Transient Directional Solidification Research Articles

Solidification, α-Al morphologies, and tensile properties of low-Sc modified Al-Mg alloys

Al alloys are very attractive in engineering applications that require a combination of low density and mechanical strength, such as in aeronautical and aerospace industry devices. Therefore, the search for alloys with higher specific strength is justified from environmental and economic points of view. This work aims to study the influence of cooling rates on the phase morphology and tensile properties of ternary Al-Mg-Sc alloys, whose composition covers the spectrum of low-Sc Al-Mg-Sc alloys. The ternary Al-3, −5, and − 10 wt% Mg-0.1 wt% Sc alloys were processed by transient directional solidification. This process induces a wide range of cooling rates which causes the formation of a microstructural variety along the longitudinal cooling direction and, consequently, different tensile properties. Several characterization methods were employed, including: thermal analysis, x-ray diffraction (XRD) structural analysis, SEM-EDS, optical microscopy, CALPHAD, tensile tests, and fractography. Two major findings should be noted after fully characterizing the microstructures, the formed phases, and the tensile properties. Solidification evolution revealed that more Mg added resulted in higher solidification rates due to higher fluidity with more Mg. Furthermore, cell growth was mapped in the three alloy compositions while competing with dendrites. Solidification scaling relationships between dendritic or cellular spacing and cooling rate for all alloys were examined. Although the Al-10 wt% Mg-Sc alloy had more Mg in solid solution, the Al-5 wt% Mg-Sc alloy showed superior tensile strength due to the predominance of cells and thinner primary dendrites trunks forming its microstructure.

Microstructure and Hardness of an Al–8 wt%Si–2.5 wt%Bi Alloy Subjected to Solidification Cooling Rates from 0.1 to 800 K s−1

This work explores the effect of the addition of bismuth (Bi) to Al–8 wt%Si alloys. Bi in Aluminum based alloys works as a self‐lubricating agent, improving machining and wear properties. As Bi is a soft material, it is essential to evaluate how it affects microstructural features and the resulting properties of Al–8 wt%Si alloys. Herein, this hypoeutectic alloy is modified by the addition of 2.5 wt%Bi and subjected to three solidification techniques: differential scanning calorimetry, transient directional solidification, and impulse atomization. Thus, this work investigates the effect of Bi in samples solidified under a wide range of cooling rates. Of specific interest is how Bi modifies the eutectic silicon morphology and alloy hardness compared with a hypoeutectic Al–10 wt%Si alloy from the literature. The silicon (Si) morphology of Al–8 wt%Si–2.5 wt%Bi transitions from flaky (coarse) to fibrous (fine) at a critical cooling rate of 1100 K s−1. Through the combination of Bi addition and processing through impulse atomization, the ternary Al–Si–Bi alloy achieves improvements in hardness of up to 20% compared to Al–10 wt%Si. This is despite having a coarser eutectic microstructure than the binary hypoeutectic Al–Si alloy. This is due to Bi modifying the morphology of the eutectic Si.

Assessing Microstructure Tensile Properties Relationships in Al-7Si-Mg Alloys via Multiple Regression

The development of Al-based alloys presumes a detailed understanding of the microstructure evolution during solidification since the as-solidified microstructure also has effects on the subsequent thermo-mechanical processing. In the present investigation Al-7wt.%Si-xMg (x = 0.5 and 1 wt.%) alloys are subjected to transient directional solidification with a view to characterizing the microstructure evolution, with special focus on both dendritic evolution and the inherent features of the Mg2Si and π-AlSiFeMg intermetallics. Experimental power-type functions relating the primary, secondary and tertiary interdendritic spacings to the solidification cooling rate and growth rate are developed. It is observed that the Mg content added to the Al-7wt.%Si alloy and the consequent increase in the Mg2Si fraction tends to increase the values of the primary dendritic spacing. However, this same behavior is not verified for the growth evolution of dendritic side branches. A multiple linear regression (MLR) analysis is developed permitting quantitative correlations for the prediction of tensile properties and hardness from microstructural parameters to be established. The increase in the Mg alloy content from 0.5 to 1 was shown to promote an increase in both the ultimate tensile strength (σu) and elongation.

