Abrasive water jet cutting and its applications at The Welding Institute

TWI Bulletin, February 1988

Ian Harris, BSc (Hons), is a Senior Research Engineer in the Gas Shielded Section of the Arc Welding Department at The Welding Institute.

Abrasive water jet cutting can be used to cut most materials including composites and ceramics. Thermal deformation stresses and the need to sharpen cutting tools do not exist with such a process, and dust and fire hazards are negligible. The process's potential is assessed.

The abrasive water jet (AWJ) process has been developed comparatively recently from high pressure water jetting which is used for material removal, e.g. for cleaning buildings and for cutting a range of non-metallic materials. Although industrial exploitation of the AWJ process is still in its infancy, there are three types of system available commercially ^[1-3] and the technique has been applied to cutting a wide range of materials, both metallic and non-metallic. ^[4-6]

The essential difference between the two techniques is that the addition of an abrasive powder to the water jet substantially increases the cutting action, extending the range of applications to metals and other hard materials.

The advantages of waterjetting compared with thermal and mechanical methods ^[1] include:

- No thermal or deformation stresses;

- No dulling of the cutting tool;

- Minimal or no dust;

- No fire hazard.

However, cutting with water alone has limitations in that hard materials such as metals, ceramics and high strength composites cannot be cut and for other materials high power levels are required for acceptable cutting rates, with water pressures of 200-340N/mm ² (30000-50000lbf/in ²).

To increase the capabilities of water jet cutting, abrasives were introduced to produce the abrasive water jet system.* The addition of abrasives greatly increased the cutting capability and the performance of the process, extending its use to metals and other hard materials in a wide range of thicknesses. Thus AWJ complements the more orthodox mechanical and thermal cutting processes, but with the added advantage that most materials can be cut.

*One of the earliest applications involved the cutting of cast iron. ^[7]

The AWJ facility at The Welding Institute, which is based on equipment from Jetin Industrial, was set up in early 1985 to investigate the application of this new technique to areas of special interest in the welding and fabrication industries. Since its installation, the equipment has been used both for basic process research and for application studies for members.

The results of application studies which have been conducted to generate cutting data on a diverse range of materials including various metals and composite materials are discussed in this article. In addition, the potential of the process is described for other cutting operations, e.g. gouging and weld toe dressing.

The AWJ process

Process and principles

The crucial part of the abrasive water jet cutting system is the abrasive jet nozzle. Water is pressurised up to 100 N/mm ² (14 500 lbf/in ²) (or more depending on the power of the pumping system) and passes through a jet orifice to form a coherent high velocity jet. In one method the water is mixed with a supply of dry abrasive, in another, an abrasive water slurry is used. The abrasive is introduced separately into the mixing chamber ( Fig.1). The momentum of the water is transferred to the abrasive particles, rapidly increasing their velocity. The momentum transfer between the water jet and the abrasive particles is a complex process. One of the mechanisms is associated with the limited dynamic stability of the high pressure water jet; the coherent water jet breaks into droplets which accelerate the particles. A second mechanism involves hydrodynamic forces imposed by the water on the abrasive particles. It is likely that both mechanisms play a part in momentum transfer, the first being dominant in the mixing chamber, and the second in the nozzle and during the cutting action.

The cutting action is performed by the focused high velocity jet of water and abrasives emerging from the nozzle orifice, as shown in Fig.2. Cutting of the workpiece results from a combination of erosion, shearing failure under rapidly changing localised stress fields and micro-machining effects, depending on the specific properties of the material. Micromachining refers to removal of small amounts of the base materials by impact of abrasive particles. The stages in the development of the cut edge are illustrated in Fig.3, ^[8] including initial through-thickness penetration, steady state cutting and exit of the jet from the end of the cut. The cutting mechanism results in a striation effect on the surface of the cut edge, and the drag angle of the striations depends on the relationship between traverse speed and cutting power. As the cutting speed increases for a given cutting power (water pressure and abrasive flow rate) the angle of the striations will increase until through-thickness cutting is no longer achieved. In Fig.3, X represents distance along the cutting direction and h the depth of cut. Through-thickness penetration is achieved at (X4, h4) in the sketch. The cutting performance of the equipment is governed by a number of factors ( Fig.1) which include the traverse rate, abrasive type and feed rate, stand-off distance and nozzle geometry angle to the surface which is usually 90°.

