Magnesium Alloys

Magnesium (Mg) is one of metal that found in some minerals such as Dolomite (CaMg(CO₃)₂), Magnesite (MgCO₃) and Carnallite (KMgCl₃.6H₂O), other source for extracting Mg  also found in sea water. Extraction process of those minerals usually using calcinations, pidgeon and down process. Some of countries that produce and have enough big deposite of Mg are USA , England, Germany, Rusia, Italy. From the extraction process the purity of Mg is able to achiev untill 99.8%, but rarely used in this purity for engineering application. Crystal structure of Mg is Hexagonal Close Packed (HCP) with the atomic  diameter and density is 0.32 and 1.74 g/cm ^-3 . 

For applications Mg can be divided into cast magnesium alloys and wrought magnesium alloy. Untill now the application for magnesium alloys are dominated by casting product with te percentage more than 90 %. But the current research and development of magnesium alloys are trying to make Mg alloy with  novel processing such as semi-solid processing, rapid solidified and continuous casting. Main commercial magnesium alloys use special designating those can be grouped with the AZ series (Mg-Al-Zn), AM series (Mg-Al-Mn), AE series (Mg-Al-RE), EZ series (Mg-RE- Zn), ZK series (Mg-Zn-Zr), and WE series (Mg-RE-Zr). For designating Magnesium alloys there are no international code only they have been a trend towards adopting the method used by American Society for Testing Materials. In this system, the first letter is the highest amount and second letter indicate the second highest amount of element. Beside letter usually followed by numbers those mean are the amount of two major element in that alloy. The letters  alloying elements according to the following code : A- Aluminium, B- Bismuth, C- Copper, D – Cadmium, E-Rare Earths, F-Iron, G-Magnesium, H-Thorium, K-Zirconium, L-Lithium, M-Manganese,N-Nickle, P-Lead, Q-Silver, R-Chromium, S-Silicon, T-Tin, W-yttrium, Y-antimony, Z-Zinc. Example designation AZ91D means 9 wt% Aluminium, 1 wt% zinc and the alloy is D the modification .

The element additions in wrought magnesium alloys can be divided into three categories:  (1) Elements that can improve both strength and ductility of magnesium. Ranging in increasing strength, they are Al, Zn, Ca, Ag, Ce, Ni, Cu, Th. Ranging in increasing ductility, they are Th, Zn, Ag, Ce, Ca, Al, Ni, Cu. (2) Elements that can only improve ductility, but with a little effect on strength of magnesium. Ranging in increasing ductility, they are Cd, Ti, and Li. (3) Elements that decrease the ductility but increase strength of magnesium. In increasing strength, they are Sn, Pb, Bi, and Sb. For complete information about the elements addition describe on the table below.

Kind of Mg	Element	Effect of Elements	Additional Information
Cast Mg Alloy	Ca	Increasing creep resistance. With 2% Ca can reduce casting defects(hot-cracking, die-sticking and cold shuts.	Addition for Mg-Al based Alloy
	Ca-Sr	Increasing the  corrosion resistance, compressive creep resistance, tensile strength and castability
	RE-Ca	Improve creep strength, creep resistance, corrosion resistance, can use until 190-200 0C condition.
	Zn-Ca-RE	Increasing creep resistance, high temperature tensile properties, eliminate hot tearing problem.
	Sn	Increaisng strength in room temperature but decreasing strength at 250 0C, increasing creep resistance
	Sb	Increasing yield and creep resistance, decrease ductility, making fine grain structure
	Bi, Ca, Ba, Co	Increasing thermal stability because of the discontinuous precipitation and accelerate Mg17(Al,Bi)12 Precipitates
	Zn	Improving creep performance, good castability but high cost due to high RE contents	Mg – RE – Zn
	Gd	Increaing Yield strength, increasing elevated mechanical properties.	Mg – RE – Zr
Wrought Mg Alloy	Zn	Incresing yield strength, decreasing the plasticity, causes the discontinuous precipitation (Solid solution strengthening and precipitation hardening).
	Li	Increasing ductility and impact strength, reducing the density
	Mn	Improving creep resisance and tensile strength.
	Zr	Refinement grain size,  improve castability, increasing hot working pocess
	Sc – Mn – Gd	Improving high temperature properties,  creep resistance and hardness
	Gd – Y – Zr	Increaing tensile strength and specific strength, increase creep resistance,

In the as-cast condition, β phase Mg₁₇Al₁₂ appears in alloys containing more than 2% Aluminium.  A network of β forms around grain boundaries as the aluminium content is increased and ductility decreased rapidly above 8%. In more slowly cooled castings, discontinuous precipitation of the β phase may occurs at grain boundaries with the formation of a cellular or pearlitic structure. Annelaing at temperatures around  420 ⁰C causes the cellular constituent and all or part of β phase along grain boundaries to redissolve leading to solid solution strengthening. Interdendritic  coring is also reduced and both tensile strength and ductility are significantly improved. Discontinuous  precipitation of lamellar of the β phase is considered to be undesirable in Mg-Al alloys subject to creep conditions and attemps have been made to prevent its formation by adding microaloying. 

Casting method for making magnesium alloys can be devided become 3 big groups there are high pressure die casting, squeeze casting and thixocasting. For die casting process the most method widely used are hot chamber and cold chamber. Differences between both of them are hot chamber has a casting case that integrated with casting chamber and always stay in the casting furnace however cold chamber, the casting case is outside of the melt metal. Hot chamber uses for making thin walled parts and cold chamber for large casting and heavy wall thickness and needs higer pressure than hot chamber.

Others casting methods those widely used are  firstly is squeeze casting which is uses punch for controlling the pressure and temperature so that porosity can be reduced and prevent the hot-cracking problem. Secondly is Thixo-casting method which is using semi-liquid process ( around 20 ⁰C below the melting temperature). This process uses thixo-casting (special thixo-forming machine) or moulding (uses injection moulding machine). With thixo-casting method production can be fully automated, excellent mechanical because produce fine grained and cost saving duet o low energy consumption.

Magnesium alloys are weldable and possible to be joined by some joining process such as arc welding (TIG, MIG), laser- beam welding, electron beam welding, friction welding and diffusion bonding.  But, for the joining process because of the dissolved gas and due to prior enclosing during high pressure casting, usually the joining process creates porous. For prevent that porous, better for magnesium alloy joining using method which is avoid involving fluid phase i.e. diffusion/fraction welding. In other hand, reducing pore formation during casting process using vacuum casting also important for prevent the porous when joining process.

Magnesium and its alloys are becoming widely recognized as playing an increasingly important role in automotive, aircraft, and electronic consumer products due to the attractiveness of their mechanical properties such as low density (only 2/3 of aluminum), excellent specific strength and stiffness, high damping capacity, high thermal conductivity, easily gas-shield arc welded, good machinability and high recyle ability. 

For automotive applications type of Mg alloys those usually use are AZ and AM serial alloys (AZ91D, AM50A, AM60B) these series have excellent combination mechanical properties, corrosion resistance and die-castability. In aerospace application Mg alloy have been produced for some assembled part such as passenger cars, gear box housing, cranck cases.

Surface coating application also have  been produced with Mg alloys material, Surface coatings produced for magnesium die-casting by hexavalent chromium baths have been used to provide stand-alone protection and as a pretreatment for painting. Teflon resin coating has been developed for Mg alloys; initially the coating is obtained with an aluminum vapor deposition and finish treatment with a Teflon resin coating. The Teflon resin coating is a low cost, chromium-free corrosion resistant coating for magnesium alloys. The coating not only has corrosion resistant properties, but also good lubricity, high frictional-resistance and non-wetting properties