Cast Aluminum Alloy A356 Research Articles

Fracture Toughness and Fracture Mechanisms of Cast A356 Aluminum Alloys

The present study aims at investigating the effects of microstructure on fracture toughness of two A356 Al alloys. These A356 alloys were fabricated by casting processes such as rheo-casting and casting-forging, and their mechanical properties and fracture toughness were analyzed in relation with microfracture mechanisms. All the cast A356 alloys contained eutectic Si particles mainly segregated along solidification cells, and the distribution of Si particles was modified by the casting-forging process. Microfracture observation results revealed that eutectic Si particles segregated along cells were cracked first, but that Al matrix played a role in blocking crack propagation. Tensile properties and fracture toughness of the cast-forged alloys having homogeneous distribution of eutectic Si particles were superior to those of the rheo-cast alloy.

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.

Effect of microstructural variables on tensile behaviour of A356 cast aluminium alloy

The effects of microstructural variables, including secondary dendrite arm spacing (SDAS), the size of primary α phase, the aspect ratio of eutectic Si particle and the thickness of eutectic wall structure, on tensile behaviour of A356 cast aluminium alloy, were quantitatively identified using linear regression analysis method. For systematic microstructural control of A356 specimen, directional solidification method was used with different solidification rates of 5, 25, 50 and 100 μm s−1 respectively. The linear regression analysis suggests that each microstructural variable affects tensile strength and tensile elongation of A356 cast aluminium alloy in a similar fashion. The change in tensile behaviour with varying microstructural variables in A356 cast aluminium alloy is discussed based on fractographic and micrographic observations.

Effect of equal channel angular pressing on the microstructure of a semisolid aluminum alloy

The microstructure evolution during semisolid equal channel angular pressing and semisolid isothermal heating of a cast A356 aluminum alloy is reported. It is demonstrated that partially melted dendritic cast structure undergoes substantial plastic deformation during semisolid equal channel angular pressing. Coarsening of the fragmented dendritic microstructure during subsequent semisolid isothermal heating gives rise to spheroidal solid phase surrounded by liquid metal.

Effect of pre/post T6 heat treatment on the mechanical properties of laser welded SSM cast A356 aluminium alloy

Aluminium alloy A356 was treated by the Rheo semi solid metal (SSM) process, developed recently by CSIR-Pretoria, and cast in plates using a high pressure die casting (HPDC) machine. Plates in as cast condition (F) and heat treated T6 condition (pre HT) were butt welded, using an Nd:YAG laser. In another experiment, as cast welded samples were heat treated to T6 condition (post HT). The base metal and weld microstructures were presented. The effect of heat treatments on microstructure and mechanical properties of the welds was investigated. It was found that the fine dendrite structure of the weld metal contributed to the mechanical properties of the joint. The mechanical properties of the post HT samples were found to be higher than the pre HT and as cast materials.

Microstructural modification of as-cast Al-Si-Mg alloy by friction stir processing

Friction stir processing (FSP) has been applied to cast aluminum alloy A356 plates to enhance the mechanical properties through microstructural refinement and homogenization. The effect of tool geometry and FSP parameters on resultant microstructure and mechanical properties was investigated. The FSP broke up and dispersed the coarse acicular Si particles creating a uniform distribution of Si particles in the aluminum matrix with significant microstructural refinement. Further, FSP healed the casting porosity. These microstructural changes led to a significant improvement in both strength and ductility. Higher tool rotation rate was the most effective parameter to refine coarse Si particles, heal the casting porosity, and consequently increase strength. The effect of tool geometry was complicated and no systematic trend was observed. For a standard pin design, maximum strength was achieved at a tool rotation rate of 900 rpm and traverse speed of 203 mm/min. Post-FSP aging increased strength for materials processed at higher tool rotation rates of 700 to 1100 rpm, but exerted only a marginal effect on samples prepared at the lower rotation rate of 300 rpm. Two-pass FSP with 100 pct overlapping passes resulted in higher strength for both as-FSP and post-FSP aged conditions.

Laser Welding of SSM Cast A356 Aluminium Alloy Processed with CSIR-Rheo Technology

Samples of aluminium alloy A356 were manufactured by Semi Solid Metals HPDC technology, developed recently in CSIR-Pretoria. They were butt welded in as cast conditions using an Nd:YAG laser. The base metal and weld microstructure were presented. The effect of different heat treatments on microstructure and mechanical properties of the welds were investigated. It was found that the fine dendrite structure of the weld metal contributed for equalizing the mechanical properties of the joint.

