Abrasive Water Jet Machining

Process Principle

• An abrasive jet starts out the same as a pure water jet.

• As the thin stream of water leaves the nozzle,

abrasive is added to the stream and mixed.

• The beam of water accelerates abrasive particles to speeds fast enough to cut through much harder materials.

• The mixing of abrasive particles in water jet is in such a manner that water jet’s momentum is transferred to the abrasives.

• The coherent, abrasive water jet that exits the AWJM nozzle has the ability to cut various materials, such as metals, glass, ceramics and composites.

Dynamics of an abrasive water jet

• Two material removal mechanisms have been identified.

• The first mechanism typically occurs at the uppermost portion of the kerf where the abrasive impact angle is shallow.

• In this portion of cut, material is removed primarily through erosion.

• Deeper into the kerf where the abrasive impact angle is larger, deformation wear becomes the primary material removal mechanism.

• Although the mechanism are not fully understood, the ability for AWJM to penetrate very thick material may be due to reentrainment of abrasive particles in the jet after the initial impacts at the top of the cut.

• The cutting action of an abrasive jet is two-fold. The force of the water and abrasive erodes the material, even if the jet is stationary (which is how the material is initially pierced).

• The cutting action is greatly enhanced if the abrasive jet stream is moved across the material and the ideal speed of movement depends on a variety of factors, including the material, the shape of the part, the water pressure and the type of abrasive.

• Controlling the speed of the abrasive jet nozzle is crucial to efficient and economical machining.

Equipment

Pumping system

• Identical as WJM

Abrasive Feed system

• Purpose: Controlled flow of abrasive particles to the abrasive jet nozzle.

• AWJM abrasive feed systems deliver a stream of dry abrasives to the nozzle.

• Drawback with dry abrasive delivery systems is that the delivery of abrasives over long distances is difficult.

Water jet

• Water jet used for this process is essentially the same as used for WJM

Abrasive Jet Nozzle

• Purpose of the abrasive jet nozzle is to provide efficient mixing of the abrasives and the water jet and to form the high-velocity abrasive water-jet combination.

Abrasive Jet Nozzle

• There is a difference between a pure water jet nozzle and an abrasive jet nozzle. With the abrasive jet nozzle, an opening in the side of the nozzle allows for the introduction of the abrasive to the high-pressure water stream. The two are mixed in a mixing tube and then exit the nozzle. With a pure water jet nozzle, there is no opening and no mixing tube and the high-pressure water is directed to the material after it exits the jewel.

Abrasive Jet Nozzle

• To minimize abrasive wear, the nozzle is usually made from either tungsten carbide or boron carbide.

• Two major design concepts are currently used for the design of abrasive jet nozzles.

Single-jet side feed nozzle

• This design is based on a central water jet with abrasives fed into the mixing chamber from the side.

• This configuration is easily machined and can be made quite small, which is an advantage when cutting in tight locations.

• But this concept does not provide for optimal mixing efficiency and usually experiences rapid wear of the exit section.

• The major advantage with this system is this that, it incorporates a central, conventional water jet, the abrasive flow can be stopped and the system will function as a conventional WJM system.

Multiple jet

• While cutting the same type of material, one can choose the nozzle that works best for that material.

Which nozzle is best for which material?

Water Jet Nozzle AbrasiveJet Nozzle

Soft rubber Hardened tool steel Plastic

Foam Titanium Nylon

Extremely thin stuff like Foil Aluminum Graphite

Carpet Hard Rubber Many ceramics

Paper and cardboard Stone Carbon Fiber

Soft Gasket material Inconel® Composites

Candy bars mild steel Copper

Diapers Stainless Steel

Soft, or thin wood

Limitations of abrasive jet nozzles

• Despite their simple design, abrasive jet nozzles can be troublesome at times. There are many designs, but they share the same problems:

• Short life of the mixing tube The abrasive jet can cut through just about anything—including itself. This mixing tube is expensive and wears out in only a few hundred hours of use. Replacing mixing tubes will be a large part of your operating cost.

• Occasional plugging of mixing tube Plugging is usually caused by dirt or large particles in abrasive. This used to be a big problem with abrasive jet nozzles, but has been getting better as manufacturers fine-tune mixing tube designs.

• Wear, misalignment, and damage to the jewel The jewel needs to be precisely positioned in the nozzle while water and thousands of pounds of pressure impacts it.

Process Parameters

Water jet pressure

• Pc is the minimum critical pressure required to cut the material.

• A minimum critical pressure Pc exits because of the minimum abrasive particle velocity required to cut specific materials.

• The value of Pc for mild steel is between 20.7 and 27.5 Mpa.

Water Flow Rate

• fig. Shows the depth of cut is affected by varying the water flow rate (increasing the nozzle diameter) while maintaining the constant pressure.

• As the flow rate increases, the slope of the curve decreases because the saturation point is reached.

• As the nozzle diameter increases and the water flow rate increases, the rate of increase in the particle velocity is reduced, thus reducing the depth of cut.

Abrasive flow rate

• Abrasive flow rate versus depth of cut is a linear relationship up to a point

• Above a critical flow rate, the cutting efficiency decreases.

• This is because of the fact that, as the abrasive flow rate increases( with a fixed water flow rate), particle velocity begins to decrease faster than the rate at which the number of abrasive particle impacts increase.

Abrasive Particle Size

• The most common abrasive particle sizes used for AWJM range from 100 to 150 grit

• An optimum abrasive particle size also exists for each particular nozzle mixing chamber configuration.

Abrasive type

• The type of abrasive used is also an important parameter.

• Garnet, silica and silicon carbide are the most commonly used abrasives.

• Selection of abrasive type is usually determined by the hardness of the material that is being cut.

Traverse rate

• When traverse rates are increased the depth of cut decreases.

• There is also a minimum critical traverse rate below which further increases in depth of cut are not obtained.

• If the traverse rate is not maintained at a relatively uniform velocity, a rough edge will result because of the nature of the process.

Stand-off-distance

• Data generated by some researchers indicate that depth of cut is approximately linear relative to SOD.

• Increasing SOD decreasing the depth of cut. • When mixing is efficient and process parameters

are correct, a deviation in SOD of up to +-12.7mm can be tolerated without degradation of the cut quality.

• If SODs are increased to a distances of about 80mm, the process will no longer cut but will efficiently clean and de-scale surfaces.

Process Capabilities

• AWJM can be thought of as a combination of WJM and AJM principles.

• But in terms of capability, AWJM combines the best of both processes, resulting in a new process that can cut materials whether they are hard or soft at high rates and in very thick sections.

• AWJM can cut materials as thick as 200mm and still maintain a comparatively narrow kerf.

• Kerf width is a function of the material thickness and usually is between 1.5 and 2.3mm.

• The resulting taper on the cut edge is a function of the material hardness,

• Where hard materials have the widest kerf at the top of the cut and

• Soft materials have the widest kerf at the bottom of the cut.