High Pressure Die Cast Magnesium Research Articles

Analysis of Fractures and Microstructures on Different Injection Speeds in High-Pressure Die-Casting Magnesium Alloy

In this study, to clarify the unknown physical properties of the Mg-Al-Th-RE alloy, the relationship between the injection conditions and the internal porosities, and the mechanical properties exerted by the solidification microstructure was investigated. The obtained cast samples were investigated using X-ray CT internal measurements, tensile tests, Vickers hardness tests, and solidification microstructure observations. The tensile strength and the elongation at the injection speed of 5.0 m/s were higher than at 2.0 m/s. The number of porosities affected the tensile strength and the elongation even at the same fracture position. In addition, it was confirmed that segregation affected the destruction smaller the porosity size and the greater the variability of porosity. As the injection speed increased, the amount of heat transferred between the molten metal and the wall surface also increased, resulting in quick freezing and solidification. The tensile strength increased at the injection speed of 5.0 m/s because the interface between the scattered primary crystals and eutectic systems was narrow. On the other hand, at the injection speed of 2.0 m/s, the tensile strength decreased because the molten metal was delayed in solidification and dendrite growth became remarkable.

Recent developments in high-pressure die-cast magnesium alloys for automotive and future applications

The use of magnesium alloy high pressure die cast (HPDC) components for structural applications, especially in the automotive and transportation industries, where weight reduction is of a great concern, is increasing. As new applications are developing and existing applications are becoming more complex, there is a need for improved properties from magnesium HPDC alloys. This paper reviews the recent developments in HPDC magnesium alloys for transportation applications. Compared to the conventional HPDC magnesium alloys, i.e. AZ91D, AM50A/AM60B and AE44, these new alloys have one or more of the following properties: higher strength, higher ductility, superior high-temperature properties or higher thermal conductivities. In this work, characteristics which are important in product manufacturing or product performance will be evaluated and discussed, including die castability of powertrain component or thin-walled structural component, mechanical properties at elevated temperatures and ductility. Results indicate that these alloys have great potentials to be added to the current magnesium HPDC alloy family and being used in actual automotive and other transport applications.

Influence of Defects on Deformation Behavior of High-Pressure Die-Casting Magnesium Alloys

Influence of Defects on Deformation Behavior of High-Pressure Die-Casting Magnesium Alloys

An ICME Method for Predicting Phase Dissolution During Solution Treatment in Advanced Super Vacuum Die Cast Magnesium Alloys

An integrated computational materials engineering (ICME) methodology was applied in this study to systematically and quantitatively study the second phase dissolution kinetics during the solution treatment process of a high pressure die cast magnesium sample. The study was conducted on Mg–9 wt% Al, Mg–5 wt% Al, and Mg–11 wt% Al binary alloys after isothermal solution treatments ranging from 380 to 420 °C. The experimental measurements revealed an exponential decrease of the volume fraction of the second β-phase (Mg17Al12) during the solution treatment. A CALPHAD-based computational tool (Thermocalc DICTRA Diffusion Module) was used to simulate the dissolution process. In this study, measurements from as-cast samples were used as input parameters to improve the accuracy of the predicted results. An analytical, physics-based micro-model, based on the Johnson–Mehl–Avrami type equation, was also applied to study the dissolution kinetics. Both the simulation and micro-model quantitatively agree with the experimental results at all solution treatment temperatures. Calibration and verification of a few input parameters help to understand the assumptions made in the modeling, improve the accuracy of the prediction, and can be applicable in different Mg–Al binary alloys. Adopting such an ICME methodology for modeling development and verification in predicting the dissolution kinetics during solution treatment will allow for more rapid optimization of solution treatments procedures to be used in industrial applications.

An integrated two-dimensional modeling method for predicting ductility of thin-walled die cast magnesium

In this paper, a two-dimensional (2D) microstructure-based modeling method is developed in order to predict the ductility of a thin-walled high pressure die cast magnesium (Mg) by considering the three-dimensional (3D) thru-thickness pore distributions. For this purpose, a series of 3D synthetic microstructure-based finite element models and the corresponding 2D models are first generated with various pore volume fractions, pore size distributions and pore shapes. The input material properties for the 2D models are determined based on the 3D cubic model with a spherical pore and the generalized Neuber’s rule. Based on the resulting ductility of the 3D and 2D models, a 3D/2D ductility correlation curve is developed as a function of the characteristics/shape of the input fracture strain curve used in the 3D model. The validity of the ductility correlation curve is examined with casting samples with actual microstructures. The results show that the suggested 2D modeling methodology can be used together with the ductility correlation curve in predicting the ductility of thin-walled Mg castings.

