Magnesium-based Alloys Magnesium is HCP at all temperatures up to its melting point of 649 oC It has relatively high strength – but limited ductility at room temperature It can be easily worked at high temperatures – i.e., at 400 oC Mg is a highly reactive metal It reacts with air and moisture – so must be covered with a flux during melting For covered crucibles the flux is 20% KCl, 50% Mg2Cl, and 15% CaF2. For open pots the flux is 55% KCl, 34% Mg2Cl, 9% BaCl2 and 2% CaF2. - strong reducing agents.
Magnesium-based Alloys Magnesium reacts with the SiO2 in clays to form Mg2Si – but it can be safely melted iniron or graphite crucibles. To obtain a bright, clean casting – the mold is covered with sulphur boric acid or KBF4. To dissolve magnesium alloy precipitates, it is solution treated at 390 – 410 oC If the solution temperature is too high – 1) It will “burn” where low melting grain boundary phases are exuded at the surface. 2) A grey-black powder appears on the surface 3) Internal voids form due to evolution of gaseous phases.
General Properties of Mg-Alloys The corrosion resistance of Mg alloys is improved by using high purity starting materials and modifying practices with respect to caustic fluxes. Mg alloys are still susceptible to corrosion in salt atmospheres – a problem for “mag” wheels in snow belt regions – and for marine applications. Aircraft are not so critical – but low flying over the ocean – or the use of reactive de-icing fluids – can create problems
General Properties of Mg-Alloys Mg is also “notch sensitive” – so care has to be taken in design to remove sharp corners – and abrupt changes in section Mg has excellent machining properties - but poor machining practice can introduce severe notch brittle effects Mg has a modulus of elasticity of 45 GPa – compared to 71 GPa for Al and 200 GPa for steel
General Properties of Mg-Alloys Mg density is 1.8 g/cm – compared to 2.8 g/cm for Al and 7.9 g/cm for steel – on a mass basis, Mg has the greatest stiffness/weight – and steel the least Mg is relatively difficult to weld – as it must be protected from the atmosphere by an inert gas – using a tungsten arc or consumable Mg It can be welded - like Al – using a gas torch with suitable flux – for temporary repairs in the field.
Mg-Al Alloy System Al is soluble up in Mg up to ~12.6 wt% Alloys containing up to 3 wt% Al are solution strengthened Alloys with 6-9 wt% Al can be precipitation hardened
Mg-Al Alloy System At 437 oC aMg forms a eutectic with an intermediate phase d – which has a mean composition of 32 wt%Al – or 33 at%Al – Its chemical formula is actually Mg17Al12 As d is brittle - the eutectic is also brittle – as d is the major constituent – the eutectic contains 71.4% d and 28.6% aMg
Mg-Zn Alloy System Zn is soluble in Mg up to 8.4 wt% At 341 oC aMg forms a eutectic with an intermediate phase MgZn – which has a mean composition of 54 wt%Zn As MgZn is brittle – the eutectic is also brittle – as MgZn is the major constituent
Mg-Zn Alloy System The eutectic contains 71%d and 29% aMg (d)
Mg-Mn Alloy System Mn is soluble in Mg up to 3.4 wt% The aMg phase forms by a peritectic reactionat 652 oC. As there are no intermediate phases for precipitation hardening – Mg-Mn alloys are strengthened by solid solution hardening alone.
ASTM Designation for Mg Alloys • Two capital letters indicate the two principal alloying elements. A Aluminium M Manganese B Bismuth N Nickel C Copper P Lead D Cadium Q Silver E Rare Earth R Chromium F Iron S Silicon H Thorium T Tin K Zirconium Z Zinc L Beryllium
ASTM Designation for Mg Alloys • Two digits indicate the rounded off percentages of the alloying elements, e.g., AZ63 = Mg + 6%Al + 3%Zn • A following capital letter – indicates the chronological order of an alloy – with the same major constituents – but with different minor elements. • A letter and number – indicate condition and properties F As fabricated O Annealed H10, H11 Slightly strain hardened H23, F24, H26 Strain harden and partially annealed T4 Solution treated T5 Artificially aged T6 Heat treated and artificially aged Similar to Al alloys
Compositions of Mg-Alloys Mg – Mn (1.2 – 1.5%) – solution hardening Mg – Al (3-6%) + Zn (0.4 – 1.5%) – solution hardening Mg – Al (6 – 10%) + Zn (2 -3%) – precipitation hardening Mg – Zn (3.5 – 6.5%) + Zr (0.55 – 1.0%) – precipitation hardening Mg – Rare Earths* (0.75 – 1.75%) + Zn (3.5 – 5.0%) + Zr (0.4 – 1.0%) – precipitation hardening Mg – Ce (6%) – precipitation hardening These alloys are solution treated at 390 – 410 oC and then air cooled. Due to the low melting temperature – this allows ageing at room temperature, i.e., natural ageing – after solution treatment – they do not have to be tempered. * - Mo, Nb, Ta, W
* ** • - as fabricated • ** - artificially aged We will discuss these alloys in turn
Wrought Mg-Alloys All solid solution Mg alloys can be hot forged at 300 – 400 oC in hydraulic presses – rather like hammers. Extrusions can also be made from all alloys – to obtain a fine grain size extrusions are made from very fine pellets M 1A, AZ31B and AZ61A – can be rolled into sheet at temperatures ~200 oC These alloys are not heat treatable.
