Materials Science Forum Vols. 654-656

Abstract: In this study, non-uniform heating approach in warm T-joint forming process is attempted for the AZ31 magnesium alloy tube. For this purpose, finite element simulation is performed to analyze the appropriate temperature distribution. The validity of the finite element(FE) model of T-joint tube hydroforming(THF) is verified by comparing the FE simulation and experimental results. Using this FE model, appropriate temperature distribution was suggested. In addition, it was showed that the wall thickness could be more uniform by optimizing the temperature condition.

Abstract: The sound insulation experiments were conducted to magnesium alloy sheet and magnesium honeycomb panels. It is found that the sound insulation coefficient (transmission loss, TL) of honeycomb panel is higher than that of the sheet in full sound frequency spectra, and TL spectra of both single sheet and honeycomb panel is similar with the sound frequency, thus the TL is increased obviously with the frequency. Comparably, TL of honeycomb panels is always higher than that of the sheets, especially at middle range of frequencies 6 kHz~10 kHz, the sheet has a TL of 27 to 31dB, and the honeycomb panel has 34-39 dB. The parameters of honeycomb cores is discussed briefly.

Abstract: This paper deals with the effect of temperature on plastic buckling strength of cylindrical pipe of some magnesium alloys subjected to shot peening process. Workpieces are three cylindrical pipes of AZ31, AZ61 and AZ80 magnesium alloys. The shape of pipe is 30mm in height, 16mm and 14mm in outer and inner diameters. The shot peening treatment was given on the surface of cylindrical pipe using an air-type peening machine. Plastic buckling strength test was performed under axial compression in a case of both ends with fixed condition, at several temperatures between 293K and 573K and at a crosshead speed of 1 mm/min using an Instron-type testing machine. From the experimental result, buckling stresses for all kinds of AZ31, AZ61 and AZ80 alloy pipe were strengthened after the shot peening treatment, but a few of differences for the increasing ratio. The buckling stress of the shot-peened pipe was kept higher than that of as received one up to the test temperature of 473K, however it tended to be around equal at 573K.

Abstract: Friction stir processing (FSP) of cast AM60 magnesium alloy has been studied using the breaking pin method, freezing the microstructure during the process. Strong evidence is seen that partial melting plays a key role in removing the β-Mg17Al12, whereas the manganese rich intermetallics persist during FSP. This is consistent with temperature predictions made using a process model adapted to FSP of AM60 castings that show the eutectic melting point is exceeded.

Abstract: Magnetic pulse welding was applied to the lap joining of similar (Al/Al) and dissimilar materials (Al/Fe, Al/Cu, and Al/Ni). The magnetic pulse welding is a kind of impact welding represented by explosive welding. The impact energy is induced by electromagnetic force generated by interaction among discharge pulse, induced magnetic flux, and eddy current produced at the plate surface. The welding was achieved within 10 microseconds with a negligible temperature increase. The welding interface exhibited a characteristic wavy morphology, which was similar to that of the explosive welding. In the Al/Fe, Al/Cu, and Al/Ni joints, an intermediate phase layer was produced along the wavy interface. In order to investigate microstructure of the intermediate phase layer, TEM observation of the welding interface was carried out. TEM observation revealed that the intermediate phase layer consisted of amorphous phase and fine crystal grains.

Abstract: Modification of the microstructure of ZE41A-T5 magnesium alloy substrates was investigated by laser surface re-melting and solidification using a 2.5 kW Nd:YAG laser. The effects of laser power, high scan rate and beam configuration were examined. The microstructure of laser treated ZE41 consisted of small precipitates dispersed in a fine dendritic α-magnesium matrix at high scan rates. The redistribution of chemical elements depended mainly on the dwell time in the liquid stage. At high scan rates, long dwell times were achieved by splitting the laser beam into two spots trailing in the scan direction which resulted in a more homogenous distribution of Mg, Zn and Zr. Cracking due to thermal shrinkage during solidification was prevented by reducing the temperature of the melt pool. This was achieved by lowering the laser power, increasing the scan rate and laser spot size. Increasing the laser spot size in the scan direction was conducive to producing homogeneous microstructures without cracks.

Abstract: Maintenance of ageing military aircraft structures encompasses both engineering and scientific management. In support of this, surface modification and repair methods are used on an opportunity basis to extend the life of aircraft in terms of fatigue and safety. Often, certain surface modification technologies have proved to be both cost effective and amenable for safe application. Some candidate technologies include shot peening, cold spray, deep surface rolling, friction stir welding, laser shock peening, and laser cladding. Whilst some technologies have been successfully applied to Australian Defence Force (ADF) aircraft in the past, some newer technologies are also being considered. The supersonic particle deposition (SPD) technology also known as cold spray coating has been recently approved for application on a helicopter gear box. Another technology of significance to ADF application is Laser Cladding (LC) technology. This paper briefly summarises the research work on these technologies at DSTO and discusses potential applications for aircraft components in the near future. It also provides an analysis of technologies and their potential advantages and disadvantages.

Abstract: This paper deals with the influence of fourth element addition to the high strength Mg-Zn-Y extruded alloys with long period stacking ordered (LPSO) phase on corrosion resistance and surface film formation. Salt water immersion test, SEM observation, and EDX analysis are performed on Mg97Zn1Y2(at.%), Mg97Zn1Y1.9La0.1 and Mg97Zn1Y1.9Al0.1 alloys. The Al or La addition to the extruded Mg97Zn1Y2 alloy is effective for the formation of thin, dense and uniform surface layer, resulting in improvement of corrosion resistance.

Abstract: Magnesium alloys possess many advantageous functional properties. Use of magnesium alloys, mainly for vehicle parts as well as electronic appliances, has been booming in recent years because of their lightweight compared to aluminum alloys and good creep resistance relative to plastics. Most the use of magnesium for structural applications has been die-cast components and most of this in one alloy, AZ91D. Since magnesium has the lowest electrochemical potential out of all the common commercial metals and is extremely prone to corrosion, it is necessary that it undergoes surface treatment. It is well known that fatigue cracks start near the free surface. Surface microstructure, therefore, should have a significant effect on the fatigue strength. This study was carried out using a mainly phosphate solution without heavy metal onto various AZ magnesium alloys. The effect of anodizing on mechanical properties and microstructure was examined by repeated tension fatigue tests, tensile tests, hardness tests and electron microscopy.

Abstract: Characterization of nickel oxide layers on the AZ91 Mg alloys by plasma electrolytic oxidation (PEO) was investigated. For this purpose, PEO coating was carried out on the present sample in the alkaline electrolyte with nickel ion at three different voltage responses, i.e., 150, 250, 350 V. The amount of nickel oxide increased by increasing process time, and changed the color of sample from light grey to moderate olive brown. It was observed from potentio-dynamic polarization analysis that polarization resistance of oxide layer was increased with increasing nickel ions. Also microhardness of the sample processed at 350 V was higher than those of the samples.