3. Conclusions

The primary motivation of this study was to investigate, analyse, characterise and compare laser and mechanical forming processes. The study focussed on the main parameters that influence the bending of plates and their effect on the microstructure and mechanical properties. New theories and discoveries are discussed in the context of contribution to the subject and body of knowledge which is wide and immense in scope. Theories and conclusions are as follows:

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

The use of thinner gauge material: The study has come up with a new application for the laser formed titanium. Laser formed titanium plates become extremely hard and could be used in the defence industry for bullet-proof body vests and applied on armoured vehicles. Titanium is light in weight and coupled with a hardened surface resulting from laser forming could be a viable solution. Current armoured vehicles are heavy and slow due to the materials used, and venturing into materials like titanium could be a breakthrough to the defence industry. The laser forming process has the ability to customise the mechanical properties of any material, and therefore with this possibility thinner gauge material could be used for the benefit of this industry.

The control of the radius of curvature: Controlling the radius of curvature using the laser forming process is complex and results in uncontrollable bending of the material. Magee et al. saw a great potential for accuracy and controllability on the amount of forming with the laser, but the current study contradicts what he thought possible with the process. Titanium has proved its unpredictability in this study resulting in no proper control of the radius of curvature as envisaged.

The line energy: This is the fraction of the laser power and traverse speed. According to Magee there is a critical energy input below which no plastic straining occurring in each experiment. The study agrees with Magee on the fact that a higher line energy results in more pronounced bending of the material. Maintaining a constant line energy does not result in same bending of plate specimens from the irradiated batch of plates. An increase in laser power increases the line energy and thermal gradient which all determine the extent of bending in titanium. The use of higher line energies compromises fatigue properties of titanium as there is no proper control of temperature, and therefore precise thermo-mechanical control is needed for the success of this process.

The industrial use of laser forming: Contrary to what was envisaged on initiating this study, the laser forming process does not pose a challenge to current popular forming methods. At the moment the best process for forming is mechanical forming due to the ease with which any desired shape can be formed in minimal time. Laser forming is much slower than mechanical forming, and changes brought by the laser forming process could be undesirable to other industrial applications.

The thermal gradient: The low thermal conductivity of titanium means higher thermal gradients are needed for pronounced bending of CP grade 2 titanium. Higher thermal gradients result in higher residual stresses in the material and a complete change in the physical properties of the material. Changes in physical properties could be desirable or less desirable depending on industrial application.

Surface hardening: The laser forming process resulted in surface hardening of CP grade 2 titanium plates. This had a negative effect on the fatigue life of specimens, as there was a reduction in fatigue life. This is contrary to the findings by Konstantino and Altus [11] who reported improvements in fatigue life of Ti-6Al-4 V which behaves in the same manner as CP grade 2 titanium plates. The improvement in fatigue life was achieved by laser heating based on reduced fraction of α (alpha) in the microstructure and a reduction in grain size. In this study there was an increase in grain size as a result of laser heating which completely changed the granular structure of the material. A significant microstructural refinement was observed during this study resulting in the formation of α-martensite. Hardness values are dependent on the line energy generated during the laser forming of titanium. The higher the line energy, the higher will the hardness be for CP grade 2 titanium plates. The laser-irradiated surface hardens as a result of the laser forming process making it difficult to polish and prepare plates for residual stress measurements. In laser formed CP grade 2 titanium plates, the hardness changes with specimen depth as a result of the effects of thermal energy from the laser, which is the heat source.

life as they changed the location of the fracture line. The maximum principal stress at a power of 3000 W is obtained at a depth of 2 mm. The changes in residual stress and strain are closer to the surface of the irradiated plate. The laser forming process increases the hardness of titanium. Residual stresses in this study are a result of interactions between time, temperature and the material. These factors played a major role in the resulting residual stress layout on all laser formed plates. The effect of the thermal gradient is evident when these plates are compared with plates not affected by thermal energy. The highest temperature gradient was obtained at a power of 3500 W. The thermal gradient became a deciding factor in microstructural layout. Even though the line energy was the same from a power of 2500 W up to a power of 3500 W, the effects on the microstructure were not uniform. This led to the conclusion that the thermal gradient is the most influential factor in laser

