The Effect of Nanostructured Functional Ceramics Additives on the Properties of Welding Electrodes

Abstract

The present work aimed to develop a new coated electrode weld based on ceramic materials. Photocatalysts of nanostructured functional ceramics (PNFCs) were synthesized by means of concentrated solar radiation on ceramic materials. This new process allows the efficient conversion of the energy of the primary source into pulsed radiation, through the adjustable and judicious choice of parameters. The photocatalysts also showed high activity when they were introduced into an electrode coating. The main problem is the complexity of the production of NFC photocatalysts on an industrial scale. A new method was developed for producing ZB-1-grade NFC catalysts using conventional technology, with subsequent activation via pulsed radiation generated by functional ceramics. The results of the addition of NFC photocatalysts obtained using this technology on the welding and technological properties of welding electrodes of the MR-3 brand are presented. The introduction of 1% to 8% NFC of the ZB-1 grade into the coating enhanced the stability of arc burning (L_bla) and increased the diameter of the weld point (ø_dp). Moreover, the addition of 1% to 8% NFC of the ZB-1 brand into the coating contributed to a reduction in the losses factor for waste and splashing of electrode metal (ψ) leading to a reduction in the height of the visor at the end of the electrode (h_k).

1. Introduction

Welding processes and equipment are in constant evolution to meet industry needs and requirements. Welding processes and equipment can only be valid if they meet the regulations and standards that ensure the quality and safety of joints. Many research works have been dedicated to their development to achieve safer and economically efficient products with respect to standards and regulations [1].

The manual metal arc (MMA) welding process, usually named coated electrode welding, is when the heat provided to the workpiece is generated by an electric arc established between the end of the electrode and the near surface of the workpieces to be joined. The electrode consists of a metal core and a flux coating. The electrode is readily melted in the form of droplets and consumed. The coated flux ensures the arc stability and protects the weld pool from air gases [2,3,4,5]. The MMA process is the cheapest and most popular technique used in many industries [6,7].

Owing to the varieties and properties, the use of nanomaterials in different industrial fields, such as the power plant, building, agriculture, medicine, industry, defense, radio-electronics, power engineering, information systems, transport, and biotechnology sectors, has grown significantly during this last decade. Moreover, nanomaterials can be fabricated to the desired size, shape, and properties [8].

The application of nanopowders would greatly improve the currently available technological processes and would create qualitatively new production processes. A study conducted by Nauka [9] shows that a nanosized powder of a refractory compound, such as titanium carbide or nitride, produces laser welding joints with greatly improved strength properties. The structure of the welded joints changes from needle-dendritic to quasi-equiaxed and fine-dispersion. The mechanical properties (strength and plasticity) of the weld metal are enhanced. The application of nanostructured materials has been expanded to the diffusion bonding of creep-resisting nickel alloys in pressure welding. The nanostructured, crystalline monoliths of Ni₃Al and NiAl₃ intermetallic compounds were used. The main findings show that the application of an intermediate layer in the form of films activates the process of diffusion bonding of nickel alloys [10].

The joint produced via diffusion bonding, without the nanolayer contained a brittle interlayer reducing the strength, and the structure of the joint with the nanolayer (Ti/Al, thickness 20 mm) were similar to those of the parent metal. The absence of the pores and cracks in the weld zone and the HAZ indicates that the quality of the welded joint is high [11]. Nanopowders are also used in other areas of welding production, such as welding electrodes. Sapozhkov et al. suggest varying the production process of MMA welding electrodes via adding nanopowder Ti, Zr, and Cs to electrode components through liquid glass. These additives support arcing stability, improve strength of a weld joint, and, as a consequence, increase the quality of a weld joint and the structure on the whole [12]. There is not much information published on the use of nanostructured materials in welding production. However, in recent years, research into the use of nanopowders in the welding industry has increased. Adding nanopowder to liquid glass is one of the ways to enhance the mechanical properties of weld metal in applying nanodisperse materials (nanopowders). There are a number of methods of adding nanopowder to a weld pool, but the most effective is to add nanopowder to liquid glass, that is, at the stage of electrode production. So, nanopowders are powders with characteristic nanosizing causing distinct changes in their properties [13]. Nanopowders are used to obtain fine-grained-structured weld metal; when crystallized, these additives ensure grain refinement and ultimately improve the mechanical properties without changing the chemical composition of the alloy. It was determined that the structure of weld metal welded with experimental electrodes is more disperse and homogenous than the uneven structure of metal welded with serial electrodes [14]. Ceramic materials have improved thermal, chemical, and physical stability. Ceramic materials of the formula RCrO₃, where R is a rare earth oxide such as yttrium oxide, are typically used. These materials, although useful in electroconductive applications such as electrodes, suffer from low chemical stability when exposed to temperatures above 1600 °C, low resistance to thermal cycling at temperatures above 1500 °C, and the inability to be heated at high heating rates. These deficiencies have limited the use of these materials in applications where property stability is important. A need therefore exists for rare earth oxide ceramic materials that have improved stability and that are useful in electroconductive applications, as well as in high-temperature environments [15].

