Surface finish of the bulk metallic glass using abrasive jet polishing process on the machining center

21 Figure 2.7 The relationship between surface roughness and normal burnishing force.. The optimal burnishing parameters were a grouping of the tungsten carbide ball, the burnishing spee

Surface finish of the bulk metallic glass using abrasive jet polishing process on

the machining center

Nguyen Hai Dang

Abstract

This thesis aims to investigate the optimal abrasive jet polishing parameters for based bulk metallic glass (BMG) material by using the Taguchi method An abrasive jet polishing (AJP) system has been newly designed and installed on a machining center In order to determine the optimal polishing parameters for the BMG sample, four polishing parameters, namely the hydraulic pressure, the impact angle, the stand-off distance, and the polishing time were chosen as the factors of experiments The optimal AJP parameters have been determined after carrying out the experiments based on the Taguchi’s L9 orthogonal array experimental results These optimal parameters are the combination of the hydraulic pressure of 2 kg/cm2, the impact angle of 50o, the stand-off distance of 15

Zr-mm, and the polishing time of 60 minutes The surface roughness can be improved from about Ra0.13 m to 0.044 m by using the AJP optimal parameters Besides, an analysis

of variation (ANOVA) of the experimental data indicated that the polishing time and hydraulic pressure was the dominant parameters of the AJP process for the BMG material

Keywords: Abrasive jet polishing, bulk metallic glass, Taguchi method, orthogonal array, ANOVA

Acknowledgements

First and foremost I would like to show my gratitude to my advisor, Professor Jung Shiou, Director of Opto-Mechatronics Technology Center, National Taiwan University of Science and Technology, who has supported me throughout my thesis Without his patience, encouragement, guidance and immense knowledge this thesis would not have been possible Besides my advisor, I would like to thank the rest of my thesis committee: Professor Geo-Ry Tang, and Doctor Wei-Yao Hsu for the valuable comments

Fang-Furthermore, my sincere thanks are due to Mr Arif, who guided me in operating the CNC machining center, Vincent, who had made his support in a number of ways and Mr Son, Mr Loc, who helped me in all the time of research and writing of this thesis Their assistance and guidance have been of great value in this study

In my daily work, I am indebted to many of my other lab mates to provide me a happy and peaceful environment

I would also like to thank the Library staffs who helped me in gathering a lot of information for this project I am also very appreciative of the Taiwan Government for providing me the financial support I offer my regards to all of those who supported me in any respect during my studying time

Lastly and most importantly, I owe my deepest gratitude to my family for everything they have done for me Without their love, this thesis would not be finished

