6th lecture Magnesium alloys4442223.pptx

Magnesium alloys

1. MAGNESIUM Magnesium (Mg) is number 12 in the periodic table of the elements. Magnesium is the lightest of all commonly used structural metals, with a density, 1.738 g/cm³, approximately two thirds that of aluminum and one quarter that of steels. Magnesium has a relatively low melting temperature 650 C o ,& high specific heat. The strength of pure magnesium is too low for most industrial applications, The yield strength of the pure metal, as cast, is about (20 MPa), (Steel is 266.6 MPa) with tensile strength of (90 MPa), elongation of 6 per cent, and Brinell hardness of 30, magnesium alloy can reach up to 115 Brinell hardness and 21.0 - 460 Mpa yield strength. Magnesium must be alloyed with other metals for engineering applications. Magnesium has potential as a scaffold material for bone substitute applications.

2. Magnesium is an abundant element,, and it is available commercially with purity exceeding 99.8% but rarely used in this stage for engineering applications. Found 2.8% in sea water and other forms, dolomite (CaMg(CO 3 ) 2 ), magnesite (MgCO 3 ) and Carnallite (KMgCl 3 .6H 2 O). Physical properties of magnesium •Alloyed with Al, Zn, Mn, rare earth metals to produce alloys with high strength to weight ratios. • Tends to form compounds with negative valence ion (due to strong electropositive) rather than solid solution. • Not readily plastically deformed at RT due to HCP structure. • Cast magnesium alloys dominate 85-90% of all magnesium alloy products, with Mg-Al-Zn system being the most widely used.

3. With the same volume, Mg weight only a quarter of steel and two-thirds the weight of Al. The melting point of Mg is 660 °C.

4. M agnesium alloys Magnesium alloys are utilized in engineering design mainly because of their high strength-weight ratios , excellent machinability, and relatively low cost on a piece basis. Magnesium alloys have received an increasing interest in the past 12 years for potential applications in the automotive, aircraft, aerospace, and electronic industries

5. Magnesium alloys  are mixtures of  magnesium  (the lightest structural metal) with other metals (called an  alloy ),often aluminum zinc, magnesium, silicon copper ,  rare earths  and  zirconium . Magnesium alloys have a  hexagonal lattice   structure, which affects the fundamental properties of these alloys.  Plastic deformation  of the hexagonal lattice is more complicated than in  cubic latticed  metals like aluminum, copper and  steel ; therefore, magnesium alloys are typically used as  cast  alloys , but research of  wrought  alloys has been more extensive , so that the Magnesium alloys can be categorized into two groups: cast alloys and wrought alloys .

6. I- Cast alloys  are basically made by pouring the molten liquid metal into a mold, within which it solidifies into the required shape. Common cast alloys of magnesium consist of different amounts – but not exceeding 10% – of aluminum, manganese and zinc as principal alloying elements. Other alloying elements have been recently used, as well, mostly to enhance creep resistance, such as zirconium and rare-earth metals. Besides, mechanical properties of cast alloys are augmented by heat treatments. II- Wrought alloys , on the other hand, are alloys subjected to mechanical working, such as forging, extrusion, and rolling operations, to reach the desired shape. Aluminum, manganese and zinc are also the main alloying elements. Wrought alloys of magnesium are sorted into heat treatable and non-heat treatable alloys.

7. Classification of magnesium alloys In order to understand the compositions of the alloys, designation systems have been created showing the alloying elements and their relative information. One of the most widely used designation systems is the ASTM Standard Alloy Designation System. It is made of four parts, described in the following. Example:- Magnesium Alloy: AZ91E-T6 First part (AZ): designates the two main alloying elements ( aluminium , zinc) Second part (91): designates the percentage amount of the main alloying elements (9% and 1%, respectively) Third part (E): differentiates alloys having the same amounts of the main alloying elements (fifth standardised alloy with the above percentages) Fourth part (T6): designates the condition of the alloy (temper)

