Smaller Secondary Dendrite Arm Spacing Research Articles

Numerical study of thermal fluid dynamics and solidification characteristics during continuous wave and pulsed wave laser welding

Pulsed wave (PW) mode laser welding is often preferred under conditions where specific heat input and tailorable cooling rate are required. Previous works on numerical modelling of laser welding mainly focus on continuous wave (CW) mode, while physical understanding of PW mode from numerical investigation is scarce. In this work, a 3D multi-physics thermal fluid model is developed considering both CW and PW modes laser welding. The main physical factors involving heat transfer, thermal fluid flow, Fresnel absorption, recoil pressure induced by metal evaporation, Marangoni effect driven by surface tension, free surface tracing, and laser multiple reflections inside the keyhole are included in this model. The model is tested and validated against the corresponding experimental results for laser welds of stainless steel 316 L. The simulated weld fusion cross sections and surface molten pool agree well with the experimentally observed results for both CW and PW modes. The results show that PW mode exhibits a higher depth of penetration and a smaller molten pool at the same heat input with CW mode. In CW mode laser welding, the shape of keyhole is unstable, while its depth keeps relatively stable. There are two main flow patterns in the molten pool of CW mode welding: an outward flow from keyhole outlet to the rear weld pool and a clockwise flow from the keyhole tip to the rear weld pool. While for PW mode, keyhole and molten pool show periodic behaviors with a period of the laser energy output used. The periodic fluid flow pattern is mainly affected by the gradually changed surface tension and suddenly changed recoil pressure. Furthermore, it is found that a finer microstructure with a smaller secondary dendrite arm spacing is obtained in PW mode due to a higher cooling rate from both the calculated and experimentally measured results.

Effect of melting rate on microsegregation and primary MC carbides in M2 high-speed steel during electroslag remelting

Large-size primary MC carbides can significantly reduce the performance of M2 high-speed steel. To better control the morphology and size of primary MC carbides, the effect of melting rate on microsegregation and primary MC carbides of M2 steel during electroslag remelting was investigated. When the melting rate is decreased from 2 kg·min−1 to 0.8 kg·min−1, the columnar dendrites are gradually coarsened, and the extent of segregation of Mo and V is alleviated, while the segregation of Cr becomes severe. At 2 kg·min−1, the number of primary MC carbides per unit area with the sizes in the range of 2 µm to 6 µm accounts for about 75% of all MC carbides, while the carbides are mainly concentrated on the size larger than 8 µm at 0.8 kg·min−1. Thermodynamic calculations based on the Clyne-Kurz (simplified to C-K) model shows that MC carbide can be precipitated in the final solidification stage and a smaller secondary dendrite arm spacing caused by higher melting rate (2 kg·min−1 in this experiment) facilitates the refinement of primary MC carbides.

Microstructure characteristics and mechanical properties improvement of gravity cast Al-7Si-0.4Mg alloys with Zr additions

Al-7Si-0.4Mg alloys are one of the most commonly applied aluminum alloys in various casting processes. In this study, 0.06%, 0.14% and 0.20% Zr are added to gravity cast Al-7Si-0.4Mg alloys. The microstructural analysis of the as-cast alloys with Zr shows the formation of (Al,Si)3(Zr,Ti) and π-Fe phases. Compared with alloys without Zr, the alloys with Zr have finer grains; additionally, a small secondary dendrite arm spacing (SDAS) is obtained with a proper Zr content. After T6 heat treatment, the Mg in Fe-rich intermetallics diffuse into the matrix, decreasing the size of the intermetallic phases. Nanoscale precipitates of the (Al,Si)3(Zr,Ti) phase are found in α-Al grains. Due to the finer grains and smaller SDAS as well as the presence of (Al,Si)3(Zr,Ti) precipitates, the tensile strength and elongation of the alloys with 0.06% Zr are 332 MPa and 8.7%, respectively, whereas those of the Al-7Si-0.4Mg alloys without Zr are 302 MPa and 4.6%, respectively.