Taiwan

July 2011

Table of Contents

Abstract i

Acknowledgements ii

Table of Contents iii

List of Figures vi

List of Tables ix

Chapter 1 INTRODUCTION 10

1.1 Research motivation 10

1.2 Literature review 10

1.2.1 BMG properties and machining ability of BMGs 10

1.2.2 Ball burnishing process 11

1.2.3 Abrasive jet polishing (AJP) process 13

1.3 Thesis objectives 14

1.4 Outline of thesis 15

Chapter 2 BACKGROUND INFORMATION 16

2.1 Milling process and milling parameters 16

2.1.1 Milling process 16

2.1.2 Milling parameters 18

2.2 Ball burnishing process 19

2.2.1 The simplified theory of ball burnishing deformation 20

2.2.2 Effect of burnishing force on surface roughness 22

2.2.3 Effect of feed on surface roughness 23

2.2.4 Effect of ball material on surface roughness 23

2.2.5 Effect of burnishing speed on surface roughness 24

2.3 AJP process 24

2.3.1 AJP system 26

2.3.2 Model for the ductile and brittle mode material removal 28

2.3.3 Parameters of the AJP process 30

2.3.4 Footprint on the workpiece after AJP process 33

2.4 Surface roughness measurement 34

Chapter 3 TAGUCHI METHOD AND ANOVA ANALYSIS 38

3.1 Introduction 38

3.2 Control factors and noise factors 39

3.3 Orthogonal array 39

3.4 ANOVA and S/N Analysis 40

Chapter 4 EXPERIMENTAL WORK 43

4.1 Introduction 43

4.2 BMG sample 44

4.3 Sample preparation 44

4.3.1 Milling process 44

4.3.2 Ball burnishing process 46

4.4 AJP process 50

4.4.1 System setup 50

4.4.2 Procedure to conduct the experiments 58

Chapter 5 EXPRIMENTAL RESULT AND DISCUSSION 66

5.1 Experimental result and analysis 66

5.2 Discussion 69

Chapter 6 CONCLUSION AND RECOMMENDATION 73

6.1 Conclusion 73

6.2 Recommendation for future work 73

Appendix A 77

Appendix B 79

Appendix C 80

Appendix D 81

List of Figures

Figure 2.1 (a) Schematic drawing of the milling process (b) A 3-axis milling machine 16

Figure 2.2 The classification of milling process (a) Peripheral milling (b) Face milling (c) End milling 17

Figure 2.3 (a) Up milling (conventional milling) (b) Down milling (climb milling) 18

Figure 2.4 The illustration of axial and radial d.o.c 19

Figure 2.5 The illustration of terminologies in the ball burnishing process 20

Figure 2.6 The illustration drawing of the deformation zone in the ball burnishing method 21 Figure 2.7 The relationship between surface roughness and normal burnishing force 22

Figure 2.8 The relation between feed and height of irregularities 23

Figure 2.9 The relationship between the burnishing speed and surface roughness 24

Figure 2.10 The collision between abrasive liquid slurry jet and the workpiece surface with terminologies of impact angle , nozzle diameter d, stand-off distance s 26

Figure 2.11 The schematic drawing of AJP setup 27

Figure 2.12 (a) The premixing pumping system and (b) separate pumping system 27

Figure 2.13 The model of brittle material removal mode according to Lawn in eight steps (a), (b), (c), (d), (e), (f), (g), (h) 29

Figure 2.14 The model of ductile material removal mode in four steps (a), (b), (c), (d) 30

Figure 2.15 Schematic illustration of abrasive fluid jet in AJP 31

Figure 2.16 The footprints of a spot after AJP process under varies of impact angles (a) 90o (b) 60o (c) 45o (d) 30o 33

Figure 2.17 The footprints in case of (a) fixed nozzle (b)(c) moving along x and y axis nozzle and (d) rotating nozzle 34

Figure 2.18 The illustration of surface texture main components 35

Figure 2.19 The illustration of Ra and Rq 36

Figure 2.20 The illustration of Rmax and Rt 37

Figure 2.21 The illustration of Rz 37

Figure 3.1 The steps of Taguchi method to find the optimal parameters 38

Figure 3.2 The illustration of control and noise factors 39

Figure 4.1 The schedule of experimental work 43

Figure 4.2 The BMG sample 44

Figure 4.3 The milling path is generated by the Pro/Engineer software 46

Figure 4.4 The milling process of BMG sample in the machining center 46

Figure 4.5 The innovation ball burnishing tool embedded with a load cell 47

Figure 4.6 The ball burnishing process of BMG sample in machine center 48

Figure 4.7 The drawing of ball burnishing mechanism for BMG material (a) The debris are separated from the crack area, (b) the rubbing lines and fracture zones are created along the burnishing direction 49

Figure 4.8 The image of practical rubbing line and fracture zone in burnishing BMG material observed with 100x optical microscope 50

Figure 4.9 The completed AJP system is installed in the machine center 51

Figure 4.10 The tank and stirring device in (a) rest state (b) in process 52

Figure 4.11 The inverter control pump 53

Figure 4.12 The AJP tool head 54

Figure 4.13 The container with some of outlets The arrows show the direction of the slurry flow which comes back to the tank 55

Figure 4.14 The slurry was inside the tank (a) without mixed hydraulic oil and (b) with mixed hydraulic oil after 2 hours of the AJP process 56

