Papers by Author: Laurens Katgerman

Abstract: Research activities on aluminium production technology focus on the successive steps in the production chain of aluminium wrought products. Direct-chill casting of aluminium alloys is a well-developed technology with a long history. But only in the last 20 years, the development of computer modelling offered a means of better understanding of the physical phenomena involved in solidification. The main scientific challenge is to obtain a fundamental insight into the processing of aluminium alloys and to establish quantitative relationships between materials, processes, and performance. A systems approach is employed, covering theoretical and experimental studies on processing into semi-finished products. Special emphasis is placed on experimental verification and industrial applicability with the availability of pilot scale experimental facilities at M2i-TUD. This facilitates the design of process conditions as desired for experimental validation studies. In this paper we will consider main mechanisms of structure and defect formation during solidification of DC cast billets, mostly based in the results obtained in cooperation between Delft University of Technology and Materials Innovation Institute (M2i).

Abstract: Direct chill (DC) casting of high strength 7xxx series aluminum alloys is difficult mainly due to solidification cracking (hot cracks) and solid state cracking (cold cracks). Poor thermal properties along with extreme brittleness in the as-cast condition make DC-casting of such alloys a challenging process. Therefore, a criterion that can predict the catastrophic failure and cold cracking of the ingots would be highly beneficial to the aluminum industry. The already established criteria are dealing with the maximum principal stress component in the ingot and the plane strain fracture toughness (KIc) of the alloy under discussion. In this research work such a criterion was applied to a typical 7xxx series alloy which is highly prone to cold cracking. The mechanical properties, constitutive parameters, as well as the KIc values of the alloy were determined experimentally in the genuine as-cast condition and used as input data for the finite element package ALSIM5. Thermomechanical simulations were run for billets of various diameters and the state of residual thermal stresses was determined. Following the contour maps of the critical crack size gained from the model, the casting conditions were optimized to produce a crack-free billet.

Abstract: Non-homogenous cooling rates and solidification conditions during DC-casting of high strength aluminum alloys result in the formation and accumulation of residual thermal stresses with different signs and magnitudes in different locations of the billet. Rapid propagation of micro-cracks in the presence of thermal stresses can lead to catastrophic failure in the solid state, which is called cold cracking. Numerical models can simulate the thermomechanical behavior of an ingot during casting and after solidification and reveal the critical cooling conditions that result in catastrophic failure, provided that the constitutive parameters of the material represent genuine as-cast properties. Simulation of residual thermal stresses of an AA7050 alloy during DC-casting by means of ALSIM5 showed that in the steady-state conditions large compressive stresses formed near the surface of the billet in the circumferential direction. Stresses changed sign on moving towards the centre of the billet and became tensile with high magnitudes in radial and transverse directions, which made the alloy prone to hot and cold cracking.

Abstract: Longitudinal weld seams are an intrinsic feature in hollow extrusions produced with porthole dies. The formation of longitudinal weld seams is a solid bonding process, controlled by the local conditions in the extrusion die. Being the weakest areas within the extrusion cross section, it is desirable to achieve adequate properties of these weld seams. In our research, the concept of a weld seam integrity indicator as a means of quantifying bonding efficiency is introduced. The value of this indicator depends on a number of factors: the material flow within the die weld chambers, an adequate pressure level acting on the weld planes and finally the evolution of the metal microstructure. Optimisation of the welding conditions leads to a higher value of the weld seam integrity indicator and thus to improved weld seam properties. The objective of the research presented in this paper is to assess the feasibility of this concept. In lab-scale experiments, AA6060 and AA6082 aluminium alloy billets were formed into strips by means of the direct hot extrusion process. By utilising porthole dies a central longitudinal weld seam is formed. The effect of different geometries of the weld chamber and the processing conditions on the quality of the weld seam are investigated. Characterisation of these weld seams through mechanical testing, focusing on the ability of the weld seam area to accommodate plastic deformation following the onset of plastic instability, and microstructural analysis provides insight into bonding performance. The outcome of this characterisation provides a basis for an estimation of the weld seam indicator. Through computer modelling, the particular process conditions related to weld seam formation are calculated and correlated with the experimental results. The experimental results clearly demonstrate that weld seam formation is controlled by a combination of factors as described above. Inadequate fulfilment of these conditions, verified by the FE-simulations, is the cause of inferior weld seams, associated with low values of the weld seam integrity indicator. Through further elaboration of the concepts presented in this work, the weld seam integrity indicator is to be developed, with the future aim of predicting the weld seam performance through finite element simulations.

