Materials Science Forum Vols. 828-829

Abstract: Weld defects - such as porosity and hot cracking - occur especially during the laser beam welding of high-alloyed Al-Zn alloys. This significantly limits the application range of these promising high-strength alloys. In the present study the laser weldability of different Al-Zn alloys was investigated regarding the used welding parameters and the chemical composition of the alloys. In addition, the novel approach of the Helmholtz-Zentrum Geesthacht for overcoming the weldability problems was applied to the different Al-Zn alloys in order to assess its capability. It was shown that the laser weldability of Al-Zn alloys deteriorates with an increasing amount of Zn, Mg and Cu. The variation of laser beam welding parameters did not lead to any improvement of weldability. Only the use of the new approach resulted in promising welding results even for the high-alloyed Al-Zn alloys.

Abstract: Reducing vehicle weight and emissions by lightweight design is a major goal of the automotive industry. Magnesium as the lightest structural metal offers a significant weight saving potential compared to steel and aluminum. However, the poor formability of magnesium semi-finished products (e.g. sheets) has hindered the massive application of this metal. This poor formability arises due to the formation of strong textures. Typically the basal planes of the HCP-structure align parallel to the sheet plane. Such preferred orientation of the basal planes limits the ability of basal <a> slip to accommodate plastic strain in the sheet plane and is thus unfavourable for ductility and formability of the sheets, especially at room temperature. A combination of new technologies could help to alleviate the strong textures formed in Mg sheets. In this regard the utilization of twin-roll casting (TRC) in combination with processing techniques which apply additional shear strain such as differential speed rolling (DSR) and/or equal channel angular pressing (ECAP) could refine the microstructure and modify the resulting deformation texture. In this study an AZ31 strip (3Al-1Zn-Mg Bal. wt.%) produced by TRC was hot rolled at 400 °C using a rolling speed of 10 m/min. A speed ratio of 10% between rolls was used. Rolled samples with dimensions of 200 x 200 x 1.8 mm receive a single pass of ECAP at 200 °C. After processing the microstructure of the samples were analyzed by means of optical microscopy, electron backscatter diffraction and X-ray diffraction. The results showed that the microstructure can be refined and the texture altered in comparison to conventionally rolled AZ31 sheets. The samples processed by ECAP show an increase in formability up to 50% higher than conventionally rolled sheets. This demonstrates the potential of using shear deformation for processing Mg sheets.

Abstract: The surface reactivity of magnesium alloys at high temperatures required for their processing in solid, semisolid and liquid states is reviewed emphasizing presently existing barriers of its understanding and control. In addition to general aspects of magnesium oxidation, other forms of reactivity such as ignition and flammability are discussed. Since surface oxide, composed of pure MgO, does not offer a sufficient protection, operations of raw alloy melting and component manufacturing require protective atmospheres what has detrimental implications on process economy and product performance. Efforts to develop ignition resistant magnesium alloys through modification of their chemistry are described with a particular role of rare earths and other elements with high affinity to oxygen. It is concluded that all forms of magnesium reactivity in solid and liquid state could be explained through diffusion characteristics of surface oxide formed during processing and service.

Abstract: The effect of strain rate and strain on the hot compression behaviour of Ti6Al4V has been analysed to understand the microstructural evolution and restoration behaviour. Cylindrical samples with partially equiaxed grains were deformed in the α+β region at different thermo-mechanical conditions. EBSD has been used to study the microstructural behaviour and the restoration mechanisms. The microstructural evolution showed a complex restoration behaviour, where both fragmentation and nucleation of new grains have been observed. The volume fraction of the equiaxed grains increased with an increase in the strain, but oppositely decreased with the strain rate. At the same time the average grain size of the equiaxed grains decreased with an increase in both the strain and strain rate. The measured activation energy for deformation revealed a good agreement with reported values in the literature.

Abstract: The precipitation of primary Fe-rich intermetallics (sludge) in AlSi9Cu3(Fe) type alloys has been investigated for different Fe, Mn and Cr contents and cooling rates. Differential scanning calorimetry was used in order to assess the nucleation temperature and the enthalpy of sludge particles as well as to follow their evolution. The results show that the sludge nucleation temperature and the release of latent heat during sludge formation are functions of the initial concentrations of Fe, Mn and Cr in the molten alloy, i.e. the sludge factor, and the cooling rate. Being able to predict the temperature of sludge formation will support the foundries to set the right molten metal temperature in order to prevent sludge precipitation during the entire high-pressure die-casting process.

