2.4 Microstructure

Figure 4 shows microstructures from the parent material specimens, and this material has equiaxed α-grains usually developed by annealing cold-worked alloy above recrystallization temperature. The microstructure has shown results from the manufacturing process of CP grade 2 titanium, which cannot be altered in the plates without the addition of heat or cold deformation processes.

The microstructure of mechanically formed plates contains the same equiaxed alpha grains found in the parent material. Mechanical forming produces no heat, and therefore the similarities in microstructure are to be expected. There were no major changes to the microstructure as a result of this process when compared to the as received material [10]. The microstructure of a mechanically formed plate is shown in Figure 5.

Figure 6 shows the fine structure of titanium from the plates irradiated at a power of 1500 W using line energies of 35 and 47 kJ/m, respectively. There is a variation in the depth of the heat-affected zone (HAZ) for both line energies [10]. The unaffected material in both cases has equiaxed α-grains similar to those in the parent material. Based on microstructural observations, it becomes clear that the temperature generated at 35 kJ/m was less than that generated at 47 kJ/m as it could

not penetrate as deep to effect changes deeper in the microstructure away from the

Microstructure and Mechanical Properties of Laser and Mechanically Formed Commercially…

The resulting microstructure from a line energy of 35 kJ/m points to a higher scanning velocity. The thermal energy from the laser managed to effect changes to a quarter of the plate's thickness [10]. The cycle took around 18 minutes to irradiate all the 10 plates in each batch at this power and line energy setting. The laser forming process resulted in a semi-circular-shaped heat-affected zone in the lower line energies (35 and 47 kJ/m). The area not affected by the heat in both cases shows smaller grains than those in the heat-affected zone. Grain size depends largely on temperature attained during the laser forming process, and grain growth proceeds more quickly as temperature increases. All the figures shown clearly reveal the influence of temperature on the microstructure, and the portion affected by laser energy has grains which are much bigger than those not affected by heat [10].

Figure 7 shows a major change in the microstructure of CP grade 2 titanium plates with enlarged primary α-grains and enlarged β-grains (2.5 kW). The structure consists of much bigger equiaxed alpha grains in the structure. The resulting microstructure is a result of thermal energy developed by the process parameters on the plates irradiated. Thermal energy is the main initiator in microstructural layout in all the laser formed plates. The microstructure managed to change only halfway through the plate which explains why there was minimal bending on the plates irradiated. Alpha titanium is cooling rate sensitive as seen by differences between the top section (laser-facing side) and the middle section. The microstructure

This is the reason why there is variation in the microstructures.

laser-irradiated surface.

Microstructure of laser formed plates [1.5 kW, 35 and 47 kJ/m].

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

Figure 6.

Figure 7.

33

Microstructure of a laser formed plate [2.5 kW, 90 kJ/m] [10].

Figure 4. As received material [parent material].

Figure 5. Mechanically formed microstructure.

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

Figure 6. Microstructure of laser formed plates [1.5 kW, 35 and 47 kJ/m].

2.4 Microstructure

shown in Figure 5.

Figure 4.

Figure 5.

32

Mechanically formed microstructure.

As received material [parent material].

Figure 4 shows microstructures from the parent material specimens, and this material has equiaxed α-grains usually developed by annealing cold-worked alloy above recrystallization temperature. The microstructure has shown results from the manufacturing process of CP grade 2 titanium, which cannot be altered in the plates

The microstructure of mechanically formed plates contains the same equiaxed alpha grains found in the parent material. Mechanical forming produces no heat, and therefore the similarities in microstructure are to be expected. There were no major changes to the microstructure as a result of this process when compared to the as received material [10]. The microstructure of a mechanically formed plate is

Figure 6 shows the fine structure of titanium from the plates irradiated at a power of 1500 W using line energies of 35 and 47 kJ/m, respectively. There is a variation in the depth of the heat-affected zone (HAZ) for both line energies [10]. The unaffected material in both cases has equiaxed α-grains similar to those in the parent material. Based on microstructural observations, it becomes clear that the temperature generated at 35 kJ/m was less than that generated at 47 kJ/m as it could

without the addition of heat or cold deformation processes.