Microstructures and mechanical properties of a high pressure die-cast Mg–4Al−4Gd−0.3Mn alloy

A new high pressure die-cast (HPDC) magnesium (Mg) alloy with high performance, namely Mg–4Al–4Gd–0.3Mn (AGd44), was developed by using the heavy rare earth element Gd. The microstructures under as-cast and peak-aged states were investigated in detail, especially by transmission electron microscopy (TEM). The studied alloy has finer cells although clearly less intermetallic phases than the conventional Mg–Al–RE based alloys. However, the structures of intermetallic phases in AGd44 alloys are fairly complicated. Most of the blocky phases are Al2Gd while minor of them are Al10Gd2Mn7 and Al8GdMn4, and the petaloid and the lamellae phases are Al2Gd and Mg17Al12, respectively. Peak-aging treatment can further enhance the yield strength of the studied alloy, which is mainly attributed to more coarse lamellae Mg17Al12 phase and fine lath-shaped Mg17Al12 precipitate appearing at cell boundaries and in α-Mg cells, respectively. Furthermore, the relatively smaller cells as well as the novel intermetallic phase component contribute to the more excellent strength-ductility balance of the studied alloy than the commercial/experimental HPDC Mg–Al-based alloys. The results provide new insight into developing HPDC Mg alloys with high mechanical properties.

Strain Rate during Creep in High-Pressure Die-Cast AZ91 Magnesium Alloys at Intermediate Temperatures.

During creep, magnesium alloys undergo microstructural changes due to temperature and stress. These alterations are associated with the evolution of the present phases at a microstructural level, creating different strain rates during primary and tertiary creep, and with the stability of the inter-metallic phase Mg17Al12 formed at these temperatures. In this paper, the results of creep testing of high-pressure die-cast AZ91 magnesium alloys are reported. During creep, continuous and discontinuous precipitates grow, which influences creep resistance. The creep mechanism that acts at these intermediate temperatures up to 150 °C is termed dislocation climbing. Finally, the influence of the type of precipitates on the creep behavior of alloys is determined by promoting the formation of continuous precipitates by a short heat treatment prior to creep testing.

Effects of heat treatment on the thermal properties of AZ91D magnesium alloys in different casting processes

The thermal diffusivity and specific heat capacity of semi-continuous casting (SC) and high-pressure die-casting (HPDC) magnesium alloys before and after T4 and T6 heat treatments were measured at room temperature, and their thermal conductivity was calculated. The effects of heat treatments on the thermal properties of AZ91D alloys in different casting processes were investigated, and the mechanism of the effects was also discussed. Before heat treatment, the HPDC alloys had higher thermal conductivity than that of the SC alloys. Moreover, heat treatment after forming had a remarkable influence on the thermal properties of the SC and HPDC alloys. T4 treatment had negative effects on the thermal conductivity of the SC and HPDC AZ91D alloys, and the T6 heat-treatment SC and HPDC alloys had higher thermal conductivity than that of the T4 treatment alloys.

Quantitative Experimental Study of Defects Induced by Process Parameters in the High-Pressure Die Cast Process

A quantitative experimental study of the effects of process parameters on the formation of defects during solidification of high-pressure die cast magnesium alloy components is presented. The parameters studied are slow-stage velocity, fast-stage velocity, intensification pressure, and die temperature. The amount of various defects are quantitatively characterized. Multiple runs of the commercial casting simulation package, ProCAST™, are used to model the mold-filling and solidification events. Several locations in the component including knit lines, last-to-fill region, and last-to-solidify region are identified as the critical regions that have a high concentration of defects. The area fractions of total porosity, shrinkage porosity, gas porosity, and externally solidified grains are separately measured. This study shows that the process parameters, fluid flow and local solidification conditions, play major roles in the formation of defects during HPDC process.

