KAKO ODREDITI STABILNOST LUKA?

HOW TO DETERMINE THE ARC STABILITY?

Marjan Suban, Janez Tušek

Adresa autora / Author's address: Institut za varilstvo (Welding Institute), Ptujska 19, 1000 Ljubljana, Slovenia

Ključne reči:

• MIG/MAG zavarivanje

• stabilnost luka

• dinamička karakteristika Izvod

U radu so prikazane raznolike metode procene stabilnosti MIG/MAG zavarivanja. Stabilnost procesa zavarivanja utjiče na zavarljivost i zavisi od mnogobrojnih parametara. Najnepogodniji rezultati loše stabilnosti luka su izštrcki, koji su problematični u smislu gubitka materijala, produženje vremena izrade zbog dodatnog čiščenja, kao i zbog lošeg izgleda šava. Metode, koje su opisivane u ovom radu baziraju na merenju ovisnosti jačine struje zavarivanja i napona zavarivanja od vremena. Rezultati eksperimentalnog rada baziraju na analizi stabilnosti električnog luka sprovedene kod zavarivanja u različitih zaštitnim atmosferama gasa. Takoñe su uporeñena dva različita režima zavarivanja. Ispitivana su zavarivanja sa kratkim lukom i sa prskajučim lukom.

Keywords:

• MIG/MAG welding process

• arc stability

• dynamic characteristics Abstract

The paper treats several methods of evaluating the stability of MIG/MAG welding processes. The stability of the welding process influences weldability and is affected by numerous parameters. The most unfavourable results of poor arc stability are spatters which are problematic in terms of material losses, extension of production times due to cleaning, as well as unaesthetic appearance. The methods described in the paper are based on measurement of time-varying welding current and welding voltage. The results of the experimental part of the paper are based on stability analyses carried out with different gas-shielding atmospheres. Also two different welding regimes were compared. The first was short-circuit material transfer, and the second spraying material transfer.

1 INTRODUCTION The introduction of automation and robotisation in industry requires real-time monitoring and control of welding processes. At the same time a more stable welding processes are required, which ensure a simpler monitoring and control. A question is if a simple and rapid method can be found to assess the process stability. A literature review indicates several methods. The majority of them are based on an analysis of the signals provided by a monitoring system. The monitoring system is based on real-time measurement of several physical quantities, of which the two predominating are measurement of

welding current intensity and that of welding voltage [3,

4]. The stability of the welding process can be determined also on the basis of an analysis of noise

emitted [5] or arc light [6], acoustic emission from a material or by means of a high-speed camera recording occurrences in the arc. In the experimental work described, a comparison of the above-mentioned methods for the analysis of welding-process stability on the basis of measurements of welding current intensity and welding voltage was made. The results of signal processing and their

comparison will be interpreted in terms of welding-process stability.

2 MATERIAL TRANSFER AND WELDING-PROCESS STABILITY Depending on the forces occurring in the arc and, first of all, welding current intensity, differing material transfer modes may occur in the welding arc. On the basis of a study of the phenomenon, the International Institute of Welding worked out a classification of

material transfer modes [7]. For MIG/MAG welding processes the following material transfer modes are characteristic:

• short-circuit transfer,

• drop transfer,

• spray transfer,

• pulse transfer. The stability of the welding process is a property of the welding arc. An ideal welding arc, i.e. a stable welding process, shows the following properties:

• a uniform material transfer,

• with the short-circuit transfer, the arc burning time and the short-circuit time should be uniform,

• with the spray transfer, time between the transfer of two subsequent drops should always be the same,

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• with the pulsed transfer, the transfer of one drop per pulse is preferred,

• constant arc length,

• no spatters. The most unwanted result of poor welding-process stability are spatters. Spatters are formed of the drops of molten metal which are not a part of the weld but solidify at the weld surface. In addition to producing a loss of material, spatters are unwanted because of a poorer appearance of the weld. They are a problem not only for the quality of the weld, but they also influences the welding equipment negatively. Adhesion of spatters to a welding nozzle will reduce shielding-gas flow, which may become turbulent. Cleaning of the welding nozzle thus is required during the operation, which additionally extends the production time. The stability of a welding process can be assessed by the control and analysis of the results obtained with measurements. The least complicated and most frequently used method of analysis of the welding process stability is based on the measurement of the dependence of welding current intensity and welding voltage on time, which is followed by a statistical

analysis of the signals measured [8, 9, 10].

