Magnesium and its alloys Subjects of interest ppsx

Lecture Magnesium and its alloys Subjects of interest • Introduction/Objectives • Production of magnesium and magnesium alloys • Extraction of magnesium • Melting and casting of magnesium alloys • Classification of magnesium alloys • Commercial magnesium alloys • Engineering designing with magnesium alloys • Joining of magnesium alloys Suranaree University of Technology Tapany Udomphol May-Aug 2007 Objectives • This chapter provides fundamental knowledge of different methods of productions / heat treatments of magnesium alloys and the use of various types of cast and wrought magnesium alloys • The influences of alloy composition, microstructure and heat treatment on chemical and mechanical properties of magnesium alloys will be discussed in relation to its applications Suranaree University of Technology Tapany Udomphol May-Aug 2007 Introduction • Found 2.8% in sea water and other forms, i.e., dolomite (CaMg(CO3)2), magnesite (MgCO3) and Carnallite (KMgCl3.6H2O) • Found in USA, England, Australia, Germany, Russia, Italy • In Thailand: Dolomite Kanchanaburi, Chonburi Magnesite Chanthaburi, • Magnesium with 99.8% purity are readily available but rarely used in this stage for engineering applications Dolomite Suranaree University of Technology Magnesite Tapany Udomphol Carnallite May-Aug 2007 Introduction – Applications Cam and mobile phone bodies Magnesium side panels Aerospace applications Alloyed wheel Gearbox housing in the VW-Passat Main application is for cast parts in automotive industry Suranaree University of Technology Tapany Udomphol May-Aug 2007 Physical properties of magnesium Crystal structure Atomic diameter Density (g.cm-3) Melting point (oC) HCP 0.320 1.74 650 a = 0.3202, c = 0.5199, c/a = 1.624 easily alloyed with Al, Zn, Ag, Zr •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 • Low strength and toughness and corrosion resistance • Easily flammable with oxygen Suranaree University of Technology Limit applications of magnesium alloy Tapany Udomphol May-Aug 2007 Production of magnesium alloys • Extraction of magnesium - Calcination - Pidgeon process - Dow process • Fabrication of magnesium alloys - Casting – sand casting, die casting, thixo-casting - Forming processes : rolling, forging , extrusion Suranaree University of Technology Tapany Udomphol May-Aug 2007 Extraction of magnesium • Calcination Heating MgCO3 to produce MgO and mix with petroleum coke and then heat to separate O from Mg • Pidgeon process (Thermal reduction method) Powdered ferrosilicon and magnesium oxide are charged in a retort and heated under vacuum at T~1200oC, giving Mg vapour, which is then condensed into crystals • Dow process (Electrolysis process) Precipitate dolomite and seawater and treated with HCl to give MgCl2 and put in electrolytic cell to give Mg metal at cathode Suranaree University of Technology Tapany Udomphol May-Aug 2007 Calcination MgCO3 Calcination MgO Mixed with petroleum coke and press Briquet Heated at T~2500oC in H2 atmosphere • MgCO3 is calcined to produce MgO • MgO is then mixed with petroleum coke and pressed into solid block, called briquet • Briquet is heated to ~2500oC to give Mg gas and cooled down to ~120oC to give Mg solid Mg (gas) Fast cooled to T~120oC MgO + coke → Mg + CO Mg (solid) Note: Boiling point of Mg ~ 1090oC Suranaree University of Technology Tapany Udomphol May-Aug 2007 Pidgeon process Dolomite Calcination at 1000-1100oC MgO+CaO Briquet Mixed with ferrosilicon and pressed Heated at T~1200oC Mg (gas) Condensed Mg (solid) Suranaree University of Technology • Dolomite is calcinated to produce MgO and CaO • Powdered ferrosilicon and magnesium oxide are charged in an air-tight steel retort and heated under vacuum at T~1200oC to give Mg vapour, 2MgO + 2CaO + Si ( Fe) → Ca SiO4 + Mg + Fe • Mg vapour is then condensed into crystals Tapany Udomphol May-Aug 2006 Electrolysis of magnesium • Dolomite and seawater is precipitated as insoluble magnesium hydroxide Mg(OH)2 which is subsequently treated with HCl to give MgCl2 • MgCl2 is fed into electrolysis cell to produce Mg metal at cathode and Cl2 at anode cwx.prenhall.com/ Electrochemical process for the extraction of magnesium Suranaree University of Technology Tapany Udomphol Electrolysis of magnesium May-Aug 2007 High-temperature magnesium casting alloys • Primarily used for aerospace applications due to light weight (major consideration) • Application range 200-250oC with tensile strength ~240 MPa • Mg-Ag-RE alloys QE22 has been used for aerospace applications, i.e., landing wheels, gear box housings • Mg-Y-RE alloys WE43 has been developed for improved elevated temperature tensile properties • Mg-Ag-Th-RE-Zr alloys Thorium is best known to improve high temperature properties, due to age hardening and refined grain but slightly radioactive not commercially available Suranaree