激光熔覆316L涂层晶粒生长取向与形貌对其耐蚀性能的影响

Effect of Grain Growth Orientation and Morphology of Laser-cladded 316L Coating on Its Corrosion Resistance

目的表征316L激光熔覆层的晶粒取向、形貌与其耐蚀性之间的关系,阐明晶粒形态对316L熔覆层耐蚀性能的影响原理。方法在Q235钢板上激光熔覆制备搭接率为20%、35%、50%的316L不锈钢熔覆层,获得具有不同取向与含量柱状晶的熔覆层。采用光学显微镜、扫描电子显微镜、能谱仪、X射线衍射仪、电化学工作站对熔覆层的显微组织、元素分布、物相组成及耐蚀性能进行表征。结果熔覆层的晶粒形态和耐蚀性能受到搭接率影响。熔覆层因搭接发生部分重熔和回火,产生了横向柱状晶,随着搭接率从20%增大至50%,横向柱状晶含量从25%提升到41%。搭接率不影响熔覆层物相组成,熔覆层主要物相均为γ相。横向柱状晶含量在38%和41%时的熔覆层自腐蚀电流密度较25%含量的熔覆层下降1个数量级,分别为3.73、1.34μA·cm^(-2),熔覆层耐蚀性能随横向柱状晶含量的增加而提升。结论熔覆层物相与搭接率无关,但搭接率会导致熔覆层晶粒生长取向与形貌发生变化:随着搭接率的增大,晶粒由等轴晶转变为横向柱状晶。横向柱状晶降低了腐蚀性介质的传输效率,导致熔覆层耐蚀性大幅度提升。

Laser cladding is an advanced surface modification technology,which can be used to prepare coatings wear resistance,corrosion resistance,oxidation resistance,and other properties on the surface of metal substrates.Coatings prepared by laser cladding can change the orientation and morphology of the tissue.In this paper,the effects of grain orientation and morphology on the corrosion resistance of the 316L cladding layer were studied.The Q235 steel plate of 160 mm×60 mm×8 mm was polished clean at laser power of 900 W,scanning speed of 3 mm/s,spot diameter of 2 mm/s,and powder feeding rate of 6 g/min,and with argon as the protective gas.Under the same conditions,the 316L cladding layers with 20%,35%,and 50%overlap ratio were prepared by laser cladding on the substrate with the equipment of model JHM-1GX-3000P.Specimens were cut with wire cutting and the sections polished and etched with aqua regia.Then,the microstructure of the cladding layer was observed with metallographic microscope and scanning electron microscope.The element distribution of the cladding layer was analyzed through EDS,and the phase composition of the cladding layer was analyzed through XRD.The corrosion resistance of the cladding layers with different overlap ratios was tested by electrochemical workstation in 3.5wt.%NaCl solution at room temperature,and the corrosion morphologies of the cladding layers after the test were observed through SEM.The results showed that the overlap ratio affected the forming quality of the cladding layers.The cladding layers with the 20%and 35%overlap ratios had no obvious pores,but there were large pores at the bottom of the cladding layer with 50%overlap ratio due to poor fusion.And the increase of the overlap ratio increased the transition layer area between the coating and the substrate but had no effect on the phase composition of the cladding layers,both of which wereγ-phase.Due to the partial remelting and tempering brought about by the lap joint,the cladding structure was recrystallized,and transverse columnar crystals and cellular crystals appeared in the overlapping part of the cladding layer.For the cladding layers with overlap ratios of 20%,35%,and 50%,the content of transverse columnar crystals gradually increased to 25%,38%,and 41%.The electrochemical test results showed that the self-corrosion potentials of the cladding layer under different lap ratios were-441,-508,and-331 m V,respectively,and the self-corrosion current densities were 19.90,3.73 and 1.34μA·cm^(-2).The impedance value increased with the increase of the overlap ratio,and the corrosion resistance of the coating increased with the increase of the transverse columnar crystal content.The overlap leads to the transformation of the grain orientation and morphology in the overlapped region of the cladding layer.The grains are transformed from equiaxial to transverse columnar crystals,and the content of transverse columnar crystals increases with the increase of the overlap ratio.Transverse columnar crystals replace equiaxial crystals,and the grain boundaries of transverse columnar crystals are much larger than those of equiaxial crystals.Transverse columnar crystals increase the transmission distance of the corrosive medium,resulting in a significant improvement in the corrosion resistance of the cladding layer.