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Aluminum alloy 7050 is known for its superior mechanical properties, and thus finds its application in aerospace industry. Vertical direct-chill (DC) casting process is typically employed for producing such an alloy. Despite its advantages, AA7050 is considered as a "hard-to-cast" alloy because of its propensity to cold cracking. This type of cracks occurs catastrophically and is difficult to predict. Previous research suggested that such a crack could be initiated by undeveloped hot tears (microscopic hot tear) formed during the DC casting process if they reach a certain critical size. However, validation of such a hypothesis has not been done yet. Therefore, a method to produce a hot tear with a controlled size is needed as part of the verification studies. In the current study, we demonstrate a method that has a potential to control the size of the created hot tear in a small-scale solidification process. We found that by changing two variables, cooling rate and displacement compensation rate, the size of the hot tear during solidification can be modified in a controlled way. An X-ray microtomography characterization technique is utilized to quantify the created hot tear. We suggest that feeding and strain rate during DC casting are more important compared with the exerted force on the sample for the formation of a hot tear. In addition, we show that there are four different domains of hot-tear development in the explored experimental window-compression, microscopic hot tear, macroscopic hot tear, and failure. The samples produced in the current study will be used for subsequent experiments that simulate cold-cracking conditions to confirm the earlier proposed model. This research was carried out within the Materials innovation institute (www.m2i.nl) research framework, project no. M42.5.09340.

A computer model has been developed to describe the in-flight dynamic and thermal histories of gas atomized droplets as a function of distance during spray forming. The model has been used to investigate the effects of the dynamic and thermal behaviour of individual gas atomized droplets and the cooling and solidification behaviour of the overall spray. The most influential parameters for a given alloy system, in order of importance, are: (i) droplet diameter and, therefore, the droplet size distribution within the spray; (ii) initial axial gas velocity at the point of atomization and the subsequent gas velocity decay profile; (iii) melt mass flow rate; (iv) melt superheat at the point of atomization; and (v) alloy composition. Experimental measurements of gas velocities and droplet size distributions during spray forming allow the spray solid fraction at deposition to be calculated and used in a subsequent computer model of billet heat flow to predict the billet top surface temperatures and solid fractions.

The knowledge on constitutive mechanical behavior at the temperatures close to the solidus is essential for predicting high-temperature deformation and fracture, e.g. cold and hot cracking of aluminum alloys. In this work we studied the tensile mechanical properties of an as-cast AA7050 alloy in a near-solidus temperature regime. Tensile tests were carried out using Gleeble-3800TM system at temperatures from 400 to 465 °C and at strain rates from 0.0005 to 0.05 s-1. The results show that the strength decreases with increasing temperature and decreasing strain rate. Meanwhile, ductility decreases with the increase of temperature and strain rate. The constitutive parameters were extracted by fitting the test data to the extended-Ludwik and creep-law equations. The parameters for the extended-Ludwik equation are continuous with the values from a lower temperature regime obtained earlier, while the parameters for the creep-law equation are comparable with those obtained on other 7XXX aluminum alloys. We observed a change in fracture mode at 450 °C; from ductile transgranular to intergranular. This temperature coincides with the discontinuity point of the temperature-ductility slope. On the fracture surface of a sample that was deformed at 465 °C with a strain rate of 0.0005 s-1, we observed features characteristic of micro-superplasticity. Considering the test conditions, viscous flows of incipient melt or liquid-like substances are suggested to be responsible for the formation of this feature.

Direct chill (DC) casting of high strength 7xxx series aluminium alloys is difficult mainly due to solidification cracking (hot cracks) and solid state cracking (cold cracks). Poor thermal properties along with extreme brittleness in the as-cast condition make DC-casting of such alloys a challenging process. Therefore, a criterion that can predict the catastrophic failure and cold cracking of the ingots would be highly beneficial to the aluminium industry. The already established criteria are dealing with the maximum principal stress component in the ingot and the plane strain fracture toughness (KIc) of the alloy under discussion. In this research work such a criterion was applied to a typical 7xxx series alloy which is highly prone to cold cracking. The mechanical properties, constitutive parameters, as well as the KIc values of the alloy were determined experimentally in the genuine as-cast condition and used as input data for the finite element package ALSIM5. Thermomechanical simulations were run for billets of various diameters and the state of residual thermal stresses was determined. Following the contour maps of the critical crack size gained from the model, the casting conditions were optimized to produce a crack-free billet.