Figure 4.15 The simple cooling setup The arrows show the direction of water which flows in

the hoses 56

Figure 4.16 The CNC machining center 57

Figure 4.17 The procedure of performing the AJP process 58

Figure 4.18 The CNC machine center based AJP setup 59

Figure 4.19 Determining the (a) x and (b) y coordinate of the workpiece zero point using an indicated probe 60

Figure 4.20 The illustration of the distance from the nozzle to the size of the polishing head 60

Figure 4.21 The drawing of the L9 experiment positions in numerical order 61

Figure 4.22 The Toolmaker’s microscope connected with a computer 64

Figure 4.23 The Taylor Hobson Form Talysurf PGI 1240 instrument 64

Figure 4.24 Measuring the surface roughness of BMG sample 65

Figure 5.1 The main effects plot for S/N ratios 67

Figure 5.2 The main effects plot for means 67

Figure 5.3 The surface of polished area 70

Figure 5.4 The surface quality of (a) milling surface (b) polished surface 70

Figure 5.5 The over-polished surface 71

Figure 5.6 The surface roughness for short time and low pressure case 71

Figure 5.7 The load cell measuring value 72

List of Tables

Table 2.1 Several influencing parameters in AJP technology 25

Table 3.1 The regular Orthogonal Arrays for Experiment Design 40

Table 4.1 The chemical and mechanical properties of BMG workpiece 44

Table 4.2 The parameters of milling BMG sample 45

Table 4.3 The parameters of ball burnishing trials on BMG material 47

Table 4.4 The summary of AJP system elements 52

Table 4.5 The fixed factors in Taguchi design experiment 62

Table 4.6 The control factors and levels in AJP experiments 62

Table 4.7 The L9 orthogonal array 62

Table 4.8 The measured roughness to Cut-off 63

Table 5.1 The measured surface roughness after AJP experiments 66

Table 5.2 The average S/N ratio by factor levels (dB) 68

Table 5.3 The average mean value by factor levels ( m) 68

Table 5.4 ANOVA table for S/N ratio for polished surface roughness 68 Table 5.5 The surface roughness value of the tested specimen after verification experiment 69

Chapter 1 INTRODUCTION

1.1 Research motivation

Metallic glass, first discovered in 1960 [1], has been a hot topic in field of material science The development of new bulk metallic glasses (BMGs) from 1980s [2] has opened the door for many applications of this new class of engineering materials In recent years, the new type of BMGs with the excellent mechanical properties including their superior strength and hardness, and corrosion and wear resistance has been developed Furthermore, a number

of research publications have been available on the difference aspects of BMGs such as the glass-forming ability, physical properties, mechanical behavior, magnetic properties, etc However, a review of literature revealed that there were less of attempt to explore the correlation between the machining and polishing conditions and the surface quality after the machining and polishing process Consequently, this thesis aims to investigate the deeper knowledge about the machining and polishing ability of the BMG materials

1.2 Literature review

1.2.1 BMG properties and machining ability of BMGs

BMGs have strange properties They have the randomly arranged atoms and the most other metals have the crystalline structure The mechanical properties of BMGs were reviewed by M M Trexler and N N Thadhani in 2010 [3] Other properties of BMGs such

as the glass-forming ability, physical properties, mechanical behavior, magnetic properties, etc were introduced by C Suryanarayana, A Inoue in 2010 [1] and M Miller, P Liaw in

2008 [2]

M Bakkal [4] in 2003 researched about the chip formation, cutting forces, and tool wear

in the turning process of Zr-based BMGs He also studied about the machinability of BMG materials on the milling and drilling process in 2009 [5] This paper showed that the best surface finish result Ra of 0,115 m were obtained with the low feed rate of 0.25 m/tooth in the BMG milling process with the 1 mm uncoated solid WC end mill tool

M.Q Jiang, L.H Dai [6] in 2009 investigated the formation mechanism of lamellar chips during the machining process of BMGs This research results provided some important references for the BMG cutting technique

A review of literature reveals that no research about the relation between the polishing conditions and the surface quality after polishing of BMG could be found

