Dendritic Primary Phase Research Articles

Thermal analysis via horizontal solidification of Al3Cu2Si (mass%) alloy: thermal and microstructural parameters, intermetallic compounds and microhardness

A transient horizontal solidification experiment was performed to investigate the role of the thermal parameters, such as growth and cooling rates (GR and CR, respectively), on the microstructure and microhardness (HV) of the Al2Si3Cu (mass%) alloy. Thermal data were generated using a water-cooled solidification device. This allowed to determine a large range of GR and CR. As-cast samples from the cooled surface were characterized by optical microscopy and scanning electron microscopy (MO and SEM). The resulting microstructure was characterized by an Al-rich dendritic primary phase (Alα) and by an interdendritic eutectic mixture composed of Alαeutectic + Si particle + (Al2Cu and Fe) intermetallic compounds. The Alα phase was characterized by primary, secondary and tertiary dendrite arm spacings (λ1, λ2 and λ3, respectively). The λ1, λ2 e λ3 dependence on GR and CR was characterized by power-type mathematical equations. It was evidenced the tertiary branches occurrence for GR > 0.95 mm s−1 and CR > 6.5 °C s−1. Microhardness (HV) was measured at the center of the dendritic primary phase and within the interdendritic regions. Higher HV values were observed in the eutectic mixture. Experimental power and Hall–Petch mathematical equations were proposed to characterize HV variation within the interdendritic region as a function of λ1, λ2 and λ3. A parametric factor given by the expression $$\varepsilon = \frac{\text{CR}}{{\left( {\mathop \sum \nolimits_{1}^{\text{n}} C_{\text{on}} } \right)/n}}$$ that allows to predict the appearance of tertiary dendritic arms for the assumed solidification conditions was proposed by the present work, for future predictions. A comparison with the literature was conducted.

Microstructure development during directional solidification of Sn–Ag–Cu ternary alloys

Replace of Pb–Sn with Pb-free solders (LFS) is one of the most important issues in the electronic industry. Eutectic and near-eutectic Sn–Ag–Cu (SAC) alloys are recommended as lead-free replacement in welding processes of electronic devices. Close to the eutectic ternary composition, the solidification occurs with three distinctive phases: Sn-rich dendritic primary phase, and the intermetallic phases and , showing a limited solubility into the Sn-rich phase. Given the key role of the solidification process in the microstructure development of the alloy, samples of Sn–Ag–Cu with a composition close to the ternary eutectic were directionally grown to analyze the mechanisms involved in the solidification process of these alloys. The typical microstructure agrees withe above description, but shows two different mechanisms, depending on the interface advance velocity, resulting in different distribution of the intermetallic phases.

EFFECT OF THIXOFORMING ON THE WEAR PROPERTIES OF AL-SI-CU ALUMINUM ALLOY

In this study a hypoeutectic Al-6Si-3Cu aluminium alloy was synthesized using two different routes: the thixoforming process and the conventional mould casting process. The microstructural features, hardness and wear behavior of thixoformed alloy have been evaluated and compared with that of as-cast alloy. Cooling slope method was used to produce the non-dendritic microstructure feedstock for thixoforming. Thixoforming was carried out at 50% liquid fraction. The dry sliding wear behaviour of the produced alloys was investigated under two loads; namely, 10 and 50 N and 1m/s sliding speed for 9 km sliding distance. The thixoformed alloy exhibited globular primary phase morphology with fine and uniform distributed Si and intermetallic particles. On the contrary, dendritic primary phase, coarse flaky silicon particles and segregated microstructure has been observed in conventional cast alloy. Thixoformed alloy exhibited improvement in the wear resistance in comparison to the conventional cast alloy, which may be attributed to the microstructural enhancement resulting in improved hardness. The thixoformed samples displayed lower volume loss of ~16.20 mm3 and ~42.40 mm3 at loads of 10 and 50 N respectively compared with that of conventional cast samples. On the basis of observations and analyses on the wear rates and worn surfaces, the wear mechanism of the alloys was dominantly controlled by abrasive, adhesive and minor delamination.