Relationship between Microstructure Evolution and Tensile Properties of AlSi10Mg Alloys with Varying Mg Content and Solidification Cooling Rates

This work explored and contrasted the effect of microstructure on the tensile properties of AlSi10Mg alloys generated by transient directional solidification depending on variations in cooling rate and magnesium (Mg) content (i.e., 0.45 and 1 wt.% Mg), with a focus on understanding the dendritic growth and phases constitution. Optical and scanning electron (SEM) microscopies, CALPHAD, and thermal analysis were used to describe the microstructure, forming phases, and resulting tensile properties. The findings showed that the experimental evolution of the primary dendritic spacing is very similar when both directionally solidified (DS) Al-10 wt.% Si-0.45 wt.% Mg and Al-10 wt.% Si-1 wt.% Mg alloys samples are compared. The secondary dendritic spacing was lower for the alloy with more Mg, especially considering the range of high growth velocities. Moreover, a greater fraction of (Al + Si + Mg2Si) ternary eutectic islands surrounding the α-Al dendritic matrix was noted for the alloy with 1 wt.% Mg. As a result of primary dendritic spacings greater than 180 μm related to lower cooling rates, slightly higher tensile properties were attained for the Al-10 wt.% Si-0.45 wt.% Mg alloy. In contrast, combining dendritic refining (<150 μm) and a larger Mg2Si fraction, fast-solidified DS Al-10 wt.% Si-1 wt.% Mg samples exhibited higher tensile strength and elongation. The control of cooling rate and fineness of the dendritic array provided a new insight related to the addition of Mg in slightly higher levels than conventional ones, capable of achieving a better balance of tensile properties in AlSi10Mg alloys.

Comparing the roles of Sb and Bi on microstructures and application properties of the Al-15% Si alloy

The roles of Bismuth (Bi) and Antimony (Sb) additions to hypereutectic Al-Si alloys with significant variation in solidification cooling rates have yet to be unraveled. Both elements are considered of importance in the production chain of Al-based metallurgical products: while Sb is a refiner of the eutectic Si, Bi remains important in applications requiring better levels of alloy machinability. Based on these verified realities, the present research aims to modify the Al-15 wt% Si hypereutectic alloy with 1.0 wt% Bi as well as with 1.0 wt% Sb. The implications of these incorporations on the characteristics of the microstructure phases and the tensile/wear properties are examined. Various samples characterized by distinct solidification rates have been generated through transient directional solidification experiments. Several methods were used to examine and compare the two alloys with each other: microstructure and segregation are investigated by CALPHAD simulations, optical microscopy, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), whereas wear and tensile properties have been determined through standardized measurements. While adding Sb provides the growth of broken Si in the form of lamellae and some fraction of fibers for higher solidification rates, Bi alloying preserves lamellar growth for all rates examined. Adding Bi is better than Sb in reaching higher tensile properties, but Sb presence provides greater wear resistance. Both alloying elements ductilize the brittle Al-15 wt% Si alloy.

Microstructure characterization and tensile properties of directionally solidified Sn-52 wt% Bi-1wt% Sb and Sn-52wt% Bi-2wt% Sb alloys

