Metal Oxide Nanoparticle-Based Coating as a Catalyzer for A-TIG Welding: Critical Raw Material Perspective

Besides a wide application in corrosion protection, wear resistance increase, providing thermal properties and power conversion, oxide coatings have found an alternative application in welding technology as catalysts of the tungsten inert gas (TIG) welding process. In this paper, the novel approach of fabricating a coating containing nanoparticles based on nanosized SiO2 and TiO2 and their mixtures was applied to the austenitic stainless-steel base metal. It was found that coatings increased depths of penetration, enabling a consumable-free welding. Using this method, the use of several critical and near-critical raw materials (e.g., Si and Cr), as well as the relatively expensive Ni can be completely avoided. The most effective coating in terms of weld penetration consisted of a mixture of nanoparticles, rather than unary oxide coatings based on nanoparticles. A model for liquid weld metal flow is proposed based on the metallographic examination of recrystallized grains and microhardnesses measured near the weld metal, supporting the reversed Marangoni convection theory.


Introduction
The traditional form of oxide coatings has several purposes, mainly to protect the base material against corrosion [1][2][3] and wear [4][5][6], to obtain certain thermal properties [7,8], and to be used for power conversion [9,10]. However, a special type of metal-oxide coating can be used in welding technology with a high potential for application in the industrial sector.
One of the most widely used welding processes is gas tungsten arc welding (GTAW) or alternatively called tungsten inert gas (TIG). This process is well known for producing high-quality welds in different metals and alloys, most commonly for different types of stainless steels and non-ferrous alloys based on aluminum, copper, nickel, titanium, etc. [11][12][13]. Compared to the metal inert gas (MIG), which is used for welding a similar array of materials, TIG suffers from a lower productivity due to a lower welding speed and relatively low penetration, even in the case of currents when an excess of 300 A