Directional Solidification of Sn-Ag-Cu Alloys

The alloys of Sn-Ag-Cu family (SAC) are candidates for lead-free replacement of Sn-Pb eutectic used in welding processes of electronic devices. These alloys, with eventual adding of Zn, In and Bi highly improves the morphological stability and the corrosion resistance. Close to the eutectic ternary composition, the solidification occurs with three distinct phases: Sn-rich dendritic primary phase, Ag3Sn and Cu6Sn5 intermetallic phases, which have limited solubility in the Sn phase, in such a way that some authors consider these alloys as compounds of Sn with the mentioned intermetallic phases. In this aspect, the directional solidification is a suitable tool to study the solidification under strictly controlled conditions. In this work, samples of Sn-Ag-Cu composition were grown to analyze the mechanisms involved in the solidification of these alloys. The typical microstructure agrees the above description, with a spacing that depends mainly on the advance interface velocity of the growth.

Microstructure Evolution and Mechanical Properties of Rheocast A319 Aluminum Alloy Using Cooling Slope

A319 aluminum alloys are commonly used in automotive industry due to a combination of good fluidity and mechanical strength. In this present work, cooling slope (CS) rheocasting process was employed to produce A319 billets with near spherical morphology of primary Al phase. The dendritic primary phase in the cast A319 alloy had readily transformed into non-dendritic when the ingots were cast over a cooling plate from pouring temperatures between 620°C and 640°C and with cooling lengths of between 300 mm and 400 mm. The shear driven flow of the solidifying melt on the cooling slope wall promotes heterogeneous nucleation of α-Al phase and subsequent separation from there due to shear driven flow of the solidifying melt produced nearly spheroidal morphology of the primary phase in the microstructure. The results show that the best combination of pouring temperature and cooling length was found to be 630°C and 400 mm respectively. The hardness of the rheocast ingots improved to 85.3 HV from 81.8 HV in as-cast condition.

Experimental investigation of the effect of temperature in semisolid casting using cooling slope method

In this work, the cooling slope method is employed to produce the A356 feedstock in semisolid processing. This paper discusses the effect of temperature on the final microstructure of A356 aluminum alloy in this method. These temperatures are casting temperature, mold temperature, and cooling slope plate temperature. The dendritic primary phase in the conventionally cast A356 alloy is transformed into a non-dendritic one in ingots cast over a cooling plate at casting temperature of 600 ℃, 615 ℃, 625 ℃, 650 ℃, and 680 ℃, and at condition of using a cooling system or without it. After pouring, the melt which becomes semisolid at the end of the plate is consequently poured into cylindrical steel mold at temperatures of 25 ℃, 200 ℃, and 400 ℃. The cooling slope is adjusted at the angle of 50° with respect to the horizontal plane and 500 mm length. It is found that the pouring, plate, and mold temperatures affect the size and morphology of α-Al phase. The effects of these are 22% decrease on average grain diameter and 8.5% increase on the shape factor at the weakest case. The results illustrated that by modifying the temperature the hardness of the specimens is increased. The results indicated that the highest uniformity with the grain size of ∼88 µm and maximum shape factor of 0.68 are related to pouring and mold temperature of 625 ℃ and 200 ℃, respectively using water cooling system.

Optimization of semi-solid metal processing of A356 aluminum alloy

Semi-solid metal (SSM) processing has been recognized as an advanced process to produce high-quality and low-cost engineering components. The cooling slope method is a simple route that can develop non-dendritic slurry for various purposes with reduced equipment and processing costs. In this study, the cooling slope method was employed to produce the A356 feedstock in SSM processing. The dendritic primary phase in the conventionally cast A356 alloy has transformed into a non-dendritic one through the use of ingots cast over a cooling plate with a different pouring temperature. After pouring, the melt that becomes semi-solid at the end of the plate is consequently poured into cylindrical steel molds with different mold temperatures. Also, the process has been conducted in different cooling slopes and different cooling lengths. Then, a back-propagation neural network has been designed to correlate the process parameters to grain size. Finally, genetic algorithm (GA) has been used to optimize the process parameters. Results indicate that the pouring temperature, mold temperature, cooling slope, and cooling length have significant effects on size and morphology of α-Al phase. The GA can optimize the process as well.