In this study, the effect of adding nanostructured functional ceramics of the ZKHM brand [13], obtained at the Big Solar Furnace (BSF) of the Institute of Materials Science of the Academy of Sciences of the Republic of Uzbekistan, on the charge of electrode coatings, consisting of Fe₃O₄-CaO-TiO₂ oxides and calcining welding electrodes with infrared (IR) radiation, generated by functional ceramics on the welding and technological properties of the welding electrode, were studied [16,17,18]. The results of these works revealed the beneficial effect of small “ZKHM” additives and heat treatment of welding electrodes using IR radiation on the welding and technological properties of welding electrodes such as the breaking length of the arc “L_bla”, the diameter of the deposited point “Ø_dp”, and especially the losses factor for the waste and splashing of electrode metal “ψ”. However, from practical and economic points of view, BSF cannot ensure the production of nanostructured functional ceramics on an industrial scale. In this regard, the Institute of Materials Science has developed technology for obtaining a nanostructured functional ceramic (NFC) photocatalyst using conventional technology via the mixing and thermos-synthesis of oxides, followed by activation using pulsed radiation generated by functional ceramics based on lanthanum chromite [15,18,19]. This technology makes it possible to synthesize the target material in the required volumes. The purpose of this work was to study the effect of an NFC photocatalyst (ZB-1 brand), synthesized in an of IR radiation furnace, when added in small amounts (from 0 to 8%) to the composition of a well-known brand of welding electrode, MR-3, on its welding and technological properties. Nanomaterials in the welding production area are still new; therefore, the publications in the field are scarce. This paper is an interesting contribution regarding nanostructured functional ceramics in consumable coating electrodes. On the other hand, the novelty in this work is the methodology used to produce such electrodes.

2. Materials and Methods

In this study, the MMA welding process was used. MMA is a fusion welding process. Arc welding is established between a consumed electrode and the weld pool. The protection of the weld pool comes from the electrode itself. The coating forms slag, protecting the transferring drops and the weld pool from deleterious atmospheric gases, such oxygen, nitrogen, and hydrogen [20,21,22,23].

Both direct current as well as alternating current can be used for MMA welding, but not all of the stock electrode’s coating types can be welded with alternating current, e.g., purely basic electrodes cannot be used [24,25].

During MMA welding, only the current is adjusted. Arc voltage is derived from the arc length, which the welder must maintain. The current-carrying capacity of the electrode diameter must be taken into consideration when adjusting the current. As a rule, the lower limits apply to the welding of root passes and for position flat position (FP), while the upper limits apply to the other positions, filler passes, and final passes. As current increases, the deposition rate and the associated welding speed also increase. Penetration also increases with current.

The specified currents apply only to non-alloyed and low-alloy steels. When working with high-alloy steels and nickel-based alloys, lower values must be selected due to the higher electrical resistance.

The following rules for calculation of individual intensity currents must be respected:

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At a diameter of 3.2 mm, current should be between 90 and 150 A.

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At a diameter of 4.0 mm, current should be between 120 and 200 A.

-

At a diameter of 5.0 mm, current should be between 180 and 200 A.

