天津职业技术师范大学2013届本科生毕业论文
Fig. 1. Schematic diagram of the welding systems.
3.Results and discussion
3.1. Droplet transfer in GMA welding
3.1.1. Droplet transfer with 1.2 mm diameter wire Fig. 2 shows droplet transfer behaviour with a 1.2 mm diameter welding wire for various average welding currents and wire feed speeds (WFSs). Increasing the average welding current also increases the heat input and wire melting rate. Stable drop transfer was observed at low average currents such as 100 A and 150 A. At medium average currents such as 200 A and 250 A, the droplets became ragged while hanging from the wire and became spherical during transfer owing to surface tension. At these medium average currents, a tapered electrode tip was inferred from the increased side condensation of electrons [12]. At high average currents such as 300 A and 350 A, in-flight explosion was observed during droplet transfer. The temperature of the welding arc increased with the arc current, and radiation heat from the high-temperature arc overheated the droplets to above the boiling point (1560 ??C) of Al 5183 alloys [6].
3.1.2. Droplet transfer for different wire diameters
31
天津职业技术师范大学2013届本科生毕业论文
Fig. 3 shows successive high-speed images for different wire diameters with the same average welding current of 150 A. Under these conditions, although the average heat input and melting rate per unit time are almost the same, droplet transfer behaviour is greatly affected by the diameter of the filler wire. The current at which the transition from stable transfer to explosive transfer occurs is inversely proportional to the wire diameter. As shown in Fig. 4, transition average currents for 1.2 mm, 1.6 mm, and 2.4 mm filler wire diameters are 300 A, 200 A, and 150 A, respectively.There are two possible causes of this phenomenon: pulse parameters and arc heating mode.
For GMA welding, different pulse shapes were employed for different wire diameters. At the same average current of 150 A, the peak current and peak time for 1.2mmwirewere 377 A and 1.5ms,respectively, and those for 2.4 mm wire were 450 A and 3.8 ms, respectively. A higher heat content is thus applied in the peak time to melt a larger diameter wire.
A nearly ?at wire tip was observed for the 2.4 mm wire, and tapered tips were observed for the 1.2 mm and 1.6 mm wires. In case of the 2.4 mm wire, arc heat was transferred to the wire tip through the hanging droplet. For smaller wires, arc heat was partially transferred to the solid wire by direct side condensation of electrons, which resulted in a tapered tip and reduction in drop overheating temperature [12]. 3.2. Droplet transfer in plasma-GMA hybrid welding
Figs. 5 and 6 show droplet transfer behaviours for plasma GMA hybrid welding with 1.2 mm and 1.6 mm wires. As given in Table 3, the plasma currents were set as 250 A and 260 A for1.2 mm and 1.6 mm wires, respectively. With increasing GMA current, the droplets became smaller but stable drop transfers were observed for all GMA currents.Compared with GMA welding,plasma-GMA hybrid welding had a lower GMA current,and globular droplet transfer was observed in plasma-GMA hybrid welding rather than the spray droplet transfer in GMA welding.
In Ref. [6], a linear relationship between heat input to droplet and drop temperature
32
天津职业技术师范大学2013届本科生毕业论文
was proposed for GMA welding with Al wire. In GMA welding, the total power input to the base material consists of direct arc heating from the welding arc to the base material and heating from droplets impinging into the weld pool. The heating from droplets is proportional to the total heat input, given by the multiplication of arc current and voltage,which was determined for GMA welding with steel and Al filler wires using calorimeter measurement [13,14].
As shown in Fig. 6(c), droplet transfer was stable even at wire feed speeds as high as 20 m/min because of the relatively low GMA current. In GMA welding with 1.6 mm wire, the wire feed speed for the transition current (200 A) was 7.2 m/min. Owing to the additional heat input from the surrounding plasma arc, plasma-GMA hybrid welding afforded three times the wire feed speed (i.e.,melting rate) afforded by GMA welding.
Table 2 Welding parameters for GMA welding.
Wire Wire feed Peak current (A) Peak time (ms) Base current (A) Frequency (Hz) diameter (mm) speed 1.2 1.6 2.4
(m/min) 6.4-22 3.9-12.6 1.7-6.3 340-480 380-420 410-477 410-477 2-2.3 3.8 64-150 54-210 46-198 105-360 65-240 20-192 Table 3 Welding parameters for plasma-GMA hybrid welding.
Wire diameter (mm) Plasma current(A) Wire feed speed(m/min) Peak time (ms) Peak Base Frequency (Hz) 1.2 1.6 250 250 12-20 12-20 5 5 140-250 230-380 100 100 100 100 current (A) current (A) 33
天津职业技术师范大学2013届本科生毕业论文
GMA current:100A,WFS:6.4m/min(Peak current;339A,Base current:64A,Peak current
time:1.3ms,base current time:7.8ms)
GMA current:150A,WFS;9.3m/min(Peak current;377A,Base current;80A,Peak current
time;1.5ms,base current time4.5.ms)
GMA current200A,WFS:13.2m/min(Peak current:390A,Base current:91A,Peak current
time:1.5ms,base current time:2.8.ms)
GMA current250A,WFS:16.7m/min(Peak current:414A,Base current:107A,Peak current
time:1.5ms,base current time:1.8.ms)
GMA
current300A,WFS:19.2m/min(Peak current:452A,Base current:129A,Peak current
time:1.6ms,base current time:1.1.ms)
34
天津职业技术师范大学2013届本科生毕业论文
GMA current350A,WFS:22.0m/min(Peak current:480A,Base current:150A,Peak current
time:1.6ms,base current time:0.8.ms)
Fig. 2. Droplet transfer in GMA welding with 1.2 mm diameter wire
1.2 mm wire,WFS;9.3m/min(Peak current;337A,base current;80A,Peak current time;1.5ms,base current time;4.5ms)
1.6 mm wire,WFS;5.7m/min(Peak current;394A,base current;76A,Peak current time;2.1ms,base current time;7.1ms)
2.4 mm wire,WFS;2.7m/min(Peak current;450A,base current;75A,Peak current time;3.8ms,base current time;12.2ms)
Fig. 3. Droplet transfer in GMA welding at same average welding current (150 A)
35