Abstract: This review paper summarizes the results of recent studies on different mechanisms of macrosegregation upon direct-chill (DC) casting of aluminium alloys. In general, the main mechanisms of macrosegregation have been identified quite some time ago as thermo-solutal convection, free-moving crystals, shrinkage- and deformation-induced flow, and forced convection. Despite this general knowledge, the separation of the effects of these mechanisms on the overall macrosegregation pattern and the ratio of their contribution remained largely unexplored. With the advances in computer simulations and in experimental techniques it becomes possible to look at the impact of individual mechanisms in relation to the macroscopic parameters of the transition region of a DC cast billet and to the microscopic parameters of billet structure. Our systematic research helps in interpreting the apparently contradictory experimental macrosegregation profiles reported in literature. Paper is illustrated by own experimental and computer-simulation results.

Abstract: In the present investigation, serial sectioning and 3D reconstructions are made on samples quenched at selected temperatures during unconstrained solidification in order to observe the evolution in morphology of coarse dendrites in 3D. The 3D microstructure reconstruction during the solidification of an Al−7 wt.% Cu alloy allowed the identification of a complex coarse morphology of dendrites. High-ordered branches present different morphologies at different temperatures and locations in the microstructure due to coarsening and coalescence. 3D visualization of complex dendritic structures is discussed in the present investigation.

Abstract: Hot tearing is a significant problem upon direct-chill casting of high-strength aluminum alloys. The occurrence of hot cracks is related to the thermal contraction of the solid phase and to the lack of feeding by the liquid phase during solidification. It has been identified that structure features such as grain size and amount of nonequilibrium eutectics influence both phenomena involved in hot tearing. Experimental and computer-simulation results are presented for a range of model and commercial aluminum alloys. The results are obtained both during special small-scale experiments and during industrial-scale direct-chill casting. It is shown that grain refinement reduces hot tearing susceptibility of aluminum alloys through the related decrease of the temperature of thermal contraction onset and increased permeability of the mushy zone. The effects of process parameters on hot tearing are also discussed.

Abstract: The interaction between flow and progressing solidification front is of great importance, since it occurs in all casting processes. The present paper provides a better understanding of the flow phenomena and associated complex effects on solidification in a rectangular cavity under forced flow conditions, by means of experiments and computer simulations. It is shown that the cavity-driven flow with solidification is determined by several interacting features. The variation in bulk flow velocity and initial superheat dramatically changes the macro- and microstructure, promoting grain refinement, formation of peculiar grain and dendrite morphologies, etc. In particular, twinned feathery grains are found in the structure formed under certain heat and flow conditions during solidification. Some correlations between twinned feathery morphology, flow and solidification parameters are obtained. The effect of flow vortices on progressing solidification front and their effects on structure evolution are analyzed. Finally, the quantitative correlations between microstructure, solidification and flow parameters are established.

Abstract: It was shown on laboratory and industrial scale that ultrasonic melt treatment (UST) significantly refines structure of aluminium alloys and improves the quality of castings. However, despite considerable efforts which have been made over decades in the field of ultrasonic processing of aluminium melts, quite a few problems remain unclear. One of them is addressed in this project. The aim of the project is to understand which mechanism is responsible for cavitation-aided grain refinement. It is expected that the knowledge gained as a result of this work can be used in directchill, shape and die casting. The paper describes an experimental setup and first results on the correlation between parameters of UST, solidification conditions and degree of structure refinement. In separate experiments, a model Al-Cu alloy with different amount of solidification sites is solidified with and without UST. The final microstructure is analyzed.

Abstract: Experiments on combustion synthesis for the Ti-2B and Ti-C systems diluted with an inert metal are presented. The paper shows the influence of geometry, composition, density and particle size of diluent on the combustion front velocity. A Ti-2B reactant mixture diluted with Al and Cu and a Ti-C reactant mixture diluted with Al are studied. The metallic diluent and its concentration are varied. Besides, each experiment is based on a stack of cylinders with decreasing diameter in order to vary the heat losses. In some experiments the eventual quenching of the combustion reaction has been observed. Furthermore these experimental results are compared with theoretical calculations based on analytical expressions derived for such systems.