Abstract: In this study, carbon nanotube (CNT) reinforced aluminum (Al) composite was fabricated by powder metallurgy (P/M) technique and the evolution of CNTs within the matrix were traced, characterized and discussed. It was found that the separation of CNTs was affected by both the powder mixing operation as well as the secondary processing. CNTs were damaged during mechanical powder mixing and sintering process, whilst the graphitic structures were not damaged during the secondary processing. In addition, CNTs were subjected to substantial compression stress in both powder mixing and sintering process.
Abstract: Sheet metal forming experiments have been carried out on AZ31 and ZE10 sheets produced by rolling conventionally DC cast slabs as well as twin roll cast (TRC) strips. Nakajima tests were performed on the various sheet materials over the temperature range from RT to 200 °C using Hasek type samples of specified geometries to generate various strain paths. The strain path data were used to derive the forming limit curves as plotted in forming limit diagrams for the two alloys. The temperature dependence of the sheet formability is discussed in terms of the operating deformation mechanisms and the roles of alloy composition, initial texture and processing history.
Abstract: Magnesium alloy sheets bear significant potential in replacing conventional materials such as aluminium and steels in ultra lightweight designs. High specific strength and stiffness, combined with the lowest density of all structural metals make magnesium alloy sheets candidates to face the challenges of reducing vessel weight in the transportation industry and thus, green house gas emissions. For forming components from sheet metal, deep drawing is a well established and commonly applied process. Due to the limited formability of magnesium sheets at room temperature, deep drawing processes have to be conducted at elevated temperatures. In the present study, hot deep drawing experiments on an industrial scale hydraulic press were successfully conducted. Forming was done at moderately low temperatures from 150°C to 250°C. Sheets of the magnesium alloy AZ31B (Mg-3Al-1Zn-Mn) were drawn to symmetrical cups according to Swift. For AZ31, distinct basal type textures are formed during hot rolling. The influence of texture on earing is displayed. The microstructural evolution of the material is dominated by the formation of twins and dynamic recrystallisation. By optimising the process, a drawing ratio of 2.9 was achieved for AZ31 sheet, outperforming conventional materials at ambient temperature.
Abstract: The role of ceramic particles, calcium and rare earth elements on magnesium alloys during solidification from the melt and after hot rolling has been studied by microstructural investigations, texture measurements and mechanical tests. Different ceramic inoculants like silicon carbide or zirconium diboride and two different rare earth elements (cerium and lanthanum) forming intermetallic compounds were used. Both, ceramic particles and intermetallic phases, modify the texture evolution during hot forming. The rolled alloys exhibited a basal fibre texture which is weakened by ceramic particles and intermetallic phases respectively. This weakening of the basal texture is capable of lowering the anisotropy of the yield stress and improving the formability of sheet material. Influences of the alloying contents on microstructure, texture evolution and mechanical properties will be discussed.
Abstract: In the present study, SPD processing was combined with annealing in order to obtain synergic effect of grain size refinement and precipitate strengthening. Samples of 7475 alloy were solution heat treated, water quenched and then subjected to hydrostatic extrusion with a total true strain of about 4. Hydrostatic extrusion resulted in a significant grain refinement from 70 mm to about 70 nm. The samples were subsequently annealed at temperatures inducing the formation of nano-precipitates. The investigations of the structure and mechanical properties of the samples subjected to SPD and annealing revealed different precipitation path in micro- and nano-grained samples. Also, it was found that the combination of HE processing and low temperature annealing results in the formation of nano-precipitates in nano-grained structures which effectively strengthen nano-aluminium alloy.
Abstract: A comprehensive numerical model has been developed for the calculation of the final microstructure and mechanical properties of aluminium casting alloys after heat treatment. The calculation is chained in such a way that the resultant microstructure of the previous processing step, such as casting, is used as the initial condition of the following step. In this paper an A357 alloy is chosen to study the microstructure and the evolution of the mechanical properties through the process from casting to heat treatment. The microstructure and mechanical properties are predicted and compared with experimental measurements.
