## 2 Materials and Methods
The device used for the MIG welding was a Daihen Varstroj WB-P500L (Lendava, Slovenija). The specimens were welded in the flat position, and a butt weld was used as the type of weld joint. The welding torch angle for the TIG process was 15\({}^{\circ}\), while for the MIG process the welding torch was perpendicular to the base material because of the automated linear displacement. The welding procedure was performed at room temperature, in a room environment. All specimens were brushed and cleaned with acetone prior to welding. After a few preliminary welding tests, with the proposed heat input according to the literature, some excess heat problems occurred. It was evident that porosity developed if too much heat was used (Figure 2). Also, as shown in Figure 2, some instances of delamination were visible on the aluminum machining chips, which indicates that the bonding of the machining chips could also be improved (Figure 2). Excessive porosity visibly developed at the end of the delamination lines. Therefore, the welding parameters, in this case, are strongly connected with the quality of the SSR specimens or the level of chip bonding during the solid-state recycling process. This was the main motivation for employing TIG with a controlled and minimized heat input, to reduce possible delamination and porosity development to a minimum. The used device for TIG welding was the Daihen AVP-300 (Osaka, Japan). Because of the 4 mm aluminum sheet thickness, one-sided TIG was performed with the aim of further reducing the heat input. To simulate different welding conditions, two-sided TIG welding was also performed. Considering an arc efficiency of 80% for the MIG process, the heat input was calculated to be 3.528 kJ/cm. For the one-sided TIG welding procedure and an arc efficiency of 60%, the heat input was 2.7 kJ/cm, and for the two-sided TIG welding procedure it was 1.62 kJ/cm for each side. The formula for this calculation is as follows: heat input = (current \(\times\) voltage \(\times\) efficiency factor \(\times\) 60)/(travel speed (cm/min) \(\times\) 1000). For the welding process, AlMg5, or 5356, filler metal was selected. It is the most widely used welding alloy and can be classified as a general-purpose type of filler alloy. The chemical composition of the filler metal is as follows: Si, max. 0.25; Fe, max. 0.4; Cu, max. 0.1; Mn, 0.05-0.2; Mg, 4.5-5.5; Cr, 0.05-0.20; Zn, max. 0.1; Ti, 0.06-0.2; Be, max. 0.0003; Al, bal. It is recommended for EN AW 6082 aluminum alloy welding because using filler metal with a higher Mg content can significantly reduce the sensitivity to solidification cracking [31]. Table 2 summarizes all important parameters used in the welding process. After welding, metallography, microhardness, and tensile test specimens were cut from each welded sheet. The macrostructure was characterized using an Olympus SZ61 optical stereomicroscope. The samples were etched with a reagent that consisted of 0.5 mL of 40% HF (hydrofluoric acid) dissolved in 100 mL of water for a duration of 40 s at room temperature. Scanning electron microscopy, accompanied by energy-dispersive X-ray spectroscopy, was performed, which served to determine the intermetallic distribution and size, as well as possible welding errors due to the increased heat input and melting of  
Figure 2: Excessive heat input’s influence on an SSR specimen welded with the MIG process: appearance of porosity and delamination. the metal. An FE-SEM FEI Quanta FEG 250 (FEI Company, Eindhoven, The Netherlands) scanning electron microscope and an Oxford EDS PentaFET (Oxford Instruments PLC, Abingdon, UK) energy-dispersive X-ray spectroscopy (EDS) device were utilized for this purpose. Microhardness testing was performed according to the HRN EN ISO 6507-1:2018 standard [32] for microhardness testing of metal material. The used device was a Shimadzu HMV-2T (Shimadzu, Kyoto, Japan) universal machine with an integrated DynoLITE camera with DynoCapture 2.0 software for computer analysis.