Materials Science Forum Vols. 828-829

Abstract: Precipitation hardening has been used before as one of the most effective strengthening methods for many metallic alloys. However, this method has not been studied completely in magnesium alloys, and the numbers of precipitation hardenable wrought Mg alloys are still very limited compared to aluminum alloys and steels. The age hardening responses of Mg-Al-Sn alloys in cast-homogenized condition were investigated by isothermal aging at 200°C for prolonged time. It was found that hardness can be improved significantly for the alloy with higher amounts of tin. The improvement in hardness was reasoned by the formation of precipitates. The shapes and morphology of the precipitates were different depending on the orientations of the grains. The precipitates were characterized by scanning electron microscope.

Abstract: Quasi-static/dynamic three-point bending tests were conducted to assess the crash performance of magnesium alloy AZ31B extruded and sheet tubes at the German Aerospace Centre (DLR) – Institute of Vehicle Concepts in Stuttgart. Different foam-filled AZ31B beams with a variation of foam density and thickness were fabricated through several manufacturing processes: cold bending, tungsten inert gas welding, cathodic dip painting and polyurethane foam injection. The experimental results were compared with those from mild steel DC04 tubes. It shows that empty magnesium alloy AZ31B outperforms steel DC04 in terms of specific energy absorption for the empty tubes with equivalent volume when subjected to bending loads. It was found that the foam-filled tubes achieved much higher load carrying capacity and specific energy absorption than the empty tubes. Moreover, there is a tendency showing that a foam-filled beam with a higher foam density reaches higher load carrying capacity, but fractures earlier. The foam-filled AZ31B tube with 0.20 g/cm³ foam obtained the highest specific energy absorption, but this outperformance was weakened due to the earlier fracture. In addition, the numerical simulation utilising material model MAT_124 in LS-DYNA explicit FEA package was performed. The simulation results indicate that using calibrated stress-strain curves and failure parameters, material model MAT_124 yields a general good agreement with the experimental results.

Abstract: Rapid solidification of molten metals has been recently used to generate a new group of alloys having ultra-fine microstructures and high end mechanical properties. Therefore, such alloys can be successfully used in the optics industry to produce diamond machined mirrors and mould inserts for plastic lens injection. Rapidly solidified aluminium grades characterised by their ultra-fine grains can be used to replace traditional optical aluminium such as 6061-T6 which has coarse microstructure when making optics. However, there is currently no data available on the performance of these new grades in terms of diamond tool wear when machined in single-point diamond turning operation. This paper reports on the wear mechanisms of natural diamond tools when turning RSA 443 which is a new aluminium grade produced by rapid cooling process. Although this new aluminium grade enjoys fine microstructure, it is harder than traditional optical aluminium because of its increased content of silicon (about 40%). Therefore, there is a need to establish a deeper understanding of diamond tool performance when using diamond turning of optical components from this material. In this study, three machining parameters, namely cutting speed, feed rate, and depth of cut, were varied at three levels and the edge wear of the diamond inserts was observed using scanning electron microscopy after 4 km of cutting distance. The first observations from this preliminary study show that tool wear of diamond is more sensitive to the change in cutting speed than it is for other cutting parameters. Wear is relatively high (12 µm) at the lowest cutting speed (500 rpm). However, at high cutting speed (3000 rpm) the edge wear was small (3 µm). This could be attributed to the increased impacts of cut the material on the cutting edge. The study also reports on the surface finish obtained at the different combinations of cutting parameters.

Abstract: This paper reports on investigations of the beneficial effects of electron beam alloying (EBA) and electron beam dispersion alloying (EBDA) on the wear behavior of AZ91D Mg alloy under mild wear conditions with applied normal loads of 1…10 N. The layers generated had a thickness of 1.5 mm with Al contents of 30 wt.%. For dispersion alloyed layers, TiC was added with particle sizes of 20…100 µm. At a sliding distance of 20 m, the wear rates of alloyed layers (150 HB) and dispersion alloyed layers (180 HB) were almost the same and could be reduced by half compared to the untreated AZ91D (60 HB). Due to their large size and the large spaces between them, TiC particles were pressed into the layer matrix, or were torn out and acted as additional abrasives. Therefore, at a sliding distance of 50 m, the wear rate of dispersion alloyed layers increased to the level of the base material.

Abstract: Compared to Al alloys Mg alloys are generally slower to extrude and this makes them expensive to process. However, making alloys easier to extrude usually equates to reduced extrudate strength. The effects of extrusion parameter, billet heat treatments and alloying element concentration on extrudability and extrudate mechanical properties are compared using four lean ZK alloys. By weight percent there was a greater increase in peak extrusion pressure and extrudate yield strength from increasing Zr than from Zn. Homogenising the extrusion billets had no effect on the lean alloys and only a minor effect on the richer alloys. Alloying element concentration has the most effect on alloy mechanical properties, while changing the extrusion speed and temperature has little influence on the extrudate mechanical properties.