To conduct this research, a photocatalyst of the ZB-1 brand [18] was used as the addition of NFC photocatalyst to the coating charge the welding electrode MR-3. The ZB-1 photocatalyst consisted of a mixture of powders based on iron oxide (35%), with additives of silicon dioxide (28%), calcium oxide (15%), chromic anhydride (13.5%), aluminum oxide (3.3%), magnesium oxide (3%), and copper oxide (2%), with a particle size distribution of 2–10 microns with a fraction of 5 microns of at least 50% (see Figure 1).

The photographs show how metastable phases are formed from an amorphous state. The particles consist of nanostructures operating in cascades.

The ZB-1 photocatalyst was synthesized using the following steps:

1.

Preparation of the mixture of the specified composition.

2.

Mixing in a planetary mill in an aqueous medium for 5 h.

3.

Subsequent drying and annealing at a temperature of 800 °C for 2 h and subsequent cooling together with the furnace.

4.

Grinding on a planetary mill for 5 h.

5.

Annealing at 1000 °C for 2 h, followed by cooling together with the furnace.

6.

Annealing at 1200 °C for 2 h, followed by cooling together with the furnace.

7.

The resulting powder was placed for 15 min under emitters coated with functional ceramics based on lanthanum chromite (MC-1), which generates pulsed radiation.

The NFC photocatalyst of the ZB-1 brand prepared in this way was added to the mixture of the coating of the welding electrode of the MR-3 brand.

The coating for the welding electrodes of the MR-3 brand was obtained from the manufacturer. The production of electrodes of the MR-3 grade is regulated by the requirements and provisions of GOST 9466-75, GOST 9467-75, and TУ 1272-002-58965179-2006. In accordance with these standards, the filler material belongs to the E46 type. The above-cited electrodes are used for the welding of structural low-alloy carbon steels with a carbon content of greater than 0.25%. The slag base of the MR-3 welding electrode is a ternary system of TiO₂-SiO₂-CaO oxides with a basicity index (BI) of 0.27.

Coatings on welding electrodes were obtained by dipping Sv08A wire, 4 mm in diameter, into a coating mass consisting of a coating mixture of an MR-3 welding electrode, with additions of NFC photocatalyst ZB-1 ranging from 0 to 8% (see Figure 2).

The coating mass was obtained as a result of mixing the investigated charge mixture (with a granulometric composition of not more than 315 μm) with liquid glass (density 1.4 g/cm³ and module 2.5) in a ratio that promotes the formation of a coating layer on the surface of the metal rod, with thickness (D/d) 1.4–1.6 mm (see Figure 3).

After applying the coating, the electrodes were dried at a temperature of 80 °C for 40 min and calcined at a temperature of 180 °C for 60 min. Surfacing was carried out on a plate made of St3sp steel, 4 mm thick, using an inverter-type rectifier of the Jasic TIG-200P type from Jasic Technology Company, Shenzhen, China. The welding arc was powered by alternating current at a welding current level of 140 A.

The welding and technological properties of welding electrodes include the stability of the burning of the arc of the welding electrode or the breaking length of the arc “L_bla”, the diameter of the deposited point “ø_dp” or the quality of its formation, and the size of the visor at the end of the electrode “h_k”. These welding and technological properties of welding electrodes were determined following the method described in precedent research works [16,26].

An indicator of the stable arc burning of welding electrodes is the breaking length of the arc “L_bla”, which was determined using the installation shown in Figure 4a. The value of “L_bla” was determined by measuring the distance between the end of the electrode and the plate formed after surfacing (see Figure 4b).

The influence of the method and calcination modes on the tendency of welding electrodes to form a peak was assessed using the height of the peak at the end of the electrode “h_k” (see Figure 5).

The quality of the formed welded joints was assessed using the deposited beads (see

Table 1. The appearance of weld beads made with experimental welding electrodes obtained from a mixture of MR-3 charge and ZB-1 photocatalyst additives.

№	Composition of the Coating of the Welding Electrode, %		Appearance of Welded Joints
	MR-3	ZB-1
1	100	0
2	99	1
3	98	2
4	96	4
5	98	8

), as well as using the shape and diameter of the deposited point “ø_dp” (see Figure 6). The correct formation of the deposited point without external defects such as undercuts, burns, cracks, and pores indicates the high-quality formation of a welded joint.

