T. D. Berman

Microstructure and Texture Through Thixomolding and Thermomechanical Processing and the Role of Mg17Al12 Particles

Thixomolding and thermomechanical processing (TTMP) is a pathway through which it is possible to produce Mg alloy sheet with both a fine grain size and a weak basal texture. Following static recrystallization, the texture of TTMP AZ61 is comparable to that of rare-earth Mg alloy sheets. The β-Mg17Al12 particles in the alloy serve several important roles. Pinning during rolling retards dynamic recrystallization, thereby preventing the development of the typical Mg sheet deformation texture. Recrystallized grains form near clusters of β-particles and in the grain mantle regions. Larger β-particles pin boundaries during subsolvus thermal exposures and therefore provide grain size stability during annealing. The size and spatial arrangement of the β-particles are relatively stable during processing, so modifications to optimize the texture reduction or grain size stability would require changes in composition or modifications in the Thixomolding parameters.

The Influence of Al Content and Thickness on the Microstructure and Tensile Properties in High-Pressure Die Cast Magnesium Alloys

The influence of Al content and section thickness on the microstructural features and tensile properties of high-pressure die cast AM series magnesium alloys is quantified in order to better understand the relationship between microstructure and tensile properties. It is found that with increasing aluminum content, the yield strength increases and the ductility decreases. Increasing the plate thickness results in a decrease in both the yield strength and ductility. The grain size, β-Mg17Al12 phase volume fraction, and solute content are all quantified through the thickness of the plates. It is found that the plates have a skin with increased hardness, due to a fine grain structure. The primary factors affecting strengthening in these alloys, including microstructural variations through the thickness, are accounted for using a linear superposition model. We conclude that yield strength is dominated by grain boundary strengthening and solid solution strengthening effects. The through-thickness grain size and solute concentration were quantified and these variations were found to play an important role in controlling the yield strength of these alloys.

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.

Microsegregation in High Pressure Die Cast Mg Alloys

Expanding Integrated Computational Materials Engineering (ICME) capabilities for High Pressure Die Cast (HPDC) and Super Vacuum Die Cast (SVDC) magnesium alloys will result in significant reductions in the time and cost required to develop components and optimize the casting and heat treatment processes. In this work, a description of segregation in SVDC Mg–Al binary and Mg–Al–Mn ternary alloys is obtained by combining quantitative Electron Probe MicroAnalysis (EPMA) mapping in conjunction with an EPMA forward simulation model. This approach was used to demonstrate that solute trapping near the casting surface can be described though use of a solidification front velocity dependent partition coefficient within the Scheil solidification model framework.

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.

Microsegregation in High Pressure Die Cast AM70

High pressure die casting (HPDC) is the predominant manufacturing method for magnesium alloy automotive components. Cooling rates during HPDC may reach 300°C/s; however, the phase transformation kinetics in this regime are poorly understood. In this study, a description of microsegregation in HPDC AM70 is obtained by electron probe micro-analysis (EPMA) mapping. It has been found that the solute profiles are in good agreement with a Scheil solidification model at mid-thickness, but diverge from Scheil solidification at the edges of the plate. The skin is Al-rich compared to the bulk of the plate; this macrosegregation persists through solution treatment. As a consequence, less Al is available in the bulk for strengthening.

Prediction of Magnesium Alloy Formability: The Role of Texture

Control of crystallographic texture is critical for the development of wrought magnesium sheet product with sufficient levels of ductility and formability. However, it is not feasible to systematically study mechanical behavior as a function of texture experimentally because texture cannot be varied independent of other parameters important for formability. A protocol for generating synthetic textures in conjunction with a viscoplastic self-consistent (VPSC) polycrystalline plasticity model to predict deformation behavior and formability has been developed. The ability to isolate and systematically vary a single texture characteristic is demonstrated for the strength of the basal peak. The changes in slip system activity, texture stability, and predicted formability are presented.

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.