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Eingeladener Vortrag
- nein (13)
Im Rahmen eines AiF-Forschungsvorhabens wurde die Beeinflussung des Schweißprozesses mittels generierter Wechselstrom-Magnetfelder (ACMagnetfelder) beim Laserstrahlschweißen von Aluminiumlegierungen untersucht. Das vorrangige Augenmerk galt hierbei der Entfernung von Poren sowie der Stabilisierung der Schweißnahtoberfläche zur Vermeidung von rauen Schweißnähten. Das Schweißen mit Einsatz des AC-Magneten erzeugte im Vergleich zu den Referenznähten deutlich flachere Schweißnähte mit reduzierter Porenanzahl, die typische Schuppenstruktur wurde unterdrückt. Das Magnetfeld bewirkte bei entsprechenden Parametern der magnetischen Flussdichte und der Frequenz eine Halbierung der Rauigkeit der Schweißnahtoberfläche. Neben der Oberflächenberuhigung wurde auch die Verteilung von Poren in der Schmelze beeinflusst. Es konnte nachgewiesen werden, dass bei einer geeigneten Auswahl der Parameter von AC-Magnetfeldern die Porositätsanteile in der Schweißnaht auf ein Zehntel gegenüber den Referenzschweißnähten reduziert werden können.-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Within the framework of an AiF research project, investigations were conducted into the influences exerted on the welding process using generated alternating current (AC) magnetic fields during the laser beam welding of aluminium alloys. In this respect, attention was principally paid to removing pores and to stabilising the weld surface in order to avoid rough welds. In comparison with the reference welds, welding utilising the AC magnet produced considerably flatter welds with a reduced number of pores and the typical ripple structure was suppressed. With corresponding parameters for the magnetic flux density and the frequency, the roughness of the weld surface was halved using the magnetic field. In addition to the surface stabilisation, influences were also exerted on the distribution of pores in the molten metal. It was possible to prove that, by selecting suitable parameters for AC magnetic fields, the porosity proportions in the weld can be reduced to one tenth of those in the reference welds.
The corrosion resistance of aluminium surfaces is closely linked to the surfacecstate after a grinding process. For years, iron‐containing abrasive materials were suspected to lead to increased corrosion susceptibility after processing of aluminium surfaces. To prove a possible correlation between the iron content of an abrasive and the corrosion behaviour of aluminium components, scientific investigations and experimentally practical corrosion tests are necessary.
For the current investigation, specimens of a technical Al‐Si alloy from the same batch were used. The test specimens were mechanically ground with various resin‐bonded model abrasives containing different iron contents. The performed corrosion tests did not reveal a negative influence of the different iron‐containing abrasives on the corrosion behaviour of the Al–Si alloy.
However, the most sensitive measuring method (electrochemical noise)
showed differences in the surface activity depending on the type of abrasive.
Aluminiumlegierungen gehören neben Stahl zu den im Maschinenbau am häufigsten verwendeten Werkstoffen. Für innovative Baustrukturen in vielfältiger Formensprache aber auch in den klassischen Bereichen wie im Schiffsbau oder bei der Fertigung von Reaktorbehältern vom Kraftwerksbau bis zur Lebensmittelindustrie verwenden Ingenieure Aluminiumwerkstoffe.
Controlling the dynamics in the weld pool is a highly demanding challenge in deep-penetration laser beam welding with modern high power laser systems in the multi kilowatt range. An approach to insert braking forces in the melt which is successfully used in large-scaled industrial applications like casting is the so-called Hartmann effect due to externally applied magnetic fields. Therefore, this study deals with its adaptation to a laser beam welding process of much smaller geometric and time scale. In this paper, the contactless mitigation of fluid dynamic processes in the melt by steady magnetic fields was investigated by numerical simulation for partial penetration welding of aluminium. Three-dimensional heat transfer, fluid dynamics including phase transition and electromagnetic field partial differential equations were solved based on temperature-dependent material properties up to evaporation temperature for two different penetration depths of the laser beam. The Marangoni convection in the surface region of the weld pool and the natural convection due to the gravitational forces were identified as main driving forces in the weld pool. Furthermore, the latent heat of solide-liquid phase transition was taken into account and the solidification was modelled by the Carman-Kozeny equation for porous medium morphology. The results show that a characteristic change of the flow pattern in the melt can be achieved by the applied steady magnetic fields depending on the ratio of magnetic induced and viscous drag. Consequently, the weld bead geometry was significantly influenced by the developing Lorentz forces. Welding experiments with a 16 kW disc laser with an applied magnetic flux density of around 500 mT support the numerical results by showing a dissipating effect on the weld pool dynamics.
