Secondary Dendrite Arm Spacing Research Articles

Effects of magnesium on the microstructure and properties of Al–Si alloy deposited by wire and arc-based additive manufacturing

ABSTRACT The effects of Mg on microstructure, defects and properties of Al–Si alloy deposited by Wire and Arc-based Additive Manufacturing (WAAM) were investigated. The WAAM alloys were characterised by the metallographic microscope, X-ray diffraction, scanning electron microscope, computed tomography and electronic universal testing machine. The results show that the secondary dendrite arm spacing (SDAS) and hydrogen content of as-deposited WAAM alloy increase, and the tendency of pore formation increases with an increase in Mg content. The impurity Fe in the WAAM alloy with low Mg content is mainly in β-Fe phase, while with high Mg content is mainly in the π-Fe phase. After solution treating and ageing treatment (T6 heat-treat), the microstructure of WAAM alloy tends to be uniform. With the increase of Mg content, the size of eutectic silicon particles increases gradually. Furthermore, the UTS and YS of the alloys increased, and the elongation decreased.

Microstructure and mechanical properties of Al-7.9Zn-2.7Mg-2.0Cu (wt%) alloy strip fabricated by twin roll casting and hot rolling

High Zn content Al-7.9Zn-2.7Mg-2.0Cu (wt. %) aluminum alloy strip with 5.5 mm thickness was successfully fabricated by twin roll casting (TRC). Fine grain size, second phase and second dendrite arm spacing (SDAS) were obtained in the cast strip due to the high cooling rate during TRC process. Short-time annealing treatment of 480 ºC-1 h could effectively reduce the second phases, and the elongation of the strip was improved. The TRC alloy sheet with 2.0 mm thickness was fabricated by the annealing treatment (480 ºC-1 h) and hot rolling. The yield strength, tensile strength and elongation along the rolling direction of the TRC sheet after solution treatment, pre-aging and natural aging were 398 MPa, 590 MPa and 15.74%, respectively. After bake-hardening, the yield strength, tensile strength and elongation along the rolling direction were 550 MPa, 605 MPa and 10.07%, respectively.

Evolutions of Micro- and Macrostructure by Cerium Treatment in As-Cast AISI M42 High-Speed Steel

The as-cast M42 high-speed steels containing different contents of cerium (Ce) were manufactured to investigate the effects of Ce on the solidification structures at the micro- and macroscale. The results indicate that the addition of Ce could modify MgO·Al2O3 and MnS into the Ce-containing inclusions. The addition of Ce refined the dendrite structure and eutectic carbides, which can be ascribed to the heterogeneous nucleation of primary austenite on Ce2O2S or Ce2O3 and the increase in both the compositional supercooling at the dendrite forefront and the restriction on the dendrites coarsening. Both the secondary dendrite arm spacing (SADS) and total eutectic carbides content decrease gradually with increasing Ce content. M2C and M6C carbides are the predominant precipitates. The increase of Ce content made the morphology of M2C carbides change from long lamellar or straight-rod morphology into shorter curved-rod or honeycomb morphology because of the overgrowth of eutectic austenite, but it had no significant effect on the morphology of M6C carbides. Ce2O2S and Ce2O3 can serve as the very effective heterogeneous nucleation sites for M6C carbides because of the low lattice disregistry between them, hence Ce addition improved markedly the macroscopic distribution of M6C carbides in the cast ingot and promoted the formation of M6C carbides at the expense of M2C carbides.