The abrasive most commonly used in conjunction with the Jetin cutting equipment is Flintag, which is a commercial name for crushed flint particles. The composion of flint is over 98% silica (Si0 ₂) with a small percentage of alumina Al ₂0 ₃) and small proportions of other oxides. Flint fractures in a characteristic way producing sharp edges. Flint particles therefore provide an effective abrasive medium. Two grades of flint abrasive are used with the equipment at The Welding Institute; Flintag No 5 which has a size range of 0.4-1.4mm, and Flintag No 7, which has a size range of 0.1-0.4mm. The coarser grade is more effective for cutting metals and other hard materials, whilst the finer type is generally used when cutting softer materials.

Zircon or alumina can also be used but these cost considerably more than the flint abrasive which costs about £100 per tonne. Consequently, they are rarely used in practice with the Jetin system.

Alumina and zirconia abrasives with fine particle sizes are commonly used with high pressure/small nozzle diameter equipment types such as those produced by Flow Systems.

Equipment

After cutting through the workpiece, the water jet is contained within the cutting tank which is generally filled with water to a level just below the workpiece. The considerable power remaining in the jet is dissipated by the water in the tank, creating considerable turbulence as can be seen in Fig.7.

The used abrasive builds up in the tank over a period of time. With flint abrasives, the particles are fractured during the cutting process, so that if the abrasive were reused it would be of a finer grade. In practice, since the abrasive is cheap and would have to be dried if it were to be reused, it is not recirculated or otherwise reused. The simplest and most cost effective solution is to drain the tank periodically and remove the used abrasive.

The cost of a commercial system such as that used at The Welding Institute is comparable to other commercial systems such as the BHRA Diajet (operating pressure 35 N/mm ² (5100 lbf/in ²). The Flow Systems or Ingersoll Rand equipments which use specialised intensifier pumps to produce high operating pressures of around 200 N/mm ² (30 000 lbf/in ²) are more expensive.

Table 1 Cutting speeds yielding good edge quality for a range of metallic materials

There is a distinct difference in design philosophy between the 'low' pressure, 'large' nozzle systems developed mostly in the UK, and the high pressure, fine nozzle systems developed mostly in the USA. This has largely arisen on the basis of equipment costs which rise sharply at pressures above about 100N/mm ² (14 500lbf/in²).

Process control

The performance of an abrasive water jet cutting system is influenced by a range of factors and parameters:

1. Hydraulic parameters - water jet orifice diameter, water flow rate and supply pressure;
2. Abrasive parameters - feed rate, grain size, material (density, hardness, shape), feed method;
3. Mixing nozzle parameters - mixing chamber geometry, nozzle diameter, nozzle material;
4. Cutting parameters - traverse rate, stand-off distance, workpiece material, jet impingement angle.

Application studies

Over two hundred and fifty enquiries have been received since the adoption and installation at TWI of the abrasive water jet process for material removal. About forty of these have led to specific application studies.

Most of the studies involve through-thickness cutting trials but interest has also been shown in the use of the process for groove and joint preparation, gouging, and weld toe dressing. The following sections describe the results of some studies on the abrasive water jet process and indicate the scope of the process.

Cutting

Through-thickness cutting has been the most widely used mode of operation to date and this is likely to continue in future. Simple linear cutting, profiling, and bevel cutting for joint edge preparation have all been successfully achieved. A wide range of materials have been cut, both metallic and non-metallic, in thicknesses ranging from 2-230mm. Cutting speeds yielding optimum edge quality are shown in Tables 1 and 2 for a variety of materials. These include mild, carbon-manganese, pressure vessel and armour steels, Stellite-clad steel, stainless steel, aluminium alloys including aluminium armour, copper and titanium. The range of non-metallic materials includes concrete, reinforced and unreinforced, marble, ceramics, carbon fibre composites and Kevlar.

