PULSED ARC ALTERNATING CURRENT WELDING OF LIGHT ALLOYS IN SHIELDING GASES

B. K. PANIBRATTSEV, A. F. KNYAZKOV, S. G. VARFALOMEEV

(Presented by the Scientific and Technical Seminar of the Static Converters Department of the AEM Research Institute at TPI)

The use of energy pulses for argon-arc welding of thin-sheet metals has been known since 1953. Pulsed-arc welding with a nonconsumable electrode in shielding gases on direct current of steels, copper, titanium, nickel, etc., has found wide application in industry.

It is known that, in welding light alloys, the use of the effect of cathodic sputtering of oxide films facilitates fusion of the edges. A pulsed direct-current arc of reverse polarity, while producing cathodic sputtering, has not found application for welding aluminum and magnesium because of melting of the tungsten rod already at currents of 10–15 amperes.

The use of alternating current for welding aluminum, magnesium, and alloys based on them with a nonconsumable electrode is the most expedient. Owing to the fact that during the half-period when the workpiece is negative, cathodic sputtering of the oxide films and considerable heat evolution at the anode—the tungsten—take place, and in the following half-period, corresponding to straight polarity, some cooling of the tungsten electrode occurs, it is possible to use tungsten rods 1–1.5 mm in diameter, welding workpieces 1 mm thick at currents of 30–40 amperes.

However, argon-arc welding on alternating current of thin-sheet structures made of light metals presents great difficulties. The use of existing power sources and the technology of argon-arc welding with a nonconsumable electrode for light alloys less than 1 mm thick is practically impossible because of burn-throughs and lack of fusion. Reducing the welding current to 10–20 amperes leads to unstable arc burning and disruption of the process.

The arc becomes spatially unstable; the cathode spot appears now on one, now on the other edge of the parts being welded. The edges melt, tending to occupy the minimum surface, while the thickness of the oxide film increases still more with heating. Reducing the arc gap in order to increase the steadiness and stability of arc burning gives no effect and leads to frequent short circuits and contaminates the weld.

The use of heat-conducting backing strips leads to great technological difficulties; however, this measure also does not provide stable quality or the possibility of welding thin-walled structures.

Since the stability of the cathode spot is disturbed, so too is

stability of cathodic sputtering, which leads to lack of fusion at the edges of the parts being welded. The disruption of the stability of the process is explained both by the difference in the thermophysical properties of the electrode materials, leading to the appearance of a constant component, and by the insulating properties of the oxide film on the surface of aluminum. It is known that at 20°C aluminum oxide has a resistance of \(4 \cdot 10^{15}\ \Omega/\text{cm}\) (for comparison, the resistance of porcelain is \(3 \cdot 10^{14}\ \Omega/\text{cm}\)); moreover, as the temperature rises the resistance decreases only slightly, compared with other insulators. Therefore, probably, in the half-periods of reverse polarity, because of the high cooling rate of thin-sheet aluminum and the high temperature strength of the oxide film, the arc power at currents of 10–12 amperes is insufficient for complete destruction of the oxide film and the formation of a stable cathode spot. In the oscillogram shown in Fig. 1, interruptions of the arc current are visible in the half-periods corresponding to reverse polarity. Only when the effective value of the alternating current was increased to 20 amperes did the arc burn comparatively stably.

Fig. 1. Absence of arc current in the half-periods of reverse polarity. Arc current 10 A

At the Research Institute of Automation and Electromechanics at the Tomsk Polytechnic Institute, a method has been developed for pulsed-arc welding of light metals and their alloys on alternating current with a nonconsumable electrode in shielding gases, with cathodic sputtering of oxide films. The developed method makes it possible to weld, in free position, without clamps or backing strips, thin-walled structures 0.5–1.5 mm thick. Figure 2 presents oscillograms of the process of pulsed-arc welding on alternating current. The pulses are a group of sinusoids. By increasing or decreasing the number of periods, and also by changing the amplitude of the sinusoids, it is possible to regulate the thermal power of the pulse; moreover, the developed power source for pulsed welding makes it possible to change the pulse parameters directly during the welding process. The use of pulses for welding makes it possible to raise the current to a value at which the arc burns stably, i.e., up to 20 amperes. Owing to the discreteness of the heat-input process, burn-throughs do not form and warping is significantly reduced.

A characteristic feature of pulsed-arc welding on alternating current of light metals and their alloys is the possibility of controlling the process of melting and crystallization of the metal, which makes it possible to fill the crater, to carry out the start and end of welding with full penetration and without run-on tabs. The technique of manual welding is considerably simplified, which reduces the cost of manufacturing structures.

With pulsed heat input, the molten pool crystallizes in the form of a circle. The duration of the pause is selected so that the mol...

Fig. 2. Oscillogram of the pulsed-arc welding process with alternating current

the molten volume of metal has managed to crystallize by 80%. The pulse repetition frequency and the welding speed are selected so that the pools melted during the pulse overlap by \(2/3\).

The metal volumes that are melted and crystallize during one cycle \(T_{\text{c}} = T_{\text{p}} + T_{\text{pa}}\) form peculiar bridges, significantly reducing sagging of the reverse side of the weld when welding with through penetration. In addition, this is aided by the strength of the oxide film, which is heated considerably less than in the continuous welding mode.

Reduction of the heat-affected zone as a result of concentrated heat input during the pulse \(q_{\text{p}} = 0.24UI t_{\text{p}}\cdot \eta\) increases the mechanical properties of the weld by 4–8%.

Thin-walled structures 0.5–1 mm thick are welded butt-to-butt without filler metal and without flanging the edges, which significantly increases the cyclic strength of the weld, since in this case there is no lack of fusion at the weld root. By applying edge flanging and filler wire, it is possible to weld elements with a wall thickness of 0.2 mm.

The structure of welds made by a pulsed arc is dense; the recrystallization zone is significantly narrower than in ordinary argon-arc welding.

Fig. 3. Photographs of the microstructure. Alloy AMg-6: a) base metal; b) weld; ×600

The microstructure is characterized by uniform dispersion of the \(\beta\)-phase in the solid solution, with finer precipitates of intermetallic compounds. Figure 3 shows photographs of the microstructures of the base metal and the weld. The results obtained in pulsed-arc welding

on alternating current for various types of thin-walled products made of aluminum alloys indicate the high reliability of the method and its advantages in comparison with other arc methods. Pulsed-arc welding has been successfully introduced into production. The use of pulsed-arc welding opens up broad possibilities for creating fundamentally new thin-walled products from light alloys (aluminum, magnesium, and alloys based on them) in apparatus engineering, instrument making, radio engineering, machine building, and other branches of industry.