Sn-Bi-based Thermal Interface Materials (TIM) are adequate alloys to promote heat dissipation in power electronics. However, despite the necessary thermal connection, mechanical support for different components and substrates are of prime importance in microelectronic devices. In this framework, the effects of Antimony (Sb) additions on the microstructure and tensile properties of the Sn-52 wt% Bi alloy are investigated. Various Sn-Bi(-Sb) samples solidified at different cooling rates and two levels of Sb-containing alloys allow a comprehensive examination of length scales of either dendritic or eutectic microstructures. A number of experimental techniques are used here to permit a sound analyses of the ternary Sn-Bi(-Sb) alloys: transient directional solidification, optical microscopy (OM), triangle and intercept quantification methods, scanning electron microscopy (SEM), x-ray fluorescence (XRF), x-ray diffraction (XRD), tensile tests and fractography. The addition of Sb enhances the nucleation of primary dendritic trunks, which resulted in a decrease in the primary dendritic arm spacing (λ1) by about 5 times for the Sn-52 wt% Bi-2 wt% Sb alloy as compared to the results for the binary Sn-Bi alloy. The relationships found for tensile properties as a function of the secondary dendritic arm spacing (λ2) demonstrate that Sb additions increase the alloy strength while preserving the ductility. This is due to very thin SnSb intermetallic particles formed in the Sn-rich dendritic matrix. The influence of λ2 variation on both the yield and ultimate strengths is roughly insignificant while the ductility varies strongly between 14.4% and 52% for samples solidified from 0.05 °C/s to 5.0 °C/s respectively. When 2.0 wt% Sb is added, there is a maintenance in the levels of ductility as those found for the binary Sn-Bi alloy. This occurs especially for very refined dendritic and eutectic microstructures samples, which also exhibit a ductile fracture mode.

Effects of Bi addition on Si features, tensile properties and wear resistance of hypereutectic Al-15Si alloy

Al-Si alloys are very important in the automotive industry. This is thanks to their high strength to weight ratio, suitable fluidity, good corrosion resistance and high productivity. The shape of the eutectic Silicon (Si) has been widely studied since it is considered an index of quality regarding many application properties. Understanding the effects of alloy additions on the morphology of Si has drawn researcher’s attention. One of these elements of interest is Bismuth (Bi), which may be part of the composition of Al-Si alloys either as an addictive or a contamination. Hence, the microstructure aspects related to the presence of Bi are important requiring further consideration. In the present study, Al-15wt% Si hypereutectic alloy is modified with 1.0 wt% Bi. The goal is to compare the effect of this addition on the morphology of the phases forming the microstructure and on tensile/wear properties. In order to pursue that comparison, previously available data for the binary Al-15wt% Si alloy will be used. Various samples characterized by distinct cooling rates were generated by transient directional solidification of the ternary Al-Si-Bi alloy. The experimental variations of the eutectic spacing will be compared to each other based on their trends. It was found that the addition of minor Bi content (∼1%) permitted tensile strength and ductility of 195 MPa and 14%. Furthermore, wear resistance was improved thanks to Bi. The reasons for that will be outlined.

Effects of Bi Addition on Si Features, Tensile Properties and Wear Resistance of Hypereutectic Al-15Si Alloy

Bismuth (Bi) alloying in Aluminum (Al) alloys is considered of importance in special applications requiring improvements in the alloy machining. Despite that, the influence of Bi either on evolution of the solidification or on resulting microstructure of hypereutectic Al-Si alloys is still to be determined. The present research work is devoted to these requirements. In the present study, Al-15wt.% Si hypereutectic alloy is modified with 1.0 wt.% Bi. The effects of this addition on the morphology of the phases forming the microstructure and on the tensile/wear properties are investigated. Various samples characterized by distinct cooling rates are generated by transient directional solidification of the ternary Al-Si-Bi alloy. Microstructure is examined by optical microscopy and scanning electron microscopy (SEM) analyses, segregation by energy-dispersive X-ray spectroscopy (EDS) whereas wear and tensile properties have been characterized by standardized tests. The experimental variations of the eutectic spacing are compared to each other based on their trends. It is found that the addition of minor Bi content (~1 wt. %) permitted tensile strength and ductility of 195 MPa and 14%. Furthermore, wear resistance is improved by up to 20% due to the Bi addition. The reasons for that will be outlined.

Research on Grain Refinement in Hypoeutectic Al-Si Alloy during Solidification under an Alternating Electric Current Pulse

Experiments on transient directional solidification were carried out to study the columnar to equiaxed transition (CET) in Al-7Si alloy without and with an alternating electric current pulse (AECP). Without AECP, the macrostructure consists of typical columnar and equiaxed zones, separated by a near horizontal plane. As the AECP is applied during solidification, an additional fine equiaxed zone (FEZ) occurs in the as-cast macrostructure. From measured temperature profiles, cooling rate and temperature gradient are determined. It is found that CET occurs for a critical value of the cooling rate, which is observed to be about 0.14 K·s−1 in the present investigation. Furthermore, the macrostructural observation with mold for embedding the mesh plate demonstrates that the major factor responsible for the formation of fine equiaxed grains is the detachment of crystal nuclei from the upper contact surface and the lateral wall. The detachment is in turn ascribed to electric current-associated free energy change (ΔGe)-induced the driving force F.

