In keyhole PAW, as more metal is melted, the depth of the partial keyhole increases such that the arc length and arc voltage increase (Ref. 24). Unfortunately,in GTAW, such positive correlation between the arc voltage and weld penetration is not observed as the authors originally expected. Analysis suggests that the free arc in GTAW does not severely deform
the weld pool surface to form a significant cavity. Instead, as more metal is melted, the increase in the volume elevates the weld pool surface toward the electrode such that the arc length and arc voltage are decreased. The authors thus propose an assumption on the dynamic development of the weld pool and weld pool surface as illustrated in Fig. 2 for GTAW on a square butt joint with zero root opening.The dynamic development of the weld pool in GTAW is divided into two stages by the voltage/arc length minimum that is caused by the establishment of complete joint penetration. In Stage 1, the increased volume due to metal melting reduces the
distance from the weld pool surface to the electrode. In Stage 2, the arc pushes the liquid
metal through the bottom surface, and the ability of the arc to push increases as the bottom width of the weld pool increases.More specifically, when the base metal is melted, its volume increases due to thermal expansion. The increased volume Δv increases as the melted metal volume v increases.Before the bottom surface of the base metal is melted, i.e., before complete joint penetration is established, the increased volume Δv is placed as an elevation of weld pool surface above the original base metal surface. The elevation of the front weld pool (volume beyondthe original surface),efv, equals Δv and thus increases as the melted metal increases
and the weld penetration depth increases.Because of the free arc in GTAW, the arc length and arc voltage are expected to decrease.This is different from keyhole PAW where a significant cavity
(partial keyhole exists)such that the arc length increases as the cavity depth increases
and the bottom of the cavity is close to the bottom of the liquid metal pool. However,
after the complete joint penetration is established,the arc pushes part of the liquid
metal beyond the original bottom surface of the base metal to form an elevation of the bottom weld pool surface. Denote the volume of this part of the liquid metal as ebe. As a result, efv = Δv – ebv. Before comlete joint penetration is established, ebv = 0. Hence, this equation also holds before complete joint penetration is established.Before complete joint penetration is
established, efv = Δv. Since Δv increases as penetration increases, efv also increases
with the penetration. Because the arc length is defined approximately as the average
length from the electrode to the weld pool surface in the arc region, the arc length and its measurement (arc voltage) decrease as vfv increases. After complete joint penetration is established, ebv > 0 such that efv < Δv. When d(Δv)/dt > d(ebv)/dt, efv still increases as Δv increases but at a reduced rate. The arc voltage will thus reduce at a reduced rate. However, as
the penetration increases such that the width of the bottom weld pool surface increases,
d(Δv)/dt < d(ebv)/dt may occur. As a result, efv will reduce instead. Because the capability of the arc to push the liquid metal to produce ebv increases with the penetration, efv will reduce at an increased rate as the penetration increases.This discussion and analysis of the dynamic
development of the weld pool is illustrated using Fig. 3. As can be seen, complete joint penetration is established first and the minimum arc length is realized when the change from d(Δv)/dt > d(ebv)/dt to d(Δv)/dt < d(ebv)/dt occurs. The authors would argue that the arc length minimum is more significant than the establishment of complete penetration. This is because,
for any practical application, a perfect alignment of the arc center with the weld joint and a perfect symmetry of the weld pool are not realistic. Without an adequate bead width on the bottom surface, the two members of the base metal would not be fully joined through their entire interface.The authors would also argue that the penetration achieved when reaching the arc length minimum should not be excessive.In fact, d(ebv)/dt is controlled by the width of the bottom weld pool surface.The arc length minimum thus must occur before the width of the bottom weld pool surface becomes excessive such that more metal moves to the bottom. As a result, the
arc length minimum should be a simple yet effective indicator for adequate complete penetration. If the degree of complete joint penetration needs to increase or reduce, the slope of the arc voltage can be analyzed.In GTAW, the arc voltage can be considered proportional to the arc length with certain accuracy and linearity. Assume the GTAW torch has a constant standoff distance from the pipe surface, which is common in welding practice. By measuring the
arc voltage signal, the arc length, or distance from the tungsten electrode to the weld
pool surface, can be determined with certain accuracy. The arc length minimum can thus be detected as the voltage minimum.A series of pulsing GTAW experiments was designed to verify the above observations,analyses, and assumptions on the dynamic development of the weld pool.
The welding current was switching between lower level base period (20~40 A)and higher level peak period (90~140 A).To better monitor the weld pool, the GTAW torch moved for a constant small
distance during each base period, typically 1~2 mm, and then stopped during the next peak period to gain penetration.Schedule 10 AISI 304 stainless steel pipes with 3.5-in. OD (outer diameter) were used as base metal. Two pieces of pipes were butt jointed with zero nominal root opening. A tentative set of welding parameters is listed in Table 1.During the experiments, a data acquisition system measured the welding signals(e.g., arc voltage, welding current) at
a sampling rate of 1 kHz. A typical peak period is illustrated in Fig. 4. All stages are marked in Fig. 4. It can be observed that the arc voltage is decreasing continuously during stage 1,
which indicates the increase in the front weld pool elevation above the pipe surface.The decrease in the arc voltage then slows down and finally reaches the voltage minimum. At this point, the arc voltage reaches the lowest level during the entire peak period. It indicates that the front weld pool has achieved its largest elevation.At the same time, the bottom side of
the pipe joint must have been adequately melted. If the peak current continues to be applied, the volume of molten base metal pushed beyond the bottom surface of the pipe may become more than necessary. As expected for Stage 2, the arc voltage starts to increase, which means the weld pool surface starts to drop down and adequate complete joint penetration is said to be
achieved. The arc voltage signal in the peak period (except for the sharp spike at the beginning) is curve fitted. This signal can be considered to follow a second order parabola, as indicated in Fig. 3. The vertex of the parabola indicates the voltage minimum.Therefore, an algorithm can be proposed to continuously measure arc voltage signal and find the vertex. By
switching the current from the peak level to the base level at the vertex, adequate complete penetration can be produced as a result of a feedback control.