High-pressure Die-cast Alloy Research Articles

Frequency based fatigue analysis and temperature effect

This paper proposes a temperature modified Dirlik method to estimate the high cycle fatigue damage for uniaxial loadings caused by random vibrations directly from a power spectral analysis. Besides, the methodology for combining the frequency based fatigue analysis with the temperature effect is represented. This approach is based on a new definition of loading as a random Gaussian process. The fatigue damage estimation of the high pressure die-cast aluminium alloy AlSi 9Cu 3 is investigated at elevated temperatures. Finally, numerical simulations on the known power spectral densities with different shapes at different temperatures are performed in order to establish proper dependence between the temperature modified Dirlik method, the rainflow cycle counting, the linear cumulative fatigue damage and the spectral bandwidth parameters. The proposed method enables computationally fast fatigue damage estimation for the random loadings and the temperature histories.

Atmospheric corrosion of field-exposed AZ91D Mg alloys in a polluted environment

Abstract The atmospheric corrosion behaviour of cast and high pressure die-cast AZ91D alloy in a polluted environment were investigated by SEM, XRD and FTIR. The atmospheric corrosion rate of the ingot was higher than that of the die-cast specimen. SO 2 gas played an important role in atmospheric corrosion. The effects of dew condensation on the ingot were greater than that on the die-cast specimen. The corrosion was initiated in the less noble α phase of the samples. The β phase, on the other hand, remained and acted as a barrier to corrosion.

A neural-network approach to describe the scatter of cyclic stress–strain curves

In order to predict a product’s durability in the early phases of development it is necessary to know the stress–strain behaviour of the material, its resistance to fatigue and the loading states in the material. These parameters, however, tend to exhibit a considerable degree of uncertainty. Due to a lack of knowledge of the actual circumstances in which the product is used, during the early development phase, simulations based on statistical methods are used. The results of the experiments show that the cyclic stress–strain curves demonstrate not only a large amount of scatter, but also a dependence on the temperature, the size of the cross-section, the content of alloying elements, the loading rate, etc. This article presents a method for modelling cyclic stress–strain curve scatter using a hybrid neural network for an arbitrary selection of the influencing factors. In an example of the measured data for a high pressure die-cast aluminium alloy it is clear that the suggested method is suitable for describing cyclic stress–strain curves. The main advantage of a hybrid neural network in comparison with a conventional method is the neural network’s ability to precisely describe the influence of various factors, and their combinations, based on the form and scatter of the cyclic stress–strain curve families. Defining the model parameters, i.e., training the neural network, is a procedure that does not require any additional user interventions; however, it enables us to gather knowledge that would otherwise require a lot of research. Thus, the trained neural network is a robust tool that can be used to predict cyclic stress–strain curves for random values of influencing factors. The capabilities of the presented method are only limited by the quantity of the measured data used for the neural-network training.

Rapid Heat Treatment of Aluminum High-Pressure Diecastings

Recently, it has been demonstrated that common high-pressure diecasting (HPDC) alloys, such as those based on the Al-Si-Cu and Al-Si-Mg-(Cu) systems, may be successfully heat treated without causing surface blistering or dimensional instability. In some compositions, the capacity to exploit age hardening may allow the proof stress values to be doubled when compared to the as-cast condition. This heat treatment procedure involves the use of severely truncated solution treatment cycles conducted at lower than normal temperatures, followed by quenching and natural or artificial aging. The potential therefore exists to develop and evaluate secondary HPDC alloys designed specifically for rapid heat treatment, while still displaying high castability. This article reports results of an experimental program in which responses of various alloy compositions to age hardening have been investigated with the primary aim of further reducing the duration and cost of the heat treatment cycle while maintaining high tensile properties. Composition ranges have been established for which values of 0.2 pct proof stress exceeding 300 MPa (i.e., increases of ~100 pct above as-cast values) can be achieved using a procedure that involves a total time for solution treatment plus age hardening of only 30 minutes. This rapid aging behavior is shown to be related to precipitation of the complex Q′ phase, which forms primarily when Mg contents of the alloys are above ~0.2 wt pct.

The correlation between microstructure and mechanical properties of high-pressure die-cast AM50 alloy

Scanning acoustic microscopy was used to characterize the micro-voids distribution for specimens in non-destructive mode. In addition, the in-situ scanning electron microscopy observation was performed during tensile deformation of high-pressure die-cast (HPDC) of AM50 alloy to obtain the mechanism of fracture induced by micro-voids. The effects of micro-voids on the mechanical properties were discussed. The results obtained from the examination suggest that fracture tends to occur at bigger micro-voids or in the cluster micro-voids area.

