RO129439B1 - Process for producing porous aluminium alloy - Google Patents

Abstract

The invention relates to a process and an installation for preparing porous aluminium alloys with reduced specific weight by treatment with stabilizing elements and by blasting gas into the melting charge. The claimed process for preparing AlSi12Ca1.5Mg alloy with a high porosity of 20...30%, an apparent density of 1.8...2.2 g/cmand a pore size of 300...500 μm, uniformly distributed in the alloy, consists in melting the AlSi12 alloy, after which the temperature is raised to 650°C, the AlMg10 pre-alloy is added for generating small size oxide particles with low surface tensions which stabilize the metallic foam, 3...5% metal Ca is added while stirring, in order to obtain thermodynamically stable oxides and increase the viscosity of the melting charge, the melting charge is kept for homogenization for 10...15 min and then gaseous nitrogen is bubbled at temperatures of 650...700°C at time intervals of 5 min, the metallic foam formed at the surface of the melting charge is collected and the porous material is solidified at the ambient temperature. The claimed installation comprises a vertical electric furnace () with a dense graphite crucible () built for injecting the gas to the bottom part, a mechanical stirrer () made of dense graphite built from a long rod provided with a screw and a gas bubbling device () placed at the bottom part of the crucible (), consisting of a leading tube and a porous plug of ceramic material with a pore size smaller than 1 mm.

Description

The invention relates to a process for obtaining a porous aluminum alloy type AlSi-Ca-Mg, with low specific weight, by treating with stabilizing elements and blowing gas in the melt.

The porous alloys are known by the multitude of interesting combinations of physical and mechanical properties, such as: high mechanical strength / density ratio, high acoustic and mechanical damping properties, high thermal conductivity in convective regime, etc. Due to these properties, porous alloys can be used in various industrial fields, from building materials to ornaments. especially due to the low density, combined with a higher mechanical strength, porous materials are preferred in the construction of aircraft (fuselage, passenger seats), space technology (thermal insulation, vibration dampers), car construction (shock absorbers, bodywork, protection) noise), military industry (radar detection mitigation, weapons), medicine (prostheses), construction (structural pillars, walls), etc.

Unlike metallic foams, high porosity materials have higher densities (40 ... 80% compared to 10 ... 30% of the density of castings), with superior mechanical properties (mechanical strength, hardness, fatigue resistance, resistance) to shock).

The methods of obtaining high porosity materials are similar to those for obtaining metal foams with closed pores and differ only by choosing process parameters. A classification criterion of these methods is according to the state of the metallic material (solid, liquid, powders) and the nature of the foaming agent (gases, chemical elements, hydrides, etc.). If the precursor material is in liquid form, we distinguish three types of production methods: by inert gas (argon, nitrogen) or reactive (oxygen, air) in the metal melt (Metcomb method), by introducing compounds which, by dissociation, they produce sparkling gas (TiH ₂ , CaCO ₃ ) (Alporas process) or by precipitating the gas previously introduced into the melt (Gazar method).

The process of obtaining metal foams by inserting gases into the metal smelter (Cymat / Metcomb) is currently industrially operated by Cymat Aluminum from Canada and was patented in its original form by Alean (I. Jin, LD Kenny , H. Sang, “Stabilized metal foam bodies,” US 5112697/1992). In this process, the metallic material is pre-inoculated with silicon carbide, aluminum oxide or magnesium particles, in weight percentages of 10 ... 20% and with dimensions of 5 ... 20 pm. In this way, a composite material with matrix of aluminum alloy reinforced with stable ceramic particles is made. The melt of composite material thus formed is bubbled with nitrogen to produce metal foam. The ceramic particles introduced into the melt have the role of increasing the viscosity of the melt and to establish centers of foam support on the surface of the melt. The liquid foam thus formed is transportable through special conveyor belts and is allowed to cool and solidify. The products obtained are in the form of strips 10 cm thick. The porosity of the aluminum foams produced in this way ranges from 80 to 98%, which corresponds to densities between 0.069 and 0.54 g / cm ³ , at an average pore size of 25 to 3 mm, and wall thicknesses. from 50 to 85 mm.