1.2.2 Ball burnishing process

Recently, surface finishing has been a more and more important field A good surface finish has an effect on a lot of aspects of the machine parts such as the resistance to wear, load-carrying capacity, tool life, corrosion, and fatigue In addition, the ball burnishing process, a method of surface finishing, has been studied by some researchers In 1988, N H Loh, S C Tam [7] had made a literature survey and discussion about the correlation between the ball burnishing parameters such as the burnishing force, the feed rate, the ball material, the ball diameter, the lubricate etc and the surface roughness after the burnishing process

N H Loh, S C Tam [8] in 1989 introduced a study of the effects of ball burnishing parameters on the surface roughness after processing The AISI 1045 specimens were applied the ball burnishing process with 34 factorial design experimental works Besides, the F-test indicated that the significant factors were the ball material, lubricant, feed and depth of penetration The surface roughness of the specimen after ball burnishing process could reach about Rtm0.772 m from the pre-machined surface roughness of Rtm4 m

N H Loh, S C Tam [9] in 2003 proceeded to the ASSAB XW-5 steel (high-carbon, high-chrome steel) with varies of ball burnishing parameters such as the burnishing speed, ball material, lubricant, burnishing forces, depth of penetration, and feed The research showed that the WC (Tungsten carbide) ball provided the best surface finish, grease was better than cutting oil, and some other effects of the ball burnishing parameters on the surface roughness

L Luca, S Neagu-Ventzel, I Marinescu [10] in 2004 used the heat-treated steel components up to 65 HRC for the ball burnishing process In this paper, the surface roughness in the range of grinding (Ra 0.5 m) was presented

F J Shiou, C H Chen [11] in 2003 applied the L-18 Taguchi’s orthogonal array method

to optimize the ball burnishing parameters on the PDS5 moulding steel The optimal burnishing parameters were a grouping of the tungsten carbide ball, the burnishing speed of

200 mm/min, the burnishing force of 300 N and the feed of 40 m The surface roughness of the specimen could be achieved to Ra 0.07 m from the initial value of Ra 1 m with the optimal ball burnishing parameters

F J Shiou, C H Chen [12, 13] in 2008 used the ball burnishing process on the STAVAX and NAK80 mould tool steel The Taguchi’s orthogonal table was employed to optimize the ball burnishing parameters After the ball burnishing process, the ball polishing process was applied to achieve a better surface roughness

F.J Shiou, C H Chuang [14] in 2010 developed an innovation ball burnishing tool with an embedded load cell In this paper, the PDS5 plastic injection mold steel was used as the burnishing specimen material The surface roughness could be increased from Ra 3.0 m on average to Ra 0.08 m The sliding-contact-type of the innovative burnishing tool with the combination of parameters: the lubricant of water-soluble oils (1:50), the ball material of WC

(Co 6%), the burnishing force of 470 N, the feed of 800 mm/min, the step over of 60 m, and the burnishing path orthogonal to the ball milling direction was recommended for the PDS5 mold steel

1.2.3 Abrasive jet polishing (AJP) process

Polishing has been commonly used to improve the surface quality of machine parts and components in the final step of machining AJP is a method of polishing using a flow of abrasive combined with air, water, or water with oil or polymer addictives [15] F Li [16] in

1996 had a dissertation about “experimental and numerical investigation of abrasive water jet polishing technology” In this research, the AJP process was applied to machining the material such as alumina ceramic and stainless steel The effects of AJP process parameters

on the surface quality were examined It demonstrated that the particles size and the jet impact angle was the significant variable in the polishing process Therefore, the surface roughness of Ra 0.3 m was achieved for ceramic and metal using #500 garnets In addition, the trajectory of particles in the abrasive jet was simulated in varies of the impact angle

In 2003, S M Booji [17] in her dissertation inspected possibilities and limitations of the fluid jet polishing technique in BK7 optical glass material The influence of the polishing process parameters such as the particle type and size, particle velocity, nozzle type and impact angle… on the removal rate and final surface roughness was examined An increase

of the surface roughness from average roughness of Ra 300 nm to Ra 3 nm was recorded in one-step process

