Determination of Thermal Boundary Conditions for the Casting and Quenching Process with the Optimization Tool OptCast

OptCast is an integrated thermal optimization tool coupling commercial casting simulation software with optimization software iSIGHT. Two types of optimizations are realized in OptCast: one uses an inverse modeling approach to determine boundary conditions such as interfacial heat-transfer coefficients (HTCs), and the other optimizes casting process variables by minimizing cycle time while maintaining casting quality. In this article, the global architecture and the development of OptCast for the commercial casting simulation software MagmaSoft is described. The determination of interfacial HTCs with OptCast for the casting process and the water quenching process is discussed. The optimized HTCs for a specific casting process are applied to a V8 engine block for thermal analysis with sufficient accuracy to enable prediction of microstructures and mechanical properties. The optimized HTCs for a water quenching process were applied to a cylinder head for residual stress analysis. This article is based on a presentation made in the symposium entitled “Simulation of Aluminum Shape Casting Processing: From Design to Mechanical Properties,” which occurred March 12–16, 2006 during the TMS Spring Meeting in San Antonio, Texas, under the auspices of the Computational Materials Science and Engineering Committee, the Process Modeling, Analysis and Control Committee, the Solidification Committee, the Mechanical Behavior of Materials Committee, and the Light Metal Division/Aluminum Committee.     

A Methodology to Predict the Effects of Quench Rates on Mechanical Properties of Cast Aluminum Alloys

The mechanical properties of age-hardenable Al-Si-Mg alloys depend on the rate at which the alloys are cooled after the solutionizing heat treatment. Quench factor analysis, developed by Evancho and Staley, was able to quantify the effects of quenching rates on the as-aged properties of an aluminum alloy. This method has been previously used to successfully predict yield strength and hardness of wrought aluminum alloys. However, the quench factor data for aluminum castings is still rare in the literature. In this study, the time-temperature during cooling and hardness were used as the inputs for quench factor modeling. The experimental data were collected using the Jominy end quench method. Multiple linear regression analysis was performed on the experimental data to estimate the kinetic parameters during quenching. Time-temperature-property curves of cast aluminum alloy A356 were generated using the estimated kinetic parameters. Experimental verification was performed on a cast engine head. The predicted hardness agreed well with that experimentally measured. The methodology described in this article requires little experimental effort and can also be used to experimentally estimate the kinetic parameters during quenching for other aluminum alloys. This article is based on a presentation made in the symposium “Simulation of Aluminum Shape Casting Processing: From Design to Mechanical Properties” which occurred March 12–16, 2006, during the TMS Spring meeting in San Antonio, Texas, under the auspices of the Computational Materials Science and Engineering Committee, the Process Modeling, Analysis and Control Committee, the Solidification Committee, the Mechanical Behavior of Materials Committee, and the Light Metal Division/Aluminum Committee.     

Study on Macro- and Micromodeling of the Solidification Process of Aluminum Shape Casting

Numerical methods to improve the computational efficiency and to extend the computational scale of the mold filling and solidification of the aluminum die casting process were studied. For molding filling simulation, the parallel computation method was studied, while for solidification simulation, an implicit finite difference scheme and a transient surface layer concept were studied. In addition, the modified cellular automaton method was used to simulate the microstructure formation and evolution of the aluminum alloy, including the grain structure and the dendritic microstructure. The experimental results show that the models in the article are reasonable for describing the formation and evolution of the microstructure formation. This article is based on a presentation made in the symposium entitled “Simulation of Aluminum Shape Casting Processing: From Design to Mechanical Properties,” which occurred March 12–16, 2006 during the TMS Spring Meeting in San Antonio, Texas, under the auspices of the Computational Materials Science and Engineering Committee, the Process Modeling, Analysis and Control Committee, the Solidification Committee, the Mechanical Behavior of Materials Committee, and the Light Metal Division/Aluminum Committee.     

Microporosity Simulation in Aluminum Castings Using an Integrated Pore Growth and Interdendritic Flow Model

A new computational model for predicting microporosity in aluminum alloys is described. The model was calibrated against literature data for binary Al-7 pct Si alloys, and subsequently applied to a chill plate test casting in A356 alloy and to an engine block in 319 alloy. The new model allows spherical micropores to nucleate and grow by hydrogen diffusion from a material volume surrounding the pores. This differs from a conventional interdendritic flow computational model for calculating porosity that assumes spherical pores have a diameter proportional to the secondary dendrite arm spacing (SDAS). The new integrated pore growth and interdendritic flow model predicts larger pore diameters and a volume fraction of microporosity that is in better agreement with experimental observations than the interdendritic flow model. This article is based on a presentation made in the symposium “Simulation of Aluminum Shape Casting Processing: From Design to Mechanical Properties,” which occurred March 12–16, 2006, during the TMS Spring Meeting in San Antonio, Texas, under the auspices of the Computational Materials Science and Engineering Committee, the Process Modeling, Analysis and Control Committee, the Solidification Committee, the Mechanical Behavior of Materials Committee, and the Light Metal Division/Aluminum Committee.     

