Globular-to-needle Zn-rich phase transition during transient solidification of a eutectic Sn-9%Zn solder alloy

The aim of this article was to investigate the microstructural evolution of a eutectic Sn-9%Zn solder alloy as a function of growth rate during transient unidirectional solidification. It was found that globular-like and needle-like Zn-rich phases prevail at growth rates ranging from 0.5 to 2 mm/s and 0.3 to 0.1 mm/s, respectively, with a transition region occurring between these growth rate ranges. The microstructure control in soldering processes can be accomplished by manipulating solidification processing variables such as the cooling rate and the growth rate, since the resulting morphological microstructure depends on heat transfer conditions imposed by solidification, and as a direct consequence will affect the final properties.     

Improvement of tensile properties of Al-Si alloys through directional solidification

This study examines the tensile properties in directionally solidified (DS-route) Al-Si alloys and in conventionally die casting (DC-route) of the same composition. An attempt has been made to correlate the results with the fracture and microstructure of the alloys. One of the most important findings in this work is that a marked improvement in ductility is observed for DS samples over their DC-processed counterparts.     

Effect of silicon content on microstructure and electrochemical behavior of hypoeutectic Al-Si alloys

The correlation of mechanical properties and corrosion behavior with microstructure parameters can be very useful for planning solidification conditions in order to achieve a desired level of final properties. The present study aims to investigate the influence of silicon content on the microstructural pattern, i.e., dendrite spacings and distribution of interdendritic phases on the corrosion behavior of Al-Si alloys castings. The corrosion resistance was analyzed by both the electrochemical impedance spectroscopy (EIS) technique and Tafel's plots carried out in a 0.5 M NaCl test solution at 25 °C. The increase on silicon content has provided a dendritic refinement and a more extensive redistribution of the eutectic mixture which has provoked a decrease on the corrosion resistance.     

Effect of isothermal extrusion parameters on mechanical properties of Al-Si eutectic alloy

In this paper, mechanical properties of a deformed Al-Si eutectic alloy processed by isothermal extrusion at temperature from 573 K to 773 K with reduction ratio from 25% to 85% were investigated at ambient temperature. The results showed that a banded structure composed of matrix region and accumulation region of second phase particles was formed and a few cracks were generated in particles and evolved to voids among particles. The tensile strength of test specimens ranged from 250 MPa to 400 MPa and was directly related with temperature from 623 K to 773 K. The elongation of test specimens ranged from 2.8% to 13.1%, and had a peak value at 673 K under each section reduction ratio. A reduction in elongation occurred at section reduction ratio larger than 75% because of particle bands splitting aluminum matrix severely. The effect of temperature on mechanical properties was more significant than that of section reduction ratio. Excellent balance between strength and ductility can be obtained by extrusion at temperature 623-723 K and section reduction ratio 40-70%.     

Graphite Nucleation in Cast Iron Melts Based on Solidification Experiments and Microstructure Simulation

Microstructure strongly influences the mechanical properties of cast iron. By inoculating the melt with proper inoculants, foreign substrates are brought into the melt and eventually the graphite can crystallize on them. The elements and substrates that really play a role for nucleation are yet unknown. Until now there is very little knowledge about the fundamentals of nucleation, such as composition and morphology of nuclei. In this work we utilized EN-GJL-200 as a base material and examined several produced specimens. The specimens were cast with and without inoculants and quenched at different solidification states. Specimens were also examined with a high and low oxygen concentration, but the results showed that different oxygen contents have no influence on the nucleation in cast iron melts. Our research was focused on the microscopic examination and phase-field simulations. For studying the samples we applied different analytical methods, where SEM-EDS, -WDS were proved to be most effective. The simulations were conducted by using the software MICRESS, which is based on a multiphase-field model and has been coupled directly to the TCFE3 thermodynamic database from TCAB. On the basis of the experimental investigations a nucleation mechanism is proposed, which claims MnS precipitates as the preferred site for graphite nucleation. This theory is supported by the results of the phase-field simulations.     

