Materials Science Forum Vols. 794-796

Abstract: The serrated flow phenomena in Al-Mg alloys with and without Zn were investigated after aging on several conditions, focusing on the role of precipitates. Al-6mass%Mg-0~3mass%Zn alloys were solution treated at 753~803K, quenched, and then aged at room temperature. Further artificial aging at 323~573K for 86.4ks was performed for some of them after natural aging for 2.6Ms. The serrated flow behavior was evaluated by tensile test. Microstructure was characterized by differential scanning calorimetry, transmission electron microscopy, atom probe tomography, and positron annihilation lifetime spectroscopy. The increase in the amount of Zn addition and the natural aging time lead to a delayed onset of serrated flow. The artificial aging at higher temperatures after natural aging, on the other hand, decreases the onset strain. A large number of small coherent Zn-Mg clusters are formed during natural aging in the Al-Mg-Zn alloys, which are transformed to the larger incoherent meta-stable precipitates during subsequent artificial aging. These results suggest that the mechanism of interfering with serrated flow is related to the vacancy trapping effect, which is enhanced by the coherent clusters.

Abstract: Cast aluminum alloys are an important group of materials which find wide application in the automotive industry. Insufficient studies have been carried out to date with regard to the mechanical performance of the aged A354 alloy. Therefore, the present work investigates the Quality index charts with the purpose of setting the limits of the tensile properties, as well as for comparing the mechanical behavior of cast alloy A354, to delineate the effect of the solution treatment applied. Tensile properties upon artificial aging in the temperature range of 155–350^oC for times ranging from 2 to 100 hours are also investigated. The results showed that the use of quality index charts is a satisfactory method for presenting tensile test results and, for assessing the effect of solution and aging treatment conditions subjected to the modified and grain-refined A354 alloys. It is also observed that the quality index, Q, is more sensitive to variations in the tensile ductility than to tensile strength.

Abstract: Tensile properties and fracture behavior of an AA6060 alloy were investigated at room temperature (295K) and cryogenic temperature (77K). It was found that both ultimate tensile strength and elongation increased with decreasing temperature. In particular, the latter increased with increasing strain rate at 77K. The changes in mechanical properties were thought to be due to a higher working hardening rate at low temperature, while the effect of strain rate on strain hardening was obscure at both temperatures. The hardness after tensile testing at 77K increased due to an increased accumulated dislocation density. Fracture occurred in a semi-ductile transgranular manner at 295K, while a mixture of intergranular and transgranular mode with less slip localization occurred at 77K. Moreover, a decreased testing temperature led to a decreased size of dimples. The rotation of grain orientations can lead to increased Schmid factors and change of the latter was strongly dependent on the deformation temperature, which was clarified by compression tests.

Abstract: In this research, Al-5.5Zn-2.5Mg-0.5Cu was investigated. Compaction pressure at 700 MPa was carried out to make green body materials. Strong interlocking between particles is expected after high compaction process. Sintering of aluminum composites powder is sensitive to the humidity. High purity nitrogen gas was used to sinter this material from 560^o-585°C for 1 hour. Sintering density increased with increasing sintering temperature and reached 96% relative sintering density at 580°C. Mechanical properties investigation of this material was done with hardness and tensile test. At optimum sintering conditions, this material has 40 HRB hardness and 329 MPa tensile strength. Based on chemical composition, this material has possibility to improve mechanical properties by T6 heat treatment. Strengthening precipitates such as MgZn₂ and CuAl₂ were expected as results from T6 heat treatment. Mechanical properties improved with this treatment with reaching 513 MPa of tensile strength. SEM, DSC-TGA and X-Ray Diffraction were used to characterize this material.

