Mathematical Modeling of the Twin Roll Casting Process for Magnesium Alloy AZ31

Although Twin Roll Casting (TRC) process has been used for almost 60 years in the aluminum industry, TRC of magnesium is relatively new. In TRC, molten metal is fed onto water-cooled rolls, where it solidifies and is then rolled. Solidification of the molten metal starts at the point of first metal-roll contact and is completed before the kissing point (point of least roll separation) of the two rolls. The unique thermo-physical properties inherent to magnesium and its alloys, such as lower specific heat and latent heat of fusion and larger freezing ranges (in comparison with aluminum and steel) make it challenging for TRC of this alloy. Therefore, a comprehensive understanding of the process and the interaction between the casting conditions and strip final quality is imperative to guarantee high quality twin roll cast strip production. A powerful tool to achieve such knowledge is to develop a mathematical model of the process. In this thesis, a 2D mathematical model for TRC of AZ31 magnesium alloy has been developed and validated based on the TRC facility located at the Natural Resources Canada Government Materials Laboratory (CanmetMATERIALS) in Hamilton, ON, Canada. The validation was performed by comparing the predicted exit strip temperature and secondary dendrite arm spacing (SDAS) through the strip thickness with those measured and obtained by experiments. The model was developed in two stages, first a thermal-fluid model was developed followed by validation and then a thermal-fluid-stress model was developed. This is the first time a comprehensive thermal-fluid-stress model has been developed to simulate the TRC process for magnesium alloys. The work has led to new knowledge about the TRC process and its effects on magnesium strip quality including the following: 1) Using ALSIM and ANSYS® CFX® commercial packages a 2D mathematical model of thermal-fluid-stress behavior of the magnesium sheet during TRC was successfully developed and validated. 2) An average value of 11 kW/m2°C for the Heat Transfer Coefficient (HTC) was found to best represent the heat transfer between the roll and the strip during TRC casting of AZ31 using the CanmetMATERIALS facility. 3) Modeling results showed that increasing casting speed, casting thicker strips and applying higher HTCs led to less uniform microstructure through thickness in terms of SDAS. 4) Simulations showed the importance of casting parameters such as casting speed and set-back distance on the thermal history and stress development in the sheet during TRC; higher casting speeds led to deeper sumps and higher exit temperatures as well as lower overall rolling loads and lower total strains experienced during TRC. 5) The effect of roll diameter on the thermal history and stress development in the strip was also studied and indicated how larger roll diameters increased the surface normal stress and rolling loads but had little effect on the mushy zone thickness. 6) The correlation between the mechanisms of center-line and inverse segregation formation and thermo-mechanical behavior of the strip was performed. The modeling results suggested that increasing the set-back distance decreases the risk of both defects. Moreover, increasing the roll diameter reduces the propensity to inverse segregation but has a minor effect for center-line segregation formation

Mathematical modeling of thermo-mechanical behavior of strip during twin roll casting of an AZ31 magnesium alloy

The definitive publication is available at Elsevier via https://doi.org/10.1016/j.jma.2013.04.001 © 2013. This publisher's version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/The effect of set-back distance on the thermo-mechanical behavior of the strip during twin roll casting (TRC) of an AZ31 magnesium alloy was modeled using finite element method (FEM). Model validation was done by comparing the predicted and measured exit strip surface temperature as well as the secondary dendrite arm spacing (SDAS) through the thickness of the sheet to those measured during experiments. Model results showed as the set-back distance increases, the strip exit temperature decreases and the solidification front moves toward the entry of the roll gap. The cast strip also experiences more plastic deformation and consequently, the normal stress on the strip surface and effective strain at the strip center-line increase. Moreover, higher separating forces were predicted for longer set-back distances. Model predictions showed that changing the set-back distance by varying the final thickness has a more significant effect on the temperature and stress-strain fields than altering the nozzle opening height.Natural Sciences and Engineering Research Council of CanadaMagnesium Strategic Research Networ

Bimodal grain microstructure development during hot compression of a cast-homogenized Mg-Zn-Zr alloy

The final publication is available at Elsevier via https://doi.org/10.1016/j.msea.2018.03.112. © 2018. This manuscript version is made available under the CCBY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/Hot deformation of a cast-homogenized ZK60 alloy was studied by compression at a temperature of 450 °C and a strain rate of 0.001 s−1 to investigate microstructural evolution. The deformed microstructure was characterized using electron backscatter diffraction (EBSD) and high resolution transmission electron microscopy (HRTEM). EBSD observations of the deformed microstructure showed that hot deformation of this alloy resulted in a bimodal grain microstructure consisting of large pancaked unrecrystallized dendrites surrounded by recrystallized equiaxed fine grains. HRTEM studies revealed the presence of nano-(Zn-Zr)-precipitates in the deformed microstructure. Due to the coherency of precipitates/matrix, the dislocations were pinned by the nano-precipitates inside the unrecrystallized grains and the dislocation motion inside the grains was impeded, hence, a substructure evolved. Consequently, dynamic recrystallization (DRX) was suppressed and deformation was concentrated inside the DRXed region.Natural Sciences and Engineering Research Council || Automotive Partnership Canada Grant APCPJ 459269Multimatic Technical CentreFord Motor CompanyCenterline Windso

