Materials Science Forum Vols. 794-796

Abstract: In recent years, automobiles with lower fuel consumption are required because the exhaust fume is severely regulated. The weight-saving is quite effective to realize such low fuel consumption, and therefore aluminum alloy becomes attractive as an alternative material of steels due to its high specific strength. 6XXX series Al-Mg-Si alloys exhibit good bake-hardenability during paint-bake treatment in the automobile manufacturing process, but to reduce further environmental impact, the paint-baking temperature is supposed to be lowered than the present temperature of about 443K. In this study, it was aimed to investigate the attained hardness after paint-bake treatment at various temperatures of 408-443K for an Al-0.55wt%Mg-0.90wt%Si alloy with/without microalloying elements of Cu and Li. The effects of multi-step aging conditions; e.g. pre-aging, natural aging and paint-bake treatments, were also investigated through Vickers hardness test, TEM observation and DSC analysis. From the obtained experimental results, it was clarified that the addition of Cu or Li to the Al-Mg-Si alloy increases the attained hardness even at a paint-baking temperature of 408K due to the increased volume fraction of precipitates. Furthermore, pre-aging treatment at 373K for 18ks was also effective in suppressing the increase in hardness during natural aging, resulting in the highest attained hardness among the investigated multi-step aging conditions; i.e. HV100 in the Li-added alloy paint-baked at 408K.

Abstract: For understanding the distribution of plastic deformation induced by asymmetric rolling (ASR), multi-pass ASR and symmetric rolling (SR) experiments combined with the finite element simulation were used for high-strength aluminum alloy in the present study. The influence of reduction per-pass on the shear / effective strain distributions were studied via different ASR processes. By measuring the shear angle (θ, the angle between the reference mark before and after rolling) of rolled sheets, redundant shear strain and equivalent strain were calculated. It is shown that with equal total thickness reduction for ASR and SR, ASR can induce much more shear deformation through the thickness. By calculating the evolution of redundant shear strain and total equivalent strain for different ASR routines, it indicates that small pass reduction could be much favorable to the strain accumulation than that of the large pass reduction under a same total reduction in ASR process. Also, the influence of shear stress on the strain distribution and the through-thickness strain distribution were studied and evaluated with FEM analyses.

Abstract: In this paper, investigations of the softening behaviour of a supersaturated Al-Mn-Fe-Si alloy during annealing after cold rolling have been carried out. Two different homogenization conditions were considered, of which one gives a condition of a large amount of small pre-existing dispersoids, i.e. providing a significant static Zener drag, while the other gives a condition where both concurrent precipitation and dispersoid drag effects are limited. The homogenized samples with different microchemistry states were then cold-rolled to different strains before subsequent annealing at 300°C. The softening and concurrent precipitation behaviours have been monitored by hardness and electrical conductivity measurements respectively, and the microstructural evolution has been characterized by EBSD. It is clearly demonstrated that the actual microchemistry state, i.e. amount of solutes and second-phase particle structures as determined by the homogenization procedure strongly influence the softening behaviour where a fine dispersion of pre-existing dispersoids together with concurrent precipitation slow down the recrystallization kinetics considerably and give a very coarse and elongated grain structure.

Abstract: Effects of Zr addition on recrystallized structure and texture in extruded Al-Mg-Si alloy containing Mn have been investigated. Materials were homogenized at temperatures in the range 550 °C to 590 °C and extruded at 500 °C at the speed of 10m/min. In extruded Al-Mg-Mn-Si-Zr alloy, the fraction of Cube oriented grains reached 40% as homogenization temperature raised to 590 °C. On the other hand, the fraction of Cube oriented grains in extruded Al-Mg-Mn-Si alloy was limited to 20-30 %. In addition, to clear the formation process of recrystallized grains in these alloys, observation of hot-compression deformed and recrystallized grain structures were carried out. It was suggested that moderate Zener drag promoted the preferential recrystallization of cube oriented grains

Abstract: Aging phenomena of Al-Zn alloys have been much investigated and many studies have been made on the change of mechanical properties on aging. Appearance of saw-tooth yielding (serration) in the stress-strain curve of tensile deformation has been reported in several reports. In this study, the relationship between occurrence of serration and reversion heat treatment was studied by tensile test on Al-Zn alloys. Serration was observed for the furnace cooled specimen, but not for the as-quenched one and the shortly aged one, which result is difficult to be interpreted by the Cottrell theory. The stress amplitude in the serration increased with increasing the time of annealing for reversion and then stayed at a constant value which might be interpreted by the Cottrell theory. The stress amplitude in the serration decreased with increasing the annealing temperature, which is contrary to the prediction by the Cottrell theory. The specimen directly annealed without aging and the one annealed after aging agreed well with each other in the tensile strength and the stress amplitude in the serration, which suggests solute clusters as the cause of serration.