Formation of core-type microstructure in Al-Bi monotectic alloys

The macroscopic morphologies and core-shell dimensional accuracy for Al-Bi immiscible alloys were investigated. Irrespective of compositions, the shell always consists of a Bi-rich phase due to surface segregation. With decreasing the superheat degree above the highest critical point of Al-65.5 wt.% Bi alloy, the shell of particles changed from irregular, annular to crescent shapes under the actions of Marangoni motion and gravity. The simulation suggests that the superheating and particle size affect markedly the temperature gradient and cooling rate of droplets, and thus influence the Marangoni and Stokes motions. At a superheat degree of 100 K, a perfect core-shell microstructure of Al-65.5 wt.% Bi particles with a diameter of 0.9 mm was successfully obtained. The dimensional relationship between the core and whole particle could be described by a function of D_core = 0.9137 D_particle − 0.0312.     

Study on influence of surface coating during cooling slope rheocasting process

Abstract

The cooling slope process is a simple method to produce feedstock materials for semisolid processing. In this work, the influence of surface coating of the cooling slope on grain morphology was examined. The investigation showed that the coating on the surface of the slope can affect the forming of both the solidification shell and the morphology of α-Al phase. A zebra form coating was proposed and tested, which cannot only reduce the solidification shell but also improve the grain microstructure.     

Microstructure, Compression Property and Shape Memory Effect of Equiatomic TaRu High Temperature Shape Memory Alloy

The microstructure, phase transformation, compression property and strain recovery characteristics of equiatomic TaRu super high temperature shape memory alloy have been studied by optical microscope, XRD, DTA, compression tests and TEM observations. When cooling the alloy specimen from high temperature to the room temperature,β(parent phase)→β′(interphase) →β"(martensite) two-step phase transformations occur. The microstructure at room temperature show regularly arranged band morphology, with the monoclinic crystal structure. The twinning relationship between the martensite bands is determined to be (101) of Type I. Reorientation and coalescence of the martensite bands inside the variant happened during compression at room temperature. The β′→β reversible transformation contributes mainly the shape memory effect, with the maximum completely recovery strain of 2%.     

Effects of trace Mn addition on the elevated temperature tensile strength and microstructure of a low-iron Al-Si piston alloy

The effects of different amount of trace Mn addition on the elevated temperature tensile strength and microstructure of a low-iron Al-Si piston alloy were investigated by tensile testing, EPMA, SEM and EDX. It has been found that with the increase of Mn addition, the elevated temperature tensile strength of the examined alloy exhibits an interesting change: from primary 67.07 MPa, increased to 75.62 MPa and then decreased to 69.67 MPa, 63.72 MPa; finally increased again to 71.92 MPa. Through microstructural analysis, it is found that there occurs an evolution of Mn-bearing intermetallic phases in alloy with the increase of the Mn addition into the alloy. When the amount is 0.15 wt.%, most of manganese exists in the dendritic Al₉FeNi phase; further addition of Mn can lead to the formation of plate-like Mn-bearing phases, whose strengthening effect is poor.     

Effect of Heat Treatments on Austempered Ductile Iron

This work presents the influence of austempering heat treatment carried out in one-step and two-step processes on the microstructures and mechanical properties of ductile cast iron. The samples were extracted from as-cast pieces and heat treated by austempering. For the one-step process the samples were heated at 910°C for 90 min for austenitization and cooled in salt bath at a temperature of 300°C for 30 min. For the two-step process the samples were cooled from 910°C to 245°C, kept at this temperature for 5 min in salt bath, then heated in another salt bath at a temperature of 300°C for 30 min. The samples were analyzed by optical microscopy and mechanical tests. After the one-step austempering, microscopic analysis of the samples showed ausferrite microstructure matrix and graphite in nodules surrounded by fine pearlite. For the two-step austempering, the presence of ausferrite matrix with graphite in nodules and retained austenite was observed. As to mechanical properties, the results showed that, with the two-step process there was gain (4.7%) in the average hardness and loss (3.5%) in the impact resistance. The microhardness of the ausferrite was 6.2% higher in the one-step austempering when compared to the two-step process.     

