Effects of squeeze casting parameters on the microstructure of LM13 alloy

Effects of applied pressure and melt and die temperatures on the microstructure of squeeze cast LM13 alloy were examined. The results showed that application of pressure during solidification decreased the grain size and SDAS of the primary α phase and modified the eutectic silicon particles. With application of an external pressure of about 100 MPa, the average SDAS and the average aspect ratio of eutectic silicon particles were reduced from 47 μm and 5 to about 34 μm and 1.5, respectively. SDAS of the primary α phase and the average aspect ratio of eutectic silicon particles decreased slightly with a drop in the melt or die temperatures, reaching to 32 μm and 1.25, respectively, for the best conditions.     

Prediction of cooling curves during solidification of Al 6061–SiCp based metal matrix composites using finite element analysis

In recent years, aluminium based cast composites have gained popularity in all the emerging fields of technology owing to their superior high stiffness and strength. The properties of cast composites are dictated largely by the solidification phenomenon, which needs to be well understood by foundry technologists. Information on the solidification studies of cast composites is scarce. However, the theoretical prediction of the solidification behaviour of cast composites by the use of commercially available finite element analysis (FEA) software has not yet been reported. The theoretical prediction can definitely yield good lot of information as regards the cooling rates of the cast composites saving enormous time in experimentation. In light of the above, the present investigation is aimed at the prediction of cooling curves of Al 6061–SiC_p composites using finite element analysis. L-shaped composite castings were prepared using stir cast technique. The temperature of the composite during solidification was measured by K-type thermocouple, from which the cooling curves were constructed. Experiments were carried out over a range of particle weight percentage of 2–6 wt% in steps of 2 wt%. Comparison of the cooling curves of Al 6061–SiC_p composite with the un-reinforced alloy reveals significant decrease in cooling rate with the addition of SiC particles. A two-dimensional transient heat transfer model was used in commercial finite element analysis software to predict the cooling curves of composite castings. The predicted cooling curves are compared with results obtained from experiments and found to be in good agreement.     

Influence of the casting and mould temperatures on the (micro)structure and compression behaviour of investment-cast open-pore aluminium foams

Current literature offers only limited information about the influence of the casting and mould temperatures on the properties of cast aluminium foams. In order to close this current research gap, the casting and mould temperatures were varied during the production of investment-cast open-pore A356 foams. The results show that a decreased mould temperature and an increased casting temperature lead to increased mechanical properties during the static compression testing of open-pore 10 and 15 ppi (pores per inch) A356 foams. This increase is due to a reduced number of eutectic silicon particles and increased strut diameter resulting from the better mould-filling ability of the melt. The effect of the process parameters is statistically significant for the mechanical properties of the 15 ppi foams. This is probably due to this foam type’s lower strut diameter; compared to the bigger 10 ppi foam struts, since these thinner struts are more sensitive to the high number of the silicon phase and to the decreased strut diameter due to a reduced mould-filling ability. In addition to this, a change of the cooling rate has a larger impact on small strut diameters. With the aid of the present results, the influence of the process parameters on the mechanical properties of open-pore 15 ppi foams was modelled and confirmed by reference test runs.     

An alternative approach in ceramic shell investment casting of AZ91D magnesium alloy: In situ melting technique

In this research, the possibility of ceramic shell investment casting of a magnesium alloy using in situ melting technique was explored. AZ91D granules were charged into shell investment mould and in situ melted under various processing parameters including heating temperature, flux application, shell mould thickness and permeability. Scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction techniques were used to characterise the cast samples. Thermal analysis was employed to further investigate the effect of mould thickness on the solidification behaviour of the metal. It was found that mixing flux with the granules not only reduced the temperature at which melting can be achieved, but it also contributed to produce castings with acceptable surface quality. The use of thinner mould provided higher solidification rate, which is believed to favour in situ melting of the granules. It enabled melting of the granules at 650 °C, which in turn helped to suppress the mould–metal reaction and produce castings with good surface quality. Shell mould permeability showed no influence on suppressing the mould–metal reaction at 650 °C.     

Cooling wheel features and amorphous ribbon formation during planar flow melt spinning process

A numerical model incorporating actual production conditions of planar flow melt spinning process is presented. Knowledge of the process conditions at which an amorphous ribbon with uniform thickness can be obtained is important to reduce the manufacturing costs. Cooling wheel conditions play a significant role during rapid solidification of the melt. Hence a heat transfer analysis is performed to investigate the influence of cooling wheel temperature on ribbon formation. Nusselt's correlation is used for the first time for selecting a convective heat transfer coefficient between rotating wheel and surrounding air. This approach assists in predicting the conditions at which a continuous/broken/no ribbon formation is obtained during the process. The model could predict whether the ribbon to be obtained is amorphous or non-amorphous before the experiment is actually performed at a set of process conditions. Various internal and external conditions of the cooling wheel are tested, and they show little influence on the ribbon thickness up to 0.5 s (10 rotations). Broken and nonamorphous ribbons are obtained for poor cooling conditions of the wheel with increase in time of cast from 0.5 s to 1 s (20 rotations). Amorphous ribbon with uniform thickness can be obtained for a wheel of 20 mm wall thickness, when the inner and outer surfaces of the cooling wheel are maintained at a temperature of at least 300 K.     