Also, studies were carried out on the effect of ZB-1 photocatalyst on the metal melting coefficient “α_p”, the deposition coefficient “α_H”, and the losses factor for waste and splashing of electrode metal “Ψ”, characterizing the processes of welding and surfacing and determined following the method described in [27].

To estimate the magnitude of these losses due to spattering, oxidation, and evaporation (fumes) during the burning of the arc, the so-called loss coefficient for waste and spatter “Ψ” was used, which is determined by the formula:

$$\Psi =\frac{{\alpha }_{p }-{\alpha }_H}{{\alpha }_p}\times 100\%$$

(1)

where the metal melting coefficient “α_p” shows how much electrode metal is melted per unit time per ampere of welding current, which is determined by the formula:

$${\alpha }_p=\frac{G_p}{I·t}$$

(2)

where G_p is the mass of electrode metal melted during time t, g; I is the welding current value, A; t is the arc burning time, hours; α_p is the metal melting coefficient, g/(A·h).

The melting coefficient depends on the electrode material, the composition of its coating, type, polarity, and current density. In addition, during the welding process, the electrode heats up, which also affects the intensity of melting of the electrode metal. Before welding begins, the electrode is at room temperature; by the end of welding, it can heat up to 500 °C.

To evaluate the surfacing process, the surfacing coefficient “${\alpha }_H$” is used, determined by the formula:

$${\alpha }_{H=\frac{G_H}{I·t}}$$

(3)

where G_H is the mass of deposited electrode metal during time t, g; I is the welding current value, A; t is the arc burning time, hours; α_H is the metal melting coefficient, g/(A·h).

The loss coefficient for waste and spatter “Ψ” depends on the composition of the electrode and its coating, the welding mode, and the type of welded joint. For example, the loss factor increases with increasing current density and arc length. Typically, the value of “Ψ” lies in the range:

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From 1 to 3% for submerged arc welding;

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From 3 to 6% when welding in shielding gases;

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From 5 to 10% when welding with thickly coated electrodes;

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From 10 to 20% when welding with thin-coated electrodes.

For values greater than 20% of the loss coefficient, it is not advisable to use electrode welding.

The degree of influence of ZB-1 photocatalyst additives in the electrode coating on the welding and technological properties of welding electrodes was evaluated using the coefficient of determination, R² [28]. The coefficient of determinism is in the range of 0 < R² < 1 and indicates the strength of the linear correlation between the modes and properties under study. When constructing dependency graphs, polynomial approximating curves were used.

3. Results and Discussion

The appearance of the welding beads made with experimental welding electrodes obtained from a mixture of MR-3 charge and ZB-1 photocatalyst additives is presented in Table 1.

We used the surfacing parameters recommended by the manufacturer of the MR-3 electrodes. However, due to the lack of a hydraulic extruder, we manufactured electrodes using the dipping method. Manufacturing electrodes using this method does not allow the production of uniform-thickness coating. Therefore, the produced welded lines are not as stable as when surfacing with electrodes obtained by a hydraulic extruder. Considering that all studied welding electrode compositions were manufactured using the same method and under the same conditions, we believe that these results can be considered correct and industrially accepted.

The results of our studies of the effect of coating compositions on the welding and technological properties of welding electrodes are shown in

Table 2. Results arising from altering the amount of ZB-1 additive in the composition of the MR-3 coating.

No.	Amount of Additive ZB-1 in Composition of MR-3 Coating, %	Welding Electrode or Breaking Length of arc (Lbla), mm	Size of Visor at End of the Electrode (hk), mm	Diameter of Deposited Point (ødp), mm	Loss Coefficient for Waste and Spatter (Ψ), %
1	0	8.2	3.2	11.7	11.1
2	1	8.4	3.1	11.9	6.4
3	2	7	2.6	11	9.2
4	4	7.6	2.6	11.1	9.3
5	8	7	2.4	11.2	6.9

and Figure 7, Figure 8, Figure 9 and Figure 10.

