Abrasive water jet

CHAPTER 1 INTRODUCTION

CHAPTER – 1INTRODUCTION

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CHAPTER 1 INTRODUCTION

CHAPTER - 1 INTRODUCTION1.1 HISTORICAL BACKGROUNDHigh pressure water jets are in continuous development from 1900 onwards. In USA

these jets were introduced in mining applications to washout valuable materials like gold.

In the early 60's O. Imanaka, University of Tokyo applied pure water for industrial

machining. The idea was based on the destruction of shell structures of airplanes by

rain particle impact.

In the late 60's R. Franz of University of Michigan, examine the cutting of wood with

high velocity jets. He got the idea from the way steam leaks were detected on invisible

spots. A broom was moved through the locations where the leak was expected. By the

damage to the broom the idea came up that a jet of high velocity water could also cut

materials.

This led to the first industrial application manufactured by McCartney Manufacturing

Company and installed in Alto Boxboard in 1972.

From that time high-pressure water jets were utilized in cutting soft materials like

wood and leather. But also hard and brittle materials like granite and bricks and

even some tough materials like titanium were cut with pure water.

Research led to the invention of the abrasive water jet in 1980 and in 1983 the first

commercial system with abrasive entrainment in the jet became available. The added

abrasives increased the range of materials, which can be cut with a Watergate drastically.[35]

1.2 INTRODUCTION

1.2.1 CONVENTIONAL MACHININGConventional machining usually involves changing the shape of a workpiece using an

implement made of a harder material. Using conventional methods to machine hard

metals and alloys means increased demand of time and energy and therefore increases

in costs; in some cases conventional machining may not be feasible.

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CHAPTER 1 INTRODUCTION

Conventional machining also costs in terms of tool wear and in loss of quality in the

product owing to induced residual stresses during manufacture. With ever increasing

demand for manufactured goods of hard alloys and metals, such as Inconel 718 or

titanium, more interest has gravitated to non-conventional machining methods.[27][28]

1.2.2 NON-CONVENTIONAL MACHININGNon-conventional machining utilizes other forms of energy. Water Jet Machining

(WJM) and Abrasive Water Jet Machining (AWJM) are two non- traditional or non-

conventional machining processes. They belong to mechanical group of non-

conventional processes like Ultrasonic Machining (USM) and Abrasive Jet Machining

(AJM). In these processes (WJM and AJWM), the mechanical energy of water and

abrasive phases are used to achieve material removal or machining. The general

grouping of some of the typical non-traditional processes are shown below

Mechanical Processes

Ultra Sonic Machining (USM)

Abrasive Jet Machining (AJM)

Water Jet Machining (WJM) and Abrasive Water Jet Machining (AWJM)

Thermal Processes

Electron Beam Machining (EBM)

Laser Beam Machining (LBM)

Plasma Arc Machining (PAM)

Electric Discharge Machining (EDM) and Wire Electric Discharge Machining

(WEDM)

Electrical Processes

Electro Chemical Machining (ECM)

Electric Discharge Grinding (EDG)

Chemical Processes

Chemical milling

Photo chemical machining[29]

1.3 ABRASIVE WATER JET SYSTEMS

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CHAPTER 1 INTRODUCTION

AWJM can be achieved using different approaches and methodologies as enumerated

below:

AWJM – entrained – three phase – abrasive, water and air

AWJM – suspended – two phase – abrasive and water

Direct pumping

Indirect pumping

Bypass pumping

As discussed earlier there are two ways or systems in w hich the abrasives are mixed

with the water to form an AWJ, one is the entrainment system and the other is the

suspended pumping system. In an entrainment system, a high pressure waterjet is

formed first by an orifice; abrasives are then entrained into the waterjet to mix with

the watejet and form an abrasive waterjet. In a suspended pumping system, abrasives

are pre-mixed with water to form slurry that is then pumped and expelled through a

nozzle to form an abrasive slurry jet (ASJ). A description of these two system is given

below.[29]

1.3.1 ENTRAINED SYSTEMThe entrainment abrasive waterjet system is being widely used in the manufacturing

area. The water pressure in this system is in the order of 400 MPa. This high-pressure

water is focused through a small precious stone orifice to form an intense water stream

or waterjet, as shown in Figure 1.1. The stream moves at a velocity of up to 2.5

times the speed of sound, depending on the water pressure. As the waterjet passes

through a mixing chamber, it mixed with the abrasive particles that are entrained into

the mixing chamber through a separate inlet to the vacuum created by the waterjet.