The method developed by T. Miyoshi of Shinko Wire Co., Japan, involves the introduction of a TiH ₂ foaming agent into the alloy melt, which was previously brought into a high viscosity state by the addition of metallic calcium (Alporas). ). The process consists of heating the melt at a temperature of 680 ° C, introducing metallic calcium in percentages of 1.5 ... 3% in the molten alloy bath with the formation of oxides and intermetallic compounds that raise the viscosity of the melt to desired values, followed by the introduction TiH ₂ powder which, by dissociation, produces the gas needed to form the porous structure of the material; through this

EN 129439 Β1 method, metal foams (at industrial level) with homogeneous structure and with 1 small pore size can be obtained. A similar method was proposed by Nakamura, where the foaming agent was replaced with CaCO ₂ particles coated with CaF ₂ and metal foams 3 were obtained with densities close to the previous method, but with smaller pore sizes.

The Gas / Lotus method exploits the difference between the gas solubility limit in 5 metals in liquid or solid state. The method consists of injecting a gas (hydrogen or nitrogen) into an alloy melt at pressures of 50 bar, followed by the solidification of the material with the inclusion of 7 gas bubbles and the formation of metal foam. The morphology of the resulting pores is influenced by the amount of gas introduced, the gas pressure, the rate of solidification and the chemical composition 9 of the alloy. In general, the resulting pores have elongated configurations in the direction of solidification of the material. The size of the pores obtained varies between 10 pm ... 10 mm in width and 11

100 pm ... 300 mm in length, and the porosity can reach up to 75%.

These processes for obtaining porous materials based on aluminum or aluminum alloys 13 present a number of difficulties and disadvantages, including: the high cost of the raw materials used (pre-made and pre-treated reinforcing particles), demanding installations (in 15 especially for the Alporas method), the high number of technological steps (obtaining a composite material with low particle size beforehand) and the reduced quality of the foams produced 17 (fragility induced at the ceramic / alloy interface, low homogeneity both in the pore distribution, as well as in their size, the appearance of internal tensions upon solidification). 19

It is also known, through the work of the authors A. Knuutinen, K. Nogita, S.D. McDonald,

A.K. Dahle, "Porosity formation in aluminum alloy A356 modified with Ba, Ca, Yand 21 Yb", (Journal of light Metals 1 (2001) 241-249), a process for producing an AI-7% Si-Mg alloy with increased porosity by adding an amount of Ca, made by melting the 23 base alloy in a crucible inserted in an induction furnace, keeping the melt at about 730 ° C, then bubbling argon into the metal melt for 10 min through a 25 graphite core immersed in the melt, periodic cleaning of the metal foam bath surface and adding calcium as a modifier, cleaning the bath surface and then pouring the alloy into 27 preheated steel shapes, coated with boron nitride.

Also in the work: TB Kim, S. Suzuki, H. Nakajima, "Fabrication of a 29 lotus-type porous Al-Si alloy by continuous casting with a thermal decomposition method", (J. Phys .: Conf. Ser. Vol. 165, No.1 (2009), presents the production of a 31 porous alloy type AI-14% Si by melting the base alloy in vacuo and adding powder or Ca (OH) ₂ pellets. 33

Parts specific to the installation for carrying out such a process are presented in the documents: RO 108432 B1 and RO 113870 B, which describe a process for treating the liquid metal in the melting pot at the bottom of the pot of preheated inert gas. , by a porous plug, respectively a process and a liquid metal treatment plant 37 in the pouring pot by the introduction of aluminum as a deoxidant, in the form of a pipe, through which an inert gas is introduced, the uniform distribution of aluminum in the liquid metal 39 being made by a column provided with a rotary immersion device, with ceramic clothing and some pallets at the bottom. 41