University of Technology Tapany Udomphol May-Aug 2007 Mg-Ag-RE alloys • Ag was found to have a positive effect on precipitation behaviour of Mg-RE alloys development of QE series to improve elevated temperature strength and creep resistance • Mg9R compound is produced at GBs of Mg solids solution embedded with a fine precipitate of Mg12Nd2Ag precipitates QE22 (2.5% Ag and 2% RE such as Nd) and other heavy RE gives • Outstanding age-hardening response • Good tensile properties up to 200oC Ag addition Finer and higher Vf of precipitates Precipitation sequence Suranaree University of Technology Tapany Udomphol May-Aug 2007 Mg-Y-RE alloys Advantages: • Mg-Y alloys are capable of age-hardening with solid solubility of Y up to 12.5 wt% • Good strength and creep resistance upto 300oC Drawbacks: • Y is expensive • Difficult to alloy due to high Tm ~ 1500oC • High affinity for oxygen WE43 (4% Y, 2.25% Nd, 1% heavy rare earths, 0.3% Zr) • Improved high temperature properties www.magnesium-elektron.com/ • Maintained RT tensile strength of 250 MPa after long-term exposure at 200oC • For advanced aerospace applications Mg-Y containing alloy Suranaree University of Technology Tapany Udomphol May-Aug 2007 Wrought magnesium alloys • Deformation is limited due to HCP structure, only occur on 1) By slip on the {1000} basal planes in the direction 2) Twining on the {1012} pyramidal planes • At T>250oC slip can occurs on pyramidal and prismatic planes basal Pyramidal Prismatic Suranaree University of Technology • More workable at elevated temperatures (300-500oC) rather than at RT • Magnesium alloys are normally produced in sheets, plates, extruded bars, shapes , tubes, and forgings Tapany Udomphol May-Aug 2007 Wrought magnesium alloy products Dirtworld.com Sport device VINOS housing Forged parts Extruded parts Suranaree University of Technology Sheet, strips http://ccmg.cqu.edu.cn/English/Achievements.asp May-Aug 2007 Forging of magnesium alloys Advantages of forged magnesium components compared to commonly used die cast magnesium parts: 1) Excellent strength, especially with the fibres lying parallel to the main load direction 2) Very good properties for pressure-sealed components because of a forging process in preventing a porous microstructure • Grain size and multiphase microstructure are the main problems in magnesium forging This can be overcome by additional extrusion process to give a sufficient grain size for forging • Complex component geometries are usually produced in several forging steps Suranaree University of Technology Tapany Udomphol Drop-forged gearbox cover for a helicopter May-Aug 2007 Forging of magnesium alloys Prototype of a magnesium forged wheel(ZK30) Produced on the basis of the corresponding aluminium part in series production for the Audi A8 Suranaree University of Technology Tapany Udomphol May-Aug 2007 Forging of magnesium alloys Prototype of a magnesium forged wheel(ZK30) Characteristics of forged magnesium wheels (ZK30/ZK60) compared to other concepts Suranaree University of Technology Tapany Udomphol May-Aug 2007 Joining of magnesium alloys • Arc-Welding (TIG, MIG) • Laser-Beam Welding Friction Welding • Electron-Beam Welding • Friction Welding • Diffusion Bonding EBW Suranaree University of Technology Tapany Udomphol LBW May-Aug 2007 Pore formation in joining of magnesium alloys Serious problem is pore formation • Due to dissolved gas • Growth of pores in the melt (due to prior enclosing during high pressure casting solution • Avoid joining method involving fluid phase, i.e, use friction/ diffusion welding • Suppress pore formation during casting process, i.e., vacuum or thixo- casting Suranaree University of Technology Tapany Udomphol Friction Welding May-Aug 2007 Weldabilty of different magnesium alloys Conventional magnesium alloys • Weldability is influenced by alloying elements and different production methods • Wrought Mg alloys are highly weldable whereas cast alloys might have problems due to porosity Suranaree University of Technology Tapany Udomphol May-Aug 2007 Engineering design with magnesium alloys Advantages of magnesium alloys for engineering designs: Ability to die cast at high productivity rate Good creep resistance to 120oC High damping capacity due to ability to absorb energy elastically High thermal conductivity permitting rapid heat dissipation Good machinability Easily gas-shield arc-welded Disadvantages of magnesium alloys for engineering designs: High tendency to galvanic corrosion when contact with dissimilar metals or electrolyte Difficult to deform by cold working High cost Suranaree University of Technology Note: Damping capacity : the ability of a material to absorb vibration and convert the mechanical energy into heat May-Aug 2007 Corrosion rate of magnesium • High susceptibility to impurities • Oxide film of magnesium are not stable in solution with pH