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

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The dynamic effect was realized, excluding the ECP passing through the metal treated. The treatment was performed by a single ECP of specimens of 150 × 30 × 4 mm sizes using EDT mode at charge voltage U ch = 350 V and capacity of CES C = 6600  F. As the results of investigations showed (Fig. 5), the grains of the untreated metal are characterized by a substructure (Fig. 5a) with sizes d s in the range of 1.8…5  m, as well as uniform distribution of density of dislocation structure between the volume  vol and boundary  b of grains. Value  vol reaches 6 · 10 9 cm -2 and  b reaches value of 8 · 10 9 cm -2 , that leads to the absence of dislocation density gradient  vol in the volume of grains. a b

Fig. 4. Fractograms of fractures of alloy AMg6, produced at fracture of specimens, where zone A is the treated region near the metal surface, zone B is a middle region of fracture: without EDT – (a) ×33; after EDT – (b) ×33.

b

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a

Fig. 5. Fine structure of alloy AMg6, where  b and  vol are the gradients of density of dislocations along the boundaries and in the volume of grains: (a) in initial state; (b) after dynamic effect; (c) after electrodynamic.

In metal after dynamic effect (Fig. 5b) a substructure is observed both of dispersed d s  1.1  m, and also of large size d s  3.2  m without formation of distinct sub-boundaries. Increase in level of dislocation density near the intergranular boundaries of  b and also gradient  vol between the inner volume of grains  vol  6 · 10 8 …4· 10 9 cm -2 and  b  2 · 10 11 cm -2 were observed. After the electrodynamic effect the metal is characterized by the formation of substructures (Fig. 5c) with clear boundaries d s = 0.8…2.5  m. Moreover, there is a reduction in density of dislocations  b as compared with metal after dynamic effect, as well as uniform their distribution within all the volume of metal (without abrupt gradients along the grain boundaries  b ) between the inner volume of grains  vol  2…3· 10 10 cm -2 and near intergranular boundaries  b  6…8· 10 10 cm -2 . Formation of this structure confirms the suggested conception, based on the theory of electron-dislocation interaction, about contribution of ECP to relaxation of residual stresses, see Baranov et al. (2001). To evaluate the effect of electrodynamic effects on residual stresses, the treatment was performed on specimens of butt welded joints of alloy AMg6 of 400 × 300 × 4 mm sizes with a central butt weld, made by the automatic TIG(Ar) welding at mode of arc voltage U a = 18 V, welding current I a = 250 A and speed V w = 3.1 mm/s. The double-sided treatment of welded plates was made by series of ECP using EDT mode at charge voltage U ch = 550 V and capacity of CES C = 6600  F. The distance between zones of applying the electrodynamic effects did not exceed 5 mm. Measurements of residual stresses were made by using the method of electron speckle-interferometry (Lobanov at al. (2006, 2016b)). 2.4. Residual stresses and fatigue resistance of specimens