Abrasive water jetting

TWI Bulletin, October 1985

by J A Padilla

J A Padilla, CEng, MRAeS, MWeldl, is Principal Research Engineer in the Control Engineering Department.

The use of abrasive water jetting as a cutting technique complements the more orthodox mechanical or thermal methods. This article describes the new facility set up at Abington, with some of the applications areas currently being researched.

The Welding Institute has recently set up an abrasive water jet (AWJ) facility to investigate the application of this novel technique in areas of special interest to the welding and fabrication industry. ^[1] This equipment will not only be used as a research tool, but is also available for use by Member companies for evaluation of their applications.

The equipment was supplied by a Member company, Jetin Industrial Ltd, formerly a subsidiary of F A Hughes and comprises:

1. A high pressure water pump - output 25 litre/min at 965 bar, max working pressure;
2. A water jet lance - comprising an orifice housing, plenum chamber, and tungsten carbide nozzle;
3. A cutting tank - with a cutting area of 1 .4m x 2.4m;
4. An abrasive grit feeder.

The cutting head positioner is a BOC unit formerly used for gas cutting. A general view of the facility is shown in Fig.1 and a close up view of the lance assembly in Fig.2.

The equipment will be fully instrumented to monitor water pressure, water flow, nozzle reaction load and traverse rate on a continuous basis so that a precise parametric specification may be drawn up for any application. The measurement of nozzle reaction load is particularly relevant to robotic applications where dimensional stability under variable loading is vital to match the robot's positional integrity. This has led to the design of a special lance holder incorporating a load cell, which can also be swivelled in two planes to provide a variation in jet impingement angle.

The process

Advantages

High pressure water jets are already used as a standard technique for cutting a wide range of non-metals and for material removal as a cleaning process for buildings and structures. The advantages offered include:

- Minimal, or no, dust;

- High cutting speeds;

- Multidirectional cutting capability;

- No dulling of the cutting tool;

- No thermal or deformation stresses;

- No fire hazards associated with the cutting process.

There are also some shortcomings of pure water jet systems:

- Hard materials such as metals, ceramics and high strength composites cannot be cut;

- Very high power levels are necessary for reasonable cutting rates;

- Delamination or striation can occur.

To increase water jet cutting capabilities, abrasives were introduced to form the abrasive water jet system. This greatly increased the cutting capability and performance providing a technique which complements more orthodox mechanical and thermal cutting methods. Advantages of the abrasive water jet include those listed above for pure water jets with the following additions:

- Any material can be cut;

- Power requirements are decreased;

- Delamination does not occur and striation is reduced.

Principles

The heart of the abrasive water jet cutting system is the abrasive jet nozzle. Water is pressurised up to 2500 bar and expelled through a nozzle to form a coherent, high velocity jet. The water jet and a stream of solid abrasives are introduced into the specially shaped abrasive jet nozzle from separate feedports. Here, the water jet's momentum is transferred to the abrasive particles, whose velocities rapidly increase.

The momentum transfer between the water jet and the abrasives is a complex phenomenon. One of the mechanisms by which this occurs is associated with the limited dynamic stability of the high pressure water jet: the initially coherent water jet breaks into droplets that accelerate the solid particles. A second mechanism corresponds to hydrodynamic drag forces imposed by the water phase on the solid particles.

As a result of momentum transfer between water and abrasive, a focused, high velocity stream of abrasive particles exits the accelerator nozzle and performs the cutting action. Cutting or controlled depth penetration of the target material occurs as a result of erosion, shearing, failure under rapidly changing localised stress fields, or micromachining effects, depending upon the specific properties of the material being cut. The performance of the erosion mechanism is subject to a wide range of criteria. The principal operating criteria are illustrated in Fig.3, which shows the lance arrangement. Traverse rate, water input, abrasive feed and nozzle geometry all combine in varying degree to affect erosion of the target material.

Abrasive water jet system components

The key components are the high pressure pump (which may be powered by electric motor or internal combustion engine), the water jet, the abrasive feed system, and the abrasive jet nozzle. Secondary components include such accessories as hoses, control valves, couplings and swivels, where necessary.

