Effect Of Macrosegregation Research Articles

Effects of macrosegregation on mechanical and tribological properties of squeeze casting immiscible bearing alloys

Effects of macrosegregation on mechanical and tribological properties of squeeze casting immiscible bearing alloys

Modelling the effect of macro-segregation on the fracture toughness of heavy forgings using FFT based crystal plasticity simulations

The local approach has been successful in evaluating the brittle fracture probability of nuclear pressure vessel steels by establishing a link between microstructural defects and the macroscopic fracture behaviour. The evaluation of fracture probabilities relies on the applied stress on the smallest representative elementary volume. A proper description of the stress heterogeneities in polycrystals helps refine the prediction. The current work investigates the effect of carbon macro-segregation in heavy forgings and demonstrates a workflow combining crystal plasticity with the Microstructure Informed Brittle Fracture (MIBF) local approach model in fracture toughness prediction. The microstructural and mechanical properties of low alloy steels with different segregation levels were evaluated. A dislocation-density based crystal plasticity model which contains carbide strengthening contribution was identified and applied for modelling microstructure influence on local stress distributions. Results show that the microstructural evolution observed at high carbon levels has a significant influence on local stress distributions, which in turn affects the fracture toughness. The simulation results also demonstrate that, with proper input of microstructural information, the MIBF model is capable to predict the shift of the brittle-to-ductile transition zone with the variation of carbon and alloying elements and gives insights about factors affecting the resistance of materials.

Residual stress distribution of a locally and inductively quenched and tempered 50CrMo4 steel analysed by synchrotron transmission techniques

This work investigates the in-depth residual stress distribution and retained austenite content in plates that were wire eroded from one locally quenched and one locally quenched-tempered 50CrMo4 cylinders by the means of high-energy X-Ray synchrotron transmission techniques. The main challenge was to interpret the results from all diffraction angles to obtain meaningful validation data for future computer simulations of induction hardening and tempering. The results were discussed in relation to hardness distribution. With the help of the applied measurement technique, even the effects of macro-segregation near the longitudinal sample axis can be detected. In inductively heated and quenched cylindrical specimens of 50CrMo4 specimens, macro-segregations near the longitudinal sample axis lead to a reduced retained austenite content, which shifts the axial stresses towards tensile stresses. Comparing the results with those from the literature, it can be seen that variations in the chemical composition of the sample within the specification range of the steel grade have less influence on the residual stress distribution in the induction hardened samples than the sample geometry and/or the quenching rate.

Nondestructive evaluation of macro segregation in creep strength enhanced 9Cr–1Mo–V–Nb steel

Effects of macro segregation and homogenizing heat treatment on creep strength of 9Cr–1Mo–V–Nb (Grade T91) steel have been investigated and it has been concluded that texture is useful feature to evaluate creep strength of the steel as a straightforward index of macro segregation.

δ-Ferrite Formation and Its Effect on the Mechanical Properties of Heavy-Section AISI 316 Stainless Steel Casting

δ-Ferrite formation across the thickness of a heavy-section AISI 316 casting slab and the effect of its decomposition on the bending properties have been studied. Cellular-shaped δ-ferrite with the faction of less than 3 pct is observed at the surface position of the cast while skeletal-shaped δ-ferrite with the faction of up to 8 pct is present at the center position, which is attributed to the combined effects of macro-segregation and cooling rate. Extensive decompositions of δ-ferrite through direct transformation (δ → σ) and eutectoid transformation (δ → σ + γ2) are demonstrated. Micro-cracks could be observed within the decomposed δ-ferrite on the convex surface of bending samples from both the surface and center position of the cast, but occurrence of cracking is only present in the sample from center position of the cast. Removal of the brittle σ phase within δ-ferrite by a preheat treatment at 1050 °C could effectively improve the bending performance.

