Melt treatment
Melt treatment is a technique that is used wherever metals and their alloys are liquefied by melting . It should optimize the quality of the melt .
Melt treatment and secondary metallurgy
As a rule, it takes place in the melting or pouring furnace , but it is also possible in the ladle as a “ladle treatment ” and even by acting on the pouring stream when pouring the melt. In the iron and steel casting industry, the term “secondary metallurgy” is used instead of the term “melt treatment”. However, this is only a special form of melt treatment of steels (→ metallurgy ).
Tasks of melt treatment
The melt treatment of ferrous and non-ferrous metals and alloys includes numerous chemical and physical measures that are aimed at influencing the state of the melt. For castings, the castings must meet the quality standards prescribed for them, such as hardness, strength and elongation. In the case of semi-finished casting , the production of bars (rolling bars, round bars) and wire bars (→ continuous casting ), freedom from hydrogen and oxides , the removal of interfering elements and the promotion of fine-grained solidification are required. Depending on the type of input material and the quality requirements for the cast products, there are different measures for melt treatment.
A standardized terminology makes it easier to understand problems and how to solve them.
Melt treatment practiced
Melt treatment is in principle necessary for all metallic melts in terms of its necessities and possibilities.
Limiting or avoiding hydrogen uptake
The main task is to limit the hydrogen uptake that results from the melting process. Aluminum melts are particularly sensitive here . Certainly, hydrogen uptake can only be avoided in the vacuum melting process and, to a certain extent, also when melting in electrically heated furnaces, because all organic fuels are hydrogen carriers. Regardless of the furnace technology, the melt can also absorb moisture from this in contact with the ambient air, which leads to the release of hydrogen. Hydrogen in the form of aluminum hydroxide can already be contained in the melting material: With scrap aluminum and block metal that has been stored for a long time, a hydroxide layer always forms on the surface . When scrap with adhering fats , oils and paints is recycled , these burn to carbon dioxide and water , which is broken down by the melt and contaminated with hydrogen. The task is therefore to keep hydrogen as far away as possible and, if this is not possible, to remove it from the melt by an appropriate treatment. The technical statement is correct that the melt treatment in fuel-heated furnaces starts with the setting of the burner. In the case of recycling material, it is state of the art to burn off buildup of the type mentioned in an upstream process. In the simplest possible way, this can be a melting bridge as part of the melting furnace. The gases produced during burning must be captured and neutralized.
The harmful effects of hydrogen contamination can be seen in all melts, but especially in aluminum and its alloys, as well as in zinc-free copper alloys. In comparison to the solid state, liquid aluminum holds almost twenty times the amount of hydrogen in solution. This difference is known as the "solubility jump". If a melt saturated with dissolved hydrogen solidifies in the poured-off mold, the hydrogen that separates out is prevented from escaping by the design and a casting skin that has already formed. After complete solidification, it appears in the form of smaller or larger bubbles, the "hydrogen porosity". Since porous parts cannot meet higher strength requirements, avoiding hydrogen absorption is an urgent task. If this does not succeed sufficiently, the removal of the hydrogen, known as “degassing”, becomes the main task of melt treatment for all melts that contain it in dissolved form. The useful measures of supplying suitable melt treatment agents, be it salt or gas mixtures, are widely adopted by automated impeller systems. Due to the nature of the system , they bring both solids and gas mixtures, such as nitrogen with argon added , into the melt in such a way that distribution and residence time lead to the best possible reaction.
Limiting or avoiding the oxidation of alloying elements
Just as important as the limitation of gas uptake (predominantly hydrogen) is to limit the oxide formation that cannot be avoided during the melting process as a result of partial overheating on bath surfaces or furnace walls. Oxidation of alloying elements always means a loss, be it of basic components or important alloying elements, which becomes noticeable qualitatively if it is not corrected, but in any case remains a factor that burdens the calculation. If, however, oxides are already brought into the melt with the feedstock, measures to limit oxidation can no longer be of any help; it is now a matter of removing or at least reducing the undesired oxides, which are in fact metal losses. Oxide-dissolving, dissolved oxides slagging, by faster melting (due to the melting point lowering caused by them as part of the batch) the oxidation-reducing flux are used especially in recycling.
In short, it can therefore be said that all action in the course of melt treatment is geared towards avoiding undesirable properties of the melt, in particular gas absorption and oxidation, or at least noticeably limiting and instead by no means the only ones desired, including freedom from gas and oxides Result, but with the primary aim to bring about.
Contrary measures
In addition to the main tasks described here, there are also secondary tasks which, although not generally applicable, are necessary in certain cases to improve the melt quality. The fact that, instead of degassing, in special cases it may also be necessary to gas the melt, is only mentioned for the sake of completeness.