The technical problem that the invention solves is to obtain a porous Al-Si alloy, low density, homogeneous and resistant to mechanical stresses. 43

The proposed process for obtaining porous metallic materials based on aluminum alloy by gas bubbling in the melt solves this technical problem in that it involves the creation in melt of thermodynamically stable intermetallic compounds and oxides, which contributes to the formation of the porous structure of the alloy. 47

RO 129439 Β1

More specifically, the process achieves a porous aluminum alloy, type Al-Si-CaMg, through the phases of production of an intermediate alloy type Al-Si-Mg, maintaining the melt for homogenization, adding calcium as a modifier, bubbling argon in the melt. metal and then collecting the metal foam from the surface of the bath and cooling it. The intermediate alloy type Al-Si-Mg is obtained from a molten AISi 12 base alloy, heated to 650 ° C, by the addition of AlMg 10 pre-alloy, the calcium is added 3-5% under stirring, and the gas bubbling inert is made by introducing nitrogen gas into the molten alloy AISi12Ca1,5Mg obtained, at

650 ... 700 ° C, after maintaining it for 10 ... 15 minutes for homogenization, the metal foam formed on the surface of the melt being collected at intervals of about 5 minutes and solidified at room temperature.

The process according to the invention has the following advantages:

- achieves obtaining a porous Al-Si alloy with low density, homogeneous and resistant to mechanical stresses;

- does not require the presence of ceramic particles for stabilizing the metal foam;

- does not require the addition of TiH ₂ type gasifying compounds, which may damage the mechanical properties of the resulting porous material;

- uses the usual and cheap raw materials, conventional alloys in a molded state, instead of deformed semi-finished products or in the form of powders;

- eliminates the need to introduce stabilizing ceramic particles that require preliminary surface treatments to improve particle / alloy wetting and which are complicated and expensive;

- Magnesium present in high percentages (> 1%) determines the appearance of small oxide particles with low surface tension, which contribute to the stabilization of metal foams with small pores (<500 pm) and with a homogeneous distribution in the alloy, which constitutes a premise of a mechanical properties / high density ratio;

- involves a small number of operations, with reduced durations and low energy consumption.

The invention is presented in detail below, in relation to the figure, which has a vertical section through the installation application process.

The process according to the invention for obtaining a porous aluminum alloy type Al-Si-CaMg contains the following main steps:

- melting the alloy in the oven; maintaining at 650 ° C for homogenization;

- introduction of magnesium (1 ... 2%) as a pre-alloy in the melting and maintenance with mixing;

- the introduction of metallic calcium (1 ... 2%) for creating a viscous melt and maintaining it with mixing;

- gas bubbling in the melt with metal foam formation; collecting the metal foam from the surface of the melt and solidifying the porous material.

Melting of the melt is done at the bottom of the crucible. During bubbling, the melt is stirred with a mechanical propeller shaker, for better dispersion of the bubbles in the alloy mass. The process parameters are: working temperature, bubbled gas flow, bubbling duration and magnesium content of the alloy. The efficiency of the process is estimated by the properties of the material produced (apparent density, homogeneity) with a minimum consumption of materials and energy.

The raw materials used to obtain the porous material AISi12Ca1,5Mg are: AISÎ12 alloy, AIMg30 pre-alloy and Ca of 99.99% purity.

RO 129439 Β1

For example, in order to obtain a porous material of the type AISi12Ca1,5Mg, the pre-1 alloy in liquid state is bubbled, at temperatures of 650 ... 700 ° C with N ₂ gas; The bubbles produced are uniformly dispersed in the alloy mass by shaking and form a dense metal foam 3 on the surface of the melt. The resulting foam is collected from the surface of the melt with solidification at room temperature. 5

The proposed installation (figure), for obtaining a porous material based on AISi12Ca1,5Mg by nitrogen bubbling in the metal bath, is composed of a vertical electric furnace 7, a crucible 2, a mechanical stirrer 3 and a device of gas bubbling 4.

