Effects of moulding sands and wall thickness on microstructure and mechanical properties of Sr-modified A356 aluminum casting alloy

The effects of different cooling conditions on the mechanical properties and microstructures of a Sr-modified A356 （Al-7Si-0.3Mg） aluminum casting alloy were comparatively investigated using three moulding sands including quartz, alumina and chromite into multi-step blocks. The results show that the mechanical properties and microstructures using chromite sand are the best. As the cooling speed increases, the dendrite arm spacing （DAS） decreases significantly and the mechanical properties are improved, and the elongation is more sensitive to the cooling speed as compared with the tensile strength. The increase of the properties is primarily attributed to the decrease of the DAS and the increase of the free strontium atoms in the matrix. In particular, the regression models for predicting both the tensile strength and the elongation for Sr-modified A356 aluminum casting alloy were established based on the experimental data.

Influence of serpentine channel pouring process parameters on semi-solid A356 aluminum alloy slurry

Semi-solid A356 aluminum alloy slurry was prepared by using serpentine channel pouring process, and the influences of the channel diameters and pouring temperatures on the semi-solid A356 aluminum alloy slurry were investigated. The experimental results show that when the channel diameter is 20 and 25 mm, respectively, and the pouring temperature is 640-680 ℃, the average diameter of primary α（Al） grains in the prepared A356 aluminum alloy slurry is 50-75 and 55-78 μm, respectively, and the average shape factor of primary α（Al） grains is 0.89-0.76 and 0.86-0.72, respectively. With the decline in the pouring temperature, the microstructure of semi-solid A356 aluminum alloy slurry is more desirable and a serpentine channel with smaller diameter is also advantageous to the microstructure imProvement. During the preparation of semi-solid A356 aluminum alloy slurry, a large number of nuclei can be produced by the chilling effect of the serpentine channel, and owing to the combined effect of the chilled nuclei separation and melt self-stirring, primary α（Al） nuclei can be multiplied and spheroidized finally.

Gating system optimization of low pressure casting A356 aluminum alloy intake manifold based on numerical simulation

To eliminate the shrinkage porosity in low pressure casting of an A356 aluminum alloy intake manifold casting, numerical simulation on fi lling and solidifi cation processes of the casting was carried out using the ProCAST software. The gating system of the casting is optimized according to the simulation results. Results show that when the gating system consists of only one sprue, the fi lling of the molten metal is not stable; and the casting does not follow the sequence solidifi cation, and many shrinkage porosities are observed through the casting. After the gating system is improved by adding one runner and two in-gates, the fi lling time is prolonged from 4.0 s to 4.5 s, the fi lling of molten metal becomes stable, but this casting does not follow the sequence solidifi cation either. Some shrinkage porosity is also observed in the hot spots of the casting. When the gating system was further improved by adding risers and chill to the hot spots of the casting, the shrinkage porosity defects were eliminated completely. Finally, by using the optimized gating system the A356 aluminum alloy intake manifold casting with integrated shape and smooth surface as well as dense microstructure was successfully produced.

Effect of various melt and heat treatment conditions on impact toughness of A356 aluminum alloy

The microstructure and impact behavior of A356 aluminum alloy were studied after melt treatment processes of grain refinement and modification under both non-heat treated and T6 heat treated conditions. The modification and grain refinement were done with the addition of Al-10%Sr and Al-5Ti-1B master alloys, respectively. All casting parameters were kept constant in order to focus on the influence of mentioned treatments. The results indicate that the eutectic silicon morphology is the main parameter to control the impact behavior of alloy. Consequently, the individual grain refinement of as-cast alloy does not improve the impact toughness as the modification does. While, simultaneous grain refinement and modification provide higher impact toughness in comparison with individual treatments. T6 heat treatment of the alloy improves the impact toughness under all melt-treated conditions. This is related to the further modification of eutectic silicon particles. To verify the results and clarify the mechanisms, three-point bending test and fractography were used to interpret the improvement of impact toughness of the alloy.

Filling ability of semi-solid A356 aluminum alloy slurry in rheo-diecasting

The effects of slurry temperature, injection pressure, and piston velocity on the rheo-filling ability of semisolid A356 alloys were studied by the reho-diecasting methods. The results show that the slurry temperature of the semi-solid A356 aluminum alloy has an important effect on the filling ability; the higher the slurry temperature, the better is the filling ability, and the appropriate slurry temperature for rheo-filling is in the range of 585-595℃. The injection pressure also has a great effect on the filling ability, and it is appropriate to the rheo-filling when the injection pressure is in the range of 15-25 MPa. The piston velocity also has a great effect on the filling ability, and it is appropriate to the rheo-filling when the piston velocity is in the range of 0.072-0.12 m/s. The filling ability of the slurry prepared by low superheat pouring with weak electromagnetic stirring is very good and the microstructural distribution in the rheo-formed die castings is homogeneous, which is advantageous to the high quality die casting. 2008 University of Science and Technology Beijing. All rights reserved.

