Externally Solidified Crystals Research Articles

Modifying the Microstructure and Mechanical Properties of Non-Heat Treated HPDC AlSi10MnMg Foundry Alloy via Incorporation of TiB<sub>2</sub> Particles and Sc

The demand for structural lightweight in a variety of industries, particularly the automobile industry, has driven the development of heat-free die-cast aluminum alloys with excellent properties. Utilizing lightweight materials, such as Al-Si alloys has several benefits, including higher overall performance in automobiles and other industries, increased heat resistance efficiency, decreased emissions, and reduced weight. The purpose of this study is to modify the microstructure and enhance the mechanical properties of high-pressure die-casting (HPDC) AlSi10MnMg foundry alloy by incorporation of TiB2 and Sc without any heat treatment. The results showed that the HPDC process significantly refines the grain structure and AlSiMnFe intermetallic compounds, transforming the eutectic morphology from sharp to rounded, and 93% enhancement in elongation at the optimum content (0.018 wt.%) of TiB2. While the hardness of the alloy was improved by 15.7% with the addition of 0.03wt.% TiB2. TiB2 incorporation refines the grain structure and AlSiMnFe phases, while depressing externally solidified crystals (ESCs). The HPDC process refines Al3Sc phases as well as AlSiMnFe phases while increasing yield strength due to Al3Sc strengthening effects. After 0.5wt.% Sc addition in 0.018wt.% TiB2-AlSi10MnMg alloy, the YS, and EL reached the maximum of 196MPa and 9.93% respectively.

Formation mechanism of the microstructural heterogeneity in a die-cast Al-Mg-Si alloy and its effect on mechanical properties

Al-Mg-Si series alloys fabricated by high pressure die casting (HPDC) have a broad application prospect in the production of large-scale and thin-walled die-castings. However, the die-cast Al-Mg-Si series alloys usually exhibit obvious microstructural heterogeneity. This study investigated the microstructural heterogeneity of the die-cast Al-7 Mg-3Si alloy along the thickness direction, and the corresponding formation mechanism was verified. In this study, the die-cast sample can be divided into four distinct regions (R1 ~ R4) from the surface to the center in turn according to the area fraction of the primary α-Al. Two aspects were considered to be the main reasons for the formation of microstructural heterogeneity. On the one hand, the externally solidified crystals (ESCs) preferentially formed in the shot sleeve and aggregated in the casting center. On the other hand, the cooling rates were quite different in each region, leading to different solidification behaviors. Based on the combination of the microstructural observation and calculated cooling rates, a physical model has been proposed to describe the formation mechanism of the microstructural heterogeneity. The heterogeneous microstructure of the die-cast sample resulted in the inhomogeneity of mechanical properties along the thickness direction. The R2 exhibited the highest micro-hardness than the other regions because of its highest amount of eutectics. Moreover, the casting surface with fine grains (R1) is conducive to improving the EL, whereas the center (R4) showed the lowest tensile properties due to the existence of ESCs and porosity band.

The characterization of porosity and externally solidified crystals in a high pressure die casting hypoeutectic Al-Si alloy using a newly developed ceramic shot sleeve

The effective methods to regulating porosity and externally solidified crystals (ESCs) have been remained as a challenging task for high pressure die casting (HPDC) automobile parts. In this work, a newly designed ceramic shot sleeve is proposed for reducing porosity and optimizing ESCs. As a result, both porosity and ESCs achieve an effective reduction after applying the ceramic shot sleeve. This study provides insights into regulating porosity and coarse ESCs of die castings and it is expected to expand its application in industrial manufacture.

Microstructure heterogeneity optimization of HPDC Al-Si-Mg-Cu alloys by modifying the characteristic of externally solidified crystals

The microstructure heterogeneity of high pressure die casting (HPDC) Al-8Si-0.5Mg-0.2Cu-2Zn alloy was modified by optimizing the characteristic of externally solidified crystals (ESCs), and their effect on mechanical behavior were also discussed. The results revealed that the formation of microstructure heterogeneity in HPDC alloys was significantly depended on the characteristic of ESCs. In addition, the characteristic of ESCs could be modified by the melt superheat treatment before slow shot process. The morphology of ESCs is dendritic and the distribution show serious segregation at pouring temperature of 685 °C and 725 °C, but evolved to regular and uniformly at pouring temperature of 705 °C. The uniform distribution and regular ESCs leads to uniform stress concentration during tensile deformation at pouring temperature of 705 °C. The cracks mainly initiated at interfacial region of the clustered ESCs, and the obtained yield strength161MPa and ultimate tensile strength 307 MPa, meanwhile, the elongation maintain a high value of 11.4%.