Titanium Alloys - Novel Aspects of Their Manufacturing and Processing

The maximum stress obtained at 3.5 kW was the second highest at 181.9 MPa (T) and a minimum stress of 1.4 MPa (T). The hardness of plates was equivalent to the parent material, but this is where similarities end. The difference in stress was 185 MPa on the laser-processed plates. This difference in stress is related to changes in hardness of titanium as a result of laser forming process. The differences in temperature between 3000 W and 3500 W played no role in influencing minimum and maximum residual stress. The optimum settings for a line energy of 90 kJ/m

The primary motivation of this study was to investigate, analyse, characterise and compare laser and mechanical forming processes. The study focussed on the main parameters that influence the bending of plates and their effect on the microstructure and mechanical properties. New theories and discoveries are discussed in the context of contribution to the subject and body of knowledge which is wide and

immense in scope. Theories and conclusions are as follows:

forming [8] (Figure 15).

Figure 15.

Relieved stress (3.5 kW).

are at a power of 2.5 kW [8].

3. Conclusions

42

The residual stress: In all the forming processes analysed, changes in residual stress are greatly influenced by process specifications. In laser forming however these changes are dependent on the process parameters used, as these differ with each laser power. Thermal gradient influences the development of residual stresses. In mechanical forming changes in residual stress are determined by the complexity of the formed shape. It was envisaged at the beginning of the study that residual stress would be enhanced but the laser forming process made undesirable changes to the underlying residual stress distribution. According to Norton [5] good design requires that an engineer try to tailor the residual stresses to a minimum, not create negative effects on the strength and preferably to create positive effects. Fatigue failure is a tensile residual stress phenomenon. The laser forming process resulted in increased tensile residual stress in the specimens due to higher line energies. The use of lower line energies on CP grade 2 titanium results in no bending of the material, and therefore high tensile residual stress remains part of the process if there is forming to be done. The parent material and mechanically formed plates had low residual stress values which was an advantage during fatigue testing as these had a higher fatigue life.

Microstructure: The mechanical forming process has a minimal influence on the microstructure of CP grade 2 titanium plate specimens compared to the effects of laser forming. The laser forming process results in changes in grain size as thermal energy is increased. Changes in the microstructure influenced the mechanical properties of CP grade 2 titanium plates.

Beam interaction time: The beam interaction time is significant in the analysis of resulting mechanical properties as a result of the laser forming process. The time taken to heat up an area influences the mechanical properties and the microstructure of plate samples.

Heat flux: The heat flux is significant in changes observed with titanium plates, and each laser power setting evaluated had a different reading, resulting in varying microstructures on the plates evaluated. The depth of laser penetration depends on the amount of line energy generated.

Laser power: An increase in laser power leads to the oxidation of the passive layer on CP grade 2 titanium plates as a result of the concentrated thermal energy generated by the laser.

Forming parameters: The changing of forming parameters in the laser forming of CP grade 2 titanium succeeded in obtaining optimum operating parameters (in the case of this research, a power of 2.5 kW, a line energy of 90 kJ/m and a scanning velocity of 1.67 m/min) for titanium.

Author details

45

Kadephi Vuyolwethu Mjali<sup>1</sup>

Port Elizabeth, South Africa

Cape Town, Western Cape, South Africa

provided the original work is properly cited.

\* and Annelize Botes2,3

Microstructure and Mechanical Properties of Laser and Mechanically Formed Commercially…

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

1 iThemba LABS—Laboratory for Accelerator Based Sciences, Somerset West,

2 Council for Scientific and Industrial Research (CSIR), Pretoria, South Africa

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

3 Department of Mechanical Engineering, Nelson Mandela University,

\*Address all correspondence to: vuyo.mjali@gmail.com

Plates: Laser formed titanium plates should not be used in applications requiring prolonged fatigue life. The process is only beneficial in applications where hardness is a priority without a need for high fatigue life, and perhaps the process could be beneficial in military defence applications. Laser formed titanium plates do not bend to the same radius of curvature as proved in this study.

Process control: The process needs precision control of processing parameters as they play a major role in the final microstructural layout and mechanical properties. For good fatigue properties, thermo-mechanical/laser processing of titanium needs to be conducted using lower line energies but then these do not bend the material.

### Acknowledgements

Nelson Mandela University (NMMU) for financial assistance and laboratory facilities. Mr. Victor Ngea-Njoume for technical assistance.

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