Taper barrel rheomoulding process for semi-solid slurry preparation and microstructure evolution of A356 aluminum alloy

The self-developed taper barrel rheomoulding (TBR) machine for light alloy semi-solid slurry preparation was introduced.The semi-solid slurry was obtained from the intense shearing turbulence of the alloy melt in the cause of solidification, which was further caused by the relative rotation of the internal and external taper barrel whose surface contained wale and groove.The heat transmission model of TBR process, the flow rules and the shearing model of the alloy melt were deduced.Taking A365 as experimental material, the microstructure evolution rules under different slurry preparation processes were analyzed.The results show that decreasing the pouring temperature of A365 alloy melt properly or increasing the shearing rate helps to obtain ideal semi-solid microstructure with the primary particle size of about 70 μm and the shape factor of above 0.8.

Numerical simulation on the rheo-diecasting of the semi-solid A356 aluminum alloy

The effect of injection pressure, piston velocity, and the forming temperature of semisolid slurry on the filling behavior of the semi-solid A356 aluminum alloy was investigated by simulation methods. The simulation results show that these processing parameters have an important effect on the filling behavior of the semi-solid A356 aluminum alloy. The slurry flows steadily in the cavity when the injection pressure, the piston velocity, and the forming temperature are low, but it is prone to turbulent flow when the injection pressure, the piston velocity, and the forming temperature are much higher. Therefore it is necessary to determine the appropriate processing parameters to get a steady flow of the slurry in the cavity.

Influence of melt pouring temperature and plate inclination on solidification and microstructure of A356 aluminum alloy produced using oblique plate

The preparation of semisolid slurry of A356 aluminum alloy using an oblique plate was investigated. A356 alloy melt undergoes partial solidification when it flows down on an oblique plate cooled from underneath by counter flowing water. It results in continuous formation of columnar dendrites on plate wall. Due to forced convection, these dendrites are sheared off into equiaxed/fragmented grains and then washed away continuously to produce semisolid slurry at plate exit. Melt pouring temperature provides required condition of solidification whereas plate inclination enables necessary shear for producing semisolid slurry of desired quality. Slurry obtained was solidified in metal mould to produce semisolid-cast billets of desired microstructure. Furthermore, semisolid-cast billets were heat treated to improve surface quality. Microstructures of both semisolid-cast and heat-treated billets were analyzed. Effects of melt pouring temperature and plate inclination on solidification and microstructure of billets produced using oblique plate were described. The investigations involved four different melt pouring temperatures (620, 625, 630 and 635 °C) associated with four different plate inclinations (30°, 45°, 60° and 75°). Melt pouring temperature of 625 °C with plate inclination of 60° shows fine and globular microstructures and it is the optimum.

Effects of cooling rate on solution heat treatment of as-cast A356 alloy

The effect of cooling rate of the solidification process on the following solution heat treatment of A356 alloy was investigated,where the cooling rates of 96 K/s and 3 K/s were obtained by the step-like metal mold.Then the eutectic silicon morphology evolution and tensile properties of the alloy samples were observed and analyzed after solution heat treatment at 540 °C for different time.The results show that the high cooling rate of the solidification process can not only reduce the solid solution heat treatment time to rapidly modify the eutectic silicon morphology,but also improve the alloy tensile properties.Specially,it is found that the disintegration,the spheroidization and coarsening of eutectic silicon of A356 alloy are completed during solution heat treatment through two stages,i.e.,at first,the disintegration and spheroidization of the eutectic silicon mainly takes place,then the eutectic silicon will coarsen.

Effect of individual and combined additions of Al-5Ti-B,Mn and Sn on sliding wear behavior of A356 alloy

The effect of grain refiner, Mn and Sn additions on the sliding wear behavior of A356 aluminum alloys was investigated. The microstructure and worn surfaces of the studied alloys were characterized by optical microscopy（OM）, scanning electron microscopy（SEM）, and transmission electron microscopy（TEM）. The experimental results indicate that the alloy refined by Al-5Ti-B alloy exhibits equiaxed α（Al） dendrites and performs better wear resistance compared with the alloy without the grain refiner. Moreover, the addition of Mn can change the β-Al5 Fe Si phase to α-Al（Mn,Fe）Si phase and reduce the possibility of crack formation, thus improving the wear resistance. Sn added to A356 aluminum alloy forms Mg2 Sn precipitates after heat treatment. Therefore, the unrealizable precipitation hardening Mg2 Si phase and the softening β-Sn phase can reduce the hardness of the alloy, and finally reduce the wear resistance.

