Non-dendritic Microstructure Research Articles

Effect of TiC addition on semisolid isothermal heat treated microstructure of ZA84 magnesium alloy

The semisolid metal processing now is becoming an important forming process for the fabrication of magnesium alloys parts. Among the various methods of obtaining semisolid billet with non-dendritic microstructure, semisolid isothermal heat treatment is thought as a potential method. To provide a theoretical guide for the semisolid processing of ZA84 magnesium alloy, the semisolid isothermal heat treated microstructure of ZA84 magnesium alloy with and without TiC modifications is investigated. The results indicate that it is possible to produce a non-dendritic microstructure of ZA84 alloy by semisolid isothermal heat treatment, and that adding 0·5 wt-%TiC to ZA84 alloy benefits the non-dendritic microstructural evolution of the alloy. After treatment at 570°C for 120 min, the ZA84+0·5TiC alloy gives a non-dendritic microstructure with a higher liquid fraction (0·22 to 0·25) and finer unmelted primary solid particles (57 to 48 μm) than the ZA84 alloy. The formation of finer unmelted primary solid particles in the semisolid ZA84+0·5TiC alloy is possibly related to the refinement of the initial as cast microstructure and the higher liquid fraction of semisolid.

Effect of Sb on microstructure of semi-solid isothermal heat-treated AZ61-0.7Si magnesium alloy

The microstructure of semi-solid isothermal heat-treated AZ61-0.7Si magnesium alloy was investigated, and compared with that of the Sb-modified AZ61-0.7Si alloy. The results indicate that it is possible to produce the AZ61-0.7Si alloy with a non-dendritic microstructure by semi-solid isothermal heat treatment, and adding 0.4% Sb can accelerate the non-dendritic microstructural evolution of the alloy. After being treated at 580 °C for 120 min, the Sb-modified AZ61-0.7Si alloys can obtain a non-dendritic microstructure with a higher liquid content (16%–18%) and finer unmelted primary solid particles (43–53 μm) compared with the alloy without Sb modification. In addition, after being treated at 580 °C for 120 min, the Mg 2Si phases in the AZ61-0.7Si alloys with and without Sb modification change from initial Chinese script shape to granule and/or polygon shapes.

Effect of solid solution treatment on semisolid microstructure of dendritic zinc alloy ZA27

The effect of solid solution treatment on semisolid microstructure of a Zn alloy ZA27 with developed dendrites was investigated and a new approach to produce non-dendritic semisolid microstructure available for thixoforming was developed. The results indicated that a semisolid microstructure with small and spherical primary particles could be obtained when the alloy with developed dendrites was solution treated for 3 h at 370°C and then partially remelted at a semisolid temperature of 460°C. This method could not only overcome some inevitable shortcomings resulting from using some present popular approaches, but also simplify the procedure of thixoforming. Too short or too long solution treatment duration could increase the shape factor, size and inhomogeneity of the primary particles. All of these microstructural changes were attributed to the dendrite coalescence and phase transformation which occurred during solid solution treatment.

Numerical Simulation of Semi-Solid Roll Strip-Casting Process for AZ91D Magnesium Alloy

High quality magnesium alloy sheets with non-dendritic microstructure can be manufactured by the technique which incorporates semi-solid processing with continuous roll stripcasting. In particular, the process is believed to be cost-effective in mass production of magnesium sheet products. The processing parameters, such as roll speed and temperature, have significant influence on roll strip-casting process. In this paper, the commercial software, DEFORM-3D, for deformation simulation of bulk materials was used to carry out the simulation of roll strip-casting process for AZ91D magnesium alloy in semi-solid state, and the distribution of equivalent stress, equivalent strain and temperature was obtained. Based on the results, the optimization of processing parameters was discussed.

The Natural and Artificial Aging Response of Semi-Solid Metal Processed Alloy A356

The heat treatment cycles that are currently applied to semi-solid processed components are mostly those that are in use for dendritic casting alloys. These heat treatments are not necessarily the optimum treatments for non-dendritic microstructures. For rheocast alloy A356, it is shown that natural aging prior to artificial aging causes the time-to-peak-hardness to be longer compared to the time when only artificial aging is used. Furthermore, a hardness plateau is maintained during artificial aging at 180oC between 1 and 5 hours without any prior natural aging. A natural aging period as short as 1 hour results in a hardness peak (rather than a plateau) to be reached during artificial aging after 4 hours at 180oC.

