Mohamed A. Hashish

State of the Art of Research and Development in Abrasive Waterjet Machining

Thermodynamic analysis of material removal mechanisms indicates that an ideal tool for shaping of materials is a high energy beam, having infinitely small cross-section, precisely controlled depth, and direction of penetration, and does not cause any detrimental effects on the generated surface. The production of the beam should be relatively inexpensive and environmentally sound while the material removal rate should be reasonably high for the process to be viable. A narrow stream of high energy water mixed with abrasive particles comes close to meeting these requirements because abrasive waterjet machining has become one of the leading manufacturing technologies in a relatively short period of time. This paper gives an overview of the basic research and development activities in the area of abrasive waterjet machining in the 1990s in the United States.

Mathematical Modeling of Ultra-High-Pressure Waterjet Peening

Waterjet peening is a recent promising method in surface treatment. It has the potential to induce compressive residual stresses that benefit the fatigue life of materials similar to the conventional shot peening process. However, there are no analytical models that incorporate process parameters (i.e., supply pressure, jet exposure time, and nozzle traverse rate, etc) to allow predicting the optimized peering process. Mathematical modeling of high-pressure waterjet peening was developed in this study to describe the relation between the waterjet peening parameters and the resulting material modifications. Results showed the possibility of using the proposed mathematical model to predict an initial range for effective waterjet peening under the variation of waterjet peening conditions. The high cycle fatigue tests were performed to validate the proposed model and fatigue test results showed good agreement with the predictions.

Fatigue Performance of High-Pressure Waterjet-Peened Aluminum Alloy

An experimental study of high-pressure waterjet peening on 7075-T6 aluminum alloy was conducted to investigate the effects of waterjet on high-cycle fatigue life and fatigue crack growth. Unnotched hourglass-shaped circular cross section test specimens were fatigue tested in completely reversed rotating bending (R =S min /S max = -1) to determine fatigue life behavior (S-N curves). Single-edge-notched flat tensile test specimens were tested in the tension-tension fatigue crack growth tests (R=S min /S max =0.1) to determine fatigue crack propagation behavior (da/dN versus AK). Surface characteristics and fracture surfaces were evaluated by scanning electron microscopy (SEM). Results show that waterjet peening can increase high-cycle fatigue life, delay fatigue crack initiation, and decrease the rate of fatigue crack propagation.

Residual Stress Induced by Waterjet Peening: A Finite Element Analysis

The concept of multiple droplet impacts resulting from ultra high-pressure waterjet (UHPWJ) was used to develop a mathematical model to describe the effect of interfacial pressure on the underlying workpiece material. A non-linear elastic-plastic finite element analysis (FEA) was carried out in this study using the interfacial pressure model to predict residual compressive stresses. This three-dimensional FEA model was based on quasi-static considerations to provide prediction of both magnitude and depth of residual stress fields in a 7075-T6 aluminum alloy (A17075-T6). Results of the FEA modeling were in good agreement with experimental measurements. Effects of applied pressures on the residual stress fields are also presented and discussed as a method of estimating high-pressure waterjet induced compressive stresses under varying process conditions for peening.

Waterjet and Water-Air Jet Surface Processing of a Titanium Alloy: A Parametric Evaluation

High-pressure waterjets have emerged as a viable method for surface texturing, cleaning, and peening of metallic materials. As the material advancements continue, research into alternate surface processing methods must strive to keep pace. One material in particular that has experienced an increase in use in the biomedical and aerospace industries is titanium—due largely to its high strength to weight ratio and corrosion resistance. In this paper, surface preparation of a titanium alloy using waterjet and the water-air jet nozzles at pressures up to 600 MPa was evaluated. A parametric study was performed based on the supply pressure, standoff distance, traverse rate, and applied air flow rate. An analysis of variance was performed on the resulting experimental set to identify the key parameters contributing to the material erosion rates and resulting surface roughness parameters. The supply pressure was found to be the primary contributor to the erosive characteristics of the waterjet followed by the traverse rate. These parameters together govern the total energy per unit area transferred to the workpiece.

Energy Based Modeling of Ultra High-Pressure Waterjet Surface Preparation Processes

Ultra high-pressure waterjets (UHP-WJ) have been emerging as a viable method for surface texturing, cleaning, and peening of metallic materials. Previous experimental studies have suggested that removal of material can be related to the energy density of the waterjet impinging upon the workpiece, rather than the net energy. The net energy transferred to the workpiece is a function of four key process parameters, namely, (i) orifice diameter, (ii) orifice geometry, (iii) supply pressure, and (iv) traverse rate. The energy density also incorporates jet spreading as well as flow rate and impulse pressure distributions within the waterjet. In this paper, a novel representation of the power distribution within the waterjet is presented, as well as a relationship governing jet-material interaction. Empirical validation on a Ti-6Al-4V titanium alloy is presented, with good correlation noted between the predicted and experimental results.