Titanium Alloys - Novel Aspects of Their Manufacturing and Processing

not penetrate as deep to effect changes deeper in the microstructure away from the laser-irradiated surface.

The resulting microstructure from a line energy of 35 kJ/m points to a higher scanning velocity. The thermal energy from the laser managed to effect changes to a quarter of the plate's thickness [10]. The cycle took around 18 minutes to irradiate all the 10 plates in each batch at this power and line energy setting. The laser forming process resulted in a semi-circular-shaped heat-affected zone in the lower line energies (35 and 47 kJ/m). The area not affected by the heat in both cases shows smaller grains than those in the heat-affected zone. Grain size depends largely on temperature attained during the laser forming process, and grain growth proceeds more quickly as temperature increases. All the figures shown clearly reveal the influence of temperature on the microstructure, and the portion affected by laser energy has grains which are much bigger than those not affected by heat [10]. This is the reason why there is variation in the microstructures.

Figure 7 shows a major change in the microstructure of CP grade 2 titanium plates with enlarged primary α-grains and enlarged β-grains (2.5 kW). The structure consists of much bigger equiaxed alpha grains in the structure. The resulting microstructure is a result of thermal energy developed by the process parameters on the plates irradiated. Thermal energy is the main initiator in microstructural layout in all the laser formed plates. The microstructure managed to change only halfway through the plate which explains why there was minimal bending on the plates irradiated. Alpha titanium is cooling rate sensitive as seen by differences between the top section (laser-facing side) and the middle section. The microstructure

Figure 7. Microstructure of a laser formed plate [2.5 kW, 90 kJ/m] [10].

formed as a result of the heat and cooling rate is not the same throughout the sample as witnessed on the as supplied parent plate. There are differences in microstructure between the top, the middle and bottom sections of the plate samples.

On the section of the plate closest to the source of laser irradiation and as thickness of the plate increases, the effect of thermal energy diminishes. The different microstructures shown are also an indication of different hardness values. The forming parameters at this power level led to plastic deformation on the laserfacing side. Before getting to plastic deformation, the grains were similar to those of the as received material (parent and mechanically formed plates). The scanning velocity used here happens to be the lowest in this study. The low scanning speed meant that the laser got more time to effect changes per unit area of the material resulting in the microstructure shown. The cooling of the plates also contributed to the microstructure. All the plates were naturally cooled. Thermal measurements have also shown the effect of the scanning velocity on the material. In multiple scan scenarios, each scan effects change on the microstructure. Differences in microstructure are brought about by the laser intensity power of 2.5 kW which makes a significant change in the microstructural layout [10].

increase in scanning velocity. The change in scanning speed was done to achieve a line energy of 90 kJ/m and resulted in the decline of process time by 18%. These numbers show that changes in the microstructure are to be expected as the new heat flux generated amounts to higher temperatures on the surface of the plate. In as much as cooling rates determine the resulting microstructure on titanium alloys, the

Microstructure and Mechanical Properties of Laser and Mechanically Formed Commercially…

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

Figure 9 shows the microstructure layout as a result of laser forming at the highest power setting. The grains from this setting were the biggest in all the plates evaluated in this study. Twin bands can be seen throughout the microstructure of the plate. All sections of the plate had different grain sizes attesting to different cooling rates in the plate. The surface facing the laser did not cool down at the same time as the opposite side of the plate [10]. Acicular alpha can also be seen on the side opposite the laser-irradiated surface. The complete laser irradiation for these samples took about 20 minutes, which explains the changes in the microstructure when compared to a power of 2.5 and 3 kW, respectively. Changing the power from 2500 to 3000 W resulted in a 40% increase in heat flux. The heat flux increased by 16% from a power of 3000–3500 W. This means that by changing the power values, there was a related increase in the heat transferred per unit area, per unit time. These changes contributed to changes in the accompanying microstructure and mechanical properties. The study on the microstructure and mechanical properties helped in understanding the behaviour of titanium in different forming scenarios. The information gathered also made it easier to analyse the hardness results.

