**4. AWJ machining operations and process parameters**

AWJ technology is being utilized by many modern industries such as automotive, aerospace, chemical processing, environmental engineering, construction engineering, medical, etc., for performing different operations such as industrial cleaning, surface preparation, paint, enamel and coating stripping, manufacturing operations, etc. Since this chapter is mainly focused on machining aspects, only linear cutting, drilling, turning, and milling machining operations are discussed. AWJM involves a number of process parameters to achieve qualitative and quantitative results. Geometry and quality of the machined surfaces depend mainly on the appropriate selection process parameters. **Table 1** shows detailed AWJ process parameters for different machining operations.

In the present chapter, in order to identify the machining capability of Mg-based materials, experimental investigations on AWJ linear cutting and AWJ drilling operations were carried out. The effect of input parameters on the depth of penetration and surface integrity of AZ91 alloy and nanocomposites was analyzed. AWJ drilling of AZ91 Mg alloy was carried out and compared with conventional and jig boring processes. Overview of AWJ is turning, and AWJ milling operations were discussed. Further research possibilities in AWJ machining Mg-based materials are also highlighted.


**Table 1.**

*AWJM process parameters for different operations [33].*

#### **4.1 AWJ linear cutting**

Being most important in industrial applications, depth of penetration and surface quality decides the efficiency, process capability, and performance of AWJ. During past decades theoretical and experimental investigations were made to determine the cutting performance of ductile materials such as aluminum and its alloys, mild steel, stainless steel, copper, brass, etc., with AWJ machining technology [34–36]. However, limited studies are available on AWJ machining of Mg-based materials. Therefore in the present section, experimental investigations were carried out to know the penetration capability and surface quality of AZ91 Mg alloy and nanocomposites by AWJ linear cutting. OMAX 1515 state-of-the-art AWJ machine equipped with 30 Hp direct drive pump to create water pressure ranging 100 MPa to 345 MPa shown in **Figure 1** was used to carry out experiments. The machine also supported with gravity feed-type abrasive hopper and pneumatically controlled three axis movements with traverse speed capacity of 1 mm/min to 8000 mm/min.

AZ91 Mg alloy and nanocomposites with volume percentage of 1, 1.5, and 2% Al2O3 nanoparticles (<50 nm) produced by stir casting were used to conduct experiments. Elemental composition of AZ91 is shown in **Table 2** (obtained by EDS) analysis. **Figure 2** shows EDS spectrum of AZ91-nanocomposites containing 1% Al2O3.

**67**

*Abrasive Water Jet Cutting: A Risk-Free Technology for Machining Mg-Based Materials*

**Al Zn Mn Si Cu Fe Be Magnesium** 9.41 1.42 0.25 0.03 0.003 0.015 0.001 Rest

In the present study, influence of only major dynamic parameters such as water pressure, traverse speed, and abrasive mass flow rate on the depth of penetration and surface integrity is considered. Topography and microstructural features of cut surfaces were examined using SEM technique. All the linear cutting experiments were carried out using standard 80 mesh garnet particles. L18 orthogonal array was used to conduct cutting experiments. Specimen was prepared into trapezoidal shape to make through cuts. This method of making trapezoidal shape is popular among different methods to determine the DOP. Profile projector was used to obtain exact depth of cuts since this method gives accurate values compared to

The effect of control factors such as water pressure, traverse speed, and mass flow rate on the depth of penetration was analyzed by considering mean values of depth of penetration. **Figure 3** shows the effect of input parameters on the depth of penetration in AZ91 magnesium alloy and nanocomposites. It can be observed that depth of penetration increases with an increase in water pressure, due to the increase in kinetic energy of water and abrasive particles with an increase in water pressure. In contrast, more energy of the particles as well as water was able to remove more material. It is also observed that, when compared to Az91 Mg alloy, penetration ability of Mg nanocomposites decreases with increase in vol. % of Al2O3, since MMCs offer resistance to the jet penetration due to the presence of harder

other methods [37]. **Table 3** shows detailed experimental conditions.