Abstract: Over the past years, the blended elemental powder metallurgy (PM) approach has been identified as one of the most promising strategies to reduce the cost of titanium-based components. However, oxygen pick-up, inhomogeneity of the microstructure and chemical composition are sometimes reported for PM parts. This work compares properties of a blended elemental Ti-6Al-4V alloy obtained by sintering under argon gas atmosphere with those of a vacuum cast alloy. Argon was purified by passing it through a series of oxygen and moisture traps prior to being introduced into the sintering furnace. Casting was performed under vacuum (1 x 10^-3 mbar). The starting material in both processes was the cold isostaticaly pressed blended elemental (BE) Ti-6Al-4V powder compact. The BE powder was prepared by mixing 60Al-40V master alloy powder with commercial Grade 4 titanium powder (0.377 wt.% O₂). The sintered and cast alloys were compared on the basis of oxygen pick-up, density, microstructure, chemical composition and hardness to determine which method is better. Although the BE approach could not eliminate the common challenges associated with powder metallurgy processing of Ti alloys, oxygen pick-up and additional contamination was lower compared vacuum casting. Sintering at 1350°C for 1 h could not achieve full density compared to casting, but the microstructure appeared more homogeneous. Both sintered and cast Ti6Al4V alloys were harder than wrought Ti6Al4V due to a high concentration of interstitial oxygen. The sinterered and sintered plus HIPed Ti6Al4V alloys were softer than as-cast Ti6Al4V due to lower oxygen pick-up and incomplete densification. From the contamination and homogeneity perspective, the BE approach is an attractive technique for processing of Ti6Al4V alloy.

Abstract: The Plasma Electrolytic Oxidation (PEO) is a surface modification process that allows producing protective oxide coatings on light metal alloys. These coatings are characterized by high microhardness and wear resistance. Changes of electrical conditions of alternate current during process, such as duration and frequency of anodic and cathodic pulses, influence on layer growth rate, its microstructure and final properties. In this study aluminium samples were oxidized at different electrical parameters in sodium silicate electrolyte. The growth rates of oxide layers were determined by mass change and thickness measurements. The microstructure of the obtained coatings was investigated by using scanning electron microscope. Additionally, phase compositions of coatings was determined by X-ray diffraction on top surfaces of layers. Generally, shorter duration and higher frequency of anodic pulses provides more uniform structure at lower growth rate. Nevertheless, there are certain ranges of electrical parameters that provide decent uniformity of oxide coatings.

Abstract: This paper describes a typical 3D X-ray micro computed tomography (microCT) analysis of light metal parts, from a radio controlled airplane engine. This case study shows the power of 3D X-ray inspection and analysis for this type of material, including information about the size and location of casting defects, the location of turbine blade balancing weights and dimensional measurements indicating the axle was not perfectly centre. Advantages and limitations of the method for light metals are described in general

Abstract: In some commercial titanium extrusion practices, twisting of the extrudate can occur, which can result in the need to crop the back and front end of the extruded material, thereby reducing yield and increasing material losses. Understanding more about the behaviour of material during the extrusion process, and investigating the cause of defects such as twisting by use of finite element (FE) modelling techniques could help to reduce these losses, improve the productivity of the extrusion process, and the overall quality of the material produced. One of the most important components of FE techniques for hot deformation is the type of flow stress model that is used in the simulations. In this investigation isothermal uniaxial compression testing was performed on cylindrical specimens of Ti-6Al-4V at temperatures ranging from 950 °C to 1200°C and strain rates of 0.1 s^-1 to 50 s^-1, to produce true stress against true strain and load against die travel curves which were subsequently used to develop a new specific flow stress model for use in hot deformation above the beta transus, which can ultimately be applied to the hot extrusion of Ti-6Al-4V. From analysis of this data it was concluded that flow softening and work hardening do not occur during deformation, and that low friction conditions exist between the material and the tooling. The activation energy for deformation was found to be 193178 J.mol^-1, and the flow stress model was shown to give a good fit to the raw data at low strain rates, but this relationship broke down at higher strain rates. Finally the importance of generating a flow stress model specific to a particular operation, and set of experimental data, rather than relying on existing data available in the literature is demonstrated.

Abstract: Direct powder rolling (DPR)/roll compaction has been labelled a complex and sample sensitive process. As such the design of the instrument and the determination of the optimal processing conditions for a given feed are very challenging. The challenge is attributed to a wide range of operating parameters and material properties. Several theoretical models can be used to evaluate the interaction of the different parameters and properties and how their changes affect the rolling process. In this study, the Johanson theory was used to determine the rolling parameters of titanium powder. Preliminary results of the nip angle, nip pressures and maximum horizontal pressures of the mill for the powder rolled on a 55mm diameter roll with roll gap sizes of 0.175, 0.15 and 0.05 mm were obtained. The results were found to be acceptable for the nip angle estimation, however improvement on predicting the maximum horizontal pressure is required.