VOL. 10 January ISSUE YEAR 2009
in Vol. 10 - January Issue - Year 2009
Abrasive Water Jet Cutting
Fig. 1: Commercial sandwich-structure (left), used sample geometry (right)
Fig. 2: Visualised particle traces in the initial area of the focusing tube, reference (left), prototype (right)
Fig. 3: Final particle velocities within the range of the focusing tube orifice, reference (left), prototype (right)
Fig. 4: Quality and performance criteria in the second material layer as a function of the secondary work distance
Fig. 5: Worn focusing tube material
Increased Performance in the Machining of Hollow Shaped Structures
Cutting with high pressure Abrasive Water Jets (AWJ) offers process-specific advantages compared to other conventional thermal or mechanical separation processes. The process cycle is almost independent of material properties and causes no thermal loads or structure deteriorations in the workpiece. Almost any material combination can be machined. Therefore, the AWJ is a highly flexible tool that can be used for a wide range of industrial applications.
The industrial use of composite materials, such as sandwich structures, rises continuously due to the adjustability of component properties. It has therefore become increasingly essential to offer recycling concepts for these materials due to increasingly scarce resources and disposal capacities . Because of the large machinable material spectrum, a significant potential for the AWJ technology exists, for instance in the field of industrial disassembly.
However, machining with AWJ is restricted by material inhomogenities, such as cavities. When cutting hollow shaped structures with conventional AWJ systems, the cutting jet expands in the inner cavity. This leads to insufficient cutting qualities in subsequent material layers and to damages of inner structures .
Under this aspect, investigations to increase the efficiency of AWJ have been undertaken. To generate an operating stable and efficient abrasive water jet, a cutting-head prototype, working according to the injection principle, was developed. The focal point was set on the optimization of relevant cutting-head components, in order to improve the jet focusing. Thus the primary energy can be kept up over a longer distance, which makes the machining of hollow shaped structures more efficient.
Weakness of conventional cutting-heads
The limited cutting performance of conventional cutting-heads is caused by the admixing and acceleration process of the abrasives. The lateral feeding of particles to a central water jet causes energy losses, which are characterized by the wear of the particles and cutting-head components as well as the warming of the cutting-head. Furthermore, an unfavorable and asymmetric particle distribution of the abrasives related to the cross section of the water jet occurs, which has a negative influence on the cutting jet stability and thus on the machining result.
In abrasive water jet cutting, the abrasives are mainly responsible for the erosion process, which makes the mixing and acceleration process of the particles one of the central points in increasing the process efficiency. Therefore the initial area of the focusing tube was set to the focal point in this study to improve the particle distribution and to increase the velocity.
Modification of relevant cutting-head components
In order to achieve the points described above, different concepts of abrasive feedings and mixing chamber geometries were designed. The development process was continuously accompanied by Computational Fluid Dynamic simulations (CFD), with FLUENT 6.3. To ensure a broad and fast modification of existing systems, standardised parts such as water nozzles and focusing tubes were used.
The water jet generated by the developed cutting-head is loaded with abrasive particles by several feeding channels distributed around the circumference. These channels are evenly fed by a special mixing chamber which is located upstream. Thus a more favorable distribution of the abrasives related to the jet cross section as well as an improved impulse exchange between the water jet and the particles can be realized. Beyond that, an axle adjustment between the water jet and the focusing tube was defined as a further demanded sub-function to minimise friction and flow losses. This sub-function has been fulfilled by a central arranged spherical joint, where the central water jet serves as reference system. In this way the angle deviations caused by assembly or manufacturing tolerances can be compensated .
Particularly of interest was the cutting performance of hollow shaped structures. This can be evaluated qualitatively by the macroscopic dimension of the cutting kerf in the secondary material layer . But when using commercial sandwich-structures as a sample geometry, problems with regard to reproducibility and comparability can occur. The position of the inner honeycomb structure in relation to the cutting jet cannot be predicted and/or adjusted exactly. Due to the fact that the honeycomb structure has an effect on the machining result, a suitable hollow shaped structure was defined, figure 1.
Therefore, aluminium plates (AlMg3) with a thickness of 5 mm were aligned in a defined angle ? to each other. Thus for all experimental series constant sample conditions were guaranteed. In addition it was possible to simulate different dimensions of hollow shaped structures (secondary work distances a) depending on the cutting length l.
The flank angle as well as the average of the cutting kerf width in the secondary material layer was used as the quality and performance criteria. Cutting results, produced by a conventional cutting-head made by KMT, served as references. The measurement of the cutting kerf profiles were done with a tactile linear measuring instrument by TESA.
For wear investigations, the prototypically realized cutting-head as well as the reference cutting-head were actuated with constant pressures and abrasive mass flow rates over a fixed period of time. The mass difference of the worn focusing material served as a reference value.
The simulation results indicate that the modified mixing chamber geometry led to improved flow conditions in the range of the initial area of the focusing tube with otherwise identical boundary conditions. Compared to the reference geometry it can be seen that in the initial area of the focusing tube the abrasive particles come in to fewer wall contacts, figure 2, which can be interpreted as preventing early wear of the focusing tube. In addition to that, more particles reach the jet core, which leads to higher final particle velocities within the range of the focusing tube orifice, figure 3.
In figure 4 the experimental results of the analyzed quality and performance criteria are represented as a function of the secondary work distance. Compared to the reference cutting-head, a significant reduction in the cutting kerf width as well as in the cutting kerf angle could be realized due to the prototype.
As expected from the simulation results, a reduction of the focusing tube wear could be evaluated, figure 5. A wear reduction of approx. 20 % could be realized, which demonstrates the positive influence on the mixing process by the mixing chamber modifications.
Conclusion and Outlook
The investigations demonstrate an increased efficiency of the developed cutting-head compared to the conventional cutting-head. Besides the guarantee of an operating stable and efficient generation of an abrasive water jet, an improved distribution of the abrasives could be realized. Thus on the one hand the power density as well as the stability of the cutting jet could be increased and on the other hand component wear could be reduced. The developed cutting-head represents improvements in the defined quality and performance criteria of hollow shaped structures in comparison to the reference. Due to the increased performance of this water jetting process, not only industrial disassembly, but also conventional manufacturing applications will profit, which opens the way for new application possibilities.
The presented work was made possible by the Deutsche Forschungsgemeinschaft DFG that provides the funding for the research project “Water abrasive cutting-head for machining composite materials”.
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