Corrosion Control of Friction Stir Welded AA2024-T351 Aluminium Alloys

Abstract

Friction stir welding (FSW) is a modern solid state welding technique developed at thewelding institute (TWI) in 1991. The joining is achieved by heat generation, materialsoftening and plastic deformation following the travelling of non-consumable pin throughthe gap between the two workpieces to be joined.In present study, joining of AA 2024-T3 aluminium alloy, is achieved by FSW. Theinfluence of the FSW on the alloy microstructure and corrosion behaviour is determined.The effect of laser surface melting (LSM) treatment on the improvement of corrosionresistance of friction stir welded alloys is investigated. Further, heat treatments tosimulate the welding process with controlled cooling rate are performed to assess theeffect of cooling rate on the microstructure, consequently, the corrosion performance ofthe welds.It is revealed that FSW process introduces elevated temperatures at the weldment,resulting in distinct regions with modified microstructures. The regions are named as theTMAZ (thermomechanically affected zone) and the HAZ (heat affected zone). TMAZ,positioned at the weldment centre, is featured by a central nugget with dynamicallyrecrystallised fine, equiaxid grains, that is surrounded by heavily deformed grains. HAZ,positioned as narrow bands just outside TMAZ, has grain size similar to parent alloy.Corrosion testing shows that the as-welded alloy is highly susceptible to corrosion,particularly at the bands just out side the TMAZ (i.e. HAZ). Welding process resulted inthe preferential precipitation of copper and magnesium rich particles at the grainboundaries within the HAZ, which reduces the corrosion resistance as a result of thegalvanic coupling of the sensitised grain boundaries and the adjacent matrix.Laser treatment resulted in a melted near-surface layer, up to 6 micro metre thick, where normalconstituent particles are absent. Corrosion testing showed that laser treatment reduces thedegree of localized corrosion due to the removal constituent particles. However, scrutinyof the melted near-surface layer revealed continuous segregation bands, approximately 10nm thick, containing mainly copper. The presence of such segregation bands promotedlocalised corrosion of the laser melting layer due to microgalvanic action. From the areaswhere melting layer is corroded, localised corrosion propagated further into the weldintergranularly. The severe intergranular corrosion beneath the laser melting layerundermines the laser melting layer, resulting delamination of the surface layer from theunderlying bulk alloy.The simulated heat treatments show that the cooling cycle of the welding process has asignificant influence on the alloy's microstructure and corrosion behaviour. Slow coolingcan result in formation of a continuous network of second phase particles at the grainsboundaries, leading to significantly reduced corrosion resistance. Rapid cooling tends toprevent the formation of second phase particles at grains boundaries, resulting inimproved corrosion resistance.

Date of Award	1 Aug 2010
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Xiaorong Zhou (Supervisor)