The same applies to selective oxidation processes in the melt which are brought about by supplying oxygen-releasing mixtures and which serve to remove undesired, but oxidizable elements. An example of this is a low aluminum content in some copper alloys, which is returned through targeted oxidation of the aluminum and the associated slagging of the aluminum oxide.
When melting copper alloys in fuel-heated furnaces - with the exception of brass - any hydrogen content in the melt can be noticeably reduced by setting the burner to excess air. Oxygen-releasing mixtures can also be used.
Excess oxides of copper are reduced again by adding phosphorus copper in the form of a 10 or 15 percent master alloy .
Removal and addition of elements present in the melt
Melt treatment also includes measures that deal with the “removal” or “addition” of elements. What is removed is what affects the structure or the strength values.
The opposite of removing is adding elements. These can be mere corrections, mostly to compensate for oxidation losses, but there are also additives that determine the alloy character in the form of master alloys based on copper or aluminum, briquettes or compacts.
For use with aluminum alloys, see the section “Influence of the structure”. For cast iron there is a wide range of “cupola accessories”.
The role of phosphorus in copper and copper alloys
With copper alloys, with the exception of brass, phosphorus takes on a hybrid position in the form of a master alloy (usually CuP10, i.e. 90 parts copper and 10 parts phosphorus). It is added on the one hand to ensure a small excess of phosphorus in the melt, which prevents its tendency to oxidize during casting, but it is also intended to reduce copper oxide suspended in the melt in order to increase its degree of fluidity to such an extent that irreducible impurities, such as tin oxide, in the slag can rise.
With basically the same purpose, no phosphorus copper may be used in copper melts where a certain electrical conductivity of the cast parts is expected. Boron copper additive is only one of the possible alternatives here.
Special measures
Protect and Remove
This includes all treatment steps that, depending on the input material, be it pure metal or a more or less specialized alloy, primarily relate to the purity of the melt. In the simplest stage, this would be letting the melt stand so that, depending on their density, impurities can either sink to the bottom or rise into the dross. A further step is a treatment that provides protection against the formation and removal of existing impurities through the introduction of reactive salt mixtures, stirring in and blowing in are common methods. The effect can be brought about mechanically and physically (flushing out) or chemically, through slagging of the contamination. This also includes the results of a thermodynamically and kinetically occurring oxidation or reduction.
Impurities to be eliminated can be oxidic / non-metallic or metallic. Metallic impurities are to be assessed as such depending on the alloy and use. In an AlMg alloy, sodium and calcium are considered to be disruptive, in AlSi alloys - with the exception of hypereutectic alloys - sodium is desirable. For the melt treatment, therefore, not only the composition of the melt to be treated is decisive, but also the optimization of the fine structure ( structure formation ) by removing all interfering elements.
Degassing by introducing purging gas
The flushing of the melt, with the purpose of removing impurities, is a variant of the introduction of salt mixtures. It can be done with air (blown steel). A fundamental distinction is made between reactive and inert purge gases. Among the “reactive”, chlorine is extremely effective for flushing and removing hydrogen from aluminum melts, but where it is used it requires complex measures to protect the environment, such as B. Wet scrubbing of exhaust air. In contrast, flushing for ingot casting of certain melts with an argon- chlorine mixture is state of the art. Common inert purging agents include nitrogen or argon.
All gaseous flushing agents can be distributed in the melt under pressure by means of inlet pipes or nozzle stones. A process that was specially developed for the treatment of light metal melts is the introduction of an impeller adapted to the furnace type into the melt. It rotates at a controllable speed and allows both the supply and fine-bubble distribution of a single flushing gas as well as a mixture of these, whereby powdery reaction carriers can be introduced at the same time to reduce undesired contents of alkali and alkaline earth metals, primarily sodium and calcium.
Influence on alloy and solidification structure
The objectives addressed here are in part in accordance with the measures mentioned in the Protection and removal section. They are supplemented by those that can be briefly characterized as “add” (add).
In the case of cast iron, as already mentioned, the type and quality of the alloy is brought about by adding further elements in the form of briquettes that dissolve easily in the melt (cupola furnace briquettes), compacts, or “packets”.
In the case of copper alloys, the microstructure is influenced by adding master alloys based on copper. For some that are used for conductive copper, there is a simultaneity of deoxidation effect and structural influence (including lithium copper, beryllium copper, boron copper).