Materials and Methods
In this study, welding was done on AISI 304 austenitic stainless-steel base metal with the following chemical composition: <0.03% C, 0.5% Si, 1.3% Mn, <0.008% S, 18.03% Cr, 0.003% P, 0.010% Al, 0.41% Cu, 9.51% Ni, 0.012% Sn and 0.07% V, with Fe making up the balance. The plates used for welding-re-melting were 10-mm-thick, were water-jet cut to 50 mm width and obtained from a single stainless-steel plate used as provided from the manufacturer, without any heat treatment.
Overall, six different coatings were prepared using 20 nm TiO 2 and SiO 2 nanoparticles. Nanoparticles at 5 wt. % were mixed with acetone ((CH 3 ) 2 CO), having a pH of 7, to produce a uniform mixture. Nanoparticles were weighed using a Tehtnica Type 2615 analytic balance (Zelezniki, Slovenia) and subsequently mixed with the solvent using a Tehtnica mm530 magnetic stirrer (Zelezniki, Metals 2019, 9, 567 3 of 12 Slovenia) for 10 min. To determine the size of the particles in the liquid component, a Zetasizer Nano ZS analyzer (Malvern Instruments, Malvern, UK) was used.
Welding was done using an EWM Tetrix 230 AC/DC welding power source (Mündersbach-Westerwald, Germany) with 200 A DCEN current and by the application of a 12.7 mm diameter nozzle. The electrode tip was sharpened to 90 • . Tungsten with a 2% thorium electrode having a 2.4 mm diameter was used. The distance between the electrode tip and the base material was 2 mm, while the electrode was 6 mm out of the torch. Welding speed was 100 mm/min for all specimens. In specimen 0, the base metal was prepared using a 2-mm-deep V-groove with an angle of 90 • . Consumable material in the form of a 0.8-mm coil wire made of AISI 308 (≤0.08% C; ≤2%; ≤0.045% P; ≤0.03% S; ≤1% Si; 19%-21% Cr; 10%-12% Ni) was continuously added to the weld pool. Specimens welded with a coating were welded without the V-groove and consumable material.
The characterization of welds in order to evaluate the effectiveness of produced coatings comprised the following techniques: macro testing, microstructure examination, and microhardness, done in various weld areas (weld material, heat-affected zone, and base material). Macro and microstructural examination was done after a common metallographic preparation procedure consisting of the following: cutting, mounting into a 20 mm diameter polyethylene cup, grinding (using 150-2000 grit abrasive paper) and polishing (using 6, 3, 1, and 0.5 µm diamond suspension). Etching was done using aqua regia. Metallographic specimens were examined using a Leitz Orthoplan light microscope (Oberkochen, Germany), which was also used to measure the crucial weld dimensions (width-W and depth-D). Microhardness was measured using a Wilson Tukon 1102 (Uzwil, Switzerland) Vickers measurement device with a 1 kg load. Vickers microhardness measurement was done in parallel to the surface, 1 mm under it and 1 mm from the bottom of the weld metal. Microhardness was also measured through the weld metal, from the weld face to the bottom of the weld metal. The schematic depiction of the microhardness measurement is shown in Scheme 1.
Welding was done using an EWM Tetrix 230 AC/DC welding power source (Mündersbach-Westerwald, Germany) with 200 A DCEN current and by the application of a 12.7 mm diameter nozzle. The electrode tip was sharpened to 90°. Tungsten with a 2% thorium electrode having a 2.4 mm diameter was used. The distance between the electrode tip and the base material was 2 mm, while the electrode was 6 mm out of the torch. Welding speed was 100 mm/min for all specimens. In specimen 0, the base metal was prepared using a 2-mm-deep V-groove with an angle of 90°. Consumable material in the form of a 0.8-mm coil wire made of AISI 308 (≤0.08% C; ≤2%; ≤0.045% P; ≤0.03% S; ≤1% Si; 19%-21% Cr; 10%-12% Ni) was continuously added to the weld pool. Specimens welded with a coating were welded without the V-groove and consumable material.
The characterization of welds in order to evaluate the effectiveness of produced coatings comprised the following techniques: macro testing, microstructure examination, and microhardness, done in various weld areas (weld material, heat-affected zone, and base material). Macro and microstructural examination was done after a common metallographic preparation procedure consisting of the following: cutting, mounting into a 20 mm diameter polyethylene cup, grinding (using 150-2000 grit abrasive paper) and polishing (using 6, 3, 1, and 0.5 µm diamond suspension). Etching was done using aqua regia. Metallographic specimens were examined using a Leitz Orthoplan light microscope (Oberkochen, Germany), which was also used to measure the crucial weld dimensions (width-W and depth-D). Microhardness was measured using a Wilson Tukon 1102 (Uzwil, Switzerland) Vickers measurement device with a 1 kg load. Vickers microhardness measurement was done in parallel to the surface, 1 mm under it and 1 mm from the bottom of the weld metal. Microhardness was also measured through the weld metal, from the weld face to the bottom of the weld metal. The schematic depiction of the microhardness measurement is shown in Scheme 1.

Particle Size Fractions
Particle size fractions are shown in Figure 1. In Figure 1, only fractions of up to 4 µm are shown, balance values of up to 17 µm were omitted to enable compactness of the chart. The largest number of particles were up to 1 µm, while larger fractions were progressively smaller as the particle size range was higher. This occurred in spite of the nominal nanoparticles that were used, indicating that intensive agglomeration was present. The fraction of particles having a size up to 1 µm was higher in all TiO2 compared to all SiO2 nanoparticle coating solvents, but the highest number of detected particles in this range was in coatings based on mixtures of SiO2 and TiO2 particles. The maximum particle size fraction was reached in specimen 2Si3Ti.

Particle Size Fractions
Particle size fractions are shown in Figure 1. In Figure 1, only fractions of up to 4 µm are shown, balance values of up to 17 µm were omitted to enable compactness of the chart. The largest number of particles were up to 1 µm, while larger fractions were progressively smaller as the particle size range was higher. This occurred in spite of the nominal nanoparticles that were used, indicating that intensive agglomeration was present. The fraction of particles having a size up to 1 µm was higher in all TiO 2 compared to all SiO 2 nanoparticle coating solvents, but the highest number of detected particles in this range was in coatings based on mixtures of SiO 2 and TiO 2 particles. The maximum particle size fraction was reached in specimen 2Si3Ti.