The hardness number is a resistance for the local plastic deformation, and the hardness is closely related to residual stresses [9]. The average Vickers hardness obtained for the parent material is 160 5Hv0.3, and whilst the average hardness number for the parent material is higher than that obtained in mechanically formed samples, the laser formed specimens show higher values. The average hardness results of the mechanically and laser formed CP grade 2 titanium specimens are

Mechanically formed plates did not behave like laser formed samples as there was a slight increase in hardness moving away from the top section resulting in an average hardness of 130 5Hv0.3. This is a result of changes in the material structure caused by the die during mechanical forming. The microstructure of plates irradiated at 1.5 kW (35 kJ/m) indicate that heat energy could only penetrate

processing temperatures also play an important role as well [10].

Microstructure of a laser formed plate [3.5 kW, 90 kJ/m] taken from the top surface.

2.5 Hardness

Figure 9.

shown in Table 5.

35

Figure 8 also shows the microstructure of a sample irradiated at 3 kW, and with this plate an increase in power results in gradual change to the microstructure of titanium. The microstructure has much bigger equiaxed-α (alpha) and (beta) βgrains compared to a power of 2.5 kW and the supplied parent material. The initial microstructure has an effect on the mechanical properties of titanium. During the process, changes in temperature affect the microstructure which in turn influences the mechanical properties of titanium. The changes in temperature and cooling rates also play a role in resulting mechanical properties. The high temperatures attained effected the top and bottom sections of the plates. An increase in power from 2500 to 3000 W meant that scanning speeds had to be adjusted in order to get to a line energy of 90 kJ/m. This reduced the time taken to irradiate the batch of samples. The heat flux increases by about 18% when the power is adjusted to 3000 W. There was also a reduction of 18% to the process time. The changes in heat flux indicate higher temperatures on the plate surface [10]. The alpha and beta grains are bigger closer to the centre of the irradiated plates and elongated closer to the laser-facing surface. The thermal energy generated resulted in different microstructures between the top and bottom halves of the sample. It should also be eminent that with an increase in power from 2.5 to 3 kW, there is a reduction in time taken to achieve irradiating the plate samples. The altering of power from 2.5 to 3 kW results in an 18% increase in the amount of heat flux generated and a 19%

Figure 8. Microstructure of a laser formed plate [3 kW, 90 kJ/m] [10].

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

Figure 9. Microstructure of a laser formed plate [3.5 kW, 90 kJ/m] taken from the top surface.

increase in scanning velocity. The change in scanning speed was done to achieve a line energy of 90 kJ/m and resulted in the decline of process time by 18%. These numbers show that changes in the microstructure are to be expected as the new heat flux generated amounts to higher temperatures on the surface of the plate. In as much as cooling rates determine the resulting microstructure on titanium alloys, the processing temperatures also play an important role as well [10].

Figure 9 shows the microstructure layout as a result of laser forming at the highest power setting. The grains from this setting were the biggest in all the plates evaluated in this study. Twin bands can be seen throughout the microstructure of the plate. All sections of the plate had different grain sizes attesting to different cooling rates in the plate. The surface facing the laser did not cool down at the same time as the opposite side of the plate [10]. Acicular alpha can also be seen on the side opposite the laser-irradiated surface. The complete laser irradiation for these samples took about 20 minutes, which explains the changes in the microstructure when compared to a power of 2.5 and 3 kW, respectively. Changing the power from 2500 to 3000 W resulted in a 40% increase in heat flux. The heat flux increased by 16% from a power of 3000–3500 W. This means that by changing the power values, there was a related increase in the heat transferred per unit area, per unit time. These changes contributed to changes in the accompanying microstructure and mechanical properties. The study on the microstructure and mechanical properties helped in understanding the behaviour of titanium in different forming scenarios. The information gathered also made it easier to analyse the hardness results.
