Papers by Author: Vesselin Michailov

Abstract: The thermomechanical properties of an AlMg0,8Si1 (corresponds to EN AW-6181) alloy in T4 state (Ecodal^® 608) were investigated under coldArc^® welding conditions using Gleeble 3500 concerning the numerical simulations of residual welding stresses and distortions. Thereby, tensile tests were carried out during the heating phase, cooling phase and after natural aging up to 10 days. Thus, the effect of weld cycle and corresponding dissolution of β''(Mg₂Si)- precipitates on the mechanical properties was physically simulated. Furthermore, two phase transformation models were used to simulate the dissolution of hardening precipitates as a result of weld temperature cycles. The used models were compared, considering their capabilities and accuracy.

Abstract: Cold cracks occur during the cooling down of welded joint at low temperatures or later at room temperature after the end of welding. It is associated with the formation of brittle microstructures as martensite in the presence of diffusible hydrogen as well as of tension stresses. By using an enhanced Simulation-und Testing Center Gleeble 3500, a procedure for physical simulation of cold cracking under laser beam welding conditions is suggested. The approach reproduces combinations of the cold crack main parameters, a brittle microstructure, tension stress and high local hydrogen concentration under welding conditions which induce a cold crack. A specimen geometry and technique were developed to enable the gaseous hydrogen charging from pure hydrogen atmosphere. The amount of charged hydrogen can be adjusted through varying the charging parameters like temperature, gas pressure and charging time. The hydrogen charging technique and the cold crack testing procedure were proven with high strength steel specimens.

Abstract: In this work structured sheet metals were investigated using electrochemical measurement techniques. The main purpose is obtaining fundamental information about the corrosion resistance of structured sheet metals in comparison to smooth sheet metals as well determination of the influence of the structuring process on the corrosion properties. The corrosion resistance of structured sheet metals is affected by manufacturing process. One of the main influence factors is the change of the surface roughness. In this study the low carbon steel DC04 and the stainless steel 304 (X5CrNi18-10) were investi-gated. The electrochemical tests were carried out in 3%-NaCl solution. Potentiodynamic linear polarization was used to determine such electrochemical characteristics as the free corrosion potential, the corrosion current, the pitting and protection potential. Furthermore, the corrosion rate was calculated for smooth and structured sheet metals of the low carbon steel DC04. For the stainless steel 304 the pitting density was estimated. The surface roughness was measured for both materials. The electrochemical corrosion tests show a small difference in the corrosion behaviour of structured and smooth sheet metals. Structured sheet metals have a lower corrosion resistance than smooth sheet metals. The steel DC04 shows the worst corrosion properties at the structure location “negative” in comparison to the structure location “positive”. The corrosion resistance of the stainless steel 304 is better at the structure location “negative” than at the structure location “positive”. Moreover, the results show the correlation between the surface roughness and the corrosion resistance for structured sheet metals.