Mg alloys Flashcards
where can we find magnesium
Mg is found in sea water and in most of the natural brine solutions. It is found very often in combined form in minerals and rocks as well
how can we produce Metallic Mg
through an electrochemical route:
✓ we obtain MgCl2 by dehydration of acqueous solutions or by reaction of the Mg oxide derived from minerals with Chlorine.
✓ Extraction of metallic Mg in electrolytic cells. Generating Cl as by-product
General features of Magnesium
- atomic diameter of Mg allows to create a large amount of solid solutions with most of the elements (However the number of alloys commercially available is quite low)
- Mg alloys feature poor formability and ductility at room temperature owing to their HCP structure that only allows slip along basal planes
- At temperatures exceeding 250°C, other slip systems are activated for dislocation on prismatic planes so that the overall formability significantly increases
is magnesium suitable for casting? why?
Mg and related alloys are particularly suitable as casting alloys:
✓ Low melting temperature
✓ High fluidity at the liquid stage
✓ Low specific heat that allows rapid cooling cycles
✓ Low density that makes more effective the pressure action to accelerate the melt toward the mould in diecasting
Temperature of plastic forming operations with Mg
Plastic forming operations (extrusion, forging, rolling) are carried out only in the range 300-500°C, cold forming cannot be performed successfully with Mg
Mg casting alloys (typical Mg-Al system)
✓ Segregation induces the formation of non-equilibrium constituents, often the Mg-Al12Mg17 eutectic in most popular Mg-Al alloys
✓ Solution annealing and aging are potentially feasible but the response to aging makes it often not fully
convenient
in Mg casting alloys if we don’t have Al what we usually have? what are the effects of this element?
In Al-free alloys, Zr is almost always present to
✓ refine the as cast grain structure
✓ Zr remove hydrogen from liquid melt by forming ZrH2
✓ To achieve high strength Additional use of Zn, RE, Th, Ag, Y
✓ Zr is incompatible with Al and Mn. If Al and Zr are both present, they form ineffective intermetallics
✓ The liquid Mg and solid Zr (Tm = 1855°C) can react on cooling by a peritectic reaction:
wrought Mg alloys
✓ The thermo-mechanical treatments induced by plastic deformation (recrystallization, refining of microstructure, homogeneization) improve the overall properties
✓ plastic deformation can generate sharp anisotropy in tension vs. compression behaviour is created
reasons for anisotropy in wrought Mg alloys
✓ Stress-strain anisotropy in a wrought Mg alloy, is favored by the development of a texture whereby the basal planes of the cells are preferentially oriented parallel to loading direction and twins can develop more favourably in compression
thermal treatments on Mg alloys
✓ In Al-containing alloys response to solution annealing and aging is very poor (Possible benefits could come from solution annealing aimed at removing most of the eutectic segregation)
✓ In Zr-containing alloys (e.g. Mg-Zr-RE or Mg-Zr-Zn) the potential strengthening on aging is much higher
Corrosion behaviour of Mg
✓ Mg critical when exposed to acid solutions ( surface oxide layer is unstable under** pH lower** than 10,5, so)
✓ An exceptions is when exposed to HF (hydrofluoric acid), which generates a passivating film
(contrarily to Al which is much more sensitive to HF)
✓ Mg occupies the lowest position in the ranking of the corrosion potentials for the most common structural metals
✓ Its potential is decreased owing to the generation of a surface oxide/hydroxide Mg(OH)2 allowing a fairly good protection in rural ad
industrial environments
✓ On the microscopic scale, there is a strong electrochemical effect produced by impurities generating more noble constituents (rich in Fe, Cu, Ni)
✓ They behave like cathodes in presence of a corrosive media, setting microgalvanic cells
✓ In hyper-pure Mg alloys a very strict control of critical elements is carried out (Fe<170 ppm, Ni<5 ppm, Cu<1300 ppm)
✓ For high quality castings, production scraps are not directly remelted to avoid any contamination. Only fresh master ingots are used
how do we protect molten Mg from oxidation
✓ Contrary to Al, the presence of a Mg oxide layer on the surface of the melt does not protect the underlying metal form oxidation
✓ protection of the molten bath is carried out by the use of special shielding gas mixtures that allows the modification of the oxide layer making it more protective
Mg machining practice
✓ Mg is one of the best machinable metals owing to the excellent surface finishing achieved, the generation of interrupted chips and the low cutting forces
✓ caution has to be paid since fine chips tend to burn in presence of air when ignited by accidental sparks
✓ Dry machining is feasible, alternatively lubricant/cooling agents free of water are adopted to avoid release of hydrogen
applications of magnesium and why we use it
- Standard use has been formerly related to aerospace applications (especially during world wars)
- In modern cars several kilos of Mg are used, used because reduction in weight for a car corresponds to fuel saving and because it is possible to achieve significant reduction in
manufacturing - Laptop and cell phones can benefit from light frames, thin profiles and from the shielding action of electromagnetic
radiations
