Vpliv parametrov varjenja z gnetenjem raznorodnih visokotrdnostnih aluminijevih zlitin na zaostale napetosti in korozijsko odpornost ; Impact of friction stir welding parameters of highstrength aluminum alloys on residual stresses and corrosion resistance

V letalski industriji so aluminijeve zlitine pogosto v uporabi, predvsem zaradi ugodnega razmerja med trdnostjo in maso. Varjenje s trenjem in mešanjem (angl. Friction Stir Welding – FSW) oz. varjenje z gnetenjem počasi izpodriva klasično kovično spajanje zaradi boljših mehanskih lastnosti spojev, odprave zareznih učinkov ter lažje izvedbe. Vendar pa ima ta način spajanja tudi neugodne lastnosti. Med samim varjenjem se generira toplota, kar privede do sprememb v mikrostrukturi ter vnos nateznih in zaostalih napetosti, kar se odraža tudi v slabši korozijski odpornosti. V diplomski nalogi smo analizirali elektrokemične korozijske lastnosti in zaostale napetosti visoko-trdnostnih Al zlitin serije 2017A-T451 in 7075-T651, spojenih z FSW varjenjem, ob uporabi različnih varilnih parametrov. Iz vidika analize učinkov varjenja smo izmerili zaostale napetosti tako v osnovnih materialih kot tudi v samem zvarnem spoju. Meritev smo opravili trikrat, in sicer pred ter po odrezu z abrazivnim vodnim curkom in po izvedenih elektrokemijskih meritvah. S tem smo neposredno preučili vpliv različnih postopkov. Za analizo korozijske odpornosti smo uporabili različne elektrokemijske metode v agresivnem kloridnem okolju. Ugotovili smo, da se ob uporabi višjega razmerja obratov na podajanje (ONP) orodja med procesom varjenja generirajo manjše natezne zaostale napetosti v zvarnem spoju. Hkrati smo z elektrokemijskimi metodami potrdili najboljšo korozijsko odpornost zvarnega spoja v neposredni coni mešanja, kjer nastopa mikrostruktura z izredno finimi, enako-osnimi kristalnimi zrni. Meritve potenciala odprtega tokokroga so potrdile najplemenitejši korozijski potencial pri zlitini 2017A-T451, pri čemer pa FSW zvarni spoj izkazuje najboljšo korozijsko odpornost z najvišjo polarizacijsko upornostjo in možnostjo pasivacije ter repasivacije. ; In the aviation industry, aluminium alloys are often used, mainly because of their favorable strength to mass ratio. Friction stir welding (FSW) is slowly replacing classic rivet joining, because of better mechanical properties of joints, elimination of notching effect and easier construction. However, this joining also has some disadvantages. During the welding process, heat is generated, which leads to changes in the microstructure and the introduction of tensile residual stresses, which is also reflected in the lower corrosion resistance. In the thesis we analyzed electrochemical corrosion properties and residual stresses of high strength Al alloys series 2017A-T451 and 7075-T651 coupled with FSW welding using different welding parameters. From the point of view of the analyses of the effects of welding, residual stresses in the base materials and in the weld joint were measured. The measurement was performed three times, i.e. before and after the abrasive water jet cut and after electrochemical measurements. This enabled the full insight of the effect of the specific procedure. Various electrochemical methods have been used for corrosion resistance analysis in an aggressive chloride environment. It has been found that smaller tensile residual stresses in the weld are generated during the welding process using higher rotaion per feed (RPF) ratio. At the same time, the electro-chemical methods confirmed the best corrosion resistance of the weld joint, in the mixing zone with extremely fine, equiaxed grains. Open circuit potential measurements have confirmed the noblest corrosion potential with 2017A-T451 alloy, whereas the specimen taken from the FSW weld exhibited the best corrosion resistance with the highest polarization resistance and passivation and repassivation ability.