Three-dimensional visualization and quantification of microporosity in aluminum castings by X-ray micro-computed tomography

Porosity is a major issue in solidification processing of metallic materials. In this work, wedge die casting experiments were designed to investigate the effect of cooling rate on microporosity in an aluminum alloy A356. Microstructure information including dendrites and porosity were measured and observed by optical microscopy and X-ray micro-computed tomography (XMCT). The effects of cooling rate on secondary dendrite arm spacing (SDAS) and porosity were discussed. The relationship between SDAS and cooling rate was established and validated using a mathematical model. Three-dimensional (3-D) porosity information, including porosity percentage, pore volume, and pore number, was determined by XMCT. With the cooling rate decreasing from a lower to a higher position of the wedge die, the observed pore number decreases, the porosity percentage increases, and the equivalent pore radius increases. Sphericity of the pores was discussed as an empirical criterion to distinguish the types of porosity. For different cooling rates, the larger the equivalent pore radius is, the lower the sphericity of the pores. This research suggests that XMCT is a useful tool to provide critical 3-D porosity information for integrated computational materials engineering (ICME) design and process optimization of solidification products.

Multi-component numerical simulation and experimental study of dendritic growth during solidification processing

A multi-component model was developed to simulate microstructure evolution during solidification of a quinary alloy system. The thermodynamic and kinetic parameters including liquidus slope, partitioning coefficient, diffusion coefficient etc. as functions of temperature and solute concentration were obtained using CALculation of PHAse Diagrams (CALPHAD) modeling. The effect of solute interactions on diffusion and dendrite growth velocity was fully considered in this model. The model was validated by mesh size convergence, and compared with a well-accepted analytical model. The new model was shown to successfully display solidification dendrites and solute redistributions of a quinary Al-Si-Mg-Fe-Mn alloy. The effect of different solute elements on solute diffusion and dendrite growth was analyzed. Additionally, various calculation domains and cooling rates of industrial importance were tested in the simulations. Directional solidification experiments were used for model validation in secondary dendrite arm spacing measurement and simulation. The validated relationship between the secondary dendrite arm spacing and cooling rate in a complex quinary system confirmed the accuracy of this 3-D multi-component model, which provides a critical connection between solidification processing and mechanical property prediction in Integrated Computational Materials Engineering (ICME) of solidification products.

Influence of periodic pressure on dendritic morphology and sidebranching

The influence of periodic pressure with low and high frequencies on microstructure and dendritic sidebranching was studied by using 3-D phase field method. In both low and high frequency cases, the variation trend of SDAS (secondary dendritic arm spacing) with increasing pressure frequency is opposite to that of sidebranching frequency, while the variation trend of the average length of secondary arms is consistent with that of sidebranching frequency. The high sidebranching frequency indicates that more secondary arms share the whole driving force of dendrite growth, resulting in lower driving force for each one and leading to less developed secondary arms. The smallest SDAS is obtained when perturbed by the periodic pressure with the frequency of 0.157/τ0 (τ0 is the physical unit of time in the dimensionless phase field model) and 2.200/τ0 in low and high frequency cases, respectively. Comparisons of dendritic morphology and secondary arms are made between the low and high frequency cases. Firstly, in the low frequency case, secondary arms are luxuriant especially when pressure frequency is low, with many high-order side branches stretching out. Secondly, the average length of secondary arms in primary dendrite is longer in the low frequency case than that without pressure, and much longer than that in the high frequency case. Thirdly, the dendrite tip without side branches in the high frequency case is much longer than that in the low frequency case. All of the differences in dendritic morphology and sidebranching in the two cases can be attributed to the different modulation mechanism. In the low frequency case, periodic pressure determines tip velocity and then modulates sidebranching directly. While in the high frequency case, periodic pressure cannot determine sidebranching directly, but via modulating tiny protuberances in dendrite tip, part of which evolves into side branch. In this case, the tiny protuberances take part of the whole driving force, leading to less developed secondary arms.