Porosity Prediction in A356 Wheel Casting

A FLUENT™-based thermal-fluid-compositional model has been developed and applied to a low-pressure, die-cast, A356 aluminum alloy wheel to explore the effects of macro-segregation of hydrogen and late-stage liquid encapsulation on pore formation during solidification. Based on the evolution of temperature, pressure, and hydrogen concentration in the liquid output from FLUENT, the porosity size distribution has been predicted at a number of locations in a commercially produced wheel using an in-house micro-porosity model. The porosity model incorporates pore nucleation kinetics based on hydrogen supersaturation, and two modes of pore growth: the first, occurring at low solid fractions is based on hydrogen diffusion; and the second, occurring at high solid fractions is based on conservation of volume. Samples from a commercially cast wheel have been analyzed using X-ray micro-tomography to provide basic validation of the micro-porosity model. The predicted results showed that the evolution in pressure has the dominant effect on pore growth, but only under conditions where pores have nucleated prior to late-stage liquid encapsulation. Otherwise, the cooling rate appears to have the dominant effect. The accuracy of the model is discussed in terms of the predicted pore size distribution and number density of pores. Areas for continued development are presented.

Near-eutectic Zn-Mg alloys: Interrelations of solidification thermal parameters, microstructure length scale and tensile/corrosion properties

Zn-Mg alloys are considered to have potential application in bone implants, since both metals are biocompatible and have biodegradable characteristics. Adding Mg to Zn can boost mechanical and corrosion resistances. However, the literature is very limited on quantifying the interrelation of solidification parameters, microstructural features and mechanical/corrosion properties of Zn-Mg alloys. The present study examines the interrelations of alloy Mg content, macrosegregation effects, morphology and scale of the matrix and eutectic phases, nature of intermetallics and tensile and corrosion properties of near-eutectic Zn-Mg alloys. The alloys samples are obtained by unsteady-state directional solidification resulting in a wide range of solidification thermal parameters and microstructures. We examine microstructural features of both dendritic and complex regular eutectic phases. It is shown that the eutectic exhibits a bimodal pattern with neighboring areas of coarse and fine lamellae. Experimental growth laws relating the primary, secondary and eutectic spacings to the solidification cooling rate and growth rate are proposed. Hall-Petch type equations are derived expressing tensile strength and elongation to dendritic and eutectic spacings. Electrochemical parameters determined by polarization curves during corrosion tests and SEM analyses of corroded areas have shown that the alloy having an essentially eutectic microstructure is associated with better corrosion resistance.

Effects of Macrosegregation and Microstructure on the Corrosion Resistance and Hardness of a Directionally Solidified Zn-5.0wt.%Mg Alloy

This study aims to analyze the influence of macrosegregation on microstructure evolution, and of microstructure length scale on the corrosion resistance of a Zn-5.0wt.%Mg alloy casting. The analyzed samples were taken along the length of castings unidirectionally solidified in unsteady state conditions of heat extraction. Microstructure characterization, microhardness, linear polarization and electrochemical impedance spectroscopy (EIS) tests were performed. A unique type of microstructure was observed, characterized essentially by a morphology typified by idiomorphic MgZn2 crystals in a “complex eutectic mixture” [coexistence of stable (Zn+Mg2Zn11) and metastable (Zn+MgZn2) eutectics]. The correlation between thermal and microstructural parameters, permitted experimental growth laws, correlating the evolution of the lamellar eutectic spacing with the cooling rate to be established. Vickers microhardness and electrochemical corrosion tests showed that more refined microstructures associated with higher experimental cooling rates, are related to a better set of higher hardness and corrosion resistance.

Numerical Simulation of Water Quenching of Large Size Steel Forgings: Effects of Macrosegregation and Grain Size on Phase Distribution

In this paper, water quenching of large ingots was simulated using FORGE NxT 1.1® Finite Element code. Simulations were carried out for as-forged medium-carbon low-alloy steel. A novel method is proposed to simulate the different parts of a large size forged block with different chemical compositions and grain sizes using the multiple materials method. The effects of macrosegregation, grain size variation and cooling rate on phase distribution through the volume of the forged block were investigated. The delay in transformation kinetics, which is due to the effect of grain size variation and carbon content, was analyzed. Results show that macrosegregation and grain size variations significantly influence transformation start points and the volume fraction of phases that are present in each location of the forged ingot. The proposed prediction method was validated using high-resolution dilatometry experiments and X-ray diffraction measurements to evaluate accurately the volume fraction of martensite, bainite and the percentage of retained austenite for each condition.