Morphology based domain partitioning of multi‐phase materials: a preprocessor for multi‐scale modelling

AbstractThis paper develops a microstructural morphology‐based domain partitioning method (MDP) as a comprehensive pre‐processor for multi‐scale simulation of heterogeneous multi‐phase materials. The MDP method systematically creates a multi‐scale image simulation–characterization methodology to enable domain partitioning that can delineate homogenizable regions from those where explicit representation of phases is necessary for analysis. The methods are strictly based on geometric features of the material morphology and do not use any mechanical response functions. The first step in this development simulates high‐resolution microstructural information from low resolution images of the domain and only a limited high resolution micrographs from optical or scanning electron microscopy. The second step uses quantitative characterization of these high resolution images with, e.g. phase distribution functions, to create effective metrics that can relate microstructural features to the material's physical behaviour. The third step invokes domain partitioning to demarcate regions corresponding to different length scales in a concurrent multi‐scale model. Partitioning criteria are defined in terms of descriptors of microstructural characteristics and these are used to adaptively create multi‐level domain partitions. The method developed is tested on a micrograph of a cast aluminium alloy A356. Copyright © 2006 John Wiley & Sons, Ltd.

Tear Toughness of Permanent Mold Cast and DC Cast A356 Aluminum Alloys

Cast products of A356 with different microstructural features were prepared; permanent-mold cast (PM) and direct-chill cast (DC) products. For each casting, a sharp notched plate specimen was subjected to static tensile loading until a crack initiated at the notch root and propagated across the width of the specimen. Both maximum load and energy to fracture (the integrated area under the load-displacement curve) increased with decreasing dendrite arm spacing (DAS). The curve of DC was smooth and the energy to fracture was quite large. The load-displacement curve was divided into two segments by a vertical line through the maximum load. The area under the first segment is a measure of the energy necessary to initiate the crack. The second segment represents the energy necessary for crack propagation. Unit energy was computed by dividing the measured energy by the net area of the specimen. Refinement of DAS and grain size increased unit energies for crack initiation (UEi) and propagation (UEp). Comparison among PM products revealed that DAS refinement was effective for increasing UEi. Among the present castings, the DC product with the finest DAS exhibited a significant increase in UEp. Observation of the crack propagation path revealed that the fracture surface was normal to the loading direction for PM. In contrast, for DC, a slanted crack path was dominant through the specimen ligament. The features of the crack propagation path are considered to have affected quantitative balance between UEi and UEp. The increased UEp in DC is considered to be due to the introduced slanted crack. Tear tests were confirmed to provide useful information concerning the effect of solidification structure on toughness, which can serve as a guide for further toughening of aluminum alloy castings.

Tear Toughness Evaluation of a Permanent Mold Cast A356 Aluminum Alloy Using a Small-size Specimen

Tear toughness evaluation of a permanent mold cast A356 aluminum alloy was carried out by using two kinds of specimen with different size. One was equivalent to the specimen size designated in ASTM B871. The other one was about 30% as large as that of standard one in volume. Unit energies for tear fracture were obtained from load-displacement curves, and their specimen size, thickness and microstructure dependency were examined. Unit crack initiation energy (UEi) increased with increase in specimen thickness. Meanwhile, unit crack propagation energy (UEp) monotonically decreased in accordance with increase in specimen thickness. In order to make sure if the UEp values reflected the characteristics of local microstructure difference, the small-size tear specimens were collected from various parts in a single cast product. Larger UEp was obtained in the specimen with finer dendrite arm spacing (DAS). These findings suggest that the tear test using a small-size specimen is useful for toughness evaluation of cast aluminum alloys.

An Anisotropic Damage Model for Ductile Metals

An anisotropic ductile damage description is motivated from fracture mechanisms and physical observations in Al-Si-Mg aluminum alloys with second phases. Ductile damage is induced by the classical process of nucleation of voids at inclusions, followed by their growth and coalescence. These mechanisms are related to different microstructural and length scale parameters like the fracture toughness, the void size, the intervoid ligament distance, etc. The classical thermodynamic constraints of irreversible processes with material state variables are used to model the tensorial damage evolution coupled to the Bammann-Chiesa-Johnson (BCJ) rate-dependent plasticity. The damage-plasticity coupling is based on the effective stress concept, assuming the total energy equivalence, and written through a deviatoric damage effect tensor on the deviatoric part and through the trace of the second rank damage tensor on the hydrostatic part. The damage rate tensor is additively decomposed into a nucleation rate tensor, a void growth rate scalar, and a coalescence rate tensor. The induced damage anisotropy in mainly driven by the nucleation, which evolves as a function of the absolute value of the plastic strain rate tensor. Finally, some experimental data of cast A356 aluminum alloy are correlated with predictive void-crack evolution to illustrate the applicability of the anisotropic damage model.