Abstract: Free cutting alloys of Al-Cu (AA2011 and AA2111B) in T6 temper and Al-Mg-Si system (AA6023 and AA6262) in tempers T6 and T8 were subjected to Charpy U - notch impact testing at the temperatures ranging from 20°C to 350°C. The microstructure of the materials was characterized by light metallography, fracture surfaces were observed using scanning electron microscope (SEM). The alloys showed a significant decrease in the impact energy KU at temperatures ~125°C (AA2011, AA2111B), ~170°C (AA6023), and ~250°C (AA6262), respectively. This decrease of KU was caused by melting of disperse phases containing low-melting point metals (Pb, Sn, Bi), which was confirmed by differential scanning calorimetry. Additional annealing of the AA6262-T8 alloy for 2h at 400°C followed by slow cooling led to the transformation of Pb + Bi particles accompanied by the shift of the melting temperature from ~250 to ~310 °C. Higher temperature solution annealing of the AA6023 alloy for 30 min at 540°C (as a replacement of common 30 min at 520°C) resulted in a partial transformation of Sn + Bi particles accompanied by melting point shift from ~170 to ~200°C. Chemical composition of the corresponding phases was monitored by energy dispersive X-ray spectroscopy in SEM.

Abstract: The evolution of microstructure and mechanical properties of an Al-5.4Mg-0.4Mn-0.2Sc-0.09Zr alloy subjected to rolling at 300^oC was studied. It was shown that the rolling of the alloy leads to strong anisotropy in mechanical properties. The formation of the lamellar structure occurs at a total reduction of 60% due to alignment of initial boundaries along rolling direction (RD), and appearance of geometrically necessary boundaries (GNB) aligned with {111} planes. This process is accompanied by a strong increase in the lattice dislocation density by a factor of 50. Further rolling induces the formation of subgrains within lamellar structure that diminishes the anisotropy. The GNBs have low-angle misorientations, initially. After a reduction of 80%, minor part of GNBs acquires high-angle misorientation. The formation of well-defined subgrains within lamellas leads to a decrease in the lattice dislocation density by a factor of about 10; the yield stress (YS) decrease is -25% along the RD. At the same time the YS in the transverse direction tends to increase with increasing reduction from 60 to 80%. The effect of the deformation structure on the mechanical properties and their anisotropy is discussed.

Abstract: The microstructure of pre-aged AA7085 rolled plate was studied by means of hardness tests, optical microscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction and electrical conductivity tests. The results show that supper refined and homogeneous precipitates were formed during pre-aging, but the hardness of the alloy was still relatively low. There occurred a large number of dislocations when the warm deformation was introduced, and the size of the intragranular precipitates firstly increases and then decreases and then increases with the dislocation density increasing, which was caused by the role of dislocations on inhibiting the formation of GP zone and promoting the nucleation and transformation of η phase. Meanwhile, the distribution of grain boundary precipitates also changed from continuous chain to coarsening interrupted distribution and the precipitate free zone (PFZ) broadened obviously.

Abstract: A study was conducted on the evolution of microstructure during homogenization for two Al-Mg-Si alloys with different Mn levels, i.e. 0 and 0.5wt%. The homogenization treatment was conducted over a wide range of temperatures above the Mg₂Si solvus. The holding time at the peak temperature ranged from 2 hour to one week. Microstructure evolution of the constituent particles and Mn dispersoids were characterized by means of optical microscopy and FEG-SEM. The Mn content in and out of solution was estimated using the Thermo-calc (TTAl6 database) and resistivity measurements. The micro-segregation and distribution of the main alloying elements before and after homogenization were systematically studied by electron probe micro analysis (EPMA). It was found that the Mn content together with the homogenization practice had a significant influence on the microstructure evolution. By combining all the measurements, a comprehensive quantitative dataset describing microstructure evolution during homogenization was developed.

Abstract: The effect on hardness and precipitate microstructure of elastically straining a 6060 Al-Mg-Si alloy during natural ageing or artificial ageing has been investigated. The elastic strain is here defined as 50 % of the material yield strength. All heat treatments where elastic straining was applied led to an increased hardness compared to the unstrained reference material. Quantitative investigations of the precipitate microstructure using transmission electron microscopy (TEM) did not indicate any significant difference in precipitate parameters as compared to the unstrained reference material. Therefore the increased strength in the elastically strained material is being linked to strain induced dislocations based on faster ageing kinetics compared to unstrained reference samples.