Cooling Slope Casting Research Articles

EFFECTS OF RHEOCASTING AND THIXOFORMING ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF A356 ALUMINIUM ALLOY

This paper presents the changes in mechanical properties and microstructure of aluminium alloy A356 that undergoes cooling slope casting. The alloy was cut into small cubes with an estimated weight of 400g before it was heated in a crucible induction casting machine. The temperature was set to 880 oC with a heating rate of 15 oC per min. Then, the metal was cooled to 620 oC until it turns to a semisolid, before pouring into a stainless steel mould through a 250 mm long and 60o cooling slope before it was cooled to room temperature. For the thixoforming experiments, the liquid fraction was between 30% and 50%, with various semisolid temperatures (583 oC-585 oC). The ram speed and die temperature were 85 mm/s and 200 oC, respectively. The microstructure and mechanical properties of rheocast feedstock in T6 condition were determined and compared with the metal without any heat treatment, rheocast and thixoformed alloy. It was found that thixoformed metal had the highest ultimate tensile and yield strength with reduced ductility. The microstructures are rosette, near globular and spherical, and were obtained in rheocast and thixoformed alloys, respectively. The α-Al grains were larger at higher semisolid temperatures.

Dry wear behavior of cooling-slope-cast hypoeutectic aluminum alloy

Abstract This study investigated the effect of use of a cooling slope on the microstructure, hardness and wear behavior of Al-7Si-Mg alloy. The Al-7Si-Mg alloy was cast with and without a cooling slope at a pouring temperature of 640 °C. Examination with optical and electron microscopes showed that the microstructure of cast samples using the cooling slope comprised fine and globular primary α-Al phase with homogeneous distribution of eutectic phase, while conventionally cast samples featured coarse and dendritic primary α-Al phase. The wear resistance of the cast sample was significantly better with cooling slope casting. The wear mechanism was found to be a combination of adhesion, delamination, oxidation and abrasive wear for both cooling slope- and conventionally cast samples. The wear mechanisms of cast samples both with and without cooling slope are similar and follow Archard's law. The cooling slope-cast samples with fine and globular α-Al phase, high hardness and low specific wear rate (K′) showed the highest wear resistance of 10.08 × 10−5 mm3 N−1 m−1.

Influence of Cu content on microstructure and mechanical properties of thixoformed Al–Si–Cu–Mg alloys

The effects of Cu content on the microstructure and mechanical properties of thixoformed Al–6Si–xCu–0.3Mg (x= 3, 4, 5 and 6, mass fraction, %) alloys were studied. The samples were thixoformed at 50% liquid content and several of the samples were treated with the T6 heat treatment. The samples were then examined by optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray (EDX) spectroscopy and X-ray diffraction (XRD) analysis, as well as hardness and tensile tests. The results show that the cooling slope casting and thixoforming process promote the formation of very fine and well distributed intermetallic compounds in the aluminium matrix and the mechanical properties of the alloys increase considerably compared with the permanent mould casting. The results also reveal that as the Cu content in the alloy increases, the hardness and tensile strength of the thixoformed alloys also increase. The ultimate tensile strength, yield strength and elongation to fracture of the thixoformed heat-treated Al–6Si–3Cu–0.3Mg alloy are 298 MPa, 201 MPa and 4.5%, respectively, whereas the values of the thixoformed heat-treated alloy with high Cu content (6%) are 361 MPa, 274 MPa and 1.1%, respectively. The fracture of the thixoformed Al–6Si–3Cu–0.3Mg alloy shows a dimple rupture, whereas in the alloy that contains the highest Cu content (6%), a cleavage fracture is observed.

Coarsening Kinetics of Semi-solid A356–5wt%TiB2 in situ Composite

An attempt has been made to investigate the coarsening behaviour of semisolid A356–5wt%TiB2 in situ composite produced by Cooling slope casting. It was found that the average grain size of α-Al in the gravity-cast alloy got reduced from 98 to 48 µm in the semi-solid A356–5TiB2 composite. The primary α-Al in the composite were found to undergo substantial spheroidisation after 5 min at a reheating temperature of 585 °C (f s = 0.5). Further, increase in holding time from 5 to 15 min resulted in grain growth of α-Al in both the alloy and the composites. The coarsening rate constant, K, for A356–5wt%TiB2 composite was found to be 339.73 μm3/s which is 32 % lower than that of semi-solid A356 alloy, thus proving the effectiveness of TiB2 particles towards grain coarsening.

