Materials Science Forum Vols. 539-543

Abstract: In this study, two different cross rolling processes, which are effective rolling processes for a reduction of (0002) plane texture, are introduced. In the first cross rolling process, a sheet specimen is rotated around the rolling plane normal after each pass. In the second cross rolling process, the roll axis is tilted against the transverse direction (TD) in the rolling direction (RD) - TD plane. The two cross-rolling processed were carried out on a AZ31 alloy, and the press formability of cross-rolled sheets was compared with that of unidirectionally rolled sheets determined by Erichsen tests at 433 – 493 K. Both the cross-rolled specimens exhibited a high press formability, compared to an unidirectionally rolled alloy. The high press formability of the specimen by the first cross rolling was due to a reduction in (0002) texture intensity. The high press formability of the specimen by the second cross rolling was due to not only a reduction in (0002) texture intensity but also grain refinement.

Abstract: This work presents experimental investigation of 14 different alloys with differential scanning calorimetery (DSC), scanning electron microscopy/energy dispersive spectrometer (SEM/EDS) analysis, quantitative electron probe micro-analysis (EPMA) and X-ray diffraction (XRD) techniques to identify the phases in the Mg-Al-Sr system and to determine their compositions. DSC has permitted real time measurement of the phase changes involved in these systems. The temperature ranges for the phase transformations and enthalpy of melting and enthalpy of formation of the compounds are reported. Comparison between these results and the thermodynamic findings has been discussed. The microstructure of the Mg-Al-Sr-based alloys is primarily dominated by (Mg) and (Al4Sr). The plate-like structure has been identified as Al4Sr. A new ternary intermetallic with chemical composition of 69.9 ± 1.5 at.% magnesium, 19.3 ± 2.0 at.% aluminum and 8.7 ± 0.6 at.% strontium has been identified in three different alloys. This phase was characterized as a large precipitate. Three ternary solid solutions have been observed. The solubility ranges of Al in Mg38Sr9 and Mg17Sr2 are 12.5 and 8.5 at.%, respectively, whereas the solubility of Mg in Al4Sr compound is found to be 23 at.% in the investigated samples. Further, Mg was found to dissolve 11.4 at.% Al at room temperature.

Abstract: Grain refinement taking place in a magnesium alloy AZ31 was studied in a single- and multi-directional compression at a temperature of 573K. The structural changes observed by SEM/EBSD analysis can be characterized by the evolution of many mutually crossing kink bands at low strains, continuous increase in their number and misorientation angle in moderate strain and finally full formation of a fine-grained structure in high strain. The characteristics of new grain evolution process are sensitively affected by initial grain size (D0) and strain path. New grains are developed faster with decrease in D0. Multi-directional compression accelerates the evolution of fine grains and the improvement of plastic workability. The mechanism of new grain formation is discussed in detail.

Abstract: The aspect of lightweight constructions becomes more and more important. This particularly applies to the automotive industry which wants to lower the fuel consumption by a smaller vehicle weight. Under this point of view in recent years steel has often been replaced by aluminum alloys. In comparison with this the application of lightweight magnesium alloys, whose specific density is appropriate within the range of plastics, opens further prospects for weight reduction. The pre-condition for this purpose is the supply of suitable joining processes for magnesium alloys, which are universally applicable and offer the maximum utilization of the materials of the joined parts under operating loads. Mechanical joining techniques provide the opportunity of connecting magnesium components homogeneously as well as in material mix. However, the anisotropic deformation characteristics of the hexagonal crystal structure of magnesium at room temperature contain the application for mechanical joining techniques. Only starting from temperatures of approx. 225°C a sufficient plastic deformation and thus a crack-free shaping of the magnesium material is given. Therefore preheating of magnesium substrates leads to a broad extension of deformability and offers the chance to realize a high quality mechanical joint. This article describes a process-safe realization of the mechanical joining operations clinching, self piercing riveting and clinch riveting of magnesium sheets by means of an inductive heating of the substrates in laboratory scale. In this context, feasibilities and limits of the considered joining techniques are shown.