Macro and Weld Bead Dimensions
Macro images of obtained weld beads are presented in Figure 2, along with the width (W), depth (D), depth-to-width ratios (D/W), and specimen designation. Weld bead measurements were averages of measurements taken from five macro images reported along with standard deviations. In Figure 2, both the conventionally welded-re-melted specimen (0) and specimens welded with the coating are shown. It can be seen that the introduction of the coating significantly influenced the melted profile of the weld. Firstly, the weld bead width decreased. It was roughly half of that of specimen 0's weld bead width. More importantly, weld depth was significantly increased. Specimen 5Si obtained with a SiO2 nanoparticle coating had a lower penetration depth than 5Ti, obtained with a coating containing TiO2 nanoparticles. The highest penetration was obtained with specimen 2Si3Ti. Deviations reported in Figure 2 were on the same order of magnitude for all specimens. There was no positive correlation between the increase in penetration and the narrowing of the weld, as for the depth-to-width ratios (D/W). However, D/W ratios increased from 0.28 in specimen 0 to 0.66 (specimen 2Si3Ti) and 0.79 (specimen 1Si4Ti) on average. Macro images of specimens welded without (specimen 0) and with coating (specimens 5Si, 4Si1Ti, 3Si2Ti, 2.5Si2.5Ti, 2Si3Ti, 1Si4Ti, and 5Ti), with the indicated weld bead widths, penetration depth/width ratios (D/W), and standard deviations.

Microstructures
Microstructures identified within and near the melting line are shown in . The analysis will mainly refer to the comparison between specimen 0, obtained without the coating and with consumable material, and specimens obtained with the coating and without consumable material. Weld metal microstructures of specimens 0, 4Si1Ti, 3Si2Ti, and 5Ti are shown in Figure 3. It can be seen that the microstructure was typically dendritic, a common feature for all the tested

Macro and Weld Bead Dimensions
Macro images of obtained weld beads are presented in Figure 2, along with the width (W), depth (D), depth-to-width ratios (D/W), and specimen designation. Weld bead measurements were averages of measurements taken from five macro images reported along with standard deviations. In Figure 2, both the conventionally welded-re-melted specimen (0) and specimens welded with the coating are shown. It can be seen that the introduction of the coating significantly influenced the melted profile of the weld. Firstly, the weld bead width decreased. It was roughly half of that of specimen 0's weld bead width. More importantly, weld depth was significantly increased. Specimen 5Si obtained with a SiO 2 nanoparticle coating had a lower penetration depth than 5Ti, obtained with a coating containing TiO 2 nanoparticles. The highest penetration was obtained with specimen 2Si3Ti. Deviations reported in Figure 2 were on the same order of magnitude for all specimens. There was no positive correlation between the increase in penetration and the narrowing of the weld, as for the depth-to-width ratios (D/W). However, D/W ratios increased from 0.28 in specimen 0 to 0.66 (specimen 2Si3Ti) and 0.79 (specimen 1Si4Ti) on average.