Wrought Mg-Alloys AZ80A and ZK60A are effectively solution treated after forging – because of the hot working temperature is close to 400 oC – so precipitation hardening during subsequent aging at room temperature occurs. AZ80A and ZK60A are used for “high” temperature ~150 oC – applications ZK60A – T5 –contains no Al – so is more expensive – but has greater strength and ductility than AZ80A.
Microstructures of Mg-Alloys - 1 AZ31 Alloy – Annealed after hot working AZ31 Alloy – cold rolled into sheet – work hardened
Microstructures of Mg-Alloys - 2 M 1A Alloy – Annealed Particles in grain boundaries are impurities
Microstructures of Mg-Alloys - 2 ZK60 Alloy – Extruded from pellets To obtain fine grain size (0.001 mm)
Sand Cast Mg-Alloys Mg reacts with SiO2 – causing the skin of the casting to be blackened (oxidized) to an appreciable depth below the surface. To obtain a bright surface – “inhibitors” – such as sulphur, boric acid or KBF4 – are mixed with the molding sand. The reactive nature of Mg also means that sand cast alloys are subject to microporosity – caused by evolution of hydrogen* – with a consequent deterioration of its mechanical properties Insoluble gases – such as He and Cl – are bubbled through the melt before casting to remove reactive gases such as H. *- similar to Al alloys
Sand Cast Mg-Alloys It is also evident from the phase diagrams that sand cast alloys will contain brittle networksof eutectic constituents To improve the ductility of these castings they can be solution treated to dissolve the eutectic constituents – and this treatment also increases the tensile strength Aging a solution-treated alloy strongly increases the yield point – and slightly lowers the ductility – but has relatively little effect on the ultimate strength Increasing the amount of Al increases the strength – compared AZ63 with AZ92 – but lowers the casting quality and increases the amount of microporosity The stronger Mg-Zn-Zr alloys are also more difficult to cast.
Microstructures of Sand Cast Mg-Alloys - 1 Grain boundary constituent is Mg17Al12 Grain boundary constituent is local Mg17Al12
Microstructures of Sand Cast Mg-Alloys - 2 EM62 Alloy – As Cast The eutectic constituent is Mg9Ce AZ91B Die Casting Alloy – As Cast The Mg17Al12 eutectic is very fine because of chill casting
Die Cast Mg-Alloys Die cast alloys have excellent dimensional tolerances – and can be formed in complicated shapes as the liquid is forced into a steel mold under pressure. Alloy AM60A is used for auto wheels. Alloy AS41A is used for crankcases for air cooled engines like VWs AZ91B is a general purpose alloy – recently used for dash boards in GM trucks Die cast alloys are significantly stronger than sand cast alloys – as they are not susceptible to microporosity.
(intermediate step) Mechanical Properties of Mg-Alloys
Effect of Grain Size on Mechanical Properties Superheating Mg-Al alloys to about 250 oC above the melting point just before casting refines the grain size and improves the strength. Note: This is the only metal that can be grain refined by superheating – usually it has the opposite effect!
Effect of Grain Size on Mechanical Properties • The grain size can also be refined by applying one of the following treatments at 760 oC – i.e., just before casting: • 1) Vigorous stirring • 2) Bubbling acetylene, methane, propane or carbon tetrachloride • 3) Stirring in 0.003% carbon – as graphite or lamp black – or Al4C3.