Refining the structure of AlSi alloys
In aluminum-silicon alloys - to the foundry by far the most widely used aluminum alloys - there are two similar measures in their meaning. One of them is the metallurgically indispensable refinement, also called “refining microstructure influence” ( Aladár Pácz ), without which cast parts made of AlSi alloys would mostly be brittle like glass and therefore would not be able to withstand any mechanical stress. In the case of alloys with approx. 7 to 12% silicon, it is mainly done by adding sodium or strontium to the melt. Powdery, pelletized and also briquetted mixtures which dissolve in the melt in a time-controlled manner and release sodium to avoid the coarse crystallization of the primary solidifying silicon which otherwise embrittles the cast parts, have been known for decades. In its pure, metallic form, airtight packaged sodium is used for the same purpose. Strontium, which acts like sodium and lasts longer because it is less easily oxidizable, is always added to the melt as a master alloy (e.g. AlSr10) for reasons of better solubility.
Effects that influence the structure, such as the addition of sodium or strontium, can also be achieved with antimony, but this is a process that cannot be compared with classic refinement, which also has the disadvantage that it is not separately processed return material with residual contents Antimony to impair the classic finishing considerably.
For engine castings made of light metal, an increase in performance and a reduction in consumption increasingly demand the use of highly heat-resistant AlMgSi alloys without their alloy development, which has recognized, among other things, additions of cobalt , nickel and especially silver as adding value. Finishing processes that differ from the previous technology are the subject of development. The modification of the eutectic, similar to the refinement of near-eutectic AlSi alloys, is already producing results as an object of research, especially in the area of increasing ductility.
Influence of the microstructure of AlSi alloys with 12–25% silicon
The eutectic and hypereutectic AlSi alloys with 12 to 25% silicon used for automotive pistons, among others, experience a special influence on the structure. The primary solidifying silicon is "refined" in a suitable form by adding phosphorus, the aim being a grain size of approx. 60 µm.
Purely technical measures to influence the structure
The use of ultrasound, which has been known since around 1930 for degassing the melts, also for gradual solidification ( gradient casting ) is proposed as an alternative or in addition. In the casting, zones with differentiated silicon contents from hypoeutectic to hypoeutectic and thus different solidification behavior and the resulting mechanical values arise.
Grain refinement or grain-refining structure influence
AlMgSi alloys have the same meaning as the refinement of AlSi alloys, be it cast or shape casting (continuous casting, semi-finished products), certain alloy additives responsible for achieving a fine crystalline structure and thus particularly high mechanical strength. They are either formed in situ from suitable, pelletized salt mixtures in the melt or released as finished crystallization nuclei when a master alloy dissolves. The addition can be made intermittently, as in the foundry, where it is particularly important for slowly solidifying types of casting, such as sand casting or gravity die casting . However, it is also state of the art to supply the germ-containing master alloy required in some cases in wire form, time-controlled and linked to the course of the casting process, which often takes several hours (inline treatment).
In terms of metallurgy, grain refinement, or "grain-refining microstructure influence", is predominantly the introduction of foreign nuclei, whereby those made from titanium diboride (TiB 2 ) are still preferred, but only part of a "family" based on titanium that has since been developed. Are boron and carbon. They form germs by introducing appropriately composed and tableted mixtures or aluminum-based master alloys comparable to these. As expected, nanostructured oxides of aluminum are also suitable for increasing the number of crystallization nuclei during solidification and thus improving the tightness of the cast structure.
When casting from magnesium-aluminum alloys, such as the most widely used alloy AZ 91, with the addition of aluminum, zinc and zircon, carbon is used for grain refinement in conjunction with brief overheating of the melt. The use of printed newsprint has been handed down - printing ink contains carbon - today, microstructure influencing agents are in use that release carbon in situ to the melt, which combines to form aluminum carbide with a grain-refining effect, i.e. a foreign nucleus, comparable to the titanium diboride in aluminum alloys.
Measures accompanying melt treatment
Since a 700–1500 ° C hot melt, unless it is kept in a vacuum, does not show itself statically, but through movement, such as the transferring into a transport vessel, as well as the casting process itself, both oxides can be re-formed and hydrogen can be absorbed different ways to avoid this. In detail, it begins with removing the protective slag layer from the melt as late as possible and limiting transfer processes or, where they are unavoidable, avoiding turbulence, which is always associated with oxide formation and turbulence in the liquid metal.
The optimization of the process control is also the subject of model approaches, especially those that deal with the flow simulation and the associated oxide formation (and gas absorption).
In order to retain oxides formed and entrained when pouring into the molds, regardless of their type and size, numerous possibilities are known and used, all of which are based on the principle of a sieve preventing the oxides from entering the mold. There are glass silk fabrics as filter material, as well as wire meshes, also ceramized, cut to size for insertion in molds for large series. Also widespread, especially in the area of so-called foundries (ingot casting), are ceramic foam filters, each with a special, temperature-adapted design for non-ferrous metals, such as iron and steel casting, with a standardized number of pores, mostly 20–30 ppi . Further developments provide for a multi-stage filtering, in which the pre-filtering or coarse filtering (separation of oxide-rich foams) is followed by grain refinement of the usual type and this in a cyclone called third and at the same time final stage the retention of oxides still present after the pre-filtering, as well as any agglomerations from the Grain refining treatment.