Modeling studies on divorced eutectic formation of high pressure die cast magnesium alloy

The morphology and content of the divorced eutectic in the microstructure of high pressure die casting (HPDC) magnesium alloy have a great influence on the final performance of castings. Based on the previous work concerning simulation of the nucleation and dendritic growth of primary α-Mg during the solidification of magnesium alloy under HPDC process, an extension was made to the formerly established CA (Cellular Automaton) model with the purpose of modeling the nucleation and growth of Mg-Al eutectic. With a temperature field and solute field obtained during simulation of the primary α-Mg dendrites as the initial condition of the modified CA model, modeling of the Mg-Al eutectic with a divorced morphology was achieved. Moreover, the simulated results were in accordance with the experimental ones regarding the distribution and content of the divorced eutectic. Taking a cover-plate die casting with AM60 magnesium alloy as an example, the rapid solidification with a high cooling rate at the surface layer of the casting led to a fine and uniform grain size of primary α-Mg, while the divorced eutectic at the grain boundary revealed a more dispersed and granular morphology. Islands of divorced eutectic were observed at the central region of the casting, due to the existence of ESCs (Externally Solidified Crystals) which contributed to a coarse and non-uniform grain size of primary α-Mg. The volume percentage of the eutectic β-Mg17Al12 phase is about 2%-6% in the die casting as a whole. The numerical model established in this study is of great significance to the study of the divorced eutectic in the microstructure of die cast magnesium alloy.

Friction Stir Processing at High Rotation Rates of a Magnesium Alloy: Mechanical Properties at High Temperatures and Microstructure

A high-pressure die-cast magnesium alloy plate was friction stir processed at high rotation rates with different advancing speeds. The stirred zone was very narrow around the tool and this made the friction stir process difficult to occur in the whole thickness of the plate. Intermetallic-phase network at grain boundaries was refined due to partial dissolution and fragmentation of Mg17Al12 β-phase during the friction stir process; the likely increment of solute content in solid solution was exploited for aging to improve hardness. The ductility of friction stir processed samples deformed at 300° and 350°C substantially increased compared to the base material and to room temperature strained samples.

The Effects of Interfacial Heat Transfer Coefficient on the Microstructure of High-Pressure Die-Cast Magnesium Alloy AM60B

This paper describes the details of a quantitative experimental and numerical study on the influence of solidification conditions, including the apparent interfacial heat transfer coefficient (IHTC) between the die and solidifying metal, on the resulting local microstructure. Multiple runs of the commercial casting simulation package, ProCASTTM, are used to model the mold filling and solidification events employing a range of IHTC values. The simulation results are used to estimate the centreline cooling curve at various locations through the casting. The centreline cooling curve, together with the die temperature and the thermodynamic properties of the alloy are then used as inputs to compute the solution to the Stefan problem of a moving phase boundary, thereby providing the through-thickness cooling curves at each chosen location of the casting. Finally, the local cooling rate is used to calculate the resulting grain size and skin thickness via previously established relationships. A comparison of the predicted and experimentally determined grain size profiles enables the determination of the apparent IHTC, which, in this study, was approximately 12000 W/m2·K. Additional useful observations from the numerical study suggest that the IHTC has a significant influence on the skin thickness and grain size in both the skin and core regions of the casting, while the effect of die temperature is limited to influencing the skin grain size only.

Deep cryogenic treatment of HPDC AZ91 magnesium alloys prior to aging and its influence on alloy microstructure and mechanical properties

Studies of deep cryogenic treatment have mainly focused on ferrous alloys, despite the advantages of this form of treatment when applied to other non-ferrous alloys. This study examines the microstructural behavior of a high pressure die-casting (HPDC) AZ91 magnesium alloy, submitted to T6 and T6 with deep cryogenic treatment (DCT) prior to aging. Differences between continuous and discontinuous β-Mg17Al12 phase precipitation are analyzed by means of TEM, SEM and optical microscopy to explain the mechanical behavior following each type of heat treatment. The mechanical properties were improved by both treatments though yield strength was higher following T6 treatment and elongation was greater following T6 with deep cryogenic treatment. The conclusion is that continuous precipitation is promoted by cryogenic treatment, resulting in an improvement of elongation by 20%. This factor is important in view of the limitations of poor plasticity and its consequences for HPDC magnesium alloy applications.

Influence of Porosity in the Fatigue Behavior of the High-Pressure Die-Casting AZ91 Magnesium Alloys

The fatigue properties of high-pressure die-casting (HPDC) magnesium (Mg) alloys AZ91 exhibit a high variability, due primarily to the porosity that is inherent in the injection process. In the 94% of the studied samples, the porosity in which crack nucleation originates is at the surface or adjacent to the surface. The threshold stress intensity factor amplitude and the limit of fatigue have been calculated following the classical models of parameterization of defects. A new set of samples were prepared by machining the surface slightly, in order to conserve the microstructure, and the fatigue behavior at low level of stress was improved. All the samples were produced in molds with the final shape by HPDC process, which allowed a realistic study of the surface effect and the influence of grain size variation from the edge to the center of the samples.

