M. Hashish

A Model for Abrasive-Waterjet (AWJ) Machining

Ultrahigh-pressure abrasive-waterjets (AWJs) are being developed as net shape and near-net-shape machining tools for hard-to-machine materials. These tools offer significant advantages over existing techniques, including technical, economical, environmental, and safety concerns. Predicting the cutting results, however, is a difficult task and a major effort in this development process. This paper presents a model for predicting the depth of cut of abrasive-waterjets in different metals. This new model is based on an improved model of erosion by solid particle impact, which is also presented. The erosion model accounts for the physical and geometrical characteristics of the eroding particle and results in a velocity exponent of 2.5, which is in agreement with erosion data in the literature. The erosion model is used with a kinematic jet-solid penetration model to yield expressions for depths of cut according to different modes of erosion along the cutting kerf. This kinematic model was developed previously through visualization of the cutting process. The depth of cut consists of two parts: one due to a cutting wear mode at shallow angles of impact, and the other due to a deformation wear mode at large angles of impact. The predictions of the AWJ cutting model are checked against a large database of cutting results for a wide range of parameters and metal types. Materials are characterized by two properties: the dynamic flow stress, and the threshold particle velocity. The dynamic flow stress used in the erosion model was found to correlate with a typical modulus of elasticity for metals. The threshold particle velocity was determined by best fitting the model to the experimental results. Model predictions agree well with experimental results, with correlation coefficients of over 0.9 for many of the metals considered in this study.

Pressure Effects in Abrasive-Waterjet (AWJ) Machining

Abrasive-waterjets (AWJs) are formed by mixing high-pressure (up to 400 MPa) waterjets (0.1 to 1 mm in diameter) with abrasive particles in mixing tubes with typical 1/d ratios of 50 to 100. The pressure of the waterjet influences the overall performance of the abrasive-waterjet cutting system through operational and phenomenological effects. Higher pressures result in lower hydraulic efficiency, more frequent maintenance, high wear rates of mixing tubes, and fragmentation of particles before they exit the nozzle. However, with high pressures, deeper cuts can be obtained and higher traverse speeds can be used. Consequently, the hydraulic power is best utilized at an optimum pressure, which is a function of all other parameters as well as the application criteria. This paper presents data and analyses on the effect of pressure on nozzle operational characteristics, i.e., jet spreading characteristics, abrasive particle fragmentation, suction capability, wear of mixing tubes, and mixing efficiency. The effect of pressure on the parameters of cutting performance is discussed with example data. These parameters are depth of cut, specific area generation, maximum cutting traverse rate, surface waviness, and cost of cutting. Optimal pressure examples presented in this study indicate that pressures over 240 MPa are required for efficient abrasive-waterjet performance in metal cutting.

Optimization Factors in Abrasive-Waterjet Machining

Machining applications such as cutting, milling, and turning are considered along with sample data. The effects of different AWJ parameters on both the functional performance of the AWJ system components and the material removal process are discussed

Visualization of the Abrasive-Waterjet Cutting Process

Cutting with abrasive waterjets was visualized in three types of materials: Lexan, Lucite and glass. Movie cameras were used at speeds of 64 and 1000 frames/s to record sequences of the jet penetration in these materials. It was found that the cutting process consists of two basic modes of erosion. The first, known as the cutting-wear mode, occurs at relatively shallow angles of impact. This mode results in a steady-state jet-solid interface. The other mode, the deformation-wear mode, occurs at large angles of impact and results in an unsteady penetration zone. The relative contribution of each of these two modes or mechanisms to material removal depends on the process parameters. The cutting process is cyclic in nature when the deformation-wear mechanism is partially or totally contributing to cutting. Qualitative and quantitative results of these visualization experiments suggest a mechanistic model for the penetration process. The results of this work may also be expanded to explain other ‘stream-like’ cutting-tool processes, such as laser and flame cutting.

Waterjet Machining and Peening of Metals

An experimental study was conducted to determine the influence of high-pressure waterjet (WJ) peening and abrasive waterjet (AWJ) machining on the surface integrity and texture of metals. A combination of microstructure analysis, microhardness measurements, and profilometry were used in determining the depth of plastic deformation and surface texture that result from the material removal process. The measurement and evaluation of residual stress was conducted with X-ray diffraction. The residual stress fields resulting from treatment were analyzed to further distinguish the influence of material properties on the surface integrity. It was found that waterjet peening induces plastic deformation at the surface layer of metals as good as shot peening. The degree of plastic deformation and the state of material surface were found to be strongly dependent on the peening conditions applied.

On the Modeling of Surface Waviness Produced by Abrasive-Waterjets

A simple physical model was developed to describe the waviness (striation) phenomenon associated with abrasive-waterjet cutting. Model predictions were compared to experimental surface waviness data showing good qualitative agreement. Quantitative discrepancies were observed, however, and were attributed to kerf taper effects. When experimental kerf taper data were also included, good quantitative agreement was observed. This suggests that the inclusion of kerf width variation in the model will result in an accurate model. The basic AWJ waviness model is expressed as:

Effect of Abrasive Waterjet Parameters on Volume Removal Trends in Turning

{{2{R_w}} \over {{d_m}}} = 1 - \sqrt {1 - {{\left( {{\pi \mathord{\left/ {\vphantom {\pi 4}} \right. \kern-\nulldelimiterspace} 4}} \right)}^2}{{\left[ {{{{d_m}\left( {h - {h_c}} \right)u} \over {0.5{m_a}{{V_a^2} \mathord{\left/ {\vphantom {{V_a^2} \varepsilon }} \right. \kern-\nulldelimiterspace} \varepsilon }}}} \right]}^2}}