Cold Flakes Research Articles

Effects of die-casting defects on the blister formation in high-pressure die-casting aluminum structural components

The following work aims to present the casting defects effect in AlSi10MnMg alloy structural components on the blister’s possibility. Structural components must exhibit high mechanical properties, specifically high ductility. That requires the heat treatment process. However, the component’s exposure to high temperatures can cause blistering on the surface, which is unacceptable in the final product.For microstructure studies, the samples were cut from regions close to the gating system (hot areas) and those away from it (cold areas). For comparison, the paper studied castings in the as-cast state and after T7 heat treatment. The results suggest that the blistering process begins at casting defects, which create microstructural discontinuities. If gases are present in a discontinuity, they expand during solution treatment, forming surface blisters. The largest blisters form on cold flakes, mainly in casting areas near the gating system.However, smaller blisters form on porosity - both gas and shrinkage. The last casting defects that act as blistering sites are cold drops, cold shots, and thin oxide films occurring mainly in areas away from the gating system.

Particle Method Simulation for Formation and Flow of Cold Flakes in High-Pressure Die Casting

In high-pressure die casting (HPDC) processing of aluminum alloys, solidified layers generated in the sleeve of a die casting machine that flow into the mold cavity are known as “cold flakes.” The prediction and control of them are a crucially important issue for HPDC. This study developed a method to simulate their formation and flow using smoothed particle hydrodynamics. First, a solidified layer was modeled as a set of solid particles with behaviors defined by mechanical constitutive equations. Second, this study proposed an algorithm for ascertaining the phase of particles and for calculating liquid–solid particle interaction. Numerical results demonstrated that the method can predict the formations of solidified layers in the sleeve, their peeling and folding during the plunger movements, their inflow into the runner and the mold cavity, and flow disturbances caused by solidified layers trapped at the gate.

Effects of Different Parameters on Porosity Defects Between the Horizontal and Vertical Shot Sleeve Processes

The effects of different casting parameters on porosity defects between the horizontal and vertical shot sleeve processes are quantitatively studied based on experiments and simulations. The castings made using the two processes are characterized in terms of porosity defects by X-ray radiographic inspection and blistering formation by T4 heat treatment process. Experimental results indicate that much fewer porosities and blisters are formed in the castings made using the vertical process than those made using the horizontal one. Meanwhile, numerical simulations have been performed to quantitatively investigate the flow dynamics behavior of molten metal in the horizontal and vertical shot sleeves. The simulation results demonstrate that compared with the conventional horizontal process, the vertical shot sleeve process is more efficient in elimination of waves and reduction of air entrapment and oxide formation. Moreover, the vertical process needs a lower pouring temperature which can decrease hydrogen solubility in the molten metal; it has a smaller temperature gradient of molten melt which can reduce the formation of cold flakes and pre-solidified materials. Therefore, the vertical shot sleeve process is more competitive in improving the internal integrity of die castings.

Method for reducing aluminum alloy heat loss in high pressure die casting shot sleeves

ABSTRACT To reduce the amount of casting defects in diecast components, a new shot-sleeve is proposed. Cold flakes are generated by solidification of the casting material in the shot-sleeve before the injection process. To solve this problem, a thermal insulation system is introduced on the inner shot-sleeve surface, with tiny grooves being machined to create an air gap between the melt and the sleeve surface. Various casting materials with different material properties are used to examine the insulating effect of the shot-sleeve. The degree of thermal insulation is found to be influenced by the material properties, with the surface tension being significant factors. This thermal insulation effect allows the production of high-quality samples and therefore with good mechanical properties. When casting materials with low surface tension are used, the melt is able to penetrate deeply into the grooves of the shot-sleeve, lessening the insulation effect and leading to the defect.

The Effects of Microstructure Heterogeneities and Casting Defects on the Mechanical Properties of High-Pressure Die-Cast AlSi9Cu3(Fe) Alloys

Detailed investigations of the salient microstructural features and casting defects of the high-pressure die-cast (HPDC) AlSi9Cu3(Fe) alloy are reported. These characteristics are addressed to the mechanical properties and reliability of separate HPDC tensile bars. Metallographic and image analysis techniques have been used to quantitatively examine the microstructural changes throughout the tensile specimen. The results indicate that the die-cast microstructure consists of several microstructural heterogeneities such as positive eutectic segregation bands, externally solidified crystals (ESCs), cold flakes, primary Fe-rich intermetallics (sludge), and porosities. In addition, it results that sludge particles, gas porosity, as well as ESCs, and cold flakes are concentrated toward the casting center while low porosity and fine-grained structure is observed on the surface layer of the castings bars. The local variation of the hardness along the cross section as well as the change of tensile test results as a function of gage diameter of the tensile bars seem to be ascribed to the change of porosity content, eutectic fraction, and amount of sludge. Further, this behavior reflects upon the reliability of the die-cast alloy, as evidenced by the Weibull statistics.

