Study on surface topography model of abrasive water jet polishing

With the increasing demand for precision optical components with the complex surface in aerospace, nuclear engineering, medical equipment and other applications, more stringent requirements of smooth surface are put forward for precision polishing. As a non-contact polishing method, abrasive jet water polishing (AWJP) technology carries forward the advantage of micro-abrasive’s collision and erosion that available micro-level removal by low-pressure control. With other unique features like no temperature rising, high flexibility, inexpensiveness and environment-friendly process, AWJP has now become a promising polishing method for ultra-precision free-form surface optical components. In fact, the topography of rough surface affected by abrasive particles cannot be predicted precisely. It is inefficient to continuously adjust the process parameters such as jet velocity, abrasive size and impact angle through experiments to obtain the smoothest polished surface. The impact deformation form irregular abrasives should be controlled in elastic plastic deformation to avoid surface damage caused by brittle fracture. In this paper, the purpose was therefore to establish impacted surface topography model. It can simulate topography of irregular abrasives impacting original rough surface in flow field. First, the model under three-dimensional abrasive impingement was constructed by the dimension reduction method. In other words, impacted surface was simplified to consist of independent elastoplastic elements theoretically. A single abrasive particle impact deformation process was constructed by combining the Popov’s dimension reduction method with Thornton's plastic deformation model. It calculated elastoplastic deformation of a single abrasive’s effect considering abrasive size and surface morphology. The acceleration and impact depth of abrasive were figured up by displacement iteration. The deformation was updated in the current three-dimensional topography of the surface, which as the initial for the next impact calculation. Moreover, abrasive density and velocity distribution were calculated by computer hydrodynamics technology. According to them, the abrasive impact depth in each region was calculated, and then the surface morphology and roughness below the whole jet beam spot are obtained. The applicability of the surface roughness model was verified by AWJP experiments. According to model, the larger tangential velocity of abrasive particle, the smaller impact depth and the larger impact length. Different angles effect on surface topography signally. When it at 90°, the surface topography caused by the velocity stagnation zone was obviously convex, and the polishing morphology is W-shaped. In contrast, 60° presented a single pit appearance and 30° presented a double pit appearance, due to increasing abrasive tangential impact. The surface roughness to be polished by AWJP was reduced from Ra 0.4 um to Ra 0.11 um. In conclusion, a micro impacted surface topography model was established. The theoretical model and experimental results prove flexibility for abrasive jet polishing with variable parameters. Through it we can determine polishing effects of particle size and velocity on rough surfaces, and then the appropriate polishing parameters can be selected. In addition, this model can be used in various abrasive impact machining situations.