Synergistic effect of externally solidified crystals and Fe-rich intermetallic on the fracture behavior of HPDC alloy

The synergistic effect of externally solidified crystals (ESCs) and Fe-rich intermetallic on the fracture behavior of high pressure die casting Al–8Si-0.4 Mg–2Zn alloy was investigated using in-situ tensile test. The results revealed that the localized stress concentration during tensile test were significantly depended on the characteristic of ESCs and Fe-rich intermetallic. The crack preferentially initiated in the region where fine Fe-rich phase was surrounded by the clustered ESCs, this mainly attributed to the inconsistency of stress distribution in the clustered ESCs during tensile loading. Meanwhile, the primary Fe-rich phase with high aspect ratio is also beneficial to the crack initiation. The cracks were propagated both in the grain boundary and internal region of ESCs. The final fracture occurred in both the interfacial region between ESCs and other intermetallic and the internal region of ESCs.

Effect of Zn Addition on the Microstructural Heterogeneity and Mechanical Properties of Vacuum‐Assisted High‐Pressure Die Casting Al–Si–Mg–Cu alloys

Herein, Al–8Si–0.5Mn–0.5 Mg–0.2Cu–xZn alloys (x = 0, 1, 2, and 3 wt%) are produced by a vacuum‐assisted high‐pressure die casting process. The effects of Zn addition on the surface layer thickness and segregation band, the number density of externally solidified crystals (ESCs), and mechanical properties of the alloys are systemically investigated. As the Zn content increases, the size and number density of the ESCs increase. The morphology and size of the eutectic Si phase could be modified by the addition of Zn. In addition, the microstructural heterogeneity indicates that the thickness of the surface layer and segregation band zone increase with increasing Zn content. Proper Zn addition can effectively promote the improvement of strength and elongation in the as‐cast state. When the Zn content is 2 wt%, the yield strength, ultimate tensile strength, and elongation of the alloy are 167, 311 MPa, and 10.8%, respectively.

The effect of slow shot speed and casting pressure on the 3D microstructure of high pressure die casting AE44 magnesium alloy

3D reconstruction and 2D observation were conducted to characterize the microstructure of the castings produced through high pressure die casting with different parameters. Our results indicate that shrinkage pores generally co-existed with externally solidified crystals (ESCs). In specimen produced without fast slow shot speed, big net-shrinkage pores accompanied with ESCs were found in the center of the specimen. When the casting pressurization was introduced, the shrinkage pores gathered to the specimen center and became much less due to the optimization of melt feeding. Much more porosity was found near the gate rather than in the middle of the rod bar, especially gas pores. The filling process simulation reveals that the middle position of the bars was firstly filled and followed by the near gate position accompanied with one intense turbulent flow.

Defect band formation in high pressure die casting AE44 magnesium alloy

The characteristics of defect bands in the microstructure of high pressure die casting (HPDC) AE44 magnesium alloy were investigated. Special attention was paid to the effects of process parameters during the HPDC process and casting structure on the distribution of defect bands. Results show that the defect bands are solute segregation bands with the enrichment of Al, Ce and La elements, which are basically in the form of Al11RE3 phase. There is no obvious aggregation of porosities in the defect bands. The width of the inner defect band is 4–8 times larger than that of the outer one. The variation trends of the distribution of the inner and outer defect bands are not consistent under different process parameters and at different locations of castings. This is due to the discrepancy between the formation mechanisms of double defect bands. The filling and solidification behavior of the melt near the chilling layer is very complicated, which finally leads to a fluctuation of the width and location of the outer defect band. By affecting the content and aggregation degree of externally solidified crystals (ESCs) in the cross section of die castings, the process parameters and casting structure have a great influence on the distribution of the inner defect band.