Development of Non-Dendritic Microstructure of Aluminum Alloy in Semi-Solid State under Ultrasonic Vibration

The effect of processing parameters on the semi-solid microstructure has been investigated in the course of semi-solid slurry preparation of A356 Al alloy by ultrasonic vibration method. The A356 melt in temperature of 630660°C was poured into a special metal cup, and exposed to ultrasonic vibration at 20 kHz or isothermal holding for a certain time after vibration. The results show that the semi-solid slurry with primary α-Al crystals smaller than 90 μm and an average shape coefficient F S (SF was defined as SF4 A/L2P) above 0.5 could be prepared by ultrasonic vibration for a time less than 144 s. In the isothermal holding period after a short time of ultrasonic vibration, the average particle diameter increased with the increase of holding time, but F S decreased at first, and then increased a little. It was discovered that ultrasonic vibration is a good method to prepare semi-solid slurry with fine and relatively round primary crystals due to cavitation and acoustic streaming effects.

Effects of holding temperature and time on semi-solid isothermal heat-treated microstructure of ZA84 magnesium alloy

The feasibility of fabricating ZA84 magnesium alloy with non-dendritic microstructure by a semi-solid isothermal heat treatment process and the effects of holding temperature and time on the semi-solid isothermal heat-treated microstructure of the alloy were investigated. The results indicate that it is possible to produce ZA84 alloy with non-dendritic microstructure by suitable semi-solid isothermal heat treatment. After being treated at 560-575 °C for 120 min, ZA84 magnesium alloy can obtain a non-dendritic microstructure with 14.2%-25.6% liquid fraction and an average size of 56-65 μm of the unmelted primary solid particles. With the increasing holding time from 30 to 120 min or holding temperature from 560 to 575 °C, the average size of unmelted primary solid particles decreases and globular tendency becomes more obvious. Under the experimental condition, the microstructural evolution of ZA84 alloy during semi-solid isothermal treatment is mainly composed of three stages of initial coarsening, structural separation and spheroidization. The subsequent coarsening of spheroidal grains is not observed.

Effect of predeformation manner on semi-solid structure of AZ61 magnesium alloy in sima process

The effect of predeformation manner, predeformation ratio and isothermal heat-treat parameter on the non-dendrite structure of AZ61 magnesium alloy in SIMA process was studied. Under coequal heat-treat condition, the impact of the hot upsetting predeformation on semi-solid microstructure in SIMA process was compared with that of the cold compressive predeformation. The results indicate that non-dendrite microstructure in AZ61 magnesium alloy billets can be obtained by hot or cold upsetting predeformation in SIMA process, although their mechanisms of evolution are different. Increasing hot or cold upsetting predeformation ratio can enhance the effect and quality of the non-dendrite microstructure formed before storage energy up to saturation, but the proper isothermal temperature and holding time should be selected.

Recrystallisation in the Semi-Solid State in 7075 Aluminium Alloy

7000 series aluminium alloys are currently machined from the wrought state with much waste. There is therefore a motivation to identify effective near net shaping routes. Semi-solid processing is one such potential route. It relies on the thixotropic behaviour of alloys with non-dendritic spheroidal microstructures. In the semisolid state the material thins when sheared and will flow to fill the die. When worked material is reheated, it recrystallises and moving into the semi-solid state, the required spheroidal microstructure develops. Here we examine the early stages of spheroid formation in 7075 aluminium alloy reheated from the as-supplied T6 condition. This alloy is very resistant to recrystallisation in the solid state due to the presence of dispersoid particles pinning grain boundaries. There is a sudden increase in the appearance of spheroidal grains. There is a close association with the position of the first liquid to form. On reheating as-supplied material to around 580°C (fraction of liquid ∼5%), a fully spheroidal microstructure can be obtained.

Effect of semi-solid isothermal heat treatment on the microstructure of Mg–6A1–1Zn–0.7Si alloy

In this paper, the effect of semi-solid isothermal heat treatment on the microstructure of Mg–6A1–1Zn–0.7Si alloy, especially on the semi-solid microstructural evolution and the modification of Chinese script shaped Mg 2Si phase, are first investigated. The research results indicate that it is possible to produce the Mg–6A1–1Zn–0.7Si alloy with non-dendritic microstructure by the semi-solid isothermal heat treatment. After treated at 575–585 °C for 120 min, the experimental alloy can obtain a non-dendritic microstructure with a 12–21% liquid content and an average size range of 48–67 μm of the unmelted primary solid particles. In addition, the semi-solid isothermal heat treatment can modify the Chinese script shaped Mg 2Si phases in experimental alloy. After treated at 580 °C or 585 °C for 120 min, the morphology of Mg 2Si phases in the experimental alloy changes from the initial Chinese script shape to granule and/or polygon shapes.