Effects of cooling rate and TiC nanoparticles on the microstructure and tensile properties of an Al–Cu cast alloy

In this work, the effects of cooling rate and the addition of TiC nanoparticles on the as-cast microstructure, precipitation behavior during subsequent heat treatments and mechanical properties were investigated. The grain size (GS), secondary dendritic arm spacing (SDAS) and primary particle size are the function of cooling rate v, following a similar power law, D = Av-n (D is the size of grains/SDAS/primary particles, A and n are constants). The TiC addition reduced the A value for grain size and primary particles but increased A for SDAS. The addition of TiC nanoparticles promoted the coarsening of secondary dendritic arms, leading to the reduced microsegregation and smaller interdendritic/intergranular secondary phase (Al2Cu). Consequently, the precipitation of the θ′ phase during ageing treatment was promoted, and the precipitates were refined and more dispersive, contributing to the increased tensile strength. The improved ductility was ascribed to the combination of reduced secondary phases and micropores. With the combination of high cooling rate and TiC addition (cooling rate: 93 °C/s; 0.5% TiC addition), the yield strength, ultimate tensile strength and uniform elongation of the Al–5Cu cast alloy were simultaneously improved, and reached those of Al–5Cu rolled sheet.

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.

Effect of Heat Input on Evolution of Microstructure and Tensile Properties of Gas Tungsten Constricted Arc (GTCA) Welded Inconel 718 Alloy Sheets

The significant effect of heat input on the microstructural characteristics and tensile properties of 2-mm-thick Inconel 718 alloy sheets joined by gas tungsten constricted arc welding process was investigated systematically. It involves the application of magnetic arc constriction technique to solve the heat input-related metallurgical problems in gas tungsten arc welding of Inconel 718 alloy such as segregation of Nb and laves phase evolution in weld metal region which drastically reduces the weld mechanical properties. Heat input range was used from 495 to 585 J/mm by varying Main Current from 60 to 80 A at distinct levels of 5 A. The average secondary dendrite arm spacing was increased from 6.31 to 8.92 μm, revealing the corresponding cooling rate between 2087 and 720 °C/s. Superior tensile properties were obtained at an optimum heat input of 518 J/mm. It exhibited 98.16 and 78% of base metal strength and ductility. It is attributed to the grain refinement in fusion zone microstructure and the evolution of lower amount of laves phase with finer and discrete morphology. The average size and volume fraction of laves phase for the current sample were 3.12 μm and 7.68%. The benefits of magnetic arc constriction and Delta Current pulsing were not achieved at higher heat input.

Effects of Zn on Microstructure Modification and Mechanical Properties Improvement of Al-Si-Cu-Mg Alloys

In this research, the influence of Zn on the microstructure and mechanical properties of Al-Si-Cu-Mg alloys under different solidification rates was investigated. Differential scanning calorimetry was used to detect the nucleation and growth of eutectic Si, while scanning electron microscopy was used to analyze Fe-rich phases. Two growth models were deduced to explain the modification of the eutectic Si and Fe-rich phases upon the addition of Zn. The results show that an addition of 0.6 wt pct Zn modified the eutectic Si from flake like and fibrous to micron- and submicron-granular structures, depending on the different secondary dendrite arm spacings of ~ 33 μm and ~ 16 μm, respectively. For the Fe-rich phases under steel mold casting conditions, β-Al5FeSi was modified into π-Al8FeMg3Si6 upon adding 0.6 wt pct Zn. Under sand mold casting conditions, the 0.6 wt pct Zn addition only refined the size of the β-Al5FeSi. The Zn addition significantly improved the ultimate tensile strength and elongation. The excellent modification effect of Zn on the eutectic Si and Fe-rich phases was attributed to the fact that the reaction of the Zn-Si eutectic on the {111}Si planes forced the Si atoms from these phases into the eutectic droplets.

