| A double-layer coating of WC-Ni outer coating and Ni60 A interface layer was prepared on a 17-4PH martensitic precipitation hardening stainless steel substrate by supersonic flame spraying technology,and the coating interface was strengthened by ultra-high frequency induction cladding.There are two methods for cladding: first,pre-cladding the sprayed Ni60 A coating,and then spraying a 200 μm WC-12 Ni outer coating on the surface of the cladding layer,which is called pre-cladding double-layer coating;second.Spray a 200 μm WC-12 Ni outer coating on the surface of the sprayed Ni60 A coating,and then perform induction cladding on the double-layer coating,which is a co-cladding double-layer coating.The effects of induced current,initial cladding position,and cladding energy on the microstructure uniformity and mechanical properties of the coating were studied,and the interface strengthening mechanism of the coating was analyzed.The main findings are as follows:(1)The influence of the induced current and initial cladding position on the microstructure characteristics of the Ni60 A pre-cladding interface layer was studied,and the effective input cladding heat energy was adjusted by the process parameters,and the induction remelting of the interface layer with two thicknesses of 50 μm and 100 μm was analyzed.Regularly,as the current increased from 360 A to 410 A,the melting degree of the top and bottom coatings of the sample gradually became serious,and when the current reached 400 A,the coating flowed due to over-melting;Moving up,the distribution of the effective input cladding heat energy was changed,the over-melting phenomenon at the top of the sample was alleviated,an interface layer with a relatively uniform macroscopic cladding morphology was obtained,and the surface over-melting area was reduced or even disappeared.It is smoother and the coating is denser.The surface roughness of the sprayed coating is 3.4～3.8 μm,and it is reduced to 0.6～0.8 μm after cladding.The macroscopic morphology of the samples is relatively uniform under the two cladding currents of 360 A and 380 A.When the thickness of the Ni60 A interface layer is the same,increasing the current can increase the thickness of the heat-affected zone of the matrix,from 18 μm at 360 A to 380 A.31 μm,the average grain size also increased from about 3 μm to about 6 μm;the increase in the thickness of the Ni60 A interface layer can reduce the heat-affected zone and grain growth of the matrix.There are many strip-shaped pores at the interface between the sprayed Ni60 A and the substrate,which are mainly mechanical bonding;after cladding,the interface porosity between Ni60 A and the substrate is reduced,and a small amount of circular pores are formed,resulting in obvious element diffusion.Metallurgical bonding.(2)According to the relationship between the cladding thermal energy and the pre-cladding microstructure of the Ni60 A interface layer,two cladding currents of 400 A and440 A were selected to prepare a co-cladding double-layer coating,and Ni60 A with two thicknesses of 50 and 100 μm was studied.Formation regularity of the microstructure characteristics of the eutectic cladding coating of the interface layer.After co-cladding,the outer coating of the sample WC-12 Ni did not melt,so a cross-sectional sample was prepared to observe the cladding state of the interface layer.There are also many pores at the interface of the sprayed Ni60 A and WC-12 Ni coatings.After co-cladding,the interface morphology has changed greatly,resulting in obvious element diffusion and metallurgical bonding,and local WC particles are combined with Ni60A;Under the same thickness of the Ni60 A interfacial layer,the increase of the induced current leads to an increase in the thickness of the heat affected zone of the substrate,corresponding to 27～35 μm and 34～43 μm,and the average grain size also increases from 4.6～5.8 μm to 9.1～9.5 μm;Similar to the change law of the pre-cladding structure HAZ and grain size with current,the increase of Ni60 A interfacial layer thickness can reduce the matrix HAZ and grain growth.(3)The effects of the cladding process parameters and the thickness of the Ni60 A interfacial layer on the microhardness of the coating were studied.With the increase of the induced current,the microhardness of the pre-cladding Ni60 A interfacial layer increased,while the microhardness of the eutectic interfacial layer increased.decreased;50 μm pre-cladding Ni60 A interface layer increased from 360 A to 380 A,hardness increased from about 6.7 GPa to 7.0 GPa,100 μm pre-cladding interface layer of 380 A was 7.7 GPa;50 μm eutectic cladding interface layer microhardness From 8.3 GPa at 400 A to 4.7 GPa at 440 A,the 100 μm co-cladding interface layer decreases from 8.8 GPa to 5.7 GPa;Significant elemental diffusion for the interfacial layer results in a reduction in hardness.(4)The variation law of residual stress on the surface of the coating was studied,and the residual stress of the pre-clad double-layer coating and the sprayed double-layer coating WC-Ni outer coating were measured,and they were both shown as tensile stress,which were about 33 MPa and 50 MPa;the surface residual stress after double-layer coating cladding is transformed into compressive stress,which is-70～-30 MPa,and the thickness of the interface layer and the induced current have little effect.The cross-section of the coating was tested by the interfacial indentation method with a small load hardness tester with a maximum load of98 N.The results showed that the mechanically bonded interface of the pre-clad double-layer coating formed a crack that propagated along the interface under a load of 44.1 N.,the half-crack length from the center of the indentation to the crack propagation tip is about 100μm,and the half-crack length of the Ni60 A and WC-12 Ni interface propagation of the metallurgical bond of the eutectic double-layer coating is significantly reduced,and the length is 53～ 62 μm,440 A length is 74～78 μm.The metallurgical bond formed between the WC-12 Ni outer coating and the Ni60 A interfacial layer of the clad double-coated layer can inhibit the propagation of interfacial cracks.In addition,residual compressive stress transformation may also be beneficial for coating crack suppression. |
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