Breaking strength-ductility trade-off in laser-powder bed fused Fe–Cr–Ni–Al maraging stainless steel: Controlled precipitation and preserved dislocations

In the current study, cylindrical rods of Fe–Cr–Ni–Al maraging steel (with the brand name CX) are fabricated using the laser-powder bed fusion (L-PBF) technique. The material is analyzed using the differential scanning calorimetry (DSC) technique under continuous heating at different heating rates. The DSC results are employed to identify the heat flow peaks associated with the precipitation and austenite reversion phase transformations. The peaks are then analyzed and processed to determine the onset, peak, and end temperatures associated with each phase transformation. These results are used to calculate the degree of phase transformation under non-isothermal conditions and to evaluate the activation energy of transformation through the Kissinger method. In the next step, the modified Johnson-Mehl-Avrami-Kolmogorov (JMAK) model is employed to model the kinetics of phase transformations during non-isothermal heating. The validated model is then used to predict the kinetics of precipitation and austenite reversion phase transformations under isothermal aging heat treatments. The isothermal modeling results are employed to design a direct aging heat treatment to promote the evolution of nanometric β-NiAl precipitates and meanwhile preserve the pre-existing dislocation networks. Such a heat treatment results in the enhancement of both yield and tensile strengths from 929 MPa and 1032 MPa in the as-built condition to 1659 MPa and 1738 MPa after heat treatment. Meanwhile, ductility slightly changes from 15.3% in the as-built condition to 14.7% after heat treatment. Such an achievement in breaking the strength-ductility trade-off is due to a controlled heat treatment process that preserves the dislocation networks

Twin roll casting (TRC) of magnesium alloys - Opportunities and challenges

Twin Roll Casting (TRC) has been successfully employed for the past sixty years to produce aluminum, steel and, in the past ten years, magnesium sheet. Although the TRC process is relatively simple, its application for commercial-scale magnesium strip production has proven difficult. This is primarily due to inherent characteristics of magnesium alloys, such as their high reactivity to oxygen, low specific heat and latent heat of fusion, and large freezing ranges, which can induce formation of casting defects if various TRC processing parameters, such as metal delivery design, heat transfer in the roll gap, and casting speed, aren\u27t tightly controlled. Research is underway worldwide to concurrently gain a better understanding of TRC processing variables in order to provide optimum casting conditions which will reduce defects, and develop new magnesium alloys with properties tailored to the TRC process. The opportunities and challenges associated with magnesium TRC will be outlined and include: 1) defect formation during TRC of magnesium alloy AZ31, 2) the feasibility of producing clad magnesium strip via TRC and 3) the effect of scale-up (moving from a laboratory unit to commercial production) will have on the TRC process for magnesium. © (2014) Trans Tech Publications, Switzerland

HAZ softening behavior of strain-hardened Al-6.7Mg alloy welded by GMAW and pulsed GMAW processes

Gas metal arc welding (GMAW) process was used to weld plates of strain-hardened Al-6.7Mg alloy. It was observed that HAZ softening issue occurred extensively for the current material using the GMAW process. So, as a solution, pulsed current was employed and the plates were welded by pulsed GMAW (PGMAW) process. The effects of peak current (93, 120, 140, and 160 A) and pulse frequency (0.5, 2.0, and 5.0 Hz) on the strength of the weldments were investigated. For high peak currents (160 A), catastrophic defects were formed in the weld metal. It was observed that for the lowest pulse frequency (0.5 Hz), increasing the peak current increased the weld strength. The peak current did not change the strength for higher frequencies (2.0 and 5.0 Hz). Furthermore, increasing the frequency from 0.5 to 2.0 Hz for peak currents of 93 and 120 A led to strength improvement. For peak current of 140 A, frequency changing was ineffective. The overall enhancement in the strength of welds and reduction of HAZ softening by employing pulsed current offers a promising opportunity for further application of GMAW process with controlled heat input for welding of strain-hardened Al-6.7Mg alloy