We notice that each value given in the Table 2 corresponds to the arithmetic mean of the results of three measurements. According to the results presented in Table 1 and Table 2 and in Figure 7, the length of the deposited bead after spontaneous arc decay increases, and the formation of the bead improves, when welding with electrodes with the addition of ZB-1 photocatalyst up to 1%. A further increase in the amount of additive in the composition of the coating worsens both the formation of the bead and the stability of arc burning, which is characterized by the value of the breaking length of the arc “L_bla”. Also, it should be noted that the curve of the dependence of the breaking length of the arc “L_bla” on the content of the additive ZB-1 in the electrode coating (see Figure 4) shows a strong correlation between these indicators (R² = 0.548).

Similar results with a strong correlation (R² = 0.606) were revealed when studying the influence of the content of ZB-1 additives in the composition of the MR-3 electrode coating on the diameter of the deposited point “ø_dp” (see Figure 8) and the quality of the bead formation (see Table 1).

By increasing the stability of the arc of welding electrodes and improving the formation of the deposited bead, small additions to the composition of the ZB-1 electrode coating play the role of a surfactant and help to reduce the surface tension of the visor at the end of the electrode. This facilitates the separation of drops from the end of the electrode and transfers the process of metal transfer during welding from large-drop to small-drop [29,30]. In addition, this promotes an increase in the breaking length of the arc (L_bla) and the diameter of the deposited point (ø_dp). However, an increase in the content of ZB-1 additive to more than 1% leads to an increase in the emission during the welding of the gas phase and a decrease in the effect of the surfactant, worsening the stability of the arc and the formation of the deposited point. A very strong correlation (R² = 0.882) was found between the content of ZB-1 additive in the composition of the MR-3 electrode and the size of the visor at the end of the electrode “h_k” (Figure 9). At the same time, the value of “h_k” reduced to 25% when the content of the NFC additive increased to 8%.

Figure 10 shows the results of studies of the effect of the introduction of nanostructured functional ceramic ZB-1 additive into the composition of the electrode coating MR-3 on the loss coefficient for waste and spatter “ψ”, which was characterized by a weak correlation (R² = 0.223). According to the data obtained, the addition of ZB-1 to the mixture of MR-3 electrode coatings reduced this indicator over the entire range of its content. The maximum decrease in the loss coefficient for waste and spatter “ψ” (by 44.5%) was observed when the content of ZB-1 additive was 1%.

The reduction in the loss of waste and spatter via the introduction of NFC photocatalyst is associated with the ability of the additive ZB-1 to generate pulsed radiation during the heating of the coating during welding, which contributes to the effective removal of the moisture remaining in the coating after the heat treatment of welding electrodes. As is known [31], the residual moisture of the coating worsens the welding and technological properties of welding electrodes, producing spatter and uneven melting.

4. Conclusions

The aim of this work was to study the effect of the addition of NFC photocatalysts on the welding and technological properties of welding electrodes of the MR-3 brand. This study focused on the effect of the introduction of 1% to 8% NFC of the ZB-1 grade into the coating on the stability of arc burning (L_bla), the diameter of the weld point (ø_dp), and the losses factor for waste and splashing of electrode metal (ψ). The following main findings can be drawn:

-

The formation of the weld beads improves when welding with electrodes with the addition of ZB-1 photocatalyst up to 1%.

-

The stability of the welding arc (L_bla) improves when introducing up to 1% ZB-1 additive into the coating of welding electrodes.

-

An increase in the diameter of the deposited point (ø_dp) occurs when surfacing with welding electrodes with additions of up to 1% ZB-1 photocatalyst.

-

A decrease in the height of the peak at the end of the “h_k” electrode is detected throughout the entire range of ZB-1 additive content in the coating. At the same time, the “h_k” value is reduced to 25% when the NFC additive content is as high as 8%.

-

The losses due to waste and spattering of the electrode metal (ψ) to 8% decrease with the content of the ZB-1 photocatalyst throughout the entire studied interval, with a maximum at a content of 1%.

-

On the practical side, the use of welding electrodes with additives of nanostructured functional ceramics of the ZB-1 brand in industry not only increases the stability of the welding arc but also reduces the loss of electrode metal due to waste and spatter during welding. This increases the productivity and reduces the consumption of welding electrodes in the production of welding structures and products.