Macro and Weld Bead Dimensions
Macro images of obtained weld beads are presented in Figure 2, along with the width (W), depth (D), depth-to-width ratios (D/W), and specimen designation. Weld bead measurements were averages of measurements taken from five macro images reported along with standard deviations. In Figure 2, both the conventionally welded-re-melted specimen (0) and specimens welded with the coating are shown. It can be seen that the introduction of the coating significantly influenced the melted profile of the weld. Firstly, the weld bead width decreased. It was roughly half of that of specimen 0's weld bead width. More importantly, weld depth was significantly increased. Specimen 5Si obtained with a SiO2 nanoparticle coating had a lower penetration depth than 5Ti, obtained with a coating containing TiO2 nanoparticles. The highest penetration was obtained with specimen 2Si3Ti. Deviations reported in Figure 2 were on the same order of magnitude for all specimens. There was no positive correlation between the increase in penetration and the narrowing of the weld, as for the depth-to-width ratios (D/W). However, D/W ratios increased from 0.28 in specimen 0 to 0.66 (specimen 2Si3Ti) and 0.79 (specimen 1Si4Ti) on average. Macro images of specimens welded without (specimen 0) and with coating (specimens 5Si, 4Si1Ti, 3Si2Ti, 2.5Si2.5Ti, 2Si3Ti, 1Si4Ti, and 5Ti), with the indicated weld bead widths, penetration depth/width ratios (D/W), and standard deviations.

Microstructures
Microstructures identified within and near the melting line are shown in Figures 3-5. The analysis will mainly refer to the comparison between specimen 0, obtained without the coating and with consumable material, and specimens obtained with the coating and without consumable material. Weld metal microstructures of specimens 0, 4Si1Ti, 3Si2Ti, and 5Ti are shown in Figure 3. It can be seen that the microstructure was typically dendritic, a common feature for all the tested Macro images of specimens welded without (specimen 0) and with coating (specimens 5Si, 4Si1Ti, 3Si2Ti, 2.5Si2.5Ti, 2Si3Ti, 1Si4Ti, and 5Ti), with the indicated weld bead widths, penetration depth/width ratios (D/W), and standard deviations.

Microstructures
Microstructures identified within and near the melting line are shown in Figures 3-5. The analysis will mainly refer to the comparison between specimen 0, obtained without the coating and with consumable material, and specimens obtained with the coating and without consumable material. Weld metal microstructures of specimens 0, 4Si1Ti, 3Si2Ti, and 5Ti are shown in Figure 3. It can be seen that the microstructure was typically dendritic, a common feature for all the tested specimens. Some differences appeared to exist, but they were the result of a different orientation of dendrites and various cross sections that were obtained. specimens. Some differences appeared to exist, but they were the result of a different orientation of dendrites and various cross sections that were obtained.  Figure 4. Specimens obtained without the coating (specimen 0) and with the coating (specimens 5Ti, 1Si4Ti, and 2Si3Ti) are shown with significant differences. The specimen obtained without the coating had significantly larger austenite polygonal grains in this area (Figure 4a, indicated by an arrow), which were not present in specimens obtained with the coating (Figure 4b-d).  Differences between microstructures under the weld of specimens 0, 5Si, 4Si1Ti, and 2.5Si2.5Ti are shown in Figure 5. It can be seen that, as in Figure 4, certain differences existed, consisting mainly of coarser grains observed in specimens obtained with the coating (Figure 5b-d, indicated by arrows) as compared to the specimen obtained without the coating (Figure 5a). Note that such a microstructure was not obtained directly under the weld metal, but rather slightly to the side. Differences between microstructures under the weld of specimens 0, 5Si, 4Si1Ti, and 2.5Si2.5Ti are shown in Figure 5. It can be seen that, as in Figure 4, certain differences existed, consisting mainly of coarser grains observed in specimens obtained with the coating (Figure 5b-d, indicated by arrows) as compared to the specimen obtained without the coating (Figure 5a). Note that such a microstructure was not obtained directly under the weld metal, but rather slightly to the side.   Figure 4. Specimens obtained without the coating (specimen 0) and with the coating (specimens 5Ti, 1Si4Ti, and 2Si3Ti) are shown with significant differences. The specimen obtained without the coating had significantly larger austenite polygonal grains in this area (Figure 4a, indicated by an arrow), which were not present in specimens obtained with the coating (Figure 4b-d).

Vickers Microhardness Results
Differences between microstructures under the weld of specimens 0, 5Si, 4Si1Ti, and 2.5Si2.5Ti are shown in Figure 5. It can be seen that, as in Figure 4, certain differences existed, consisting mainly of coarser grains observed in specimens obtained with the coating (Figure 5b-d, indicated by arrows) as compared to the specimen obtained without the coating (Figure 5a). Note that such a microstructure was not obtained directly under the weld metal, but rather slightly to the side.