Multiple pulse investigations of 130-fs Ti:sapphire laser-induced damage of a high reflecting mirror consisting of alternating ?/4-layers of Ta2O5 and SiO2 and a single 500-nm Ta2O5 film were performed. In both cases, fused silica served as the substrate. For a fixed number of 1000 laser pulses per spot, a decrease in the damage threshold fluence of the mirror by a factor of two was observed by changing the repetition rate from 10 Hz to 1 kHz. A single 500-nm Ta2O5 film shows higher damage resistance compared to the mirror. The mirror and the Ta2O5 film samples were partially coated with a 300-nm-thick aluminium layer. The aluminium coating does not influence the damage threshold of the dielectrics underneath.
Despite the massive literature and the efforts devoted to understand the creep behavior of aluminum alloys, a full description of this phenomenon on the basis of microstructural parameters and experimental conditions is, at present, still missing. The analysis of creep is typically carried out in terms of the so-called steady or secondary creep regime. The present work offers an alternative view of the creep behavior based on the Orowan dislocation dynamics. Our approach considers primary and secondary creep together as solid state isothermal transformations, similar to recrystallization or precipitation phenomena. In this frame, it is shown that the Johnson-Mehl-Avrami-Kolmogorov equation, typically used to analyze these transformations, can also be employed to explain creep deformation. The description is fully compatible with present (empirical) models of steady state creep. We used creep curves of commercially pure Al and ingot AA6061 alloy at different temperatures and stresses to validate the proposed model.
Für den schadensfreien Einsatz passiver, metallischer Werkstoffe muss sich die Passivschicht vollständig ausbilden können und die Repassivierung von Defektstellen unter den Einsatzbedingungen muss möglich sein. Dabei müssen folgende Randbedingungen beachtet werden:
- glatte, blanke, saubere Oberfläche
- Grenzen der Beständigkeit bzgl. pH-Wert und Potential
- Elementverteilung, Ausscheidungen, Verformungen
- chemische Nachbehandlung bzw. Konditionierung
- geeignete Werkstoffauswahl bzw. Legierungszusammensetzung
- unplanmäßige Betriebszustände und Störungen
- Ausbildung von Bimetallelementen
Characterization of multiphase metal matrix composites by means of CT and neutron diffraction
(2016)
The present study examines the relationship between the microstructure of multiphase MMC and their damage mechanisms. The matrix AlSi12CuMgNi was combined with 15% vol. Al2O3 (short fibres) and with 7% vol. Al2O3 + 15% vol. SiC (short fibres and whiskers, respectively). The experimental approach encompasses 3D microstructure characterization by means of computed tomography of samples (a) as-received, (b) after heat treatment, and (c) after compression tests at room temperature. The volume fraction of different phases, their distribution, their orientation and the presence of defects and damage are studied.
Influence of addition of SiC whiskers on mechanical properties of composite was investigated. Phase-specific load partition analysis for samples with fibre plane parallel to load was perform by using neutron diffraction measurements during in-situ compression. It shows damage in the Si phase, while Al2O3 short fibres carry load without damage until failure. The computed tomography observations confirm the load partition analysis.
Certified water reference materials are currently not available for most of the hydrophobic organic pollutants listed in the EU Water Framework Directive. To find the most suitable container type for subsequent reference material productions, feasibility studies for the preparation of waters with polycyclic aromatic hydrocarbons (PAHs), polybrominated diphenyl ethers (PBDEs) and tributyltin (TBT) close to environmental quality standards in water have been performed. Due to the hydrophobic nature of these compounds and their tendency to adsorb onto container walls, an adequate selection of the most appropriate material for containment, storage and transport of water reference materials is crucial. Three different materials (aluminium, amber glass and fluorinated polyethylene, FPE) and three volumes (500/600 mL, 1000/1200 mL and 2000/3000 mL, depending on commercial availability) were tested at ng L-1 level of the target compounds. FPE shows by far the highest loss of analytes due to adsorption onto the container walls for all compounds studied. Aluminium and glass are equally suited for PAHs and PBDEs, but aluminium is unsuitable as container material for TBT due to acid cleaning requirements. The volume of the containers had no dramatic effect on the adsorption behaviour of target compounds for the different volumes tested.
Two metal matrix composites, both consisting of a near-eutectic cast AlSi12CuMgNi alloy, one reinforced with 15%vol. Al2O3 short fibers and the other with 7%vol. Al2O3 short fibers + 15%vol. SiC particles were studied. Distribution, orientation, and volume fraction of the different phases was determined by means of synchrotron computed tomography. The load partitioning between phases was investigated by in-situ neutron diffraction compression tests. The internal damage of the eutectic Si phase and Al2O3 fibers after ex-situ compression tests was directly observed in CT reconstructed volumes. Significant debonding between Al-matrix and SiC particles was found. Those observations allowed rationalizing the load transfer among the constituent phases of two different composites. Finally, based on the Maxwell scheme, a micro-mechanical model was utilized for the composite with one and two ceramic reinforcements. The model rationalizes the experimental data, and predicts the evolution of principal stresses in each phase.