Abstract: The influence of crystallographic texture on high cycle fatigue behaviour has been studied using an extruded rectangular profile of the AZ31 (Mg-Al-3wt%-Zn-wt1%) alloy. The fatigue samples, cut at 0, 45 and 90° to the extrusion direction correspond to different initial textures. Besides high cycle fatigue tests, quasi-static tensile and compression tests were performed to assess the tension-compression asymmetries as a function of the initial texture. The micro-mechanisms of fatigue crack initiation were investigated using scanning electron microscopy and electron backscatter diffraction. Differences in the mechanical properties and the endurance limit for the different sample directions are related to the initial texture and, subsequently, the easiness or difficulty of slip/twinning.
Abstract: Three alloys of the magnesium AZ-series (AZ31, AZ61 and AZ91) were processed by multi-temperature equal channel angular pressing (ECAP) with five passes using route BC. The ECAP temperature was decreased from 275 °C to 250 °C during the process for better grain refinement. The mechanical behaviour was investigated over a wide range of strain rates (10-3 s-1 up to 103 s-1) under compressive loading at room temperature. The investigations show that significant grain refinement takes place during the ECAP-process. The initial grain size of 12 μm, 9 μm and 5.8 μm for extruded AZ31, AZ61 and AZ91, respectively, could be refined to 2.5 μm. The grain refinement occurs by dynamic recrystallisation. Compared to extruded initial Mg-alloys, the yield stress increases slightly for all selected alloys after ECAP processing, while the amount of strain hardening decreases at the same time, due to reduced grain size and texture effects. Furthermore, the flow stress of extruded and ECAPed material is less affected by strain rates within a range of 10-3 s-1 to 10-1 s-1.
Abstract: The continuous miniaturisation of products needed e.g. in the automotive or the micro electronic sector requires process chains which allow the manufacturing of microscopically small components in high quantities. The development of the required processes and technologies is the aim of the Collaborative Research Centre 747 “Micro Cold Forming” of the German Research Foundation. A necessary step in the manufacturing process chain is the heat treatment, which enables the adjustment of the semi-finished micro components to cold forming. Finally the usage properties have to be adjusted e.g. by precipitation hardening of aluminium alloys in order to increase the strength above the strain hardened level. To achieve a high strength aluminium alloy with elevated temperature stability in form of thin foils a new Al-Sc alloy with 2 mass-% scandium had to be developed within the Collaborative Research Centre. In comparison to conventional casting techniques it was possible to suppress Al3Sc precipitation and to ensure the supersaturated solid solution by using a Physical Vapour Deposition (PVD) Magnetron Sputtering process and low sputtering temperatures. Samples of the material are then artificially aged and characterised by ultra micro hardness measurements to evaluate the optimal ageing parameters. Besides this the microstructure was determined by light microscopy and Transmission Electron Microscopical (TEM) analysis.
Abstract: The use of wrought magnesium for automobile structural components is an important component of the mass reduction strategy for automobiles to improve their fuel efficiency. Compared to Direct chill casting, Twin Roll Casting (TRC) allows major reduction of hot rolling steps in the production of Mg sheet due to the thin thickness of the as-cast strip. This TRC route can substantially reduce the time and cost to produce Mg alloy sheet product. In this work, AZ31 magnesium alloy was casted to 5 and 6 mm thick strips under different process conditions. Microstructure of these strips was analyzed using optical microscopy, SEM and EPMA. TRC strip was annealed under two different conditions: 2 hours at 330 and 1 hour at 400°C. It has been found that heat treatment at 400°C for 1 hour reduces centerline segregation significantly. TRC strips were rolled down to 2 mm and annealed at 450°C for 2 minutes. The average grain size was 4-6 µm and mechanical properties were comparable with commercial AZ31 sheet.