Typical components are shown in Fig.4-6. The components illustrated form part of a commercial system operating at pressures up to 1000 bar, water flow of 25 litre/min and an abrasive feed rate up to 3 kg/min. The abrasive can be fed dry to the nozzle or as a slurry. The slurrying of abrasives may provide better control of the abrasive feed rate but adds the complication of a separate slurrying circuit. Because slurries result in less efficient mixing of the abrasives with the water jet in the mixing chamber because of the additional mass of the carrier fluid, higher water flow rates with consequent increases in power are required.

The performance of an abrasive water jet is influenced by:

1. Hydraulic - water-jet orifice diameters/flow; parameters supply pressure.
2. Abrasive parameters - flow or feed rate;
   grain size;
   feed method (positive or suction);
   abrasive condition (slurry or dry);
   material (density, hardness, shape).
3. Mixing nozzle parameters - mixing chamber geometry and dimensions;
   nozzle diameter; nozzle material.
4. Cutting - traverse rate;
   standoff distance;
   jet impingement angle; material being cut.

Thus, the establishment of optimum conditions for a particular cutting or erosion application could require study of a wide range of operating variables.

Applications and current research areas

The full potential of the technique in the context of cutting and material removal in metal components and structures has yet to be realised. In particular, the technique could prove to be superior to conventional methods for cutting hard metals such as hardened or hard-clad components, armour plating, or parts hard-surfaced by welding. Also, the fact that no significant heat is generated suggests that abrasive water jet erosion would be more suitable than flame cutting or gouging for removing metal prior to repair, particularly for excavating cracks which might extend during gouging, such as lamellar tears, or for introducing weld preparations in metals which could degrade as a result of local heating. Finally, the method could prove useful for removing large areas of metal for residual stress measurement. It is intended to study the value of water jet erosion in such applications in the future.

A limited investigation has already been carried out ^[2] into the use of an abrasive water jet on welded structures for removal of weld toe regions to improve fatigue life of fillet welded joints (patent applied for). The tests were carried out on specimens of structural C-Mn steel over a range of erosion conditions. Fatigue life increases were obtained for all conditions although the increase was only significant for applied stress ranges below 200 N/mm ² . The improvement was comparable with that achieved by other methods (grinding or remelting) but the dressing speed was typically ten times faster. As the process lends itself readily to automation, robotic manipulation will be an area for further investigation.

In trials using The Welding Institute's facility, a limited range of specimens has been processed by cutting or erosion. These include 50mm thick mild steel, 50mm mild steel clad with 8mm Stellite 6, 3 and 25mm titanium, 3 and 6mm aluminium, 36mm aluminium armour and 100mm thick reinforced concrete. A novel application currently being investigated is the possibility of cutting a specimen of mild steel 50mm thick chilled to a temperature of -165°C such that the specimen temperature does not rise above -50°C.

Cutting or erosion to a controlled depth without through penetration has been carried out to gouge out an entire weld fillet section. This is of particular interest for gouging defective weld runs for repair, particularly since no heat is involved thereby maintaining the material properties.

The abrasive water jet technique is included in the Institute's proposed research programmes to study cutting processes and also to investigate methods of fatigue enhancement of welded structures. In addition a range of Group Sponsored Programmes is being drawn up targeted at specific areas of interest to member companies. These will include:

1. The cutting and eroding of aluminium, titanium and other light weight non-ferrous materials and alloys.
2. The cutting and eroding of corrosion resistant and alloy steels.
3. The fatigue enhancement of welded structures and machined components by surface dressing or erosion.

The abrasive water jet process is not the universal answer; as with other techniques it has its limitations. However, it does offer significant advantages over other processes in certain areas, and is a useful and economic complement to other methods of material processing.

Acknowledgements

The co-operation of Jetin Industrial Ltd for permission to publish the photographs in Fig.4-6 is acknowledged. The AWJ trials on the various materials were conducted by G M Courtman of the Control Engineering Department.

References