An Evaluation of the Effect of Ultrasonic Degassing on Components Produced by High Pressure Die Casting

  • Österreichisches Giesserei Institut, Austrian Foundry Research Institute
  • Österreichisches Gießerei-Institut (ÖGI), Austria, Leoben
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Ultrasonic treatment is known to be efficient for aluminium melt degassing with the additional benefits of being both economical and environment friendly. This paper describes the effect of ultrasonic degassing on the preparation of an AlSi9Cu3(Fe) alloy for High Pressure Die Casting (HPDC). The degassing efficiency was assessed in terms of the indirect evaluation of the melt, by means of the reduced pressure test and the porosity evaluation of the cast parts. Additionally, the corresponding hydrogen content was estimated with an experimental equation reported in the literature. Ultrasonic degassing shows greater efficiency in terms of hydrogen removal from the melt than conventional N2 + Ar lance bubbling. Components produced by HPDC without degassing, with ultrasonic degassing and with lance degassing, were analysed by computed tomography and by metallography. The results show that the components produced by HPDC after ultrasonic degassing have a similar porosity level to components degassed with conventional lance bubbling, both showing an important improvement over components produced without degassing treatment. Hardness values were similar for all different treatment conditions and well over the minimum value established for the alloy by the corresponding standard.

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With the use of differential scanning calorimetry (DSC), the characteristic temperatures and enthalpy of phase transformations were defined for commercial AlSi9Cu3 cast alloy (EN AC-46000) that is being used for example for pressurized castings for automotive industry. During the heating with the speed of 10°C·min Complimentary to the calorimetric research, the structural tests (SEM and EDX) were conducted on light microscope Reichert and on scanning microscope Hitachi S-4200. As it comes out of that, there are dendrites in the structure of α(Al) solution, as well as the eutectic (β) silicon crystals, and two types of eutectic mixture: double eutectic α(Al)+β(Si) and compound eutectic α+Al
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The paper concerns the problem of discontinuity in high pressure die castings (HPDC). The compactness of their structure is not perfect, as it is sometimes believed. The discontinuities present in these castings are the porosity as follow: shrinkage and gas (hydrogen and gas-air occlusions) origin. The mixed gas and shrinkage nature of porosity makes it difficult to identify and indicate the dominant source. The selected parameters of metallurgical quality of AlSi9Cu3 alloy before and after refining and the gravity castings samples (as DI - density index method), were tested and evaluated. This alloy was served to cast the test casting by HPDC method. The penetrating testing (PT) and metallographic study of both kinds of castings were realized. The application of the NF&S simulation system allowed virtually to indicate the porosity zones at risk of a particular type in gravity and high-pressure-die-castings. The comparing of these results with the experiment allowed to conclude about NF&S models validation. The validity of hypotheses concerning the mechanisms of formation and development of porosity in HPDC casting were also analyzed.
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In this work, an experimental investigation was carried out on the grain refinement of molten AA5754 Aluminum alloy through ultrasonic treatment. The cavitation induced heterogeneous nucleation was suggested as the major mechanism for grain refinement in the AA5754 aluminum alloy. A numerical simulation was performed to predict the formation, growth and collapse of cavitation bubbles in the molten AA5754 Aluminum alloy. Moreover, the acoustic pressure distribution and the induced acoustic streaming by ultrasonic horn reactor were investigated. It is suggested that the streaming by ultrasonic could transport the small bubbles formed in the ultrasonic cavitation zone into the bulk of melt rapidly. These micro-bubbles are collapsed due to acoustic vibrations where the resulting micro-jets are strong enough to break the oxide layer and to wet the impurities. These exogenous particles, intermetallics and oxides could contribute to the formation of fine, uniform and equiaxed microstructure across the treated melt. The experimental results confirmed the simulation predictions.
The effect of heat treatment on gas porosity and mechanical properties was investigated in a step test part manufactured by vacuum assisted high pressure die casting using a new AlSi10MnMg(Fe) secondary alloy. Porosity and mechanical properties were investigated in the T6 temper with different solution heat treatments. Porosity was characterized by metallographic and fractographic analysis of the tensile specimens. Mechanical properties comparable to the corresponding primary alloy have been achieved for each solution heat treatment. It was found that increasing the solution temperature and time increased the yield and ultimate tensile strength, while elongation decreased and porosity observed on the metallographic sections and fracture surfaces increased. Porosity evaluation on the fracture surfaces indicates that the local area fraction rather than the one observed on the whole fracture surface determines ductility. The size of the largest pore appears to become more relevant above values of 200 μm.
Ultrasonic processing is known to be an efficient means of aluminium melt degassing with additional benefits of being economical and environment friendly. This paper describes the performance of ultrasonic degassing in preparing melt for low pressure die casting (LPDC). Efficiency of ultrasonic degassing is compared with conventional Ar rotary degassing by direct measurements of hydrogen concentration in the melt with a Foseco Alspek-H probe and by reduced pressure test in different stages of the casting process. Significant reduction in dross formation along with similar efficiency of hydrogen degassing was shown for ultrasonic degassing as compared with conventional Ar rotary degassing. Mechanical properties, microstructure and porosity level of the components produced by LPDC after both degassing techniques are determined. Results show that the components produced after ultrasonic degassing treatment have similar hardness, tensile properties, porosity level and microstructure as the components degassed with conventional Ar rotary degassing.