8. So, in the ASTM designation system, magnesium alloys are named and grouped by means of their main alloying elements. T he principal alloying elements and their relative designations. Principal Alloying Element ASTM Designation Manganese M Aluminum-Manganese AM Aluminum-Zinc-Manganese AZ Zirconium K Zinc-Zirconium ZK Zinc-Zirconium-Rare Earth Metal ZE Rare Earth Metal-Zirconium EZ Zinc-Copper-Manganese ZC Aluminium -Silicon-Manganese AS

9. AZ91D: The magnesium alloy contains 9 wt % aluminium , 1 wt % zinc and the alloy is the D modification. QE22A-T6 The magnesium alloy contains 2 wt % silver, 2% rare earths and in the A modification. T6 – solution heat-treated, quenched and artificially aged Some of the Commercial magnesium alloys • Mg-Al casting alloys • Mg-Al-Zn casting alloys • Mg-Zn and Mg-Zn-Cu casting alloys • Mg-Zn-Zr and Mg-RE-Zn-Zr casting alloy

10. Mg-Al casting alloy • Al is alloyed to increase strength, castability and corrosion resistance. • Maximum solid solubility is ~ 12.7% at 473 oC. • Light weight and superior ductility. • Solid solution treatment of these alloys however produce non-coherent, coarse precipitates of equilibrium Mg17Al12 (lying on the basal plane of the matrix) without the formation of the GP zone→ no solid solution strengthening. → Zn addition Mg-Al phase diagram

11. Mg-Al-Zn casting alloys • Light weight, strength and relatively good corrosion resistance and easily cast. • Zn addition increases strength by solid solution strengthening and precipitation hardening. • σTS ~ 214-241 MPa with 1-8% elongation. cast alloy with the β phase (Mg 17 Al 12 ) at grain boundaries . A network of Mg 17 Al 12 or β phase is formed around GBs in the as-cast condition, which reduce the σTS, %E. More slowly cooled alloy appears discontinuous β phase at GBs with a cellular or pearlitic structure. T6 temper → Mg 17 Al 12 is refined and uniformly distributed→ improved properties. Note: AZ91 is the most widely used (die cast) more slowly cooled alloy. due to fine and uniform as-cast structure

12. Mg-Zn alloy • Response to age hardening (MgZn 2 forms from GP zones) • not amendable to grain refining • susceptible for microporosity. Not used for commercial castings. Mg-Zn-Cu alloy • Cu addition notably improves ductility and large response to age hardening . • σy ~130-160 MPa, σTS ~215-260 MPa • Ductility 3-8%. • Cu addition also raises eutectic temp. → give maximum solution of Zn and Cu.

13. Binary alloy, and b) Ternary alloy Mg-6%Zn-1,5%Cu. The Mg-Zn compounds formed around GBs and dendrite arms,,,,, Cu addition Lamella structure. ( In materials science, lamellar structures or microstructures are composed of fine, alternating layers of different materials in the form of lamellae ).

14. Wrought magnesium alloys • Deformation is limited due to HCP structure, only occur :- 1) By slip on the {1000} basal planes in the <1120> direction. 2) Twining on the {1012} pyramidal planes. • At T>250oC slip can occurs on pyramidal and prismatic planes. • More workable at elevated temperatures (300-500oC) rather than at RT. • Normally produced in sheets, plates, extruded bars, shapes , tubes, and forgings

15. Advantages of magnesium alloys for engineering designs: 1. Ability to die cast at high productivity rate. 2. Good creep resistance to 120 o C. 3. High damping capacity due to ability to absorb energy elastically. 4. High thermal conductivity permitting rapid heat dissipation. 5. Good machinability. 6. Easily gas-shield arc-welded. Disadvantages of magnesium alloys for engineering designs: 1. High tendency to galvanic corrosion when contact with dissimilar metals or electrolyte. 2. Difficult to deform by cold working.  Note: Damping capacity : the ability of a material to absorb vibration and convert the mechanical energy into heat.