Microstructure evolution in equiaxed dendritic growth

Microstructure evolution in equiaxed dendritic solidification is investigated through the study of free dendritic growth in a supercooled melt. A detailed measurement of microstructural features (such as side-branch spacings, envelope shape, projection area, and contour length) of freely growing succinonitrile dendrites is performed using images from the microgravity experiment of Glicksman and co-workers. The measurements show that the microstructure evolution of an equiaxed dendrite is divided into two regimes: an initial linear regime and a subsequent non-linear coarsening regime. It is found that unique scaling relations exist between the measured geometry parameters and the primary tip radius or speed for both regimes. The underlying mechanisms involved in dendritic structure evolution are discussed. In addition, using the phase-field method, we perform numerical experiments to investigate the effects of melt convection on equiaxed dendritic growth. The dendrite tip operating state (i.e. the tip velocity and radius) is quantitatively evaluated as a function of the flow velocity and dendrite orientations and compared with Microscopic Solvability Theory. Other structural features (such as the side-branches) of an equiaxed dendrite in the presence of flow are also examined in order to show how convection influences microstructure evolution in equiaxed dendritic growth.     

Thermodynamic–kinetic model for the simulation of solidification in binary copper alloys and calculation of thermophysical properties

An earlier developed thermodynamic–kinetic solidification model for binary copper alloys is extended to take into account the formation of the bcc phase via the peritectic transformation and the formation of binary compounds from the fcc phase. Also the eutectic and eutectoid transformations are simulated but only approximately, by modeling the movement of the fcc/eutectic and fcc/eutectoid interfaces due to the diffusion kinetics of the fcc phase only. The new model can handle binary copper alloys containing solutes Ag, Al, Cr, Fe, Mg, Mn, Ni, P, Si, Sn, Te, Ti, Zn and Zr. Depending on the alloy composition, cooling rate and dendrite arm spacing, the model determines the fractions and compositions of the phases (liquid, fcc, bcc, compounds) and calculates thermophysical material properties (enthalpy, specific heat, thermal conductivity, density and liquid viscosity), needed in heat transfer models, from the liquid state down to room temperature. The model is applied to Cu–Sn and Cu–Zn alloys but also to some other binary alloys to show the effect of cooling on the phases formed. Depending on the alloy system, the solidification structures obtained after real cooling processes are shown to be quite different from those estimated from phase diagrams.     

Modelling of Mould Filling and Solidification of Castings

An experimental casting for validation has been designed. The casting is composed of two 50×600×2.5 (width×length×thick) thin-wall pieces. One downsprue is located in the middle. A pouring cup with a stopper is used. This design allows to using two different types of moulds simultaneously. An Al-10%Si alloy has been poured at different temperatures. Two effects have been studied: one is the pouring temperature and the other is the moulding method (namely by machine or manually). The filling length is proportional to the pouring temperature. The influence of different moulding methods on mould filling is more complicated. The filling length in the manual-made mould is 1.5 times as long as the one in the machine-made mould due to the different thermal conductivities. Vents have little influence. A finite volume based computer code which can simulate fluid flow during mould filling coupled with heat transfer as well as solidification has been developed in WTCM Foundry Center.. The code can predict cold shut during mould filling and shrinkage defects during solidification. The simulated results are in good agreement with the experiments.In the second part of the paper, an example is given which illustrates how to use computer simulation to aid designing the casting system. The final computational result is compared with the industrial casting. The process of designing castings by using simulation is completely different from the traditional way. The computer aided casting design offers the possibility to obtain a sound casting from the first time.     