Influence of Electropulsing Treatment on the Initial Solidification of Molten Steel During Continuous Casting

An electropulsing-assisted mold simulator (EPMS) technique was developed to investigate the effect of pulsed electric current on the initial solidification behavior of molten steel during the process of continuous casting. The results indicated that the variation of mold hot surface temperatures in the meniscus area decreased from 375.8 K to 394 K for the case without electropulsing treatment (EPT) to 363.5 K to 377.3 K for the one with EPT. The mold surface heat fluxes fluctuated around the baseline of 0.83 and 2.42 MW/m2 in the cases with and without EPT, respectively, which indicated that a higher thermal resistance between the initial shell and copper mold was introduced by employing the EPT. The crystal fraction of slag films increased from 69.8 to 77.9 pct, whereas the grain size of mold flux film decreased, when compared the case of EPT treatment with the benchmark one, which suggested that the pulsed electric current promotes the movement of molten clusters and particles, leading to the enhancement of mold flux crystallization and the improvement of thermal resistance. Moreover, the solidification structure of steel along the direction of solidification (from the mold to melt) includes a fine chilled grain layer close to the mold, a columnar dendrite layer in the middle, and equiaxed grains next to the melt core, in which a compacted microstructure with smaller secondary dendrite arm spacing was observed in the case of the EPT. The results demonstrate that the EPT could refine the as-cast solidification microstructure.

Microstructure, Texture and Mechanical Properties of Al Alloy A380 Prepared by Directional Solidification Method

The mechanical properties of Al-Si alloys are affected by several microstructural features such as secondary dendrite arm spacing (SDAS), size and shape of eutectic Si-particles, presence of intermetallics as well as by porosity. In the current study, Al-Si-Cu alloy A380 was prepared by a unique directional solidification method to produce samples with two different SDAS of 9 μm and 27 μm. The lower solidification rate resulted in larger SDAS, larger grain size, larger eutectic Si and larger intermetallics including Fe-rich β phase. The microstructure with higher solidification rate was found to be finer and more homogeneous with smaller eutectic Si and intermetallics. The specimen with larger SDAS exhibited stronger texture than the one with smaller SDAS. The specimen with smaller SDAS showed improved mechanical properties including YS, UTS and ductility.

Effect of chemical composition on microstructure, strength and wear resistance of wire deposited Ni-Cu alloys

Two Ni-Cu alloys (Monel K500 and FM 60) having various Mn, Fe, Al, Ti and C contents were deposited on a Monel K500 plate at three different speeds using wire arc additive manufacturing technique. Microstructure characterisation, in particular a detailed study of precipitates, was carried out using optical and scanning electron microscopy. Mechanical properties were assessed using hardness, tensile and wear testing. For similar deposition conditions, Monel K500 has exhibited smaller secondary dendrite arm spacing and higher number density of Ti-rich particles, although the Ti concentration in FM 60 was higher. Finer microstructure and Ti precipitation led to superior hardness, tensile and wear resistance of Monel K500 compared to FM 60. The variation in microstructure-properties relationship with alloy composition is discussed.

Effect of Dendritic Morphology and Central Segregation of Billet Castings on the Microstructure and Mechanical Property of Hot-Rolled Wire Rods

Microscopic dendritic morphology and central macrosegregation in continuously cast high carbon steel billets with a cross section 150 × 150 mm2 are investigated to elucidate their effect on the microstructure and mechanical property of the following hot-rolled wire rods, whose diameters are 10 and 6.5 mm, respectively. Experimental observations and numerical simulations for the steel billets under different casting conditions are carried out. It is found that the secondary dendrite arm spacing (SDAS) in the billets increases with the increase of superheat temperature, while decreases with the increasing specific water flow. Smaller SDAS in the billet center leads to lower segregation index of carbon (rC) and manganese (rMn) due to the low permeability in the final solidification stage. The measured rMn and rC in billets are in an approximately linear relationship. In addition, for the same rolling and cooling process, the severer carbon segregation in the center of the billets is, the lower the sorbite rate of the rods and the worse stability in the mechanical property of the rods. The tensile strength of wire rods can be improved by adjusting the casting parameters to decrease the SDAS and reduce the central carbon segregation of the billets.

