2. Commercially pure grade 2 titanium

Table 1 shows the chemical composition of the titanium used in this study which is weldable and formable and has excellent corrosion resistance properties. The tensile and yield strength values go up with grade number for pure grades.

Titanium can be cold rolled at room temperature to above 90% reduction in thickness without serious cracking [1]. Titanium undergoes allotropic transformation from the hexagonal close-packed (hcp) alpha phase to the body-centred cubic (bcc) beta phase at a temperature of 883°C. At room temperature, its properties are controlled by chemical composition and grain size. The presence of these elements determines the nature of the alloy and its chemical properties. The density of alpha titanium alloy falls between that of aluminium alloys (2.7 g/cm<sup>3</sup> ) and steel (7.8 g/cm<sup>3</sup> ) at 4.51 g/cm3 as indicated. Due to the high-yield stress values of titanium, which are similar to steels and twice the strength of aluminium, makes this metal a choice in areas where weight is an important consideration [2]. An inhibiting factor especially in the automotive industry is the cost involved in using titanium as the main structural metal, whereas in the aviation industry, the manufacturers are able to include the cost of titanium in the final price of their products. The physical properties of CP titanium and properties like linear expansion coefficient, thermal conductivity and specific heat capacity playing a major role in the laser forming process are shown in Table 2.

## 2.1 Laser forming

Laser forming (LF) evolved from more mature, but less sophisticated thermomechanical forming processes. Specifically, manual application of an oxyacetylene


Table 1.

Chemical composition of commercially pure grade 2 titanium in % wt.


#### Table 2.

and a continued search for improved methods in metal forming. This contributed to the development in forming techniques, materials, processing and understanding of changes a metal goes through during forming. There has always been room for improvement in the forming of materials due to the widespread use of forming operations in the automotive, aviation and shipbuilding industries. An in-depth study into the effects of laser and mechanical forming processes on the mechanical properties of commercially pure grade 2 titanium plates was conducted. This was achieved by producing a radius of curvature of approximately 120 mm on the plates with the aid of the mechanical forming machine. The plate samples were then subjected to mechanical testing to evaluate changes in mechanical properties. A Nd:YAG laser was used to replicate what had been achieved using the mechanical forming machine to bend titanium to the same radius of curvature. It was anticipated that this would lead to an extension of applications of laser forming and the possibility of increasing strength of thin commercially pure (CP) grade 2 titanium plates due to the heat treatment characteristics induced by the process. The laser forming study used established parameter settings which greatly influence the microstructure and bend radii. The intention of the study was to use both mechanical and laser forming to bend titanium plates to a final radius of curvature of 120 mm.

Titanium Alloys - Novel Aspects of Their Manufacturing and Processing

Table 1 shows the chemical composition of the titanium used in this study which

) at 4.51 g/cm3 as indicated. Due to the high-yield stress values of titanium,

) and steel

is weldable and formable and has excellent corrosion resistance properties. The tensile and yield strength values go up with grade number for pure grades.

of alpha titanium alloy falls between that of aluminium alloys (2.7 g/cm<sup>3</sup>

Titanium can be cold rolled at room temperature to above 90% reduction in thickness without serious cracking [1]. Titanium undergoes allotropic transformation from the hexagonal close-packed (hcp) alpha phase to the body-centred cubic (bcc) beta phase at a temperature of 883°C. At room temperature, its properties are controlled by chemical composition and grain size. The presence of these elements determines the nature of the alloy and its chemical properties. The density

which are similar to steels and twice the strength of aluminium, makes this metal a choice in areas where weight is an important consideration [2]. An inhibiting factor especially in the automotive industry is the cost involved in using titanium as the main structural metal, whereas in the aviation industry, the manufacturers are able to include the cost of titanium in the final price of their products. The physical properties of CP titanium and properties like linear expansion coefficient, thermal conductivity and specific heat capacity playing a major role in the laser forming

Laser forming (LF) evolved from more mature, but less sophisticated thermomechanical forming processes. Specifically, manual application of an oxyacetylene

Grade C O2 N2 Max Fe Max H Max Ti Commercially pure titanium (as supplied) 0.005 0.155 0.009 0.04 0.003 Bal

Chemical composition of commercially pure grade 2 titanium in % wt.

