Study On Mechanical Performance Of High Frequency Pulse Electroforming Ni-Co Alloy

Stricted by manufacture technology of the pulse power, frequency of the pulse power used in the study of pulse electroplating was under 1kHz, studies on high frequency pulse electroplating of Ni-Co that the frequency ranging from 20 kHz to 140kHz cannot be found. In this text, mechanical performance of Ni-Co alloys prepared by high frequency, ranging from 20kHz to 140kHz, pulse electroplating in Watt solution was studied. The effects of process conditions such as pulse frequency, t_on/off, current density, temperature and concentration of CoSO₄·7 -H₂O on the weight loss rate of the Ni-Co platings immersed in 15%H₂SO₄ solution and the depositing rate of the plating were studied through orthogonal experiments and the technology of electroplating was optimized: pulse frequency 140 kHz, t_on/off 0.25, current density 3A·dm^-2 , pH value 3, temperature 55℃, concentration of CoSO₄·7H₂O 40g/L. Based on the condition, the effect of pulse power's parameters and the solution's concentration on morphology, micro-hardness, tensile intensity and internal stress. Ni-Co alloys obtained by the high frequency pulse electroplating features higher compactness, better uniformity, lower porosity and higher micro-hardness as compared with that electroplated with direct current.The micro-hardness of the as-plated Ni-Co alloy plating increased with the increasing pulse frequency and concentration of CoSO₄·7H₂O in plating bath. The concentration of cobalt in the plating was higher than the concentration of Co⁺ in the bath. This phenomenon acted in accord with the anomalous co-deposition principle. The Micro-hardness of deposits prepared with direct current and 20-40 kHz pulse increased with the increasing heat treatment temperature from 200 to 400℃, but decreased with further increasing temperature over 400℃. Micro-hardness of deposits prepared at pulse frequency of 120kHz and 140kHz decreased with the increasing heat treatment. The concentration of cobalt in the plating decreased with the increasing current density, and that the micro-hardness of the as -plated Ni-Co alloy coating also decreased.In the experiment the effect of surfactants on the micro-hardness of high frequency pulse electroplating was studied. Sodium allylsulfonate, Saccharin sodium and Thiourea improved the micro-hardness evidently, especially micro-hardness of the plating that was prepared in the bath, added into Thiourea, reached 526HV. As the frequency increased, limit intension of the plating increased at first and then decreased with the maximum at 80kHz. The internal stress of the plating got the minimum at 80kHz, and the maximum at the concentration of CoSO₄·7H₂O 30g/L in the bath. In addition, according to the effect of each surfactants on internal stress of the platings, two schemes for the adding of the surfactants was advanced. Internal stress of the platings prepared according to the schemes above go near to zero, and that the platings were bright and smooth. Micro-hardness of the platings prepared according to scheme 1 was up to the value of 635HV, scheme 2 was 689HV, however that of the platings without the surfactants can only reach 411HV.