Influence of laser beam guiding and overlapping on residual stress in remelting process

Abstract

The laser remelting process is a very promising hardening method for ferrous and non-ferrous alloys which do not show a homogeneous microstructural state after being heat treated in a traditional way. Consequently, the alloys concerned do not achieve the required hardness of the surface layer and do not provide the required wear resistance. The present paper deals with residual stress and strain changes in thin flat specimens of nodular cast iron 500-7 in the laser remelting process. In the process of gradual remelting of the specimen surface, i.e. during its heating and cooling, volume changes in the specimen occur due to thermal and microstructural changes. To find out more about volume changes in the thin surface layer of specimens, the latter were monitored by strain measurement at the lower side of the specimens during the remelting process and by residual stress measurement in the thin surface layer after the remelting process. The strain was measured using a resistance rosette, placed on the opposite side of the remelted layer, and the temperature was measured in the same place simultaneously. In laser remelting of a workpiece, such remelting conditions should ensure that a uniform thickness of the modified layer is obtained, in spite of several laser beam passages across the workpiece surface. Owing to the multiple passage of the laser beam across the workpiece surface, the state of internal stress in the workpiece changes and, consequently, undesired strain may occur. Such strain may result in greater consumption of material and additional machining which, in turn, may result in higher cost of the machined parts. The results obtained confirmed that on the basis of knowing the strain occurring during the remelting process and the residual stress, the process could be successfully optimised. On the basis of knowing the strains occurring during the remelting process and the residual stress, optimal laser surface remelting conditions were chosen, while the method of guiding the laser beam over the surface of the flat samples and the degree of overlap were varied.