2.5 Hardness

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

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%

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

between the top, the middle and bottom sections of the plate samples.

Titanium Alloys - Novel Aspects of Their Manufacturing and Processing

significant change in the microstructural layout [10].

Figure 8.

34

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

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 shown in Table 5.

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


those obtained from the parent plate by a bigger margin. The improvement in hardness as a result of the laser forming process could help in the preparation of titanium for other engineering applications in need of hardened titanium

Microstructure and Mechanical Properties of Laser and Mechanically Formed Commercially…

The graphs plotted from the analysed plates were a result of residual stress information gathered by the MTS3000 machine on each plate sample evaluated. Comparisons are made between the plates based on the graphs obtained. The relieved strain from the parent material differs to that obtained from other evaluated plates. Figure 10 shows relieved strain measured on the parent material, and all the micro-strain values (ɛ1, ɛ2, ɛ3) show a slight reduction in strain as the depth of

The parent material shows minimum values in both residual stress and strain. Even when the drill depth increases, residual stress and strain remain constant. The graph obtained is totally different when compared to other plates evaluated in this study. With the other power levels in laser formed plates, there were changes in residual stress and strain with changes in drill depth [8]. This figure also shows an even distribution of residual strains on the material, and, unlike the laser formed plates, it seems possible that the temperature gradient on the parent plates during fabrication was not steep. The residual strains are not modified in any way but result from the manufacturing procedure used to produce titanium. The other forming operations witnessed in the study show a marked change to the residual stress/strain distribution. Residual stress from as received parent material shows steep residual stress versus drill depth gradient. The gradient is typical of stress induced by the manufacturing process. Surface residual stress is of high importance to mechanical design engineers as they show areas of high residual stress. The high residual stress areas help contribute to fatigue failure of the material [8]. All values obtained in the analysis of residual stress and strain of CP grade 2 titanium plates are shown in Table 6, and results obtained allude to the performance of these plates

The readings obtained from the parent material form the base for the analysis of

residual stress, and strain results for the forming process utilised in this study. Results from the parent material show a difference between the maximum and minimum stresses of 12.9 MPa which is tensile. The stress values also give an indication as to why the parent material performed better than other plates during fatigue testing. The laser formed plates showed higher values of stress than both mechanically formed and the parent materials. The effect of these stresses is

plates [10].

2.6 Residual stress

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

the hole increases.

during fatigue testing.

Figure 10.

37

Relieved strain (A) and stress (B) for parent material.

#### Table 5.

Hardness profile of laser and mechanically formed plate samples.

to a third of the depth of the sample (changing a small portion of the microstructure). Due to the low amount of heat generated, there was a minor change in the microstructure, and this translated to minimal changes in the hardness values. The increase in line energy from 35 to 47 kJ/m also contributed to an increase in hardness. The increase in hardness values could be traced back to the change in the size of the microstructure grains when compared with the parent material [10].

On examining the microstructure of specimen irradiated at 47 kJ/m (1.5 kW), the change in the structure is more remarkable than the plate samples irradiated at 35 kJ/m (1.5 kW). The processing speed at 47 kJ/m was slower, making it possible for the laser to effect changes on the microstructure on a much improved scale resulting in a bigger heat-affected zone. Values obtained at this power level and line energy are higher than those obtained from a line energy of 35 kJ/m. These values also show the importance of thermal energy in the laser forming process. The power of 2.5 kW had the highest average hardness values in all the plate samples evaluated. This could be linked to the low scanning velocity at this power level. More heat was dissipated per unit area per unit time resulting in the high hardness values. Hardness values obtained at a power of 2500 W had high values than that of the parent material. Process parameters at this power level proved to be the optimum settings for this study. For those engineering applications in need of improved hardness properties on this grade of titanium, these settings could be used. The optimum settings resulted in the highest value of Vickers hardness in this study at 410Hv0.3. The hardness value obtained shows a 100% increase in hardness when compared to both the parent and mechanically formed plates [10].

The reduction in hardness values at this power setting could be traced back to the grain structure found in samples irradiated. The microstructure contained acicular alpha and beta phases which have a significant effect on the mechanical properties of titanium. An average Vickers hardness value of 349Hv0.3 was obtained at this power setting, and it was the lowest on the samples evaluated. Plates irradiated at 3000 W had the hardness value of 349Hv0.3 in plates formed at a line energy of 90 kJ/m. The same plates showed a marked improvement in hardness at the middle section of the plates. The forming process effected physical changes on the surface of the plates. These changes translated to changes in the hardness of the material. The results show an improvement of more than 100% when compared with the as received material. These changes also made the material hard to polish during the preparation of residual stress samples [10].

A hardness value of 311Hv0.3 was obtained at a power setting of 3500 W. This is the third highest value in samples irradiating a line energy of 90 kJ/m. The size of grains and their structure were different when compared to other laser formed plates. Readings taken from the top section of the laser-irradiated side indicate a considerable increase in the average hardness of titanium. An average Vickers hardness value of 311Hv0.3 was obtained from the top section which indicates a 40% increase in the hardness of titanium. The Vickers hardness readings taken closer to the surface show increased hardness values which are much higher than

those obtained from the parent plate by a bigger margin. The improvement in hardness as a result of the laser forming process could help in the preparation of titanium for other engineering applications in need of hardened titanium plates [10].
