PSI - Issue 16

Leonid Lobanov et al. / Procedia Structural Integrity 16 (2019) 27–34 Leonid Lobanov, Nikolai Pashсhin / Structural Integrity Procedia 00 (2019) 000 – 000

30

4

deformations, caused by welding, has an effect on the reduction in stress-strain state of welded structures, that will be shown below.

Fig. 3. Distribution of lines of equal radial plastic deformations  r (%) in thickness of plate of AMg6 alloy at L = 5  H, C = 5140  F, U ch = 500 V at the moment of time t = 71.2  s, which corresponds to value I max in the circuit.

2.3. Investigations of structure

Investigations of evolution of structure of structural materials as a result of EDT allowed determining the peculiarities of effect of electrodynamic effects on the mechanism of plastic deforming of metals and alloys as a result of treatment. Peculiarities of structure and relief of fractograms were examined by using the scanning electron microscopy. Effect of EDT on peculiarities of fracture of flat specimens of aluminium alloy AMg6, treated by EDT by a single channel scheme, was investigated. Treatment was performed by a single ESP of base metal and welded joints with 150 × 30 × 4 mm sizes of test part using EDT mode at charge voltage U ch = 500 V and capacitance of CES C = 6600  F. To determine the effect of electrodynamic effect on changing the structure of material the comparison of topography of microrelief of fracture of alloy AMg6 specimens was carried out in initial state (Fig. 4a) and after EDT (Fig. 4b) at the given-above mode. The comparison showed that fractures have mainly a fibrous structure with tear peaks, formed by the mechanism of a mixed fracture (see Kishkina at al. (1988)). To study the relief of fractures after the electrodynamic effect, the specimens with a single-sided treatment of the material were used. It can be seen in Fig. 4b (zone A) that the fracture on the treated side has the more developed fibrous structure as compared with an initial state (Fig. 4a). The depth of spreading the fibres reaches 3.0 mm across the specimen thickness, which characterizes the electrodynamic effect as a volumetric one. At a thorough study of the fracture relief after EDT (zone B in Fig. 4b) the developed groups of plane lines of sliding were observed, the orientation of which coincided with the treated surface of the specimen. Moreover, the sliding has features of a rotation mechanism, which proves the intensive proceeding of the process of volumetric plastic yielding of material in the conditions of electrodynamic effects, see Yakovleva (2003). With growth of density of treated regions of polycrystalline structure the deformational hardening takes place that was confirmed by the results of measurements of hardness HV, which were made in unit M- 400 of “Leco” company at load value of 0.1 N. Value HV for the untreated material (Fig. 4a zone B) was 824 MPa. Maximum values HV = 1290 … 1310 MPa were observed in the treated surface (Fig. 4b, zone A), where together with a plane sliding the rotational sliding was observed. Thus, the hardness of alloy AMg6 after EDT is increased by 35 – 40% as compared with untreated one. Using the method of “thin foils” a fine initial structure of alloy AMg6, as well as its evolution as a result of dynamic and electrodynamic effect by a single-channel scheme of EDT, were investigated (Darovsky et al. (1985)).