2. Commercially pure grade 2 titanium

(7.8 g/cm<sup>3</sup>

process are shown in Table 2.

2.1 Laser forming

Table 1.

26

Physical properties of pure titanium.

torch for forming steel plates for the ship building industry has been used for some time and is seen as the precursor to LF [3].

The laser forming process has become a choice to fabricators of metallic components and as a means of rapid prototyping and of adjusting and aligning [4]. Laser forming is of importance to industries that previously relied on expensive stamping dies and presses for prototype evaluations. Industry sectors making use of this process include aerospace, automotive, shipbuilding and those in microelectronics. The laser forming process involves no mechanical contact, which is a requisite in mechanical forming and is considered a virtual manufacturing kind of method. The laser forming process can be used to produce predetermined shapes. The process results in minimal distortion on the formed components [5]. The laser forming process can produce metallic, predetermined shapes with minimal unwanted distortion, and investigations are also ongoing in the removal of unwanted distortion resulting from the procedure.

A successful and significant research in the laser forming of materials needs a good understanding of thermal transfer concepts as they play a crucial role in the process. Concepts like conduction and thermal radiation need to be understood fully to balance all the process variables. Thermal radiation is the transfer of energy by electromagnetic waves, whereas thermal conductivity, on the other hand, is the property a material possesses indicating its ability to conduct heat. Thermal conductivity of titanium is lower than most competing metals like steel, magnesium and aluminium. This means that in order to cause changes in the microstructure, a higher intensity of heat would have to be emitted by the heat source and in this scenario by the laser. The ability of the plate material to absorb and transfer heat is the major underlying factor. This factor plays a major role in the forming of plates as the effect of conduction affects the microstructure, thereby influencing the mechanical properties. The heat flux (power density), which plays a considerable role in the laser forming process, is the amount of energy flowing through a particular surface area per unit of time and is represented by the following formula:

$$q = \frac{Q}{\pi r^2} \tag{1}$$

where q is the heat flux, Q is the laser beam power (W), r is the beam radius (m), and π is the constant.

According to Ion et al., a large number of variables influence the interaction between a laser beam and a material; over 140 variables can be identified for welding alone. In this instance, the power density will be considered when a beam is switched on. The heat flow becomes steady state, and the energy absorbed by the surface is balanced by that conducted heat into the plate, and the temperature field becomes constant. The principal process variables are the beam power, the beam radius and material properties. The power density can be increased four times by quadrupling the power or by reducing the beam radius to a half. When this variable group is identified, a smaller subset of experiments can be undertaken to establish that the power density determines the peak surface temperature attained. These factors determine the principal mechanism of thermal interaction—which could either be heating, melting or vaporisation [6]. The laser powers used, the thermal conductivity, the line energy, scanning velocities, beam interaction time and also the heat flux (power density) generated during the laser forming process are all shown in Table 3.

Power and scanning velocities were adjusted during the preparation of plate specimens used in this study and the other given parameters resulted from these adjustments (beam interaction time, heat flux and thermal gradient). The heat flux formula was considered for analysis in order to understand the concepts involved in this process. The laser power ranged from 1.5 to 3.5 kW for the specimens evaluated, and an increase in power resulted in an increase to the heat flux and line energy generated. With the arrangement used, the samples are not clamped in any way, and the line heating application alternates in succession from each end incrementally moving towards the centre of the plate. The open mould method shown in Figure 1 was used in the laser forming of the CP grade 2 titanium plates.

The beam interaction time was an important factor in the analysis of the resulting microstructure and can be determined by the formula

$$t = \frac{2r\_b}{v} \tag{2}$$

The variables 2rb and v represent the beam radius and the scanning velocity, respectively. The power density, beam radius and beam interaction time play a considerable role as they determine whether the material will be cut, welded, melted or hardened. The heat flow in laser processing can be complex, but for many processes it may be approximated to three fundamental conditions: steady state, transient or quasi-steady state. Fourier's first law describes steady state conditions as

$$F = -\lambda \Delta T \tag{3}$$

Laser power

29

Thermal

conductivity

Line energy

Scanning velocity

Beam interaction

Heat flux

Thermal gradient

Beam

Average radius of

DOI: http://dx.doi.org/10.5772/intechopen.81807

curvature (mm)

diameter

(mm)

(106 W/m2

)

(103 K/m)

> time (sec)

(kJ/m)

(m/min)

(W/m.K)

(kW)

1.5 1.5 2.5

3 3.5 Table 3. The various parameters

 involved in the laser forming process.

