Thermo-mechanical Processing of Ca Containing Highly Creep-resistant Mg-Al-Ba and Mg-Zn Alloys for High-strength Light-weight Applications

Description

Magnesium (Mg) alloys are being developed for light-weight applications includingautomobile and aerospace components and are largely used in cast condition. For obtaininghigh strength, these alloys are prepared with high amounts of alloying additions whichinclude Al, Zn, Sn, Mn and rare-earth elements. The high temperature creep strength of suchalloys is not satisfactory and therefore further alloying is being attempted recently. Neweralloys based on Mg-Al-Ba-Ca (ABaX) system and Mg-Zn-Ca (ZX) have shown promisebecause both Ba and Ca form thermally stable intermetallic compounds that strengthen thematerial better than the conventional creep-resistant Mg-Al-RE alloys. Barium incombination with Mg and Al forms a ternary phase Mg21Al3Ba2while Ca forms (Al/Mg)2Ca,both reside at grain boundaries. Ba and Ca are less expensive as alloying elements comparedwith rare-earth elements and are also flame retarding. These alloys in as-cast condition havethe problems of non-uniform precipitate distribution, chemical segregation, micro-porosityand casting defects. Many of these problems can be overcome by thermo-mechanicalprocessing which includes hot working under optimum conditions and heat-treatment.However, Ca containing magnesium alloys are difficult to deform and require controlledprocessing schedules.In recent years, techniques for the optimization of hot workability have been developedwhich not only pin-point the exact combination of the processing parameters but also suggestthe methods for microstructural control. One such technique which became popular overyears is the processing map. Various domains in which certain microstructural mechanismsdominate are revealed by the map along with the limiting conditions for undesirable regimeslike cracking and flow instabilities. ABaX633 and ZX11 alloys have exhibited far superiorcreep resistance compared to some of the best known magnesium alloys, and are chosen forstudy in this project since increase in strength due to Ba and Ca decreases workability. Thehot working characteristics of these alloys will be established by developing processing maps.The mechanisms of microstructural evolution under a broad range of hot working conditionswill be established. The alloys will be forged on a laboratory scale forging tool set-up toproduce a cup-shape component so as to validate the hot working results and safe workingwindows revealed by the respective processing maps. The study will give the technology forproducing wrought ABaX633 and ZX11 alloys with better strength, ductility and creepresistance than the as-cast alloys, for light-weight applications requiring high strength andreliability.