Materials Science Forum Vols. 794-796

Abstract: Age-hardening of Al-Zn alloy after quenching develops inhomogeneously due to the effect of surface as a vacancy sink and grain boundary as an easy path. In this study, reversion of the age-hardened Al-Zn alloys, in which ellipsoidal GP zones were formed, was investigated by Vickers micro-hardness test. Ellipsoidal GP zones were reverted more quickly near the surface and grain boundary than in the interior, as spherical GP zones in Al-10%alloy did. It is considered that the surface and grain boundary plays a role of effective source for vacancies, in addition to the interior source such as dislocations, as in the case of the reversion of spherical GP zones.

Abstract: Among multiple applications 6XXX-series aluminum alloys are widely used as electrical conductor for overhead power lines. For that application Al-Mg-Si alloys are produced mainly in the form of wire rod in the continuous casting and rolling lines (CCR). There is many technological routes possible to transform the Al-Mg-Si wire-rod into precipitation hardened, ready to use Al-Mg-Si wires. One of these routes involves using the heat of continuous casting and rolling process to artificial ageing of wire-rod, which is motivated by ecological premise. In the current paper the results of continuous heat treatment of the wire-rod within the CCR line (involving quenching of the alloy to artificial ageing temperature) on the microstructure and properties has been examined. It was shown the structure of wire rod formed in such process is very irregular. TEM investigations has shown that many heterogeneous particles are present and the arrangement of the β main hardening phase is very inhomogeneous with wide precipitate free zones occurred. However this type of microstructure has a positive effect on electrical properties of the material in question, which is important taking into consideration application of the alloy.

Abstract: In this paper a self-hardening Al-based alloy (AlZn10Si8Mg) is proposed for automotive applications as an alternative to the A357 (AlSi7Mg0.6) T6 heat treated alloy. The properties of the AlZn10Si8Mg alloy have been monitored and compared to those of the A357 alloy which is already employed in the targeted automotive industry. The samples have been submitted to microstructural analysis and mechanical characterization, while the presence of defects on the fractured surface has been identified by fracture surface analysis. A relationship between defects and mechanical performances has been identified. The corrosion resistance of the alloys has also been investigated according to the Standard BS 11846. On the basis of the results obtained till now, the AlZn10Si8Mg alloy is a good candidate for the proposed automotive application.

Abstract: Recrystallization control over the whole process route is strictly needed in order to achieve the required texture and properties. An important factor for this is the different type of particles formed in the material, their size and volume fractions. This study addresses the role of the Mn-bearing dispersoids and the Mg-Si (-Cu) particles, forming during different phases of the sheet processing, thus significantly influencing the recrystallization. It will be shown that, when present, the latter ones are playing an important role in the recrystallization control, not only in promoting the recrystallization by means of Particle Stimulated Nucleation, but also retarding it, if the particle size is sufficiently fine. Moreover, they can completely block the recrystallization, if concurrently precipitating during it. The Mn-dispersoids is comparable in its pinning effect, but takes over only when the Mg & Si atoms are in solid solution. Additionally, an update of the Diagram of Humphreys on the effect of particles on the recrystallization of 6xxx alloys is proposed and discussed.

Abstract: Distribution of microstructure and hardness by RBT (Rotary Bending and Tensile) loading at ambient temperature is presented. Grain size is one of the important parameters of microstructures of alloys, and affects mechanical characteristics depending on deformation conditions. At higher temperatures, coarsening of grain size improves creep strength, while the finer improve tensile strength at ambient temperature. Grain size shows opposite effect on strength of alloys depending on temperatures and not always possible to improve strength both at ambient and high temperatures. Authors have attempted microstructural control by formation of distribution of plastic strain prior to heat treatment of aluminum alloys to obtain well-balanced strength both at high and ambient temperatures. In this report, distribution of grain size and hardness in 2017 aluminum by RBT loading are presented, and compared with results in 1070 reported previously. RBT loading equipment is designed for combined loading by rotary bending and static tensile loading to distribute plastic strain. In 2017 alloy, obtained microstructure after suitable heat treatment show distribution of hardness, while grain size show homogeneous distribution. The distributions, however, are different from that in 1070 alloy.