Influence of Semi-Solid Casting and Equal Channel Pressing on Microstructure of a Hypoeutectic Al-Si Alloy

The aim of this work was to develop understanding of microstructural evolution of the alloy casted in semi-solid condition using a cooling slope and conventional casting followed by ECAP in a 120o die. Feed materials were prepared by cooling slope casting and conventional casting for ECAP process. The microstructures and Vickers hardness of the worked materials extruded by two routes (A and BC) were evaluated. The primary α-Al phase tends to be elongated after processing by route A. However, its morphology was similar in nature to the microstructure of the as-cast sample after processing by route BC. The Si particles become fragmented during ECAP processing and are more nearly globular in shape and uniform in size than in the as-cast sample. The microstructure of the semi-solid cast ECAPed samples was more homogenous than that of the conventional cast ECAPed sample followed by ECAP for both routes. The hardness of semi-solid cast ECAP samples was also higher than that of conventional cast ECAPed samples for both routes.

Effect of Thixoforming on the Microstructure and Tensile Properties of A356 Alloy and A356-5TiB2 In-situ Composite

In the present work, A356 alloy and in situ A356-5TiB2 composite feedstock was produced by employing Cooling slope casting technique. The technique resulted in near spherical morphology of primary α-Al phase in the feedstock of both the alloy and composite. A fine distribution of eutectic Si phase within the matrix was observed in the composite feedstock. The rheocast billets of both the alloy and composite were then thixoformed successfully at 50 % solid fraction temperatures of 580 and 585 °C respectively. Further, tensile properties of thixoformed alloy and composite were measured and compared with those of gravity-cast samples. It was observed that the % increase in yield strength and tensile strength of thixoformed alloy increased by 43 and 36 % respectively with respect to the gravity-cast alloy. The thixoformed composite attained the highest ultimate tensile strength of 211 MPa which is about 40 % higher as compared to gravity-cast alloy. Interestingly, the ductility of the composites is comparable to that of alloy after thixoforming.

EFFECT OF COOLING SLOPE CASTING PARAMETERS ON THE THIXOTROPIC MICROSTRUCTURE OF A356 ALUMINUM ALLOY

In the present paper, the effect of cooling slope casting (CSC) parameters process, typically, pouring temperature (T), cooling slope length (L) and tilt angle (θ), on the microstructure of A356 cast aluminum alloys was studied. The analysis of variance (ANOVA) statistical technique was carried out to clarify the relationship between the aforementioned parameters and the microstructure variables, typically, the shape factor (S.F.) and the grain size (G.S.) of the α-Al primary phase. The uniformity of the microstructure in both the axial and radial positions of the ingots was also evaluated statistically. The results revealed that the tilt angle has the highest significant effect on both shape factor and grain size. The shape factor exhibited maximum value of about 0.812 for ingots poured at θ = 30o, L = 300 mm, T = 630 oC. The minimum grain size was about 32.85 μm for ingots casted at θ= 60o, L = 200 mm, T = 630 oC. Ingots poured at θ = 60o, L = 300 mm, T = 630 oC exhibited the highest grain size uniformity. While ingots casted at θ = 30o, L = 400 mm, T = 620 oC, exhibited the highest shape factor uniformity.

On the age hardening behavior of thixoformed A356–5TiB2in-situ composite

The present article investigates the age hardening behavior of thixoformed A356–5TiB2in-situ composite prepared by a salt-metal reaction. The non-dendritic feedstock for thixoforming was prepared by a cooling slope casting technique. The results show that the thixoformed A356–5TiB2 composite attains the peak hardness in only four hours. The detailed microstructural analysis of thixoformed composite shows that enhanced ageing can be attributed to high dislocation density generated in α-Al matrix due to thermal mismatch and the thixoforming process itself. Furthermore, it was found that thixoforming not only increases the tensile strength of thixoformed composite in peak-aged condition but also increases the ductility by ~7.4%.

