Materials Science Forum Vols. 794-796

Abstract: It has been reported that the elongation of the AA1050-H26 sheet annealed at 200°C is lower than that of the AA1050-H18 sheet. In this study, the effect of the annealing time at 250°C on the elongation of AA1200 alloy sheets was investigated, and the cause of the low elongation was discussed by observing the change in the microstructures before and after tensile deformation. The elongation of the samples annealed at 250°C for less than 50 min was below 1%, and this elongation was lower than the elongation of the as-rolled and annealed ones at 250°C for more than 150 min. In the samples annealed for a short time, the subgrains with diameters of 0.5~2μm formed, and Fe and Si, which were a solid solution, became segregated at the sub-boundaries. These samples were locally deformed in a stress concentrated area during the tensile deformation, and there was no significant increase in the dislocation density near the fracture part after the deformation. The cause of the low elongation was considered to be due to the dynamic recovery that locally occurred in a stress concentrated area during the tensile deformation, because dislocations introduced into the subgrains by the deformation easily moved to the sub-boundaries due to the low solute levels within the subgrains.

Abstract: To overcome the low room temperature formability of AA7075-T6 aluminum sheet, without sacrificing the high strength properties of this alloy, a hat section beam member was formed and quenched within a cold die immediately after a 20 minute solutionizing treatment. Natural aging for 24 hours followed the forming process which was then followed by various heat treatments that included a typical precipitation hardening (PH) and industrial paint bake (PB) temperature-time treatment. Tensile specimens were extracted from the beams to evaluate their mechanical properties. When compared to the as-received AA7075-T6 mechanical properties, the beams heat treated with the PH, PHPB and PB treatment resulted in a 5%, 13% and 20% reduction in ultimate tensile strength respectively. A similar trend was shown for the yield strength measurements. There was little effect of the heat treatments on the total elongation, with the PH condition showing a slight improvement. A backing plate was riveted to the beams and a quasi-static 3 point bend test was conducted to evaluate the crush performance. The peak load for the PH, PHPB and PB beams was 9.2, 8.5 and 7.3 kN respectively, but the calculated energy-displacement (or energy absorption) curves were similar for the PH and PHPB parts due to a more ductile fracture behavior for the PHPB material condition.

Abstract: Aluminium is already extensively used in car production to reduce the CO₂ emissions by weight reduction. A further beneficial effect of lightweight design can be generated in components of the chassis by reducing the weight of unsprung mass thereby enhancing the driving comfort and reducing the noise level. The medium strength alloys of the type AlMg3Mn (EN AW-5754) and AlMg3.5Mn (EN AW-5454) are currently the aluminium sheet material choice for application in chassis components. The newly developed alloy AMAG AL6-CHA was optimized with regard to chassis applications and shows the potential of significant increase of the mechanical properties compared to state-of-the-art 5xxx series alloys. AMAG AL6-CHA, which is a 6xxx series alloy with balanced Mg/Si-ratio, is characterized with regard to mechanical properties and intergranular corrosion resistance in delivery temper T4 and after artificial aging with the typical heat treatment cycle 205 °C/60 min in peak aged temper T6. Furthermore we will show the results of the Charpy-V-notch impact test and the formability is described per bend test and grain size analysis.

Abstract: The high temperature tensile behaviour of the A354 casting aluminum alloy was investigated also evaluating the influence of secondary dendrite arm spacing (SDAS). Cast specimens were produced through a gradient solidification equipment, obtaining two different classes of SDAS, namely 20-25 µm (fine microstructure) and 40-50 µm (coarse microstructure). After hot isostatic pressing and T6 heat treatment, the samples underwent mechanical characterization both at room and high temperature (200 °C). Results of tensile tests and hardness measurements were related to the microstructural features and fractographic characterization, in order to investigate the effect of microstructure and high temperature exposure on the mechanical behaviour of the alloy.

Abstract: Line tension simulations were implemented to study the glide of a single dislocation through an array of randomly distributed point obstacles. The strength of each obstacle in the glide plane was populated from an assumed distribution, mimicking what might be expected in engineering alloys containing precipitates. The effect of changing the width of the distribution was investigated. It was found that the assumption of a uniform strength gives a lower bond to the critical resolved shear stress.