H Liu, C Z Huang, and J Wang [18] in 2008 had reviewed about the polishing process using abrasive liquid jet as the flexible tool The overview of principles, systems, polishing mechanisms, simulation, and mathematical models… of AJP technology was introduced in this paper

B.H Yan, F.C Tsai, L W Sun, R T Hsu [19] in 2008 applied the AJP on discharge-machined and ground SKD61 mold steel In this research, the #3000 SiC abrasives coated with wax were used as a new technique to improve the surface quality after polishing Consequently, the surface roughness was improved from Ra 0.36 m of the ground surface to

electro-Ra 0.049 m after polishing within 45 min

H T Zhu, C Z Huang, J Wang, Q L Lia, C L Che [20] in 2009 had a study about AJP for hard–brittle materials included Silicate Glass, 96% Alumina and Silicon Nitride The experiments were conducted with low pressure and small erosion angle As a result, the mode

of material removal by a single abrasive particle was the ductile erosion Therefore, the final results were the surface roughness of Ra 93.195 nm of Silicate Glass, Ra 131.22 nm of Alumina, Ra 38.616 nm of Silicon Nitride

1.3 Thesis objectives

The main research objective of this thesis is the study of the surface quality after the processing of milling, ball burnishing and abrasive jet polishing In addition, the effects of the AJP parameters on the surface quality of polished surface are investigated A Taguchi analysis is executed in order to determine the optimal AJP processing condition

With the intention to accomplish the main research objective, some fundamental tasks are formulated below:

- Designing the fixture to clamp the workpiece and pre-machining the surface of the BMG sample with the milling process

- The series of experiments based on Taguchi’s method are executed to determine the optimal ball burnishing and AJP parameters for BMG material

- Analysis of the result to establish the effects of the AJP parameters on the surface quality after processing

Chapter 5 is the final result and the analysis of all experiment data including S/N ratio and ANOVA Moreover, the optimal parameters of the AJP process are determined to achieve the finest surface roughness for BMG material

Chapter 6 is the conclusion of the author and some comments for the future works in the research field

Chapter 2 BACKGROUND INFORMATION

2.1 Milling process and milling parameters

2.1.1 Milling process

Milling machines have a wide range of material cutting capabilities and versatility for executing a variety of machining functions Milling is the process of removing material by a feeding movement of workpiece through a rotating cutter Additionally, most of the milling machine has the column and knee structure for holding the workpiece, rotating the cutter, and feeding the table Figure 2.1 is the schematic illustration of milling process and a typical milling machine

Figure 2.1 (a) Schematic drawing of the milling process (b) A 3-axis milling machine [22]

Milling process has been classified into three types (figure 2.2)

1 In peripheral milling, the axis of the cutter is parallel to the workpiece surface The milled surface is created by the peripheral teeth of the cutter

2 In face milling, the axis of the cutter is perpendicular to the machining surface The peripheral cutting edges and face of the cutter produce the milled area

3 In end milling, the axis of the cutter is vertical to the workpiece The milled surface is generated by the end face and periphery of the cutter

Figure 2.2 The classification of milling process (a) Peripheral milling (b) Face milling (c)

End milling [23]

In addition, there are also two methods of milling basing on the direction of the cutter rotation and feed movement In up milling (conventional milling), the cutter rotates in the opposite direction of the feed travel Contrarily in down milling (climb milling), the cutter rotates in the same direction of the feed movement Besides, the backlash between the lead screw and the nut of the machine table are eliminated in conventional milling Climb milling provides a better milled surface and a longer cutter tool life than conventional milling

n = Spindle speed (rev/min)

vc = Linear cutting speed of material (m/min)

D = Cutter diameter (mm)

2 Feed rate is the rate of the movement of workpice in mm/min

(2.2) Where

f = Feed rate (mm/min)

fz = Feed per tooth (mm/tooth)

z = Number of teeth of cutting tool

3 Depth of cut (d.o.c) is the thickness of material removing from the workpiece in one pass of the cutting tool The illustration of axial d.o.c and radial d.o.c are shown in figure 2.4