Results check
Melt treatment of any kind will only make sense if its results are checked. This is relatively easy if there are externally visible defects such as large voids , sink marks or surface pores . In aluminum alloys, the gas content can be checked in the simplest way by solidifying a sample in a vacuum ( Straube-Pfeiffer method ). However, the state of the art today is the negative pressure density test (UDT) after a previous impeller treatment of the melt with the aim of rinsing out fine and extremely fine oxide particles, since these hydrogen can accumulate. A density index DI of 1 on the treated melt, determined using an untreated reference sample, corresponds to a hydrogen content of 0.1 ml / 100 g and represents a practically optimal value.
In the case of AlSi alloys, the degree of refinement can be checked by breaking, grinding or, with greater accuracy, by means of thermal analysis (TA). X-ray testing is an alloy-independent standard for many cast parts; computed tomography (CT) is also used, as it enables the three-dimensional evaluation of defects.
For every melt, irrespective of the alloy, the analytical control for any alloy defects or burnout of important elements can be carried out relatively easily with an X-ray spectrometer. A metallographic examination of the structural condition of castings exposed to particular stress should also be carried out at least on a random basis, even if they have previously been X-rayed.
Individual evidence
- ↑ H. Riedelbauch: On the terminology of the melt treatment of non-ferrous metals and their alloys . In: foundry practice . No. 1-2 , 1977, pp. 16 f .
- ↑ Melt treatment . In: Common representation of the iron and steel industry . 17th edition. Schiele & Schön, Berlin, Düsseldorf 1997, ISBN 3-7949-0606-3 .
- ↑ The historical term of flux, already used in ore processing and in blast furnace operation, is broadly defined and not limited to metallurgy. (Editor's note)
- ↑ Franz Hofer Prill, Gernot Lukesch: Melt treatment of aluminum alloys in channel induction casting furnace. In: Giesserei-Rundschau. 56th year, issue 3/4, 2009, p. 38 f. Furthermore: Degassing of aluminum melts - influence of the rotor design on the effectiveness of the hydrogen removal. In: VÖG Gießereirundschau. jhg59, issue 7/8, 2012, p. 201.
- ↑ Georg Dambauer: High-strength AlMg2Si alloys. Dissertation. Montan University Leoben, 2010; Short version in: VÖG Gießerei-Rundschau. Jhg. 58, issue 7/8, 2010, p. 176.
- ↑ Peter Schumacher: Basic research as a basis for innovations. In: Foundry review. Jhg. 58, Issue 5/6, 2010, p. 88 (Lecture given on the occasion of the 54th Austrian Foundry Conference on April 23, 2010 in Leoben).
- ↑ Thomas Pabel: Modification of the eutectic magnesium silicide phase of AlMgSi cast alloys. Dissertation; Short version in: VÖG Gießerei-Rundschau. Jhg. 58, issue 5/6, 2010.
- ↑ Notes from research, science and business . In: China Foundry . tape 4 , no. 3 , 2008, ISSN 1672-6421 , p. 194 (translated).
- ↑ W. Vogel: Use of nanostructured oxides to avoid casting defects caused by shrinkage. In: Österreichische Giesserei Rundschau. Vol. 58, issue 7/8, 2010, p. 148.
- ↑ A. Schiffl, K. Renger, R. Simon, W. Kättlitz: Grain refinement of the Al-Mg alloy AZ 91 with Nucleant 5000 . In: Foundry Practice. No. 250, 2008, p. 17 ( PDF , accessed on September 20, 2010).
- ↑ Andreas Buchholz: Flow simulation in melting furnaces . In: Erzmetall . tape 61 , no. 3 , 2008, p. 146-151 .
- ^ John H. Courtenay, Laurens Katgermann, Frank Reusch: Development of an improved system for filtering ... In: Erzmetall . tape 61 , no. 5 , 2008, p. 303-317 .
literature
- Trade journals: "METALL", "ALUMINUM", "GIESSEREI" (organ of the VDG and the Association of German Ironworkers (VdEh) ); "Giesserei-Praxis" published by Schiele & Schoen, Berlin; "Giesserei-Rundschau", organ of the VOeG (Association of Austrian Foundry Experts) published by Strohmayer KG, A 1100 Vienna, all with related articles.
- Melt treatment agent for non-ferrous metals and alloys . In: VDG leaflet . R 50.
- H. Jaunich: Carbon parts for degassing and melt treatment with impeller devices . In: GDMB-LM technical committee . October 10, 1991 (talk).