The Reseach On The Technology Of High-Frequence Induction Cladding For Ductile Iron-Bronze Bimetal

This paper paid attention to the ductile iron/bronze bimetal. It clad a tin layer of bronze on the surface of ductile iron by the technology of high-frequency induction cladding and achieves good combination.The effects of single-component flux on the cladding were firstly researched. Based on the effects, an orthogonal test was applied to get the best composition of mixed flux which is effective to promote the combination. The result showed that: defects such as pore, impurity and crack easily occurred on the interface and the shearing strength was very low without flux. During the four single-component fluxes, Na2B4O7 had the best competence of wiping impurities out and the cladding owned the least defected, so the shearing strength was on the top, up to 133MPa. The result of the orthogonal test demonstrated that the best mixed flux was constitutes by 60% Na2B4O7, 20% H3BO3, 10%AlP03 and 10% NaF.Using optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), electron probe (EPMA), and micro hardness tester, the interface morphology, the microstructure and phase structure of cladding layer, the distribution of alloying elements and the micro hardness were studied, and bi-metal's bonding strength were measured.Results showed that: when the frequency was 20 KHz, the reasonable range of heating current was 300A ~ 450A and holding time was 0 ~ 60s. The diffusion of elements occurred on both sides of interface when two materials combined at high temperature. The maximum diffusive distance of Fe in the copper was 20μm and the one of Cu in the substrate was 8~10μm. The bond of the substrate and the cladding was metallurgical bond, but new phase wasn't formed on the interface. The micro hardness of the interface organization was between the one of the cladding and the substrate. Increasing the current and extending the holding time, the bi-metallic bonding strength continuously increased. When the heating current was 420A, the holding time was 60s, the shear strength of the bi-metal was 186MPa and the shear section located within the bronze, also the times of repeated bending was up to 8.Tafel curves of three samples in 3.5% wt NaCl solution were respectively tested. The analysis result showed that the corrosion current density of the cladding without defects was 0.886μA / cm2, much less than the one of the substrate, 15.39μA /cm2.So the cladding's corrosion resistance was much higher than the substrate.The impacts on the combination of the bimetal of three interlayer materials including aluminum zinc and phosphor bronze was explored. The result showed that severe oxidation occurred when aluminum foil was heated, and the combination between the copper and the substrate was prevented by the oxide. so the specimen's shear strength was only about 40MPa; after adding Zinc the organization of the Cladding changes from dendrite segregation to equiaxed segregation, and the shear strength of the sample was about 100MPa, lower than the bronze's shear strength, and the fracture occurs at the bonding interface; A transition layer was formed on the interface after adding phosphor bronze, the metallurgical bonding of the bi-metal was promoted, and the average shear strength was up to 152MPa, and the fracture mode was ductile fracture.