16. Physical properties of Magnesium alloys Magnesium alloys are materials of interest mostly due to their high strength-to-weight ratios, exceptional machinability and low cost. They have a low specific gravity of 1.74 g/cm 3  and a relatively low Young’s modulus (42 GPa ) compared to other common alloys such as aluminum (69 Gpa ) or steel alloys (between 190 and 215 Gpa ). ( The Young's modulus represents the material's stiffness or ability to resist deformation when subjected to tensile or compressive forces . It is a measure of the material's elasticity, indicating how much it will stretch or compress under stress ) . They suffer, however, from brittleness and poor formability at room temperature. Their formability increases with increasing temperature, but that requires high energy. Furthermore, studies have shown that formability can be enhanced at the expense of strength, by weakening the basal texture of the Mg alloys. .

17. The inverse relationship between the Index Erichsen (IE) – the measure of ductility in a sheet metal – and the yield strength of different Mg alloys at room temperature. This shows that as the yield strength increases, the IE value decreases, thus demonstrating the poor formability of Mg alloys at room temperature.

18. The physical properties of the alloys change based on their chemical compositions. Adding different alloying elements would result in different properties at different conditions. Aluminium   improves strength, hardness and ductility , facilitating the alloy’s casting process. Zinc   increases room-temperature strength, fluidity in casting, and corrosion resistance. Manganese  increases the resistance of AM and AZ alloys to saltwater corrosion by forming intermetallic compounds with iron-like metals, to be removed during melting. Rare earth metals   help increase strength and resistance to high-temperature creep and corrosion, and decrease porosity and weld cracking . Zirconium  is a strong grain refiner when added to alloys containing zinc and rare earth metals.

19. Precipitation and Hardening in Magnesium Alloys Magnesium alloys have received an increasing interest in the last two decades for potential applications in the automotive, aircraft, aerospace, and electronic industries. Many of these alloys are strong because of solid-state precipitates that are produced by an age-hardening process . Although some strength improvements of existing magnesium alloys have been made and some novel alloys with improved strength have been developed, the strength level that has been achieved so far is still substantially lower than that obtained in counterpart aluminum alloys . Improvements in the alloy strength require a better understanding of the structure, morphology, orientation of precipitates, effects of precipitate morphology, and orientation on the strengthening and microstructural factors that are important in controlling the nucleation and growth of these precipitates.

20. Many magnesium casting and wrought alloys achieve their useful mechanical properties via age hardening, which involves. (1) solution treatment at a relatively high temperature within the α -Mg single-phase region, (2) water quenching to obtain a supersaturated solid solution of alloying elements in magnesium, and (3) subsequent aging at a relatively low temperature to achieve a controlled. Decomposition of the supersaturated solid solution into a fine distribution of precipitates in the magnesium matrix. The decomposition of the supersaturated solid solution often involves the formation of a series of metastable or equilibrium precipitate phases that have a different resistance to dislocation shearing. Therefore, the control of the precipitation is important if the maximum precipitation strengthening effect is to be achieved.

21. For precipitation-hardened magnesium alloys, their microstructures often contain a distribution of plate-shaped or lath-rod shaped precipitates of intermediate or equilibrium phases formed parallel or normal to the basal plane of the magnesium matrix phase. the crystal structure, composition, and orientation relationship of these precipitates have been characterized primarily using conventional transmission electron microscopy (TEM).

22. Welding metallurgy of magnesium alloys Magnesium alloy , as a light metal, has specific physical and chemical characteristics that affect its welding property significantly, especially its boiling and melting temperatures, surface tension, corrosion potential and expansion coefficient, which will directly influence its welding process. The liquidus temperature of most Mg alloys is between 500 °C and 680 °C, and some melt at only 443 °C and others at 700–800 °C. The boiling temperature of Mg alloys at 1100–1107°C is lower than that of the other structural materials; accordingly these alloys will be overburnt and the chemical components of the molten pool will be changed during welding .

23. As an engineering material, wider applications of Mg alloy will meet the needs for dissimilar Mg alloy welding and dissimilar welding of Mg alloy to other metals like steel,  Al , Cu and even nonmetals. The main difficulties in the welding process are the great difference in crystallographic features, physical and mechanical properties, dissolving into each other in liquid phase and separation from each other in the cooling process.