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Välkommen till ett doktorandseminarie i teknisk fysik där Razaz Ghadir försvarar sin avhandling ”Problems in the Aluminium DC Casting Process Associated with Melt Treatment Operations”. Disputationen är på engelska.
Opponent: Professor Arne Dahle, Jönköpings Universiitet
Supervisor: Professor Torbjörn Carlberg
The quality of DC cast Al alloys is highly dependent on melt batch
composition and impurity level in the molten alloy. The chemical composition
and cleanliness of a melt is controlled through the melt treatment operations,
carried out while the melt is still in the furnace before casting starts. The
present work has studied some of these operations and associated problems
such as slow dissolution of alloying elements, non-reproducibility in chemical
composition analysis and inclusions.
The results of the dissolution of the alloy elements Mn and Fe showed
different behaviors. For Mn three intermediate phases were involved, all of
which exhibited a smooth interface between Mn and the liquid. These three
phases were identified as the γ2, Al11Mn4, and µ phases, which grow slowly
towards the dissolving Mn particles. The results from the Fe dissolution
revealed that only one phase dominates the process, Al5Fe2, which penetrates
the Fe particles with an irregular interface.
The interaction between Mn and Ti additions to AA3003 alloys and
consequences for the solidification and precipitation behavior was
investigated. The study could map the limits for formation of an earlier
unknown AlMnTi phase, which formed large particles, detrimental for
subsequent rolling operations.
Different sampling procedures for chemical composition analysis were
studied, and a novel approach was proposed. A mould with an insulated
periphery provided one-dimensional solidification, which gave compositions
close to nominal.
Inclusion distributions along as-cast billets were studied as a function of
different holding times, and thus different grades of sedimentation. Holding
times longer than 30 minutes did not show any improvements. It was also
shown that if melt remaining in the furnace at end of casting is less than about
3000 kg, the sedimented inclusions are stirred into the bulk again, and can
enter into the end of the billet.
The impact on hot tearing susceptibility of different Cu and Fe contents for
AA3000 alloys was studied. Cu contents in a range from 0.3 to 1.2 wt%
significantly increase the hot tearing tendency, which was attributed to bad
feeding at end of solidification. Decreasing of the Fe content below 0.2 wt%,
gives a strong cracking tendency, owing to decreased precipitations of the
Al6(Mn,Fe) phase, which contributes to early bridging and thus reinforcement