Interrelationship of thermal parameters, microstructure and microhardness of directionally solidified Bi–Zn solder alloys

Due to the health and environmental concerns associated with lead usage, the research on alternative lead-free alloys for replacing lead-based solders is a global demand. Despite numerous studies on Sn-based lead-free solders in recent years, there is not yet a standard solder alloy able to cover the spectrum of properties furnished by the classic Sn-Pb alloy. In this sense, particular lead-free alloys compositions have been suited for specific needs and the options have been broadening when elements other than tin are used as the base component, such as indium, gold and bismuth. The later element is well known in the hall of lead-free alloys as an alloying option rather than as a base component. This study aims to establish interrelations of solidification thermal parameters (growth and cooling rates), microstructure features (primary and secondary dendrite arm spacings – λ1 and λ2; eutectic spacing – λE and interphase spacing – λint) and hardness of Bi-Zn alloys (1.5wt% Zn-hypoeutectic, 2.7wt% Zn-eutectic, and 5wt% Zn-hypereutectic alloys) samples, which were directionally solidified in unsteady-state conditions under cooling rates similar to those of found in industrial soldering practice. Examination of the resulting microstructures by scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) permitted the different phases morphologies to be characterized: Bi-rich trigonal dendrites and long Zn fibers, as primary phases of, hypoeutectic and hypereutectic alloys, respectively, immersed in a fiber-like eutectic mixture. The combined effects of macrosegregation, Zn alloying and representative scales of the phases forming the microstructure (λint, λ2 and λE) on hardness of the Bi-Zn alloys are evaluated and Hall–Petch type equations relating λint, λ2 and λE to hardness are proposed.

Effect of Macrosegregation on the Microstructure and Mechanical Properties of a Pressure-Vessel Steel

Macrosegregation refers to the chemical segregation, which occurs quite commonly in the large forgings such as nuclear reactor pressure vessel. This work assesses the effect of macrosegregation and homogenization treatment on the mechanical properties of a pressure-vessel steel (SA508 Gr.3). It was found that the primary reason for the inhomogeneity of the microstructure was the segregation of Mn, Mo, and Ni. Martensite, and coarse upper bainite with M-A (martensite-austenite) islands have been obtained, respectively, in the positive and negative segregation zone during a simulated quenching process. During tempering, the carbon-rich M-A islands decomposed into a mixture of ferrite and numerous carbides which deteriorated the toughness of the material. The segregation has been substantially minimized by a homogenizing treatment. The results indicate that the material homogenized has a higher impact toughness than the material with segregation, due to the reduction in M-A island in the negative segregation zone. It can be concluded that the microstructure and mechanical properties have been improved remarkably by means of homogenization treatment.

Solidification with Buoyancy Induced Convection: Evaluation of Different Mushy Zone Formulations

A comparative assessment is performed between purely porous formulation of the mushy zone controlled by permeability and the hybrid formulations controlled by both permeability and viscosity of the mushy zone during solidification of a binary alloy. The Darcy’s Carman-Kozeny equation is used to model the permeability of the mushy zone in porous formulation for all the models. The first hybrid model employs switching functions to simultaneously control the permeability and viscosity of the mushy zone up to a critical solid fraction and thereafter it switches to a purely permeability controlled porous formulation. The second hybrid model assumes mushy zone to be non-newtonian slurry with the liquid viscosity following a power law up to the critical solid fraction and a permeability controlled porous medium thereafter. A twodimensional computational domain of aqueous ammonium chloride (NH 4 Cl-H 2 O) solution employing continuum mixture approach is considered for the analysis. Model with purely porous mushy zone formulation showed higher solutal gradients in the mushy zone resulting in higher solutal buoyancy driven convection in mushy zone along with higher bulk macrosegregation effects in comparison to the hybrid models. Both the hybrid models showed potential of capturing the settled free floating particles and broken dendrites with the non-newtonian slurry hybrid model showing potential of capturing the liquidus irregularities.