Some examples of materials which have been cut are shown in Fig.8-13. These include non-metallic materials such as carbon fibre composites ( Fig.8), reinforced concrete ( Fig.9), and marble ( Fig.10). Metallic materials include profiled aluminium alloy plate ( Fig.11), aluminium armour showing both square and bevel cuts ( Fig.12), and 230mm thickness steel plate ( Fig.13).

The cutting mechanism results in a striation effect on the cut surface of most materials and the drag angle of the striations indicates the cutting speed in relation to the cutting force or power, i.e. for a given water pressure and abrasive feed rate, the drag angle will increase with increasing cutting speed until through-thickness cutting is no longer achieved. The optimum cutting speed in terms of edge quality is that which results in a smooth edge with a minimum bevel angle. As the cutting speed is increased to the maximum value for through-thickness cutting, the surface generally becomes rougher, particularly along the bottom edge of the workpiece, and the bevel angle increases. The optimum cutting speed in terms of edge quality is two-thirds that of the maximum speed.

Abrasive water jet cutting has proved particularly effective for cutting hard materials such as hardened components and parts hard-surfaced by welding. Examples of such components are gears and shafts, and Stellite-clad steels. These components are difficult to cut using more conventional cutting techniques such as cold sawing e.g. for specimen preparation.

Metals which suffer metallurgical degradation at the cut edge when cut by thermal processes lend themselves to abrasive water jet cutting which has no thermal effect on material properties. So there is no need for post-cut machining, which often has to be used for heat-sensitive metals to remove the heat affected zone prior to welding. Such metals include steel armour and aluminium armour plate, and titanium alloys. However, wire brushing or light machining may be required for the softer aluminium alloys as some embedment of abrasive particle fragments has been observed. The effect of these on the quality and strength of welded joints has yet to be established.

Groove preparation

This technique involves the removal of material from the workpiece so as to form a groove ( Fig.14).

Table 2 Cutting speeds yielding good edge quality for a range of non-metallic materials

The groove can be filled with hardfacing or corrosion resistant material, or used to form a slot in a component, for example, to locate another part. The required groove profile is developed by increasing stand-off to widen the groove and by decreasing travel speed (for a predetermined water pressure) to deepen the groove. Typical parameters are shown in Fig.14.

The technique could also be used to prepare a narrow-gap joint configuration. The narrow gap is produced by welding together two plates with square edges using what is effectively a backing bead; and then removing material from the other side of the joint to form the weld preparation. This technique is currently under development and is illustrated schematically in Fig.15.

Gouging

This technique is similar in principle to that described for groove preparation but refers specifically to back-gouging or defect removal. In the former, the root of the weld is removed before welding from the reverse side. In the latter, as illustrated in Fig.16, AWJ can be used to remove a centreline crack in a fillet weld.

As the process generates no thermal or deformation stresses, it offers a major advantage compared with thermal or arc gouging techniques which can thermally damage the parent material or, in the case of crack removal, can extend the crack instead of removing it.

Weld toe dressing

This technique is particularly effective for enhancing the fatigue life of welded structures, especially for fillet welds. The AWJ process is used to remove material at the weld toes in order to remove crack-like flaws and produce a smooth transition between the plate and the weld. This technique is illustrated in Fig.17, while a typical erosion profile is shown in Fig.18, including typical parameters. This toe dressing technique is many times faster than other techniques such as grinding or remelting (TIG or plasma dressing) and has been found to have an equally beneficial effect on fatigue life. In a recent study ^[9] the following conclusions were reached:

1. Erosion of the toes in fillet welded steel specimens improved their fatigue strengths, although the improvement was only significant for applied stress ranges below 200 N/mm ².
2. The improvement in fatigue life was comparable with that obtained by local grinding, plasma dressing and shot peening, particularly in the high cycle regime.
3. The weld toe erosion rates of 20 to 45 m/hr were greater than those achieved by conventional weld toe dressing methods (0.5-2.5 m/hr for grinding, 0.9 m/hr for remelting).

