Preparation Of Small-hole Without Recast Layer By Electrical Combined Machining

Electrical discharge machining (EDM) is widely used for manufacturing micro-holes because of its advantages such as low-cost apparatus, high aspect ratio of parts and so on. Due to the rapid heating and cooling effects induced by the machining process, the recast layer will form on the surface of the part, which is very hard and contains many imperfections such as micro pits and micro cracks. On the condition of alternate load, the part may rupture due to the diffusion of the micro cracks. So a polishing operation, such as the lapping or finishing process will be normally employed after micro-hole machining by EDM in order to remove the recast layer in aerospace manufacture industry. Abrasive Flow Machining(AFM)is usually used to improve the quality of the surface of the hole, but the recast layer can't be fully removed by this way. A method which can realize the preparation of small-hole without recast layer is needed.A method that combines high-speed EDM process and an in situ electrochemical finishing using the same tool electrode is firstly proposed to produce the small-hole without recast layer in stainless steel, the tap water is used as electrolyte during the whole process. First, the small-hole is machined by high-speed EDM, and then in situ electrochemical finishing is performed to remove the recast layer produced during the high-speed EDM process.The following work has been done in this paper:1. Based on the high-speed micro-hole EDM machine, a setup of electrochemical finishing is designed and assembled;2. The effects of machining parameters (including discharge current and pulse-on time) on the dimension and shape accuracy (including machining clearance and pitch) of the small-hole processed by high-speed EDM are discussed, and the characters of the recast layer are analyzed by metallography;3. The effects of machining parameters such as voltage, machining time, and tool electrode rotating or not, on the improvements of the small-hole's dimension and shape accuracy are discussed. The finishing result is evaluated, and the mechanism of the electrochemical finishing is analyzed.The experimental results show that it's feasible that using tap water as electrolyte in electrochemical finishing process and the recast layer can be fully removed while using appropriate machining parameters. The hole with an excellent machining surface would be obtained through the whole process including high-speed EDM with a discharge current of 9A together with a pulse-on duration of 20μs, and then electrochemical finishing with a voltage 25v together with a machining time of 5min.