*2.4. Hardness Test*

Macro-hardness Rockwell scale F (HRF) was performed along a planar section of the welded joint while micro hardness Vickers (HV) was performed on the cross section of the welds. Hardness profiles were obtained by measuring the hardness at regular distances starting from the center of the weld and moving forward the base metal. For HRF measurements, a hardness tester Rockwell Rupac 500Mra (Rupac srl Milano, Italy) with a 1.58 mm steel ball indenter diameter, load of 588 N (60 Kgf), and dwell time of 15 s was used, following the ASTM E 18-03 procedures [51]. At least three measurements were made and the average value was then considered. Vickers microhardness tests were carried out under 2.94 N (0.3 Kgf) load applied for 15 s by means of a Mitutoyo HM-200 hardness testing machine (Mitutoyo Italiana srl, Lainate, Italy, according to ASTM E92-16 [52] and ASTM E140-02 [53]. The microhardness profile was obtained in the center of the cross section after preliminary analyses confirming that the effect of the position was negligible for all three couplings of materials. The BM hardness was assumed to be 66 HRF, 65–70 HV for the 6xxx-T6 extrusion and 77 HRF, 85–90 HV for the EN AC 42100-T6 casting [54].