Interconnection of Zn content, macrosegregation, dendritic growth, nature of intermetallics and hardness in directionally solidified Mg–Zn alloys

Mg and Mg-based alloys are lightweight metallic materials that are extremely biocompatible and have promising mechanical properties. Mg–Zn alloys have potential applications as biomaterials in the field of hard tissue engineering for load bearing applications, and are mainly produced by casting with limited further operations such as finishing by thermo-mechanical and machining of the surface. Thus, understanding the solidification behaviour of Mg–Zn alloys becomes an essential task to be accomplished. The aim of this work is to examine the interconnection of alloy Zn content/macrosegregation/scale of the dendritic microstructure/nature of intermetallics and hardness of samples obtained by transient directional solidification of four Mg–Zn alloys. Experimental growth laws relating the primary dendrite arm spacing, λ1, of directionally solidified samples to solidification thermal parameters such as the growth rate and the cooling rate are proposed. The experimental scatters of λ1 are shown to be mainly inside the range of theoretical λ1 values predicted by the Hunt-Lu model. Also, the combined effects of macrosegregation, Zn alloying and λ1 on hardness of Mg–Zn alloys are evaluated and functional equations relating λ1 to hardness are proposed.

A Combined Numerical and Experimental Study of the Effects of Controlled Fluid Flow on Alloy Solidification

A quantitative understanding of the effect of fluid flow on the microstructure of cast alloys is still lacking. The application of time dependent magnetic fields during solidification offers the possibility to create defined flow conditions in solidification processing. The effect of rotating magnetic fields (RMF) on the microstructure formation in cast Al-alloys (Al-7wt.%Si, Al-7wt.%Si- 0.6wt.Mg) is studied experimentally and numerically. The forced fluid flow conditions result in pronounced macrosegregation effects and affect microstructural parameters. With increasing fluid flow the primary dendrite spacing decreases whereas the secondary dendrite arm spacing increases. The experimental analysis is supported by a rigorous application of numerical modeling with the software package CrysVUn.

The Effect of Inverse Macro-Segregation on Fatigue Behavior of Die-Cast AM60 Magnesium Alloy

The normal and inverse solute macro-segregation are known to occur in Al and other nonferrous alloy castings and have been well studied and documented. However, these phenomena have not been investigated in the high-pressure die-cast Mg-alloys. Consequently, the effects of macro-segregation on the mechanical properties of cast Mg-alloys have not been characterized. The objective of this contribution is to investigate the effects of inverse macro-segregation and porosity on the fatigue behavior of high-pressure die-cast AM60 alloy. It is observed that the inverse macro-segregation of eutectic phase at the cast surfaces adversely affects the fatigue behavior: the fatigue resistance decreases substantially due to the presence of the surface segregation.

Inverse surface macro-segregation in high-pressure die-cast AM60 magnesium alloy and its effects on fatigue behavior

Abstract Metallographic evidence is presented for existence of inverse surface macro-segregation in a HPDC AM60 Mg-alloy. The process conditions that facilitate the inverse surface macro-segregation are identified, and the effects of macro-segregation on the low and high cycle fatigue behavior of the HPDC alloy are studied.

Batch Stirred Vessel Mixing Evaluated by Visualized Reactive Tracers and Electrical Tomography

Atechnique using the visualization of reactive tracer mixing in 3-D is proposed for the quantification of mixing with chemical reaction in typical batch stirred vessel reactors. Images can be conveniently and inexpensively acquired using acid–alkali pulse tracer tests with suitable indicators. A demonstration is reported for the caustic soda– hydrochloric acid–phenolphthalein system, which produces dilute brine solution for safe disposal, on a 2.3 m3 plant-scale stirred vessel. The fluid mixing and fast acid–alkali reaction have been modelled using simplified computational fluid dynamics (CFD) based on the network- of-zones model for O(105) zones. The model predictions have been image reconstructed using AVS graphics. Using flow and mixing parameters independently acquired from inert tracer mixing tests, the close correspondence between experimental and theoretical images confirms that the micromixing must be close to perfect. The approach adopted quantifies both the local zone macromixing and macro-segregation effects for reactions faster than the fluid mixing rates and is useful for analysis of the shape and size of reagent-rich regions in semi-batch operation. Electrical resistance tomography (ERT) can suitably image the tracer mixing with reaction in 3-D according to the accompanying conductivity changes.

