Effect Of Alloying Elements On The Microstructure And Properties Of Laser Cladding Fe-based Coatings

45 steel is widely used to manufacture various important structural parts,such as connecting rods,gears,bearings,etc.,but when these parts serve in harsh environments,the overall failure is often caused by serious surface wear,which greatly increases the cost of industrial applications.Therefore,it has become an important means to extend their service life by surface technology to manufacture wear-resistant coatings on the surfaces of wearing parts.Compared with traditional surface modification technologies,laser cladding has the advantages of fast deposition rate,low dilution rate of,and good metallurgical combination between coating and substrate,and it is often used to improve the wear and corrosion resistance of parts.In this thesis,the Fe-based alloy coatings with different Mo contents and B contents were prepared on the surface of 45 steel substrate by a semiconductor laser.The effects of different amounts of Mo and B alloying elements on the macro-morphology,phase composition,microstructure,wear resistance and corrosion resistance of Fe-based coatings were analyzed,and the rule of alloying element addition amount to improve the comprehensive performance of Fe-based coating was obtained,which could provide theoretical guidance for application.The results show that:（1）Mo can effectively refine the microstructure of the coating and promote the transformation from dendrite to equiaxed crystal.With the increase of Mo content,the hard phases in the coating changes from Fe₂B to Mo₂Fe B₂.With the increase of Mo content,the Mo content in the intergranular and intragranular regions of the coating increases.The addition of Mo plays a role in fine grain strengthening and solid solution strengthening,which is conducive to improving the hardness and wear resistance of the coating.When the Mo content is 4.8 wt.%,the average microhardness of the coating is 3.6 times higher than that of the substrate,and the wear loss is 44%lower than that of the coating containing 0.8 wt.%Mo.The results of polarization curve and electrochemical impedance spectroscopy show that the coating containing 4.8 wt.%Mo has the best corrosion resistance.（2）The Fe-based coatings with different B contents are composed ofα-Fe,Cr₇C₃and Fe₂B phases.The content of B in the coating has a significant effect on the morphology and quantity of boride precipitates.With the increase of B content,the proportion of boride phases in the microstructure increases and the morphology of boride undergoes a transition form reticular to cluster-shape,long strip-shape and plate-shape.Composition analysis shows that the boride is a Cr rich Fe₂B phase,and the content of Cr element in the boride phase increases with the increase of B content in the coating.The results of microhardness and friction and wear tests show that the hardness and wear resistance of Fe-based coating can be effectively improved by adding B.When the B content increases from 1.6 wt.%to 6.0 wt.%,the average microhardness of the coating increases from 680.71 HV_0.2 to 887.41 HV_0.2.The wear resistance of the coating shows the same change rule as the microhardness,and the wear loss of 6.0 wt.%B coating is only 1/5 of 1.6 wt.%B.The wear mechanism of Fe-based coatings with high boron contents are mainly abrasive wear and oxidation wear.（3）Gradient and multilayer Fe-based coating with high boron content show good metallurgical bonding with the substrate.The microhardness of 45 steel substrate is significantly improved by both gradient coating and multi-layer coating,and its surface microhardness is about 4.8 times of that of the substrate.Both coatings exhibit good wear resistance.The morphology and content of boride phase play a key role in improving the microhardness and wear resistance of the coating.