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The mechanical strength of wrought high-strength aluminum alloys is essentially based on precipitation hardening, possibly in combination with prior forming, e. g. by stretching. Important parameters for achieving an optimum combination of hardness, strength, ductility, toughness, and further properties such as corrosion resistance are age-hardening temperature and time.
During thermal (mechanical) treatment, nucleation and growth of precipitates takes place, leading to the desired degree of hardening. In aluminum alloys, precipitation sequences are usually passed through, i. e. a sequence of metastable precipitates is formed before the stable phase can precipitate. The optimum combination of properties is therefore based on a certain (optimum) microstructure, which can, however, change during the use of a component, since the microstructure is not stable. This happens in particular when the operating temperatures are close to the aging temperature and/or the operating times are sufficiently long. An external mechanical load may accelerate the processes. The presentation gives some examples for this.
Combination of high-resolution time-of-flight (TOF) Neutron diffraction, scanning electron microscopy (SEM) and small angle neutron scattering (SANS) have been used to study hardening precipitation in 2014Al and 2124Al alloys. Neutron diffraction experiment revealed presence of tetragonal CuAl2 Phase (θ/θ´ precipitates) in 2014Al alloy and orthorhombic CuMgAl2 Phase (S-precipitates) in 2124Al alloy. SEM image analysis revealed that the θ/θ´-precipitates have platelet morphology while the S-precipitates exhibit rod-like shapes. From SANS data analysis the characteristic dimensions of precipitate particles and their polydisperse distributions were estimated.
The application of welded aluminum constructions has increased over the last decades due to the high demand for lightweight design. In this context aluminum alloys have a significant importance especially the heat treatable Al-Mg-Si alloys. With regards to welding the heat input causes the dissolving of the precipitates yielding a degradation of the strength properties. Furthermore, there is the risk of solidification cracking during welding for Al-Mg-Si alloys, especially AW 6082-T6. As a consequence Al-Mg-Si alloys are welded using a dissimilar filler material based on 5xxxx aluminum alloys that yields a reduction of resulting strength properties in comparison to the base material. Therefore, the paper presents the application of a novel massive filler wire for Al-Mg-Si alloys. The wire consists of AW 6082 with additionally added AlTi5B1 master alloy fostering the grain refinement of the weld metal. Exemplarily for the base material AW 6082-T6 GMA welding experiments are performed using the novel filler wire 6082TiB. It could be shown that solidification cracks could totally be avoided. The tensile strength of the welded joints are evaluated and compared with weldments using a conventional 5183 filler wire. Artificial aging is performed after welding in order to improve the strength properties by solution hardening showing the potential of these alloys.