Effect of Solution Heat Treatments on the Microstructure and Mechanical Properties of a Die-Cast AlSi7MgMn Alloy

The influence of solution heat treatment time and temperature on the microstructure and mechanical properties and the mode of fracture of a high-pressure die-cast AlSi7MgMn alloy is reported. Metallographic and image analysis techniques have been used to quantitatively examine the microstructural changes occurring during solution heat treatment. A solution heat treatment of 15 minutes at 475 °C, or even more at 525 °C, is sufficient to spheroidize the eutectic Si, as well as coarsen and increase the interparticle distance of the eutectic Si. Increasing the solutionizing temperature from 475 °C to 525 °C improves the mechanical properties.

Fatigue Resistance of Heat Treated Aluminium High Pressure Die-Castings

High pressure die-casting (HPDC) is widely used as a cost-effective way to massproduce metal components that are required to have close dimensional tolerances and smooth surface finishes. Approximately 50%, by mass, of the aluminium castings produced worldwide are made by this manufacturing route. However, HPDC components are relatively porous compared with other types of castings and so cannot usually be conventionally heat treated to improve mechanical properties. This follows because during solution treatment (e.g. at 540°C for 8h), the pores expand, resulting in unacceptable surface blisters, distortion and poor mechanical properties. Recent work within the CSIRO Light Metals Flagship has revealed a heat treatment procedure by which the problems of blistering and distortion can be avoided [1]. As a result, large improvements in strength have been achieved, as compared with the as-cast condition. One uncertainty is the behaviour of heat treated HPDCs under cyclic stress and this paper investigates the fatigue properties of a common high pressure die-casting alloy, A380 (Al-8.5Si-3.5Cu). Comparisons are made between as-cast, T4 and T6 conditions. Fatigue strength is highest for the alloy aged to a T6 temper and ratios of fatigue strength to tensile strength for the as-cast, T4 and T6 conditions are constant at a value of approximately 0.6, which is particularly high for aluminium alloys.

Wear properties of high-pressure die cast and thixoformed aluminium alloys for connecting rod applications in compressors

Hypereutectic Al–Si casting alloys are attractive candidates for connecting rod applications in compressors. Several hypo- and hypereutectic Al–Si alloys were produced by high-pressure die casting and thixoforming in the present work. Hypereutectic Al–Si alloys wear less than the near-eutectic and hypoeutectic alloys under the severe wear conditions encountered in compressors, confirming the impact of Si on wear resistance. Cu also improves wear properties. The thixoformed alloys wear at comparable rates with high-pressure die cast alloys at lower Si levels, implying that the lower Si content of the former is compensated for by the thixoforming processing route. Hypereutectic compositions, uniform dispersion of fine Si particles and thixoforming as the production route are all good for a superior wear performance. The substantial hardening produced by the T6 heat treatment, however, does not translate into a sizable improvement in wear resistance of the thixoformed alloys.

Thermal characteristics of heat-treated aluminum high-pressure die-castings

High-pressure die-cast aluminum alloys cannot normally be heat treated because they contain pores that expand, causing surface blistering and dimensional change in components. Recent work has revealed a method by which heat treatment is now possible, resulting in significant improvements in the mechanical properties of these alloys that respond to age hardening. Now it has been shown that heat treatment has the additional desirable effect of improving thermal heat transfer characteristics in these die-cast alloys.

Study of fatigue crack incubation and propagation mechanisms in a HPDC AM60B magnesium alloy

Abstract The micro-mechanisms of fatigue crack nucleation and subsequent propagation in a commercial high-pressure die-cast (HPDC) AM60B alloy were studied. It was found that the majority of fatigue cracks were initiated due to the compressive residual stresses at the locations of large casting pores underneath the machined surfaces, not close to the cast skins, as expected. The propagation of the fatigue cracks depends on the alloy's microstructure, as well as the casting initiated defects during the manufacturing process. Close to the initiation sites, in a location of high local porosity, the cracks grow along the interdendritic regions. On the other hand, in the absence of the weak microstructual paths, the cracks propagate directly through the dendrite cells. Numerous randomly oriented serrated surfaces near the regions farther away from the initiation sites indicate a mixed interdendritic–transdendritic propagation mode. The final fracture zone is characterized by a high level of porosity and dimples surrounding the second phase particles. The resulting conclusions provide a summary of the parameters influencing crack propagation in the alloy.

