3D printed Mg-NiTi interpenetrating-phase composites with high strength, damping capacity, and energy absorption efficiency

It is of significance, but still remains a key challenge, to simultaneously enhance the strength and damping capacities in metals, as these two properties are often mutually exclusive. Here, we provide a multidesign strategy for defeating such a conflict by developing a Mg-NiTi composite with a bicontinuous interpenetrating-phase architecture through infiltration of magnesium melt into three-dimensionally printed Nitinol scaffold. The composite exhibits a unique combination of mechanical properties with improved strengths at ambient to elevated temperatures, remarkable damage tolerance, good damping capacities at differing amplitudes, and exceptional energy absorption efficiency, which is unprecedented for magnesium materials. The shape and strength after deformation can even be largely recovered by heat treatment. This study offers a new perspective for the structural and biomedical applications of magnesium.

Formation of Al-alloyed Layer on Magnesium with Use of Casting Techniques

Al-enriched layer was formed on a magnesium substrate with use of casting. The magnesium melt was cast into a steel mould with an aluminium insert placed inside. Different conditions of the casting process were applied. The reaction between the molten magnesium and the aluminium piece during casting led to the formation of an Al-enriched surface layer on the magnesium substrate. The thickness of the layer was dependent on the casting conditions. In all fabricated layers the following phases were detected: a solid solution of Mg in Al, Al3Mg2, Mg17Al12 and a solid solution of Mg in Al. When the temperature of the melt and the mould was lower (variant 1 – 670°C and 310°; variant 2 – 680°C and 310°C, respectively) the unreacted thin layer of aluminium was observed in the outer zone. Applying higher temperatures of the melt (685°C) and the mould (325°C) resulted in deep penetration of aluminium into the magnesium substrate. Areas enriched in aluminium were locally observed. The Al-enriched layers composed mainly of Mg-Al intermetallic phases have hardness from 187-256 HV0.1.

Melt Refining Technology of Highly-Contaminated Magnesium Alloy Scraps via a Sequential Refining Process

Steering wheel and headlamp housing scrap are chosen as targets for the development of a refining technology for highly contaminated magnesium melt, because they contain abundant foreign materials, which have high potential to form non-metallic inclusions and to deteriorate the corrosion resistance of recycled alloys. A sequential melt treatment technology has been investigated for refining old scrap. The headlamp housing scrap composed of AZ91D magnesium alloy covered with phosphating and Al-Si deposition layers was successfully refined by repeating gas bubbling without flux addition, whereas prefiltering and fluxing were necessary to refine the steering wheel scrap composed of AM50A magnesium alloy containing 1% polyurethane. However, the corrosion rates of the recovered ingots from the headlamp housing were relatively higher than those from the steering wheel, likely due to the greater concentration of iron with the increased amount of flux.

Twin-Roll Casting after Intensive Melt Shearing and Subsequent Rolling of an AM30 Magnesium Alloy with Addition of CaO and SiC

Intensive melt shearing is a process that can be used for mixing ceramic particles into magnesium melt. It applies shear stress to the melt and can de-agglomerate nanoparticle additions to magnesium melts without the use of electromagnetic fields or ultrasound. A wrought magnesium alloy AM30 was selected for processing with intensive melt shearing and subsequent twin-roll casting. AM30 with additions of CaO and SiC were also processed by this route and the hardness and microstructure were investigated. Sheets were rolled and their tensile strength was determined. The work was done as part of the European Union research project ExoMet. Its target includes the production of high-performance magnesium-based materials by exploring novel grain refinement and nanoparticle addition in conjunction with melt treatment by means of external fields.

Magnesium Melt Protection

Mg especially in the molten state is well known for its high affinity to O2. When O2 content of the atmosphere is larger than 4%, molten Mg will burn! To avoid this, melt protection is necessary. At present mostly SF6 is used during primary production and processing of Mg and its alloys. Unfortunately SF6 is a very potent greenhouse gas that is > 23,000 times more effective than CO2. This also affects life cycle considerations e.g. for the use of Mg alloys in transportation. However, other protective gases like SO2 or fluorinated hydrocarbons like HFC134a, Novec 612, or AMCover (=HFC134a) have been suggested to replace SF6. Additionally fluxes mixed from different salts may be used again as well to protect molten Mg. But fluxes and feasible replacements of SF6 have also disadvantages. Moreover SF6 and other fluorinated hydrocarbons are under discussion especially in Europe. There is an existing EU legislation that will ban SF6 from 2018 and there are similar discussions regarding all other fluorinated hydrocarbons. Due to this, new innovative ways have to be found or old methods have to be renewed to allow Mg industries further safe processing of molten magnesium. This contribution will report the state of the art in protecting molten Mg and alternatives to the use of SF6.