Cradle-to-grave simulation-based design incorporating multiscale microstructure-property modeling: Reinvigorating design with science

We propose a new paradigm for design that incorporates scientifically oriented research directly and feasibly into engineering design practice. The goal is to use this simulation-based tool earlier in design to achieve more optimized components and systems. The method to accomplish this bridge of science and engineering is by using thermodynamically constrained internal state variables that are physically based upon microstructure-property relations. When the microstructure-property relations are included in the internal state variable rate equations, history effects can be captured. Hence, the cradle-to-grave notion arises. The method to help determine the appropriate microstructure-property relations for the internal state variables is through a multiscale modeling methodology which includes experimentation. As such, scientifically oriented research occurs in the multiscale methodology, and the engineering design practice employs the cradle-to-grave internal state variable model. An example of the multiscale methodology is presented in terms of a cast A356 aluminum alloy used in automotive design, and an example of the cradle-to-grave simulation based design is presented in terms of a stamped product used in a crash scenario.

Using a micromechanical finite element parametric study to motivate a phenomenological macroscale model for void/crack nucleation in aluminum with a hard second phase

A multi-scale methodology that includes microscale finite element simulations, physical experiments, and a macroscale phenomenological model was used to determine the appropriate first-order influence parameters relating to void/crack nucleation. The finite element analyses were used to examine the role of seven independent features (number of silicon particle sites, uniformity of particle sizes which were micron size, shape of particles, additional microporosity, temperature, prestrain history, and loading conditions) in debonding and fracture of hard silicon particles in a cast A356 aluminum alloy. Owing to the wide range of features that can affect void/crack nucleation, an optimal matrix of finite element calculations is generated using a statistical method of design of experiments (DOE). The DOE method was used to independently screen the parametric influences concerning void/crack nucleation by second phase fracture or interface debonding. The results clearly show that the initial temperature was the most dominant influence parameter with respect to the others for both fracture and debonding. Experiments were then performed at three temperatures to quantify the void/crack nucleation from notch tensile specimen fracture surfaces. The data verified the importance of the temperature dependence on void/crack nucleation and showed that as the temperature decreased, the void nucleation rate increased. The Horstemeyer–Gokhale void/crack nucleation model was modified to include the temperature dependence and material constants were determined based on the experimental data. This study exemplifies a methodology of bridging various size scale analyses by sorting out the pertinent cause–effect relations from the structure–property relations.

Fatigue crack growth and fracture paths in sand cast B319 and A356 aluminum alloys

The effect of heat-treatment on the fatigue crack growth and fracture paths in lost foam sand cast B319 and A356 Al has been examined by in situ testing in a scanning electron microscope. Fatigue crack growth and fracture toughness experiments were performed at ambient temperature on B319 Al in T5, T6, and T7 conditions and on A356 in the T6 condition to characterize the crack path and the interaction of the crack-tip with the microstructure. High-resolution micrographs of the near-tip region were analyzed using a machined-vision-based stereoimaging technique, known as DISMAP, to determine the crack-tip displacement and strain fields. Selected-area energy dispersive spectroscopy was used to identify the particles in the wake of the fatigue cracks, as well as the alloy content of the Al-matrix ahead of the crack-tip. Microhardness tests revealed the presence of hard and soft zones in B319-T6 that the fatigue cracks tended to follow. The fatigue crack growth rate was reduced and the crack path altered when the crack-tip interacted with (1) localized shear bands, (2) interdendritic boundaries, (3) particle/matrix interface, (4) shrinkage pores, and (5) fractured particles. The decrease in the fatigue crack growth rate can be attributed to increasing crack-tip shielding due to crack branching and deflection. The fracture toughness of A356-T6 is higher than the B319 materials because of the absence of shear localization in the matrix.

Effects of HIPping on high-cycle fatigue properties of investment cast A356 aluminum alloys

This study is concerned with the effects of HIPping on high-cycle fatigue properties of investment cast A356 Al alloys. Tensile and high-cycle fatigue tests were conducted on cast alloys, two of which were HIPped, and then the test data were analyzed in relation with microstructures, tensile and fracture properties, and fatigue fracture mode. Eutectic Si particles were homogeneously dispersed in the matrix of the casting A356 Al alloys, but there were many large pores formed as casting defects. The high-cycle fatigue test results indicated that fatigue strength of the HIPped alloys was higher than that of the non-HIPped alloys because of the significant reduction in volume fraction of pores by HIPping. In the non-HIPped specimens, fatigue cracks initiated at large pores adjacent to the specimen surface and then propagated down to several hundreds micrometers depth while coalescing with other large pores. On the other hand, the HIPped specimens, where pores did not affect the fatigue much, fatigue cracks initiated at eutectic Si particles and propagated along them, thereby leading to improved fatigue strength by 40 to ∼50% over the non-HIPped specimens.