3 EXPERIMENTAL PROCEDURE The experiments were focused on measurements of welding voltage and current in the MIG/MAG welding process. Welding was fully automated. Surfacing was applied to a sheet with a quality of low-alloy structural steel. A synergic inverter power source and a suitable automated system for burner guidance were used. A filler material used was a 1.6 mm solid welding wire of quality SG 2. Three types of the shielding atmosphere were used in the experiments, i.e., pure CO2, a two-component 82% Ar/18%CO2 gas mixture, and a four-component T.I.M.E. (Transferred Ionized Molten Energy) gas mixture (65% Ar/26.5% He/8%CO2/0.5% O2). The measuring part of the system consisted of a shunt, low-pass filters, analogue-to-digital and digital-to-analogue (A/D-D/A) converters, and a personal computer. The sampling frequency of the measurement chain was 6 kHz.

3.1 Dynamic characteristic Variations in arc length produce variations in welding current intensity and arc voltage, which is indicated by the dynamic characteristic of the welding arc (see Figure 1). The following dependence is thus obtained: U = f1(t) and I = f2(t) (1). In practice the variations in current and voltage occur very fast and under the influence of various factors such as electric and thermal conductivity of the arc, electrode and workpiece, wire-extension length and arc length, the type of shielding medium used, etc.

Figure 1: Dependence of welding current and voltage

on time in short-circuit material transfer.

3.2 Probability distribution of short-circuit periods and arc burning In the short-circuit material transfer, the welding process in the direction of the time axis may be evaluated in the following manner. The short-circuit transfer welding process may be divided, along the time axis, into two characteristic phases:

• arc burning time,

• short-circuit period. The above random variables may be statistically processed and presented in terms of probability distributions (see Figure 2).

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Figure 2: Analysis of arc burning time and short-circuit period.

Figure 3: Probability distribution of short-circuit period with CO2, Ar/CO2, and T.I.M.E shielding gases. I = 90 A, U = 21 V.

Figure 3 shows three diagrams indicating the differences in short-circuit periods with three differing shielding gases but with the same other welding parameters. For pure-CO2 welding the variation of the curves during the short-circuit periods are smaller than in the two other cases. Therefore it may be concluded that the welding process is more stable in this case than in the other two cases. Two characteristics of the short-circuit periods in T.I.M.E. are a large dissipation and two extremes. They both reduce the welding-process stability.

3.3 Probability distribution of voltage and current Evaluation of stationary random processes, i.e. of time-varying welding current and voltage in our case, can be performed also in the direction of ordinate x(t). In the direction of the ordinate x(t), a mean value mx, standard

deviation σx, and probability distribution p(x) can be calculated as shown in Figure 4.

Figure 5 shows the probability distribution only for the welding current intensity but for three differing shielding gases and with low (medium welding current intensity is 100 A) and high (medium welding current intensity is 430 A) welding parameters. The occurrence of spatters primarily depends on the welding current intensity. The higher the welding current intensity in the short circuit, the stronger the pinch-effect force. Consequently, breaking of the short-circuit bridge will be more explosive. The highest welding current intensities in the short circuit were obtained in welding with the T.I.M.E. mixture (Figure 5 - left, below), and the lowest, however, in welding with the pure CO2 (Figure 5 - left, above). A stronger variation in the probability distribution with the high welding parameters and pure CO2 (Figure 5 - right, above) occurs because there is no spray transfer in this case. Consequently, spattering is stronger.

CO2 Ar/CO2 T.I.M.E.

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p [%]

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p [%]

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Figure 4: Evaluation of the process in the direction of ordinate x(t).

Figure 5: Probability distribution p(I) with CO2, Ar/CO2, and T.I.M.E. shielding gases. Left: I = 90 A, U = 21 V; Right: I = 420 A, U = 34 V.