Abstract: Some aluminium alloys with Mg-Si age-hardening are used in vehicle radiators. For cost reasons they are preferably delivered in a naturally aged temper. Estimated minimum time of natural ageing between brazing and when the radiator is taken into service is 14 days. At the service temperature of 95°C, the radiator material will continue to age harden. For accelerated durability testing it is vital to use a radiator with the strength and ageing response of a service radiator. We investigated whether the full 14 days of natural ageing were needed, or if the time could be shortened. Since a vehicle is not in constant use, the radiator temperature will vary over time. We therefore compared cyclic ageing between ambient temperature and 95°C to continuous ageing at 95°C. The Sapa Heat Transfer alloys FA7870 (for headers) and FA7850 (for tubes) were subjected to different ageing times at different temperatures. Tensile and hardness were performed to assess the ageing effect. It was found that natural ageing reduced hardening during the subsequent ageing at service temperature ageing effect, an effect that was most pronounced for the first four days. There was no difference between continuous and cyclic ageing.

Abstract: In the current study the nucleation of Cube grains during recrystallization in a commercial Al-Fe-Si alloy was investigated by in-situ and ex-situ annealing experiments at different temperatures. Both methods confirm that the Cube grains can be nucleated during RX both at large deformed Cube bands and other locations. During annealing only about one third of the Cube nuclei from these other locations developed into Cube oriented recrystallized grains owing to an unfavorable growth environment. Such nuclei needed also a longer incubation time due to their small size compared to nuclei formed at large Cube bands which can form nuclei very fast owing to the well-developed substructure and transition zones promoting nucleation. The growth rate of all Cube nuclei was found to be strongly influenced by their environment. This effect became more pronounced with decreasing annealing temperature due to an increasing amount of recovery.

Abstract: Favorable features such as finer particles and supersaturated solid solution are characterized in continuous casting. In this study, difference of recrystallization behavior was mainly evaluated on two intermediate annealing conditions in continuous cast Al-Mn alloy. One annealing condition was air furnace annealing which corresponds to batch type annealing and is conventionally used to anneal aluminum alloys. Another one was salt bath annealing which simulates CAL type annealing, which is characterized by shorter holding time with rapid heating and cooling. With consideration for application to fin stock in heat exchanger, heat treatment simulating brazing at 600°C was carried out on cold rolled sheets after respective intermediate annealing. After the brazing heat treatment, superior tensile strength and coarser grain structure were attractively obtained with the salt bath intermediate annealed fin. This tensile strength was correlated with much finer particles and more solid solution confirmed after the salt bath intermediate annealing. The coarser grain structure would be brought by changing in recovery and recrystallization behavior during brazing heat treatment.

Abstract: Twin roll casting (TRC) is an efficient process used to produce thin plates directly from molten metal. However, TRC has a much higher cooling rate during solidification compared with conventional direct chill casting. Accordingly, the constituent phases are refined and the added elements are supersaturated by this casting method. Supersaturation of the added elements hinders both recovery and recrystallization because fine dispersoids precipitate from the supersaturated solid solution during annealing [ and coarsen the grain size of the final plate, which leads to degradation of formability. Therefore, it is important to understand the influence of the solute element and dispersoids on recrystallization behavior to obtain an appropriate grain structure for forming. In this study, the effects of the homogenization conditions and cold rolling reduction on the recrystallization behavior of twin-roll cast 3XXX series aluminum alloy were investigated.

Abstract: A new Al-Mg-Si-Cu aluminum alloy was hot compressed at temperatures between 320°Cand 530°Cand strain rates between 0.001 s¹ and 10 s¹ using a Gleeble-1500 thermo-simulation machine. The effects of deformation heating on flow stress were analyzed and corrected, and a comparison was made of hyperbolic-sine constitutive equations with and without correction. The results show that the deformation heating apparently increases the flow stress at strain rates of 1 s¹, 10 s¹. The thermal activation energy values were 325.12 KJ/mol and 304.47 KJ/mol before and after correction, respectively.