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%0 Thesis %A Szczepaniak, Agnieszka %T Investigation of intermetallic layer formation in dependence of process parameters during the thermal joining of aluminium with steel %I Rheinisch-Westfälische Technische Hochschule Aachen %V Dissertation %C Aachen %M RWTH-2016-05763 %P 1 Online-Ressource (viii, 163 Seiten) : Illustrationen, Diagramme %D 2016 %Z Veröffentlicht auf dem Publikationsserver der RWTH Aachen University %Z Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2016 %X This thesis examines the formation of intermetallic layers at the interface between aluminium and low-carbon steel during thermal joining using state of the art electron microscopy techniques applied both ex-and in-situ. To sample a wide range of intermetallic layer formation conditions, the well-established coating method of hot-dip aluminising, as well as more modern welding techniques such as laser beam welding (in conduction mode) and friction stir welding, are applied. The intermetallic layer always consists of brittle phases, mainly Fe2Al5and Fe4Al13, which makes it a detrimental factor for mechanical properties of the assembly. Therefore, the possibility to control the thickness and morphology of this layer is of great importance for industrial processes. This requires in-depth knowledge about the influence of the joining parameters on the conditions of intermetallic layer formation, the factors affecting its growth, and finally, the resulting microstructure; all of which are discussed in this thesis. Hot-dip aluminising experiments show that a significant reduction of the intermetallic layer thickness can be obtained when the immersion process is performed at sufficiently high temperatures (above 900 °C for 30 s of aluminising). Thus counterintuitively, a high process temperature, even for extended holding times, is an effective alternative to the conventional case of simply limiting growth conditions to reduce the intermetallic layer thickness, owing to increased dissolution of the intermetallic layer in the melt. It also was shown that the intermetallic layer experiences significant growth under insufficiently rapid cooling from high temperatures. Furthermore, evidence of a carbon buildup in front of the growing layer (κ-phase, pearlite, martensite/acicular α-Fe) is presented and discussed in terms of producing a beneficial hardness gradient. The specific influence of the parameters related to the energy density delivered to the assembly (laser power and welding speed) as well as the parameters affecting mainly the heat dissipation in the weld microstructure (overlap size and lateral spot position) was established systematically. Additionally, a correlation with finite element method simulations of thermal cycles, performed for selected points at the aluminium/steel interface, allow determining that mainly the peak temperature and related cooling rate differ between particular welds. Moreover, the laser beam intensity distribution results in a variation of the intermetallic layer thickness and morphology along the weld interface. Regardless of the comparatively faster kinetics induced by the welding process, the intermetallic layer constitution is nearly the same as in case of aluminising: Fe2Al5and Fe4Al13. Transmission electron microscopy shows that formation of the Fe-richer phases (Fe3Al, FeAl and eutectoid: FeAl + FeAl2) occurs only in the region where the joining temperature approached the melting point of the Fe2Al5phase. The carbon build-up effect is not observed in the welds. Electron back-scatter diffraction, applied in a systematic study of this problem for the first time in this thesis, reveals how the ii width of Fe2Al5grains is strongly affected by the temperature and time of the joining process, leading to vanishing of the typical tongue-like morphology. In case of laser beam welding, the maximum width of Fe2Al5grains can reach up to 500 μm with the corresponding height of about 50 μm in comparison to few microns in the initial case. For the first time, using a novel in-situheating transmission electron microscopysetup, a characterisation of the FexAlyphase formation at the weld interface of friction-stir welded samples, used as model microstructures, is attempted in real time. It is shown that Fe4Al13is the first stable phase that forms under annealing, in agreement to the Walser-Bene model. In addition, a new method of the site-specific sample preparation and lift-out process for in-situheating measurements was proposed. Throughout this thesis, the literature data is discussed and compared to the experimentally obtained microstructures, which are characterised in detail by electron back-scatter diffraction techniques and transmission electron microscopy. Practical applications and potential microstructure optimisation for industrial processing is also discussed. Examples are the reduction ofthe intermetallic layer thickness through enhanced dissolution and combination of high temperature and quenching rates, as well as the tailoring of layer morphology. %F PUB:(DE-HGF)11 %9 Dissertation / PhD Thesis %U https://publications.rwth-aachen.de/record/661160