R. Decker

Microstructure Evolution in AZ61L During TTMP and Subsequent Annealing Treatments

Microstructure evolution is studied in Thixomolded® Thermomechanical Processed (TTMP) AZ61L sheet at various stages of processing. Transmission electron microscopy (TEM) is utilized to examine (1) grain refinement and recrystallization and (2) refinement and re-distribution of the β-Mg17Al12 phase in the as-Thixomolded, as-TTMP, and annealed conditions. Electron backscatter diffraction (EBSD) is used to study texture evolution through TTMP and annealing. The influence of microstructure produced by TTMP and annealing on the mechanical properties will be discussed.

Thixomolded® and Thermomechanically Processed Fine-Grained Magnesium Alloys

Thixomolding of Mg alloys produces fine microstructure of about 5-10 micron alpha phase grain size, surrounded by divorced eutectic phases. During the period from 1995 to 2009, this process and microstructure has captured broad applications around the globe - in markets such as electronics (lap-tops, cameras and cell phones), autos, sports and hand tools. Thermomechanical processing has been applied recently to the Thixomolded precursor to further refine the grain size and eutectic phases - providing yield strength above 300 MPa, fatigue strength of 150 MPa along with elongation of 10%. Alloys studied include AM60, AZ61L and thixoblended alloys of higher Zn content. Microstructure is related to processing and properties. Metal/epoxy fiber composites based on this Mg product have demonstrated yield strength of 900 MPa, with E of 90 GPa.

Thermomechanical Processing of Thixomolded Alloys

A wide variety of Mg alloys have been processed by Thixomolding ® followed by thermomechanical processing (TMP)—to increase tensile, creep and fatigue strength, ductility and formability. These alloys encompass variations in Al, Zn, Ca, Mn, Sr, Y, Zr and Rare Earths (RE) in the Mg base. Due to the fine microstructure and low porosity rendered by Thixomolding, TMP has been feasible using high strain warm rolling and warm pressing. Thus, grain size is further reduced and texture can be moderated. Data is presented on the above alloys along with more extensive information on the commercial alloys AM60 and AZ61; but also the newly developed AZ70L-TH and AXJ810-TH alloys. In discussing the above processing, properties are related to microstructures.

Strengthening in Thermomechanically Processed Magnesium Alloys

Commercial Mg alloys, compared to other engineering materials such as steels or aluminum materials have inferior strengths (Y.S. = ~120 MPa), limited ductility and poor formability. Furthermore, due to high costs their use in structural applications for transportation industry is still rather limited. Therefore, there is significant interest in developing microstructure modification routes to produce novel Mg base alloys with an attractive combination of strength and ductility at room temperature as-well as warm temperature formability. In order to promote use of such microstructurally engineered Mg materials, better understanding of the relationship between microstructure, texture etc. with mechanical properties must be developed for a range of different alloys. In this work, microstructure evolution and mechanical response of two thermomechanically processed Mg alloys AZ61L and AZ70-TH were investigated. Initial findings of this work are presented here. The processed materials exhibited a good combination of strength and tensile ductility at room temperature that was further enhanced (Y.S. > 250 MPa, El. % > 10%) by low temperature (180°C) annealing treatment for 1 hr. The ductility and in-plane anisotropy in mechanical property was found to be related to basal texture formation in the sheet plane. In addition to the Hall-Petch strengthening due to near ultrafine grain size, β-particles from as-molded microstructure, complement strengthening by sub-dividing and possibly solutionizing/re-precipitating into nano-sized, well-dispersed, obstacles to dislocation motion and grain growth.

Effect of Microstructure on Deformation and Fracture of Thixomolded and Thermomechanically Processed AZ61

This paper investigates the deformation, damage accumulation and fracture behavior of fine-grained AZ61 sheet produced by combined Thixomolding and Thermomechanical (TTMP) processing. In the TTMP process, warm rolling of a fine-grained, untextured Thixomolded plate with subsequent recrystallizion produces a sheet with an α-Mg grain size of 3 to 5 μm and s-particles with an average diameter of 0.5 μm. The fine grain size and weak texture in the sheet results in nearly isotropic mechanical properties. Damage during tensile straining occurs in the form of cracking in the s-particles in both the as-Thixomolded plate precursor and in the recrystallized sheet. However, the material fails due to strain localization in shear bands and microvoid coalescence.