Abstract: The age hardening response of a quasi-binary Al-Mg₂Si alloy was studied using activation distance analysis of precise strain rate sensitivity experiments at 78 and 300K. The alloy of Al-0.7Mg-0.33Si-0.024Fe-0.006Ti(at.%) has a stochiometrically balanced composition of Mg₂Si. The alloy was solutionized at 550°C and ice water quenched before ageing in one of two ways: single-step or multi-step ageing. For single-step ageing: specimens were naturally aged for 70 days at room temperature (RT); pre-aged for 16 hours at 70°C; or artificially aged for 30 min or 10 hours at 175°C. For multi-step ageing: specimens were pre-aged at 70°C for 16 hours after natural ageing for 70 days at RT; artificially aged for 30 min or 10 hours at 175°C after pre-aging at 70°C for 16 hours; or artificially aged for 10 hours at 175°C after natural aging for 70 days at RT. The activation analysis reveals rate controlling obstacle dimensions ranging from 0.3 to 10 nm depending upon the ageing condition. A comparison is made to a prior three dimensional atom probe ageing study of the same alloy [Murayama and Hono, Acta Mater., 47 (1999) 1537-1578.].

Abstract: Al-11%Si (-0.44%Fe-0.16%Cu-0.14%Mg) was cast into the strip using a vertical type high speed twin roll caster at speed of 60m/min. Cooling rate of the strip was ranging from 2000^OC/s at surface to 1000^OC/s at center of thickness. The eutectic Si was smaller than 2μm. The homogenization, cold rolling down to 1mm, and annealing were carried out before the tension test, 180 degrees bending test and deep drawing test. Tensile stress was 192MPa, 0.2%proof stress was 84MPa, and elongation was 23.8%. Roll cast Al-11%Si had excellent elongation. The specimen was not broken by the 180 degree bending. LDR (Limiting Drawing Ratio) of the deep drawing test was 1.8. The ductility was drastically improved by the high speed twin roll caster. These results show that roll cast Al-11%Si has ability to be used for sheet forming. Moreover, Fe was increased up to 1.0% as the model of recycled alloy. The elongation was 15.9% and LDR was 1.7 in the condition that Fe content was 1.0%. Al-Si-Fe intermetallic became fine by the effect of rapid solidification. As the result, deterioration of the ductility was improved.

Abstract: High strength AA7xxx alloys have been extensively used in aerospace industry. However, experience in cast house demonstrates that such alloys are particularly prone to cold cracking and ingot distortion during direct chill (DC) casting, which leads to big amount of scraping or even total rejection of ingot. Those stress induced defects are greatly affected by casting process, and fine tuning of casting parameters is critical to improve the quality and productivity. Recently, numerical modeling has been widely utilized in direct-chill casting for the purpose of thermal mechanical analysis and cracking evaluation. Parameter optimization has become convenient. The model needs the input of constitutive properties of the AA7xxx alloys, of which the microstructure should resemble those formed during DC casting. Unfortunately, these constitutive data are not yet available in literatures. In this study, the mechanical properties of two high strength AA7xxx alloys were measured at temperature range from solidus down to room temperature through on-cooling compressive tests and the effect of strain rate on stress-strain behavior was also studied. The results were fitted to extended Ludwik equation which has been proved to be able to properly describe the stress-strain response of aluminum alloys. The results of the current work shed some light on the evolution of mechanical behavior of high strength AA7xxx alloys during cooling from high temperature.

Abstract: In the aluminium wrought alloys, the solid solution strengthening is one of the dominant hardening mechanism. Every solute element shows different strengthening effect. In present work the strengthening effect of different elements in a large temperature range was studied with the microstructure and flow stress model 3IVM+. This model was developed for cell forming metals, in which the work hardening and softening effects due to interaction of dislocation densities are taken into account. Compression tests for various binary aluminium alloys (Al-Mg, Al-Mn, Al-Si) were carried out under various deformation temperatures and strain rates. The contribution of each element to solution strengthening was investigated with 3IVM+. It was found that the dislocation interaction, especially cross slip and climb, is sensitive to the solute type and concentration.

Abstract: It is well know that the 7000 series Al-Zn-Mg alloy has good age hardening ability and high strength among commercial aluminum alloys. In this study, hardness measurement, tensile test, SEM and TEM observation have been performed in order to understand the effect of the Zn/Mg ratio on age hardening behaviour in Al-Zn-Mg alloys. It was seen from hardness measurement that the peak hardness increased with increasing amount of Zn and Mg. Tensile tests were performed for the samples of peak aged condition. It was seen that UTS increased with increasing amount of the Zn and Mg. The elongation decreased with increasing the amount of the Zn and Mg. Intregranular fracture was observed in alloys with low amount of Zn and Mg.Transgranular fracture was observed with high amount of Zn and Mg alloy. TEM observation was performed for peak aged samples. The size of precipitates became finer and the number density increased with increasing Zn and Mg contents.T’ Phase and h₁ phase were observed in low Zn/Mg alloy. The h’ phase was observed in high Zn/Mg alloy.