Preparation of semisolid A356 alloy feedstock cast through vertical pipe

It is demonstrated that the fine spheroidal and equiaxed non-dendritic microstructures required for semisolid casting can be formed by using a vertical pipe. Various pouring temperatures (615–660°C) and cooling rates of the vertical pipe (no, 200, 400°C preheat) were used to investigate their influence on the semisolid microstructure of an A356 alloy. The results indicate that suitable semisolid billets can be generated at both lower and higher pouring temperatures; thus the vertical pipe technique may be readily exploited in industrial applications. The results also reveal that the microstructure is affected significantly by the cooling effect of the vertical pipe. Furthermore, in the longitudinal section, non-uniform grain sizes were observed, namely the primary α-Al at the bottom of the billet was coarser than that at the top, an effect attributed to the differences of solidification time and cooling rate. The relevant mechanisms are discussed.

Computational Modeling of Microstructure Evolution in Solidification of Aluminum Alloys

In this article, a front tracking (FT) model and a modified cellular automaton (MCA) model are presented and their capabilities in modeling the microstructure evolution during solidification of aluminum alloys are demonstrated. The FT model is first validated by comparison with the predictions of the Lipton–Glicksman–Kurz (LGK) model. Calculations of the steady-state dendritic tip growth velocity and equilibrium liquid composition as a function of melt undercooling for an Al-4 wt pct Cu alloy exhibit good agreement between the FT simulations and the LGK predictions. The FT model is also used to simulate the secondary dendrite arm spacing as a function of local solidification time. The simulated results agree well with the experimental data. The MCA model is applied to simulate dendritic and nondendritic microstructure evolution in semisolid processing of an Al-Si alloy. The effect of fluid flow on dendritic growth is also examined. The solute profiles in equiaxed dendritic solidification of a ternary aluminum alloy are simulated as a function of cooling rate and compared with the prediction of the Scheil model. The MCA model is extended to the multiphase system for the simulation of eutectic solidification. A particular emphasis is made on the quantitative aspects of simulations.

Microstructural characterisation in non-dendritic Al–Si–Mg (356/357) alloy feedstocks

Microstructural characterisation carried out on four non-dendritic feedstocks in the as received and reheated conditions has revealed that the transition from dendritic to non-dendritic occurs through the formation of ripened dendrites, rosettes and ripened rosettes. Formation of divorced eutectic is observed during the initial stages of remelting in all the alloys. Electrical conductivity measurement can be used as an instantaneous non-destructive evaluation (DE) tool to assess the presence of divorced eutectic. The eutectic entrapped inside the primary alpha globules is finer than that present outside. Feedstock, which is neither grain refined nor modified, requires longer holding time to obtain the non-dendritic microstructure on remelting.

Effect of Slope Plate Variables and Reheating on the Semi-Solid Structure of Al Alloys

Semi-solid forming (SSF) involves alloys with non-dendrite microstructure that contain spherical solid particles in the liquid matrix. This process is generally divided into three main steps: feedstock manufacturing, reheating and forming. Feedstock has the main effect on the cost and quality of product. Many researches have been carried out to reduce the cost of feedstock manufacturing. Slope plate is a simple semi-solid process that can reduce the cost of feedstock. In this study a Cu plate with water and without water circulation was used to investigate the effect of pouring temperature, cooling rate during casting and solidification in the mold on the microstructure. Cast ingots with optimized microstructure were reheated in various time and temperatures in semi-solid region to obtain kinetics of globularization and solid grain growth.

Microstructural evolution of thixomolded AZ91D magnesium alloy with process parameters variation

A prototype system of Thixomolder ® was used to manufacture the AZ91D magnesium alloy. The effect of various processing parameters on the microstructure was investigated using an image analyzing system. The solid particle size was found to be reduced with increasing barrel temperature while the particle morphology was not essentially affected. The size reduction and the increase of the solid particle sphericity followed the elevated screw rotation speed because of the accelerated solute diffusion. Furthermore, the refinement of the solid particles occurred because of the high cooling rate induced by low injection velocity or low mold temperature. The mechanism of nucleation and growth of the solid particle was put forward to explain the generation and evolution of non-dendritic microstructure including the particle with entrapped liquid, or with rosette shape in semisolid slurry under forced convection. It was believed that there was a critical solid fraction about 10–20%, at which the particle grew intensively, considered as the change point of coalescence and Ostwald ripening.

