Papers by Author: Håkon Hallem

Abstract: Aluminium is increasingly used in structural components in cars. The advantages are many, and recyclability, weight saving and energy absorption during impact and are often considered as the most important. There are also several disadvantages compared to iron and steel, i.e. material cost, low stiffness, lack of fatigue limit, high thermal conductivity, high thermal expansion coefficient, corrosion susceptibility and others. To enhance the advantages and to minimize the disadvantages, several actions can be made. To prevent recrystallization is one of them, and has to be dealt with from the melt through all thermo-mechanical, mass reducing and assembly processes – “from the ore to the car”. As recrystallization may reduce the static strength and fatigue life time in a finished component considerably, preventing recrystallization is as important as to optimize component geometry. Additionally, there is considerable risk of inter-granular cracking or melting during thermo-mechanical processing, especially those involving very high strain and forming temperatures above eutectics. To prevent recrystallization, a well balanced budget on chemical composition, strain, strain rates and temperatures is essential. Examples of successes and failures are vital ingredients of the base of knowledge in development of highly utilized aluminium car components. All process steps, from alloying and melt treatment of wrought aluminium alloys to the assembly of a finished component, can influence on product performance. AlMgSi1 alloys are used as examples in evaluating recrystallization in this paper. By focusing on recrystallization, other major factors influencing performance are not neglected, but not further discussed here.

Abstract: In the present work the precipitation behaviour and recrystallisation resistance of Alalloys containing Hf, Sc and Zr in different concentrations and combinations have been investigated. Special focus has been put on the Hf-containing alloys, as one of the objectives of this work was to find out if Hf can be used as a replacement for Sc. Additions of Sc, either alone or in combination with Zr, leads to the formation of coherent and homogeneously distributed dispersoids, which very efficiently inhibit recrystallisation. Despite these attractive properties, the high price of Sc has limited its use as an alloying element in aluminium. The present investigation has revealed that Hf cannot fully replace Sc, as only heterogeneous dispersoid distributions are obtained in the absence of Sc, i.e. in regions where the number density is low the alloys would still be prone to recrystallisation. However, as an extra addition to the already remarkably stable Sc+Zr-containing alloys, Hf can lead to further improvements and consequently open for the use of aluminium alloys at very high temperatures. Al3(Sc,Zr,Hf)-dispersoids were present at the largest f/r-ratios and also displayed lower coarsening rates than Al3(Sc,Zr)-dispersoids. Very promising results were obtained for an Al-Hf-Sc-Zr alloy, which maintained mainly an unrecrystallised structure after extrusion and large degrees of cold rolling.

Abstract: A high recrystallisation resistance is required in aluminium alloys intended for processing or use at temperatures between 450°C-600°C. Additions of Hf, Sc and Zr significantly improve the resistance to recrystallisation through the formation of Al3X-dispersoids (X=Hf,Sc,Zr), and in this work different concentrations and combinations of these elements were added to five aluminium alloys. The alloys were extruded, subjected to various degrees of cold rolling (0%-80%) and finally annealed at high temperatures in order to study the structural stability. All variants displayed a high resistance towards recrystallisation, but the best results were obtained in the alloy containing only Sc and Zr. In this alloy no signs of recrystallisation were observed even after 1 hour annealing of extruded and 80% cold rolled profiles at 600°C.

Abstract: As cast and precipitation annealed variants of Al-Mn-Zr-alloys with and without Sc have been extruded in order to study the effect of Sc on the extrudability and the recrystallisation resistance after extrusion and subsequent annealing. Both Zr and Sc form dispersoids, which retard recrystallisation very effectively in many aluminium alloys. However, while Al3Zr often is heterogeneously distributed, a dense and homogeneous distribution of Al3(Sc,Zr)-dispersoids is obtained when Sc is added. This was also the case in these alloys, and the Sc-containing variants consequently displayed a far higher recrystallisation resistance than the Sc-free variants during extrusion and subsequent annealing. Another advantage by adding Sc is that precipitation annealing no longer seems to be necessary in order to obtain a high recrystallisation resistance, as the Sccontaining variants displayed an identical structural stability. The Sc-free alloy, on the other hand, had to be precipitation annealed in order to be able to resist recrystallisation during extrusion. However, an addition of Sc leads to a lower extrudability, as the Sc-containing variants displayed significantly higher extrusion pressures than the Sc-free alloys.