On the transition from diffusion-limited to kinetic-limited regimes of alloy solidification

An abrupt transition from diffusion-limited solidification to diffusionless, kinetic-limited solidification with complete solute trapping is explained as a critical phenomenon which arises due to local non-equilibrium diffusion effects in the bulk liquid. The transition occurs when the interface velocity V passes through the critical point V = V_D, where V = V_D is the bulk liquid diffusive velocity. Analytical expressions are developed for velocity–temperature and velocity–undercooling functions, using local non-equilibrium partition coefficient based on the Jackson et al. kinetic model and the local non-equilibrium diffusion model of Sobolev. The calculated functions demonstrate a sharp break in the velocity–undercooling and velocity–temperature relationships at the critical point V = V_D. At this point the local non-equilibrium solidus and liquidus lines coincide with the T₀ temperature. The relationship to pertinent experiments and the influence of the local non-equilibrium diffusion effects on grain refinement and disorder trapping phenomena are discussed.     

Experimental study of heat transfer coefficient on hot steel plate during water jet impingement cooling

Experiments were performed, under transient conditions, to investigate the heat transfer phenomena of stationary hot steel plate under multiple top circular jets on run-out table. Based on inverse heat conduction model, a two-dimensional finite difference program was developed to calculate the local surface convective heat transfer coefficients and corresponding temperatures. The cooling water jet flow rate was varied from 15 L/min to 35 L/min and its effect on the convective heat transfer coefficient and surface temperature was analyzed. The results show that heat transfer coefficients are nonlinear functions of surface temperature. The cooling flow rate has no effect on heat transfer coefficient and surface temperature at stagnation point. Within 70 mm distance from stagnation line, heat transfer coefficient ratio changes slightly from 0.87 to 0.97. Beyond surface temperature of 350 °C, heat transfer coefficient ratio decreases with increasing distance from stagnation line.     

A thermal model of friction stir welding applied to Sc-modified Al–Zn–Mg–Cu alloy extrusions

A thermal model of friction stir welding is presented that proposes an energy-based formulation of the Johnson–Cook plasticity model in order to account for heat generation due to plastic deformation. The proposed formulation is derived from an empirical, linear relationship observed between the ratio of the maximum welding temperature to the solidus temperature of the alloy and the welding energy. The thermal model is applied to Sc-modified Al–Zn–Mg–Cu alloy extrusions joined by friction stir welding at 225, 250, 300 and 400 RPM (all other weld parameters held constant). With the incorporation of heat generation due to plastic deformation, the thermal model accurately predicts the maximum weld temperatures and temperature profiles at the higher energy weld conditions, i.e. 300 and 400 RPM. At the lower energy welds (i.e. 225 and 250 RPM) where plastic deformation contributes a larger portion to the total heat generation, the model under-predicts the maximum weld temperatures under the tool shoulder but shows good agreement with the remaining experimental temperature data.     

Heat transfer behavior of top side-pouring twin-roll casting

The interfacial heat transfer between the casting and the substrate from liquid/solid contact to solid/solid contact with pressure was investigated using a set of equipment designed according to the characteristics of the top side-pouring twin-roll casting process. The interfacial heat transfer behavior of this process consists of 4 stages: chilling, solidification shrinkage, compression and cooling. High values of the IHTC ranging from 50,000 to 90,000 W/m² °C were detected in the chilling stage, followed by a sharp decrease in solidification shrinkage stage (4000–8000 W/m² °C). Due to the pressure, which modeled the effect of rolling in twin-roll casting, the IHTC bounced back to 6000–20000 W/m² °C, according to different conditions. The influence of process variables such as pressure magnitude, compress speed, pouring temperature, surface roughness and alloy composition had been discussed. Because of the compress action, the influence of these variables performed in a different way, but it was concluded that the way to improve the contact conditions always accompanied with an increase in the IHTC.     

