Figure 1: Macroscopic appearance of welded joints

As a first analysis we proceeded to a macroscopic observation of the samples.
On the joint made with two pulses, without the use of argon, a bluish area is observed near the weld, due to titanium oxide. This feature is not present in any of the other joints considered (1 pulse with and without using argon, 2 pulses using argon).
The use of a protected atmosphere, therefore, is essential to avoid the external surface oxidation of the welded samples with two pulses.
At this point we proceeded to prepare the samples for observations by optical and electronic microscopy (as described in Figure 2). To be able to proceed to the section analysis of the junctions, without introducing artifacts due to the cutting procedure, removal of the wire abutments was performed (by truncation at the distance of about 2 mm from the junction) and the joint of the thermosetting resin joint conductive. The excess material was then removed (residues of the wire abutments near the junction) using abrasive papers, until the junction section was exposed on the surface, and the surface itself was then polished to a mirror-like finish.

Figure 2: a) sample fixed by means of a special clip, b) introduction of the sample fixed in the mold, c) addition of the resin and waiting for solidification, d) embedded joint, e) polishing of the sample to expose the joint and uniform the surface, f ) exposed and polished joint

Subsequently the samples were subjected to metallographic attack with Kroll solution (100ml distilled water, 1ml HF, 2ml HNO3) to highlight the metal microstructure.
A small buffer was immersed in the acid attack solution and rubbed on the surface of the sample (as described in Figure 3) for several times (20-60s) until the crystalline grains are visible.

Figure 3: Metallographic attack