Figure 2.4 The illustration of axial and radial d.o.c

2.2 Ball burnishing process

Ball burnishing is a finishing technique without chip removal In the ball burnishing process (figure 2.5), the surface of the workpiece is pressed down by a sliding or rolling contact ball Consequently, the plastic deformation is occurred on the surface layer As a result, an improvement of the surface roughness and mechanical properties of the surface such as the harness, corrosion resistance, and wear resistance is recorded as the advantages of ball burnishing method The main parameters of the ball burnishing process are the burnishing force, feed, ball material, burnishing speed, and type of lubrication The effect of significant ball burnishing parameters on the surface roughness of burnished surfaces is introduced in following section

Figure 2.5 The illustration of terminologies in the ball burnishing process [11]

2.2.1 The simplified theory of ball burnishing deformation

In this simplified case, a burnishing ball is pressed directly on a flat workpiece surface

As the applied force increases, the workpiece surface plastically deforms The deformation is caused by a burnishing ball on the flat surface of a ball burnishing process is demonstrated in figure 2.6 with following assumptions:

1 The burnishing ball does not rotate

2 The friction force between the burnishing ball and workpiece surface is ignored

3 The burnishing ball is undeformed during the ball burnishing process

4 Only the normal burnishing force is considered

Figure 2.6 The illustration drawing of the deformation zone in the ball burnishing method

A = Area of penetration (mm2)

(2.4)

Where

= stress (N/mm2)

2.2.2 Effect of burnishing force on surface roughness [7]

The burnishing force is split into three components: the normal force, orthogonal force and tangential force In ball burnishing process, the normal burnishing force affects significantly on the surface roughness (figure 2.7) As the burnishing force continuously increases, the surface roughness reduces to a lowest point and then starts to rise The ball burnishing parameters such as the workpiece material, burnishing speed, ball diameter, pre-machined surface finish, and feed-rate alter the value of optimum burnishing force Additionally, the effect of orthogonal force and tangential force on the surface roughness is not significant

Figure 2.7 The relationship between surface roughness and normal burnishing force

2.2.3 Effect of feed on surface roughness [7]

The height of irregularities is altered by the feed in ball burnishing process The relationship between height of irregularities and the feed parameter is showed in figure 2.8 A smaller feed produces a lower height of irregularities and then generates a better surface roughness Nevertheless, an extremely low feed causes an over-stressed metal surface layer with irregularities characteristic and hence effects on the surface roughness

Figure 2.8 The relation between feed and height of irregularities

2.2.4 Effect of ball material on surface roughness

Generally, the burnishing ball is required to ensure the following properties: the burnishing ball is negligible undeformed in the burnishing process, high toughness and fatigue strength, high wear resistance, low adhesion to the workpiece surface Diamond, sapphire, ruby, ceramics, hardened steel, carbides, etc are used as the burnishing ball materials There is a small amount of study about the effect of ball material on surface

roughness F J Shiou [11] in 2003 reported that the modification of ball material had not significantly affected on the burnished mould steel PDS5 specimen surface roughness

2.2.5 Effect of burnishing speed on surface roughness [7]

The burnishing speed directly affects on the surface roughness after burnishing The common trend of the relationship between burnishing force speed and surface roughness is illustrated in figure 2.9 As the burnishing speed increases, the surface roughness increases to

a highest point and hence starts to decrease Besides, the feed, burnishing force, ball diameter influences the place of the curve on the graph but the trend is immutable

Figure 2.9 The relationship between the burnishing speed and surface roughness

2.3 AJP process

In abrasive liquid jet polishing, the surface of a workpiece is impacted by a mixture of abrasive particles and addictives such as water, oil or polymer Therefore, the particles under high velocity affect on the surface and hence removing the material Figure 2.10 shows the

fundamental drawing of the AJP method The AJP process is a flexible process with varies of parameters (table 2.1)