Microstructure and high temperature mechanical properties of wire arc additively deposited Stellite 6 alloy

In this work, wire arc additive deposition of Stellite 6 alloy was carried out using cold metal transfer (CMT) process and the microstructural features, microhardness, and high temperature mechanical properties were studied. The as-deposited multi-layer wall was free of any defects. High temperature compression test samples were extracted parallel to the build direction, and the tensile samples were prepared along parallel and perpendicular orientation to the build direction. The hot compression test data was used to predict the flow behavior of the additively deposited alloy using an Arrhenius type equation at a strain of 0.6. The flow behavior of the additively deposited Stellite 6 alloy was similar to the conventional Co-based alloys in spite of the layer-by-layer macrostructure and microstructural variations. Microstructure, hardness, and phase analysis studies were carried out for the deformed samples after hot compression. The high temperature tensile properties were found to be anisotropic and location-specific, which were correlated with the columnar dendritic microstructure, alignment of the layer interfaces and variation in the secondary dendrite arm spacing due to repeated thermal cycles and difference in the cooling rate.

Characterization of Microstructure, Precipitations and Microsegregation in Laser Additive Manufactured Nickel-Based Single-Crystal Superalloy.

In this study, the microstructure, precipitations, and microsegregation in the laser additive manufactured thin-wall structure of a single-crystal superalloy are synthetically characterized. The influence of a subsequent heat treatment on the microstructure and precipitations is discussed. The results show that under the given processing conditions, the single-crystal microstructure is regenerated perfectly with small misorientation angles in the thin-wall structure. The crystal morphology shows obvious diversity and instability with the incremental height of thin-wall structure. With the increase of manufacturing height, both the primary and secondary dendritic arm spacings of epitaxial columnar dendrites first increase rapidly and then reach a dynamic balanced state. The distribution of precipitations and pores keeps symbiosis in the interdendritic region and shows periodic band characteristic with high density in the band region and low density in the inner region of plate layers. The microsegregation of element atoms in the microstructure shows a three-dimensional network distribution. The concentration of element atoms keeps good consistency with high value in the three-dimensional network and nearly standard value in the outside region. The subsequent heat treatment process contributes to the occupation of as-processed pores by the expanded mature precipitations with good blocky shape. Further optimization of the heat treatment process for improving the lattice coherency of precipitated γ’ phase and γ matrix in the laser additive manufactured single-crystal superalloy is needed and valuable.

Casting Simulations of Arsenical Copper: New Insights into Prehistoric Metal Production and Materials

To improve our understanding of prehistoric casting methods and materials, simulations for copper arsenic (As-Cu) alloys with up to 15 wt.% As were calculated. Cooling curves and the secondary dendritic arm spacings (SDAS) for the alloy were plotted and calculated, respectively, under non-steady-state conditions with a time-stepping procedure for prehistoric mold materials (e.g., quartz sand, sandstone, terracotta, and steatite). The cooling and microstructure of the alloy was also simulated in iron molds for immediate comparison with as-cast microstructure. The microstructure and SDAS of the as-cast alloys were studied and measured in polished samples with a metallographic microscope. The purpose of this research was to improve our ability to retroactively evaluate the influence of mold materials on as-cast microstructures and determine their materials. This article focuses on As-Cu alloy microstructure and SDAS values, and also discusses the phenomenon of “inverse segregation” and its relation to cooling rate and As concentration.

Study on the Directional Solidification Process of an Aluminum Alloy Bar in Multishell Mold Being Gradually Immersed in Water.

A multishell mold structure and water-immersion cooling method (MSMWI) is proposed for the directional solidification of castings. A four-layer-shell sand mold was designed for a bar with diameter of 40 mm. As the aluminum melt was poured, the multishell mold was gradually immersed in water, and the water level drove the advancement of the solidification front from bottom to top. The multishell mold was helpful for the heat insulation of its upper part, and its bottom was chilled by the water. Therefore, directional solidification of the bar was vertically realized. The water-cooled solidification process of the bar was 5.8 times faster than that by air natural cooling (MSMNC), and the temperature gradient was increased by 78 times. The secondary dendrite arm spacing (SDAS) and eutectic silicon were significantly refined. Its tensile strength, elongation, and hardness were increased by 56%, 185%, and 62.6%, respectively.