Slow and rapid cooling of Al–Cu–Si ultrafine eutectic composites: Interplay of cooling rate and microstructure in mechanical properties

Abstract

Processing, As-Cast Microstructure and Wear Characteristics of a Monotectic Al-Bi-Cu Alloy

Ternary Al-based monotectic alloys have a good combination of wear resistance and mechanical strength. While self-lubricating soft elements guarantee an adequate wear resistance, the modification with third elements can increase the ability to support load. In the present investigation, a collection of microstructures is generated through transient directional solidification of the Al-3.2wt.%Bi-3.0wt.%Cu alloy. Samples with different Bi spacing have been subjected to micro-adhesive wear ball tests. A relationship linking the wear volume, V, the microstructural spacing and the test time is proposed for Bi spacing higher than 16 μm, according to which V decreases with the decrease in Bi spacing. It is observed that wider and deeper grooves emerged on the surface of the samples related to more refined Bi and Al2Cu phases, that is, associated with Bi spacing and Bi diameter lower than 16 and 2.4 μm, respectively. A reverse trend is noted for these finer microstructures, for which V increases with further decrease in Bi spacing. This can be caused by the detachment of the very fine and less cohesive Al2Cu lamellas as the Al2O3 oxide breaks up forming debris, with the presence of these lamellas as loose debris at the interface acting as third-body abrasives.

Length Scale of the Dendritic Array Tailoring Strength of a 5052 Aluminum Alloy

The present research aims to characterize 5xxx alloy series, considering chemistries based on the commercial 5052 alloy with three Mg contents (2.4wt.%, 2.6wt.% and 3.2wt.%) through transient directional solidification experiments. Very representative incoming impurities to a given twin-roll casting procedure were reached. As such, the Si, Fe, Cu, Mn and Cr-contents in the tested samples typically trend in between the suitable alloying spectrum. Microstructural analyzes were performed using polarized light microscopy of samples taken from various ingot positions. Growth relationships between the secondary dendritic spacing (λ2) and the growth velocity were determined. The 5052 alloys containing higher Mg content may induce a decrease in λ2 for a certain growth velocity. The hardness values measured across the three directionally solidified castings were directly related to the λ2, which can be considered a fundamental variable affecting mechanical strength. For representative conditions vis-a-vis those employed in industry, it was shown that even relatively small changes in Mg content of the 5052 alloy may have some impact on λ2. These results open new ways to predict the final as-cast microstructure characterizing commercial 5052 alloy products, with a view to controlling not only the dendritic growth but also the Mg content during casting operations.

Wetting behavior of Sn–Ag–Cu and Sn–Bi–X alloys: insights into factors affecting cooling rate

Based on two experimental approaches: transient directional solidification and drop shape analyses, the measurements of cooling rates and contact angles (θ) of several solders of interest became practical. Three Sn–0.7Cu–(1; 2 and 3Ag) and four Sn–34Bi–(0Cu; 0.1Cu; 0.7Cu; 2Ag) alloys are investigated to determine their wetting behavior, compare to each other and bring to light their ability (or not) to control the initial cooling rate. In the case of SAC alloys, increase in Ag means decrease in both θ and cooling rate. An opposite correlation between cooling rate and θ is observed when Sn–34Bi and the Sn–34Bi–0.1Cu alloys are compared.

