Pulse Electroforming Nickel-cobalt Alloy Forming Technology And Performance Study

Electroforming technology have better imitation and high precision in duplicating. In recent years, with the development of industry, the needs of complex superfine, multifarious precision parts are drastically increasing. It leads electroform technology to be highly recognized. Ni-Co alloys have been widely used as important engineering materials in industry because of their excellent physical-chemical properties and mechanical properties, high hardness and strength, good wear resistance and corrosion-resistance, heat-conductive.In this paper, the Ni-Co alloys were electroformed from sulfate electrolyte. The influence of electrolyte component, pulse frequency, cathode current density, temperature and PH value on the hardness of electroforming layer, inner stress, corrosion-resistance, electrodepositing speed, porosity, surface appearance and microstructure were carefully studied. The results as follows:The micro-hardness of the alloy electroforming layers was used as the criteria to study the effect of the five parameters on the electroformed layers using the orthogonal test. The five parameters were COSO4concentration in electrolyte, pulse frequency, cathode current density, temperature and pH value. Meanwhile, the five parameters were analyzed by using single factor test. The structure and composition of the electroformed layers were examined by scanning electronic microscopy (SEM), energy dispersive spectrum (EDS) and transmission electronic microscopy (TEM). The best electroforming parameters were COSO420g/L, pulse frequency5000Hz, the cathode electricity density1.0A/dm2, temperature55℃, pH value3.5.The hardness of Ni-Co deposit increased firstly and then decreased with the increase of Co2+concentration and the current density with a maximum around20g/L COSO4and3.OA/dm2current density; The hardness of Ni-Co films increased with the increase of pulse frequency, but it decreased with the increase of PH. And the hardness of Ni-Co films decreased firstly and then increased with the increase of temperature with a minimum around45℃. The Co content in Ni-Co alloys increased with the increase of Co2+concentration in the electrolyte, pulse frequency and temperature, but it decreased with the increase of current density and PH.The SEM micrographs of the electroplated Ni-Co films show that the grain size of Ni-Co deposits become smaller and more homogeneous when increasing Co content from10g/L to20g/L, and the Ni-Co alloy showed a rather regularly triangle particles and the grain size of Ni-Co deposits becomes bigger with further increase of Co content, the amounts of triangle particles increases and the grain size of Ni-Co deposits becomes smaller and the triangle particles turn into branched structure when continue to increase Co content; The grain size of Ni-Co deposits becomes smaller with increase of pulse frequency and decrease of current density, and the microstructure of Ni-Co deposits worsen with increase of current density.Ni-Co alloys which were produced under the optimum conditions included44.26%Co. The SEM micrographs of the electroplated Ni-Co films became smoother, more homogeneous, better densification and fewer defects compared with pure Ni deposit. The TEM and XRD images of the electrodeposited Ni-Co alloys illustrate an isotropic polycrystalline microstructure, diffraction peak of deposited films changed with additive of Co. The crystal structure of the Ni-Co alloy changed from complete fcc lattice into a mixed (majority of fcc+minority of hcp) phase. Grain size of Ni-Co alloys films was about a quarter of the pure Ni deposit with mean size80nm of pure Ni deposit compared with mean size20nm of Ni-Co alloys films. There were a lot of dislocations, stacking faults and twin crystals, the grain boundary of twin crystals inhibited the motions of dislocations and stacking faults, which led to increase of hardness and strength. Corrosion resistance of Ni-Co alloys films was almost twice of the pure Ni deposit. Internal stress of Ni-Co alloys films illustrate tensile stress which increased firstly and then decreased with the increase of Co content. Both the porosity of Ni-Co alloys films and Ni deposit decreased with increase of depth of deposit films. Deposition rate of films decreased gradually with the increase of Co2+concentration in the electrolyte, but had no significant changes.