Abrasive-Waterjet Drilling of High Temperature Jet Engine Parts

An experimental investigation was performed to demonstrate the AWJ process capability for precision drilling of small diameter cooling holes in TBC-coated samples and full scale parts used in jet engines. All holes were drilled from the TBC side. Both qualitative and quantitative hole results issues were addressed. These include chipping of TBC, gouging inside the hole at the ceramic-metal interface, feathering at the exit side of the hole, tear drop shape, hole size and its consistency, overall drilling time, and potential systems productivity. It was observed that the AWJ produces high quality holes free from chipping or gouging. However, feathering may occur depending primarily on whether the substrate material is cast or rolled and can be substantially reduced or eliminated by adjusting the drilling parameters. It was also demonstrated that holes of about 0.025-ich can be drilled at 30 with less than 0.001-inch standard deviation. Air flow measurement results have also confirmed that less than 6% variation can be obtained from one set of holes (270 holes) to another. Drilling times below 10 seconds hole to hole were observed for drilling 0.060-inch thick metal substrates with 0.020-inch thick TBC thickness at 30 degrees. This time can be reduced to 3–4 seconds with improved systems. Although AWJ is still slower than laser, produced holes are of superior quality and do not need any additional processing. Future research efforts will focus on improving the drilling time.Copyright

Singulation of Electronic Packages With Abrasive Waterjets

Abrasive waterjets were used for the first time to commercially singulate electronic chips such as those used for flash memory cards found in digital cameras, cell phones, and USB storage devices. Cutting these components requires high cutting speed, high edge quality, accuracy, and precision. For example, a minimal accuracy needed is about 0.1-mm and a minimum Cpk of 1.33. A relatively small AWJ (~ 0.38 mm) was successfully used to accurately cut chips at speeds of 20 mm/s to 60 mm/s. It was determined that the use of machine vision is critical to meeting the accuracy requirements. The cutting process consisted of piercing starting holes and then cutting shaped pattern cuts to contour the chip components. Drilling holes was performed without delamination and the cutting speed was optimized to meet the intricate chip geometry. Because of the relatively high volume of components to be cut, requiring around the clock duty, process and machine reliability are of critical importance. This paper discusses the results and observation of the cutting process as well as the performance of the system.Copyright

A Novel Approach to Energy Based Evaluations of Ultra High-Pressure Waterjets

Ultra high-pressure waterjets (UHP-WJ) have been emerging as a viable method for surface texturing, cleaning, and peening of metallic materials. Previous experimental studies have suggested that removal of material can be related to the energy density of the waterjet impinging upon the workpiece, rather than the net energy. The net energy transferred to the workpiece is a function of four key process parameters, namely i.) orifice diameter, ii.) orifice geometry, iii.) supply pressure, and iv.) traverse rate. The energy density also incorporates jet spreading as well as flow rate and impulse pressure distributions within the waterjet. In this paper, a novel representation of the power distribution within the waterjet is presented, as well as a relationship governing jet-material interaction. Empirical validation on a Ti-6Al-4V titanium alloy is presented, with good correlation noted between the predicted and experimental results.Copyright

A Study on AWJ Trimming of Composite Aircraft Stringers

A study was undertaken to develop AWJ processes and tools for trimming carbon fiber composite stringers used in several structural air frame components. The cutting strategy was to trim the stringers flanges from the inside out to allow beveling and to minimize cutting risks. Specially designed small size side fire cutting heads were developed to meet this requirement, and they were first used to cut flat composite material samples with thicknesses up to 1 inch. These cutting tests focused on identifying the cutting speeds for producing high quality surfaces without any frayed edges or delamination and meet the Boeing BAC 5578 specifications. For example, tests were performed to determine the effects of pressure, abrasive flow rate, orifice size, and cutting speed on the taper and surface finish of the produced surface walls. It was shown that cutting stringer material with AWJ is highly feasible both qualitatively and quantitatively. An example cutting rate of 18 inch/min was determined to cut 0.65-inch thick composite to produce better than 400 micro inch surface finish with less than 1 degree taper. Trimming tests were then performed for both 90 and 45 degrees to produce straight and beveled surfaces using sample stringers. This new AWJ process is now being used for production at aircraft subcontractor facilities for the Boeing 787.Copyright