15 15 15 15 15

90

2.3

0.0101

31

2064

12

106.1

90

2

0.0121

26.53

1769

12

118.4

90

1.7

0.0141

22.11

1474

12

134.3

Microstructure and Mechanical Properties of Laser and Mechanically Formed Commercially…

47

1.9

0.0122

13.3

221

12

150.3

35

2.62

0.0091

13.3

221

12

180.1

where F is the heat flux (W/m2), ΔT is the thermal gradient (K/m), and λ is the thermal conductivity (W/m.K). In this state, the temperature field does not change with time at a location in a material. The thermal gradient is a physical quantity that describes in which direction and at what rate the temperature changes most rapidly around a particular location. The thermal gradient can lead to different amounts of contraction in different areas, and if residual tensile stresses become high enough, flaws may propagate and cause failure. A lower thermal gradient may cause bending in other engineering materials but due to differing thermal conductivities may not work in other materials. This means that each engineering material needs to be isolated in the analysis of its physical properties. For example, what may work for steel may not be applicable to titanium due to different thermal conductivities of the two materials. Line energy is a concept used by engineers and scientists in laser forming to control bending characteristics of plates. According to Magee, the energy input to the sheet-metal surface critically affects the nature of the process and forming mechanisms which take place [7].

The line energy specified by Magee is a function of laser power and the scanning velocity. In determining the process parameters for the experimental exercise, four sets of power levels believed to result in the desired curvature were chosen and are listed in Table 3 and discussed here. The laser forming process produces large thermal gradients that could either bend or shorten the material. The bending or shortening of the material is a result of the line energy produced by the laser and is given by the formula

### Microstructure and Mechanical Properties of Laser and Mechanically Formed Commercially… DOI: http://dx.doi.org/10.5772/intechopen.81807


Table 3. The various parameters involved in the laser forming process.

group is identified, a smaller subset of experiments can be undertaken to establish that the power density determines the peak surface temperature attained. These factors determine the principal mechanism of thermal interaction—which could either be heating, melting or vaporisation [6]. The laser powers used, the thermal conductivity, the line energy, scanning velocities, beam interaction time and also the heat flux (power density) generated during the laser forming process are all

Titanium Alloys - Novel Aspects of Their Manufacturing and Processing

Power and scanning velocities were adjusted during the preparation of plate specimens used in this study and the other given parameters resulted from these adjustments (beam interaction time, heat flux and thermal gradient). The heat flux formula was considered for analysis in order to understand the concepts involved in this process. The laser power ranged from 1.5 to 3.5 kW for the specimens evaluated, and an increase in power resulted in an increase to the heat flux and line energy generated. With the arrangement used, the samples are not clamped in any way, and the line heating application alternates in succession from each end incrementally moving towards the centre of the plate. The open mould method shown in

> <sup>t</sup> <sup>¼</sup> <sup>2</sup>rb v

The variables 2rb and v represent the beam radius and the scanning velocity, respectively. The power density, beam radius and beam interaction time play a considerable role as they determine whether the material will be cut, welded, melted

where F is the heat flux (W/m2), ΔT is the thermal gradient (K/m), and λ is the thermal conductivity (W/m.K). In this state, the temperature field does not change with time at a location in a material. The thermal gradient is a physical quantity that describes in which direction and at what rate the temperature changes most rapidly around a particular location. The thermal gradient can lead to different amounts of contraction in different areas, and if residual tensile stresses become high enough, flaws may propagate and cause failure. A lower thermal gradient may cause bending in other engineering materials but due to differing thermal conductivities may not work in other materials. This means that each engineering material needs to be isolated in the analysis of its physical properties. For example, what may work for steel may not be applicable to titanium due to different thermal conductivities of the two materials. Line energy is a concept used by engineers and scientists in laser forming to control bending characteristics of plates. According to Magee, the energy input to the sheet-metal surface critically affects the nature of the process