Influence of Continuous Casting Speeds on Cast Microstructure and Mechanical Properties of an ADC14 Alloy

To improve the mechanical properties of the casting alloys, various attempts have been made to use alternative casting technologies. The Ohno continuous casting (OCC) process is a unidirectional solidification method, which leads to high-quality cast samples. In this study, the Al-Si-Cu-Mg alloy was cast at casting speeds of 1 mm/s, 2 mm/s, and 3 mm/s, by the OCC process. The aim of this study is to investigate the effects of the casting process parameters, such as casting speeds and cooling conditions, on the crystallization characteristics and mechanical properties of OCC-Al-Si-Cu-Mg alloy. Particularly, secondary dendrite arms spacing of α-Al dendrites in OCC samples significantly decreases with increasing casting speed. Moreover, the mean tensile strength of the samples, produced at the highest casting speed of 4.0 mm/s, is significantly higher than that for the samples produced at a casting speed of 1.0 mm/s.

Multi-Refinement Effect of Rare Earth Lanthanum on α-Al and Eutectic Si Phase in Hypoeutectic Al-7Si Alloy

The effect of La addition on primary α-Al and the eutectic Si phase of Al-7Si alloy is investigated systematically in this work. The results indicate that La addition causes a multi-refining efficiency on the microstructure of Al-7Si alloy, including refinement of α-Al grains and secondary dendrite arm spacing as well as eutectic Si particles. The grain size, secondary dendrite arm spacing and area of eutectic Si particles are decreased by 26.8%, 7.7% and 26.7%, respectively, with the addition of 0.1 wt.% La. It is also found that La-rich phases of Al2Si2La form and distribute in the vicinity of the eutectic Si phase. The crystal structure and lattice parameter of Al2Si2La phase are determined to be hexagonal (a = b = 0.405 nm, c = 6.944 nm) based on the TEM analysis results. The multi-refinement effects are mainly attributed to the increased constitutional undercooling caused by the low solubility of La in Al alloy and the growth-restricting factor caused by the Al2Si2La phase.

Optimization of casting means and heat treatment routines for improving mechanical and corrosion resistance properties of A356-0.54Sc casting alloy

In this paper, the effect of different casting means and heat treatment routines on the mechanical and corrosion resistance properties of A356-0.54Sc casting alloy was comprehensively studied. Three different molds, including graphite mold, steel mold, and copper mold, were employed. Both the grain size and secondary dendrite arm space of primary α-(Al) in the casting A356-0.54Sc alloy with copper mold are the finest. Three different heat treatments, i.e., T4, T5, and T6, were then applied. With the T4 treatment, the eutectic (Si) particles transferred from the as-cast fibrous structure to the isolated spherical one, leading to a significant increase in ductility. After aging, the alloys in the T6 state always own the best strength properties. The strong heredity of microstructures from the casting stage to the subsequent heat treatments was observed. Moreover, the corrosion resistance of alloys was studied by the salt-spray corrosion experiments. The experiments showed that the pitting corrosion mostly occurs in the interface between the eutectic (Si) and primary α-(Al). It was finally concluded that the alloys casted using the copper mold and treated with the T6 routine maintain the best comprehensive mechanical properties after corrosion, while the alloys casted using the graphite mold are with the best corrosion resistance.