A Mechanism of Porosity Distribution in A356 Aluminum Alloy Castings

The problem of porosity and shrinkage defects in metal casting is complex. Over the years there have been debates on the mechanisms responsible for their formation. In this study A356 aluminum alloy with different hydrogen contents in the melts were cast in a permanent mold and the porosity content and thermal parameters were measured. A simple mechanism was proposed to explain the porosity distribution in the casting. In the mechanism both the roles of interdendritic feeding resistance by Darcy's law and kinetic hydrogen diffusion into the pore are involved. By differentiating one factor in the model, and comparing the prediction with the experimental data, the study suggests that both factors should be taken into consideration to satisfactorily explain the porosity distribution in the castings.

An experimental study on thixoforging of semi-solid materials and solutions for avoiding defects

commercial semi-solid cast aluminium alloy A356 and extruded wrought alloy A12024 were used to fabricate an arbitrarily shaped part by thixoforging. Thixoforging was conducted with four different die temperatures ( Td = 400, 350, 300 and 250°C) and two levels of applied pressure ( P = 100 and 150 MPa). The influence of process parameters on the defects in the thixoforging process of cast and wrought aluminium alloys is discussed in terms of the microstructure of each alloy. The fabricated components were heat treated to T6, and the mechanical properties were also investigated as a function of the process parameters. Various defects were observed, particularly at the lower die temperatures. It was necessary to increase the punch speed and to optimize the gate design. The tensile properties improved with increasing applied pressure.

On the driving force for fatigue crack formation from inclusions and voids in a cast A356 aluminum alloy

Monotonic and cyclic finite element simulations are conducted on linear-elastic inclusions and voids embedded in an elasto-plastic matrix material. The elasto-plastic material is modeled with both kinematic and isotropic hardening laws cast in a hardening minus recovery format. Three loading amplitudes (Δe/2=0.10%, 0.15, 0.20%) and three load ratios (R=−1, 0, 0.5) are considered. From a continuum standpoint, the primary driving force for fatigue crack formation is assumed to be the local maximum plastic shear strain range, Δγmax, with respect to all possible shear strain planes. For certain inhomogeneities, the Δγmax was as high as ten times the far field strains. Bonded inclusions have Δγmax values two orders of magnitude smaller than voids, cracked, or debonded inclusions. A cracked inclusion facilitates extremely large local stresses in the broken particle halves, which will invariably facilitate the debonding of a cracked particle. Based on these two observations, debonded inclusions and voids are asserted to be the critical inhomogeneities for fatigue crack formation. Furthermore, for voids and debonded inclusions, shape has a negligible effect on fatigue crack formation compared to other significant effects such as inhomogeneity size and reversed loading conditions (R ratio). Increasing the size of an inclusion by a factor of four increases Δγmax by about a factor of two. At low R ratios (−1) equivalent sized voids and debonded inclusions have comparable Δγmax values. At higher R ratios (0, 0.5) debonded inclusions have Δγmax values twice that of voids.

The effect of microscopic inclusion locations and silicon segregation on fatigue lifetimes of aluminum alloy A356 castings

The microstructural heterogeneity of aluminum alloy A356 permanent mold castings impacts fatigue performance differently depending on location in the casting. Casting conditions, which include casting temperature, gradients in the mold temperature, plunger speed and casting pressure are variables which can affect solidification rates and therefore, the microstructure within the casting. In this paper an experimental investigation of fatigue performance of specimens cut from various locations in a test bar of cast aluminum alloy A356 is reported. The number of cycles to failure for each test was recorded and the fracture surfaces and local microstructure examined. It was observed that local fatigue resistance varied substantially along the solidification path while tensile strength was little affected. The amount of Al–Si eutectic and the density of micropores increases along the solidification path. Samples located near the surface and the farthest from the gate end demonstrated the longest lifetimes. Conversely, the samples taken from the centerline of the casting closest to the gate end demonstrated the shortest lifetimes.

Damage influence on Bauschinger effect of a cast A356 aluminum alloy

The Bauschinger effect is interpreted as anisotropic work hardening that arises upon reverse loading from internal backstresses that are attributed to dislocations accumulating at obstacles. Studies to describe the Bauschinger effect have either focused on continuum mechanics modeling or on analyzing dislocation build-up at a microscopic scale. In continuum mechanics, the focus often is on the relationship of the ratio of kinematic (anisotropic) hardening to isotropic hardening. In materials science, the focus is often on determining mechanisms related to dislocation arrangement. Few have studied both aspects together and fewer yet have considered cast Al-Si-Mg aluminum alloys. To the author`s knowledge, no studies have examined the effect of void damage with regard to second phase particles on the Bauschinger effect for a cast A356 aluminum alloy that was mechanically tested in tension-followed-by-compression and compression-followed-by-tension up to different moderate prestrain levels.