The Spherical Casting WC Powder and its Application in Laser Surface Alloying Treatment

The spherical casting WC powder is a kind of new potential wear-resistance ceramic material. In this paper, its chemical composition, surface morphology, microstructure, phase, microhardness and some powder characteristics were investigated, and then the powders were applied in laser surface alloying treatment on medium carbon steel. The results indicate that the spherical WC powder consists of (wt.%) 94.06 W, 4.03 C and 1.91 other elements including Fe, V, Cr and Nb. Most of powder particles have regular spherical shape and smooth surface, and the microstructure is acicular eutectic of WC and W2C, which leads to high microhardness of 2812 HV. The fluidity of powders is 5.97 s/50g, and the apparent density of powders is 10.1 g/cm3. The laser alloyed layer is characterized by dendritic primary phase, ledeburite microstructure and some residual WC particles. The laser surface alloying treatment with adding material of spherical casting WC powder can enhanc the abrasive wear resistance of medium carbon steel by over 200%.

Wear Properties of A356/Al2O3 Metal Matrix Composites Produced by Semisolid Processing

In this study, A356/Al2O3 metal matrix composites were fabricated by conventional stirring and semisolid processing (combination of mechanical stirring in the mushy zone and cooling slope casting) and these composites were subjected to wear tests. The composites containing various weight fractions of Al2O3 particles (1%, 2.5% and 5%) were used in both conventional and rheocasting processes. In conventional stirring, the primary phase remained dendritic while in rheocasting the dendritic primary phase transformed into a globular microstructure. The influences of these processes on the hardness and wear behaviour of the composites were investigated. A pin-on-disc tribometer was used to carry out tribological tests under dry sliding conditions in air. Wear tests were carried out at 1.0 m/s sliding speed under a 20N load and four different sliding distances. A scanning electron microscope was used to examine the wear mechanism on the worn surfaces of the composites. The results showed that the volume loss of the composites produced by semisolid processing was lower than that of those produced by conventional casting. Based on wear rate ranges, all metal matrix composites exhibited moderate wear regimes. The dominant wear mechanism for all materials was ploughing, with some delimitation.

Mechanical Properties and Tensile Fracture Mechanism of Rheocast A356 Al Alloy Using Cooling Slope

A356 aluminium alloy has a wide applicability in the manufacturing of automotive parts. Cooling slope (CS) rheocasting process has been used in the present work to produce A356 billets having near spherical morphology of primary Al phase. Absence of dendritic primary phase, observed in case conventionally cast A356 alloy, and finer distribution of secondary eutectic Si phase within the matrix establishes the usefulness of the CS casting route. Near spherical primary phase in the rheocast alloy ensures better strength, elongation properties and structural integrity in the produced billets. The liquid melt is allowed to flow through the cooling slope after pouring at 6500C. Rapid heat exchange between the flowing melt and slope wall and the atmosphere facilitates heterogeneous nucleation of α-Al phase on the cooling slope wall. Shear driven flow of the solidifying melt is found responsible for separation of α-Al phase from the slope wall and generation of nearly spherical morphology of the primary phase in the microstructure. Grain refiner addition in the melt leads to enhance the primary α-Al percentage in the microstructure and also aids to the improvement of degree of sphericity and reduction of spheroid size. So, grain refining helps to improve the strength, elongation and fracture properties of rheocast billets further.

Effects of WC particle size on the wear resistance of laser surface alloyed medium carbon steel

The CO2 laser surface alloying technique was used to form wear resistance layers on medium carbon steel with a kind of spherical WC powder. The effects of WC particle size on the abrasive wear resistance were thoroughly investigated. The results indicate that the laser alloyed layer is characterized by dendritic primary phase and ledeburite microstructure, consisting of austenite, martensite and carbides of Fe3W3C, W2C and WC. The laser surface alloying with WC powder could improve the abrasive wear resistance of the medium carbon steel by over 63%. The factors such as the hardness, the amount and the distribution of WC particle determined the laser alloyed samples’ wear resistance, and the laser alloyed sample with WC powder of 88–100μm diameter presented the best wear resistance in this study. Furthermore, the wear resistance mechanisms of the laser alloyed layers were also explored.