Effects of Processing Parameters on Microstructure Evolution of Al-7Si-Mg Alloy by Cooling Slope Casting

This work investigates the effects of pouring temperature, slope length, and slope temperature in cooling slope casting on the formation of globular microstructure of Al-7Si-Mg alloy. The remnant alloy on the slope during casting was quenched and characterized at different stages of flow to evaluate the microstructure features developed in cooling slope casting. The primary α-Al dendritic phase found in conventional cast alloy was transformed into globular shape in slope-processed cast alloy. Finer and more homogenous primary α-Al phase was formed at lower pouring temperature (625 °C). The effect of slope length on microstructure of Al-7Si-Mg alloy was significant at high pouring temperatures (640 and 660 °C) but was not visible at low pouring temperature (625 °C). The microstructure of alloy became coarser with increasing slope temperature.

Production of semi-solid feedstock of A356 alloy and A356-5TiB2 in-situ composite by cooling slope casting

Cooling Slope casting process has gained significant importance for manufacturing of semi-solid feedstock Al alloys and composites which find applications in automotive and aerospace industries. The primary objective of this research is to generate the semi-solid feedstock suitable for thixoforming process. The current work discusses the phenomena involved in the evolution of microstructure of the semi-solid feed stock by Cooling Slope casting process and the effect of various parameters. The process parameters like the angle and length of inclined plate affect the size and morphology of α-Al phase. The Cooling slope process resulted in the formation of fine grain size of about 38 μm of α-Al phase, compared to that of 98 μm processed conventionally. Further, the pouring temperature played a crucial role in the generation of semi-solid microstructures in case of both the alloy and composites. Moreover, fine TiB2 particles of size 0.2-0.5 μm are uniformly distributed in the almost spherical α-Al grains and at the grain boundaries in the composite feed stock. Cooling slope casting route is a suitable and economical route for semi-solid slurry generation by effectively varying the process parameters.

Investigation on microstructure of Al–25 wt-%Mg2Si composite produced by slope casting and semi-solid forming

The effect of cooling slope casting process and semi-solid forming on the microstructure of Al–Mg2Si composite were investigated. In the current work, in order to obtain a suitable microstructure for semi-solid forming process, including good sphericity of α-Al phase with the equivalent diameter of less than 100 μm, cooling slope method was used. Cooling curve of cast composite was plotted using thermal analysis method. Nucleation temperature of each phase was determined using the first derivative curve and cooling curve. Casting was performed at three different temperatures to find the best temperature for casting on the cooling slope. Approriate temperature for reheating in semi-solid range was determined using solid fraction curve. Samples obtained from cooling slope process were thixoformed. The results showed that the microstructure was improved in the thixoformed samples.

Effect of thixoforming on morphological changes in iron-bearing intermetallics and mechanical properties of Al–Si–Cu alloys

Abstract Al–Si–Cu casting alloys are widely used in manufacturing automotive parts. Semisolid forming of these alloys can eliminate major problems in their die casting. In this study, low superheated melt was employed to modify the microstructure and to produce non-dendritic A380 alloy. Melt was poured on a cooling slope plate at 615°C and ingots were obtained through permanent mold casting. They were thixoformed using a hydraulic press after holding at 570°C for 15 min to yield a microstructure having α-Al globules, Si particles, and modified β-phase intermetallic compounds. Image analysis showed that the size and distribution of α-Al phases and iron-bearing intermetallics were modified by using semisolid processing. The results also showed that cooling slope casting and thixoforming have improved the mechanical properties including tensile strength, elongation, and hardness of the high iron content A380 alloys.

Microstructural evolution and wear characteristics of equal channel angular pressing processed semi-solid-cast hypoeutectic aluminum alloys

Abstract This work investigated the microstructural evolution of Al–7Si–Mg alloy cast semi-solid using a cooling slope as well as conventional casting followed by equal channel angular pressing (ECAP) in a 120° die. Feed materials were prepared for ECAP by cooling slope casting and by conventional casting. The microstructure of the processed alloys extruded was observed by optical microscope and by transmission electron microscope, and their hardness and wear resistance were evaluated. After ECAP processing, the primary α-Al phase tended to be elongated while the Si particles became fragmented and more nearly globular in shape and uniform in size than in the as-cast sample. The microstructure of the cooling slope-cast ECAPed samples was more homogenous than that of the conventionally cast ECAPed sample. The α-Al phase sub-grains were refined to sub-micrometer sizes for samples cast by both methods after ECAP. The hardness of the cooling slope-cast ECAPed sample was also higher than that of the conventionally cast ECAPed sample. The wear resistance of the alloy improved after cooling slope casting and ECAP processing.