Typical AWJ particle velocities range from 450 to 720m/s. The material removal takes

place due to the erosion of target material by particles. Each particle contributes to

the cutting process and the integration of all cutting actions by a large number of

particles (in the order of 105 particles per second) can remove material at a high rate.

As the vast majority of the abrasive waterjet cutting applications use the entrainment

system, this project will study the various aspects of the technology for entrainment

abrasive waterjet systems. [29]

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CHAPTER 1 INTRODUCTION

Figure 1.1 : Principle of entrainment system.

1.3.2 SUSPENDED SYSTEMSTwo different principles can be used to pump premixed abrasive suspension or slurry

through a fine nozzle to form an ASJ.

1.3.2.1 BYPASS PUMPING SYSTEM

In this system, part of the water flow is pumped through the bypass line to a storage

vessel to bring the abrasives out of the storage. The bypassed water together with the

abrasives then joins the main flow and mix with the water. The main components of

a bypass Figure 1.2 system are a plunger pump, a high pressure abrasive storage

vessel, a bypass line and hopper, and valves. This system has the disadvantages of

system wear, and low pressure level of usually lower than 50MPa, while the

maximum pressure is limited to 200MPa.

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Figure 1.2 : Bypass Pumping System

1.3.2.2 INDIRECT PUMPING SYSTEMAbrasive suspension is first formed by mixing abrasives and water. A high viscous

suspension-stabilizing additive may be added to the water abrasives slurry to suspend

the particles in the storage vessel and reduce their settling velocity. The premixed

slurry is then pumped by a pump-isolator assembly through a nozzle, where the

isolator is used to prevent unexpected mixing of the slurry and water. The pressure

level in this generation system is relatively higher than the bypass system, but lower

than the entrainment system. A comparison of these two systems indicates that the

process of ASJ formation is more energy efficient than an AWJ. In an ASJ system,

most of the energy put into the system is realized as cutting power at the nozzle.

However, an AWJ at high pressures (in the order of 400MPa) is more effective

in cutting than an ASJ at low pressures for the same abrasive mass usage. In the

ASJ system, the elimination of entrained air ensures a minimal expansion of the

exiting jet, which in turn provides maximum cutting precision. Furthermore, the ASJ

generation system has a simple nozzle and compact cutting head which enables

operation in confined spaces. The implementation of the ASJ system is associated

with hardware problems such as the requirements of a high pressure slurry check

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CHAPTER 1 INTRODUCTION

valve, pressure vessel and nozzles. Because of severe wear on the high pressure tubes

in the ASJ generation system, it is only used with relatively low pressure and

manufacturing cutting applications primarily use the entrainment system. Figure 1.3. [29]

Figure 1.3 : Indirect Pumping System

1.4 ABRASIVE WATER JET MACHINEAbrasive Waterjet (AWJ) machine uses cold supersonic abrasive erosion to cut al-

most any materials both metals and non-metals and so it is also understood as a

’blast’ erosion process in which the highly pressurized water is forced through a tiny

areas resulting in formation of waterjet. Abrasive garnet is mixed to this jet in the

mixing chamber making it an Abrasive Waterjet which erodes away the material.

Conventional machining may also use a liquid medium in conjunction with the cutting

tool but its purpose is not to deliver but to carry away the material.

Figure 1.4 shows the Abrasive water jet machine. In addition for both conventional and

abrasive water jet machining the liquid medium will also act as a heat sink, taking heat

away from the machining away from the working area.[35]

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CHAPTER 1 INTRODUCTION

Figure 1.4 : Abrasive water jet machine

1.5 WORKING PROCESSWater pressurized at 240 MPa or greater enters the cutting head at relatively slow speed,

The water is forced through an orifice that has a small diameter orifice of 0.1-0.3 mm.

These orifices are made of extremely hard material, such as diamond, sapphire or

ruby. This step converts the water stream from a high pressure stream to a high

velocity stream. At this point the water is moving in excess of 1000 m/s.

The high velocity of the jet creates a Venturi effect, or vacuum, in the mixing

chamber located immediately beneath the orifice. Abrasive, typically garnet is metered

from a mini-hopper through a plastic tube down to the cutting head and is sucked

into the waterjet stream in the mixing chamber. Cutting speed will increase with more

abrasive until a saturation point is reached where speed starts to decrease. If the

abrasive amount is increased too high, ultimately the mixing tube will clog.