Microstructure Evolution of A356 Aluminum Alloy Produced by Cooling Slope Method

In this work the cooling slope (CS) method was employed to produce the A356 feedstock in semisolid processing. The dendritic primary phase in the conventionally cast A356 alloy has transformed into a non-dendritic one in ingots cast over a cooling plate with (625°C, 650°C and 680°C) superheat. After pouring, the melt which became semisolid at the end of the plate was consequently poured into cylindrical steel mold with the various mold temperatures of (25°C, 200°C, 400°C). In this study, the cooling slope was adjusted at 50o with respect to the horizontal plane and 500mm length. It was found that the pouring and mold temperature affect the size and morphology of α-Al phase.

The Effects of Cooling Slope Casting and Isothermal Treatment on Wear Behavior of A380 Alloy

In this study, A380 aluminum alloy feedstock produced with cooling slope casting was exposed to isothermal treatment to obtain a globular microstructure which is a key feature for semisolid forming. The dendritic primary phase in the conventionally cast A380 alloy has readily transformed into a non-dendritic one in ingots cast over a cooling plate from pouring temperatures between 615, 630 and 650 °C. After the casting process, isothermal treatment was carried out at 565 °C in induction unit. Isothermal treatment yields a globular microstructure. To determine the tribological properties of this alloy, a pin-on-disc tribometer was used to carry out tribological tests under dry sliding conditions. The results showed that both cooling slope casting and isothermal treatment has an effect on tribological properties.

A357 thixoforming feedstock produced by cooling slope casting

Cooling slope (CS) casting process was employed in the present work to produce A357 thixoforming feedstock. The dendritic primary phase in the conventionally cast A357 alloy has readily transformed into a non-dendritic one in ingots cast over a cooling plate from pouring temperatures between 620 and 640 °C and with cooling lengths between 200 and 400 mm. Dissipation of the melt superheat required longer cooling lengths for higher pouring temperatures. Primary phase in the ingots thus produced has attained a globular morphology in only as much as 5 min during isothermal holding at 580 °C. The minor effect the pouring temperature and the cooling length had on the CS-cast morphologies was not evident after reheating, implying that a wide range of CS casting parameters are capable of providing the required microstructural features for thixoforming.

Solidification of FeSi2 alloy under high pressure

Influence of pressure on solidification microstructure of FeSi2 alloy is investigated. Different from the eutectic structure of the sample solidified at normal pressure, the microstructure of the sample solidified at high pressure is composed of dendritic primary phase and anomalous eutectic. The significant change in microstructure results from the change of phase diagram and solute atom diffusion induced by high pressure. By introducing the pressure parameter, the constitution undercooling criterion is attained. Formation mechanism of dendritic primary phase under high pressure is analysed according to the constitution criterion.

Solidification characteristics of Pd40Ni40P20 alloy under microgravity condition

The microstructure and solute distribution of Pd40Ni40P20 alloy solidified both on board a Chinese retrievable satellite (μg) and on the earth (1g) were studied. It was found that the dendritic primary phase formed under microgravity condition was finer and shorter. In the central area of the sample some asteroidal patterns of the primary phase were present in the microstructure. The primary spacing of the dendrites at the cooling rate of 0. 056 K/s was smaller than that measured in the ground-based experiments at the same cooling rate, but almost the same as that cooled at 0.67 K/s on the ground. With these experimental results, mass transport coefficients both in space and on the earth were evaluated.

High temperature compressive behaviour of as-cast Al-Cu alloys of varying composition

Al-Cu alloys with different copper contents ranging from 10 to 45 wt% were deformed in compression in the as-cast condition. Measurements of the strain-rate sensitivity parameterm using the crosshead speed jump technique show that alloys of composition close to the eutectic become superplastic after about 25% strain, withm increasing from about 0.2 at the beginning of deformation to about 0.5 or more, depending on composition. This transition to the superplastic state is accompanied by an important decrease of the flow stress during compression and it is associated by the breakdown of the initially lamellar structure of the eutectic phase in the highly deformed regions of the specimen. Moreover for the alloy with 37 wt% copper, the transition corresponds also to the degeneration of the dendritic primary phase and this alloy shows a particularly high value ofm and a low steady state flow stress in spite of the large volume fraction of the hard CuAl2 compound. Alloys of composition far away from the eutectic do not exhibit superplastic behaviour during compression, at least in the range of strain rate investigated and this is due to the large grain size and the important grain growth that occurs during deformation.