Correlations of microstructure and mechanical properties of the ternary Sn-9wt%Zn-2wt%Cu solder alloy

The microstructure length-scales (dendritic and eutectic), morphologies and tensile properties of a ternary Sn-9wt.%Zn-2wt.%Cu alloy are compared with those of a binary eutectic Sn-9wt.%Zn alloy. The following experimental/analytical steps were performed: transient directional solidification experiments of the Sn-9wt.%Zn-2wt.%Cu alloy; measurements of secondary dendrite arm (λ2) and interphase spacing (λ); morphology of the eutectic α-Zn phase; determination of thermal parameters such as cooling rate (ṪL) and their evolutions during solidification; and, finally, interrelations of microstructure vs. tensile properties. The addition of 2wt.%Cu causes significant refinement of the eutectic structure. Hall-Petch type experimental expressions outlined the variations of strength and ductility with both λ2 and λ. Very fine Zn globules and needles forming the eutectic in the ternary alloy seems to cause a reversal on ductility behavior, as compared to the tendency observed for the binary Sn-9wt.%Zn alloy. Here, for the ternary Sn-9wt.%Zn-2wt.%Cu alloy ductility increases with decreasing interphase spacing.

Tailoring microstructure, tensile properties and fracture process via transient directional solidification of Zn-Sn alloys

The aim of the present study is to determine interrelations of microstructure length-scale, tensile properties and fracture mechanisms of hypoeutectic Zn-Sn alloys. Three compositions were subjected to transient directional solidification: Zn-10, 20 and 40wt%Sn. Grainy-faceted cleavage has been observed as the predominant mode of fracture, which propagated across the Zn-rich plate cells. A clear influence of the formed microstructure and proportion of eutectic may be noted in the fracture features, with alveolar structures also appearing in fracture surfaces. Growth laws relating the eutectic spacing with the growth rate are proposed, which are able to encompass both steady-state and transient solidification conditions. Hall-Petch type equations are developed relating the yield and ultimate tensile strengths, σy and σu, respectively, to the cellular spacing, λ, in which smaller λ values resulted in higher σy and σu.

Examination of Thermal Parameters, Primary Dendritic Spacings and Microhardness of a Horizontally Solidified Al-Cu-Mg Ternary Alloy

In this paper, the Al-3wt.%Cu-0.5wt.%Mg alloy was grown by the transient horizontal directional solidification method in a water cooled stainless steel chill. A temperature monitoring system in the metal was used and the registered time-temperature data during the solidification process were applied to determine tip growth rates (VL) and cooling rates (TR) which have been correlated with both primary dendrite arm spacings (λ1) and Vickers microhardness (HV). Characterization techniques by optical and scanning electron microscopy (OM and SEM) were employed and the observed microstructures show an Al-rich dendritic matrix (a-Al) and θ (Al2Cu) and S (Al2CuMg) intermetallic phases within the interdendritic regions. Power and Hall-Petch type experimental equations were proposed to describe the HV profile as a function of the thermal and microstructural parameters (VL, TR, and λ1). In order to investigate the effect of the Mg alloying, a comparative analysis was also performed between the experimental values of this article and the results of previous studies carried out with the Al-3wt.%Cu binary alloy.

Tailoring Morphology and Size of Microstructure and Tensile Properties of Sn-5.5 wt.%Sb-1 wt.%(Cu,Ag) Solder Alloys

Transient directional solidification experiments, and further optical and scanning electron microscopy analyses and tensile tests, allowed the dependence of tensile properties on the micromorphology and length scale of the dendritic/cellular matrix of ternary Sn-5.5Sb-1Ag and Sn-5.5Sb-1Cu alloys to be determined. Extensive ranges of cooling rates were obtained, which permitted specific values of cooling rate for each sample examined along the length of the casting to be attributed. Very broad microstructural length scales were revealed as well as the presence of either cells or dendrites for the Ag-containing alloy. Hereafter, microstructural spacing values such as the cellular spacing, λ c, and the primary dendritic spacing, λ 1, may be correlated with thermal solidification parameters, that is, the cooling rate and the growth rate. While, for the Cu-containing Sn-Sb alloy, the β-Sn matrix is characterized only by the presence of dendritic arrangements, the Ag-containing Sn-Sb alloy is shown to have high-velocity β-Sn cells associated with high cooling rate regions, i.e., positions closer to the bottom of the alloy casting, with the remaining positions being characterized by a complex growth of β-Sn dendrites. Minor additions of Cu and Ag increase both the yield and ultimate tensile strengths when compared with the corresponding values of the binary Sn-5.5Sb alloy, with a small reduction in ductility. This has been attributed to the homogeneous distribution of the Ag3Sn and Cu6Sn5 intermetallic particles related to smaller λ 1 characterizing the dendritic zones of the ternary Sn-Sb-(Cu,Ag) alloys. In addition, the Ag-modified Sn-Sb alloy exhibited an initial wetting angle consistent with that characterizing the binary Sn-5.5Sb alloy.