*4.1.2.1 Effect of input parameters on the depth of penetration*

*DOI: http://dx.doi.org/10.5772/intechopen.85209*

*Elemental composition of AZ91 Mg alloy.*

**Table 2.**

**Figure 2.**

*4.1.1 Experimental conditions*

*EDS spectrum of AZ91/1% Al2O3 nanocomposites.*

*4.1.2 Results and discussion*

nanoceramic particles.

**Figure 1.** *State-of-the-art AWJM technology.*

*Abrasive Water Jet Cutting: A Risk-Free Technology for Machining Mg-Based Materials DOI: http://dx.doi.org/10.5772/intechopen.85209*


#### **Table 2.**

*Magnesium - The Wonder Element for Engineering/Biomedical Applications*

**AWJ parameters Machining parameters**

SOD AOI TS Material thickness Number of passes

DOP Width of cut Surface finish

*AWJM process parameters for different operations [33].*

Being most important in industrial applications, depth of penetration and surface quality decides the efficiency, process capability, and performance of AWJ. During past decades theoretical and experimental investigations were made to determine the cutting performance of ductile materials such as aluminum and its alloys, mild steel, stainless steel, copper, brass, etc., with AWJ machining technology [34–36]. However, limited studies are available on AWJ machining of Mg-based materials. Therefore in the present section, experimental investigations were carried out to know the penetration capability and surface quality of AZ91 Mg alloy and nanocomposites by AWJ linear cutting. OMAX 1515 state-of-the-art AWJ machine equipped with 30 Hp direct drive pump to create water pressure ranging 100 MPa to 345 MPa shown in **Figure 1** was used to carry out experiments. The machine also supported with gravity feed-type abrasive hopper and pneumatically controlled three axis movements with traverse speed capacity of 1 mm/min to 8000 mm/min.

Diameter Drilling time Hole shape

**Linear cutting Drilling Milling Turning**

TR Lateral increment Number of passes Number of sweeps

VMR Depth control Rotational speed Direction of rotation Angle Traverse rate Initial diameter Final diameter DOC

Turned diameter Surface finish Machining time

Angle SOD Dwell time Material thickness

AZ91 Mg alloy and nanocomposites with volume percentage of 1, 1.5, and 2% Al2O3 nanoparticles (<50 nm) produced by stir casting were used to conduct experiments. Elemental composition of AZ91 is shown in **Table 2** (obtained by EDS) analysis. **Figure 2** shows EDS spectrum of AZ91-nanocomposites containing 1% Al2O3.

**66**

**Figure 1.**

*State-of-the-art AWJM technology.*

**4.1 AWJ linear cutting**

**Table 1.**

Output variables (dependent)

Water pressure Jet diameter Abrasive particle size Abrasive material Abrasive flow rate Abrasive condition Mixing tube length Mixing tube diameter

*Elemental composition of AZ91 Mg alloy.*

#### **Figure 2.**

*EDS spectrum of AZ91/1% Al2O3 nanocomposites.*

#### *4.1.1 Experimental conditions*

In the present study, influence of only major dynamic parameters such as water pressure, traverse speed, and abrasive mass flow rate on the depth of penetration and surface integrity is considered. Topography and microstructural features of cut surfaces were examined using SEM technique. All the linear cutting experiments were carried out using standard 80 mesh garnet particles. L18 orthogonal array was used to conduct cutting experiments. Specimen was prepared into trapezoidal shape to make through cuts. This method of making trapezoidal shape is popular among different methods to determine the DOP. Profile projector was used to obtain exact depth of cuts since this method gives accurate values compared to other methods [37]. **Table 3** shows detailed experimental conditions.

#### *4.1.2 Results and discussion*

#### *4.1.2.1 Effect of input parameters on the depth of penetration*

The effect of control factors such as water pressure, traverse speed, and mass flow rate on the depth of penetration was analyzed by considering mean values of depth of penetration. **Figure 3** shows the effect of input parameters on the depth of penetration in AZ91 magnesium alloy and nanocomposites. It can be observed that depth of penetration increases with an increase in water pressure, due to the increase in kinetic energy of water and abrasive particles with an increase in water pressure. In contrast, more energy of the particles as well as water was able to remove more material. It is also observed that, when compared to Az91 Mg alloy, penetration ability of Mg nanocomposites decreases with increase in vol. % of Al2O3, since MMCs offer resistance to the jet penetration due to the presence of harder nanoceramic particles.

