Materials Science Forum Vols. 638-642

Abstract: The paper presents the results of DSC and microstructural investigations of Elektron 21 magnesium alloy in as cast condition and after solution hardening. Elektron 21 is a magnesium based casting alloy containing neodymium and gadolinium for used to at 200°C in aerospace application. The solution heat treatment was performed at 520°C/8h/water. Ageing treatment was performed at different temperatures 200, 250, 300 and 350°C, then quenched in air. The microstructure of Elektron 21 in as cast condition consists of primary solid solution α -Mg grains with eutectic α-Mg + Mg3(Nd,Gd) phase and regular precipitates of MgGd3 phase. After DSC investigations three exothermal signals has been observed. First exothermal signal at ~170÷245°C assigned to an undifferentiated formation of the metastable phases β” and β’ and the second one at ~280°C corresponded to the formation of a stable β (Mg3Nd) phase. The last signal at ~300°C was connected to the formation of Mg41Nd5 phase. Regular precipitates of MgGd3 phase have been also observed. TEM investigation confirmed that the Elektron 21 alloy precipitate from the solid solution according to the sequence of the following phases: α–Mgβ”β’β(Mg3Nd)Mg41Nd5

Abstract: We have developed the refinement process of the microstructure of metallic materials by imposition of electromagnetic vibration force during solidification. This process is effective for both wrought magnesium alloys and cast ones. By simultaneous imposition of a static magnetic field of 10 Tesla under an alternative electric current of 60A, the average grain sizes of the AZ31B wrought alloy and the AZ91D cast alloy were obtained about 50 micron in both alloys. The grain size was affected by electric current frequency and had the minimum value at the special electric current frequncy of 500 to 2000 Hz and 900 Hz for wrought alloy and cast alloy, respectively. From experimental results, we suggested the mechanism of refinement of microstructure during solidification by imposition of electromagnetic vibration force. The cavitation phenomenon in liquid phase during electromagnetic vibration was effective to break down th esolid phase. And also the difference of electric conductivity between the solid phase and the liquid one brought vigorous vibration of the solid phase. Then the solid phase was suppressed its growth.

Abstract: The development of blocks and subblocks in the lath martensite was observed with SEM/EBSD and TEM/Kikuchi diffraction patterns analyses. Morphology of lath martensite formed below Ms temperature was observed using step quenching method in 18 mass% Ni maraging steel. The development of lath martensite structure can be described as follow; lath groups with single K-S variant start to form at the austenite grain boundary. Next, other sub-blocks appear adjacent to the lath groups formed first and these lath groups form macroscopic blocks observed in optical microscopy. The morphologies of the lath martensite in Fe -23 mass% Ni alloy cooled at different cooling rates after austenitization were also observed. The packet size and block thickness decreases with increasing cooling rate, although the sub-block thickness do not change. A packet locally contains small packets with different relationship of close packed planes, and the amount of small packets increases with increasing the cooling rate.

Abstract: Corrosion protection by anodization and conversion treatment in phosphate solution was studied by microstructural and electrochemical analysis. Both the anodized and the conversion-treated layers showed sacrificial protection in a solution of sodium chloride. The corrosion current or the dissolving rate of the anodized layer was smaller than that of the conversion-treated surface. The modified layers had another mode of protection to form protective films on magnesium substrate where the original modified layers were mechanically lost. Since the state of magnesium in the anodized layer is close to magnesium oxide, phosphorus in the layer is considered to have an important role in these properties concerning the above corrosion protection.

Abstract: Lightweight Mg alloys are being widely adopted in electronic devices and automotive parts. However, the mechanical properties of Mg alloys are inferior to those of Al alloys, and their range of applications is limited. Recently, Mg-Zn-Y alloys have been found to exhibit superior mechanical properties to other Mg alloys. However, for use as industrial materials, it is necessary to improve their ductility while retaining their strength. We studied the mechanical properties of extruded Mg96Zn2Y2 alloy with long-period stacking order (LPSO) phases, in addition to the thermal stability of its structure during annealing. The elongation of the alloy improved from 5 % to 10 % with slightly loss of strength up to an annealing temperature of 623 K. Even at this temperature, it was possible to maintain good mechanical properties and a thermally stable microstructure.