Induction heating process of an Al–Si aluminum alloy for semi-solid die casting and its resulting microstructure

During induction heating, the relationship between time and temperature must be controlled exactly to obtain a homogeneous temperature distribution over the entire cross-sectional area. Because the initial solid fraction in the semi-solid die casting (SDC) process is the main parameter to achieve a homogeneous flow behavior of the liquid and solid phases and to prevent macro-segregation effects in the SDC process, an accurately controllable induction heating method must be selected for the reheating process. The purpose of this work is not just to obtain the desire solid fraction, generally about 50%, but also to ensure the optimal induction heating conditions of A356 alloy to reduce the temperature gradient of a 76 mm diameter ×90 mm length billet and to obtain a fine globular microstructure without grain coarsening (resulting microstructure). This work shows that, in the case of a three-step reheating process for the SDC process, the final holding time is the most important factor to maintain a fine globular microstructure without grain coarsening.

Influence of solidification variables on the microstructure, macrosegregation, and porosity of directionally solidified Mar-M247

The solidification microstructure is critical in determining the amount and distribution of porosity that develops during the freezing of castings. As the solidification velocity V s increases, the microstructural length scales (primary and secondary dendrite arm spacings) decrease; thus (1) the mushy zone permeability would be expected to decrease; and (2) nucleated pores would be increasingly isolated. Although the first effect would tend to increase the observed porosity, the second effect would tend to decrease the porosity. To better understand these competitive mechanisms, a series of controlled unidirectional solidification experiments were performed on bars of nickel-base superalloy Mar-M247. Samples were produced with constant dendrite arm spacing throughout an extended length of each cast bar. The axial thermal gradient and withdrawal velocity imposed on each casting were varied between castings to produce a range of microstructures from aligned cellular dendritic to aligned dendritic to mis-aligned dendritic. Macrosegregation effects along the lengths of the bars were evaluated and the resultant impact upon the density along the length of each casting was also characterized. The density measurements were found to be very sensitive to both (1) compositional macrosegregation in these castings and (2) internal porosity. Statistical analyses of microporosity in the castings were based upon metallographic measurements. The development of microporosity in the unidirectionally solidified castings is shown to be dependent upon the hydrogen gas content of the samples and the imposed solidification velocity through the sample's cast microstructures. An optimum intermediate withdrawal velocity of 0.005–0.01 cm s −1 was found, which led to closely spaced dendrite arms, a large number of very small pores and a minimum total porosity level. Lower velocities lead to increased porosity from larger pores whereas higher velocities lead to macroporosity due to centerline shrinkage.

A Study of Macrosegregation in Low-Density Polyethylene Autoclave Reactors by Computational Fluid Dynamic Modeling

Flow and reaction in a typical commercial scale autoclave LDPE reactor were modeled by a three-dimensional computational fluid dynamic (CFD) k-e model in order to shed light on the macrosegregation effects that can occur in these reactors. It is shown that the CFD model predicts significant differences from CSTR behavior. Results are discussed in terms of the effects of macro- and microscale inhomogeneities of concentration and temperature on free radical polymerization kinetics. The observed nonidealities in terms of minima in the initiator consumption curves and multiple steady states are explained on the basis of competing turbulent transport and chemical kinetics. Microsegregation effects are shown to be negligible in comparison to macrosegregation effects. Given the fact that the CFD model is based on reactive scalar and energy balances without adjusted parameters in the three-dimensional flow field of the entire reactor, it is tentatively concluded that commercial-scale LDPE vessel reactors can have significant macrosegregation effects beyond a certain steady-state adiabatic operating temperature that is specific to the initiator being used.