Study On Mechanism Of Influence Of Laser Discrete Strengthening On Bonding Properties Of 45 Steel Substrate And Ni-plated Layer

In this thesis,the surface of 45 steel plate is polished,then laser quenching is carried out on the surface of the steel plate.Finally,nickel-cobalt alloy coating is deposited on the surface of 45 steel substrate by electroplating technology to prepare samples.The prepared sample was tempered at 200?;the cross section of the sample was observed by scanning electron microscopy to characterize the bonding and microstructure of the coating and the substrate.Effect of laser discrete strengthening pretreatment substrate on thermal shock resistance of electroplated nickel alloy layer by thermal shock test.Indentation test and scratch test were used to analyze the effect of pre-laser discrete strengthening and post-electroplating composite process on the physical and mechanical properties of coating and matrix and the bearing capacity of coating/matrix system.Research indicates:(1)The laser quenching zone contains high-hardness lath martensite and very fine carbide particles,while the original substrate region is pearlite and ferrite,and the transition zone exhibits a very uneven microstructure and morphology.The original substrate/plated junction is poorly coupled and presents a rough and pitted state;However,the combination of the matrix and the coating at the junction of the transition zone is better,and the rough and pitted state is improved compared with the original substrate/plating interface;The junction between the laser quenching zone and the coating is best combined,showing a smooth and straight state.(2)The sample with no pre-laser discrete strengthening treatment has poor thermal shock resistance.With the increase of the number of thermal shock experiments,the falling area of the coating increases remarkably.After 5 thermal shock experiments,the percentage of the falling area of the coating reached a staggering 46.1%;The pre-laser discrete strengthened sample has excellent thermal shock resistance.Even after 5 thermal shock tests,the maximum percentage of the area of the nickel plated on the laser strengthened sample is only 2.48%,the percentage of shedding area is one order of magnitude lower than that of a sample that is not pre-laser strengthened.(3)The results of nanoindentation experiments show that the composite process of pre-laser discrete strengthening and post-plating of the matrix can significantly improve the hardness of the electroplated nickel alloy layer in the quenching zone and above,but has little effect on the elastic modulus of the coating.(4)The scratch test results show that the scratch width and depth of the coating on the laser quenching zone are small,and the composite process of pre-laser discrete strengthening and post-plating can greatly enhance the bearing capacity of the coating/matrix system.