The line energy specified by Magee is a function of laser power and the scanning velocity. In determining the process parameters for the experimental exercise, four sets of power levels believed to result in the desired curvature were chosen and are listed in Table 3 and discussed here. The laser forming process produces large thermal gradients that could either bend or shorten the material. The bending or shortening of the material is a result of the line energy produced by the laser and is

F ¼ �λΔT (3)

or hardened. The heat flow in laser processing can be complex, but for many processes it may be approximated to three fundamental conditions: steady state, transient or quasi-steady state. Fourier's first law describes steady state conditions as

(2)

Figure 1 was used in the laser forming of the CP grade 2 titanium plates. The beam interaction time was an important factor in the analysis of the

resulting microstructure and can be determined by the formula

and forming mechanisms which take place [7].

given by the formula

28

shown in Table 3.

Figure 1. Laser formed CP grade 2 titanium with the open mould arrangement.

$$L = \frac{P}{v} \tag{4}$$

engineering materials to desired shapes and sizes. These forming operations generally occur after the metal is cast, so it is important to understand how forming operations interact with pre-existing casting defects. Most metal forming operations reduce the severity of casting defects, such as microporosity, and break up coarse particles, such as non-metallic inclusions, that form during solidification. The mating die method was used to bend titanium alloy plates to the desired radius of curvature. The mating die method of stretch-draw forming involves an upper and lower die block mounted in a hydraulic press bed. The workpiece is securely held in tension by movable grippers. Yield stress of the finished part may be increased as much as 10% by the stretching and cold working operations. Shown in Figure 3 is how the bending of titanium plates was achieved using the mechanical forming

Microstructure and Mechanical Properties of Laser and Mechanically Formed Commercially…

The objective of this study was to bend a flat plate of titanium to a radius of 120 mm, and this would help in understanding the principles behind the mechanical forming process. The study also aimed at comparing mechanical to laser forming with regard to the microstructure and mechanical properties of the material and any

The mechanical properties of CP grade 2 titanium alloy vary with its grade as indicated in Table 4. CP grade 2 titanium plate specimens were evaluated according to the American Society for Testing and Materials (ASTM) E8/E8M test method. The tensile test was performed on the parent material (CP grade 2 titanium plates)

The resulting data were made available using computer software of the machine. The table shows average values taken from both the transverse and longitudinal directions of the plate. The ultimate tensile strength (the maximum engineering stress in tension that may be sustained without fracture) is given as 452 MPa, the

Alloy E (GPa) δ 0.2 (MPa) UTS (MPa) Elongation (%) Grade 1 105 170 240 24 Grade 2 105 275 345 20 Grade 2 (current study) 105 338 452 28 Grade 3 105 380 445 18 Grade 4 105 480 550 15

changes that happen thereafter as a result of both forming operations.

in accordance with ASTM E8, using the Hounsfield machine.

yield 338 MPa and a percentage elongation of 28%.

machine and also the resulting shape.

DOI: http://dx.doi.org/10.5772/intechopen.81807

2.3 Tensile test

Figure 3. Mechanical forming.

Table 4.

31

Mechanical properties of CP grade 2 titanium alloy.

where P represents laser power in Watts (W) and v represents the scanning velocity in metres per minute, respectively. Line energy is the most important variable in the laser forming process. It was also decided to determine what influence the variation in power levels would have on the microstructure and the mechanical properties of titanium [8]. There is a widespread belief that a line energy threshold must be exceeded in order to commence with permanent deformation by the temperature gradient method (TGM) [9]. A line energy of 90 kJ/m was considered after unsuccessful attempts to bend samples at a power of 1.5 kW where the line energies of 35 and 47 kJ/m, respectively, were used. For powers ranging from 2.5 to 3.5 kW, the line energy was kept constant at 90 kJ/m, and the scanning velocities were adjusted to suit the required line energy, in this instance 90 kJ/m.

