2. PROCESS CHARACTERISTICS AND METAL TRANSFER CHARACTERIZATION
2.2. RESEARCH OBJECTIVES
2.4.5. Analysis of Arc Stability Control using Current – Voltage Diagrams
Arc stability provides a fundamental criterion to characterize welding quality. However, the analysis of arc stability in arc welding can be quite complex due to the different variations associated with arc characteristics, power source characteristics and waveform design.
Different methods, from experimental to numerical techniques, have been applied to characterize the arc stability, but none has been successfully able to identify the unstable phenomena from only arc current and voltage waveform electrical data. As was observed during the previous section of this chapter, the analyses of metal transfer phenomena gave some valuable information about the unstable mechanisms. However, the method of synchronised high speed video-images with arc current and voltage waveform is difficult to apply to the overall welding length and also time consuming.
Hence in this thesis, a new method is proposed to characterize the arc stability by analysing solely the arc current and voltage waveforms. Previous work has been published to analyse the mechanism of metal transfer associated with cross plots (Scotti 2000) (Huang and Yapp 2000), but these methods have not been applied to characterize the arc stability and reach conclusions about welding performance. The method developed consists in correlating directly the arc voltage and current at each instant of time and observing the performance of waveform cycles, in “UI diagrams”.
The UI diagrams provide an alternative representation of the waveform cycle of arc voltage and current. The dots correspond to the arc current and voltage at different instants of time. As is observed in the following generic UI diagram, synchronized with the corresponding waveform (Figure 2.70), different analysis features of the waveform cycle can be more clearly seen in the U-I diagram. The distribution of dots which correspond to the pulse region is indicated in Figure 2.70, while short circuiting is indicated by a line of dots, where voltage is proportional to current, due to the resistive nature of current flow for this process. Furthermore, the dispersion of the dots provides information about the consistency or stability of the process.
Time [ms]
Figure 2.70 – Waveform (a) and UI Diagram (b) correspondent for a generic GMAW process.
Ib Ip
Up
Ub Short-circuiting region
Pulse region
a) b)
103 The UI diagrams are therefore useful for characterization of arc stability and also provide important information about metal transfer phenomena. The variability of arc current and voltage waveforms is a response of the stability of the process, associated with the setting conditions applied.
The evaluation of arc stability will be presented in terms of:
Variation of WFS at different shielding gases and WFS/TS ratio;
Variation of arc length parameter adjustment (trim, ALC or Base Current);
Variation of dynamics (DC, PC, FP);
Variation of CTWD.
The experimental conditions applied to this research (unless otherwise indicated) were presented in Section 2.4.1. The following subsections will present the results obtained for the different waveform designs considered.
2.4.5.1. Characterization of Arc Stability for GMAW-P
The arc stability diagrams obtained from variation of WFS, at two different shielding gases (2.5%CO2 97.5%Ar and 1.5%CO2 54%He 44.5%Ar), are illustrated in the Figure 2.71. The distribution of the points on the waveform cycle has much less scatter at high WFS levels. It is also noticed that the variability of the cycle points is wider when 1.5%CO2 54%He 44.5%Ar was used. This is due to the increase of the arc voltage. For both gases a high concentration of the points at low voltage is observed (from low to high current levels) which indicates the existence of short-circuiting phenomena.
At low travel speed level (WFS/TS ratio of 18) the distribution of the points has less scatter suggesting an increase of the stability associated with the waveform cycle. The presence of short-circuiting is, however, still apparent (Figure 2.72).
a) b)
104
I [A]
0 200 400 600 800
U [V]
0 10 20 30 40 50
WFS = 4 m/min WFS = 6 m/min WFS = 8 m/min WFS = 10 m/min
Figure 2.72 – UI diagram of the variation with WFS for GMAW-P, using WFS/TS ratio of 18.
The analysis of the effect of trim demonstrates that the variability of the points is higher at low trim levels, which suggests that higher arc stability is achieved at higher trim (Figure 2.73).
I [A]
0 200 400 600 800
U [V]
0 10 20 30 40
50 Trim = 0.50
Trim = 0.75 Trim = 1.0 Trim = 1.25 Trim = 1.50
Figure 2.73 – UI diagram of the variation with trim for GMAW-P.
The effect of CTWD is the most significant among the analyses performed for GMAW-P. It is observed that at 11mm the unstable short-circuiting phenomena is significantly high, and decreases with the increasing of CTWD (Figure 2.74).
105
Figure 2.74 – UI diagram of the variation with CTWD for GMAW-P.
