10。45    10。46    10。47    10。48    10。49     10。5     10。51    10。52    10。53    10。54    10。55      10。45    10。46    10。47    10。48    10。49     10。5     10。51    10。52    10。53    10。54        10。55

Time (s) Figure 3。9。 Weld voltage and current waveforms for different metal transfer modes

The GMAW process has three basic modes in which metal is transferred from the electrode tip to the work-piece, Figure 3。9。 The modes can be classified as short circuiting transfer, globular transfer and spray transfer。 In short-circuiting transfer, the metal is deposited during a short circuiting of the weld wire which normally lasts about 10ms。 In globular transfer the droplet diameter is larger than the wire and is considered as an unwanted transfer mode。 Spray transfer is a transfer mode where the drops are smaller than the weld wire。 The type of metal transfer mode  is a function of the weld current according to both the static force balance theory and the pinch instability theory [22]。 As the mean current increases, the metal transfer goes from stubbing, through short-circuits and globular to spray transfer mode, see Figure 3。10。

wire electrode droplet Figure 3。10。 Successive transfer modes of metal transfer in GMA welding with increasing mean current (left to right) [23]

For the three main metal transfer modes there is a correlation between the voltage and current envelope waveforms and modes of metal transfer。 The variation in the current and voltage waveform is reduced when moving from short-circuit to spray transfer and can be used to classify the transfer。 To weld in short-circuiting GMAW mode, the open circuit voltage and the electrode wire-feed rate is set to a low value and in spray transfer mode, the open circuit voltage and the wire feed rate is set to a high value。

Time Figure 3。11。 A schematic illustration of weld current and related parameters in pulsed GMA welding。 Tp and Tb denote peak pulse time and background pulse time respectively, and Ip and Ib denote the peak current and background current respectively

3。4Pulsed GMAW In pulsed GMAW the amplitude of the current alternates between two levels, see Figure 3。11。 The advantage of this method is that the mean current, and thus the average heat input to the work-piece, is lower than in spray GMAW。 Thanks to the smaller heat transfer, it is possible to weld thinner plates with pulsed GMAW than with spray GMAW and at the same time maintain a high metal deposition rate。

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[19] Adolfsson S, Bahrami A and Claesson I, “Quality monitoring in robotized welding using sequential probability ratio test”, In Proceedings of TENCO ’96。 Digital Signal Processing Applications, volume 2, pages 635-640, Perth, Western Australia, 27-29 November 1996, IEEE。

[20] Shinoda T, Nishikawa H and Shimitzu T, “The development of data processing algorithms and assessment of arc stability as affected by titanium content of GMAW wires during metal transfer”, In Proceedings of sixth International Conference on Computer Technology in Welding, Lanaken, Belgium, 9-12June 1996。 TWI, Abington Publishing。