ВИЗНАЧЕННЯ ЗАЛИШКОВИХ НАПРУГ В ІНСТРУМЕНТАЛЬНІЙ СТАЛІ У7А ПІСЛЯ ЕЛЕКТРОЕРОЗІЙНОГО ДРОТЯНОГО ВИРІЗАННЯ

Abstract

The article deals with residual stresses in thin plate of U7A steel, the working surface of which in different technological cases has been formed by different technologies, namely: main cut by wire electrical discharge machining (WEDM), additional passages with different energy of single spark discharge, sequential layer-by-layer removal of heat affected zone (HAZ) material using highprecision controlled electrochemical modes. Residual stresses were first determined by mechanical measuring of residual plate deformations directly in the machine's working bath (without removing the obtained samples) and calculating the stresses by a known equation which describes the relationship between the sample curvature and the residual stress for a one-dimensional stressed case. The obtained database after the calculations of residual stresses in U7A steel after WEDM correlates well with the results of work of leading researchers in the field of erosion technologies and processes of interaction of electric discharge plasma with metals. The results make it possible to determine the residual stress levels in the HAZ of U7A steel after WEDM and to determine their dependence on the energy of a single spark discharge. For the specific technological case, it is found that the maximum residual stresses relax over time and decrease with decreasing the thickness of the HAZ. It is possible to completely eliminate residual stresses only with the complete removal of the HAZ by non-thermal electrochemical technology, since WEDM modes, even with minimum discharge energies, form the HAZ and corresponding residual stresses. The results obtained can be used in the design of highprecision modes of electrical discharge and combined sequential WEDM and electrochemical machining with the same wire electrode. Besides, additional confirmation of the capabilities of the latest combined sequential WEDM and electrochemical technology not only to form the microgeometry of the surfaces, but also to obtain in the surface layers the required level of residual stresses or to completely eliminate them is obtained.