Abstract The effect of high intensity ultrasound based on the novel multi-frequency multimode modulated technology on the final density, porosity, mechanical and fatigue properties of an AlSi9Cu3(Fe) alloy after different processing time was studied. Reduced pressure test was used to evaluate the density of alloys. The tensile and fatigue tests were used to evaluate the static and dynamic properties for the different time of ultrasonic degassing, respectively. It is found that ultrasonic degassing is effective in reduction porosity as well as to improve the final density of castings. Furthermore, the experimental results suggest that the porosity level does not have a substantial influence on the static properties contrary to what is observed on fatigue properties.
Ultrasonic processing is known to be an efficient means of aluminium melt degassing and structure modification with additional benefits of being economical and environment friendly. The present paper reports on the kinetics of ultrasonic degassing and regassing of foundry aluminium alloys and on pilot scale degassing trials. Efficiency of ultrasonic degassing is compared with conventional Ar rotary degassing. Direct measurements of hydrogen concentration in the melt by Foseco Alspek-H probe are used along with reduced pressure test. The effects of ultrasonic processing on porosity are studied using three-dimensional X-ray tomography.
This is the first monograph to comprehensively cover the effect of using power ultrasound to refine and solidify aluminum and magnesium alloys. The author is widely regarded as a pioneer in the field, and the text is based on results obtained over the 40 years he has spent developing these techniques. Ultrasonic treatment efficiently removes hydrogen and fine solid inclusions from melts, and also helps create a refined grain structure during solidification in the ultrasonic field. Both the fundamental and applied aspects of the formation of an extremely fine nondentritic grain structure are discussed, as well as the application of ultrasound to the process of zone melting.
The microstructures and mechanical properties (impact toughness, tensile and hardness) of Al–7Si and Al–7Si–2.5Cu cast alloys were studied after various melt treatments like grain refinement and modification. Results indicate that combined grain refined and modified Al–7Si–2.5Cu alloys have microstructures consisting of uniformly distributed α-Al grains, interdendritic network of fine eutectic silicon and fine CuAl2 particles in the interdendritic region. These alloys exhibited improved mechanical properties in as cast condition when compared to those treated by individual addition of grain refiner or modifier. The improved mechanical properties of Al–7Si–2.5Cu alloys are related to breakage of the large aluminum grains and uniform distribution of eutectic silicon and fine CuAl2 particles in the interdendritic region resulting from combined refinement and modification. This paper attempts to investigate the influence of the microstructural changes in the Al–7Si and Al–7Si–2.5Cu cast alloys by grain refinement, modification and combined action of both on the mechanical properties (impact, tensile and hardness).
A symposium of the Acoustics Group of the Physical Society, held on 18th February 1949, surveyed recent advances in (a) the investigation of the fundamental structure of matter; (b) telecommunication and allied applications; (c) use of mechanical forces set up by intense waves. In (a) derivation of elastic constants of matter was an important field especially as small samples such as single crystals could be used. Losses incurred in propagating waves were surveyed with the help of an electrical transmission line model and simple versions of this were established to represent relaxation phenomena based on Maxwell's hypothesis of shear elasticity as a time function and on Kneser's treatment of loss due to delay in a storage process. There was excellent agreement of the latter with recent results on acetic acid. Available sources of ultrasonic power were surveyed and the importance of barium titanate as a powerful and strongly coupled piezoelectric transducer was emphasized. An expression for the receiver/transmitter power ratio in telecommunications systems was examined for gaseous, liquid and solid media and, from available data, optimum frequencies for various ranges were deduced. These were found to be in accord with experience in echo-sounding, earth exploration and in propagation in metals. Accounts were given of experience with flaw detectors and echo-sounding which showed that these were becoming important industrially and in navigation; work on blind aids was unpromising. Advances in timing and time delay devices were described. There is a dearth of important industrial applications of the use of intense waves in spite of the interesting phenomena which have been demonstrated in the laboratory. The importance of the study of cavitation was pointed out. Results were discussed for killing bacteria, disintegrating proteins, emulsifying, soldering aluminium and refining the crystalline structure in solidification of light alloys. Stress was laid on the wideness of the frequency spectrum over which these phenomena occurred and on the difficulties and importance of maintaining temperature constant and of measuring intensities during investigations.
In order to investigate the effects of ultrasonic vibration on degassing of aluminum alloys, three experimental systems have been designed and built: one for ultrasonic degassing in open air, one for ultrasonic degassing under reduced pressure, and one for ultrasonic degassing with a purging gas. Experiments were first carried out in air to test degassing using ultrasonic vibration alone. The limitations with ultrasonic degassing were outlined. Further experiments were then performed under reduced pressures and in combination with purging argon gas. Experimental results suggest that ultrasonic vibration alone is efficient for degassing a small volume of melt. Ultrasonic vibration can be used for assisting vacuum degassing, making vacuum degassing much faster than that without using ultrasonic vibration. Ultrasonically assisted argon degassing is the fastest method for degassing among the three methods tested in this research. More importantly, dross formation during ultrasonically assisted argon degassing is much less than that during argon degassing. The mechanisms of ultrasonic degassing are discussed.
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