Abstract: The conventional and novel routes of injection molding magnesium alloys are described, including thixo- and rheomolding, semisolid extrusion molding, near-liquidus molding, composite molding and alloy generation by mixing thixotropic slurries. For each technique the basic concepts along with their experimental verifications are explained. The examples show the universal nature of the injection molding and its capability of implementing a variety of modern processing methods.

Abstract: A feasibility study has been made to produce the Mg alloy sheet of high tensile strength and formability by strip casting and an optimized rolling process. 4mm thickness AZ31 and AZ91 strips were cast successfully. While the microstructure of the cast AZ31 strip consisted of columnar zones, the cast AZ91 strip consisted of fine, equiaxed dendrite and fine precipitations. All samples were rolled to 0.5 mm thickness using an optimized finish-rolling schedule, and their grain size was around 4μm. The ultimate tensile strength (UTS), yield strength (YS) (0.2% offset) and elongation (EL) of the rolled AZ31 sheet were higher than those of commercial AZ31B sheet, and the UTS and YS of the rolled AZ91 sheet were 30% higher than commercial AZ31B sheet. The press formability of the rolled AZ31 and AZ91 sheet was equal or superior to commercial AZ31B sheet.

Abstract: Superior mechanical properties of the recycled specimen by solid-state recycling were introduced at first. AZ31 Mg machined chips were recycled by extrusion at 673 K with the different extrusion ratios. The oxide contaminants were dispersed more uniformly in the recycled specimen with the high extrusion ratio (1600:1). There was a remarkable increase in tensile strength and 0.2% yield stress for the recycled specimen with the high extrusion ratio compared with an extrusion reference subjected to the same deformation history. Next, superior corrosion resistance of the recycled specimen was introduced. The recycled specimen with low extrusion ratio (45:1) possessed superior corrosion resistance compared with the extrusion reference. The enhancement of corrosion resistance for the recycled specimens was attributed to the presence of oxide contaminants which were distributed parallel to the extrusion direction.

Abstract: The effect of the concentration of Ni, Y and La-rich mischmetal on the thermal stability, microstructure and mechanical properties of ribbons tested in the temperature range 25-350°C have been evaluated. The low-alloyed materials were crystalline or partially crystalline while high-alloyed materials were amorphous. The amorphous alloys experienced numerous transformations during heating above 170°C. A transition in the amorphous state was observed in all alloys prior to the crystallization stage. The alloy composition determines the sequence of phase transformations during crystallization. In general, the structure of crystallised amorphous, even at high temperatures, is much finer than that of crystalline materials. A MgxREy intermetallic matrix with other intermetallic phases homogeneously distributed was observed in the crystallised ribbons with high RE contents. However, a magnesium matrix embedding other intermetallic phases was the microstructure observed for low-alloyed materials. The amorphous Mg-10Ni-2.5Y2.5-2.5La(MM) showed the higher tensile strength values up to 200 °C The crystalline Mg-2Ni-1Y-1La(MM) ribbon and the partially crystalline Mg-3Ni-1.5Y-1.5La(MM) alloy also exhibited high mechanical resistance levels which were maintained up to 250 °C. MgNi10Y2.5La(MM)2.5 amorphous and MgNi3Y1.5La(MM)1.5 crystalline broken above 500 and 400 MPa, respectively, at 100°C

Abstract: An In Situ observation was made on rolled sheet Mg-Al-Zn alloy (AZ31) in SEM. A type of mini-tensile specimens was used in three typical material orientations, RD (Rolling direction), 45 (45 degrees to rolling direction) and TD (transverse direction, 90 degrees to rolling direction). It is found that the plastic deformation occurred unevenly and limited in single grains, shown by micro extruding steps at grain boundaries. Further deformation concentrated at twinning bands and grain boundaries, more and finer twinning bands appeared as secondary twins on the original ones. In different orientations the dislocation slipping mechanism within twinning bands seems to play a role in different degrees. In RD specimens the slipping mechanism seems predominant, whereas in TD specimens the twinned steps were obvious. Micro cracks nucleated at internal interfaces: grain boundaries, interfaces of twinning bands, and intersections of twinning bands as well.