The furnace 1 is electric, of tubular form inside, with a power adequate to provide 9 with a temperature of at least 800 ° C. Crucible 2, made of dense graphite, is specially constructed to allow gas injection at the bottom and high enough to allow mechanical agitation of the melt. The mechanical agitator 3, made of dense graphite, consists of a rod long enough to reach the middle of the crucible and a propeller sized at half the diameter of the corresponding section of the crucible enclosure. The gas bubbling device 4 is placed at the bottom of the crucible and contains the gas steering tube and a porous plug 15 of ceramic material with pore size below 1 mm.

The process for obtaining a porous material based on AISi12Ca1,5Mg by bar-17 nitrogen dipping in the metal bath is based on achieving a high viscosity melt 5 by adding metallic calcium and bubbling the viscous melt with a low molecular weight 19 inert gas. . The mechanical stirring of the molten alloy bath produces a better dispersion of the bubbles 6 in the melt, thus increasing the efficiency of the process, and obtaining metallic foams 7 homogeneous, with small pores.

The process according to the invention removes some of the disadvantages of known processes 23 in that: it does not require the presence of ceramic particles for stabilizing the metal foam, it does not require the addition of TiH ₂ type gasifying compounds, which can damage the 25 mechanical properties of the resulting porous material; produces small pores (<500 pm); magnesium present in high percentages (> 1%) causes the appearance of small 27 oxide particles and low surface tension, which contribute to the stabilization of metallic foams with small pores. These advantages improve the mechanical properties of the material obtained, the energy consumption of the process and, implicitly, the cost of the final product.

The installation, according to the invention, removes some of the disadvantages of the usual installations by the fact that: the melting gas is made at the bottom of the crucible by means of a ceramic stopper with pores <1 mm, and the dispersion of the bubbles produced is realized 33 by means of a mechanical stirrer . Thus, an adequate gas flow is ensured, a small gas bubble size and an increased gas / melt contact surface, for obtaining porous alloys with small pore dimensions and homogeneously distributed in the structure.

The following is an example of the application of the procedure: 37

To obtain a batch of 100 g porous material AISi12% / Ca1,5Mg, it is started from a quantity of 314 g precursor alloy AISÎ12, placed in the graphite crucible, inside the 39 oven. Heat the batch to 650 ° C and add 35 g of AMg10 pre-alloy (contains Mg in excess of 15%, with formation of spinel particles and losses through 41 vaporization) with 15 min maintenance. Add 5% Ca metal (contains 10% calcium loss with oxide formation) with slight stirring and 10 min maintenance. The process of barboiling 43 with nitrogen at a rate of 2.4 l / min begins. In five steps, at an interval of 4 min from each other, 20 g samples of porous material are collected from the melt surface. Thus, a quantity of 100 g of high porosity aluminum alloy is obtained (20 ... 30%), apparent density of 1.8 ... 2.2 g / cm ³ and pore size of 300 .. .500 pm, evenly distributed throughout the material. 47

Claims (4)

1 Claim 3 Process for obtaining a porous aluminum alloy, type Al-Si-Ca-Mg, made through the phases of production of an intermediate alloy type Al-Si-Mg, maintaining the melt for homogeneity5, adding calcium as a modifier, bubbling argon in metallic melting, collecting the metal foam from the surface of the bath and cooling it, characterized in that 7 Al-Si-Mg intermediate alloy is obtained from a molten AISÎ12 base alloy, heated to 650 ° C, by adding AlMgIO pre-alloy, calcium is added in a proportion of 3 ... 5% under strong action9, and bubbling of inert gas is made by introducing nitrogen gas into the molten alloy AISi 12Ca1,5Mg, obtained at 650 ... 700 ° C, after maintaining it for 10 ... 15 min for 11 homogenization, the metal foam formed on the surface of the melt being collected at intervals of about 5 minutes and solidified at room temperature.