Tensile properties of high-pressure die-cast AM60 and AZ91 magnesium alloys on microporosity variation

The effect of microporosity on the variability in the tensile properties of high-pressure die-cast AM60 and AZ91 alloys was investigated, together with a theoretical prediction based on a constitutive model. The strain rate sensitivity of both alloys was measured through an incremental strain rate change test at room temperature, and the microporosity was measured through quantitative fractography analyses on the fractured surface. The variability in the tensile strength and elongation of both alloys can be empirically described as a power law relationship in terms of the microporosity variation. The defect susceptibility of the UTS and elongation to the microporosity variation in the AZ91 alloy is slightly higher than that in the AM60 alloy. The constitutive prediction on the tensile properties of the AM60 and AZ91 alloys is in good agreement with the experimental results, and it suggests that the defect susceptibility of the tensile properties to the microporosity variation is significantly decreased with the increase of strain hardening exponent.

Thermal and metallographic characteristics of the Al–20% Si high-pressure die-casting alloy for monolithic cylinder blocks

Abstract Thermal analysis data were gathered to control the hypereutectic microstructure of the die-cast Al–20% Si cylinder block at a cooling rate of 1 °C/s. The liquidus temperature was approximately 691 ± 2.2 °C, the nucleation temperature of the Al–Si eutectic was approximately 567.1 ± 1.9 °C and the nucleation temperature of the Cu- and Mg-enriched eutectic was approximately 513.6 ± 1.4 °C. The fraction solid increases linearly from 0 to 21.6% when the temperature decreases from 691 to 567.1 °C. During the Al–Si eutectic growth the fraction solid rose to approximately 70% within a very narrow temperature range of 14 °C. The solidus temperature was recorded to be 479.9 ± 3.3 °C. The total solidification range for the investigated alloy was approximately 211 °C. The rapid quenching experiments during the alloy melting cycle revealed the dissolution of the primary Si particles with the increasing temperature followed by complete melting when the liquidus temperature were exceeded (i.e., 710.9 °C). It was found that a minimum superheat temperature of 80 °C was needed to achieve the adequate refinement of the primary Si particles as a combined effect of thermal and chemical modification. The results provided an extensive understanding of the thermal characteristics for the newly developed hypereutectic Al–20% Si alloy and might be used to optimize the liquid metal handling on the foundry floor with respect to obtaining the required alloy microstructural and service characteristics.

Identification and modeling of fatigue crack growth mechanisms in a die-cast AM50 magnesium alloy

We experimentally identified the fatigue crack growth micromechanisms operating near the limit plasticity regime in a commercial high-pressure die-cast AM50 alloy with a critical review of the available literature. An existing multistage fatigue model was modified to subsequently recognize these micromechanisms in a threefold fatigue crack growth regime. The formation of the main fatigue crack occurred almost exclusively at shrinkage pores and to a lesser extent at large Mn-rich particles. At shrinkage pores, several adjacent cracks typically incubated at the edge of flat interdendritic pores, propagated along the α-Mg dendrite cells/Al-rich eutectic interface, and rapidly coalesced into a main physically small fatigue crack that advanced through the Al-rich eutectic. In the long crack regime, the crack advanced in a mixed transdendritic–interdendritic mode along persistent slip bands spreading over several tens of dendrite cells. The model predicts well the fatigue life compared to the experimental data when these observed mechanisms are accounted for.

Reconstruction and visualization of complex 3D pore morphologies in a high-pressure die-cast magnesium alloy

Visualization and representation of three-dimensional (3D) pore morphologies in the high-pressure die-cast Mg alloys are of significant interest for understanding and modeling processing–microstructure–properties relationships. In this contribution, an efficient and unbiased montage serial sectioning technique is applied for reconstruction of large volume (∼1.24 × 10 9 μm 3) high-resolution (∼1 μm) 3D microstructure of a high-pressure die-cast AM50 Mg alloy containing gas (air) and shrinkage pores. The reconstruction of 3D microstructure from montage serial sections enables visualization and characterization of detailed 3D morphologies and spatial correlations of gas (air) and shrinkage pores. The reconstructed 3D microstructure is implemented in a 3D finite elements (FE) computational framework to illustrate how realistic complex microstructural feature morphologies can be incorporated in the simulations of micromechanical response of these high-pressure die-cast cast microstructures.