Preparation of semisolid A356 alloy slurry with larger capacity cast by serpentine channel

The integral microstructure of semisolid A356 alloy slurry with larger capacity cast by serpentine channel was studied and the influence of cooling ability of serpentine channel on the microstructure was investigated. The results indicate that ideal slurry with larger capacity can be prepared through serpentine channel with good cooling ability. When the serpentine channel was continuously cooled, both the longitudinal and the radial microstructure of the slurry was composed of granular primary phase and the integral microstructure uniformity of the slurry was good. However, uncooled serpentine channel can only produce larger slurry with fine grains in positions adjacent to its centre and with a large number of dendrites in positions close to its edge, thus, the radial microstructure of larger slurry is nonuniform. The pouring temperature is set up to 680 °C and the solid shell inside the channel can be avoided at this pouring temperature.

Effect of depressurizing speed on mold filling behavior and entrainment of oxide film in vacuum suction casting of A356 alloy

The effect of depressurizing speed on mold filling behavior and entrainment of oxide film of A356 alloy was studied. Themold filling behavior and velocity fields were recorded by water simulation with particle image velocimetry. The results show thatthe gate velocity first increased dramatically, then changed with the depressurizing speed: the gate velocity increased slowly atrelatively high depressurizing speed; at reasonable depressurizing speed, the gate velocity kept unchanged; while at lowerdepressurizing speed, the gate velocity decreased firstly and then kept unchanged. High gate velocity results in melt falling backunder gravity at higher speed. The falling velocity is the main factor of oxide film entrainment in vacuum suction casting. The designcriterion of depressurizing rate was deduced, and the A356 alloy castings were poured to test the formula. The four-point bend testand Weibull probability plots were applied to assessing the fracture mechanisms of the as-cast A356 alloy. The results illuminate amethod on designing suitable depressurizing speed for mold filling in vacuum suction casting.

Evolution of microstructure and mechanical properties of A356 aluminium alloy processed by hot spinning process

The evolution of microstructure and mechanical properties of A356 aluminum alloy subjected to hot spinning process has been investigated. The results indicated that the deformation process homogenized microstructure and improved mechanical properties of the A356 aluminum alloy. During the hot spinning process, eutectic Si particles and Fe-rich phases were fragmented, and porosities were eliminated. In addition, recrystallization of Al matrix and precipitation of Al Si Ti phases occurred. The mechanical property testing results indicated that there was a significant increase of ductility and a decrease of average microhardness in deformed alloy over die-cast alloy. This is attributed to uniform distribution of finer spherical eutectic Si particles, the elimination of casting defects and to the recrystallized finer grain structure.

Microstructure and Properties of SemisolidA356 Alloy Strip Affected by Nozzle Temperature of a Novel Micro Fused-Casting

The semisolid A356 alloy strip was prepared by a novel continuous micro fused-casting process. The microstructure evolution and mechanical property of A356 aluminum alloy strip with different nozzle temperatures were investigated. The nozzle temperature had great influences on the microstructure and property primarily accompanied with the crystal change in the fused-casting area through the cooling conditions. The results showed that the semisolid A356 alloy strip samples fabricated by micro fused-casting demonstrated good performances and uniform structures with the nozzle temperature at 593 ℃ and the stirring velocity at 700 r/min. The fine grains of the primary α-Al phase with average grain size of 51 μm and shape factor up to 0.71 were obtained under the micro fused-casting process, and the ultimate average vickers hardness came up to 83.39±0.89 HV, and the tensile strength and elongation of the A356 alloy strip reached 245.32 MPa and 7.85%, respectively.

Interfacial heat-transfer between A356-aluminium alloy and metal mould

The interfacial heat-transfer coefficient at casting/mould interface is a key factor that impacts the simulation accuracy of solidification progress.At present,the simulation result of using available data is comparatively different from the practice.In the current study,the methods of radial heating and electricity measurement under steady-state condition were employed to study the nature of interfacial heat-transfer between A356 Aluminum alloy and metal mould.The experimental results show that the interfacial heat-transfer between A356 Aluminum alloy and the outer mould drops linearly with time while that of A356 aluminum alloy and the inner mould increases with time during cooling.The interfacial heat-transfer coefficient between A356 aluminum alloy and mould is inversely proportional to the electrical resistance.

Mechanical properties and microstructural evolution of rheocast A356 semi-solid slurry prepared by annular electromagnetic stirring

Nowadays,having an effective technique in preparing semi-solid slurries for rheocasting process seems to be an essential requirement.In this study,semi-solid slurry of A356 aluminum alloy was prepared by three-phase annular electromagnetic stirring(A-EMS)technique under different conditions.The effects of stirring current,pouring temperature and stirring time on microstructural evolution,mean particle size,shape factor and solid fraction were investigated.The rheocasting process was carried out by using a drop weight setup and to inject the prepared semi-solid slurry in optimal conditions into the step-die cavity.The filling behavior and mechanical properties of parts were studied.Microstructural evolution showed that the best semi-solid slurry which had fine spherical particles with the average size of~27μm and a shape factor of~0.8 was achieved at the stirring current of 70 A,melt pouring temperature of 670℃,and stirring time of 30 s.Under these conditions,the step-die cavity was completely filled at die preheating temperature of 470℃.The hardness increases by decreasing step thickness as well as die preheating temperature.Moreover,the tensile properties are improved at lower die preheating temperatures.The fracture surface,which consists of a complex topography,indicates a typical ductile fracture.