Improvements engineered in UTS and elongation of aluminum alloy high pressure die castings through the alteration of runner geometry and plunger velocity

Improvements in ultimate tensile strength (UTS) and elongation were engineered in as-cast tensile specimens made using cold chamber high pressure die casting (HPDC) by increasing the melt flow velocity in the runner. Such improvements were achieved irrespective of casting size, runner geometry and alloy condition (virgin vs. recycled) as long as melt flowed faster. Round tensile specimens were cast using Australian alloy CA313 (an A380 equivalent). Two sets of geometries were used for the tensile castings—with the first satisfying dimensions stipulated by the ASTM standard for HPDC test specimens and the other the ASTM standard for permanent mold casting (PMC) test specimens. In the HPDC set of casting trials, melt velocities were increased by using a higher plunger velocity. By contrast, in the PMC set of castings, the melt was accelerated by introducing a constriction within the runner, upstream of the gates. The reasons for the higher properties are proposed to be a result of the refinement and more homogeneous dispersion of externally solidified crystals and defect-forming suspensions such as cold flakes, oxides and gas bubbles that flow along with the molten alloy and their more homogeneous dispersion. It is suggested that the fragmentation of the suspensions is caused both by the greater rates of shear achieved in the melt, particularly when it flowed through a constriction, and by the viscous dissipation of turbulent energy. The increased dispersion of these suspensions is proposed to be caused by the higher levels of turbulence. The above suggestions are supported with calculations performed using computational fluid dynamics (CFD) simulations which reconstructed some of the physical experiments involving the PMC set. Importantly, CFD modeling indicates that the tensile piece castings were well vented due to the fill patterns achieved, indicating the results may be generalized for such situations where sufficient venting is available including when vacuum assisted venting is employed. The 0.2% proof strength of the alloy was the least affected by the engineered change introduced here, as this property is more dependent on the bulk microstructure including the α-Al grain size which is determined predominantly by cooling rates.

Effect of cold flakes on mechanical properties of aluminium alloy die casts

The cold flake was detected in aluminium alloy die casts ADC12 by using the scanning acoustic microscope, and was visualised in the acoustic image as bright and dark regions, which were corresponding to the oxide layer and the body of the cold flake respectively. By using the specimen which contains the cold flake, the tensile testing and the fatigue testing were carried out. The in process ultrasonic measurement was also carried out with a water bag in the fatigue testing to detect detachment of the cold flake from the matrix in the fatigue process. From these results, it was found that the cold flake was detached from the matrix in the fatigue process to form a crack, and cold flakes, especially exposed cold flakes, reduce the tensile strength and the fatigue life of die casts. The effect of the cold flake on the strengths was discussed from the point of crack propagation.

Influence of Casting Defects on Tensile Properties of ADC12 Aluminum Alloy Die-Castings

Cold flakes, one of casting defects, were observed through acoustic microscopy in aluminum alloy die-cast plates (ADC12), and specimens having a cold flake of different sizes were prepared and subjected to the tensile testing. The tensile strength linearly decreased with the size of the cold flake, and the decreasing rate of the strength was greater for the specimen with the exposed cold flake than that with the embedded cold flake. The in-process ultrasonic measurement was carried out during tensile testing, and it was found that the oxide layer of the cold flake was

Melt flow velocity in high pressure die casting: its effect on microstructure and mechanical properties in an Al–Si alloy

Analysis of as cast tensile specimens of Australian alloy CA313 (an equivalent of A380) made using the high pressure die casting process at three different melt flow velocities has revealed that increasing melt velocity resulted in a finer microstructure and a reduced pore volume fraction and, consequently, better mechanical properties. Mechanical properties are shown to increase more than proportionately with reducing pore volume fraction and the reason for this is demonstrated to be the contribution from the more refined microstructure. The gate used in the investigation was unusual in that its geometry encouraged and prolonged the shearing of the melt. It is proposed that the microstructure was refined by flow shear, the rate of which increased with melt velocity and consequently enhanced the fragmentation of the externally solidified α-Al grains and inclusions such as oxides, cold flakes and gas bubbles.