Abstract: Modern lightweight construction containing hybrid materials and structures allow the improvement of a wide range of density specific properties. In this regard, the great potential of structures consisting of carbon fibre reinforced plastics (CFRP) and aluminium (Al) is far from being exhausted. In order to exploit this potential, novel joint concepts are necessary, enabling simultaneously an economic structure manufacturing. As a more advantageous alternative to conventional riveting, the development of integral joint concepts with material oriented design that includes high load capacity and improved corrosion resistance has currently high priority for aviation industry. The research group Schwarz-Silber (FOR 1224) funded by the Deutsche Forschungsgemeinschaft (DFG) and based at the University of Bremen set itself the goal to explore and develop interface structures for advanced CFRP-Al compounds. Considering textile, welding and casting techniques, novel joint concepts will be designed, dimensioned and produced within six interdisciplinary projects. Experimental and numerical investigations support the validation and enhancements of the developed solutions. Regarding a joint concept combining textile and welding techniques, basic investigations have been performed. The concept of coupling Al sheets with CFRP panels by means of transition structures made of titanium (Ti) sheets, being bonded to the CFRP side and welded to the Al side, has already been analysed under static tensile loads. This paper presents the failure behaviour of such entire Al-Ti-CFRP joints under cyclic loads. The failure probability is calculated by using the approach of maximum likelihood.

Abstract: Monotonic compression testing was conducted on AZ31B-F magnesium alloy in both the as-received and forged conditions. Sigmoidal stress strain behaviour was the key feature in the majority of material conditions and directions corresponding to plastic behaviour where twinning de-twinning is the dominant deformation mechanism. More conventional monotonic hardening (slip deformation mechanism) was exhibited in certain material directions which initially were orthogonal to the extrusion direction in the as-received condition, but once forged are coincident with the direction of forging once forged. It was shown that in the forged condition, there is potential for significant increases in both ultimate tensile strength as well as strain to failure.

Abstract: In the present work the mechanical behaviour of laser beam welded AZ31B alloy was studied, by changing systematically the spot size of the used fibre laser system between 200 µm and 1000 µm at different power levels between 2 kW and 8 kW. Maximum welding velocities with respect to imperfections were determined. The characterization of the obtained welds - in terms of Vickers hardness, UTS, A_f and weld width, resp. weld area - was correlated with the micro-texture in dependence of the different Focus Spot Diameters and Laser Beam Power levels as well as the resulting cooling rates. Highest UTS of 94% of the base material was achieved with 200 µm Focus Spot Diameter and Laser Beam Power of 4 kW at welding velocity of 100 mm/s. By increasing the Focus Spot Diameter to 600 µm, the tensile strength was reduced to 86 % of the actual strength of the base material.

Abstract: Selective Laser Melting (SLM) presents a modern manufacturing process with an innovative technology which allows the production of full-density objects or fine-structured parts with complex geometry and inner structures. Stability and certification of the properties of SLM parts are important tasks for all producers and end-users. One of the drawbacks of this technology is high residual stress in as-made SLM objects. In this study X-ray diffraction technique was used for investigating the residual stress induced into SLM Ti6Al4V alloy samples. Principal stresses were estimated for the cut rectangular specimen. Two types of the cantilevers were produced and numerical simulation of the stress was performed. The bending of cut cantilevers was measured before and after heat treatment. Next series of the samples had rectangular shapes and different thicknesses from 1 to 46 layers. All as-manufactured specimens attached to the substrate showed the presence of tensile residual stresses near the top surface. Residual stress along the laser scanning direction had magnitudes twice that of the stress in the perpendicular direction. Conclusions regarding directions and values of stresses in SLM objects from Ti6Al4V powder are given.

Abstract: Due to their high specific strength, good corrosion resistance and high temperature strength Magnesium alloys containing Rare Earth additions are promising candidates for structural and engine applications in the transportation industry. Also medical applications, like bone screws and nails, benefit from their moderate corrosion rate and biocompatibility. All applications need materials which show a high strength, ductility and fracture toughness in case a crack has formed during service to keep safety against rupture. In this study four extruded Mg10Gd based alloys modified with Nd and La have been 3-point-bend tested at low a deformation speed to evaluate the influence of the microstructure on crack growth. A comparison to the cast material (subjected to T4 to increase ductility and to reduce the dendritic microstructure) shows an increase in strength and ductility due to the fine grained microstructure as a result of recrystallization during extrusion. The maximum bending strength and outer strain to crack initiation is also strongly influenced by the alloying system itself. The influence of Nd and La to the binary alloy Mg10Gd is discussed in using tensile, compression and bending tests. The increase in strength results in reduced elongation to fracture in tension loading as well as the outer strain for the crack initiation during bending tests. Tensile tests are often discussed to be not a reliable method for determining the Young’s modulus of magnesium. Therefore resonance frequency damping analysis has been applied to determine the dynamic modulus of elasticity, which is compared with the flexural (bending) modulus. Crack growth is discussed using light microscopy and correlated with bending stress-strain curves. The crack growth rate of the extruded, fine grained material is many times higher than of the cast, coarse grained material. Crack propagation is mostly transgranular and assisted by twinning.