### *Magnesium - The Wonder Element for Engineering/Biomedical Applications*


#### **Table 3.**

*Detailed machining conditions selected for cutting experiments.*

#### **Figure 3.** *Effect of input parameters on depth of penetration.*

Relationship between traverse speed and depth of penetration indicates that increasing the traverse speed lowers the impact of numbers of abrasive particles and results in decreasing the depth of penetration. The influence of traverse speed on the depth of penetration, therefore, lies on the exposure time of abrasive water jet. The lesser the exposure time, the more the depth of penetration. The effect of mass flow rate on the depth of penetration indicating an increase in abrasive mass flow increases the depth of penetration due to the participation of large number of abrasive particles in cutting. This trend remains stable till the value of mass flow rate reaches to a critical level. After that depth of penetration decreases since the higher abrasive flow rates sometimes block the nozzle and at higher abrasive mass

**69**

**Figure 4.**

*Material removal mechanism.*

*Abrasive Water Jet Cutting: A Risk-Free Technology for Machining Mg-Based Materials*

flow rates. Due to some damping mechanisms such as generation of water-solid films and abrasive particles, collision in mixing chamber significantly reduces the specific energy of abrasive particles [38]. It is evident from the previous studies that the velocity of abrasive particles decreases with an increase in abrasive mass flow

Literatures have witnessed solid particle impact in the material removal process by abrasive water jets by erosion [42]. In ductile materials, material removal process is divided into two zones such as micro cutting zone observed at the top surface and deformation zone, which occurs at the bottom surface [43]. Similar observations were noted down in surfaces of MMC [44]. In the micro cutting region, material removal takes place by sharp-edged and angular abrasive particles at low cutting depths. Impact of the abrasive particles at shallow angles promotes micro cutting, termed as "cutting wear." At larger cutting depths, the impact angle of abrasive particles becomes more obtuse and causes "deformation wear." Plowing is responsible for material removal in the bottom region due to spherical abrasive particles [45]. Deformation wear zone is affected by surface waviness caused due to instabilities of the water jet. Loss of jet energy reduces the capacity of material removal creating waviness and thus dividing

A surface characteristic of cut surfaces is evaluated in two regions, i.e., smooth cutting zone (SCZ) and rough cutting zone (RCZ) as shown in **Figure 5** [46, 47]. Surface integrity of cut surfaces can be obtained by measuring surface roughness in both regions using either a contact-type or noncontact-type measuring device. Most of the researchers have used optical profiler for measuring surface roughness which is the noncontact type. Several researchers have used contact-type roughness measuring devices, which use a stylus tip to measure waviness of the surface. In the SCZ, initially the surfaces are clear and smooth but affected by the formation of uniform wear tracks in the direction of the jet traverse at the bottom portion of the SCZ. Since this zone is shared by both micro cutting and deformation modes, surface roughness increases further down the surfaces. In RCZ the surface roughness is affected by striations caused by deflected water jet. **Figure 5** shows the presence of deviated wear tracks with waviness produced by a stream of the

*DOI: http://dx.doi.org/10.5772/intechopen.85209*

*4.1.3 Topography of cut surfaces*

*4.1.4 Integrity of the cut surfaces*

rate [39–41]. This reduces the depth of penetration.

cutting and deformation wear zones as shown in **Figure 4**.

#### *Abrasive Water Jet Cutting: A Risk-Free Technology for Machining Mg-Based Materials DOI: http://dx.doi.org/10.5772/intechopen.85209*

flow rates. Due to some damping mechanisms such as generation of water-solid films and abrasive particles, collision in mixing chamber significantly reduces the specific energy of abrasive particles [38]. It is evident from the previous studies that the velocity of abrasive particles decreases with an increase in abrasive mass flow rate [39–41]. This reduces the depth of penetration.

