Influence of Frequency and Induction of Longitudinal Magnetic Field on The Electrode Metal Loss and its Spattering During MAG-Welding

It is possible to increase the MAG-welding efficiency by controlling the electrode metal mass transfer at the reduction of discharge coefficient on spattering by influence of longitudinal magnetic field on the arc. The paper identifies a range of longitudinal magnetic field frequencies and induction which provide the discharge coefficient reduction of the electrode metal; it has also been found the characteristics of their mutual influence on electrode metal mass transfer process; mathematical models correlating the frequency and induction of longitudinal magnetic field length with loss coefficient of electrode metal on spattering are presented; technological recommendations, the implementation of which will allow to improve the efficiency of MAG-welding in industrial environments, are given.


Introduction
Mechanized welding method in active shielding gas by solid cross-section wire -MAG-weldingcertainly has a number of advantages over the manual arc welding process. However, along with the advantages this method has drawbacks that reduce the effectiveness of its use: the low productivity compared to the automatic welding methods; quality dependence of the welded joint on the skill of the welder; significant metal loss on spattering, which constitutes up to 10 ... 20% of welding wire weight.
Spattering is accompanied by the ejection from arc zone the sprays of molten metal of various size, which come into physical and chemical interaction with the surface layers of the weld metal [1]. The protection of metal surface from spray sticking and / or its cleaning leads to the need of additional works in a volume of 20 ... 40% of the overall complexity of the welding operations [2].
Among the main reasons of metal droplets ejection from the welding zone the following are pointed out: unstable metal transfer, when the power which separetes a drop from the electrode is directed away from the bath and a drop is ejected beyond its bounds; local explosive gases expelling in the volume of metal of welded bath caused by metallurgical reactions; destruction of molten metal bridge formed during metal transfer with the short circuits as a result of a sharp increase in current density under narrowing of the tie plate (pinch effect) [3].
The metal spattering intensity depends on the metal wire composition, the protective mixture components and the surface state of the base metal edges; the size and the ratio of welding variables; characteristics of the power supply (the relationship between the dynamic characteristics of the power supply and loss on spattering of the electrode metal), and others. [4,5]. Therefore, we have made an assumption about the possibility of a control over the process of electrode metal mass transfer by the influence of the external magnetic field on the arc. However, the accurate data about the spattering indicators under MAG-welding with the use of additional influence of an external electromagnetic field on the arc are practically absent or are controversial [4,6].

Problem setting
The research object is the process of arc welding by smelting electrode in the medium of active protective gases with an additional influence of an external electromagnetic field on the arc.
The subject of study is the influence of the frequency and the induction of a longitudinal magnetic field (further in the paper -LgMF) on the processes of drop-formation and mass transfer of electrode metal during MAG-welding.
The aim of this work is to study the impact of LgMF induction and frequency on the loss coefficient of the electrode metal on spattering ψ and droplet distribution of electrode metal into fractions during MAG-welding, as well as the development of technological recommendations that will improve the technological and economic efficiency of this welding process.
The main objectives of the study are: -determination of the frequency range and inductions of LgMF, providing the reduction of ψ coefficient; -characterization of their mutual influence on the process of mass transfer of electrode metal; -development of mathematical models relating the LgMF frequency and induction with a discharge coefficient on electrode metal spattering.
The introduction of new technologies in a production environment will improve the efficiency of MAG-welding and will lead to material, labor and energy reduction.