In hollow aluminium extrusions produced with porthole dies, longitudinal weld-seams are present throughout the entire extruded length, due to rejoining of metal streams inside the extrusion tooling. These weld-seams, formed by a solid-state bonding process at elevated temperatures and under conditions of interfacial pressure and plastic deformation, are commonly the weakest areas in the extrusion cross-section. Therefore, predictive control of the weld-seam formation process is essential. For this purpose a conceptual model is developed, containing all relevant features pertaining to the solid-state bonding process as this occurs in aluminium extrusion. This paper describes an experimental programme aimed at exploring the effects of tooling geometry, process conditions and alloy composition on weld-seam performance. In lab-scale experiments, strips were extruded from AA6060 and AA6082 billets, using tooling geometries in which a bridge was incorporated to form a weld-seam. Keeping the profile geometry constant, the internal geometry of the tooling was varied, resulting in different bonding conditions. The obtained weld-seams were characterised by means of transverse tensile tests. Associated microstructural characterisation of the extrusions and inspection of fracture surfaces was performed in order to relate the mechanical performance to microstructural features. The results clearly show that the evolutionary microstructural response of the alloy, determined by the local thermomechanical conditions inside the die, has a significant effect on the weld-seam performance. It is therefore concluded that, besides a criterion for suitable flow conditions and interfacial pressure, a predictive tool for weld-seam quality must incorporate the evolutionary microstructural response of the alloy in question.

In hollow aluminium extrusions, longitudinal weld-seams are formed through a solid-state bonding process at elevated temperatures and under conditions of interfacial pressure and plastic deformation. For structurally loaded components, sound weld seams are imperative. In our research, a weld seam integrity indicator as a means of quantifying bonding efficiency is introduced and the feasibility of this concept is investigated by means of lab-scale experiments. Characterisation of these weld seams through mechanical testing, provides a basis for an estimation of the weld seam indicator introduced in this paper, thus demonstrating the feasibility of this concept.

Design models are required to assess the behaviour in fire of aluminium structures. These models need to be validated by comparison with test. Up to now tests results are only available for (small scale) individual components. This paper provides the results of tests carried out on frames. A finite element model in combination with a sophisticated constitutive model are used to simulate the test. The results of the simulations agree with that of the tests at room and elevated temperatures.

Shrinkage, imposed strain rate, and (lack of) feeding are considered the main factors that determine cavity formation or the formation of hot tears. A hot-tearing model is proposed that will combine a macroscopic description of the casting process and a microscopic model. The micromodel predicts whether porosity will form or a hot tear will develop. Results for an Al-4.5 pct Cu alloy are presented as a function of the constant strain rate and cooling rate. Also, incorporation of the model in a finite element method (FEM) simulation of the direct-chill (DC) casting process is reported. The model shows features well known from literature such as increasing hot-tearing sensitivity with increasing deformation rate, cooling rate, and grain size. Similar trends are found for the porosity formation as well. The model also predicts a beneficial effect of applying a ramping procedure during the start-up phase, which is an improvement in comparison with earlier findings obtained with alternative models. In principle, the model does not contain adjustable parameters, but several parameters are not well known. A full quantitative validation not only requires detailed casting trials but also independent determination of some thermophysical parameters of the semisolid mush.

Reactive mixtures based on nano aluminium and nano- or micron size molybdenum trioxide have been pressed. These energetic mixtures have been tested in the Ballistic Impact Chamber and as an energetic projectile impacting a steel plate. Al/ MoO3-based reactive mixtures can be initiated by a controlled mechanical stimulus. The sensitivity depends on deformation velocity, pressing density, addition of fluorine containing binder and particle size. Nanometric mixtures of Al and MoO3 have a shorter time to reaction compared to the corresponding mixtures with micron size MoO3. The sensitivity decreases with increase in porosity. No influence of fuel/oxidizer ratio has been found in the experiments. An extensive deformation is needed before the start of reaction is observed. An analytical evaluation of shear rate cannot be applied as a detailed numerical simulation of the deformation process in PBXN-109 shows. © 2007 WILEY-VCH Verlag GmbH & Co. KGaA.

The invention relates to a method of recovering a metal from a mixture comprising more than one metal. According to the invention, the methods comprises the steps of (a) melting the mixture to produce a melt; (b) passing the melt over a cooled surface, causing the metal to be recovered to crystallise out on the cooled surface and a desired metal-depleted melt is discharged from the cooled surface