Quench Sensitivity of an Al-7 Pct Si-0.6 Pct Mg Alloy: Characterization and Modeling

The quench sensitivity of a cast Al-7 wt pct Si-0.6 wt pct Mg alloy was characterized by tensile tests and scanning electron microscopy. Specimens were cooled from the solution treatment temperature following 58 different cooling paths including interrupted and delayed quenches. Analysis of the microstructure showed that quench precipitates were Mg₂Si (β), which nucleated heterogeneously on Si eutectic particles as well as in the aluminum matrix, presumably on dislocations. The quench sensitivity of the alloy’s yield strength was modeled by multiple C-curves, using an improved methodology for quench factor analysis. The three C-curves used in the model represented loss of solute by (1) diffusion of Si to eutectic particles, (2) precipitation of β on Si eutectic particles, and (3) precipitation of β in the matrix. The model yielded a R ² of 0.994 and a root-mean-square error (RMSE) of 7.4 MPa. The model and the implications of the results are discussed in the article. This article is based on a presentation made in the symposium entitled “Simulation of Aluminum Shape Casting Processing: From Design to Mechanical Properties,” which occurred March 12–16, 2006 during the TMS Spring Meeting in San Antonio, Texas, under the auspices of the Computational Materials Science and Engineering Committee, the Process Modeling, Analysis and Control Committee, the Solidification Committee, the Mechanical Behavior of Materials Committee, and the Light Metal Division/Aluminum Committee.     

Computational Modeling of Microstructure Evolution in Solidification of Aluminum Alloys

In this article, a front tracking (FT) model and a modified cellular automaton (MCA) model are presented and their capabilities in modeling the microstructure evolution during solidification of aluminum alloys are demonstrated. The FT model is first validated by comparison with the predictions of the Lipton–Glicksman–Kurz (LGK) model. Calculations of the steady-state dendritic tip growth velocity and equilibrium liquid composition as a function of melt undercooling for an Al-4 wt pct Cu alloy exhibit good agreement between the FT simulations and the LGK predictions. The FT model is also used to simulate the secondary dendrite arm spacing as a function of local solidification time. The simulated results agree well with the experimental data. The MCA model is applied to simulate dendritic and nondendritic microstructure evolution in semisolid processing of an Al-Si alloy. The effect of fluid flow on dendritic growth is also examined. The solute profiles in equiaxed dendritic solidification of a ternary aluminum alloy are simulated as a function of cooling rate and compared with the prediction of the Scheil model. The MCA model is extended to the multiphase system for the simulation of eutectic solidification. A particular emphasis is made on the quantitative aspects of simulations. This article is based on a presentation made in the symposium ”Simulation of Aluminum Shape Casting Processing: From Design to Mechanical Properties,” which occurred March 12–16, 2006, during the TMS Spring Meeting in San Antonio, Texas, under the auspices of the Computational Materials Science and Engineering Committee, the Process Modeling, Analysis and Control Committee, the Solidification Committee, the Mechanical Behavior of Materials Committee, and the Light Metal Division/Aluminum Committee.     

Multistage Fatigue Modeling of Cast A356-T6 and A380-F Aluminum Alloys

This article presents a microstructure-based multistage fatigue (MSF) model extended from the model developed by McDowell et al.[1,2] to an A380-F aluminum alloy to consider microstructure-property relations of descending order, signifying deleterious effects of defects/discontinuities: (1) pores or oxides greater than 100 μm, (2) pores or oxides greater than 50 μm near the free surface, (3) a high porosity region with an area greater than 200 μm, and (4) oxide film of an area greater than 10,000 μm². These microconstituents, inclusions, or discontinuities represent different casting features that may dominate fatigue life at stages of fatigue damage evolutions. The incubation life is estimated using a modified Coffin–Mansion law at the microscale based on the microplasticity at the discontinuity. The microstructurally small crack (MSC) and physically small crack (PSC) growth was modeled using the crack tip displacement as the driving force, which is affected by the porosity and dendrite cell size (DCS). When the fatigue damage evolves to several DCSs, cracks behave as long cracks with growth subject to the effective stress intensity factor in linear elastic fracture mechanics. Based on an understanding of the microstructures of A380-F and A356-T6 aluminum alloys, an engineering treatment of the MSF model was introduced for A380-F aluminum alloys by tailoring a few model parameters based on the mechanical properties of the alloy. The MSF model is used to predict the upper and lower bounds of the experimental fatigue strain life and stress life of the two cast aluminum alloys. This article is based on a presentation made in the symposium entitled “Simulation of Aluminum Shape Casting Processing: From Design to Mechanical Properties,” which occurred March 12–16, 2006 during the TMS Spring Meeting in San Antonio, Texas, under the auspices of the Computational Materials Science and Engineering Committee, the Process Modeling, Analysis and Control Committee, the Solidification Committee, the Mechanical Behavior of Materials Committee, and the Light Metal Division/Aluminum Committee.

