Microstructure Of A356 Aluminum Alloy Research Articles

Effect of Pressure on Microstructure and Cooling Curves of A356 Aluminum Alloy During Solidification

During solidification process of metals and alloys in die casting, the cooling rate of molten material and also freezing time are affected by the heat transfer coefficient of metal/mold interface which is not constant at whole casting time. Therefore, determination of the cooling curves and the heat transfer coefficient is a crucial step in optimization of casting process by simulation because the resultant microstructure of cast components is affected by cooling rate. In the present study, the effect of applied pressure on cooling curves and microstructures of A356 aluminum alloy was investigated using a specially designed apparatus. Results showed that the cooling curves obtained from the central axis of cylindrical sample clearly represent the solidus and liquidus temperatures which can be used for determination of freezing time. It was observed that the solidification time decreases with increasing applied pressure. Also, the side thermocouples showed a temperature drop at the moment of applying pressure in which its magnitude increases with the increase in applied pressure. Moreover, the metallographic examinations showed that the microstructure of as-cast samples is refined with the increase in pressure. Also, the heat transfer coefficient obtained from experiments was used in the simulation of casting process and the simulated cooling curves were matched carefully with the experimental curves.

Effects of important parameters in the production of Al-A356 alloy by semi-solid forming process

There are many routes to produce feedstock with globular microstructure. Cooling slope methods has been attracted as the result of its simplicity and also produces of the globular shape billets quicker than other methods. In this method molten metal is poured on a tilted slope that is cooled by water circulating underneath. Due to shear stress exerted to the slurry, it is solidified with globular microstructure. By simulation of semi-solid casting with cooling slope the effects of different pouring conditions on the microstructure of A356 aluminum alloy are investigated. The simulations are carried out by a CFD code called FLOW3D. The average diameter and shape factor of primary α-Al particles from experiments and the time duration of slurry presence on the slope, solid fraction of the slurry, strain rate and turbulence from the simulations were investigated. Comparing the results of simulations with experimental results showed that for having the best microstructure with higher sphericity and lowest particle size, the residence time of slurry on the cooling slope must be enough, while the shear stress and turbulence must be as high as possible. Also, combination of the process parameters including pouring temperature, tilt angle and slope length should lead to adequate value of tf, while solid fraction of slurry at the exit of slope is about 30–35%.

Solidification behavior and rheo-diecasting microstructure of A356 aluminum alloy prepared by self-inoculation method

Semisolid slurry of A356 aluminum alloy was prepared by self-inoculation method, and the microstructure and solidification behavior during rheo-diecasting process were investigated. The results indicate that the semisolid slurry of A356 aluminum alloy can be prepared by self-inoculation method at 600 °C. Primary α-Al particles with fine and spherical morphologies are uniformly distributed when the isothermal holding time of slurry is 3 min. Liquid phase segregation occurs during rheo-diecasting process of semisolid slurry and the primary particles (α1) show obvious plastic deformation in the area of high stress and low cooling rate. A small amount of dendrites resulting from the relatively low temperature of the shot chamber at the initial stage of secondary solidification are fragmented as they pass through the in-gate during the mould filling process. The amount of dendrite fragments decreases with the increase of filling distance. During the solidification process of the remaining liquid, the nucleation rate of secondary particles (α2) increases with the increase of cooling rate, and the content of Si in secondary particles (α2) are larger than primary particles (α1). With the increase of cooling rate, the content of Si in secondary particles (α2) gradually increases. The morphologies of eutectic Si in different parts of die casting are noticeably different. The low cooling rate in the first filling positions leads to coarse eutectic structures, while the high cooling rate in the post filling positions promotes small and compact eutectic structures.