Materials and methods
In the experiments we use the retrofit of UD-209-UHL4, universal rectifier VDU-506 (i.e. Multioperated Arc Rectifier-506); analytical laboratory scales. Rollers were fused on the flat carbon steel St3sp (i.e. dead-melted steel, carbon content -0,14-0,22 %) GOST 380 (all-Union State Standard) by welding wire of brand Sv-08G2S (i.e. welding wire, 0,8 -carbon content, manganese -2%, siliconless than 1 %) in the carbon dioxide medium of the highest grade (GOST 8050 (all-Union State Standard). To create an external electromagnetic field we used a specialized nozzle of welding head with integrated solenoid, powered by a laboratory device LATR-1 and specialized mode control unit .
The induction components of LgMF B Z , were measured near the plate surface at a distance from the flat end of the electrode to the 5·10 -3 m plate. The induction of constant magnetic field was determined by teslameter of EM-4305 type with the Hall sensor and the base of 1 × 1 mm, and the alterative -by teslameter of F-4356 type with a Hall sensor and the base 4 × 4 mm.
The level of electrode metal spattering was estimated by a coefficient, which is numerically equal to the percentage of the wire mass, spent respectively on spattering (m р = m п -mweld) and on the formation of a roll of 10 -1 m (m п = γ п · l п ·π·d п /4) where: mweld -weld mass, equal to the difference of sample masses before and after welding, kg; γ п -wire material density, in kilograms; l п -the length of wire needed for the formation of roll of 10 -1 m, m.
The parameter change of the metal electrode mass transfer during welding with an additional influence of external LgMF of different frequency was investigated by method based on welding on a copper disc rotating at a predetermined angular velocity. In experimental conditions the angular velocity was 0,185с -1 . Nonstick coating was periodically applied on its upper surface with a diameter of 0.5 m. To ensure the same arcing process time (15 seconds), the welding power source, the pulse generator of LgMF and the specialized drive burner were put into operation with the aid of a control device with an electronic timer. We began the experimental data processing with measuring the distances between the adjacent droplets (burns) as well as for determining the transfer frequency.
In drawing up the experiment matrix two factors, affecting the result, were taken -LgMF induction and its frequency. The loss coefficient of the electrode metal, the number of drops of electrode metal were the consequence of this matrix. To determine the mathematical models the orthogonal central composite second-order plan was chosen. Induction B Z and pulse rate fimp were changed within 0 ... 80 mT (T-Tesla) and 0 ... 50 Hz, respectively ( Experimental data processing was performed using the software StatSoft Statistica. The received dependence between the parameters was shown in the form of three-dimensional graphs (Figs 1 and  2).
A greater impact on ψ has the induction value of LgMF than its frequency according to the analysis of the dependence of electrode metal loss coefficient from LgMF induction and frequency (Fig. 1). With increasing LgMF induction from 0 to 18 mT the loss coefficient decreases in average by 25%; in absolute values (equation (1)) from 14 to 10%.
With increasing frequency of LgMF from 0 to 36 Hz with the values of its induction up to 35 mT we also observed a reduction in the loss coiefficient of electrode metal. A further increase in LgMF frequency has the opposite effect -the ψ coefficient increased from a minimum of 10% to 12.5 %. That is, the additional influence on the arc by outer LgMF is positive, as in welding without LgMF implementation under the same operation conditions the loss coefficient is equal to 14.2% as compared to 12.5%.
The influence on welding arc by a longitudinal magnetic field by 40 mT induction increases the loss coefficient of the electrode metal. It was experimentally proved that a significant impact on the number of electrode metal droplets has LgMF induction value and as well as its frequency (see Figure 2).
From the viewpoint of mass transfer of the metal electrode the number and the size of formed droplets have the importance in the process of MAG-welding. It is known that with decrease in droplet size the stability of arc welding process increases and the metal electrode loss on spattering reduces. In order to establish an optimum mode range of LgMF (frequency and induction) under which a minimum number of large (> 2 d э ) drops with the increase of the total number, a series of experiments were conducted, their results are shown in the Table 2 and 3.  Histograms of the distribution of the electrode metal droplets at MAG-welding with additional LgMF influence (Fig. 3). Fig. 3 shows the dependence of the electrode metal amount from LgMF frequency where the columns 1-6 correspond to LgMF frequencies from 0 to 50 Hz with a step 10. As we can see with increasing LgMF frequency from 0 to 10 Hz the number of droplets almost doubled. What is more the number of small droplets fraction significantly increases: fractions from 1.5 and 2.0 mm increase in 5 times; 2.5 mm -in 4 times. A number of droplets of a larger fraction is reduced: 3.0 mm and 3.5 to 30%.
With increasing LgMF frequency from 10 to 20 Hz the number of droplets also increases and reaches 158 units, i.e., in 2.15 times. Moreover, as in the previous case, the number of small fraction droplets increases significantly: 1.5 mm fractions increase in 8 times; 2.0 mm -in 9.2 times; 2.5 mmin 5 times respectively. A number of drops of large fraction is also reduced, on average by 30%.
With further increase in the LgMF frequency (from 30 to 50 Hz) we observed the stable high rates of droplets formation of small fraction, reducing the number of 3.5 mm fraction drops in three times and the lack of droplets with a diameter of 5 mm. For 40 mT values of LgMF induction the number of drops also increases in 2.15 times and reaches 168 units. What is more, as in the previous case, the number of small fraction droplets significantly increases -1.5 mm in 6.3 times; 2.0 mm in 10 times; 2.5 mm in 7.4 times. A number of large fraction drops is also reduced on average by 15%. With further increase of LgMF induction (from 40 to 60 Hz) we observed consistently high indices of small droplets formation and the reduction in number of 3.0 mm drops fraction, which halved in number. Droplets with the diameter of no more than 5 mm are absent when an arc is subjected to a magnetic field induction from 60 mT.
During the analysis of LgMF frequency and induction influence it was found that a greater influence on droplet size reduction of electrode metal had a frequency of the electromagnetic impulse than its (magnetic field) amplitude values.
The dependence graphs of the drops number of the electrode metal during MAG-welding with the additional LgMF influence (Fig. 4) are plotted; we obtained the mathematical relationships (Table. 4  Thus, it was found that: -the loss coefficient of electrode metal is minimal on condition that the influence on the arc by alternate LgMF is 16 ... 34 Hz frequency and induction is 18 ... 38 mT; -the maximum number of droplets formed while influence on the arc of alternate LgMF with 12 ... 36 Hz frequency and 70 ... 90 mT induction, in this case a transition to the jet electrode metal transfer is observed, and the total number of drops is doubled compared to the welding process without the LgMF use; -a greater influence on the reduction of droplet size of electrode metal has a frequency of the electromagnetic impulse than its (magnetic field) amplitude values.
However, it should be noted that the additional impact on the arc of external/outer LgMF with values of B Z = 70 ... 90 mT is negative, as the loss coefficient of the electrode metal is greatly increased. The LgMF modes optimization in terms of the lowest values of the loss coefficient and the maximum number of drops of electrode metal were performed graphically. Optimization results are shown in Fig. 5. As we can see in Figure 3 the achievement of the jet electrode metal transfer during MAG-welding with minimum losses is possible by influence of longitudinal magnetic field induction of 26 ... 38 ... 22 mT and frequency of 34 Hz on the arc. Appliance of LgMF induction of more than 75 mT is not recommended due to the significant loss of electrode metal.
Thus, the proposed model can be used to determine the LgMF conditions for the arc MAG-welding by steel solid wire with a diameter up to 2·10 -3 m to minimize losses of welding materials.

Conclusions
It was determined that the efficiency of MAG-welding process can be achieved by jet electrode metal transfer while minimizing its losses spray with an additional influence of longitudinal magnetic field on the arc. The mathematical dependences for determining the number of droplets formed in separate factions, and a map of LgMF modes optimization are obtained. The recommendations are presented which allow to achieve the best results saving of welding materials with MAG-welding by low-carbon and low-alloy steels using external LgMF, namely: -the core wire diameter of 1.