316L stainless steel is widely used in nuclear and chemical industries.The efficient cutting of thick plates is realized using lasers,which are high-energy-density heat sources.During the laser cutting process,the plate material melts and is blown off vertically under the action of a coaxial compressed gas.Therefore,a kerf is formed.During a rapid thermal cycle,an extremely thin recast layer(the order of microns)is formed on the surface of the kerf.During the solidification of the recast layer,a particular temperature gradient and fluid motion significantly influence the morphology and the texture of the structure at room temperature.In previous studies,researchers have mainly focused on the influence of parameters,such as laser power,cutting speed,and pressure of compressed gas,on the cut formation and its quality.Few studies have focused on the microstructural morphology and formation mechanism of the recast layer.The differences between the as-solidified microstructure and the substrate may lead to non-negligible changes in the properties of the edge,which in turn affects the overall characteristics.To study the morphology and microstructural growth of the recast layer,an 18 mm thick 316L austenitic stainless steel plate is taken as the object of laser cutting for this study.The solidification mechanism of the recast layer at different kerf sites during the laser cutting process is revealed.Methods An 18 mm thick 316L austenitic stainless steel plate was employed as the base metal for this study.A pulsed laser was used to cut the base metal to form a kerf.N2 was chosen as the compressed gas,and its flow direction was coaxial with the laser.Representative specimens were then sampled to analyze their surfaces.Transverse and surficial microstructural morphologies of the recast layer,under the laser action,were analyzed using scanning electron microscopy and electron back scattering diffraction(EBSD).In addition,the recast surfaces were cleaned using anhydrous ethanol.The transverse surfaces were treated using coarse grinding,fine grinding,and polishing techniques.The polished surface was then etched with diluted aqua regia(volume ratio of HCl,HNO3 and H2O is 3∶1∶4).Results and Discussions The distribution of the main elements on the surface of the recast layer is analyzed using energy dispersive spectroscopy.The results indicate that no significant element change occurred along the thickness,except for a slight loss of Fe(Table 5).The grain growth mode of the recast layer is further analyzed using EBSD at the 1/3 site from kerf top and the kerf bottom site.The results indicate that epitaxial growth is the primary growth mode.However,the proportion of non-epitaxial growth at the 1/3 site from kerf top(Fig.5)is observed to be higher than that at the bottom site(Fig.7).A comparison between the IPF orientation distribution and pole figures in Figs.9 and 10 also shows that the grain growth at the 1/3 site from kerf top exhibits some fluctuations with unmixed and unperturbed features.Conclusions The results show that a small amount of Fe evaporates from the recast layer surface.A variation in flow state from turbulent at the top to laminar at the bottom surface is observed,with an increase in thickness and needle-like grains.For crystal orientation,the ratio of the epitaxial growth at the top surface of the recast layer is lower than that at the bottom surface.Such a random distribution of epitaxial growth is caused by the turbulent flow at the former,whereas the dominant epitaxial growth is induced by the laminar flow at the latter.Considering the grain profiles,theγphase in the base metal is equiaxed,whereas theδphase is arranged in a banded form.The morphology of theγphase grains in the recast layer is irregular and coarsen by approximately 2 times compared to those of the base metal.However,theδphase is dispersed and refined from 1/6 to 1/2 of the base metal.Under the conditions of an extremely high-temperature gradient and a disordered disturbance owing to melting,a substantially reduced duration ofδphase formation with considerable dispersion is produced.