Microstructure characterization of weakly textured and fine grained AZ61 sheet

Formability in magnesium alloy sheet is strongly limited by a strong basal texture in the as-rolled material, which is difficulty to remove by thermal processing. We introduce a new process to the control of texture by combining Thixomolding and Thermomechanical Processing (TTMP). Plates of AZ61L with a divorced β-Mg17Al12 eutectic are produced by Thixomolding, resulting in a non-textured, fine grained (2.8 µm) precursor. Sheet produced from the plate by single pass warm-rolling exhibits a weaker texture, and more isotropic tensile deformation than generally observed in AZ-series alloy sheet. Recrystallization annealing produces a further reduction in texture and average grain size (2.3 µm) and results in nearly isotropic room temperature deformation, a yield strength of ~ 220 MPa, and an elongation of ~ 23%. Particle stimulated nucleation of new grains by the β-phase during both dynamic and static recrystallization, is critical for achieving the low levels of texture. The influence of β-phase distribution in microstructure development is discussed.

Microstructure modification and deformation behavior of fine grained AZ61L sheet produced by thixomolding® and thermomechanical processing

X-ray diffraction and hardness measurements are used to study recrystallization in fine-grained AZ61L sheet produced by warm-rolling of Thixomolded® material. The as-rolled sheet is partially dynamically-recrystallized, with a strong basal texture and a sub-micron grain size. Significant increases in ductility with moderate reductions in tensile strength were produced by annealing at temperatures greater than 250 °C. A weakening in basal texture was observed in samples annealed at over 250°C. Static recrystallization was determined to be responsible for the reduction in texture and associated increase in elongation.

The Effect of Friction Stir Processing on Microstructure and Tensile Behavior of Thixomolded AZ91 Magnesium Alloy

Friction Stir Processing (FSP) to partial sheet thickness can be utilized to engineer unique microstructures in metallic alloys. These composite microstructures consist of three distinct layers associated with stirred, transition and core micro structural regions. The stirred region is of particular interest where severe plastic deformation imparted by the rotating and translating FSP tool under frictional heat leads to grain refinement down to ~ 1 urn grain size. In this work, partial depth penetration into thixomolded AZ91 Mg plate from the top and bottom surfaces by friction stir processing is explored. Furthermore, low temperature aging treatments are applied to the processed material. The present results with AZ91 Mg show that FSP processed material exhibits higher strength (> 300 MPa), and improvement in ductility (> 7 % tensile elongation). It is found that in addition to Hall-Petch strengthening produced by ~ 1 um grain size in the stirred region, the enhanced strength levels and ductility are strongly influenced by dispersoids of the intermetallic precipitates found in this alloy.

The Effect of Thermomechanical Processing on the Creep Behavior and Fracture Toughness of Thixomolded® AM60 Alloy

Creep and fracture toughness experiments were performed on a commercially available magnesium-aluminum alloy (AM60) after three processing treatments: (1) As-Thixomolded® (as-molded), (2) Thixomolded® then thermomechanically processed (TTMP), and (3) Thixomolded® then TTMP then annealed (annealed). The conventional tensile-creep experiments were performed at applied stresses ranging between 20-75MPa and temperatures between 373-473K (100-200°C). In-situ tensile-creep tests were performed on selected samples. The as-molded material exhibited creep resistance superior to the thermomechanically processed materials. Creep experiments indicated grain boundary cracking, and grain size was expected to be an important microstructural parameter that affected the creep behavior. Fracture toughness experiments were performed at room temperature (RT) on single edge notched tension (SENT) samples. The TTMP and annealed materials exhibited fracture toughness values almost twice that of the as-molded material.