Acoustic visualization of cold flakes and crack propagation in aluminum alloy die-cast plate

Acoustic images received at different depths of aluminum alloy (ADC12) die-cast plates containing coarse cold flakes were compared with microstructures at the same depth. The bright and dark regions in the acoustic images coincided with the oxide layer on the initially solidified surface of the cold flake and the body of the cold flake, respectively. These results show the ability of nondestructive detection of cold flakes by the ultrasonic microscopy. The specimens were then fabricated to contain the cold flake at a center, and subjected to three-point bending test. When the coarse cold flake existed apart from the tensile side surface of the specimen, a crack initiated at a small cold flake on the side surface and propagated in the specimen. In this case the bending strength was almost the same as the tensile strength (285 MPa) of the die-cast plate. When the cold flake existed on the tensile side surface or at a position less than 0.5 mm inside the surface, the crack propagated along the oxide layer of the cold flake. In this case, the bending strength decreased from the tensile strength when the cold flake approached the side surface. A relation between the bending strength and the position of the cold flake in the specimen was discussed from the standpoint of the fracture mechanics.

The heat transfer during filling of a high-pressure die-casting shot sleeve

Microstructures in high-pressure die-cast products (HPDC) are important for the mechanical properties. During filling of the shot sleeve, the liquid metal dissipates superheat and solidification starts. The solidification may result in externally solidified crystals (ESCs) or cold flakes in the product [H.I. Laukli, L. Arnberg, O. Lohne, Int. J. Cast Metals Res. 18 (2005) 65–72]. The temperature of the melt in the shot sleeve is influenced by several parameters, such as liquid metal superheat, the heat transfer coefficient (HTC), surface properties of the shot sleeve and alloy composition. In order to get a better quantitative understanding of the heat loss during filling, investigations of the temperature history in both liquid metal and the shot sleeve wall have been carried out. A replica of an industrial shot sleeve was made and filling experiments were carried out. Thermocouples at different locations and depths in the melt and steel cylinder were used to measure the temperatures. The thermal field in the shot sleeve was also modelled using the simulation software MAGMASOFT ®. The results from the casting experiments and casting simulations were compared to estimate the heat transfer coefficient.

10103 破断チル層を内在するAl合金ダイカスト材におけるき裂進展の非破壊評価(疲労およびき裂進展,OS10 先進材料の力学と強度)

10103 破断チル層を内在するAl合金ダイカスト材におけるき裂進展の非破壊評価(疲労およびき裂進展,OS10 先進材料の力学と強度)

Nondestructive Detection of Cold Flakes in Aluminum Alloy Die-Cast Plate with Ultrasonic Measurement

Aluminum alloy die-cast plates (ADC 12) were subjected to ultrasonic measurement to obtain a relation between the intensity distribution of the ultrasonic wave and positions of cold flakes appearing in the plate for developing a nondestructive method to detect cold flakes in the die-casts. Die-cast plates of 170 mm in length, 50 mm in width and 6.8 mm in thickness were produced with wider gates to introduce larger cold flakes in the plate. Then the ultrasonic measurement was carried out with the immersion method by using a probe generating a longitudinal wave of 20 MHz in frequency. Intensity distributions of the ultrasonic wave were obtained from the surface to the bottom. The cross section analysis was carried out to examine the distribution of the cold flake. From the cross section analysis, three types of cold flakes were observed: the type A with a straight boundary of initial solidification with oxide thin layer, the type B with a straight boundary without oxide layer, and the type C of an irregular and wavy boundary without oxide layer. In the case of the type C, the oxide layer was thought to be out of the observed section. The ultrasonic wave was slightly reflected from the front and rear boundaries between the cold flake and the matrix, and it was found that the position and the thickness of the cold flake can be detected by ultrasonic measurement.

Thermal and flow modelling of ladling and injection in high pressure die casting process

Air entrapment and premature solidification in the shot sleeve of the cold chamber high pressure die casting process are problems affecting the casting quality. These issues are addressed from several viewpoints, including heat loss during metal ladling, turbulence generation and gas escape during pouring, wave formation and propagation during metal pouring and injection, and heat loss and solid creation in the shot sleeve during metal injection. Measurements and simulations have been conducted for a typical industrial case where the fill ratio of the shot sleeve is about 30%. It has been found that the settling time of the aluminium alloy in the shot sleeve has a profound influence on the air entrapment. The settling time also has a negative effect on the metal quality the solid formation (premature solidification or cold flakes) in the shot sleeve. Fast pouring and quick injection (no settling time) are recommended as a good approach to minimise the air entrapment and cold flakes in the shot sleeve in this particular case. IJCMR/474