Effect of Pouring Temperature and Cooling Slope Length on Microstructure and Mechanical Properties of Rheocast A319 Aluminium Alloy

Aluminium alloys are among the most prominent and well known materials used in automotive industries. Nowadays, many vehicles used aluminium engine blocks instead of cast iron to improve fuel efficiency. Among cast aluminium alloys, A319 grade alloys are normally used in automotive industries due to a combination of good fluidity and mechanical strength. In this study, A319 cooling slope rheocasting billets were produced in order to obtain near spherical morphology of primary aluminium phase. The change in the α-Al morphology upon the cooling slope casting was remarkable and the dendritic microstructure was almost replaced by α-Al globules and rosettes. The rheocasting billets were prepared for tensile testing at room temperature. It is found that, the yield strength and elongation of cooling slope rheocasting billets is higher than those from as-cast A319 alloy.

The effects of Mg addition on the microstructure and mechanical properties of thixoformed Al–5%Si–Cu alloys

In this study, the effects of different amounts of magnesium (Mg) on the microstructures and tensile properties of thixoformed Al–5%Si–Cu alloys were investigated. Three different alloys containing various amounts of Mg (0.5, 0.8 and 1.2wt%) were prepared through the cooling slope casting technique, before they were thixoformed using a compression press. Several of the thixoformed samples were then treated with a T6 heat treatment, that is, solution treatment at 525°C for 8h, quenching in warm water at 60°C, followed by aging at 155°C for 4h. All of the samples were then characterised by optical microscopy (OM), scanning electron microscopy (SEM) energy dispersive X-ray (EDX) spectroscopy and X-ray diffraction (XRD) analysis as well as by tensile tests. The results revealed that magnesium was able to refine the size of α-Al globules and the eutectic silicon in the samples. It was also observed that a compact π-Al9FeMg3Si5 phase was formed when the magnesium content was 0.8wt% and 1.2wt%. The mechanical properties of the thixoformed alloys improved significantly after the T6 heat treatment. The highest attainment was recorded by the latter alloy (i.e. with 1.2wt%Mg) with its ultimate tensile strength (UTS) as high as 306MPa, yield strength (YS), 264MPa, and elongation to fracture of 1.8%. The fracture of thixoformed alloy with a low Mg content (0.5wt%) showed a combination of dimple and cleavage fracture, whereas in the alloy that contained the highest Mg content (1.2wt%), cleavage fracture was observed.

Effects of Cu on the Microstructures and Tensile Properties of Thixoformed Al-Si Alloys

In this study, the effects of copper content on the microstructures and tensile properties of thixoformed Al-5Si-xCu-0.5Fe (x =1.0, 2.0 and 3.0 wt. %) were investigated. For this study, three different alloys having various amounts of copper were prepared using cooling slope casting before thixoforming. The semi-solid liquid range for the alloys were estimated using the diffrential scanning calorimetry (DSC) analysis. The samples were thixoformed at 40% liquid fraction. Some of these samples were treated with a T6 aging process. The thixoformed and thixoformed T6 samples were then characterized by optical microscopy, scanning electron microscope (SEM) and energy dispersive X-ray (EDX) as well as tensile tests. The different phases formed in the thixoformed and thixoformed T6 samples were throughly investigated.The results indicate that as copper content increases, the tensile strength also increases, which might due to precipitation hardening. The thixoformed T6 alloys attained an ultimate tensile strength (UTS) as high as 303 MPa when Cu content is 3 wt%.

New Semisolid Casting of an Al-25Wt.% Mg<sub>2</sub>Si Composite Using Vibrating Cooling Slope