2.4.5.2. Characterization of Arc Stability for RapidArc
The effect of WFS (when WFS/TS of 16 was applied) suggests that the distribution of the points on the waveform cycle is very stable from 4 to 8m/min but an unstable phenomena at 10m/min is associated with voltage drops at low current levels. It is also observed that the variability of the points in terms of voltage is slightly higher when 1.5%CO2 54%He 44.5%Ar was applied (Figure 2.75).
I [A] changes can be observed, when compared with WFS/TS ratio of 16 (Figure 2.76).
a) b)
106
I [A]
0 200 400 600 800
U [V]
0 10 20 30 40 50
WFS = 4 m/min WFS = 6 m/min WFS = 8 m/min WFS = 10 m/min
Figure 2.76 – UI diagram of the variation with WFS for RapidArc, using WFS/TS ratio of 18.
The analysis of the effect of trim demonstrates high instability phenomena from 0.5 until 1.25, and a uniform distribution of points is only reached at trim of 1.5 (Figure 2.77).
I [A]
0 200 400 600 800
U [V]
0 10 20 30 40
50 Trim = 0.50
Trim = 0.75 Trim = 1.0 Trim = 1.25 Trim = 1.50
Figure 2.77 – UI diagram of the variation with trim for RapidArc.
The effect of CTWD reveals a good cycle performance for the three levels studied, but a much compact distribution of the points when 11mm was applied (Figure 2.78).
107
Figure 2.78 – UI diagram of the variation with CTWD for RapidArc.
2.4.5.3. Characterization of Arc Stability for STT
The effect of WFS (when WFS/TS ratio of 16 was applied) suggests that the distribution of the points is significantly unstable at high WFS levels (8 and 8.26m/min). The application of 1.5%CO2 54%He 44.5%Ar shows significantly changes in the shape of the points distribution (current and voltage variations), when compared with 2.5%CO2 97.5%Ar, but no noteworthy stability variations can be observed (Figure 2.79).
I [A]
Figure 2.79 – UI diagrams of the variation with WFS for STT, using WFS/TS ratio of 16: a) 2.5%CO2 97.5%Ar;
b) 1.5%CO2 54%He 44.5%Ar.
At lower welding speeds (when WFS/TS ratio of 18 was applied) no significant changes, are observed, when compared to the WFS/TS ratio of 16. However, the distribution of the points suggests a slightly increase of the arc stability (Figure 2.80).
a) b)
108
I [A]
0 200 400 600 800
U [V]
0 10 20 30 40 50
WFS = 3 m/min WFS = 6 m/min WFS = 8 m/min WFS = 8.26 m/min
Figure 2.80 – UI diagram of the variation with WFS for STT, using WFS/TS ratio of 18.
The analysis of the effect of trim demonstrates that no significant changes are observed, but at 1.5 some instability is observed (Figure 2.81).
I [A]
0 200 400 600 800
U [V]
0 10 20 30 40
50 Trim = 0.50
Trim = 0.75 Trim = 0 Trim = 1.0 Trim = 1.25
Figure 2.81 – UI diagram of the variation with trim for STT.
The effect of CTWD reveals a good cycle performance for the three levels studied, but a more condensed points distribution is observed at the highest level (16mm) (Figure 2.82).
109
Figure 2.82 – UI diagram of the variation with CTWD for STT.
2.4.5.4. Characterization of Arc Stability for CMT
The effect of WFS (when WFS/TS ratio of 16 was applied) suggests that the instability phenomenon is higher at the highest WFS level (8m/min). This phenomenon is illustrated by a significant change in the shape of the cycle points. No significant changes are observed between both gases analysed (Figure 2.83).
I [A]
Figure 2.83 – UI diagrams of the variation with WFS for CMT, using WFS/TS ratio of 16: a) 2.5%CO2 97.5%Ar;
b) 1.5%CO2 54%He 44.5%Ar.
The use of WFS/TS ratio of 18 (lower welding speed, considering constant WFS) suggests a slightly increase of the arc stability, compared to the WFS/TS ratio of 16 (Figure 2.84).
a) b)
110
I [A]
0 200 400 600 800
U [V]
0 10 20 30 40 50
WFS = 3 m/min WFS = 4 m/min WFS = 6 m/min WFS = 8 m/min
Figure 2.84 – UI diagram of the variation with WFS for CMT, using WFS/TS ratio of 18.
The analysis of the effect of ALC demonstrates that no significant changes are observed in terms of arc stability, but significant changes can be observed in shape of the cycle points (Figure 2.85).
I [A]
0 200 400 600 800
U [V]
0 10 20 30 40 50
ALC = -30%
ALC = -15%
ALC = 0%
ALC = 15%
ALC = 30%
Figure 2.85 – UI diagram of the variation with ALC for CMT.