Mechanical properties and microstructures of Al alloy tensile samples produced by serpentine channel pouring rheo-diecasting process

An innovative one-step semi-solid processing technique of A356 Al alloy,the serpentine channel pouring rheo-diecasting process （SCRC）,was explored.The mechanical properties and microstructures of the tensile samples made by the SCRC technique were tested in the as-cast and T6 heat treatment conditions.The experimental results show that the as-cast ultimate tensile strength can reach about 250MPa and the elongation is 8.6%？13.2%.The ultimate tensile strength can increase approximately 30% higher than that of the as-cast one but there is some slight sacrifice of the plasticity after T6 heat treatment.Under these experimental conditions,the semi-solid A356 Al alloy slurry with primary α1（Al） grains,which have the shape factor of 0.78？0.89 and the grain diameter of 35？45μm,can be prepared by the serpentine channel pouring process.The primary α2（Al） grains are very fine during the secondary solidification stage.Compared with the conventional HPDC process,the SCRC process can improve the microstructures and mechanical properties of the tensile test samples.The advantages of the SCRC process include easily incorporating with an existing HPDC machine,cancelling the preservation and transportation process of the semi-solid alloy slurry,and a higher cost performance.

Enhancing Fe content tolerance in A356 alloys for achieving low carbon footprint aluminum structure castings

Low carbon footprint aluminum structure castings are the mainstream development direction of alu-minum alloys in the future.Enhancing the Fe content tolerance upper limit in casting aluminum alloys is considered an effective way to promote the application of recycled aluminum production.In this work,it was found that with the Ce and TiCN NPs(nanoparticles)addition simultaneously to A356 alloys,the acicular Fe-rich phases(β-Al 5 FeSi)which damages the properties can be changed into beneficial phases(core-shell heterostructures)for the alloy,and hence the mechanical properties of A356 alloys can be im-proved.The TiCN nanoparticles and rare element Ce play crucial roles in the formation of core-shell het-erostructures.The formation mechanism of core-shell heterostructure was also systematically researched from the perspective of thermodynamics and kinetics.This study provides a simple and feasible strategy to eliminate the harmful effects of Fe impurity and contributes to the industrialization of low carbon footprint aluminum structure castings.

Tribological behavior of A356-CNT nanocomposites fabricated by various casting techniques

Tribological behaviors of monolithic A356 aluminum alloy castings and A356.CNT nanocomposite castings, fabricated by fully liquid and semisolid routes were examined. Samples were prepared by melt agitation, rheocasting, stir casting, and compocasting techniques. Effects of addition of carbon nanotubes (CNTs), casting process and the applied load on wear properties and mechanisms were investigated. It was found that wear loss, wear rate and friction coefficient of nanocomposite samples remarkably declined by the addition of CNTs. Moreover, changing the casting process from fully liquid to semisolid routes, plus increasing fractions of the primary phase were the two factors that improved the wear properties of the investigated samples, especially nanocomposite ones. In addition, it was revealed that adhesion and delamination were the dominant wear mechanism of the monolithic samples produced by fully liquid and semisolid routes, respectively. However, regardless of fabrication techniques, the abrasion was the main wear mechanism of nanocomposite samples.

Nano-SiC_P particles distribution and mechanical properties of Al-matrix composites prepared by stir casting and ultrasonic treatment

Nano-ceramic particles are generally difficult to add into molten metal because of poor wettability. Nano-SiC particles reinforced A356 aluminum alloy composites were prepared by a new complex process, i.e., a molten-metal process combined with high energy ball milling and ultrasonic vibration methods. The nano particles were β-SiCp with an average diameter of 40 nm, and pre-oxidized at about 850 ℃ to form an oxide layer with a thickness of approximately 3 nm. The mm-sized composite granules containing nano-SiCp were firstly produced by milling the mixture of oxidized nano-SiCp and pure Al powders, and then were remelted in the matrix-metal melt with mechanical stirring and treated by ultrasonic vibration to prepare the composite. SEM analysis results show that the nano-SiC particles are distributed uniformly in the matrix and no serious agglomeration is observed. The tensile strength and elongation of the composite with 2wt.% nano-SiCp in as-cast state are 226 MPa and 5.5％, improved by 20％ and 44％, respectively, compared with the A356 alloy.