Materials Science Forum Vol. 877

Abstract: We follow changes in the micro structure at several distances from the weld nugget of friction stir welded AlCuLi-alloy (AA2198) plates occurring due to the tool movement and the created heat by employing different methods: Small Angle X-ray Scattering (SAXS), giving information on type, size and density of precipitates, Differential Scanning Calorimetry (DSC), giving information on formed precipitates by their dissolution signal, and positron annihilation lifetime spectroscopy (PALS), being sensitive to vacancies and dislocations as well as to the formation and growth of precipitates. We start by characterizing the base material as a reference and proceed via the heat-affected zone to the weld nugget. By the use of complementary methods, we obtain information on structure, kind and distribution of precipitates and correlate this with hardness measurements.

Abstract: This paper presents the main achievements of a research project aimed at investigating the applicability of the hot stamping technology to non heat treatable aluminium alloys of the 5052 H32 and heat treatable aluminium alloys of the 6016 T4P after six months natural aging. The formability and mechanical properties of 5052 H32 and 6016 T4P aluminum alloy sheets after six months natural aging under different temperature conditions were studied, the processing characteristics and potential of the two aluminium alloy at room and elevated temperature were investigated. The results indicated that the 6016 aluminum alloy sheet exhibit better mechanical properties at room temperature. 5052 H32 aluminum alloy sheet shows better formability at elevated temperature, and it has higher potential to increase formability by raising the temperature.

Abstract: Precipitation hardening aluminium alloys are widely used for automotive applications. To enhance the application of aluminium profiles, improved formability is needed. Tailor Heat Treated Profiles (THTP) with locally different material properties attempt to increase formability e.g. in bending processes. Tailoring of local properties is obtained by a local short-term heat treatment, dissolving the initial precipitate state (retrogression) and still allowing subsequent ageing. In the present study, the dissolution and precipitation behaviour of the aluminium alloy EN AW-6060 T4 was investigated during heating with differential scanning calorimetry (DSC). Heating curves from 20 to 600 °C with heating rates of 0.01 up to 5 K/s were recorded. Interrupted heat treatments with different maximum temperatures were performed in a deformation dilatometer. Immediately afterwards, tensile tests were carried out at room temperature. The course of the recorded mechanical properties as a function of the maximum temperature is discussed with regard to the dissolution and precipitation behaviour during heating. Finally, the aging behaviour of the investigated alloy was recorded after different typical short-term heat treatments and is discussed with reference to the DSC‐curves. The correlation of the microstructure and the mechanical properties enables the derivation of optimal parameters for the development of THTP through a local softening.

Abstract: In electrical power systems bolted joints with bus bars made of aluminium are common, whereby the tendency towards higher operating temperatures can be observed. At higher temperatures a reduction of the joint force can occur due to creep and/or stress relaxation processes, which leads to an increasing electrical resistance and, in the worst case, to failed joints. The aim of this project is to increase the creep resistance (and to minimise the stress relaxation) of aluminium conductors for electrical applications without a significant reduction in their electrical conductivity – even after long-term exposure to elevated temperatures. The effect of dispersoids in different aluminium alloys on the longterm behaviour of currentcarrying joints at high temperatures (i.e. 140 °C) was investigated. Longterm tests on bolted joints with force measuring devices were performed to monitor the joint forces and to measure the joint resistances, both with and without current supply.

Abstract: Microstructure of high strength aluminum alloy have determinant effect on the properties, thus an effort has been made to investigation the relationship between fracture toughness and quantitative characteristics of microstructure in high strength aluminum alloy. Fracture toughness was tested for aluminum alloy specimens with various microstructure. The corresponding microstructure was observed by optical metallography and electron back-scattered diffraction, and quantitative characterization was conducted by further analysis of result obtained. Correlation between fracture toughness and parameters included grain size, percentage of recrystallization, substructure content and area fraction of residual phases was investigated. It was shown that percentage of recrystallization was a crucial factor for the fracture toughness, and correlations were established with proper and reasonable correction.

Abstract: Analysis of the absorbed impact energy of an Al-Mg-Sc alloy after different thermo-mechanical processing routes was investigated between-196°C and 20°C. The material with a grain size of ∼ 22 μm in cast condition and with an average grain size of 0.7 μm produced by was produced by equal-channel angular pressing (ECAP) exhibits well-defined ductile-brittle transition in the temperature interval-60...-100°C, however, even at-196°C the value impact energy of fine-grained alloy is higher by a factor of 2 in comparison with coarse-grained state. The impact toughness of the hot rolled alloy linearly decreases with decreasing temperature. The influence of different microstructures on impact toughness and fracture behavior of alloy is discussed.

