The features of AC arc burning. The diagrams of currents and voltages for the AC arc.

It should be noted that all the properties of the DC arc are inherent in the AC arc. When the arc burns at the alternating current of 50 Hz, there are conditions that reduce the stability of the arc combustion. First of all, it is worth noting that when the polarity of the arc is changed on the electrodes, when the arc voltage (transformer) is zero, the arc goes out. Thus, the welding arc goes out, and then ignites again 100 times in 1 second. This causes special requirements to the power source of the arc (transformer) and to the welding circuit. The main influence is the Continue reading →

The surface work of the critical dimension nucleus formation.

In the formation of the crystallization nucleus, the part of the energy is released when the bulk phase occurs, and part of the work is spent on creating the surface partition, the system resists, prevents the formation of the interface. We substitute rкр values into the energy balance formula:

If the amount of energy released by the passage of the certain volume of a liquid phase with a large level of free energy into the solid phase with a lower level of free energy would be equal to the amount of energy spent on the formation of the interface, then ΔG would be equal to zero. If the energy was released (allocated) more than was expended, then ΔG would be less than zero. In our case, ΔG is positive and equal to 1/3A of the nucleus formation. This means that the system Continue reading →

The isolation of the excess phase from the supersaturated solid solution (Part 2).

With the appearance of β-phase crystals, the α-solution surrounding them is depleted by component B. Near the β phase, the solid α-solution has the Сг composition, and in the distance the composition remains the same. This heterogeneity of the solution is weakened with diffusion in the α-phase and the β-phase release. If the exposure at Тв is prolonged, the separation of the β phase will cease and the stable equilibrium between the α crystals of the Сг and β composition will be established by crystals of the composition Сд. The quantitative ratio of the phases in this case is β/α = гв/вд. To further isolate the phase β, it is necessary to cool the alloy. When it is super cooled, for example, to the Те temperature, the α phase of the composition Сг is supersaturated by the component B (the degree of Continue reading →

The amount of metal evaporating from the cathode spot (Part 2).

At present, it is considered that the value of the ion fraction of the current in the welding arcs is approximately β ≈ 0.5. Assuming that the value of β does not depend on the value of the welding current (I), we determine the number of ions transferring the ion current in the cathode region (Ni) at different currents by the formula

Ni = Ii/е = βI/е,

where e — the electron charge, Кл, and the corresponding number of atoms evaporated from the cathode spot (Nп), according to the formula Nп = Gп/ mFe, Continue reading →

The crystals shape (Part 2).

With the increase in the growth rate of crystals, the dimensions of the cells decrease. With intensive heat removal through the solidified metal, when the growth rate of the crystals is high, the cellular structure does not arise. Thus, at the low cooling rates, the surface of the crystallization front is almost smooth. At medium – the cellular structure appears, the elements of which are crushed with the acceleration of cooling. At the high cooling rate, the branches appear in place of the cells, indicating the initiation of dendritic crystallization.

In the presence of a super cooled melt zone in front of the crystallization front, in Continue reading →

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