Piercing

Piercing of most materials can be achieved rapidly in comparison with conventional mechanical techniques and can produce holes of reasonable uniformity requiring little post-cut dressing. For example, a typical piercing time for 25mm thickness aluminium is 30sec. However, the range of hole sizes which can be produced is restricted, at the lower end, by the range of nozzle sizes available and the abrasive particle size used. No specific applications have been found to date for this technique.

Discussion

The parameters used in the cutting process depend to a large extent on the tolerances imposed on cut edge quality for a particular application, the most important being the bevel in most cases. In terms of productivity and cost effectiveness, the maximum cutting speed and minimum consumable costs are required, as in any other cutting process, while edge quality is maintained within the specified limits.

Application studies

The abrasive water jet process can be used in a wide range of operating modes for cutting and material removal of both metallic and non-metallic materials. As the types of operation include through-thickness cutting, groove preparation, gouging, weld toe dressing, and piercing, AWJ represents a flexible process for material removal.

The process is particularly useful for cutting hard metals such as armour plating, hardened components and parts hard-surfaced by welding. In addition, because no heat is generated, the process is preferable to thermal techniques such as flame cutting or gouging for metal removal prior to repair. This is particularly useful for excavating cracks such as lamellar tears which may extend during gouging, or for joint preparation in metals which are particularly sensitive to localised heating.

The process can also be useful for removing sections of components and structures for residual stress measurement since the areas to be removed would not be subjected to stress relief from thermal cutting techniques.

The AWJ cutting process has been shown to be particularly useful for cutting carbon fibre composites and Kevlar, avoiding the edge damage and delamination effects resulting from mechanical cutting techniques; 'fibre-pulling' in Kevlar is also avoided using AWJ. The ability of the AWJ process to cut almost any material, makes it especially useful for the new range of multi-material composites and modern ceramic materials, many of which present considerable problems in materials processing for fabrication, and profiling of component parts. The process also offers potential in processing the ever expanding range of plastics materials where use of a non-thermal cutting technique may often be a prerequisite to achieve high cut-edge quality.

In the present state of development the AWJ process is suitable for applications involving cutting hard materials which present problems for conventional cutting techniques, e.g. gears, tool steels, ceramics etc, for cutting heat sensitive materials, and for cutting and profiling materials such as marble, reinforced concrete, and stone. Glass cutting, and profiling in particular, is another important application area.

Industrial potential

The low cutting speeds attainable for structural steels, and other metals which can be cut using oxy-fuel or plasma cutting, place the AWJ process at a disadvantage for most standard plate cutting applications when viewed in terms of cutting speed. Nevertheless, it is considered that despite its limitations, it should be considered as a new, and complementary, addition to the range of materials processing techniques available to industry. With a variety of material removal modes, and an ability to cut a wide-range of material types, the abrasive water jet process offers considerable potential for the future.

Improvements in cutting speed, through improved equipment design are required before the AWJ process can hope to rival flame cutting for structural steel. The new DIAJET equipment developed by BHRA represents a significant move in this direction using direct abrasive entrainment upstream of the jet orifice to achieve an approximate five fold increase in cutting speed compared with the conventional abrasive entrainment systems. This equipment is at a stage of development where commercial systems, operating on a continuous rather than a batch basis, will soon be available.

Summary

The results of investigations into various applications can be summarised as follows:

1. The abrasive water jet cutting technique can be used to cut most materials including metals, composites and ceramics in a wide range of thicknesses.
2. The abrasive water jet process is suitable for a range of material removal applications including weld toe dressing, gouging, weld edge preparation and cutting in general.
3. The optimum cutting speed in terms of edge quality is generally about two thirds of the maximum cutting speed.