Microstructural Evolution During Homogenization And Its Effect On Rolling And Annealing Of AA3104Aluminium Alloy

This thesis investigates the microstructural evolution of a domestic can-bodyAA3104aluminium alloy during homogenization and its effect on subsequent rollingand annealing. The promotion of ‘6-α’ transformation on particle break up duringrolling and the effect of non-homogeneous distribution of the second-phase particles onrecrystallization nucleation are studied based on the microstructural evolution duringhomogenization. The study employs multiple characterization methods including backscattered electron (BSE) image, electron channeling contrast (ECC) image, electronbackscattered diffraction (EBSD), transmission electron microscope (TEM) and3Dserial sectioning techniques. The investigation has yielded the following results:The constituent particles (eutectic phases) in the alloy appear as lamellar networksin3D. High homogenization temperature and long soaking time will be helpful forimproving the morphology and distribution of those constituent particles. The ‘6-α’transformation happens within the temperature range from400to450℃, thetransformation rate and velocity are proportional to the homogenization temperature. Asa feature of the happening of ‘6-α’ transformation, morphology of the ‘Al spots’ withinα phase changes along with the transformation. During transformation the amount of Alwithin α phase will decrease by diffusing into neighboring matrix thus leading to adecrease of area fraction of the Al spots. After long time soaking, a number of ‘pure’ αphase with no ‘feature Al spots’ will appear.Precipitations formed in the alloy are different when soaking at middle lowtemperatures (300-450℃) and high temperatures(500-600℃). The precipitation of Mnincreases with the rise of temperature within middle low temperature range. Mg2Sineedles precipitate firstly during this range of temperatures, and then AlMnSidispersoids could form through nucleation both on Mg2Si and directly from the matrix.Finally, Mg2Si precipitates are totally replaced by AlMnSi dispersoids. In contrast, theprecipitation of Mn decreases with the rise of temperature when soaking at hightemperatures. AlMnSi dispersoids are the main precipitates during this temperaturerange. Zones with large rod-like dispersoids can be easily observed at the central regionof grains. The average size of the dispersoids increases with higher temperature andlonger soaking time. It is very common to observe PFZs during high temperaturehomogenization, the area fraction of PFZs decreases first and then increases. The ‘6-α transformation’ during homogenization is helpful for the separation andbreak of the coarse particles during rolling. Higher α particle content leads to betterpromotion effect on the fragmentation, which is helpful for obtaining particles ofsmaller sizes and less clusters. However, overlong soaking time will lead to thedisappearance of many Al spots and the spheroidization of the constituent particles,which would reduce their fragmentation during rolling. A sufficient break of the coarseparticles during the early rolling stage is very significant to improve the distribution ofparticles in final sheet, because the ‘fragments’ could spread more sufficiently with theflowing metal during later rolling passes.The traditional3D serial sectioning technique is improved by combining ECC andEBSD to reveal both particle morphology and microstructure/orientation of the matrixin3D when studying the recrystallization behavior of the alloy during annealing. Theresults show that:Clusters/bands of big particles greatly influence recrystallization nucleation in thealloy. Those sites are very powerful nucleation positions stimulating more than90%ofall nuclei observed when most of the constituent particles appear as particleclusters/bands, leaving only a few nuclei formed without second phase particles. A largesize variation of nuclei is observed which relate mainly to the inhomogeneousdistribution of dispersoids in the deformed matrix. Some nuclei grow to reach a regionwith rod dispersoids with lower pinning effect thus speeding up significantly andquickly consuming large parts of the zone. Consequently large nuclei clusters/bandswhich are comparable to the size of the regions of rod dispersoids are obtained. Thegrowth of most nuclei within PFZs which beside big particle clusters/bands is restricteddue to a high nucleation density, nuclei sizes are smaller than those within roddispersoid regions. Nuclei within zones with dense spot dispersoids are the smallestbecause of the largest pinning effect. It is believed that the size variation relates to a lessdegree of orientation effects.