Abstract: Fatigue weak-link density and strength distribution are materials fatigue properties which are useful in evaluation of alloy quality in terms of the fatigue crack nucleation behavior. This paper reports: 1) an experimental methodology developed to measure fatigue weak-link density and strength distribution by measuring surface crack population as a function of the maximum applied stress, and 2) a quantitative model to quantify fatigue weak-links, based on the 3-D effects of pores in cast alloys. The experimental method could be employed to characterize the fatigue crack initiation behaviors and their anisotropy in wrought and cast Al alloys such as AA8090 Al-Li, AA2026, AA7075 and A713 alloys. It was found that the crack populations were a Weibull function of the applied maximum stress. By fitting this measured curve, fatigue weak-link density could be quantified in these alloys. The derivative of the measured Weibull function resulted in the strength distribution of the fatigue weak-links. The fatigue weak-links could also be quantified from the reconstructed microstructure of a cast Al alloy by analyzing the stress/strain fields around a micro-pore (an elasto-plastic media) under cyclic loading as a function of pore position in depth on surface using a 3-D finite element method. The incubation life for the fatigue crack initiated from a surface pore could be estimated using a microscopic scale Manson-Coffin equation. The results obtained using the 3-D pore-sensitive model were consistent with the experimental results, i.e., the crack population was a Weibull function of the applied stress. The percentage of crack initiation life was found to increase with decrease in cyclic load, i.e., at the stress just above the fatigue limit, it was over 95% the total fatigue life, compared to just 62.5% at the maximum stress of 110% yield strength.

Abstract: Microstructure and mechanical properties of a 6013 Al-Mg-Si-Cu aluminum alloy processed by a combination of equal channel angular pressing (ECAP) and preaging treatment were comparatively investigated using quantitative X-ray diffraction (XRD) measurements, transmission electron microscopy (TEM) and tensile tests. In addition, the precipitation sequences were obtained by thermodynamic calculations using the FactSage software package. Average grain sizes measured by XRD are in the range 211–501 nm while the average dislocation density is in the range 0.35-1.0 × 10¹⁴ m^-2 in the deformed alloy. TEM analysis reveals that fine needle β′′ precipitates with an average length of 4-10 nm are uniformly dispersed in the preaging ECAPed alloy. The local dislocation density in this sample is as high as 2.2×10¹⁷ m^-2. The strength is significantly increased in the preaging-ECAPed samples as compared to that of the undeformed counterparts. The highest yield strength among the preaging ECAPed alloys is 322 MPa. This value is about 1.25 times higher than that (258 MPa) of the static peak-aging sample. The high strength in the preaging ECAPed alloy is suggested to be related to grain size strengthening and dislocation strengthening, as well as precipitation strengthening contributed from both preaging treatment and ECAP deformation.

Abstract: In the 2xxx series alloys, intergranular corrosion is generally related to the strong reactivity of copper-rich intergranular precipitates leading to a copper enrichment of these particles. While the nature of the oxides formed inside the intergranular corrosion defects was assumed to strongly influence the intergranular corrosion propagation rate, it was not clearly identified due to the thickness of the oxide layer formed which required to use high resolution analytical techniques. The present work aims to characterize the intergranular corrosion defects formed for a 2024-T351 aluminum alloy after a 24 hours continuous immersion in a 1 M NaCl solution and compares the results to literature data concerning the oxide layers formed on copper-rich model alloys. A combination of focus ion beam (FIB) technique, transmission electron microscopy (TEM) observations and energy dispersive X-Ray spectroscopy (EDX) analyses was used to accurately characterize both the morphology and chemical composition of the intergranular corrosion defects. Results evidenced the dissolution of intergranular copper-rich particles, the formation of a 10-200 nm-thin metallic copper-rich layer at the oxide/metal interface and the incorporation of copper inside the amorphous alumina oxide film leading to the formation of structural defects of the oxide film.

Abstract: The microstructure, phase composition and nanomechanical properties of AlCu2Si1Mg0.6 alloy was studied where partial melting of grain boundaries during quenching. It was assumed that the following two low-melting eutectics were present along the grain boundaries: α-Al+Cu₂Al+Mg₂Si and α-Al+Al₁₅(Fe,Mn)₃Si₂+Si. The measurements of nanohardness and the Young’s modulus in the sub micro volumes were carried out by means of the nanoindentation method using a NanoScan-4D scanning nanohardness tester. An algorithm offered is for calculating the additional pressure that occurs when the mechanical action on the metal having a partial melting of grain boundaries.