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Determination of interfacial heat-transfer boundary conditions in an aluminum low-pressure permanent mold test casting

Great advances in casting modeling/simulation software have been made in the past several years. However, software predictions are only as accurate as the material and processing properties used as inputs. The process of low-pressure semipermanent mold casting is widely used by the automotive industry in the production of aluminum cylinder head castings. However, the process is highly sensitive to various casting parameters and many of the influences are not well understood. This report presents results from casting experiments and simulations using a specially designed casting, the “wing casting.” The casting experiments demonstrated the variation in temperature change in different parts of the casting for different processing parameters. Boundary conditions were determined for different areas on the mold surface and those parameters were discussed in terms of temperature sensitivity in the casting behavior. It was also demonstrated that boundary conditions play a major role in the thermal behavior in the casting during solidification. Finally, changes in the mold/casting surface geometry were found to have a large influence on the value of the boundary conditions.     

Creep strength of magnesium-based alloys

The high-temperature creep resistance of magnesium alloys was discussed, with special reference to Mg-Al and Mg-Y alloys. Mg-Al solid-solution alloys are superior to Al-Mg solid-solution alloys in terms of creep resistance. This is attributed to the high internal stress typical of an hcp structure having only two independent basal slip systems. Although magnesium has a smaller shear modulus than aluminum, the inherent creep resistance of Mg alloys is better than that of Al alloys. The creep resistance of Mg alloys is improved substantially by the addition of Y. Solid-solution hardening is the principal mechanism of the strengthening, but the details of the mechanism have not been elucidated yet. Forest dislocation hardening in concentrated alloys and dynamic precipitation in a Mg-2.4 pct Y alloy also contribute to the strengthening. An addition of a very small amount of Zn raises the dislocation density and significantly improves the creep resistance of Mg-Y alloys. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Creep strength of magnesium-based alloys

The high-temperature creep resistance of magnesium alloys was discussed, with special reference to Mg-Al and Mg-Y alloys. Mg-Al solid-solution alloys are superior to Al-Mg solid-solution alloys in terms of creep resistance. This is attributed to the high internal stress typical of an hcp structure having only two independent basal slip systems. Although magnesium has a smaller shear modulus than aluminum, the inherent creep resistance of Mg alloys is better than that of Al alloys. The creep resistance of Mg alloys is improved substantially by the addition of Y. Solid-solution hardening is the principal mechanism of the strengthening, but the details of the mechanism have not been elucidated yet. Forest dislocation hardening in concentrated alloys and dynamic precipitation in a Mg-2.4 pct Y alloy also contribute to the strengthening. An addition of a very small amount of Zn raises the dislocation density and significantly improves the creep resistance of Mg-Y alloys. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Influence of microstructure on tensile and creep properties of a new castable TiAl-based alloy

Snecma Motors has been working on the development of γ-TiAl low-pressure turbine blades, including manufacturing optimization, castability evaluation of a selected alloy called G4, and heat-treatment optimization of mechanical and physical properties. The objective of this study was to evaluate microstructure variability regarding casting conditions and aluminum content. The response of cast microstructures to hot isostatic pressing (hipping) and subsequent heat treatments was determined and quantified using tensile and creep testing. Such investigations helped define an optimized heat treatment. Tensile and creep property assessment has shown a high-temperature potential for G4 alloy with respect to other γ alloys. The G4 alloy also appears to be more creep resistant than conventional nickel-based superalloys on a specific basis. The enhanced creep properties under the optimized low-temperature treatment are mainly attributed to solid solution strengthening with Re, W, and Si elements and precipitation hardening with primary β phase decorating the primary dendrites.     

