Relationship to Time-Temperature-Transformation (TTT) diagram It is clear from the microstructures observed in the previous section that

there should be a correlation to the TTT diagram. It was difficult to try and predict the type of microstructure from the TTT diagram obtained from literature or to forecast the cooling rate required for the

Time, seconds

microstructures observed on the various samples. In order to determine the microstructure, accurate measurements of the cooling rate at a single point had to be made on the various samples. The point on all samples was taken in the centre of the sample, i.e. after half of the irradiation cycles were completed. Samples for metallurgical investigation were taken from the centre of the specimen. The time-temperature readings for the various samples are given in Appendix B. A graphic

representation of the time-temperature data for the 7,5mm beam diameter samples are shown in Figure 4.51.

Figure 4.51: Graphic representation of time-temperature data for 7,5mm beam diameter

From Figure 4.51 it is clear that the maximum temperature is reached during laser irradiation of the specific point at which measurements were taken. The first irradiation cycle adjacent (next scanning cycle) to this point seems to have an influence on the cooling rate as indicated by point A on the graph. Subsequent irradiation cycles (scanning cycles) seem to have less of an effect, except for slowing down the cooling rate (of the specific point in the center of the sample where all temperature

1,5kW, 0% overlap,

readings were taken). The subsequent irradiation cycles (scanning cycles) are indicated by points B, C, D, E and F on the graph.

The cooling rates at the various regions on Figure 4.51 are indicated by green arrows. The cooling rate is most severe when cooling from the original laser irradiation at the point of measurement (first region

indicated by 1 on Figure 4.51) and corresponds with a value of 364°C/s.

The second region (indicated by 2 on Figure 4.51) indicates a slower cooling rate of 13°C/s and the third region (indicated by 3 on Figure 4.51) shows an even slower cooling rate of only 1,1°C/s. The cooling rate is significant because if cooling is slow enough and above the temperature at which martensite forms, no martensite will be present in the final microstructure. On the other hand, if the cooling rate is fast enough, martensite will be present.

Figure 4.52 shows the location of the temperature measurement during subsequent laser irradiation cycles (cross).

Figure 4.52: Point of temperature measurement

The graphic representation of the samples formed with the 3,1kW and 14mm beam diameter settings are shown in Figure 4.53.

Time, seconds

Figure 4.53: Graphic representation of time-temperature data for 14mm beam diameter

The shape of the curve in Figure 4.51 and Figure 4.53 is very similar except that the temperature ranges differ slightly. It can thus be said that an overlap of 25% between consecutive laser irradiation lines only seems to influence the temperature experienced at the point being measured.

The cooling rate in region one (green arrows) is the fastest and

corresponds with a value of 334°C/s. In region two, the cooling rate is slower and corresponds with a value of 9°C/s. Region three has the slowest cooling rate on the graph and corresponds with a value of 1,9°C/s. The values of the cooling rates of the various regions on the graph shown in Figure 4.53 are very similar to those observed for the previous sample with a 0% overlap between consecutive laser irradiations.

Figure 4.54 shows the graphic representation of time-temperature data for the 5kW and 20mm beam diameter setting. It is clear that the graph displayed in Figure 4.54 appears different to the previous two graphs.

3,1kW, 25% overlap,

Time, seconds

Figure 4.54: Graphic representation of time-temperature data for 20mm beam diameter

The 50% overlap between consecutive laser irradiations definitely

increased the temperature at the point of measurement. This means that the point in question remains basically at a temperature above 800°C for two laser irradiation cycles. The fastest cooling rate was observed in region one (green arrows) with a value of 255°C/s. Region two exhibited a cooling rate corresponding with 22°C/s and in region three the cooling rate corresponded with a value of 2°C/s.

Table 4.8 shows the cooling rates of the different samples for the respective regions as indicated by the green arrows on the various time-temperature plots.

Table 4.8: Comparison of cooling rates of the samples 1,5kW (1250J/m)

Region 1 364°C/s 334°C/s 255°C/s

Region 2 13°C/s 9°C/s 22°C/s

Time, seconds

0 50 100 150 200 250 300 350

Temperature,

o C

0 200 400 600 800 1000 1200 1400 1600

1,5kW, 0% overlap 3,1kW, 25% overlap 5kW, 50% overlap

From Table 4.8 it is clear that the 1,5kW samples showed the fastest cooling rate in region 1, the 5kW samples the fastest cooling rate in region 2 and region 3. The comparative graph of the cooling cycles of the various samples is shown in Figure 4.55. This graph shows the trend experienced by the samples at a specific point (centre of sample’s width and length) while cooling.

Figure 4.55: Comparative graph indicating cooling rate trends for the various samples

It is clear from the microstructural analysis that both (martensitic and bainitic) transformations have taken place. Due to the percentage overlap between consecutive laser scans, it is difficult to accurately determine the heating/cooling cycle at one specific area.

Taking the TTT diagram into account, the following cooling curves can be superimposed to show the possible resulting microstructure. Figure 4.56(a), (b) and (c) shows the average cooling curves for the various samples evaluated.

Figure 4.56: (a) Superimposed cooling curve of a sample irradiated with 1,5kW; (b) 3,1kW and (c) 5kW

Figure 4.56(a), (b) and (c) shows that the samples all cool through the same region of the TTT graph i.e. the Bainitic ferrite and Martensitic (M’

– martensite formed from carbon enriched austenite) region. There is, however, a difference in the temperature to which the samples are heated during the laser forming process. It is clear that the samples irradiated with 5kW laser power, are at a temperature higher than the

a b

c

lower critical (Ac1) and upper critical temperature (Ac3) for a longer time than the other samples. This affects the amount of austenite formed during the heating cycle, and also the amount of carbides being dissolved in the austenite. This is the only reason that could be found that could attribute to the variance in microstructure observed using the different laser parameters. It is only the 1,5kW samples that exhibited a microstructure consisting of acicular ferrite (bainitic ferrite) and

martensite. It is thus proposed that the 3,1kW and 5kW samples

followed a cooling curve similar to what is shown schematically in Figure 4.57.

Figure 4.57: Schematic TTT diagram indicating probable cooling rates for 3,1kW and 5kW samples

Time, seconds

Temperature, °C

Ferrite

5kW 3,1kW

Pearlite

Depth, mm

0.25 0.75 1.25 1.75

0.00 0.50 1.00 1.50 2.00

Releived strain, µe

-250 -200 -150 -100 -50 0 50

e1 e2 e3