The prime pocket monitor shown in Figure 2 was used in this study to measure the laser power projected on the surface of titanium specimens. Readings were taken to fully understand the incident power hitting the titanium plate surface. As an example for a laser power setting of 3500 W, the pocket monitor reader would show 3250 W. This value indicates a 10% loss in power on the irradiated sample [8]. This assisted in understanding and acknowledging the presence of losses in laser irradiation in material processing. For the purpose of this study, the losses were ignored and not taken into consideration in the analysis.

#### 2.2 Mechanical forming

Metals are used extensively as engineering materials in part because of their ability to deform plastically. Various forming processes are used to form

Figure 2. Prime pocket monitor.

Microstructure and Mechanical Properties of Laser and Mechanically Formed Commercially… DOI: http://dx.doi.org/10.5772/intechopen.81807

engineering materials to desired shapes and sizes. These forming operations generally occur after the metal is cast, so it is important to understand how forming operations interact with pre-existing casting defects. Most metal forming operations reduce the severity of casting defects, such as microporosity, and break up coarse particles, such as non-metallic inclusions, that form during solidification. The mating die method was used to bend titanium alloy plates to the desired radius of curvature. The mating die method of stretch-draw forming involves an upper and lower die block mounted in a hydraulic press bed. The workpiece is securely held in tension by movable grippers. Yield stress of the finished part may be increased as much as 10% by the stretching and cold working operations. Shown in Figure 3 is how the bending of titanium plates was achieved using the mechanical forming machine and also the resulting shape.

The objective of this study was to bend a flat plate of titanium to a radius of 120 mm, and this would help in understanding the principles behind the mechanical forming process. The study also aimed at comparing mechanical to laser forming with regard to the microstructure and mechanical properties of the material and any changes that happen thereafter as a result of both forming operations.

#### 2.3 Tensile test

<sup>L</sup> <sup>¼</sup> <sup>P</sup> v

where P represents laser power in Watts (W) and v represents the scanning velocity in metres per minute, respectively. Line energy is the most important variable in the laser forming process. It was also decided to determine what influence the variation in power levels would have on the microstructure and the mechanical properties of titanium [8]. There is a widespread belief that a line energy threshold must be exceeded in order to commence with permanent deformation by the temperature gradient method (TGM) [9]. A line energy of 90 kJ/m was considered after unsuccessful attempts to bend samples at a power of 1.5 kW where the line energies of 35 and 47 kJ/m, respectively, were used. For powers ranging from 2.5 to 3.5 kW, the line energy was kept constant at 90 kJ/m, and the scanning velocities were adjusted to suit the required line energy, in this instance

The prime pocket monitor shown in Figure 2 was used in this study to measure

the laser power projected on the surface of titanium specimens. Readings were taken to fully understand the incident power hitting the titanium plate surface. As an example for a laser power setting of 3500 W, the pocket monitor reader would show 3250 W. This value indicates a 10% loss in power on the irradiated sample [8]. This assisted in understanding and acknowledging the presence of losses in laser irradiation in material processing. For the purpose of this study, the losses were

Metals are used extensively as engineering materials in part because of their

ability to deform plastically. Various forming processes are used to form

ignored and not taken into consideration in the analysis.

Laser formed CP grade 2 titanium with the open mould arrangement.

Titanium Alloys - Novel Aspects of Their Manufacturing and Processing

90 kJ/m.

Figure 2.

30

Prime pocket monitor.

Figure 1.

2.2 Mechanical forming

(4)

The mechanical properties of CP grade 2 titanium alloy vary with its grade as indicated in Table 4. CP grade 2 titanium plate specimens were evaluated according to the American Society for Testing and Materials (ASTM) E8/E8M test method. The tensile test was performed on the parent material (CP grade 2 titanium plates) in accordance with ASTM E8, using the Hounsfield machine.

The resulting data were made available using computer software of the machine. The table shows average values taken from both the transverse and longitudinal directions of the plate. The ultimate tensile strength (the maximum engineering stress in tension that may be sustained without fracture) is given as 452 MPa, the yield 338 MPa and a percentage elongation of 28%.

Figure 3. Mechanical forming.


#### Table 4.

Mechanical properties of CP grade 2 titanium alloy.