Al-Mg2Si composites have gained considerable attention because of their attractive properties such as low density, improved wear resistance and good castability. However, when in-situ routes are used for processing of these composites, the formation of coarse Mg2Si particles with sharp cornersis inevitable and is detrimental for the composite properties. This problem is intensified when a hypereutectic composition such as Al-25wt.% Mg2Si alloy is processed. In the present study, an innovative semisolid technique termed as the Vibrating Cooling Slope (VCS) has been applied to produce a sample of in-situ Al-25wt.% Mg2Si composite.This technique combines the conventional cooling slope and vibration casting methods into an integrated one for producing fine and globular structures in the as-cast condition. An inclined plate was prepared from a 10mm thickness copper plate and was coated by boron nitride. This platecould vibrate mechanically in the vertical direction at a predetermined frequency by the aid of four springs and an electric motor. The molten Al-16.5wt.%Mg-9.4%Sialloy with 100oC superheat was poured on the surface of this cooling slope (set at 45° inclined angle)while it was vibrating at the frequency of 40 Hzandamplitude of 400 μm.The semisolid alloy travelled the length of 40 cm on the slope before being poured into a steel mold (60 mm internal diameter and 35 mm in height). Also for the purpose of comparison, gravity casting (GC) and conventionally still cooling slope casting (CS)were carried outboth using the same mold and with the same superheat.The samples were sectioned, polished and subjected to metallographicstudies,porosity and hardness measurements. It was concluded that CS and VCS techniques resulted in a decrease in the size of Mg2Si particles by about 50%and 70% respectively when compared with the gravity casting. However, the increased shape factor of Mg2Si particles in the VCS and CS processed samples was insignificant as compared with GC. Although in comparison with GC, the VCS processed sample showed a higher porosity level, it exhibited a higher hardness value. These results were attributed to the finer and modified microstructure obtained via this newly developed technique.

Preparation of Globular Microstructure Aluminum Alloy Using Cooling Slope Casting as Feed Material for Equal Channel Pressing Process

Equal channel angular pressing (ECAP) could be used to achieve ultra fine grains in bulk aluminum alloy through severe plastic deformation. Typically a feed material of as-cast aluminum alloys is used with a typical hypoeutectic solidification structure, consisting of primary aluminum dendrites and interdendritic network of lamellar eutectic silicon. On the other hand, semi-solid metal casting provides non-dendritic and globular microstructure which is one of a considerable factor in obtaining homogenous microstructure after ECAP. This work is an attempt to produce aluminum alloy with globular microstructure using cooling slope semi-solid casting process which is believed suitable as a feedstock for ECAP. The aim of this work described in this paper was to understand of microstructural evolution of aluminum structure during cooling slope casting process. Two experiments were carried out. A sample was casted via a cooling slope into a vertical cold mild steel mould at pouring temperature of 640°C. Cooling slope length of 250 mm, slope temperature of room temperature and tilt angle of 60o was applied. Another sample was casted directly into a vertical cold mild steel mould at pouring temperature of 640°C. The primary α-Al phases in the sample that casted without cooling slope was mostly in dendritic throughout the section of sample whilst the primary α-Al phases transformed completely into non-dendritic in the sample that was casted via the cooling slope. Therefore, the transformation is believed resulted from the effect of cooling slope

Optimization of degree of sphericity of primary phase during cooling slope casting of A356 Al alloy: Taguchi method and regression analysis

The present work presents the results of experimental investigation of semi-solid rheocasting of A356 Al alloy using a cooling slope. The experiments have been carried out following Taguchi method of parameter design (orthogonal array of L9 experiments). Four key process variables (slope angle, pouring temperature, wall temperature, and length of travel of the melt) at three different levels have been considered for the present experimentation. Regression analysis and analysis of variance (ANOVA) has also been performed to develop a mathematical model for degree of sphericity evolution of primary α-Al phase and to find the significance and percentage contribution of each process variable towards the final outcome of degree of sphericity, respectively. The best processing condition has been identified for optimum degree of sphericity (0.83) as A3, B3, C2, D1 i.e., slope angle of 60°, pouring temperature of 650°C, wall temperature 60°C, and 500mm length of travel of the melt, based on mean response and signal to noise ratio (SNR). ANOVA results shows that the length of travel has maximum impact on degree of sphericity evolution. The predicted sphericity obtained from the developed regression model and the values obtained experimentally are found to be in good agreement with each other. The sphericity values obtained from confirmation experiment, performed at 95% confidence level, ensures that the optimum result is correct and also the confirmation experiment values are within permissible limits.

Cooling slope casting to produce EN AW 6082 forging stock for manufacture of suspension components

The potential of cooling slope casting process to produce EN AW 6082 forging stock for the manufacture of EN AW 6082 suspension components was investigated. EN AW 6082 billets cast over a cooling plate offer a fine uniform structure that can be forged even without a separate homogenization treatment. This is made it possible by the limited superheat of the melt at the start of casting and the fractional solidification that occurs already on the cooling plate. Suspension parts forged from cast and homogenized billets with or without Cr all showed a uniform structure, and the hardness reached HV 110 after the standard artificial ageing treatment.