3.4 Fourier analysis For the process evaluation in the direction of the time

axis, the discrete Fourier transform [11, 13] can be used. Signals received from digital processors are discrete time signals which are defined only at the moment of sampling. The discrete Fourier transform

F(ω) = F[f(nT)] is a sequence of (complex) samples

{F(ω)} in the frequency space defined by

∑−

=

⋅⋅⋅−⋅⋅=1

0

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n

TnjeTnfF ωω )()(

(2) where N is the number of samples, T sampling time,

and ω sampling frequency calculated as follows:

1-N , ... 2, 1, 0,=k , TN

k

⋅=ω

(3).

ωωωω [Hz] ωωωω [Hz]

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S(ωωωω)

The absolute value |F(ω)| of the complex spectrum F(ω) is the spectrum of amplitude density of the function

f(nT). The expression |F(ω)|2 is marked by a symbol

S(ω),

2

)()( ωω FS =

(4) and is called an energy spectrum of function f(nT).

Figure 6 shows a comparison of energy spectra for three different shielding gases and low and high welding parameters. Exceptionally a uniform and stable short-circuit material transfer occurs in pure CO2 welding (Figure 6 - left, above). For the T.I.M.E. mixture two frequencies of short-circuit transfer are characteristic (Figure 6, right, below). As already mentioned, there is no spray transfer in CO2 welding. Consequently, there is no characteristic frequency to be detected (Figure 6, right, above). The diagram in the right lower corner shows that spray material transfer in the T.I.M.E. gas mixture is the stablest transfer, the frequency of the drop transfer being approximately 410 Hz.

Figure 6: Energy spectra of a signal in welding with CO2, Ar/CO2, and T.I.M.E. shielding gases. Left: I = 90 A, U = 21 V; Right: I = 420 A, U = 34 V.

3.5 Stability analysis based on a cyclogramme Dynamic processes in the arc may be demonstrated also in another way, i.e., by cyclogrammes (dynamic movement of the working point) in Figure 7. A cyclogramme shows the welding voltage as a function of the welding current intensity. Cyclogrammes are a very

uncomplicated and fast presentation as far as the welding process stability is concerned. Monitoring of the process by means of a cyclogramme may be carried out in real time.

In short-circuit transfer, two characteristic zones of the U-I characteristic may be noticed in the cyclogramme. The arc burning zone is characterised by a higher welding voltage and a lower welding current intensity, and the short-circuit period by a low short-circuit voltage and a higher short-circuit intensity (Figure 7, left, above). If the three left diagrams in Figure 7 are compared, a greater stability is observed in welding with the pure CO2 (U-I characteristic occupies a smaller area).

With an increase in welding current intensity, the U-I characteristic will displace to the upper right corner of the diagram, which means higher welding voltages and a higher welding current intensity. In the case of Ar/CO2 and T.I.M.E. mixtures, the spray transfer occurs. In pure CO2 welding, a smaller number of transfers from the arc burning zone to the short-circuit zone may be observed. This considerably reduces arc stability and essentially increases spattering.

Figure 7: Cyclogrammes for welding with CO2, Ar/CO2, and T.I.M.E. shielding gases. Left: I = 90 A, U = 21 V; Right: I = 420 A, U = 34 V.

4 CONCLUSIONS

The methods described in the paper are based on measurement of time-varying welding current and

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welding voltage. A measurement chain was composed, experiments were carried out, and a comparison among three differing gas mixtures and two different welding regimes was made. The welding process stability may be determined in several ways. It was established that stability evaluation by means of cyclogrammes is extremely uncomplicated and also fast. Thanks to the process rapidity and surveyability, a system for monitoring the welding process in real time can be elaborated. The comparison of the welding process stability with three different shielding gases showed that in the case of short-circuit material transfer, the welding process is more stable when using the pure CO2, and in the case of spray transfer, when using the T.I.M.E. mixture. The T.I.M.E. mixture is for the short-circuit transfer less suitable since its development was based on high productivity of the welding process, and consequently spray material transfer. As regards stability, the common Ar/CO2 mixture, which is universally applicable, represents the golden mean.

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