Sung S. Park

Microstructure and properties of strip cast AZ91 Mg alloy

A study has been conducted on the microstructure and mechanical properties of the strip cast AZ91 Mg alloy. The microstructure of the as-cast strip is characterized by a fine equiaxed dendritic structure. There is a variation of secondary dendrite arm spacing throughout the thickness of the strip, showing the smallest value at the wheel surface and the largest value at the center. The distribution of Mg17Al12 particles is also not uniform in the as-cast strip. The microstructure of the solution treated strip consists of fine Mg grains and Al−Mn particles in the matrix with no Mg17Al12 particles. T6 treatment of the strip results in the precipitation of Mg17Al12 particles, the volume fraction of which decreases from the wheel side to the center of the strip. The strip cast AZ91 Mg alloy has the best combination of tensile properties in the T4 condition. It is believed that the good tensile properties of the T4 treated strip are due to the presence of Al−Mn particles, which induce homogeneous deformation.

Development of Mg Alloy Sheets via Strip Casting

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. These alloys were subjected to various thermomechanical treatments and their tensile properties were evaluated. 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.

Tensile properties of extruded Mg–8Sn–1Zn alloys subjected to different heat treatments

The tensile properties of extruded Mg–8Sn–1Zn alloys subjected to different heat treatments have been investigated. At room temperature, the alloy heat-treated at 500°C showed higher strength and ductility than the alloy heat-treated at 420°C. At 200°C, the former showed much larger tensile elongations than the latter, exhibiting 670% at a strain rate of 1.0 × 10−4 s−1. The better tensile properties in the former are attributed to grain refinement and the larger quantity of fine Mg2Sn particles.

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.

Development of Creep Resistant Mg Alloys

A study has been made on the tensile and creep properties of squeeze and die cast Mg alloys, with emphasis on the effect of second phase particles. Two different microstructures were compared; the microstructure with thermally unstable particles along grain boundaries (AZ91 alloy) and the microstructure with thermally stable particles along grain boundaries as well as within matrix (MX alloy). The experimental MX alloy has equivalent room temperature tensile strength to AZ91 alloy with higher ductility. Moreover, elevated temperature tensile properties and creep resistance of MX alloy are higher than those of AZ91 alloy. Such improved properties of MX alloy over those of AZ91 alloy are due to the optimized microstructure of the former.

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.

Tensile and Creep Properties of Magnesium Alloys Incorporating Dispersoids within Matrix

A study has been made on the tensile and creep properties of squeeze and die cast Mg alloys, with emphasis on the effect of second phase particles. Compositions of the alloys were controlled to have four different microstructures; 1) having thermally unstable particles along grain boundaries, 2) having thermally stable particles along grain boundaries, 3) having thermally unstable particles along grain boundaries and thermally stable particles within matrix, and 4) having thermally stable particles along grain boundaries as well as within matrix. It shows that room temperature strength of the alloys is mainly controlled by the volume fraction of second phase particles, regardless of their distribution. The distribution of second phase particles, on the other hand, has a large effect on the fracture behavior. It shows that the creep resistance of Mg alloys can be significantly improved by the incorporation of thermally stable second phase particles within matrix. Such improved properties of TAS831-X alloy over those of AZ91 alloy are due to the optimized microstructure of the former.

Development of Wrought Mg Alloys Via Strip Casting

Development of wrought Mg alloy, particularly in sheet form, are 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, 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 alloys are strip cast into 2 mm thick strips. The microstructure of the as-cast AZ31 alloy strip consists of columnar zones near the roll side and equiaxed zones in the mid-thickness region. On the other hand, as-cast MX1 alloy strip shows equiaxed dendritic structure through the thickness of strip. The cooling rate estimated from the secondary dendrite arm spacing is around 102 K/s. These alloys were subjected to various thermomechanical treatments and their tensile properties were evaluated. 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 experimental MX alloys have 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 experimental MX0 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.

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