Low cycle fatigue behavior of T4-treated Al–Zn–Mg–Cu alloys prepared by squeeze casting and gravity die casting

Gravity die casting (GC) and squeeze casting (SC) T4-treated Al–7.0Zn–2.5Mg–2.1Cu alloys were employed to investigate the microstructures, mechanical properties and low cycle fatigue (LCF) behavior. The results show that mechanical properties of SC specimens are significantly better than those of GC specimens due to less cast defects and smaller secondary dendrite arm spacing (SDAS). Excellent fatigue properties are obtained for the SC alloy compared with the GC alloy. GC and SC alloys both exhibit cyclic stabilization at low total strain amplitudes (less than 0.4%) and cyclic hardening at higher total strain amplitudes. The degree of cyclic hardening of SC samples is greater than that of GC samples. Fatigue cracks of GC samples dominantly initiate from shrinkage porosities and are easy to propagate along them, while the crack initiation sites for SC samples are slip bands, eutectic phases and inclusions at or near the free surface.

SDAS, Si and Cu Content, and the Size of Intermetallics in Al-Si-Cu-Mg-Fe Alloys

Plates of Al-(a)Si-(b)Cu-Mg-(c)Fe alloys with varying content of (mass pct) Si (a = 3, 4.5, 7.5, 9, 10, or 11), Cu (b = 0, 1, or 4), and Fe (c = 0.2, 0.5 or 0.8) were cast in sand molds with a heavy chill at one end to ensure quasi-directional solidification over a wide range of Secondary Dendrite Arm Spacing (SDAS). Statistical analysis on the size of the β-Al5FeSi, α-Al8Fe2Si, or Al2Cu intermetallics on Backscattered Electron images showed that a high Si content reduced the size of the β platelets in alloys with up to 0.5 Fe content regardless of the SDAS, whereas at small SDAS the refining effect extended up to 0.8 Fe, and involved α-phase intermetallics which replaced the beta platelets at those concentrations. At low Si contents, a high Cu level appeared to have similar refining effects as increased Si, through the formation of α-phase particles in the post-eutectic stage which agglomerated with the Al2Cu intermetallics. A high content of Si appears to make the overall refining process less critical in terms of SDAS/cooling rate.

Phase field simulation of dendrite growth with boundary heat flux

AbstractBoundary heat flux has a significant effect on solidification behavior and microstructure formation, for it can directly affect the interfacial heat flux and cooling rate during phase transition. In this study, a phase field model for non-isothermal solidification in binary alloys is employed to simulate the free dendrite growth in undercooled melts with induced boundary heat flux, and an anti-trapping current is introduced to suppress the solute trapping due to the larger interface width used in simulations than a real solidifying material. The effect of heat flux input/extraction from different boundaries was studied first. With heat input from boundaries, the temperature can be raised and the dendritic morphology changed with gradient temperature distribution caused by the heat flux input coupling with latent heat release during the liquid-solid phase transition. Also, the concentration distribution can be also influenced by this irregular temperature distribution. Heat flux extraction from the boundaries can decrease the temperature, which results in rapid solidification with small solute segregation and concentration changes in the dendrite structures. Also, dendrite growth manner changes caused by undercooling variation, the result of competition between heat flux and latent heat release from phase transition, are also studied. Results indicate that heat flux in the simulation zone significantly reduces the undercooling, thus slowing down the dendrite formation and enhancing the solute segregation, while large heat extraction can enlarge the undercooling and lead to rapid solidification with large dendrite tip speed and small secondary dendrite arm spacing, while solute segregation tends to be steady. Therefore, the boundary heat flux coupling with the latent heat release from the solidification has an effective influence on the temperature gradient distribution within the simulation zone, which leads to the morphology and concentration changes in the dendritic structure formation.

