VI Международная студенческая научная конференция Студенческий научный форум - 2014

INFLUENCE PULSED ARC WELDING

ON THE STRUCTURE WELDED JOINTS

Krampit M.A.

Scientific advisor: Krampit A.G.

The pulsed arc welding processes result not only in the controlled transfer of electrode metal but also in the formation of high-quality welded joints with the required mechanical properties and uniform chemical composition. Welding with the pulsed power supplied to the welding arc is a variety of pulsed arc welding, which increases the strength of the metal of welded joints. In this work, the authors present the results of investigations of the mechanical properties of welded joints produced by CO₂ welding with a pulsed arc.

Experimental procedure

To determine the mechanical properties and the chemical composition of the weld metal, sheets with the size of 150x300x12mm made of steel St3 with the U-shaped edge preparation were welded. Welding was carried out on a copper backing sheet in CO₂ with Sv08G2S wire with a diameter of 1.6mm with pulsed power supplied to the welding arc (pulsed arc).

The pulsed arc welding conditions were electrode wire feed rate V_f=300 m/h, welding speed V_w=20 m/h, the frequency of applied pulses f=60 Hz, pulse time p=5.5 ms, mean current I_m=250A, the amplitude value of the welding current in the pulse I_a=670 A, mean voltage U_m=36V, electrode extension 20 mm, and consumption of the shielding gas 50 l/min.

In welding with the pulsed power supply, the previously mentioned equipment was supplemented with the IRS-1200 ADM and VDU-504 power sources with an additional power section [1].

The welded sheets were used for the preparation of specimens for tensile and impact toughness testing and also for investigating shavings for the determination of the chemical composition of the weld metal by spectral analysis. All the specimens were produced in accordance with the GOST 6996 standard. The impact tests were carried out at a temperature of 20 and 2408C. The notch was made on the side of the final joint.

Experimental results and discussion

The special features of the pulsed arc welding process and, in particular, the alternation of the powerful short-term pulses of welding current influence the structure of the resultant welded joint. For example, welding with the pulsed power supply increases the flight speed of the electrode metal droplet, and this reduces the holding time of the droplet in the high-temperature zone. This influences the chemical composition of the weld metal and, consequently, the mechanical properties of the welded joint.

In welding with the pulsed power supply, the weld pool oscillates continuously with the frequency equal to the frequency of repetition of the current pulses. The metal in the pool is thoroughly stirred by the effect of the pulsed arc pressure and the impact of electrode metal droplets. The intensity of staring creates suitable conditions for the release of gases present in the liquid metal and results in refining of the grains of the solidified metal [2], as confirmed by the structure of the metal of the fusion zone and the welded joint in welding with pulsed arcs (Figures 1 and 2).

Figure 1. Microstructure (X100) of the metal of the fusion zone in welding with a pulsed arc.	Figure 2. Microstructure (X100) of the weld metal in welding with pulsed power supply for the welding arc.

The quality of the welded joint is controlled by the structure and properties of the weld metal and by the heat-affected zone (HAZ), which depends strongly on the thermal welding cycle. Since welding with the pulsed power supply is accompanied by pulsations of the thermal power of the arc, this influences the conditions of heating and cooling of the metal. In welding with the pulsed power supply, the weld metal is characterized by the equilibrium granular structure of ferrite, with small pearlite inclusions distributed along the grain boundaries. The structure is refined by the periodic effect of the pulses of welding current on the liquid metal of the weld pool and, consequently, on the solidification region. Analysis of the melting zones shows that in welding with the pulsed power supplied to the welding arc, there is no distinctive transition between the welded joint and the HAZ and the structure of the superheated zone is finer.

In welding with the pulsed power supply to the arc, the duration of formation of a droplet and the holding time of the droplet in the arc gap are considerably shorter.

The increase in the mechanical properties (tensile strength and impact toughness) in welding with the pulsed power supply is explained by the favourable conditions of solidification of the metal; the increase of the carbon content by 0.003%, the manganese content by 0.05%, the silicon content by 0.04% in the weld metal; and also by a reduction of the content of non-metallic inclusions. The tensile specimens failed through the parent metal.

Welding with the pulsed power supply has a beneficial effect on the mechanical properties of the welded joint and chemical composition of the deposited metal.

References

1. Knyaz’kov AF, et al. Improvement of the welding process in CO₂ welding in modernisation of equipment. Remont, Vosstanovlenie, Modernizatsiya. 2008; (9): 7–9.

2. Kpampit NY, Kpampit AG. The effect of pulsed power supply of the welding arc on the structure of the welded joint. Voprosy Materialovedeniya. 2003; (2): 45–51.

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