Simulation of α-Al grain formation in high vacuum die-casting Al-Si-Mg alloys with multi-component quantitative cellular automaton method

Al-Si-Mg alloys are the most commonly used material in high vacuum die-casting (HVDC), in which the morphology and distribution of α-Al grains have important effect on mechanical properties. A multi-component quantitative cellular automaton (CA) model was developed to simulate the microstructure and microsegregation of HVDC Al-Si-Mg alloys with different Si contents (7% and 10%) and cooling rates during solidification. The grain number and average grain size with electron backscatter diffraction (EBSD) analysis were used to verify the simulation. The relationship between grain size and nucleation order as well as nuclei density was investigated and discussed. It is found that the growth of grains will be restrained in the location with higher nuclei density. The influence of composition and cooling rate on the solute transport reveals that for AlSi7Mg0.3 alloy the concentration of solute Mg in liquid is higher at the beginning of eutectic solidification. The comparison between simulation and experiment results shows that externally solidified crystals (ESCs) have a significant effect for samples with high cooling rate and narrow solidification interval.

Modelling flow-induced microstructural segregation in semi-solid metals

Lightweight alloys produced by pressurized castings suffer from flow-induced microstructural segregation due to the migration of externally solidified crystals (ESCs). Although some qualitative research has been carried out on ESCs migration, quantitative prediction of it is still demanded. In this respect, an ESCs migration model based on hydrodynamics and the rheological behaviour of semi-solid metals (SSMs) is proposed to estimate flow-induced microstructural segregation. By employing the model, a plug flow region having a decisive influence on ESCs migration is observed according to the flow simulation of yield-pseudoplastic SSMs. Good agreement between the experimental data and the simulation results is obtained, suggesting that such flow-induced microstructural segregation is controllable by manipulating process parameters such as solid fraction, cavity size and filling velocity. A real-time in situ synchrotron X-ray radiography experiment is carried out to reveal the nature of ESCs migration and provide evidence for the hypothesized flow-induced ESCs migration mechanism. A semi-empirical solid fraction-related rheological (SFR) model capable of relating rheological parameters with solid fraction of SSMs is also proposed, enabling the optimization of process parameters.

Characterization of externally solidified crystals in a high-pressure die-cast AlSi10MnMg alloy and their effect on porosities and mechanical properties

The characterization of externally solidified crystals (ESCs) and porosities in a high-pressure die-cast AlSi10MnMg alloy was investigated using optical microscope (OM) and computed tomography (CT) with particular attention on the effect of shot speeds on the ESCs and fracture behavior. Results showed that ESCs tended to agglomerate at the center region and changed from fine globular grains into large size dendrites from surface to center. The formation of the ESCs is found to be coarser as the "slow shot speed" in the first stage of high-pressure die casting (HPDC) or the "fast shot speed" in the second stage of HPDC is reduced, consequently, the corresponding ESC area fraction increased. From the fractured morphology, the plate specimen with a higher ESC content exhibited a rough fracture and a large volume fraction of shrinkage porosities was discovered beneath the fracture surface especially when a lower fast shot speed was applied. In in-situ tensile test, the large size shrinkage porosity served as a crack source and it further expanded in an inter-granular mode along ESC boundaries/other porosities (which caused a tortuous crack route) or in a trans-granular mode across the ESC grains/fine (α-Al) II grains (which caused a flat crack route). In addition, the gas porosities in the specimen promoted the crack propagation as a crack concentration area and experienced almost no deformation during the tensile process.

Influence of different high pressure die casting processes on 3D porosity distribution of Mg-3.0Nd-0.3Zn-0.6Zr alloy

3D reconstruction was adopted to characterize the microstructural morphologies of Mg-3.0Nd-0.3Zn-0.6Zr alloy castings produced by high pressure die casting (HPDC) processes with different parameters, including low slow-shot speed, solidification pressurization and fast slow-shot speed. At low slow-shot speeds of 0.1 m·s−1, 0.2 m·s−1 and 0.3 m·s−1, the porosity is concentrated in the center of the castings with one spiral staggered shape along the liquid flow direction. The porosity volume simultaneously decreases with the reduction of quantity and size of externally solidified crystals (ESCs), while the shrinkage pores become more and more dispersed with the increasing low slow-shot speed. Pressurization not only reduces the porosity volume due to the improvement of feeding ability, but also transformes the center gathered porosity into one layer-by-layer distribution form. Accompanied with the increasing fast slow-shot speed, the central porosity dramatically decreases and transforms into a large-scale spiral staggered shape along the liquid flow direction. However, the porosity is much more dispersed when the speed is increased from 2 m·s−1 to 3 m·s−1.

