The presence and amount of residual stresses is very important in dynamically loaded automotive machine parts. In this investigation, the residual stresses after laser surface remelting were measured as a function of the modified layer depth on flat specimens from nodular cast iron. The results of the measured residual stresses confirm that the stresses strongly depend on the presence and quantity of the microstructure constituents in the surface remelted layer. Residual stresses have a characteristic profile in the modified surface layer. In the surface remelted layer, tensile residual stresses were found in a range between +70 and +200 MPa. The change from tensile into compressive residual stresses takes place in the lower part of the remelted layer. Maximum compressive residual stress values were found in the middle of the hardened layer in a range between -25 and -80 MPa.
F.02 Acquisition of new scientific knowledge
COBISS.SI-ID: 12119835Flame sprayed coatings are susceptible to the formation of inter-connected elongated porosity due to the lack of fusion between sprayed particles or the expansion of gases generated during the spraying process. The partial remelting of self-fluxing Ni-Cr-B-Si coating after spraying is an efficient method to reduce porosity and to improve coating microstructural and mechanical properties. The paper describes the image pre-processing procedure for noise removal which separates the pores from the background and the noise. The purpose was to obtain the lowest possible porosity. The experiment shows zero-percent porosity cannot be obtained with a partial remelting of flame-sprayed Ni-Cr-B-Si coatings; however, it can be greatly reduced. This method of coating post-processing not only favours pore size and shape but also eliminates the undesired network of inter-connected elongated pores.
F.10 Improvements to an existing technological process or technology
COBISS.SI-ID: 11805723The present paper deals with residual stress and strain changes in thin flat specimens of nodular cast iron during laser remelting process. In laser remelting process of a workpiece we have used several laser beam passages across the workpiece surface. The state of internal stress in the workpiece changes because of different ways of guiding the laser beam across the workpiece surface, and consequently strain of the workpiece may occur. In the process of gradual remelting of the specimen surface, i.e. during its heating and cooling, volume changes in the specimen surface layer 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 measuring the strain at the lower side of the specimens during and after the remelting process, and also by residual stress measurement in the thin surface layer. The results obtained confirm that the laser remelting process could be successfully optimised on the basis of knowing the residual stresses and the strain generated during the remelting process.
F.02 Acquisition of new scientific knowledge
COBISS.SI-ID: 11903259