Besides recording the sample regions' localized temperature, the thermal image helps identify the region borders because there is a thermal contrast between them. For example, to identify the weld bead region length, C & D's image coordinates must be extracted. The thermal image is viewed as a matrix of intensities corresponding to temperature, where the upper left image pixel has the row-column index zero-zero. Then the weld region's absolute total length in millimeters, for a given time t, can be calculated using the formula:
where RC and RD are the lower and upper image row indices, in pixels, of the lower and upper borders of the weld bead (CD) region, and P = 1.67 pixels/mm is the scaling factor of the thermal image found by measuring an object in the image such as the scale wire or the Inconel 625 sample width both in reality (in mm) and in the thermal image (in pixels):
Equation 1 is applied in the same manner to find the elongations of all five regions AB, BC, CD, DE, and EF, for every time instance t of interest. Given the instantaneous Lt lengths of these regions, it is possible to calculate the region lengths as a percentage of the sample's overall length at a given moment. For example, the weld bead has the relative length at a given time t, of:
The bar chart in Figure 5 illustrates the linear increase in total specimen length as the tensile test progresses, from the initial 72.3 mm to 106.4 mm when the specimen fractured. The total specimen length is composed of the localized region length, for instance, the base metal regions AB and EF, the heat-affected zone regions BC and DE, and finally, the weld bead CD. It can be observed that the value of length increase varies across the regions, which was attributed to the difference in mechanical properties and initial length.
This work also focused on the details of surface temperature evolution over the specific regions of the weld joints, heat-affected zone, and Inconel 625 base metal alloys. In other words, the question we asked: does the temperature evolution vary across the three central regions of interest: Inconel 625 base metal alloy, heat-affected zone, and weld beads, which may show a correlation to different mechanical properties of those regions. Hence, the temperature versus time was recorded for each region as marked in the methodology in-situ during tensile loaded and then analyzed the thermographic images and its correlated time, as shown in Fig. 7. A similar approach was conducted between elongation and time. It was observed that the temperature for the heat-affected and weld beads regions (BC & CD) rises abruptly right before sample failure. This was consistent with the observation that the most deformation in these regions resulted in the highest heating rate. Simultaneously, the temperature evolution for the heat-affected and weld bead zones was higher than the Inconel 625 base metal alloy, which was also consistent with the deformation profile. It was speculated that the difference in temperature development was due to the difference in mechanical behaviour where the weld beads and heat-affected zone show more ductile behaviour compared to a more brittle and rigid (plastic) behaviour in the base metal alloy. With this, it was concluded that the Inconel 625 TIG-welded specimens exhibit two behaviours of elastic and plastic acting parallel on the net specimen deformation.
As seen in the figure, the heat-affected zone (BC) is always slightly warmer than the weld bead (CD) as the tensile test progresses, indicating it is more plastic than the weld bead. The weld bead's temporal temperature profiles (CD) and heat-affected zone (BC) exhibit a similar trend as the tensile test progresses. There are four distinct regions of the temperature change rate. Namely: 0 to 4.5 min, 4.5 to 14 min, 14 to 32 min, and 32 to 34 min. The slopes of these four regions, m1 = 0.17°C/min, m2 = 0.39°C/min, m3 = 0.19°C/min, and m4 = 0.67°C/min, correspond to the weld bead’s (region CD) rate of temperature change, and plastic deformation throughout the tensile test. With this understanding, it becomes possible to predict the specimen's fracture time. In the case under study, the fracture occurred about 2.5 minutes after the weld bead heating rate rose to the level of m4. The same discussion applies to the heat-affected zone (BC region). The base metal (AB region) temperature rates of change, on the other hand, are distinguishable for the other two zones after the second inflection point at 14 minutes.
This indicates three inflection times at T1 = 4.6, T2 = 14, and T3 = 32 minutes, corresponding to the entire sample's mechanical behaviour shown in Fig. 8. The maximum temperature profile shown corresponds to the heat-affected zones (BC & EF) and the weld bead (CD). There is a clear indication that the three inflection times, which were deduced from the thermal data, correlate to the mechanical stress curve's inflection points. In the last inflection point T3, the weld bead hardens, and shortly after that, at minute 34, the fracture occurs.
After the fracture, specimen images were directly captured using a light optical microscope to show the fracture profile, the type of fracture, either ductile or brittle and to see where the fracture took place. It is well established in the research literature that a fracture with extensive plastic deformation and energy absorption before the failure is ductile. Hence, a ductile fracture surface usually has coarser precipitates mixed with flat groves and subtle dimples. Therefore, the coarse slip bands are then formed over the crack path. On the other hand, a brittle fracture shows little plastic deformation and low energy absorption before it occurs. When the material has high strengths, the fracture and cracks paths occur intergranularly, a flatter surface is observed. Figure 9 shows the thermal and optical images of the Inconel 625 after fracturing, and it can be concluded that fracture occurred precisely in the middle third of the weld bead. It is speculated that the fracture path and surface profile correspond to a ductile fracture path, where plastic deformation occurred with coarse surface profile mixed with subtle dimples. This agrees with the observation seen in the previous discussion that weld beads are the region where most deformation occurs. It can be observed from Fig. 9 (a) that the weld bead is where the fractured progress even though the temperature was maximum at the heat-affected zones, which may mean that they are still deforming and have more rigid mechanical properties. Conversely, the Inconel 625 base metal alloy shows the lowest temperature indicating it is not deforming and can still sustain higher stress than the weld beads.