Tensile properties and fracture behavior of partial squeeze added slow shot die-cast A356 aluminum alloy

In this contribution, effect of the microstructural characteristics on tensile properties and fracture behavior of partial squeeze added slow shot die-cast A356 alloy die casting in the as-cast and T6 heat-treated conditions was studied. The results show that, inferior tensile properties of the casting partial squeeze part were caused by the heterogeneity of α-Al cells with fragment, rosette, angular and globular shapes, while finer dendrites with smaller secondary dendrite arm spacing and more rounded silicon particles corresponded to higher tensile properties. After T6 treatment, tensile properties increased significantly, due to the spheroidization of silicon particle and consequently the reduction of stress concentration at silicon/eutectic matrix interface. Differences observed in the tensile fracture path were attributed to microstructural changes as well as morphological aspects of silicon phase.

Effects of Secondary Dendrite Arm Spacing on Microporosity of A357 Alloy

Microporosity in cast aluminum alloys formed during solidification of castings can be due to the evolution of dissolved hydrogen gas from the liquid metal, the inability of liquid metal to feed through interdendritic channels, or a combination of both. The secondary dendrite arm spacing (SDAS) of the casting may affect either the evolution of hydrogen gas or the ability of feeding. In this investigation, a method based on the Nearest-Neighbor-Distance (NND for short) cluster analysis of image analysis data was used for distinguishing the gas and shrinkage pores in A357 alloy, in order to study the effects of SDAS on the microporosity formation. It shows that the shrinkage pores are prone to form in the specimens with small SDAS. The discrete, isolated gas microporosity is prone to form in the specimens with large SDAS.

Effect of Minor Sb Additions on SDAS, Age Hardening and Mechanical Properties of A356 Aluminium Alloy Casting

A356 is the aluminum casting grade which has compositions that combines outstanding casting characteristics with excellent properties after heat treatment. Mechanical properties of A356 can be improved by reducing of secondary dendrite arm spacing (SDAS), precipitation hardening, and the interaction effect of both. It is generally accepted that dendrite arm spacing and fine distribution microstructure are related to each other and they also affect the precipitation hardening in a way that smaller SDAS results in shorter time required to obtain a satisfactory degree of solution of the undissolved or precipitated soluble phase constituents and to achieve good homogeneity. Minor addition of Sb was successfully used in reducing the SDAS in previous work. However, the effect of Sb addition on age hardening has not been investigated, especially in a high cooling rate condition. In this research, effects of minor addition of Sb on SDAS, age hardening and mechanical properties; i.e. hardness and tensile properties, are reported. It was found that Sb addition did not clearly affect SDAS at the high cooling rate, i.e. as in permanent mold casting process. Moreover, we found that the addition of Sb into A356 also lowered mechanical properties.

Assessment of Thermo-Mechanical Fatigue Behaviors of Cast Al-Si Alloys by Experiments and Multi-Step Numerical Simulation