Quantification of Microsegregation in Cast Al-Si-Cu Alloys

The random sampling approach offers an elegant yet accurate way of validating microsegregation models. However, both instrumental errors and interference from secondary phases complicate the treatment of randomly sampled microprobe data. This study demonstrates that the normal procedure of sorting the data for each element independently can lead to inaccurate estimation of segregation profiles within multicomponent, multiphase, aluminum alloys. A recently proposed alloy-independent approach is shown to more reliably isolate these interferences, allowing more accurate validation of microsegregation models. Application of this approach to examine solidification segregation of a 319-type alloy demonstrated that, for these slowly cooled castings, neither Sr or TiB₂ additions significantly affected coring of Cu within the primary α-Al dendrites. Comparison against predictions of CALPHAD-type Gulliver–Scheil models was less satisfactory. Consideration of back-diffusion and morphology effects through a one-dimensional (1-D) numerical model do not improve the agreement. Possible reasons for the lack of agreement are hypothesized. This article is based on a presentation made in the symposium entitled “Simulation of Aluminum Shape Casting Processing: From Design to Mechanical Properties,” which occurred March 12–16, 2006 during the TMS Spring Meeting in San Antonio, Texas, under the auspices of the Computational Materials Science and Engineering Committee, the Process Modeling, Analysis and Control Committee, the Solidification Committee, the Mechanical Behavior of Materials Committee, and the Light Metal Division/Aluminum Committee.

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Microstructure and Corrosion Characterization of Squeeze Cast AM50 Magnesium Alloys

Squeeze casting of magnesium alloys potentially can be used in lightweight chassis components such as control arms and knuckles. This study documents the microstructural analysis and corrosion behavior of AM50 alloys squeeze cast at different pressures between 40 and 120 MPa and compares them with high-pressure die cast (HPDC) AM50 alloy castings and an AM50 squeeze cast prototype control arm. Although the corrosion rates of the squeeze cast samples are slightly higher than those observed for the HPDC AM50 alloy, the former does produce virtually porosity-free castings that are required for structural applications like control arms and wheels. This outcome is extremely encouraging as it provides an opportunity for additional alloy and process development by squeeze casting that has remained relatively unexplored for magnesium alloys compared with aluminum. Among the microstructural parameters analyzed, it seems that the β-phase interfacial area, indicating a greater degree of β network, leads to a lower corrosion rate. Weight loss was the better method for determining corrosion behavior in these alloys that contain a large fraction of second phase, which can cause perturbations to an overall uniform surface corrosion behavior.     

Creep behaviors of an Ir-23Nb alloy at ultra-high temperatures

The ultra-high-temperature creep behaviors of an Ir-base, Ir-23Nb (in at. pct), two-phase refractory superalloy have been investigated. The compression creep experiments were performed at temperatures from 1650 °C to 1800 °C at initial applied stresses from 49 to 200 MPa. The results show that Ir-23Nb alloy has higher creep resistance and longer creep life in comparison to Ir-17Nb alloy under the same experimental conditions. The steady-state creep behavior can be described in terms of power-law creep with the apparent stress exponent of 4.5 and apparent activation energy of 653 kJ/mol. Basing on the investigation, the possible reasons for the great improvement on the creep resistance and creep life in Ir-23Nb alloy are discussed. This article is based on a presentation made in the symposium entitled “Fundamentals of Structural Intermetallics,” presented at the 2002 TMS Annual Meeting, February 21–27, 2002, in Seattle, Washington, under the auspices of the ASM and TMS Joint Committee on Mechanical Behavior of Materials.     

A finite element model of the effects of primary creep in an Al-SiC metal matrix composite

A two dimensional axisymmetric finite element model has been developed to study the creep behavior of a high-temperature aluminum alloy matrix (alloy 8009) reinforced with 11 vol pct silicon carbide paniculate. Because primary creep represents a significant portion of the total creep strain for this matrix alloy, the emphasis of the present investigation is on the influence of primary creep on the high-temperature behavior of the composite. The base alloy and composite are prepared by rapid solidification processing, resulting in a very fine grain size and the absence of precipitates that may complicate modeling of the composite. Because the matrix microstructure is unaffected by the presence of the SiC paniculate, this material is particularly well suited to continuum finite element modeling. Stress contours, strain contours, and creep curves are presented for the model. While the final distribution of stresses and strains is unaffected by the inclusion of primary creep, the overall creep response of the model reveals a significant primary strain transient. The effects of true primary creep are more significant than the primary-like transient introduced by the redistribution of stresses after loading. Examination of the stress contours indicates that the matrix axial and shear components become less uniform while the effective stress becomes more homogeneous as creep progresses and that the distribution of stresses do not change significantly with time after the strain rate reaches a steady state. These results also confirm that load transfer from the matrix to reinforcement occurs primarily through the shear stress. It is concluded that inclusion of matrix primary creep is essential to obtaining accurate representations of the creep response of metal matrix composites. formerly Graduate Research Assistant, University of California-Davis This article is based on a presentation made in the symposium entitled “Creep and Fatigue in Metal Matrix Composites” at the 1994 TMS/ASM Spring meeting, held February 28–March 3, 1994, in San Francisco, California, under the auspices of the joint TMS-SMD/ ASM-MSD Composite Materials Committee.     

