## 4 Discussion  
The results in Table 5 clearly show that the assumption of 100% Ni yield to the weld metal is not applicable in the application of unconstrained metal powders in SAW. The assumption of 100% Ni yield typically used in solid weld wire application in SAW is based on the low oxygen affinity of Ni for oxygen, indicating that Ni should not be oxidised from the weld pool to form NiO which ends up in the welding slag [12; 17]. Comparison to literature reported values of Ni yield in the application of different Ni-alloying consumables illustrates that differences in Ni-yield are seen for different consumable configurations such as solid weld wire vs. alloyed metal powders. For example, in the application of pre-alloyed powders in SAW the demonstrated application used iron based powder containing 27-29% Cr, 5-8% Ni, and 2-4% Mo [14]. Interestingly, the nickel yield from these pre-alloyed powders ranged from 59% to 74% when the \"direct feeding\" powder feed mode was applied [14]. This feeding mode consisted of magnetically attaching the pre-alloyed powder to the weld wire as a surface coating. In a comparative test, the same pre-alloyed powder feed material at the same feed rate, using one arcing weld wire only, was applied for two feeding modes, namely \"direct feeding\" described above vs. \"forward feeding\" of the powder onto the substrate steel plate, in advance of the welding head. The alloying element yields of Ni, Cr, and Mo to the weld metal were similar for the two feeding modes and within the following ranges: 57-59% Ni yield, 56-58% Cr yield, and 55-58% Mo yield from the pre-alloyed powder to the weld metal [14]. Here, in the presence of Al as a de-oxidiser, the Ni yield is higher than the yields from pre-alloyed powder application in SAW. In the following section a thermodynamic analysis is applied to elucidate the chemical interaction between Ni and Al, and its effects on Ni yield in the weld pool. Several complex reactions take place in the arc cavity and may contribute to Ni loss in SAW. For example, some of the Ni is expected to vaporise, Ni may react with F\({}_{2}\)(g) and/or CaF\({}_{2}\)(g) to form NiF\({}_{2}\)(g), and Ni may also oxidise to NiO. Thermodynamic calculations were applied to further investigate the relative likelihood of the possible reactions of NiF\({}_{2}\)(g) formation, relative to similar reactions from different compounds contained in the flux, metal powders, and steel. Reactions of the type in Equations (9) and (10) were considered, and the standard Gibbs free energy graphs for these reactions are displayed in Figures 4 and 5. The standard Gibbs free energy values were calculated in FactSage 7.3 thermochemical software [30]. The temperature interval of 1600 \({}^{\circ}\)C to 2500 \({}^{\circ}\)C is used to illustrate the temperature band of the main reactions in SAW as identified in previous works, namely 2000 \({}^{\circ}\)C for slag-metal equilibrium reactions and 2500 \({}^{\circ}\)C at the arc plasma-veld pool interface [2; 3; 31]. \[[\mathrm{Ni}]+[\mathrm{F}_{2}]\ =\ [\mathrm{NiF}_{2}] \tag{9}\]  
\[(\mathrm{NiO})+[\mathrm{CaF}_{2}]\ =\ [\mathrm{NiF}_{2}]+\langle\mathrm{ CaO}\rangle \tag{10}\]  
( ): liquid; [ ]: gas; \(<\) >: solid. Chemical values applied as inputs to the Figure 4 calculations were 1 atm for each gas, namely metal vapour, F\({}_{2}\)(g), and the product metal fluoride gas. Chemical values applied as inputs to the Figure 5 calculations were 1 atm CaF\({}_{2}\)(g), unit activity for the molten metal oxides, 0.10 atm for the metal fluoride gas formed, and 0.01 activity for the CaO formed from reaction (10). According to the standard Gibbs free energy values displayed in Figure 4, the formation of NiF\({}_{2}\)(g) is least likely among the metal vapour reactions with F\({}_{2}\)(g). However, in comparison to the standard Gibbs free energy values in Figure 5, all reactions of type (9) are more likely to occur than reactions of type (10).