| Magnesium alloy has been known as the most promising green engineering material of 21st century owing to the light weight and high damping resistance.While the high chemical activity or poor corrosion resistance restricts the serving lives and application fields of magnesium parts.Magnesium alloy presents a lower laser beam absorption and melting point,which makes the laser cladding process complicated and difficult.Nowadays,researchers are being devoted themselves on the equipment updating,cladding materials choosing,processing parameters optimizing and others.In this research,the influence of different cladding materials(Ni35,Al-Si/Ni35,Al-Si/(Ni35+xwt.%Al2O3))on the microstructure and corrosion resistance of AZ91D cladding coatings has been analyzed;the evolution rules of the cladding coating’s microstructure and properties have also been studied when the scanning speeds change from 200 mm/min to 300 mm/min;the solidification behaviors of the molten pool and the forming mechanism of the main phases have been revealed.SEM,XRD and EDS are used to characterize the coatings’ microstructure,phases and composition,respectively.Electrochemical workstation is used for analyzing the corrosion properties,aiming to explain the improving mechanisms of the cladding coatings’ corrosion resistance.Results indicated the cladding coatings with fine grains and excellent corrosion resistance could be obtained when the Ni35 powders were cladded on AZ91D substrates.The coatings were mainly composed of γ-Ni,Ni3B,M23C6,MgNi2,and most of them were the original phases of Ni3 5 powders.The bonding zone had been divided into the over and hardly diluting parts owing to the un-homogenized distribution of thermal and composition in the molten pool.There existed a white bonding zone in the lower part of cladding coating,and the upper part was characterized by the dendritic crystal and inter-dendritic eutectic organization.Theγ-Ni dendritic,dispersing strengthening of intermetallic compounds and the solution strengthening of Cr,Fe,Si elements played very important roles in improving the coatings’ corrosion resistance.The substrates and Ni3 5 powders could bond very well when the Al-Si eutectic alloys were used as the transition layer.The samples after laser treated could be divided into three parts against the penetration direction:substrate,Al-Si transition layer,cladding coating.The coatings were mainly composed of γ-Ni,MgNi2,AlNi3,Mg2Si,FeNi3,M23C6 phases,with fine grains and uniform microstructures.According to the line scanning analysis results,Al and Si elements mainly lay in the transition zone,and the interaction with Mg formed Mg2Si and Mg-Al phases.There also existed much Al combining with Ni in upper zone of the cladding coating,which diminished the combination between Cr and Ni.The IMC(intermetallic compound)between A1 and Ni usually nucleated around the Cr compounds and formed the petals tissue,making the Cr precipitate in the form of boride and limiting the solid solution in Ni.It had different influence on the corrosion resistance when the Cr was precipitated in different manners.The molten pool would last a long time when the scanning speed was 200 mm/min,which caused a higher dilution rate of substrate and a serious mixing between cladding powders and substrate,obtaining a coating with loose microstructure.When speed was 300 mm/min,the cladding coating was easy to break away from substrate because of the weak bonding between them.Optimization of the parameters could regulate the dilution rate,and under the speed of 250 mm/min,both the Ni35 and Al-Si/Ni35 coatings obtained the best microstructure and corrosion properties,which was also the processing parameter of fabricating the Al2O3 added coatings.It was indicated that the coating had the best corrosion resistance with the power 1500 W,the scanning speed 250 mm/min,and the cladding material Al-Si/Ni35.The coating’s corrosion potential was-0.997 V,which increased by 0.521 V compared with the substrate.The corrosion current density was 0.437×10-4 A·cm-2,which was only 1/30 of the substrate.The corrosion resistance of Al-Si/Ni35 coating was even 4-5 times as much as that of Ni35 coating under the condition of v=200 and 250 mm/min,indicicating the Al-Si/Ni35 coating could improve the corrosion resistance of aubstrate.It was clear the improvement was related to the excellent corrosion property of Ni-based alloy,the microstructure after laser treated,the solution and dispersing strengthening,then it also identified the Al-Si layer realized the metallurgical bonding between substrate and Ni35 powders.The corrosion resistance could be significantly improved when the Al2O3 were evenly distributed in the cladding coating.While,the Al2O3 and Ni35 powders were unfulfilled to mix uniformly owing to the big diffirence in physicochemical properties.And the laser absorption rate of Al2O3 was much higher than the substrate and Ni35,adding with the great thermal conductivity of Al-Si transition layer,the inhomogeneity of the thermal and composition were easily caused in the molten pool.However,the corrosion properties had been improved compared with substrate.When the weight percent of Al2O3 was 1-3%,y-Ni and Al2O3 phases could be tested,while the segregation appeared and the composition was not uniform.When the amount of Al2O3 increased to 5wt.%,the distribution of Al2O3 particles was homogeneous and there were flock-like structure Al2O3,which were beneficial to the improvement of corrosion resistance. |
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