Predicting the Influence of Microporosity on the Mechanical Properties and Fracture Behavior of High-Pressure Die-Cast AM50 Magnesium Alloy

In this paper, experimental and finite element modeling methods were adopted to investigate the effects of microporosity on the tensile properties and fracture behavior of high-pressure die-casting (HPDC) AM50 alloy. By specimen-to-specimen fractographic analysis, the variability in tensile properties could be quantitatively correlated with the areal fraction of the porosity presented in the corresponding fracture surfaces by using a simple power law equation. Numerical models of synthetic microstructures with different pore sizes, areal fractions of pores and pore distributions were established. Based on the experimental and numerical simulation results, it could be concluded that the fracture will initially occur in the region where has the highest intensity of equivalent stress field (i.e., contains the most highly localized cluster of pores and shrinkage), and then, fracture crack will fast propagate through the adjacent regions which have the relatively high intensity of stress field.

Effect of Friction Stir Processing on Microstructure and Mechanical Properties of a HPDC Magnesium Alloy

A high pressure die cast (HPDC) magnesium alloy was friction stir processed (FSP) at high rotation rates with different advancing speeds. The AZ91 plate was 3 mm thick and the pin had a truncated cone tip. The friction stir processing induced the disappearance of porosity (typical of the HPDC process) in the nugget zone but some tunnel defects were introduced. The presence of characteristic FSP zones is not obvious due to the low plastic deformation of this alloy. The stirred zone is very narrow around the tool and this makes the FSP difficult to occur in the whole thickness of the plate. Microhardness values and electrical conductivity are sensitive to stirred zone and differences arise according to FSP parameters. The grain size is refined and homogenized by FSP due to partial solubilisation and disappearance of the eutectic phase surrounding Mg-alpha. X-rays diffractometry was performed on FSP samples to index phases and calculate peak shifts of Mg-alpha.

Predicting the flow stress of high pressure die cast magnesium alloys

In the present study, we describe the characterization of three magnesium alloys, AM60, AZ91 and AE44, which were processed by high pressure die casting (HPDC) and gravity casting. Spherical microindentation was used to analyze the influence of microstructural features on the flow stress for both skin (finer grain sizes) and core (larger grain sizes and dendrites) regions of HPDC, as well as different regions of gravity step-cast plate for all three magnesium alloys. It was observed that the local yield stress and flow stress of magnesium alloys depend on grain size. The Hall–Petch slopes determined from indentation testing compared well with the results obtained from uniaxial tensile testing and literature. The Hall–Petch equation was shown to be applicable for predicting the yield strength and the flow stress at several levels of plastic strains.

Heat treatment of vacuum high pressure die cast magnesium alloy AZ91

Alloys produced by high pressure die casting (HPDC) are generally considered non-heat treatable because trapped gas pores tend to expand, causing surface blistering and bulk distortion. In this paper, vacuum assisted HPDC of magnesium alloy AZ91 was used, and the properties were assessed. The specimens produced using vacuum die casting contain less porosity. Little improvement in yield strength by applying vacuum is found, although a small increase in elongation is observed. A conventional heat treatment applied to the vacuum die cast AZ91 shows pronounced precipitation hardening during aging, especially after a prior solution treatment. However, an associated improvement in yield strength after aging is not observed, and this is related to the decreased contribution of the ‘skin’ effect as a result of grain growth.

Prediction of knit line formations in complex high pressure die cast magnesium alloy components

This work investigates the formation of as cast defects in knit line regions and their effects on local mechanical properties in a thin walled high pressure die cast AM60B magnesium alloy component. The defect distributions and the tensile and bending properties of specimens cut from knit line and other regions are compared. Numerical simulations of the mould filling process, computed X-ray tomography experiments and reports from the literature are used to formulate an initial explanation for the occurrence of metal front dispersion in knit line regions. This is an initial study for a future project investigating knit line formation and process–structure correlations in cast magnesium alloys.

Dynamic deformation of high pressure die-cast magnesium alloys

The dynamic behavior of the die-cast Mg-6 % Al-0.5 % Mn and Mg-9 % Al-1 % Zn alloys has been investigated in a wide range of temperatures and compared to the quasi-static behavior. It has been found that, at high strain rates, these alloys possess very good energy absorption capacity and that twinning is significantly enhanced. The small variability in the tensile ductility at dynamic rates is attributed to the independence of the adiabatic shear failure mechanism of the porosity distribution.