Abstract: Cooling of age-hardening Al-alloys after solution annealing is a critical step with respect to distortion and residual stresses. In order to predict their extent by simulation models, the mechanical behaviour must be known in a wide range of conditions and compositions. Therefore, experimental data is needed both for calibration and validation of the mechanical model. It is known for Al-Mg-Si alloys that supersaturation of the solid solution leads to a significant increase of strength during cooling. In order to understand the influence of single alloying elements on the strengthening effect, the mechanical properties of different binary alloys are investigated experimentally. The precipitation behaviour during cooling was investigated by Differential Scanning Calorimetry in a wide cooling rate range. A methodology to determine the degree of supersaturation of the solid solution based on the calorimetric results is presented. This approach is compared to atom probe tomography data. The mechanical behaviour of the alloys after various heat treatments was analysed by mechanical tests performed in a quenching and deformation dilatometer. Flow curves with high resolution at small strains (< 3 %) were measured at different temperatures. The results of the different experimental techniques are discussed in comparison and with respect to their testing limitations.

Abstract: The influence of artificial aging on the phase composition and the properties of sheets made of V-1213 alloy of Al-Cu-Mg-Ag system. It is shown that the phase composition, morphology, dispersion and the density of phase discharge changes with the temperature increase of artificial aging. After the aging at the highest temperature in the range from 150 to 190 °C the maximum density of S' and Ω' phase is observed. This density provides the best combination of mechanical properties when extended at room temperature and high temperature strengthcharacteristics.

Abstract: In an attempt to improve the constitutive models for 6xxx aluminium alloys at high temperatures and high strain rates, a comprehensive test series has been carried out for AA6060. Uniaxial tension tests were performed at various strain rates from 0.01 s^-1 to 1000 s^-1 and temperatures from 20 °C to 350 °C. The tests were carried out using a standard tensile machine for low to moderate strain rates and a split-Hopkinson tension bar (SHTB) system for high strain rates. In both cases, an induction apparatus was used to heat the sample while local deformation measurements were obtained with a high-speed camera and used to estimate the true strain beyond necking. Strong coupling between the influence of strain rate and temperature on the stress-strain behaviour was found. At room temperature, the strain rate has a minor effect on the behaviour of AA6060. On the other hand, a significant increase of the yield stress and work-hardening with strain-rate is observed for temperatures above (K), being the melting temperature; i.e., above 673 K.

Abstract: Super-ductile diecast aluminium alloys are critical to future lightweighting of automotive body structures. This paper introduces a diecast aluminium alloy that can satisfy the requirements of these applications. After a review of currently available alloys, the requirement of a diecast aluminium alloy for automotive body structural parts is proposed and an Al-Mg-Si system is suggested. The effect of the alloying elements, in the composition, has been investigated on the microstructure and mechanical properties, in particular the yield strength, the ultimate tensile strength and elongation.

Abstract: Aluminium alloys are known to have good cryogenic properties. However, little work is reported on aluminium alloys in the temperature range from room temperature and down to-100°C, which are likely operating temperatures in arctic regions. This work is an experimental study of the low temperature mechanical properties of an extruded AA6082 aluminium alloy in the T6 condition approved for marine applications. Quasi-static and dynamic tensile testing has been carried out at different temperatures from room temperature and down to-70°C. This decrease in temperature leads to a 10 % increase in yield strength. No significant influence of temperature was found on the area reduction to fracture. The work-hardening behaviour has been analysed for different temperatures and strain rates by fitting a generalized Voce rule to the tensile data. The initial work-hardening rate is found to depend weakly on the temperature and strain rate.

Abstract: The tensile properties of the quasi-static and the impact tests for 7075 aluminum alloys subjected to pre-fatigue deformation in a humid environment were investigated. An impact tensile test of the pre-fatigue deformed specimens was performed by means of the Split Hopkinson pressure bar method. Within the authors set of experiments, the flow stress in both the quasi-static and the impact tests was unaffected by the pre-fatigue deformation. On the other hand, it was shown that the ductility of the pre-fatigue specimen was slightly lower than that of the non-fatigue specimen. Additionally, the pre-fatigue specimen in a high-humidity environment had a lower ductility than in a low-humidity environment. The ductility of the quasi-static test was decreased as compared with the impact test. It is thought that the ductility loss occurred due to changes in the microstructure, such as dislocations caused by the presence of hydrogen in the specimen due to pre-fatigue deformation in a high-humidity environment.