A scaling analysis of alloy solidification in presence of electromagnetic stirring

Application of electromagnetic stirring (EMS) during continuous casting shears off the dendrites from the solidification front to produce billets with a non-dendritic microstructure. In the present study, a systematic approach to the scaling analysis of momentum, energy and species conservation equations pertaining to the case of the solidification of a binary alloy in the presence of EMS is outlined. With suitable choices of non-dimensionalizing parameters, the governing equations coupled with appropriate boundary conditions are first scaled, and then the relative significance of various terms appearing in them are analysed. In the physical domain two regions are identified, one where the electromagnetic forces play a dominant role in the momentum equations, and the other where the inertia and viscous effects play major roles. Using the scaling predictions, the influence of various processing parameters on the system variables can be utilized for the selection of appropriate electromagnetic forces to shear off the dendrites from the solidification area. For the sake of assessment of the scaling analysis, the predictions are validated against corresponding computational results.

Modelling the Semisolid Processing of Metallic Alloys

AbstractFor Abstract see ChemInform Abstract in Full Text.

Continuous Casting of Magnesium Billets for Semi-Solid Processing

Semi-solid processing is recognized as an attractive alternative method for the near net-shape production of engineering components. Although there has been a significant progress in semi-solid processing of aluminum alloys, very limited information is available on semi-solid processing of magnesium alloys, except for the thixomolding process. Continuous casting process has been utilized to produce the billets with the desirable cross-section at a reduced production cost for many metals, such as steel, copper and aluminum alloys. It has also been commercially utilized to produce the aluminum billets with non-dendritic microstructure for subsequent thixocasting process. However, continuous casting of magnesium billets for semi-solid processing has not yet been commercialized due to the difficulties involved in casting of magnesium alloys. In the present study, a continuous casting process has been developed for the production of the cylindrical billets of magnesium alloys for the subsequent thixocasting process. In order to obtain the desired non-dendritic microstructure with an excellent degree of homogeneity both in microstructure and composition, an electromagnetic stirring system has been utilized. A continuous casting process has been proven to be an efficient way to produce the high quality billets of magnesium alloys for semi-solid processing. A prototype air conditioner cover was produced using the continuously cast billets of AZ91 alloy.

Microstructural evolution and rheological behaviour of aluminium alloys A356, and A356 + 0.5% Sn designed for thixocasting

One of the most important parameters in any thixoforming process is the viscosity of the semi-solid raw material. The reduction of the value of this property contributes towards a more complete filling of thin mould sections and decreases the incidence of defects. The present work deals with the influence of Sn on the viscosity of an A356 alloy (A17SiO.3Mg) in the semi-solid state (SSS). A thermomechanical treatment was employed in order to obtain a non-dendritic microstructure, and the A356-base composition was modified by adding 0.5% Sn to the alloy. Thus, two A356 alloys, one conventional and the other containing Sn were compared in terms of microstructural evolution in the semi-solid state at a solid fraction of 0.5 and up to 3600s soaking time. Solid fraction, particle size, contiguity and shape factor were measured by optical microscopy. Wettability was evaluated by separately dipping an Al–1.5% Si plate in an Al–12% Si and in an Al–12% Si–l% Sn baths (thus simulating the solid and the liquid phases of the A356 and A356 + Sn alloys, at 580°C, respectively) and measuring the grain boundary penetration by the liquid. Rheology was measured with a Searle type viscosimeter and by the back-extrusion technique. It was shown that Sn increases the wettability without interfering in the morphology and coarsening rate of the solid phase. In addition, it reduces considerably the viscosity and thixotropy of the A356 alloy, leading to better flow behaviour.

The fragmentation of primary dendrites during shearing in semisolid processing

Semisolid slurries with non-dendritic microstructure can be produced by solidification under forced convection. Dendrite fragmentation is a commonly accepted mechanism for shearing semisolid slurry. However, it requires from experimental observation to confirm the fragmentation of dendrite arms occurring during shearing process. Experiments are undertaken to investigate the dendrite fragmentation in Sn-15wt%Pb alloy under shearing by virtue of a twin-screw extruder together with the wetting and penetration of pure tin grain boundaries submerged and sheared in the semisolid slurry of Sn-15wt%Pb alloy. It is found that the primary dendrites can be fragmented rapidly into small particles and there are penetration grooves existing in the partially fragmented dendrites. Such grooves are similar to that found at the grain boundaries in pre-cast pure tin polycrystals, which have been submerged and sheared in the semisolid slurry of Sn-15wt%Pb alloy.