Evaluations of residual tension states in laser welded CoCrMo alloys

Objectives: The study was focused on stress areas evaluation, induced in RPD laser welded frameworks, on the purpose of determining the durability of prosthesis rehabilitations. Material and Methods: For testing, there were used 2 types of laser welding with filling material for CoCrMo alloys (WIRONIT-BEGO, "C" VASKUT KFT) in RPD technology. In order to realize a thermal analysis with finite elements and reveal the welding residual stress, specific software (solid work 2007) was used. This analysis serves to welding procedure simulation, in which temperature, heat flux distribution and static analysis of stress state, resulting from welding, were under observation. The following thermal parameters resulted for evaluation: coefficient of linear expansion-ß=3,36•105 W/mm2•°C, coefficient of heat conductivity-λ=6,05•10-2W/mm•°C; specific heat-C=434 J/Kg•°C. Results: The Study results reveal that around the fusion zone there is a stress gradient, and this is the one that forms cracks in the fusion zone in steel. The thermal calculation applied in the rib is analyzed in connection with temperatures distribution during welding and cooling. This consists in temperature field's distribution on a period of 1-5s. Analysing the stress field of numerical simulated longitudinal section, the high values of stress from the heat affected zone and their orientation towards the welding exterior surface, where cracks usually appear, can be observed. Conclusion: The study with finite elements has the advantage of being non-invasive and of permitting a correct prognosis of cracks appearance zones in the heat affected zone. Welding is a useful method for metallic prostheses repairs and for the control of welding heat input, contributing to the decrease of welding stress and to the avoidance of cracks. Keywords: residual tension states, CoCrMo alloys, laser welding