Evaluation of microstructure of A356 aluminum alloy casting prepared under vibratory conditions during the solidification

The objective of this investigation was to evaluate the effect of vibrations (during solidification) on the metallurgical properties of A356 aluminum casting. Mechanical vibrations were applied to A356 aluminum alloy through set up. A356 melt has been subjected to mechanical vibration with the frequency range from 0 to 400 Hz with constant amplitude 5 µm. Grain refinement was obtained through mold vibration. Metallurgical properties were examined through optical microstructure, tensile fracture scanning electron microscope (SEM) and SEM image of test specimens prepared under different conditions of solidification. Results indicate that mold vibration effectively modified the microstructure of A356 casting and it has uniform and smaller grain size with fibrous silicon particle than nonvibrated casting. Grain refinement results increase in mechanical properties with increase in frequency of vibration of mold during the solidification. SEM micrograph of tensile fracture surface was carried out to study the influence of microstructure on fracture mode. SEM image of tensile fractured surface shows transgranular cleavage facets due to fracture of primary silicon particles. Fractures are brittle in nature so observation indicates low ductility and brittle fracture.

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.

Investigation of the controlled atmosphere of semisolid metal processing of A356 aluminium alloy

The cooling slope (CS) method is one of the semi-solid methods in which the molten alloy with a suitable amount of superheat is poured on a cooling slope to achieve a fine and non-dendritic structure. After pouring, the melt, which becomes semisolid at the end of the plate, is subsequently poured into a cylindrical steel mold with different mold temperatures. Also, the process has been done in different cooling slopes and different cooling lengths. This work, at first discusses the effect of these parameters on the final microstructure of A356 aluminum alloy and then the effect of the controlled atmosphere is discussed. Also, in this research, the advantages of using the controlled atmosphere system are discussed by tests such as XRD, SEM and ultrasonic test. Results indicate that the pouring temperature, mold temperature, cooling slope and cooling length have significant effects on the size and morphology of α-Al phase. Also, the controlled atmosphere could optimize the process as well and increase the mechanical properties of alloy.

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.

Production of globular microstructure of A356 aluminium alloy by cooling channel and strain induced melt activation processes: morphological and hardness studies

Processing of alloys in the semisolid state has several advantages over conventional casting processes, such as reduction in macrosegregation, porosity and improved mechanical properties. In this study, produced A356 alloy by means of cooling channel and strain induced melt activation (SIMA) processes has been investigated, and their microstructures and hardness were evaluated. At first, in cooling channel process, the grain size is smaller than that in the conventional cast processes and increases with increasing time. Before heat treatment, the microstructure of the SIMA sample is elongated towards the rolling side in contrast to spherical structure in the cooling channel process. The hardness value of sample in the SIMA process is greater than that in the cooling channel process without heat treatment. When the solution annealing was carried out within 2 h, the globular microstructure in both samples was observed. Finally, the grain size and the hardness of samples were determined as 150 μm and 60 HV respectively.

Preparation of Semisolid A356 Alloy by a Cooling Slope Processing

In order to obtain the non-dendritic feedstock for the semisolid forming process, a cooling slope processing was used. In this work, the effects of the angle, length of cooling slope and pouring temperature on the microstructure of A356 aluminum alloy were investigated. It showed that these parameters affect the size and morphology of α-Al phase to some extent. The results indicate that a pouring temperature of 650°C and a cooling slope with 45° in angle and 50 cm in length are the optimum parameters for preparing fine and globular grain structures. To eliminate the solidification shell formed in the surface of cooling slope, a nitride dope was coated on the surface of the cooling slope.

Study on the effects of the length and angle of inclined plate on the thixotropic microstructure of A356 aluminum alloy

Application of inclined plate in the casting processes is a simple process with low equipment, which is able to produce feedstock materials for semisolid processing. In the inclined plate method, several parameters such as length, angle, inclined plate materials and superheat affect final microstructure. In this work, effects of the angle and length of a copper-made inclined plate on the thixotropic microstructure of A356 aluminum alloy were investigated. Therefore, the molten alloy with 680 °C superheat was poured on the surface of the inclined plate in various angles (20°, 30°, 40°, 50°) and lengths (20, 30, 40, 50, 60 cm). It was found that the angle and length of inclined plate affect the size and morphology of α-Al phase. The results indicate that 40° and 40 cm condition is optimum inclined plate angle and length presenting the minimum grain size of 28 μm and maximum shape factor of 0.70 with highest uniformity.