Table 2.1 Several influencing parameters in AJP technology

Hydraulic

system

Abrasive particles Addictives

Process parameters Workpiece Nozzle

rotation speed

Initial surface roughness Material

Concentration

in addictives Polymer

Stand-off

The AJP technology has many of advantages such as no contact between the polishing tool and the target surface, low cost system setup in the basic form, cooling tool and removing debris during process, ability to polish the complex profiles, recyclable slurry Furthermore, several of materials such as metals, ceramics, optical glass etc can be polished

by the flow of abrasive liquid slurry Consequently, the AJP method has a wide-range applicable capability to improve the surface quality of diversified components

Figure 2.10 The collision between abrasive liquid slurry jet and the workpiece surface with

terminologies of impact angle , nozzle diameter d, stand-off distance s

2.3.1 AJP system

A basic AJP system is demonstrated in figure 2.11 This system has only few of components The water and addictives (water, oil …) is mixed in the tank by the stirrer Therefore, the mixed slurry is pumped to the polishing head in a specific pressure Consequently, the nozzle guides the slurry jet to inject the target surface The workpiece can

be rotated or travelled one direction to the nozzle The slurry will be recycled during the AJP process

Figure 2.11 The schematic drawing of AJP setup

The jetting system can be divided into two forms, premixing pumping system and separate pumping system according to the method in which abrasives are added in the addictive fluid In premixing system, the abrasives and liquid are premixed in a tank and then the slurry is pumped directly through the nozzle to the target workpiece On the other hand, the abrasives are mixed with the addictive fluid inside the polishing head in separate pumping system The abrasives and fluid are passed to the polishing head in the different lines

Figure 2.12 (a) The premixing pumping system and (b) separate pumping system

The premixing pumping system has a simple structure and easy maintainability Additionally, the slurry can be recycled during the polishing process However, this system is suitable for the low pressure jetting only due to the wear to the system and the limited capability of the slurry pump On the contrary, the separate pumping system operates properly for a high pressure case without the wear to the pump but the abrasives have to be refilled regularly The separate system also has a poor ability to reuse the slurry As a result, the premixing pumping system is used commonly in the AJP setup

2.3.2 Model for the ductile and brittle mode material removal [17]

In order to describe the crack formation of the target surface in AJP process, the ductile and brittle mode material removal mode was introduced In the case of brittle material removal, the surface is impacted by an abrasive particle Therefore, a load is applied to the surface and then a plastically deformed zone with a permanent impression on the surface is created As the load increases, a lateral crack appears due to the growth of plastically deformed zone The lateral crack grows more under effect of the load increasing Later, the load starts to decrease causing the crack to close but the crack still exists Therefore, a median crack comes out under the deformed zone and then extends until it touches another crack or target surface The material zone upper the median crack removes from the surface Figure 2.13 illustrates the process of material removal in the brittle mode

The material removal mode in ductile case is generated in a different method In this case, the load is not large enough to produce a lateral crack Consequently, an impression bounded

a plastically deformed zone is created on the workpiece surface (figure 2.14) This impression still remains after the load is removed If the load is small enough, the material will be elastically deformed and return to its original shape after the load is removed

Figure 2.13 The model of brittle material removal mode according to Lawn [17] in eight

steps (a), (b), (c), (d), (e), (f), (g), (h)

Figure 2.14 The model of ductile material removal mode in four steps (a), (b), (c), (d)

The translation between the ductile mode and brittle mode depends on the critical penetration depth If the particle impacts into the target surface a depth deeper than the critical depth, the material will be removed in the mode of brittle removal case In contrast, the ductile mode occurs when the particle penetrates to the surface a depth smaller than the critical depth Moreover the surface quality associated with the brittle removal mode is worse than that in ductile mode

2.3.3 Parameters of the AJP process

There are many of parameters in the AJP process introduced in table 2.1 In this section, the influence of some fundamental parameters such as the hydraulic system pressure, impact angle, stand-off distance, type and size of abrasive particles, and polishing time in AJP process is presented