No significant variations can be observed when DC was varied (Figure 2.86).
111
I [A]
0 200 400 600 800
U [V]
0 10 20 30 40
50 DC = -5%
DC = -2.5%
DC = 0%
DC = 2.5%
DC = 5%
Figure 2.86 – UI diagram of the variation with DC for CMT.
No significant changes are observed for the effect of CTWD on arc stability (Figure 2.87).
I [A]
0 200 400 600 800
U [V]
0 10 20 30 40 50
CTWD = 11 mm CTWD = 13.5 mm CTWD = 16 mm
Figure 2.87 – UI diagram of the variation with CTWD for CMT.
2.4.5.5. Characterization of Arc Stability for CMT-P
The analysis of the effect of WFS (using WFS/TS ratio of 16) for both shielding gases reveal that the arc instability is more significant between 8m/min and 10m/min and more pronounced when 1.5%CO2 54%He 44.5%Ar was applied (Figure 2.88). The unstable phenomenon is associated with larger voltage changes, occasional at 6m/min and more often at 8m/min, and high voltage variability for 10m/min.
112
At lower welding speeds (when WFS/TS ratio of 18 was applied) the arc instability is higher at the high WFS levels (8 and 10m/min) but similar to that observed at WFS/TS ratio of 16 for low WFS levels (4 and 6m/min) (Figure 2.89).
I [A]
Figure 2.89 – UI diagram of the variation with WFS for CMT-P, using WFS/TS ratio of 18.
The analysis of the effect of ALC demonstrates that the stability is higher at 30%. At low ALC values (-30 and -15%) is observed very high dispersion of the cycle points, while at 0% and at 15% occasional voltage excursions can be identified (Figure 2.90).
a) b)
113
I [A]
0 200 400 600 800
U [V]
0 10 20 30 40 50
ALC = -30%
ALC = -15%
ALC = 0%
ALC = 15%
ALC = 30%
Figure 2.90 – UI diagram of the variation with ALC for CMT.
The effect of PC does not suggest a strong effect on the instability phenomena. However, at high PC level higher voltage excursions can be observed (Figure 2.91).
I [A]
0 200 400 600 800
U [V]
0 10 20 30 40 50
PC = -5%
PC = -2.5%
PC = 0%
PC = 2.5%
PC = 5%
Figure 2.91 – UI diagram of the variation with PC for CMT-P.
The effect of CTWD demonstrates that in general no significant instability phenomenon can be observed. However, when the CTWD applied was 11mm an occasional voltage excursion can be identified (Figure 2.92).
114
Figure 2.92 – UI diagram of the variation with CTWD for CMT-P.
2.4.5.6. Characterization of Arc Stability for FastROOT
The results obtained on the effect of WFS (using WFS/TS ratio of 16) demonstrate that no significant variations are observed between the different WFS levels evaluated. However, the application of 2.5%CO2 97.5%Ar suggests a much more stable distribution of the points (Figure 2.93). demonstrate much higher instability in the distribution of the points (Figure 2.94).
a) b)
115
I [A]
0 200 400 600 800
U [V]
0 10 20 30 40 50
WFS = 4 m/min WFS = 6 m/min WFS = 8 m/min WFS = 9 m/min
Figure 2.94 – UI diagram of the variation with WFS for FastROOT, using WFS/TS ratio of 18.
The analysis of the effect of Base Current (BC) suggests that arc stability is higher at high BC levels (25% and 50%), where the arc current does go higher than 500A (Figure 2.95).
I [A]
0 200 400 600 800
U [V]
0 10 20 30 40
50 'Base Current' = -50%
'Base Current' = -25%
'Base Current' = 0%
'Base Current' = 25%
'Base Current' = 50%
Figure 2.95 – UI diagram of the variation with Base Current for FastROOT.
The effect of Forming Pulse (FP) also suggests higher waveform cycle stability at high FP levels (between 15 and 30%), where the arc current levels are lower and points well distributed (Figure 2.96).
116
I [A]
0 200 400 600 800
U [V]
0 10 20 30 40 50
'Forming Pulse' = -30%
'Forming Pulse' = -15%
'Forming Pulse' = 0%
'Forming Pulse' = 15%
'Forming Pulse' = 30%
Figure 2.96 – UI diagram of the variation with Forming Pulse for FastROOT.
No significant changes are observed to the effect of CTWD in the arc stability phenomenon (Figure 2.97).
I [A]
0 200 400 600 800
U [V]
0 10 20 30 40 50
CTWD = 11 mm CTWD = 13.5 mm CTWD = 16 mm
Figure 2.97 – UI diagram of the variation with CTWD for CMT-P.