Out-of-phase type thermo-mechanical fatigue tests have been performed for Al-Si cast alloys with the temperature range of 323–523 K and the applied mechanical strain range of 0.5–1.5%. Thermo-mechanical fatigue lives and stress-strain hysteresis loops are investigated by the experiments. In general, the thermo-mechanical fatigue is affected by various factors such as thermal expansion/contraction, elasto-plastic/creep deformation, softening by overaging, dynamic recovery, damaging and cracking. Multi-step finite element simulation techniques have provided an effective way of assessing the local damaging behavior of silicon particles, along with extracting the contribution from creep. Although the size and shape of the silicon particles in the material with a higher solidification rate are similar to those of a slowly cooled material, it exhibits superior thermo-mechanical fatigue property together with a smaller secondary dendrite arm spacing (SDAS, hereinafter). Since the difference in the stress-strain hysteresis loops between the two materials has vanished when the softening by overaging is almost finished, the observed difference in the thermo-mechanical properties is attributed to age-hardenability rather than the SDAS. The effects of damaging at the silicon particles to this difference are also suggested. In fact, damaged silicon particles have been observed extensively from an early stage of the thermo-mechanical loading, then forming fatigue cracks by the linkage of the damages. It has been clarified by the simulation that interfacial debonding is likely to occur rather than particle cracking in the materials used. The simulation provides valuable insights into the understanding of the spatial distribution of damage in the eutectic region. The simulation has also enabled to assess the contribution of the creep deformation, indicating that medium to high cycle fatigues are significantly affected by it.

Plastic deformation behavior of aluminum casting alloys A356/357

The plastic deformation behavior of aluminum casting alloys A356 and A357 has been investigated at various solidification rates with or without Sr modification using monotonic tensile and multi-loop tensile and compression testing. The results indicate that at low plastic strains, the eutectic particle aspect ratio and matrix strength dominate the work hardening, while at large plastic strains, the hardening rate depends on secondary dendrite arm spacing (SDAS). For the alloys studied, the average internal stresses increase very rapidly at small plastic strains and gradually saturate at large plastic strains. Elongated eutectic particles, small SDAS, or high matrix strength result in a high saturation value. The difference in the internal stresses, due to different microstructural features, determines the rate of eutectic particle cracking and, in turn, the tensile instability of the alloys. The higher the internal stresses, the higher the damage rate of particle cracking and then the lower the Young’s modulus. The fracture strain of alloys A356/357 corresponds to the critical amount of damage by particle cracking locally or globally, irrespective of the fineness of the microstructure. In the coarse structure (large SDAS), this critical amount of damage is easily reached, due to the clusters of large and elongated particles, leading to alloy fracture before global necking. However, in the alloy with the small SDAS, the critical amount of damage is postponed until global necking takes place due to the small and round particles. Current models for dispersion hardening can be used to calculate the stresses induced in the particles. The calculations agree well with the results inferred from the experimental results.

Microstructural effects on the tensile and fracture behavior of aluminum casting alloys A356/357

The tensile properties and fracture behavior of cast aluminum alloys A356 and A357 strongly depend on secondary dendrite arm spacing (SDAS), Mg content, and, in particular, the size and shape of eutectic silicon particles and Fe-rich intermetallics. In the unmodified alloys, increasing the cooling rate during solidification refines both the dendrites and eutectic particles and increases ductility. Strontium modification reduces the size and aspect ratio of the eutectic silicon particles, leading to a fairly constant particle size and aspect ratio over the range of SDAS studied. In comparison with the unmodified alloys, the Sr-modified alloys show higher ductility, particularly the A356 alloy, but slightly lower yield strength. In the microstructures with large SDAS (>50 µm), the ductility of the Sr-modified alloys does not continuously decrease with SDAS as it does in the unmodified alloy. Increasing Mg content increases both the matrix strength and eutectic particle size. This decreases ductility in both the Sr-modified and unmodified alloys. The A356/357 alloys with large and elongated particles show higher strain hardening and, thus, have a higher damage accumulation rate by particle cracking. Compared to A356, the increased volume fraction and size of the Fe-rich intermetallics (π phase) in the A357 alloy are responsible for the lower ductility, especially in the Sr-modified alloy. In alloys with large SDAS (>50 µm), final fracture occurs along the cell boundaries, and the fracture mode is transgranular. In the small SDAS (<30 µm) alloys, final fracture tends to concentrate along grain boundaries. The transition from transgranular to intergranular fracture mode is accompanied by an increase in the ductility of the alloys.