Influence of cooling rate on the microstructure and ageing behavior of as-cast Al–Sc–Zr alloy

Al–0.3Sc–0.15Zr alloy was cast using copper die, insulated alumina mould, and conventional investment shell mould to obtain a wide range of cooling rates. A novel method of quenching the investment shell mould along with the liquid metal in oil was also used which resulted in a significant increase in the cooling rate. The order in increasing average cooling rate is 0.16, 0.78, 1.28, 5.93, 7.69 °C/s. The as-cast samples were aged isothermally at 300 °C and various temperatures for 2 h. Slow cooled samples (in alumina-insulated mould) showed the presence of as-cast primary precipitates as well as rod shaped discontinuous precipitates with high density of interfacial dislocation. The amount of as-cast precipitates decreased with increase in the cooling rate. These as-cast precipitates grew at the expense of Sc in solid solution reducing the number of precipitates formed during ageing process. This results in lower increment in hardness on ageing.     

连铸大方坯凝固传热过程的数值模拟

基于凝固传热学基本理论，建立了大方坯凝固传热数学模型。该模型根据糊状区溶质传输和反扩散理论，基于变热物性参数，采用钢中多组元成分的百分含量计算液、固相线温度。以实际铸坯为对象，以二冷区铸坯表面实际水流密度分布和辊子接触传热为边界条件，充分考虑二冷区的四种不同的换热状况，研究连铸大方坯三维温度场。计算结果与实测温度吻合。     

Mechanical and microstructural properties of low-carbon steel-plate-reinforced gray cast iron

In the present experimental study, strengthening of gray cast iron by reinforcing with steel plates was investigated in the as-cast and normalized conditions. Normalizing heat treatments were applied to the specimens at temperatures of 800 °C and 850 °C. Three-point bend specimens were manufactured from gray cast iron and also from gray cast iron with reinforced steel plates. Flexural strengths of the steel-plate-reinforced cast iron were calculated for four distinct volume fractions (V_r = 0.04, 0.08, 0.12, and 0.16). The current study shows that the steel-plate-reinforced gray cast iron has higher flexural strength and flexural modulus than the cast iron without reinforcement. The flexural strength considerably increases with slight increase in normalization temperature. Optical and scanning electron microscopy analyses were used to examine flake morphology and microstructures of gray cast iron and steel-plate-reinforced gray cast iron. It is noted that carbon diffuses from the gray cast iron to the steel plates. A transition region containing partially dissolved graphite and having high hardness was observed due to the carbon diffusion.     

Boronizing of iron aluminide Fe72Al28

Boronizing of Fe₇₂Al₂₈ was carried out in a slurry salt bath composed of borax and SiC at temperatures between 900 and 1050 °C for different durations starting from 3 h up to 15 h. The microstructures and microhardness of boride layers and intermetallic matrix were analysed. Samples boronized had Fe₂B phase or Fe₂B plus FeB phases depending on the process temperatures and times. SEM-EDS studies indicated that there was major aluminium migration into the matrix leaving only 3–4 at.% aluminium in the boronized layer. The measured microhardness values of the boronized layers varied from 1014 to 1980 HV and their thickness increased from 13 up to 103 μm with increasing temperature and processing times. The kinetics of the boronizing reaction have been analysed and the controlling diffusion process determined.     

连铸大方坯凝固传热过程的数值模拟

基于凝固传热学基本理论,建立了大方坯凝固传热数学模型。该模型根据糊状区溶质传输和反扩散理论,基于变热物性参数,采用钢中多组元成分的百分含量计算液、固相线温度。以实际铸坯为对象,以二冷区铸坯表面实际水流密度分布和辊子接触传热为边界条件,充分考虑二冷区的四种不同的换热状况,研究连铸大方坯三维温度场。计算结果与实测温度吻合。     

DC pulsed plasma deposition of nanocomposite coatings for improved tribology of gray cast iron stamping dies

Automotive stamping dies are very large in size with sizes ranging up to 2 m × 2 m × 1 m. The primary material used in the manufacturing of these dies is gray cast iron that is relatively cheap with good castability, machinability and reparability. Due to its lack of adequate hardness, die wear is a major problem at highly loaded die features such as corners, bead-radii, punch-radii etc. The greater is this wear, the greater is the die-related down times and production losses. This problem of die wear has become even more significant in recent times due to the introduction of Advanced High Strength Steels (AHSS) and Ultra High Strength Steels (UHSS) in automotive stamping. Stamping of these materials is accompanied by higher contact pressures on the die surface, higher abrasion and higher surface temperature from frictional heat.This paper presents a novel DC pulsed plasma based approach to the deposition of triplex coating on cast iron substrates that can withstand high normal pressures, shears, sliding and abrasion experienced during stamping. This triplex coating consists of silicon enhanced nanocomposite a-C:H film supported by plasma nitrocarburized duplex case. This nanostructured-composite film is shown to have high toughness and extremely low coefficients of friction. A unique feature of this plasma processing is the industrial size of chambers used in the coating development. This paper also includes results of film characterization and its performance evaluation in simulated wear tests that generate stresses similar to actual stamping dies. By carefully designing the plasma processing treatment, a good hardness gradient and elastic modulus gradient is achieved that provide an improvement in life of about 5 times that of the currently used hard chrome plating.     