Vickers Microhardness Results
The microhardness results are shown in Figures 6 and 7, while average values of the microhardness data measured in the weld metal are shown in Table 1. It can be seen that the major difference between welds obtained without ( Figure 6a) and with the nanoparticle-based coating (Figure 6b-h) was in the microhardness values of the weld metals, as shown in Table 1. This was due to the application of a consumable welding wire. Additionally, based on the results shown in Figures 6 and 7 it can be seen that there were drops in microhardness occurring in the base metal near the melt line. However, there were differences between specimens obtained without and with the coating. Namely, in the specimen welded without the coating (0) drops in microhardness occurred in Line 1, right next to the weld line (Figure 6a, marked with black arrows). In specimens welded with coatings (Figure 6b-h) no such drop in microhardness was noted, but a similar drop in microhardness was observed right under the bottom of the weld (Figure 6b-h), which was not observed in the specimen welded without the coating (Figure 6a). The increased microhardnesses obtained in the weld metal were the result of the application of a consumable material in the form of a welding wire.
with coatings (Figure 6b-h) no such drop in microhardness was noted, but a similar drop in microhardness was observed right under the bottom of the weld (Figure 6b-h), which was not observed in the specimen welded without the coating (Figure 6a). The increased microhardnesses obtained in the weld metal were the result of the application of a consumable material in the form of a welding wire.

Discussion
The application of a nanoparticle-based coating applied before the TIG welding had multiple effects. The first and most important was the increase in weld penetration, even though the welding without the coating applied was used with a welding wire and a relatively small V-groove. The increase in penetration was 61% in the case of the most effective mixture containing 2% SiO2 and 3% TiO2 nanoparticles, closely followed by 1% SiO2 and 4% TiO2 nanoparticles. Unary TiO2 coatings proved clearly superior to SiO2-based coatings. These results were not in agreement with the results obtained in studies by Venkatesan et al. [22,31], who found that a SiO2-based coating was more effective compared to a TiO2-based coating at the same welding parameters. The results obtained by Venkatesan et al. [22,31] could be explained by a higher electronegativity of Si (Pauling scale 1.9) versus Ti (Pauling scale 1.54) [20,32,33]. However, one possible explanation in the discrepancy between this study and studies by Venkatesan et al. [22,31] is the size of oxide particles in the coating. Namely, when mixed with the solvent (i.e., acetone), the results suggested that particle agglomeration of TiO2 was more favorable towards a smaller size compared to SiO2. A higher amount of relatively small particles in the flux can be well correlated to a higher effectiveness of a TiO2-based coating