Compressive properties of high-pressure die cast IP 75 alloy with strengthening Laves phase

The NiAl–2Ta–7.5Cr–0.5Nb alloys (IP 75 alloy) were prepared by high-pressure die cast (HPDC), and tested for compressible strength and fracture behavior in the temperature range 300–1373 K. The fine structures with a homogeneous distribution of Laves phase at the boundary regions created by high-pressure die cast led to improvements in both the compressible yield strength and fracture strain. The high temperature (1373 K) 0.2% compressible yield strength of the HDC IP 75 alloy (160 MPa) is larger than that of the IP 75 alloys prepared by other processes. The room-temperature compressible fracture strain of the HDC IP 75 (14%) is also superior to the IP 75 alloy (5%) prepared by an ingot-casting process. The effects of size refinement and the more homogenous distribution of Laves phase and the formation of a ductile Cr-rich phase due to a rapid solidification contribute to the increments of the compressible yield strength and the fracture strain of the HPDC alloy.

Microstructural evolution of rheo-diecast AZ91D magnesium alloy during heat treatment

Microstructures and mechanical properties of an AZ91D magnesium alloy prepared with rheo-diecasting (RDC) were characterised in as-cast and various heat treatment conditions. The RDC alloy exhibited a unique microstructure featuring primary α-Mg globules uniformly distributed in the matrix of fine secondary α-Mg grains and β-Mg 17Al 12 intermetallic. Solution treatment at 413 °C caused microstructural evolution, involving four processes: dissolution of the β-phase; formation of a fine-grain structure; morphological change of the α globules into traditional grain shape; and grain coarsening. The RDC alloy exhibited a faster β-phase dissolution rate than a high-pressure diecast alloy. During aging at 216 °C, discontinuous precipitates were preferentially initiated at some of the grain boundaries and then continuous precipitates appeared in the grain interiors with accelerated age-hardening kinetics for the RDC alloy. A newly developed heat treatment, T x , carried out at a typical temperature and time of 365 °C and 2 h, produced an improved combination of mechanical properties compared to the conventional heat treatments studied in the present work.

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.

Utility of micro-indentation technique for characterization of the constitutive behavior of skin and interior microstructures of die-cast magnesium alloys

There has been increasing thrust lately on the development of lightweight cast magnesium alloy components for structural automotive and other applications. The microstructure of the high-pressure die-cast Mg alloys usually contains a fine-grained “skin” having a microstructure significantly different from that of the bulk material. Characterization of the local constitutive behavior of the skin microstructure is of interest as it can affect the overall mechanical response of the component. However, the standard mechanical tests on the macro-specimens are not useful for characterization of the local stress–strain response of the skin microstructure. In this contribution, we present a novel methodology based on a combination of micro-indentation experiments and three-dimensional (3D) finite elements based simulations that permits computation of the local stress–strain (constitutive) behavior of the skin and the interior microstructures at the length scales of 100 μm in a cast high-pressure die-cast AM60 Mg-alloy. The methodology involves development of a numerical solution to the inverse problem. The computed constitutive equations are then utilized to simulate the effect of skin thickness on the overall global mechanical response of the alloy under uniaxial compression.

Corrosion behaviour of high pressure die-cast and semi-solid cast AZ91D alloys

The microstructure and the corrosion behaviour of high pressure die-cast and semi-solid cast AZ91D magnesium alloys have been investigated. Semi-solid processing leads to a structure with large rounded grains of a solid solution of magnesium (α phase) whereas die-cast alloys are more homogeneous. Electrochemical measurements, particularly with impedance spectroscopy, have shown that the semi-solid cast alloy possesses a corrosion rate at least 35% below that of the die-cast alloy. This can be explained with considering the differences between the composition of the primary α phase and the volume fraction of the β phase, in the two types of alloy.

Microstructural characterization of a die-cast magnesium-rare earth alloy

Microstructural characterization of high-pressure die-cast alloy MEZ (Mg–2.5RE–0.35Zn–0.3Mn) reveals equiaxed dendrites of α-Mg with a partially divorced interdendritic eutectic. Detailed diffraction studies coupled with WDS analysis reveal the presence of a continuous Mg 12RE intermetallic phase in the eutectic aggregate along with fine Mg particles.