3.1. Investigation of the effects of welding parameters on tensile strength
Table 4 shows the results of the uniaxial tensile test on the joints as well as the failure site of the specimens.
Table 4
Results of uniaxial tensile test and location of joint failure.
Sample no. | Ultimate tensile strength of the joints )MPa( | Location of failure |
1 | 270 | Base Metal |
2 | 91 | Weld metal |
3 | 299 | Base Metal |
4 | 305 | Base Metal |
According to Table 4, it can be said that the sample (4) has the highest tensile strength of 305 MPa with a rotational speed of 800 rpm and linear motion speed 80 mm/min. In order to compare the strength of the welded samples and the base metal, the base metal sample was also subjected to uniaxial tensile test and it was observed that the base metal had a tensile strength of 303 MPa, therefore, it can be said that the tensile strength of the sample (4) is approximately 101% of the base metal.
3.2. Microstructure analysis of ST14 welded steel joints
Figure 3 shows the optical microscopy image of the 14ST base metal cross section. As it is known, the microstructure has ferrite phase and due to very low carbon content (0.04%) the amount of pearlite phase is negligible. According to the lever rule in the Fe-C diagram, the structure appears to contain 2% pearlite phase and 98% ferrite phase.
Figure 4 shows the optical microscopy image of the cross-section sample (4). Due to the heat generated and the thermomechanical operations, different zones with different properties have been created at the junction. Therefore, it is expected that ferrite grains grow and become larger. On the other hand, by rotating the pin and shoulder in the Stirred Zone (SZ), thermomechanical operations are performed, which results in recrystallization in the grains and fine-grained up to 10–20 times, which enhances the mechanical properties of the joint. A little further away is the Thermo Mechanically Affected Zone (TMAZ). This area is affected by heat and mechanical operations at the same time, but its intensity is much lower than in SZ. The Heat Affected Zone (HAZ), located between the base metal and TMAZ, is only affected by heat and no mechanical operations are performed in this area. As can be seen in this area, in some parts, the grain size is more elongated and larger than the base metal or at least equal to the base metal.
For comparison the effect of welding parameters, optical microscopic image of a cross section of the sample no (1) was also examined (Fig. 5).
It seems that due to lower linear motion speed and higher rotational speed in this case, the heat input to sample (1) is also higher than other samples. For this reason, it is expected that as the input heat increases, the temperature of the tungsten carbide tool will also rise due to high friction, Causing the tool to soften and also some of the tungsten carbide material to be removed and entered to the Stirred Zone, and ultimately affect the bonding properties and reduce the strength. Also, because heat is high, it is expected that grain growth in the SZ will be higher and the hardness will decrease in this area, but unexpectedly, the hardness of the area has increased significantly, which appears to be related to the tungsten carbide particles removed from the tool, that is visible in the form of black particles.
3.3. Survey hardness profile of ST14 welded joints
The hardness profile of the cross-section of the welded specimens is given in Figs. 6–9.
It should be noted that the average hardness of the ST14 steel sheet is equal to 91.5 and equal to the hardness profile of the specimens as the hardness of the weld metal increased in all four welded specimens compared to the base metal, which is the cause. It can be the microstructural changes that occur during the welding process, which greatly reduces the grain size in the weld area. As mentioned earlier, in sample (1), the hardness of the perturbation zone increased due to the separation of tungsten carbide particles from the tool and the composite formation of iron and tungsten carbide. In sample (2) also, due to the low rotational speed and the high speed of linear movement of the inlet heat to the sample, the lack of sufficient heat in the area in question caused the turbulence between the two samples to not proceed well.
Also because the structure in the SZ region is fine, the hardness has increased, but due to the lack of sufficient perturbation, the sample is not robust enough.
In Examples (3) and (4), the structure in the SZ region has been fine-grained due to thermomechanical operations, but the hardness in the area has increased, but the hardness in the TMAZ and HAZ has gradually decreased with the departure of the central welding line.