To check whether the junction section was continuous or not and the possible presence of inclusions inside the joint, it was therefore proceeded to the SEM observation of the sections, before and after metallographic attack. Images obtained by both back-scattering and secondary electrons were recorded. The first images allow to verify the chemical type continuity of the junction interface; that is to highlight the possible presence of inclusions or contamination. These images in fact have a contrast of gray in areas of different chemical composition. Vice versa the images obtained by secondary electron signal, show the continuity or not of the junction from the morphological point of view: presence of cracks, discontinuities or porosity.
Finally we proceeded to the EDS analysis which allows us to verify the chemical composition of the surface being analyzed.
All the junctions have two lateral protuberances, in correspondence of the weld, in the shape of an irregular “segment”. The lateral protuberances appear to consist of Ti, without appreciable differences with respect to the wire. The composition of the material is similar both to the welding and to the core of the wire and in both cases due to pure titanium. No contamination or inclusions were found.
1 IMPULSE WITHOUT USING ARGON
The photomicrographs on the sample before the metallographic attack and using the back-scattering signal (Figure 4 a-b) show the absence of inclusions and inhomogeneities of chemical composition. From this point of view all the samples showed a similar appearance (see the following photos).
The photomicrographs that refer instead to the sample subjected to metallographic attack and recorded with the use of the secondary electron signal, highlight a regular series of
microphones aligned at the interface line (figure 4e). This interface is observable as a line of discontinuity (figure 4f).
1 IMPULSE WITH USE OF ARGON
As regards the compositional homogeneity of the welding zone, the results are completely analogous to the previous sample (Figure 6 a-b). Also with regard to the morphology of the junction, the presence of micro-holes and a discontinuous line on the junction is observed (Figure 6 d-e).
FIRST AND SECOND IMPULSE WITHOUT USING ARGON
In this case the photomicrographs show that the junction is morphologically continuous: no micro-holes or break lines are observed. In the junction area, however, there is an altered area, which appears as a band of about 50 microns in width and covers the entire welding area (Figure 8 d-e). This area is particularly evident after metallographic attack and recording the secondary electron signal.
The EDS analysis did not reveal significant changes in the composition in this area.
FIRST AND SECOND IMPULSE WITH USE OF ARGON
From the observation of the images (Figure 10), it can be deduced that this sample (with welding carried out with 2 pulses and the use of argon) shows a continuous interface, without cracks, porosity or inclusions of foreign material. There is a perfect morphological continuity in the passage from a wire, proceeding through the junction area, until reaching the second wire. The outer surfaces of the two wires are no longer distinguishable in the junction area.