Directional solidification of Cu-20Sn alloy at low speed: From peritectic coupled growth to banding

Bridgman-type directional solidification experiments have been carried out in Cu-20Sn peritectic alloy. Peritectic coupled growth and banding structures have been observed at low growth rates (1.5 and 2 μm/s) under a temperature gradient up to 40 K/mm. The peritectic coupled growth structure, containing rod dendrite primary α phase plus peritectic β phase, forms initially. As solidification proceeds, peritectic coupled growth is overgrown by banding or island banding structures. The formation of banding structure from coupled growth is explained by a model involving Sn concentration change at nucleation of the secondary phase ahead of the solid/liquid interface. It is found that the competitive growth between the α and β phases also plays a critical role on the formation of banding structures.     

Characterization of tensile properties and microstructures in directionally solidified Al–Si alloys using linear roughness index

The tensile properties in directionally solidified Al–Si alloys and in gravity die casting of the same composition are presented. Examples of relating both the tensile and the microstructural properties of these alloys with the fracture roughness index are indicated. The roughness index was measured on vertical sections cut through the tensile fracture surface. The tensile properties examined were the yield and ultimate tensile strengths, strain at fracture, and Young's modulus. The analyzed microstructural features were porosity, dendrite area fraction, secondary dendrite arm spacing, and Si particle spacing. In almost all cases an unambiguous correlation was found between the roughness index and the tensile or the microstructural properties. A marked improvement in ductility was observed for directionally solidified samples over their gravity die casting processed counterparts. The roughness index diminished going from die cast to direction solidification, and this is likely accompanied by change in the fracture mechanisms.     

Microstructural and mechanical features of aluminium semi-solid alloys made by rheocasting technique

The rheocasting process applied by Swirled Enthalpy Equilibration Device (SEED) technique relies on rapid extraction of a controlled quantity of heat from the liquid aluminium alloy via mechanical agitation to form the semi-solid slurry that can be formed under pressure. Microstructural characteristics of both conventional and semi-solid A357 castings under T6 heat treatment conditions were examined using optical and scanning electron microscopy. The fatigue and tensile experiments were applied to evaluate the effect of SEED technique on the mechanical properties of T6-A357 semi-solid alloys and conventional castings. The results showed that the rheocasting–SEED technique has proved successful in producing optimum microstructure of Al–Si–Mg semi-solid alloys providing an excellent combination of quality and mechanical performance as compared to conventional technique.

This paper is part of a Themed Issue on Aluminium-based materials: processing, microstructure, properties, and recycling.     

Numerical Simulation of Morphology and Microsegregation Evolution during Solidification of Al-Si Alloy

A stochastic model coupled with transient calculations for the distributions of temperature, solute and velocity during the solidification of binary alloy is presented. The model can directly describe the evolution of both morphology and segregation during dendritic crystal growth. The model takes into account the curvature and growth anisotropy of dendritic crystals. Finite difference method is used to explicitly track the sharp solid liquid (S/L) interface on a fixed Cartesian grid. Two-dimensional mesoscopic calculations are performed to simulate the evolution of columnar and equiaxed dendritic morphologies of an AI-7 wt pct Si alloy. The effects of heat transfer coefficient on the evolution of both the dendrite morphology and segregation patterns during the solidification of binary alloys are analyzed. This model is applied to the solidification of small casting. Columnar-to-equiaxed transition is analyzed in detail. The effects of heat transfer coefficient on final casting structures are also studi     

Formability of two aluminium alloys

Abstract

The suitability of two recently developed aluminium alloys (an Al–Mg–Mn alloy and an Al–Li–Cu alloy) for press forming applications has been examined. The characterisation involved the experimental determination of microstructural aspects, tensile properties, and formability parameters such as average plastic strain ratio and planar anisotropy. The forming limit diagram has been experimentally evaluated. A detailed analysis of the strain distribution profiles obtained from punch stretching experiments has been attempted. An attempt has been made to correlate the crystallographic texture with the formability parameters. The fracture surfaces of the punch stretched samples were observed using scanning electron microscopy with a view to obtaining a correlation between fracture behaviour and formability. The alloys, in particular the Al–Mg–Mn alloy, have been found to possess good stretchability but both show very limited drawability. Texture analysis indicated negligible earing during deep drawing. These alloys are suitable for stamping applications where stretching constitutes the major proportion of the deformation.