Correlation of 3D defect-band morphologies and mechanical properties in high pressure die casting magnesium alloy

For understanding the processing-microstructure-properties correlation of high pressure die casting (HPDC) AZ91D alloy, 3D reconstruction was carried out to characterize the microstructural morphologies of castings produced by four designed HPDC processes. The influence of different parameters (vacuum, slow-shot speed, fast-shot speed) on porosity volume and morphology, defect band width and average externally solidified crystals (ESCs) size of castings were systematically studied. Microstructural characterization revealed that the defect band width was strongly related with the size and quantity of ESCs, rather than the porosity volume. Meanwhile, ESCs were mainly influenced by the slow-shot speed in shot sleeve (ESCs growth time) and fast-shot speed into the die cavity (shear stress of melt flow). Tensile fracture revealed that cracks easily propagated within the defect bands due to the accumulated ESCs and connected irregular pores, while the cracks were restrained in center zone. The mechanical properties of HPDC castings was inversely proportional to the defect band width, and is not dependent on the average volume fraction porosity.

The gradient microstructure and deformation heterogeneity in HPDC AM60 alloy

As an important manufacturing process for Mg-alloy components, high pressure die casting (HPDC) has attracted considerable attention. However, the application of magnesium alloy high pressure die castings is limited by variations in the mechanical properties, which can be ascribed to a heterogeneous distribution of microstructure. In this work, the through-thickness distribution of the microstructure features (i.e. grain size, β-phase and porosity) of HPDC AM60 alloy and their effects on the strain heterogeneity and the microhardness variation were carefully investigated. The results show that the HPDC AM60 alloy exhibited a gradient structured microstructure, i.e., two skin layers sandwiching a core layer. The skin was mainly comprised of fine grains (diameter ≤10 μm), whereas the core predominately consisted of externally solidified crystals (ESCs). Furthermore, the skin shows higher number and area fractions of β-Mg17Al12 particles. Heterogeneous through-thickness distribution of microhardness was observed in the as-cast sample, and the skin showed higher the microhardness than the core. β-Phase was the primary factor accounting for heterogeneous distribution of microhardness. In the as-cast sample, strain heterogeneity was measured at strain of 3% using digital image correlation (DIC) method. The strain in the skin regions was homogeneous, whereas significant strain heterogeneity was presented in the core. After T4 treatment, remarkable strain heterogeneity was still observed, indicating that β-phase did not have profound effect on deformation heterogeneity. Strain localized at the interdendritic regions of ESCs and the vicinity of grain boundaries in fine-grained regions, where porosity occurred. This illustrates that porosity was the primary factor inducing strain localization.

Fluidity of Alloys Under High-Pressure Die Casting Conditions: Flow-Choking Mechanisms

Theories on fluidity of alloys based on the solidification mode are not satisfactory in describing fluidity of alloys under high-pressure die casting (HPDC) conditions. To understand the flow-choking mechanisms under HPDC conditions, microstructure in the fluidity test coupons made using a 125-ton HPDC machine was characterized. Pre-solidified dendrites (PSDs), or externally solidified crystals (ESCs), which were formed in the shot sleeve, were found in the runners as well as in the fluidity casting. Surprisingly, a large amount of PSDs are collected in the runner adjacent to the in-gate, forming a PSD core with a thin layer of PSD-less or PSD-free region near the surfaces of the runner due to Magnus effect. Analytical calculations were performed to estimate the pressure drop for molten aluminum flowing through the mushy PSD zone. The results indicate that the pressure drop is comparable to the maximum pressure that was used for injecting molten metal to fill the casting. When the pressure drop is equal to the pressure driving the mold filling, the metal ceases to flow. Thus, it is the PSDs that are responsible for the choking of mold filling either mechanically at the in-gate or providing a pressure drop high enough to resistant fluid flow.

On the characterization of microstructure and fracture in a high-pressure die-casting Al-10 wt%Si alloy

Microstructure and fracture behavior in a high-pressure die-casting Al-10 wt%Si alloy have been investigated using optical microscope (OM), scanning electron microscope (SEM) and a high-resolution laboratory computed tomography (CT). The results showed that a typical heterogeneous microstructure of the alloy comprised α-Al rich region, eutectic silicon band region and porosity. The microstructure patterns highly dependent on fluid convection and rapid solidification. Under high filling speed, externally solidified crystals (ESCs) and the growing dendrites migrated in center and formed α-Al rich region. Si particles was discharged and enriched in the final solidified liquid, forming eutectic silicon band. Hard Si particles and brittle Fe-rich phases served as obstacles prevented dislocation migration, causing local stress concentration. Due to large movable slip systems in α-Al rich region, the propagation path of the crack was greatly extended. Net-shrinkage that induced by dense impinging dendrites led to the microcracks along the boundary of dendrites which promoted intergranular fracture.

