A Study On Milling Ceramics With Abrasive Waterjet Technology

In this paper, systemic studies on AWJ milling technique of ceramic materials is presented, which examines the milling technologies and material removal mechanisms and analyses the milled surface quality. At the same time, mathematical models for the milling depth and surface roughness models are established. These researches for AWJ milling technique have provided an effective machining technology for brittle difficult-to-cut materials, which show that the AWJ technique has valuable utility and good prospect in industrial applications.The milling performance of ceramics with AWJ is studied by experiments, and the effect of various process variables on the material volume removal rate and milling depth are evaluated. The predictive models for material volume removal rate and milling depth are constructed, which indicate the quantitative relationship between the milling results and the milling parameters can be given to practical operation. The material volume removal rate of Al₂O₃ ranges from 0.405mm³/s to 4.463mm³/s and the material volume removal rate of Si₃N₄ ranges from 0.095mm³/s to 0.177mm³/s. The volume removal rate increases with an increase in water pressure, abrasive flow rate and standoff distance, and decreases with an increase in nozzle traverse speed, lateral spacing and the hardness of the milled materials. Therefore, higher water pressure and abrasive flow rate, larger standoff distance, lower lateral spacing and nozzle traverse speed should be used to gain a higher material volume removal rate. The milling depth of Al₂O₃ ranges from 0.2mm to 2mm and the milling depth of Si₃N₄ are less than 0.1mm at the milling conditions of the experiments. The milling depth increases with an increase in water pressure, abrasive flow rate and standoff distance, and decreases with an increase in nozzle traverse speed and lateral spacing. Therefore, higher water pressure and abrasive flow rate, larger standoff distance, lower lateral spacing and nozzle traverse speed should be used to gain a larger milling depth.The micro-topography of the milled surface and chips are investigated. The milling mechanisms of ceramics is analyzed. The results show that radial cracks perpendicular to the impacted surface are formed due to plastic distortion during the initial period of abrasive impacting materials, and the lateral cracks parallel to the impacted surface are formed during the last period. Propagation of the radial cracks and lateral cracks result in the removal of material. It is shown that there are many deep and flat pits due to material fractures on the Al₂O₃ milled surface and evidence of plastic flow is also observed around the pits. The milled surface of Si₃N₄ is smooth and even, and the trace of plastic flow can be observed. It is shown that the dominant milling mechanisms of ceramics are crack and exfoliating due to brittle fracture and plastic distortion according to the form of the chips. It is shown that the abrasives are uniform in size and almost round in shape, and the surface of abrasives is relatively smooth. However, the abrasives are often broken into several pieces after milling, acute angles and obvious cleavage fracture vein can be observed on the surface of abrasives.The grooves with single-pass milling and two-pass milling tests and the surface quality with multiple-pass milling process are systemically studied. The results show that in the single-pass milling process, the milled groove depth increases with an enhancement in water pressure and decreases with an increase in nozzle traverse speed and standoff distance. The milled groove width increase with an increase in standoff distance, but is not changed with water pressure and nozzle traverse speed. The lower water pressure, larger traverse speed and the finer abrasives should be used to gain a better surface quality. The surface quality can be improved with an increase in lateral spacing at a higher standoff distance, and also improved with an enhancement in standoff distance at a larger lateral spacing. All the milled surfaces of the Si₃N₄ ceramics are smooth and the surface roughness ranges between 0.6μm and 1.0μm at the milling conditions under consideration, and the most y-directional milled surface roughness are higher than x-directional milled surface roughness. The milled surface roughness will be significantly decreased at the milling conditions of lower water pressure, higher traverse speed, medium standoff distance, larger lateral spacing and finer abrasives.The residual stress and the surface hardness of the AWJ milled surface are studied. The results show that the surface residual stress resulting from AWJ milling is compressive, and residual stress ranges from -203MPa to -19.6MPa. The effect of water pressure and standoff distance on the surface residual stress is significant, and the effect of nozzle traverse speed and lateral spacing on the surface residual stress is insignificant. The surface residual compressive stress slightly decreases and then greatly increases with an increase in the water pressure, and always increases with an enhancement in standoff distance. At the milling conditions under consideration, the surface residual compressive stress firstly decreases and then increases with an increase in the nozzle traverse speed and lateral spacing. The results also show that there is some enhancement in the hardness of AWJ milling surface, and it is affected by milling process parameters. The surface hardness of the AWJ milled surface increases with an increase in the water pressure and traverse speed, and decreases with an enhancement in lateral spacing and standoff distance. The micro-topography of the milled surface cross section shows that there is a thin deteriorative layer in milled layer, in which no obvious crack is observed. Since the surface residual stress is compressive and the surface hardness increases, the milled surface deteriorative layer by AWJ could improve the surface quality.The maximal depth model for single-pass milling process, the depth and surface roughness for plane surface are established. The milled profile of a single-pass milling of the jet can be modeled as a cosine curve of the form. Therefore, the maximal depth model for single-pass milling is established which indicates the quantitative relationship between the AWJ single-pass milling depth and the milling parameters. The mathematical model, which shows the effect of water pressure, standoff distance, lateral spacing, abrasive flow rate and the properties of the workpiece material on the AWJ milling depth, is established based on the maximal depth model for single-pass milling. The derived model is verified with AWJ milling experiments. The results show that the milling depth increases with an increase in water pressure and standoff distance, and decreases with an increase in lateral spacing and traverse speed. The theoretical milling depth predicted by the mathematical model is good agreement with the experimental result. The surface roughness model for AWJ milling is established based on the maximal depth model of single-pass milling. According to this surface roughness model, the effect of water pressure, standoff distance, traverse speed, lateral spacing, abrasive flow rate and the properties of the workpiece material on roughness of the milled surface is analyzed, which is very important in theory for further applications.