### *4.1.3 Topography of cut surfaces*

*Magnesium - The Wonder Element for Engineering/Biomedical Applications*

Abrasive mesh size 80

Angle of target 90° Standoff distance mm 1.5

*Detailed machining conditions selected for cutting experiments.*

Orifice (jewel) diameter mm 0.35 and material is sapphire Focusing nozzle diameter mm 0.76 and material is tungsten carbide

Water pressure (*Pw*) Unit Level 1 Level 2 Level 3

Traverse speed (*ts*) mm/min 150 300 400 Mass flow rate (*mf*) g/min 309 425 611

MPa 100 200 300

**Dynamic parameters**

**Constant parameters**

**Table 3.**

**68**

**Figure 3.**

*Effect of input parameters on depth of penetration.*

Relationship between traverse speed and depth of penetration indicates that increasing the traverse speed lowers the impact of numbers of abrasive particles and results in decreasing the depth of penetration. The influence of traverse speed on the depth of penetration, therefore, lies on the exposure time of abrasive water jet. The lesser the exposure time, the more the depth of penetration. The effect of mass flow rate on the depth of penetration indicating an increase in abrasive mass flow increases the depth of penetration due to the participation of large number of abrasive particles in cutting. This trend remains stable till the value of mass flow rate reaches to a critical level. After that depth of penetration decreases since the higher abrasive flow rates sometimes block the nozzle and at higher abrasive mass

Literatures have witnessed solid particle impact in the material removal process by abrasive water jets by erosion [42]. In ductile materials, material removal process is divided into two zones such as micro cutting zone observed at the top surface and deformation zone, which occurs at the bottom surface [43]. Similar observations were noted down in surfaces of MMC [44]. In the micro cutting region, material removal takes place by sharp-edged and angular abrasive particles at low cutting depths. Impact of the abrasive particles at shallow angles promotes micro cutting, termed as "cutting wear." At larger cutting depths, the impact angle of abrasive particles becomes more obtuse and causes "deformation wear." Plowing is responsible for material removal in the bottom region due to spherical abrasive particles [45]. Deformation wear zone is affected by surface waviness caused due to instabilities of the water jet. Loss of jet energy reduces the capacity of material removal creating waviness and thus dividing cutting and deformation wear zones as shown in **Figure 4**.

### *4.1.4 Integrity of the cut surfaces*

A surface characteristic of cut surfaces is evaluated in two regions, i.e., smooth cutting zone (SCZ) and rough cutting zone (RCZ) as shown in **Figure 5** [46, 47]. Surface integrity of cut surfaces can be obtained by measuring surface roughness in both regions using either a contact-type or noncontact-type measuring device. Most of the researchers have used optical profiler for measuring surface roughness which is the noncontact type. Several researchers have used contact-type roughness measuring devices, which use a stylus tip to measure waviness of the surface.

In the SCZ, initially the surfaces are clear and smooth but affected by the formation of uniform wear tracks in the direction of the jet traverse at the bottom portion of the SCZ. Since this zone is shared by both micro cutting and deformation modes, surface roughness increases further down the surfaces. In RCZ the surface roughness is affected by striations caused by deflected water jet. **Figure 5** shows the presence of deviated wear tracks with waviness produced by a stream of the

**Figure 4.** *Material removal mechanism.*

**Figure 5.** *Regions of cut surfaces.*

**Figure 6.** *3-D images of SCZ and RCZ.*

deflected water jet. In cutting AZ91 magnesium alloy, the embedment of spherical abrasive particles and pocket-like structures is observed at the shallow impact angles as shown in **Figure 4**. The surface roughness at RCZ zone is higher than SCZ zones. **Figure 6** shows the 3-D images of SCZ and RCZ generated in optical profiler.

The surface roughness of the AZ91 and nanocomposites in the SCZ zones is found between 5–20 μm and 20–40 μm in RCZ at lower traverse speeds. This is due to the increase in the impact of a number of abrasive particles and exposure time. Surface roughness is a function of water pressure because the kinetic energy of water jet increases the velocity of particles which in turn increases the surface quality, and also the influence of abrasive mass flow rate on surface roughness depends mainly on water pressure. However, at higher traverse speeds, an increase in water pressure increases the surface roughness. Hence traverse rate is the most significant parameter in deciding the quality of cut at two zones.
