Mechanical pretreatment before electroplating of aluminium alloy AlSi12

Electroplating is a coating process used to increase the durability of workpieces or to change their appearance. To form a high adhesive bonding between the coating and the workpiece, its surface has to meet certain requirements achieved by a comprehensive pretreatment. The conventional pretreatment process chain consists of different steps to increase the roughness and the surface tension of the workpieces. But besides its high costs chemicals are used, which are harmful to the environment and health. The complexity of the pretreatment and coating process chain differs depending on the used workpiece material. Particularly challenging to coat are thereby aluminium-silicon alloys. During the conventional pretreatment of these casting alloys, the Silicon phase enriches on the surface of the workpieces and has to be removed in an additional process step using nitric acid. This study compares alternative mechanical pretreatment processes for AlSi12-workpieces prior to electroplating. The investigated processes are conventional grinding, double face grinding with planetary kinematics, lapping and dry ice blasting. Subsequent to the structuring, the workpieces are cleaned by carbon dioxide snow blasting. The investigation of the different surface structures and its properties, like roughness, surface tension and wetting behaviour, allowed the determination of their impact on the adhesive strength of the coating by empirical modelling. By structuring the surfaces of the workpieces they could be coated by electroplating. Thereby, the chemicals used in the conventional pretreatment are substituted. Especially the creation of surfaces, which could be wetted homogenously, showed a satisfactory adhesive strength of the applied coatings.