Discussion
The application of a nanoparticle-based coating applied before the TIG welding had multiple effects. The first and most important was the increase in weld penetration, even though the welding without the coating applied was used with a welding wire and a relatively small V-groove. The increase in penetration was 61% in the case of the most effective mixture containing 2% SiO 2 and 3% TiO 2 nanoparticles, closely followed by 1% SiO 2 and 4% TiO 2 nanoparticles. Unary TiO 2 coatings proved clearly superior to SiO 2 -based coatings. These results were not in agreement with the results obtained in studies by Venkatesan et al. [22,31], who found that a SiO 2 -based coating was more effective compared to a TiO 2 -based coating at the same welding parameters. The results obtained by Venkatesan et al. [22,31] could be explained by a higher electronegativity of Si (Pauling scale 1.9) versus Ti (Pauling scale 1.54) [20,32,33]. However, one possible explanation in the discrepancy between this study and studies by Venkatesan et al. [22,31] is the size of oxide particles in the coating. Namely, when mixed with the solvent (i.e., acetone), the results suggested that particle agglomeration of TiO 2 was more favorable towards a smaller size compared to SiO 2 . A higher amount of relatively small particles in the flux can be well correlated to a higher effectiveness of a TiO 2 -based coating compared to a SiO 2 one. Microparticles have a lower tendency to agglomerate [34], and therefore electronegativity in A-TIG welding on penetration depth can be of greater importance.
The most convenient particle size distribution was obtained with mixtures of SiO 2 and TiO 2 nanoparticles, most particularly 2Si3Ti, which closely corresponded to the highest penetration of the specimen obtained with the application of this particular coating. These results are in accordance with the results obtained by Venkatesan et al. [22,31], who also obtained the highest penetration not with single-component coatings, but rather with mixtures-particularly the SiO 2 and TiO 2 metal-oxides, which were also used in our study.
When nanoparticles are used, as in the case of the present study, it must be noted that the nanoparticle nominal size cannot be considered as influential. Rather the distribution of nanoparticles in accordance to their size is of utmost importance, which accounts for the occurrence of nanoparticle agglomeration, the sizes of which were measured by a Zetasizer in the present study. The results observed with nanoparticle mixtures could be the result of a lower tendency to agglomerate-that is, a higher amount of relatively small particles agglomerates with size up to 1 µm. A similar result regarding a lower tendency to form agglomerates with nanoparticle mixtures was obtained by Efimov et al. [35].
The results of penetration suggest a significant advantage of the application of coatings for TIG welding versus the preparation with an applied consumable wire. The main mechanism of the increased penetration depth was the reversal of the Marangoni convection, proved by the microstructural study. The material flow model with molten metal temperature indicators are shown in Scheme 2. In the specimen welded without the coating (specimen 0), the material flowed from the center of the molten surface towards the melting lines and then towards the bottom and back to the centerline. In this process, the heat was dissipated mainly to the base metal on the sides. This means that the highest heat transfer was towards this area, which caused recrystallization and grain growth, as documented by the microstructure shown in Figure 4a (recrystallization in the horizontal plane). The unchanged microstructure under the weld suggests that the heat transfer towards this area was much less intensive, which is understandable since the melt dissipated its heat to the sides, next to the weld. This was proved by the microhardness measurements with minimum values obtained in this area (Figure 6a). On the other hand, a reversed Marangoni convection caused the material to flow inwards and towards the bottom of the weld, increasing penetration, as well as heat transfer under the weld into the base metal. This means that the area under the weld received the most heat, which resulted in the austenite grain growth-that is, recrystallization in the vertical plane (Figure 5b-d). Similarly to the specimen obtained without the coating, specimens with the applied coating showed a marked drop in microhardness under the weld (Figure 7b-h).
Metals 2019, 9, x FOR PEER REVIEW 9 of 12 compared to a SiO2 one. Microparticles have a lower tendency to agglomerate [34], and therefore electronegativity in A-TIG welding on penetration depth can be of greater importance. The most convenient particle size distribution was obtained with mixtures of SiO2 and TiO2 nanoparticles, most particularly 2Si3Ti, which closely corresponded to the highest penetration of the specimen obtained with the application of this particular coating. These results are in accordance with the results obtained by Venkatesan et al. [22,31], who also obtained the highest penetration not with single-component coatings, but rather with mixtures-particularly the SiO2 and TiO2 metaloxides, which were also used in our study.
When nanoparticles are used, as in the case of the present study, it must be noted that the nanoparticle nominal size cannot be considered as influential. Rather the distribution of nanoparticles in accordance to their size is of utmost importance, which accounts for the occurrence of nanoparticle agglomeration, the sizes of which were measured by a Zetasizer in the present study. The results observed with nanoparticle mixtures could be the result of a lower tendency to agglomerate-that is, a higher amount of relatively small particles agglomerates with size up to 1 µm. A similar result regarding a lower tendency to form agglomerates with nanoparticle mixtures was obtained by Efimov et al. [35].
The results of penetration suggest a significant advantage of the application of coatings for TIG welding versus the preparation with an applied consumable wire. The main mechanism of the increased penetration depth was the reversal of the Marangoni convection, proved by the microstructural study. The material flow model with molten metal temperature indicators are shown in Scheme 2. In the specimen welded without the coating (specimen 0), the material flowed from the center of the molten surface towards the melting lines and then towards the bottom and back to the centerline. In this process, the heat was dissipated mainly to the base metal on the sides. This means that the highest heat transfer was towards this area, which caused recrystallization and grain growth, as documented by the microstructure shown in Figure 4a (recrystallization in the horizontal plane). The unchanged microstructure under the weld suggests that the heat transfer towards this area was much less intensive, which is understandable since the melt dissipated its heat to the sides, next to the weld. This was proved by the microhardness measurements with minimum values obtained in this area (Figure 6a). On the other hand, a reversed Marangoni convection caused the material to flow inwards and towards the bottom of the weld, increasing penetration, as well as heat transfer under the weld into the base metal. This means that the area under the weld received the most heat, which resulted in the austenite grain growth-that is, recrystallization in the vertical plane (Figure 5b-d).
Similarly to the specimen obtained without the coating, specimens with the applied coating showed a marked drop in microhardness under the weld (Figure 7b-h). Recrystallization in the horizontal plane that occurred in TIG without the coating was less convenient, weakening the material perpendicular to the weld, unlike the recrystallization in the vertical plane in A-TIG with a truly multifunctional coating. This means that three crucial benefits are obtained with the application of coatings: 1. Improved penetration, along with all accompanying benefits of a lower shielding gas consumption, an increased productivity, a lower environmental impact, and lower costs; 2. A lower negative influence on the material's microstructure and subsequently higher tensile strength, which is proportional to the hardness of the material; Recrystallization in the horizontal plane that occurred in TIG without the coating was less convenient, weakening the material perpendicular to the weld, unlike the recrystallization in the vertical plane in A-TIG with a truly multifunctional coating. This means that three crucial benefits are obtained with the application of coatings:

1.
Improved penetration, along with all accompanying benefits of a lower shielding gas consumption, an increased productivity, a lower environmental impact, and lower costs; 2.
A lower negative influence on the material's microstructure and subsequently higher tensile strength, which is proportional to the hardness of the material; 3.
A completely eliminated consumable material (i.e., welding wire) containing CRMs such as silicon and a near-CRM such as chromium.
However, special attention must be paid to the root of the weld, keeping in mind that the material flow and its heat transfer was to the bottom part of the weld metal, reducing viscosity and increasing the tendency of the molten metal to flow, causing over-penetration.

Conclusions
Based on the results presented in this work, the following conclusions can be drawn:

•
Coatings containing metallic oxide nanoparticles offer a higher A-TIG welding penetration compared to the specimen without the coating; • The added consumable material in TIG results in a higher hardness of the weld metal compared to the specimens that were re-melted-that is, welded without the consumable material (A-TIG); • The penetration of A-TIG specimens depends primarily on the particle size in the coating solution, not the primary particle size, due to the agglomeration of nanoparticles; • smaller size of agglomerates in coatings with an increased TiO 2 content caused a higher coating effectiveness, leading to an increased penetration that reached a value of 61% over that of the TIG weld without the coating; • The presence of the reversed Marangoni convection was proved by the recrystallization of certain areas of the base metal near the melt line: near the surface in TIG specimen and under the weld in A-TIG specimens. These results were proved by microhardness measurements, with marked drops in microhardness values in recrystallized areas.
An A-TIG coating based on metallic oxide nanoparticles acting as catalysts for welding process gives several benefits to the resulting weld: improves technological properties and improves environmental friendliness (higher penetration, lower shielding gas consumption, improved productivity, etc.), mechanical integrity (weak parts in A-TIG specimens are theoretically under the weld rather than to the sides) and finally, a complete elimination of CRMs and near-CRMs such as Si and Cr, respectively. On the other hand, the catalytic activity of the coating may cause over-penetration as the high-temperature molten metal flows directly downwards, towards the weld root.