Stresses in laser alloyed surfaces of high-melting metals

The paper deals with stresses induced in the alloyed layers of high-melting metals in the laser treatment process with a continuous beam and a pulse of a duration tau=4 ms. It has been found that the threshold power densities of laser radiation resulting in the permanent plastic strain of the metal under treatment are greater than the ones used during treatment. Making use of an analysis of the temperature field, the cooling rates of the treated zone have been determined by the finite difference method and an analytical method. The spatial non-uniformity of the heating and cooling rate of tungsten and molybdenum can be the cause of plastic strain and fractures on the metal surface. Experimental investigations were carried out for the optimal parameters of a laser radiation beam (the maximum depth of the alloyed layer, possibly with the smallest number of fractures). A privilidged direction of fractures at an agle of 45 degrees in relation to the direction of scanning was observed (basing on the microscopic observations of 2mm x 2 mm layers made on the surface of 10 specimens). Experiments showed a considerable effect of the type of the metals treated on the likelihood of occurrence of fractures; these are distinct and numerous in the case of molybdenum alloyed with nickel, whereas they cannot be observed in tungsten alloyed with nickel at a magnification of x 20 000. The smaller concentration of stresses in tungsten is also confirmed by the results of microhardness measurements (Table 2). The different behaviour of the metals under treatment may be caused by volumetric changes brought about by metallurgical phase transitions.