Figure 2.15 Schematic illustration of abrasive fluid jet in AJP

1. Hydraulic system pressure and impact angle

The system pressure influents the velocity of the abrasives As the pressure increases, the abrasives in fluid are accelerated Therefore, a deeper penetration is formed by the abrasive particle and a higher material removal rate is established In general, the increase

of polishing pressure causes the increase of surface roughness Moreover, the method of abrasives penetrating into the target surface is also affected by the impact angle (figure 2.15) The depth of penetration of abrasives associates with the impact angle With the aim of achieving a good surface roughness after AJP process, the ductile material removal mode (figure 2.14) is expected As a result, there are two approaches of AJP system in order to make material removal process in ductile mode The first method is the combine

of high system pressure and low impact angle In this method, the abrasive particles crash

into the target surface in a small angle from 1o to 15o [18] and then remove material The other method is the applying of low pressure system The impact angle used in low pressure system is normally 30o to 50o Because of the difficulty in control and the equipment wear, the high pressure system has a limited application in the polishing technique

2. Stand-off distance

When travels along in air, the abrasive fluid jet loses the energy because of the friction between itself and the surrounding medium The slurry distribution of the jet will not modify strongly relative to the travelling path on air Moreover, a longer travel distance causes a higher decreasing of jet kinetic energy For that reason, the depth of penetration will be altered in relation to the stand-off distance

3. Type and size of abrasive particles

The abrasives in AJP technique can be Aluminum Oxide, Silicon Carbide, Garnet, Diamond, etc Each type of abrasives has the specific mechanical and geometry properties Aluminum Oxide and Silicon Carbide is used commonly in the AJP method The properties of Aluminum Oxide and Silicon Carbide Abrasives are introduced in Appendix A Besides, the size is another property of the abrasives The bigger abrasives increase the cutting force and hence enhance the material removal rate

4. Polishing time

It is clear that the polishing time will affect on the amount of material removal The removal volume linearly depends on the polishing time For a long time of AJP process, the pre-machining process marks such as milling or grinding trace can be erased

2.3.4 Footprint on the workpiece after AJP process

The footprint on the target workpiece after AJP process is influenced by many parameters such as impact angle, stand-off distance, polishing time, etc The impact angle considerably affects the geometry of the footprint of AJP method However, the footprint created by the jet

is depended on the cross-section between the jet and a plane describing in figure 2.16

Figure 2.16 The footprints of a spot after AJP process under varies of impact angles (a) 90o

(b) 60o (c) 45o (d) 30o [27]

If the nozzle moves along a line or rotates, the scan footprint will be different The types

of scanning profile are showed in figure 2.17 The darker area expresses the deeper depth of material removal The effect area of the abrasive fluid jet is generally larger than diameter of the nozzle

Figure 2.17 The footprints in case of (a) fixed nozzle (b)(c) moving along x and y axis nozzle

and (d) rotating nozzle [17]

The surface texture is evaluated by the surface profile characterization components such

as roughness, waviness, lay (figure 2.18) The roughness expresses the closely-spaced irregularities of a surface In general, these irregularities are in submicron scale A more

widely spaced component of the surface texture is described by the waviness parameter Moreover, lay is the main direction of the surface texture

Figure 2.18 The illustration of surface texture main components [28]

The surface smoothness is measured by the profilometers The waviness and roughness is filtered from the measurement data The surface roughness is calculated by the statistical method Consequently, the roughness parameters commonly used in industry are defined below:

1 Ra – Roughness Average is the arithmetical mean deviation of all profile values The equation of Ra is expressed below:

(2.5)

Where

L = the length of the profile (evaluation length)

y(x) = profile roughness of the roughness profile

2 Rq – Root mean square Roughness is the root mean square deviation of all profile values

(2.6)

Figure 2.19 The illustration of Ra and Rq

3 Rmax is the largest single peak-to-valley height within 5 of the cutoff lengths

4 Rt is the maximum peak-to-valley height within the assessment length

Figure 2.20 The illustration of Rmax and Rt Where