Al8Mn5 in High-Pressure Die Cast AZ91: Twinning, Morphology and Size Distributions

Manganese-bearing intermetallic compounds (IMCs) are important for limiting micro-galvanic corrosion of magnesium-aluminum alloys and can initiate cracks under tensile load. Here, we use electron backscatter diffraction (EBSD), deep etching, and focussed ion beam (FIB) tomography to investigate the types of Al-Mn phases present, their faceted growth crystallography, and their three-dimensional distribution at different locations in high-pressure die cast (HPDC) AZ91D. The Al-Mn particle size distributions were well-described by lognormal distributions but with an additional population of externally solidified crystals (ESCs) formed in the shot chamber analogous to α-Mg ESCs. The large Al8Mn5 particles were cyclic twinned. Differences in the particle size distributions and number density in the center compared with the HPDC skin are identified, and the spatial relationship between Mg17Al12 and Al-Mn particles is explored.

Effect of runner design on the externally solidified crystals in vacuum die-cast Mg-3.0Nd-0.3Zn-0.6Zr alloy

The microstructure and tensile properties of a vacuum-assist high-pressure die casting (HPDC) Mg-3.0Nd-0.3Zn-0.6 Zr alloy and the effect of runner design on the externally solidified crystals (ESCs) were investigated. The microstructure of the alloy mainly comprised ESCs (large primary α-Mg grains) formed in the chamber, small primary α-Mg grains crystalized in the cavity, divorced eutectic Mg12Nd and a small amount of Mg12(NdxZn1-x) or Mg12(NdxZnyZr1-x-y) phases in the region near the shrinkage. The average ESCs grain size decreased as the distance from the center increased remarkably. The employment of ESCs collector contributed to the depression of ESCs grain size, particularly in center region. On the contrary, the grain size of small primary α-Mg has overall slightly downward trend from center to skin region, and the size difference was small. The employment of an ESCs collector effectively decreased the number and size of the ESCs, and accordingly both strength and elongation of the alloy were improved. This specially design ESCs collector improve the runner system and optimize the mold design. The elongation of the alloy approached 10%.

Influence of melt flow in the gating system on microstructure and mechanical properties of high pressure die casting AZ91D magnesium alloy

The influence of melt flow in the gating system on microstructure and mechanical properties of high pressure die casting AZ91D magnesium alloy was investigated. Experimental and theoretical results showed that the externally solidified crystals (ESCs) were broken and eliminated by the shear stress caused by the turbulent melt flow in gating system. In comparison with the casting fabricated with conventional gating system, a simultaneous increase in both strength (~ 16% higher ultimate tensile strength) and ductility (~ 70% higher elongation) were achieved in the casting fabricated with modified gating system. Furthermore, different modes of crack initiation and propagation were found. For the one fabricated with the conventional gating system, the crack initiated at the shrinkage pore and propagated in a quasi-cleavage fracture form. In contrast, for the other one fabricated with the modified gating system, micro-voids appeared between the α-Mg grains and β-Mg17Al12 phase, and crack propagated in a ductile form.

Effects of process parameters on morphology and distribution of externally solidified crystals in microstructure of magnesium alloy die castings

During the cold-chamber high pressure die casting (HPDC) process, samples were produced to investigate the microstructure characteristics of AM60B magnesium alloy. Special attention was paid to the effects of process parameters on the morphology and distribution of externally solidified crystals (ESCs) in the microstructure of magnesium alloy die castings, such as slow shot phase plunger velocity, delay time of pouring and fast shot phase plunger velocity. On the basis of metallographic observation and quantitative statistics, it is concluded that a lower slow shot phase plunger velocity and a longer delay time of pouring both lead to an increment of the size and percentage of the ESCs, due to the fact that a longer holding time of the melt in the shot sleeve will cause a more severe loss of the superheat. The impingement of the melt flow on the ESCs is more intensive with a higher fast shot phase plunger velocity, in such case the ESCs reveal a more granular and roundish morphology and are dispersed throughout the cross section of the castings. Based on analysis of the filling and solidification processes of the melt during the HPDC process, reasonable explanations were proposed in terms of the nucleation, growth, remelting and fragmentation of the ESCs to interpret the effects of process parameters on the morphology and distribution of the ESCs in the microstructure of magnesium alloy die castings.