The abrasive is fully mixed in the waterjet stream and is accelerated to approximately

the speed of the waterjet stream. This step does steal some energy from the waterjet

stream, slowing it down slightly.

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The abrasive waterjet stream exits the mixing tube with extreme speed and power. The

abrasive erodes the material to be cut. The process is referred to as “abrasive waterjet

cutting” because it is the abrasive that is actually doing the cutting. [26]

Figure 1.5 : Working of AWJ

1.5.1 WATER JET CUTTERWater jet cutting works by forcing a large volume of water through a small orifice in

the nozzle Figure 1.6.The constant volume of water traveling through a reduced area

causes the particles to accelerate. This accelerated stream leaving the nozzle impacts

the material to be cut. The extreme pressure of t he accelerated water particles contacts a

small area of the work piece. In this small area the work piece develops small

cracks due to stream impact. The water jet washes away the material that “erodes”

from the surface of the work piece. The crack caused by the water jet impact is now

exposed to the water jet the extreme pressure and impact of particles in the

following stream cause t he small crack to propagate until the material is cut through.

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CHAPTER 1 INTRODUCTION

[26]

Figure 1.6 : Working of AWJ cutter head

1.6 MAIN COMPONENTS OF AWJMThe general domain of parameters in entrained type AWJ machining system is given

below:

Orifice – Sapphires – 0.1 to 0.3 mm

Focusing Tube – WC – 0.8 to 2.4 mm

Pressure – 2500 to 4000 bar

Abrasive – garnet and silica sand - #125 to #60

Abrasive flow - 0.1 to 1.0 Kg/min

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Standoff distance – 1 to 10 mm

Machine Impact Angle – 600 to 900

Traverse Speed – 10 mm/min to 1000 m/min

Depth of Cut – 1 mm to 250 mm[29]

1.7 PARAMETERS OF AWJMThere are numerous parameters involved in the AWJ process. Generally, all these

parameters can be divided into two groups, i.e. the input parameters and output

parameters. These are listed below:

1.7.1 INPUT PARAMETERSHydraulic parameters

Waterjet pressure

Waterjet diameter

Mixing parameters

Mixing tube diameter

Mixing tube length

Cutting parameters

Traverse speed

Stand off distance

Angle of attack

Abrasive parameters

Abrasive particle size

Abrasive material

Abrasive flow rate

1.7.2 OUTPUT PARAMETERS Surface roughness

Kerf width and taper

Depth of cut

Material removal rate

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In practice, the output parameters are of major concern as they represent the cut-

ting performance. The cutting and abrasive parameters as well as the water pressure

(under hydraulic parameters) need to be properly selected or optimized to enhance

the cutting performance or output parameters for a given target material.[26]

1.8 ADVANTAGES AND LIMITATIONS OF AWJM

1.8.1 ADVANTAGES AWJ can machine a wide range of materials including titanium, stainless steel,

aerospace alloys, glass, plastics, ceramics, and so on.

AWJ can cut net-shape parts and near net-shape parts.

No heat is generated in the cutting process. Therefore, there is no heat -affected

area and thus no structural changes in work materials occur.

AWJ cutting is particularly environmentally friendly as it does not generate

any cutting dust or chemical air pollutants.

The abrasives after cutting can be reused which allows for possible reduction

of the AWJ cutting cost.

Only one nozzle can be used to machine various types of work materials and

work piece shapes.

AWJ machining can be easily automated and therefore can be run with

unmanned shifts.

1.8.2 LIMITATIONS The total cutting cost is relatively high.

The cutting quality is not always satisfying and unstable in nature. [29]

1.9 APPLICATIONSThe applications and materials, which are generally machined using AWJM are given

below

Application

Paint removal

Cleaning

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Cutting soft materials

Cutting frozen meat

Textile, leather industry

Mass immunization

Surgery

Peening

Cutting

Pocket milling

Drilling turning

Nuclear plant dismantling

Some common applications:

Fast and precise cutting of fabrics

Vinyl, foam coverings of car dashboard panels

Plastic and composite body panels used in the interior of cars

Cutting glass and ceramic tiles

Materials like:

Steels

Nonferrous alloys

Ti alloys, Ni alloys

Polymers

Honeycombs

Metal matrix composite

Ceramic matrix composite

Concrete

Stone- granite

Wood

Reinforced plastics

Metal polymer laminates

Glass fiber metal laminates. [29]

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