Abstract: Interests in magnesium alloys increase as eco-material for its lightweight, and many investigations have been carried out on the development of manufacturing processes and alloy development. In 2001, Mg97Zn1Y2 (at%) alloy with a tensile yield strength of 610 MPa and an elongation of more than 5 % has been developed by rapidly solidified powder metallurgy (RS P/M) processing. The developed alloy was characterized by a novel phase with long period stacking ordered (LPSO) structure. Recently, we have investigated new compositions for LPSO RS P/M Mg-Zn-Y-X alloys in order to improve the corrosion resistance of the RS P/M Mg97Zn1Y2 alloy with maintaining the superior mechanical properties. Consequently, we have developed a RS P/M Mg96.7Zn0.85Y2Al0.45 alloy with high strength and high corrosion resistance. The RS P/M Mg96.7Zn0.85Y2Al0.45 alloy contained the LPSO phase and exhibited a tensile strength of 525 MPa, an elongation of 9 % and a fatigue strength of 325 MPa, which were similar to those of the RS P/M Mg97Zn1Y2 alloy. However, the corrosion resistance of the RS P/M Mg96.7Zn0.85Y2Al0.45 alloy was 1.5 times that of the RS P/M Mg97Zn1Y2 alloy. The specific tensile yield strength, the specific fatigue strength and the corrosion resistance of the RS P/M Mg96.7Zn0.85Y2Al0.45 alloy were about 1.7 times, 1.8 times, and twice those of extra-super-duralumin (7075-T6 or 7075-T73), respectively.

Abstract: The high temperature response in torsion and creep of two extruded Mg-Zn alloys was investigated in the present study. The alloy 0 (Mg-2Zn-1Mn) was found to exhibit a lower strength than the alloy 2 (Mg-0.55Zn-0.79Mn-0.75Al-0.17Ca), even if the activation energy for creep was similar for both materials (170-180 kJ/mol). The difference in flow stress was here preliminarily attributed to the precipitation of fine Al2Ca particles.

Abstract: In this paper there are presented results of the optimization of heat treatment conditions, which are temperature and heating time during solution heat treatment or ageing as well the cooling rate after solution treatment for MCMgAl12Zn1, MCMgAl9Zn1, MCMgAl6Zn1, MCMgAl3Zn1 cast magnesium alloys. A casting cycle of alloys has been carried out in an induction crucible furnace using a protective salt bath Flux 12 equipped with two ceramic filters at the melting temperature of 750±10°C, suitable for the manufactured material. The heat treatment involve the solution heat treatment and cooling in different cooling mediums as well water, air and furnace. The improvement of the manufacturing technique and chemical composition as well as of heat treatment and cooling methods leads to the development of a material designing process for the optimal physical and mechanical properties of a new developed alloy.

Abstract: The understanding of the weld formation process in FSW is difficult due to: (1) the complexities resulting from coupled physical phenomenon’s (thermo mechanical and metallic process); (2) the difficulties characterizing exactly the interactions between the tool and the materials (friction, 3D material flow, …); (3) the collecting of complete and relevant information during the generation of the weld. Technological studies prove that FSW process is easy to realise. In addition to that, the welds may have good mechanical behaviours, in comparison with those resulting from traditional fusion welding as GMAW. The challenge is to relate the main and relevant parameters of the process (geometry of the tool, topologies of the assembly, others parameters such as axial force, torque, tool positioning, the nature of materials to be assembled,..) to the mechanical properties expected for that weld The study proposes an analogical simulation in a modeling clay (plasticine) workpieces environment. The goal is to obtain a better understanding of the joining creation in the case of butt join of homogeneous assembly of two flat plates made of aluminium alloy AA2017T351. By varying different parameters such as geometrical conditions of assembly, geometry of the tool, other kinematics parameters, we can measure the consequences on the dimensional morphology of the weld. The latter is then characterized by geometrical parameters such as penetration, area of the join, geometrical offset between tool axis and weld interface, and chromatic parameters… . A comparison between the assembly of two aluminium alloys sheets and the equivalent in plasticine workpieces will be discussed, so as to outline differences and analogies between them.

Abstract: 6 mm thick SiCp/2009Al composite and 2024Al-T351 alloy plates were successfully joined by friction stir welding (FSW) with and without the tool pin offsetting to the 2024Al side (denoted as NOS and OS samples, respectively), producing defect-free joints. The SiC particles from the composite were distributed along a ring structure in the nugget and the volume fraction of the SiC particles decreased as the tool pin offset to the 2024Al side. The Al-clad layer on the 2024Al plate was aggregated on the retreating side of the nugget after FSW. For the OS sample, the Al formed a layer along the nugget boundary. The strength of the NOS sample reached up to 85% of the 2024Al alloy with the joint failing in the heat affected zone on the 2024Al side. The strength of the OS samples was 47% of the 2024Al alloy due to the aggregated Al layer on the retreating side of the nugget which decreased the strength of the joint.