The evolution of the growth morphology in Mg–Al alloys depending on the cooling rate during solidification

The comprehensive microstructural evolution of Mg–3, 6 and 9 wt.% Al alloys with respect to the solidification parameters such as thermal gradient (G), solidification velocity (V), cooling rate (G·V) and solute (Al) content were investigated in the present study. Various solidification techniques, including directional solidification, wedge casting, sand and graphite mould casting, gravity casting in a Cu mould and water quenching, were employed in order to obtain wide ranges of cooling rates between 0.05 and 1000 K s^–1. The microstructural length scales of Mg–Al alloys, such as secondary dendrite arm spacing and primary dendrite arm spacing, were determined experimentally and compared with published models. In addition, the solidification parameters of morphological transitions such as cellular to columnar dendrite and columnar to equiaxed dendrite were also determined. Based on all the experimental data and the solidification model, a solidification map was built in order to provide guidelines for the as-cast microstructural features of Mg–Al alloys.     

Alloy design concepts for refined gamma titanium aluminide based alloys

The influence of the Al content and the addition of further alloying elements on the cast microstructure of γ(TiAl) + α₂(Ti₃Al) alloys has been examined. The results show that particularly fine and homogeneous microstructures without strong segregation can be obtained for certain alloy compositions solidifying through the β phase. This behavior can be attributed to the avoidance of peritectic solidification and to the alloying influence on the kinetics of the β ⇒ α transformation following solidification. The experimental findings were used to propose a design concept for γ-TiAl + α₂-Ti₃Al alloys. This concept aims at the production of high-quality castings as well as at ingot material for wrought processing routes because the chemically homogeneous and fine-grained microstructures would be a good precondition for improved workability.     

Effect of ultrasonic treatment on the Fe-intermetallic phases in ADC12 die cast alloy

The ultrasonic treatment temperatures were varied from about 100 °C above the liquidus temperature down to the Al–Si eutectic temperature, for different treatment times (0–15 s). The results showed that the ultrasonic melt treatment was very effective to convert the long plate-like Fe-intermetallic phases (up to 200 μm length) to a highly compacted fine polyhedral/globular form (<15 μm size). The critical ultrasonic treatment temperature to affect the morphology of Fe intermetallics was found to be in the range of 596–582 °C. The eutectic Si was mostly not affected by ultrasonic treatments carried out in this study (in the temperature range of 670–581 °C and for up to 10 s). It was also observed that the nucleation undercooling, which is a measure of nucleation efficiency, at the start of solidification was lowered from ～2.9 to ～0.4 °C by ultrasonic treatment. The variation of horn temperature within 20 °C above pouring temperature to 10 °C below it had no noticeable effect. The ultrasonically treated samples showed better tensile properties than the untreated samples, due to the change in morphology of the Fe-intermetallic particles.     

Effect of filtration on the morphology and mechanical properties of Mg molten alloy entering the mould cavity

In this research, the experiments were carried out in two methods. In the first, the foam filter has been placed at the gate or runner of the mould, and in the second, no filter was used. In both methods, the casting temperatures were 680 °C and 750 °C. A pyrex glass plate was used on one side of the mould cavity in order to evaluate the morphological behaviour of entering melt into the cavity. The height of filtered molten Mg–Al–Zn alloy in mould was increased step-by-step with the filling time. This showed that the filtered molten alloy flows in mould cavity with different velocities. In case of no filtered molten alloy this was not observed. The reliability of the results of bending tests has been evaluated by Weibull analysis. The filters had absorbed the inclusions very well. A fracture toughness of 260 MPa was obtained in specimen poured at 680 °C with ceramic filters. The Weibull plots showed that the castings filtered at the gate at temperature of 680 °C had the best reliability.     

Simulation of cooling of liquid Al–33 wt.% Cu droplet impinging on a metallic substrate and its experimental validation

In the present work a model for heat transfer during collision of a falling liquid Al–33 wt.% Cu droplet on a 304 stainless steel substrate has been developed on a FLUENT 6.3.16 platform. The model simultaneously takes into account the fluid flow and heat transfer in the liquid droplet and the surrounding gas, and the heat transfer in the substrate. The liquid–gas interface was tracked using the volume of fluid method and the contact resistance between Al–33 wt.% Cu and the substrate was taken into account. The comprehensive model correctly predicted the total spread in the droplet. As per the predicted transient thermal field, the solidification front speed oscillated along the radius of the spread droplet. Based on the estimated front speeds at these locations and Jackson–Hunt plot for Al–33 wt.% Cu, the variation of interlamellar spacing along the radial direction was found. It matched well with the variation of the experimentally measured interlamellar spacing at different locations along the radius.