Analysis of the hot deformation of ZK60 magnesium alloy

Hot deformation of cast-homogenized and extruded (in both the extrusion and transverse directions) ZK60 magnesium alloy was conducted using the Gleeble® 3500 thermal-mechanical simulation testing system. A new approach to model the high temperature constitutive behavior of the alloy was done using two well-known equations (i.e. hyperbolic sine and Ludwig equations). For this approach, the deformation conditions were divided into regimes of low and high temperature and strain rate (four regimes). Constitutive model development was conducted in each regime and the material parameters (P) were evaluated as strain, strain rate and temperature-dependent variables; P(ε, ε˙, T). Using this approach, the flow curves were predicted with high accuracy relative to the experimental measurements. Moreover, detailed information on the evolution of hot deformation activation energy was obtained using the modified hyperbolic sine model. Using the modified Ludwig equation, details of strain hardening and strain rate sensitivity of the ZK60 material during hot deformation were obtained. Keywords: ZK60 magnesium alloy, Hot deformation, Constitutive modeling, Zener–Hollomon, Hyperbolic sine, Ludwig equatio

The effect of gas tungsten arc welding and pulsed-gas tungsten arc welding processes\u27 parameters on the heat affected zone-softening behavior of strain-hardened Al-6.7Mg alloy

The heat affected zone (HAZ) softening behavior of strain-hardened Al-6.7Mg alloy welded by gas tungsten arc welding (GTAW) process was investigated. Increasing the heat input during welding led to formation of a wider HAZ. Moreover, the size of the precipitates was increased at higher heat inputs. Consequently, by increasing the heat input, lower strength was obtained for the welding joints. At the second stage of the study, pulsed-GTAW (PGTAW) process was employed to improve the strength of the joints. It was observed that the overall strength of the welding joints was improved and the fracture during tensile test was moved from the HAZ to the fusion zone. Moreover, the effect of duration ratio and pulse frequency was studied. For the current study, the duration ratio did not have a significant effect on the strength and microstructure of the weld, but increasing the frequency led to higher strength of the weld and finer microstructure. © 2013 Elsevier Ltd

Development of a mathematical model to study the feasibility of creating a clad AZ31 magnesium sheet via twin roll casting

A previously developed and validated thermalfluid mathematical model of the twin roll casting (TRC) process for magnesium alloy AZ31 was used to quantitatively study the feasibility of producing a clad magnesium strip via the TRC process. The clad material was varied to identify the effect of material composition on the feasibility of producing a clad strip. The clad alloys chosen included pure Zn, pure Al, AA3003, and AA5182 aluminum alloys. In the analysis, the effect of casting speed and clad sheet thickness (100 and 500 μm) on the thermal history in the magnesium strip and clad layer was analyzed. Assessment of the process feasibility was determined based on the exit temperature of the clad strip at the centerline, temperature of the clad sheet prior to the roll bite entry, and fraction solid of both the core (magnesium sheet) and clad along the core/clad interface. The results indicated that using pure Zn as a clad material is not feasible due to premelting of the clad strip prior to introduction into the TRC apparatus. All three aluminum alloys studied proved to be feasible in terms of a cladmaterial, and it was found that the effect of clad thickness and clad material chemical composition on the thermal history (temperature distribution) of the clad strip was negligible. It was also predicted using the thermodynamics package FactSageTM that the intermetallic phase at the core/clad interface will be primarily α-Mg (Mg17Al12). For AA5182 clad material, formation of β-Mg (Al3Mg2) is also possible. © Springer-Verlag London 2014

Evading strength-ductility trade-off in directed energy deposited precipitation hardenable stainless steels: A pathway through precipitation kinetics modeling, design of heat treatment, and evolution of clusters

The kinetics of precipitation and austenite reversion phase transformations in an as-built arc-directed energy deposition (arc-DED) PH13–8Mo stainless steel are studied. The material in the as-built condition is analyzed using the differential scanning calorimetry (DSC) technique to determine the fraction of transformed precipitates and reverted austenite at any time/temperature during non-isothermal transformations. The Johnson-Mehl-Avrami-Kolmogorov (JMAK) kinetic equation is employed to model the transformed phase volume fraction during precipitation and austenite reversion processes. Using the non-isothermal phase transformation kinetics, an isothermal kinetics model is developed. The isothermal kinetics modeling results are then employed to design and develop direct aging heat treatments to enhance the hardness and strength of arc-DED-PH13–8Mo steel. The developed heat treatment results in the concurrent enhancement of strength and ductility in the material. Such an achievement is a result of the evolution of nano-scaled β-NiAl clusters. It appears that the formation of β-NiAl clusters increases the dislocation storage capability in the material, resulting in the evasion of the strength-ductility trade-off. The results of the current study provide great insight into the crucial role of clusters in the strength-ductility trade-off in PH stainless steels