A Brief Review of Multiaxial High-Cycle Fatigue

In this article, three classes of criteria (stress-strain, energy, and critical plane) used for life estimation in multiaxial high-cycle fatigue are discussed with emphasis on nonproportional loading, and the merits or demerits of each fatigue criterion are indicated. This paper also reviews mechanisms of multiaxial high-cycle fatigue, especially under nonproportional loading, and introduces studies in China on micromechanisms in high-cycle fatigue under multiaxial loading. Suggestions for further work are proposed in the conclusions. This article is based on a presentation made in the symposium entitled “Simulation of Aluminum Shape Casting Processing: From Design to Mechanical Properties,” which occurred March 12–16, 2006 during the TMS Spring Meeting in San Antonio, Texas, under the auspices of the Computational Materials Science and Engineering Committee, the Process Modeling, Analysis and Control Committee, the Solidification Committee, the Mechanical Behavior of Materials Committee, and the Light Metal Division/Aluminum Committee.

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Plastic deformation of aluminum under repeated loading

The plastic deformation behavior of high purity (99.999 pct) polycrystalline and single crystal aluminum under repeated stressing was investigated by studying the creep behavior. The creep behavior under repeated stressing (cyclic creep) was compared with the static creep behavior at identical peak stresses. The influence of such experimental variables as the applied stress, the amplitude of cyclic stress, the test temperature and the static creep rate prior to stress cycling were systematically examined. The most important experimental observation in this study was that the cycling of the creep stress could either enhance or retard the creep deformation, depending upon the combination of the experimental variables. The experimental variable that had the most significant influence on the cyclic creep behavior was the applied stress; the enhancement of the creep rate was observed above a threshold stress, while the cyclic stress retarded the creep deformation at lower stresses. The threshold stress was found to depend sensitively on temperature. The implications of the threshold stress were examined by an analysis of the work-hardening behavior.     

Internal stress superplasticity in anisotropic

Internal stress superplasticity is assessed in powder metallurgy and wrought polycrystalline Zn and in polycrystalline α-U. These materials are anisotropic in their thermal expansion coefficients, and, as a result, internal stresses are generated during thermal cycling. A creep model is developed based on the contribution of internal stress to the enhancement and inhibition of normal plastic flow. This creep model, which has no disposable parameters, is shown to describe quantitatively the flow behavior of anisotropic materials under thermal cycling conditions, and correctly predicts the attainment of Newtonian flow characteristics at low stresses. It is predicted that certain polycrystalline ceramics can be made superplastic in tension when thermally cycled under small applied stress.     

AC4B 合金低压铸造生产4G1发动机缸盖

低压铸造应用非常广泛，调整浇注压力与铝液速率，细化成分要求改善高现场铸造条件的稳定性和兼容性，对提高4 G1发动机缸盖的铸造质量具有重要作用。     

Compressive creep properties of Ir-base refractory superalloys

Ir-base alloys with the fcc and L1₂-Ir₃X (X = Nb, Zr) two-phase structure have been developed as next-generation high-temperature materials. The compressive creep behavior of Ir-Nb and Ir-Zr alloys was investigated at 2073 K under 137 MPa. The effect of addition of the third element, Zr, on the creep behavior of an Ir-Nb alloy was also investigated at 2073 K for 137 MPa. The creep rate became two orders lower by addition of a small amount of Zr. The lattice misfit change between the fcc and L1₂ two phase by addition of Zr and the deformation structure in binary and ternary alloys after a creep test were also investigated. The creep behavior is discussed in terms of the lattice misfit, precipitate shape, and their distribution. This article is based on a presentation made in the symposium entitled “Beyond Nickel-Base Superalloys,” which took place March 14–18, 2004, at the TMS Spring meeting in Charlotte, NC, under the auspices of the SMD-Corrosion and Environmental Effects Committee, the SMD-High Temperature Alloys Committee, the SMD-Mechanical Behavior of Materials Committee, and the SMD-Refractory Metals Committee.