Thermal stresses in the tungsten and molybdenum surface layer following laser treatment

Abstract

Very high rates of change in temperature ( A T / A T ~107-109 Ks -I) occurring in the process of laser treatment of metal surface layers result in a change in the initial macrostructure of thus-fused metal layers [1-3]. The investigations carried out have proved that such a treatment can have a favourable effect on selected mechanical properties (e.g. microhardness) and electrical properties (e.g. resistance to electro-erosion in an electric arc) [1, 3]. It has also been found that in the newly formed layers of some metals considerable thermal stresses occur, which is evidenced by the presence of fractures in these areas of the fusion. This letter presents the results of microscopic examination of surface and metallographic cross-sections of tungsten, molybdenum and molybdenum with 1% silicon doping, subjected to laser treatment. The results of microhardness measurements performed by means of the Vickers method are also presented. In addition, selected results of the calculations of temperature fields and thermal stresses are given. The process of the local heating of the metal to temperature T < Tevaporating by the action of a single laser pulse on its surface and of the cooling (crystallization) of the fused area was analysed based on a model of a half-space heated with a surface, pulsed source of heat [1]. The results of the analysis have made it possible to determine intervals of the source power density and values of the heating time corresponding to these densities (Table I). An analysis of the cooling process shows that the rate of temperature change in the fusion, immediately after completion of the laser pulse, ranges from about 6 x 108 to 107 Ks -1 [1]. Such a high rate of cooling enables "freezing" of the dynamic state of the surface. This is confirmed by microscopic examination (Fig. 1) and literature reports [2]. Investigations of metallographic cross-sections of areas fused with laser radiation have revealed a change in the direction of arrangement of crystalline grains in relation to the initial fibrous structure (Fig. 1). Oblong grains occur here, oriented towards the point which, during the fusion, reaches the highest temperature.