Advanced Manufacturing Processes: Waterjet

Water jet machining

Index

Water jet machining ......................................................................................................................................... 1

Introduction ................................................................................................................................................. 1

Process parameters ...................................................................................................................................... 3

Outflow regimes ........................................................................................................................................... 9

Kerf shape and cutting quality .................................................................................................................... 11

Optimal selection of parameters and innovative solutions ........................................................................ 15

Introduction

The physical principle of this process is the conversion of the fluid pressure energy into kinetic energy. Hypotheses are incompressibility of the fluid, no friction with walls and laminar flow. Abrasive waterjet exploits the presence of particles: in this case, water has the only function of accelerating them. The temperature of the process must be lower than 80°C. WJ and AWJ are used for a great number of processes such as cutting, drilling, surface cleaning, decoating and forming¹.

The great advantage of WJ is the possibility of a cold cut: water is a vector for energy but also a cooling system, impeding structural alterations in metallic materials and degradation in polymers; cutting doesn’t leave burrs and there is not contact between tool and workpiece. Even if material removal is possible, forces are small and the clamping system is of low importance; the process can start everywhere and achievable geometries are different, with small tolerances. The process can be automatic. Alignment is of high importance: all the components are grinded for high precision and nylon inserts help to keep positions stable.

The main components are:

• Water treatment system: its principle is to use softening, deionization, filtering and reverse osmosis² to depurate water. A good quality assures hundreds of hours of operation without changing the tool.
• Pumping system: a hydraulic fluid is pumped into the intensifier, whereas depurated water is transported into the system.
• High-pressure water transport system and connection, made of both rigid and flexible pipes.
• Cutting head: it contains the primary nozzle or orifice that transforms the pressure energy into kinetic energy. It is really important that the jet has coherence in order to keep the energy on a small area: a focuser is here introduced³. In case of AWJ, the mixing chamber is present, otherwise particles would be rejected by the water flux. To assure wear, corrosion and high pressure resistance, the primary nozzle is made of sapphire, the mixing chamber and the focuser in carbides, typically WC or BC (100 h of tool life). Misalignment between focuser and primary nozzle is cause of asymmetrical erosion. Generally, the focuser is subjected to erosion in the upper part in contact with the mixing chamber and to abrasion in the lower part where particles embedded in the flux have lower impact angles. A small divergence of pure water from the primary nozzle may decrease rejection and reflection of particles entering the focuser. Regular shapes and perfect holes are needed to avoid much friction and to guarantee perfect alignment⁴.
• Abrasive feeding system if AWJ. A rubber canal inside a rigid chamber composes it. Increase the pressure oil in the rigid chamber allows the deformation of the canal and the modulation of abrasive quantity. The particles must have high hardness, homogeneous dimension and low hygroscopic behaviour.
• Intensifier: it increases the pressure at low frequency, 2 Hz, (400 = 20 → = 200 → = 4000). By exploiting the surfaces ratio: highest-pressure intensifiers lead to 600 MPa. Nowadays also electric intensifiers are available: elimination of the oil circuit, best mechanical efficiency and reduction of power consumption are possible.
• Attenuator (high-pressure vessel for accumulation): water at 400 MPa is 15% compressible, so it smooths the pressure oscillations and supplies water flow when needed.
• Catcher: it has the function to dissipate the jet residual energy, to avoid jet back reflection, to reduce noise and to catch and flush the machining scraps.

¹ Water jet forming exploit the impact of a jet to give plastic deformation to the workpiece.

² The particles suspended in the water pipes would lead to an accelerated wear of the mechanical parts of the intensifier, valves and the sapphire orifice; moreover, calcium and magnesium salts dissolved in the water may deposit thus damaging the parts and the hydraulic efficiency of the system; finally the chlorides and sulfates are responsible for the corrosion of metal parts. The osmosis process is the passage of water from a less concentrated solution to a more concentrated solution through a semi-permeable membrane (i.e. able to stop only the particles above a certain size). To purify the water it is necessary to get a reverse stream to extract fluid from a solution yet full of impurities. To do this a pressure higher than the osmotic pressure is applied. The diluted solution is collected in a tank while the more concentrated stream is drained.

³ Function: promote the momentum transfer from water jet to abrasive particles and create a homogeneous and collimated outflowing jet. The distance between the focuser and the workpiece is of few mm in order to have a collimated jet. In AWJ, the focuser must be long enough to have a regular flux of particles (there is a threshold below that the flux is irregular, particles are in a turbulent motion and erosion of walls occurs).

⁴ The velocity can reach 9000 m/s.

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Process parameters

The principal parameters are:

• Pressure: 300-400 MPa⁵;
• Water-orifice diameter: 0.1-0.3 mm;
• Traverse rate: 100-200 mm/min;
• Standoff distance;
• Abrasive mass flow rate: 100-200 g/min;
• Mesh: #80-120;
• Abrasive mean dimension: 170 μm;
• Focuser diameter: 0.3-1 mm;
• Focuser length: 76-100 mm (3 inch);
• Garnet / olivine.

⁵ Grains with higher pressure give more energy to the impact.

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The values are found with experiments. We can have 3*2 combinations = 1536 tests would be required to have a preliminary idea. Moreover, there are fixed external factors, e.g. humidity of air in the environment. Any variation can cause a different outcome: for example, roughness may depend on pressure or on mass flow rate in a very different way. To evaluate quality, comparison with samples is possible. Striations are more evident with high velocity.

Theoretical compressible water jet velocity

1−2 √ , = [(1 + ) − 1]ℎ (1 − ) 0 = 300 = 0,1368

Where and at ambient T.

The compressibility coefficient can be expressed as the ratio of the theoretical velocity of a compressible flow⁶ with the theoretical velocity of an incompressible flow, derived from Bernoulli’s approach:

1− ℎ, =√Ψ= [(1 + ) − 1](1 − ) ℎ

The velocity coefficient takes into account friction losses in the orifice and it is defined as: = ℎ,

The contraction coefficient takes into account the vena contracta phenomenon: = = 0

The theoretical water flow rate is given by:

2 2√ = ∙ = ℎ 0 ℎ 4 0

The actual flow rate is:

= ∙ = ( ∙ )( ∙ ) 0 ℎ,

The overall discharge coefficient is:

= Ψ = ℎ = 0 ℎ

The overall coefficient of discharge depends also on the orifice edge conditions and on the inlet radius (on the geometry, actually): sharp edge entry decreases it, leading actually to more losses. From experimental observation, sapphire orifices with sharp edges are dominated by the following:

−4 = 0,785 − 1,4 ∙ 10 ∙