Technology Study For Ni-Co Alloy Cold-shield Electroforming

At present, the cold-shield material used in the first generation infrared detector's Dewar subassembly is mainly pure Ni. It is formed by electroforming, but has some gaps in hardness, surface brightness and thickness homogenization compared with the part of foreign's. The aim of this thesis is to make out Ni-Co alloy cold-shield which has high hardness, low stress and even thickness, and meet the need of second generation infrared detector.The cold-shield is heterotype and thin-wall part, and ordinary machining is hard to meet the quality requirement, so the electroforming is the first choose for it; Ni-Co alloy has higher hardness and corrosion resistance, Ni and Co are easy to codeposit from aqueous solution and control the alloy composition, so the second generation cold-shield uses Ni-Co alloy by electroforming. In experiment, simply using the cheap sulfate bath, it can't meet the need of designing due to its high stress and low hardness. So the additive is used to decline the stress, to increase the hardness and surface brightness. The thickness homogenization is the key to electroforming. Improving the homogenization by two ways: first is to improve the throwing power by additive and reasonable processes control; second is to use computer analogy to adjust the distributing of electrode and baffle's shape and place, then inproving the current distributing of cathodal surface, at the same time using second electroplating to improve the thichness homogenization drastically.Through lots of parallel and orthogonal test, the final cold-shield satisfies the requirements of designing by electroforming, the content of Co 5～25%; hardness HV>400; homogenization>90%, and thickness in 100+10μm; and low stress; semi-bright surface.The tests show:additive TN2 can improve the hardness, decline the stress, fine the grain and make the surface semi-bright; TN3 can improve the hardness further and get full bright surface, but it will enlarge the tensile stress; TN1 can improve the throwing power evidently, but it can make the anode to passivate when it is overcharged, so its content is controlled under 20g/L, current density and pH has a little effect to the throwing power, so it can be changed in a large range. When the wave is full, the bath has best throwing power, the reason is that wave form can affect the current efficiency, electric double layer and conductivity of bath; The results of ANSYS analogy show that reasonable electrode distributing and baffle can improve the current distributing of cathode surface evidently, but the thickness of inside corner is thinner comparatively, so using second electroplating to repair its thickness.The electrolyte in test can be used in small batch production for its conposition simpleness, easy to maintain and good stability.