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Development of Strip Casting Process for Fabrication of Wrought Mg Alloys

AZ31 and experimental ZMA611 alloys were strip cast into 2 mm thick strips. The as-cast AZ31 alloy strip consists of columnar dendrites. On the other hand, as-cast ZMA611 alloy strip shows equiaxed dendritic structure through the thickness of strip. Strip cast AZ31 alloy in H24 condition has equivalent yield and tensile strengths with similar ductility compared to commercial ingot cast AZ31-H24 alloy, indicating that strip casting is a viable process for the fabrication of Mg alloy strips. The ZMA611 alloy has a large volume fraction of fine dispersoid particles in the microstructure, resulting from the beneficial effect of strip casting on microstructural refinement. It has been shown that the ZMA611 alloy has superior tensile properties compared to commercial ingot cast AZ31-H24 alloy, suggesting the possibility of the development of new wrought Mg alloy sheets by strip casting.

Microstructure and Mechanical Properties of Twin-Roll Strip Cast Mg Alloys

Development of wrought Mg alloys, particularly in sheet form, is essential to support the growing interest for lightweight components in the automotive industry. However, development of Mg alloy sheets has been quite slow due to the complexity of sheet production originated from limited deformability of Mg. In this respect, twin-roll strip casting, a one-step processing of flat rolled products, can be an alternative for the production of Mg alloy sheets. In this study, AZ31 and experimental ZM series alloys are twin-roll strip cast into 2 mm thick sheets. The microstructure of the as-cast AZ31 alloy sheet consists of columnar zones near the roll side and equiaxed zones in the mid-thickness region. On the other hand, as-cast ZM series alloy sheets show equiaxed dendritic structure through the thickness of sheet. These alloys were subjected to various thermo-mechanical treatments and their tensile properties were evaluated. Twin-roll strip cast AZ31 alloy in H24 condition has equivalent yield and tensile strengths with similar ductility compared to commercial ingot cast AZ31-H24 alloy, indicating that twin-roll strip casting is a viable process for the fabrication of Mg alloy sheets. The experimental ZM series alloys have a large volume fraction of fine dispersoid particles in the microstructure, resulting from the beneficial effect of twin-roll strip casting on microstructural refinement. It has been shown that the experimental ZM series alloys have superior tensile properties compared to commercial ingot cast AZ31-H24 alloy, suggesting the possibility of the development of new wrought Mg alloy sheets by twin-roll strip casting.

Twin-Roll Cast Al-Clad Magnesium Alloy

An attempt has been made to clad Mg alloy with Al by twin-roll casting. This was done by inserting an Al sheet between the roll and the Mg alloy melt during twin-roll casting. Microstructural investigation across the transverse section of the as-cast Al-clad Mg alloy sheet reveals a very good interfacial bonding between Al and the base Mg alloy. Annealing of the Al-clad Mg alloy sheet results in the formation of layers of various intermetallic phases along the Mg/Al interface. Subsequent rolling of the as-annealed sheet significantly improves the formability of the reaction zone, as evidenced by the cracking of the base Mg alloy before the cracking of the reaction zone.

Room Temperature Formability of Mg Alloys

Room temperature formability of twin-roll cast Mg alloys has been investigated and correlated with their work hardening behavior. Tensile properties of these alloys were measured and their work hardening behaviour was analysed by using constitutive equations. Room temperature formability of the alloys was evaluated by the limiting dome height (LDH) value, obtained by the Erichsen cupping test. It shows that there is a linear relationship between LDH value and the inverse of yield ratio, which is a function of work hardening exponents. An increase in grain size increases work hardening exponent and concurrently increases LDH.

Friction-Stir Welding and Surface-Friction Welding of Twin-Roll Strip Cast ZMA611 Mg Alloy

Twin-roll strip cast Mg-6Zn-1Mn-1Al (ZMA611) alloy sheet was subjected to friction stir welding (FSW) and surface friction welding (SFW) and their microstructure and mechanical properties were investigated. It shows that stir zones (SZs) of both FSWed and SFWed sheets are dynamically recrystallized, but the grain size of SZ of FSWed sheet is finer than that of SZ of SFWed sheet. Moreover, the structure is inhomogeneous through the thickness in SFWed sheet. Such differences in the microstructures between FSWed and SFWed sheets affect their fracture behavior and tensile properties. Introduction Mg alloys are the lightest commercial structural alloys and have the excellent specific strength and stiffness [1-11]. Due to the low workability of Mg, however, there is a lack of competitive Mg wrought products, especially sheet materials, which are much needed for numerous weight-sensitive applications. Recently, it has been shown that twin-roll strip casting can produce high quality Mg sheet products which have equivalent mechanical properties to conventional ingot cast Mg alloys [12-17]. For the successful application of twin-roll strip cast Mg alloys, however, cost-effective and reliable ways of joining are needed. Frictions stir welding (FSW), a solid-state joining process invented in 1991, is a potential candidate since it can produce a high quality joint compared to other conventional weld processes [18,19]. Being the solid state joining processes, FSW is an environmental-friendly joining technology since neither shielding gas nor consumable material is required and no fumes are produced during the processing. One of the main drawbacks of FSW is that the key-hole trace from pin after stirring can give the harmful effect on the properties. Also, FSW cannot be applied to the thin sheets having less than 1.2 mm thickness. Surface friction welding (SFW) is a newly developed solid state welding technology for joining of thin metal sheets [20]. In case of SFW, thin plates are joined by frictional heat and plastic flow generated by friction between rotating tool and surface of plate. SFW is very similar to FSW, but there is no profiled pin which is injected in the welded materials. Therefore, SFW is capable of welding the thin sheets and closed curve shape with the advantages of FSW. The present research is aimed at investigating the response of twin-roll strip cast Mg alloy to FSW and SFW. In the present study, twin-roll strip cast ZMA611 alloy was welded by FSW and SFW and its microstructure and tensile properties were investigated. Twin-roll strip cast ZMA alloy is characterized by the presence of fine, thermally stable Al8Mn5 dispersoids in microstructure and has superior tensile properties compared to commercial Mg alloys [12,13]. Advanced Materials Research Online: 2006-02-15 ISSN: 1662-8985, Vols. 15-17, pp 333-338 doi:10.4028/www.scientific.net/AMR.15-17.333