Directional solidification of a Sn-0.2Ni solder alloy in water-cooled copper and steel molds: Related effects on the matrix micromorphology, nature of intermetallics and tensile properties

The present investigation is focused on, firstly, performing transient directional solidification experiments with a Sn-0.2 wt.% Ni solder alloy using two different substrates as mold sheets separating the alloy casting from the cooling fluid: copper and low carbon steel. Secondly, the examination of the obtained microstructures is carried out highlighting not only the micromorphology aspects of the formed β-Sn phase but also the nature and the shape of the intermetallic compounds (IMCs) developed. The purpose of this research work is to verify the influences that different substrate materials may have on the alloy solidification kinetics, resultant microstructures and tensile properties of the Sn-0.2 wt.%Ni solder. The microstructure characteristics may be correlated with thermal solidification parameters such as the eutectic cooling rate and eutectic growth rate along with a qualitative evaluation of Fe and Cu dissolutions into the alloy. The results display that the dissolution of Cu into the Sn-Ni alloy provided the prevalent growth of the (Cu,Ni)6Sn5 fiber-like eutectic phase along the length of the casting. Other than, the Cu-containing Sn-Ni alloy allowed the growth of high-velocity β-Sn cells only for very high cooling rates, associated with positions closer to the bottom of the alloy casting. Farther positions are characterized by a complex growth of β-Sn dendrites. On the other hand, for the alloy solidified against the steel mold, a predominance of the non-equilibrium NiSn4 eutectic phase with plate-like shape has been identified by SEM/EDS and XRD. In this case, the predominant growth of β-Sn cells associated with the presence of the plates of the NiSn4 IMC allowed lower tensile strength and higher ductility to be attained.

Macrosegregation of Impurities in a Metallurgical Silicon Ingot After Transient Directional Solidification

Metallurgical grade silicon was melted and directionally solidified in transient conditions by extracting heat from the base of a cylindrical ingot and inserting heat at the ingot top. A heat-transfer mathematical model was implemented to predict the solidification velocity and temperature gradient using cooling curves measured directly in the silicon melt. Nearly 70% of the resulting ingot displays a region of columnar grains aligned with the ingot axis. In this region, the concentration of metallic impurities is usually below the quantification limit of the analytical technique and intermetallic particles are absent, strongly indicating significant purification. The transition from the purified region of the ingot to the ingot top, where impurity concentrations increase and intermetallic particles are seen, is consistent with a change of the solid-liquid interface morphology from planar to cellular/dendritic, as similarly reported in the literature and as indicated by a preliminary analysis with the constitutional undercooling criterion.

Thermal Parameters, Tertiary Dendritic Growth and Microhardness of Directionally Solidified Al-3wt%Cu Alloy

The main purpose of this paper is to evaluate both tertiary dendritic arm growth and microhardness of Al-3wt%Cu alloy during horizontal directional solidification under transient heat flow conditions. Experimental thermal profiles recorded during solidification process allowed to determine growth rate and cooling rate values which are associated with both tertiary dendritic arm spacings and microhardness. The results show that initial tertiary branches growth only occurs when a cooling rate value of 1.14 K/s is reached. Variation of tertiary spacings is expressed as-1.1 and-0.55 power law functions of growth rate and cooling rate, respectively. A comparative analysis with other studies published in the literature that analyze tertiary dendritic growth of Al-Cu alloys considering transient directional solidification is carried out. Dependence of microhardness on dendritic arrangement is evaluated by experimental laws of power and Hall-Petch types with a view to permitting the applicability of the resulting expressions.