Influence of B and Nb Additions and Heat Treatments on the Mechanical Properties of Cu-Ni-Co-Cr-Si Alloy for High Pressure Die Casting Applications

During the high pressure die casting process (HPDC), it is necessary to develop new designs and alloys for the copper plungers. In this research, two alloys Cu-9Ni-1Co-1.6Cr-2Si-1.3Fe-0.25B wt.% (A1) and Cu-9Ni-1Co-1.6Cr-2Si-0.1Fe-0.2Nb wt.% (A2) under different heat treatments (HT) were studied. Optical microscopy technique was applied to reveal the regions of dendritic morphology, also lower Secondary Dendrite Arm Spacing (SDAS); and different grain orientations. The results reveal that the solidification sequence is primary Cu dendrites and secondary intermetallics; heat treatments increase the redistribution of alloying elements in the interdendritic regions. During the heat treatments, some precipitates were found in the grain boundary after aging heat treatments for both alloys, which were determined by X-ray diffraction. Hardness test HRB presented a decrease with the solution heat treatment and an increase with the aging heat treatments proposed for both alloys. Finally, the wear resistances for both alloys were compared with a commercial alloy C17530, with decreased A1 with B additions having the best result in the as-cast condition 4.07 × 10−4 mm3/Nm, while for A2 with Nb additions wear resistance increased, with the best result in the one with aging heat treatment 1.69 × 10−4 mm3/Nm while for the C17530 alloy this was 2.74 × 10−4 mm3/Nm.

Study on primary carbides precipitation in H13 tool steel regarding cooling rate during solidification

This study aims to investigate the primary carbides precipitation in H13 steel solidified at relatively high cooling rates, ranging from 300 to 6,000 °Cmin−1, based on in situ observations with a high temperature confocal laser scanning microscope. In the cooling rate range investigated, the solidification microstructure becomes more refined as cooling rate increases and the relationship between the secondary dendrite arm spacing (SDAS), λ2, and cooling rate, $${\dot T}$$ , can be expressed as $${\lambda _2} = 128.45{{\dot T}^{ - 0.124}}$$ . Regardless of cooling rates, two kinds of primary carbides, i.e., the Mo-Cr-rich and V-rich carbides, are precipitated along the interdendritic region and most of them are the Mo-Cr-rich carbides. The morphology of Mo-Cr-rich carbide is not obviously influenced by the cooling rate, but that of V-rich carbide is obviously affected. The increasing cooling rate markedly refines the primary carbides and reduces their volume fractions, but their precipitations cannot be inhibited even when the cooling rate is increased to 6,000 °C·min−1. Besides, the segregation ratios (SRs) of the carbides forming elements are not obviously affected by the cooling rate. However, compared with the conventionally cast ingot, the SDAS and primary carbides in the steel solidified at the investigated cooling rates are much finer, morphologies of the carbides have changed significantly, and SRs of the carbides forming elements are markedly greater. The variation of primary carbide characteristics with cooling rate is mainly due to the change in SDAS.

Investigation on Microstructures and Mechanical Properties of the Hypoeutectic Al-10Si-0.8Fe-XEr Alloy

In this paper, the effect of Er addition (0.2, 0.5, 0.65, 0.8, 1.0, and 1.5 wt. %) on the microstructure evolution and tensile properties of as-cast hypereutectic Al-10Si-0.8Fe alloy was investigated. The phases and their morphologies in these alloys were identified by XRD and SEM equipped with EDX with the help of metallographic analysis techniques; the length of the secondary phase (LSP) and secondary dendrite arm spacing (SDAS) of α-Al grain were quantified. The results indicated that the second phases (primary Si, eutectic Si, and iron-rich phases) and α-Al grain were significantly refined when the addition of Er increased from 0 to 0.8 wt. %. The mean LSP and SADS values were decreased to a minimum value when the Er addition reached 0.8 wt. %. However, the second phases and α-Al grain became coarser when the level of Er increased more than 0.8 wt. %. The analysis of XRD shows that Er mainly exists in the form of Er2Si compound. The microstructure modification also has a significant effect on the mechanical properties of the alloy. The yield strength (YS), ultimate tensile strength (UTS), and elongation (EL) increase from 52.86 MPa, 163.84 MPa, and 3.45% to 71.01 MPa, 163.84 MPa, and 5.65%, respectively. From the fracture surface, the promotions of mechanical properties are due to the dispersion and pinning reinforcement caused by the Er2Si phase.