Aluminium alloy inclusions
Encyclopedia
An inclusion is a solid particle
in liquid aluminium alloy
. It is usually non-metallic and can be of different nature depending on its source.
when they are large and in too high concentration. Here are examples of problems related to inclusions:
and form an oxide film
layer (gama-Al2O3). This layer becomes thicker with time. When aluminium is moved, this oxide film gets mixed inside the melt.
s (Al4C3) originates from the reduction of alumina where carbon anodes and cathode
s are in contact with the mix. Later in the process, any carbon tools in contact with the liquid aluminium can react and create carbides.
, magnesium oxide
s (MgO), cuboids (MgAl2O4-cuboid) and metallurgical spinel
(MgAl2O4-spinel) can form. They result from the reaction between magnesium and oxygen in the melt. More of them will form with time and temperature.
Spinel can be highly detrimental because of its big size and high hardness.
material in contact with aluminium can detach and become inclusions. We can find graphite
inclusions (C), alumina inclusions (alpha-Al2O3), CaO, SiO2, …
After some time, graphite
refractory in contact with aluminium will react to create aluminum carbides (harder and more detrimental inclusions).
In aluminium alloy containing magnesium
, the magnesium reacts with some refractories to create rather big and hard inclusions similar to spinels.
Unreacted refractory particles coming from the degradation of refractory materials which comes in contact with the melt.
inclusions (MgCl2, NaCl, CaCl2, …) are a special type of inclusion as they are liquid in liquid metal. When aluminium solidifies, they form spherical voids similar to hydrogen gas porosity
but the void contains a chloride crystal formed when aluminium became colder.
(TiB2) is intentionally added to the melt for grain refinement to improve mechanical properties.
Phosphorus
is added to the melt hypereutectic alloys for modification of the silicon
phase for better mechanical properties. This creates a AlP inclusions.
Boron
treatment inclusions ( (Ti, V)B2 ) form when boron is added to the melt to increase conductivity
by precipitating vanadium
and titanium
.
s (FeO), manganese oxide
s (MnO), fluoride
s (Na3AlF6, NaF, CaF2, …), aluminium boride
s (AlB2, AlB12), borocarbides (Al4C4B).
Bone ash
(Ca3(PO4)2) sometimes added to patch cracks in the through can be found as inclusions in the melt.
, and similar parameters on melt cleanliness.
efficiency.
The PoDFA method was developed by Rio Tinto Alcan in the 70s. The metallographic analysis method has been optimized for over the years on a wide variety of alloys.
The measurement principle is the following: A predetermined quantity of liquid aluminum is filtered under controlled conditions using a very fine porosity filter. Inclusions in the melt are concentrated at the filter surface by a factor of about 10,000. The filter, along with the residual metal, is then cut, mounted and polished before being analyzed under an optical microscope
by a trained PoDFA metallographer.
, metal temperature
, ...), the only parameter affecting the filtration speed is the inclusion content. One can determine the cleanliness level from the filtration curve (weight of metal filtered as a function of time).
test method. Liquid metal is cast into a mold containing notches. Once solidified, the resulting bar is bent to expose a fracture surface. The visual observation of inclusions on the fracture is used to determine a K-value for the melt and compared to a preset standard. This method is rather imprecise and therefore only suitable when metal contents large inclusions and inclusion clusters.
of a melt at time intervals in the order of one minute. It can therefore monitor, in real-time, the evolution of cleanliness along a cast as a function of process parameters and melt-handling practices.
The heart of the LiMCA measuring system consists of a closed glass tube (electrically insulating material) bearing a small orifice at its bottom. The tube is positioned in liquid metal
. By creating a vacuum
inside the tube, the metal with the suspended inclusions to be detected is forced through the small orifice. Two electrodes are necessary: one inside the tube and the other outside. Both electrodes are immersed in the liquid metal. A constant electrical current is applied between the electrodes. The current flows through the
liquid metal by the small orifice in the tube. When an inclusion enters the orifice, it displaces its volume of conducting fluid, temporarily rising electrical resistance. The increase of resistance generates a voltage pulse. The magnitude of the voltage pulse is a function of the volume of the particle. The duration of the pulse is related to the
transit time of the inclusion. The voltage pulses are amplified and their amplitude
measured digitally. The size distribution and total concentration are displayed in real-time on a computer screen.
media is an efficient way to clean the metal. Different types of ceramic media are used in-line in foundries, such as, ceramic foam filters, porous tube filters, bonded ceramic filters, and deep bed filters.
Particle
A particle is, generally, a small localized object to which can be ascribed physical properties. It may also refer to:In chemistry:* Colloidal particle, part of a one-phase system of two or more components where the particles aren't individually visible.In physics:* Subatomic particle, which may be...
in liquid aluminium alloy
Aluminium alloy
Aluminium alloys are alloys in which aluminium is the predominant metal. The typical alloying elements are copper, magnesium, manganese, silicon and zinc. There are two principal classifications, namely casting alloys and wrought alloys, both of which are further subdivided into the categories...
. It is usually non-metallic and can be of different nature depending on its source.
Problems related to inclusions
Inclusions can create problems in the castingCasting
In metalworking, casting involves pouring liquid metal into a mold, which contains a hollow cavity of the desired shape, and then allowing it to cool and solidify. The solidified part is also known as a casting, which is ejected or broken out of the mold to complete the process...
when they are large and in too high concentration. Here are examples of problems related to inclusions:
- Pinholes in light gauge foil
- Flange cracks in Beverage containers
- Surface streaks in bright automotive trim and lithographic material
- Breakage in wire drawingWire drawingWire drawing is a metalworking process used to reduce the cross-section of a wire by pulling the wire through a single, or series of, drawing die. There are many applications for wire drawing, including electrical wiring, cables, tension-loaded structural components, springs, paper clips, spokes...
operation - Increased tool wear and tear
- Increased porosityPorosityPorosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...
- Loss of pressure tightness of engine blocks
- Poor machinabilityMachinabilityThe term machinability refers to the ease with which a metal can be machined to an acceptable surface finish. Materials with good machinability require little power to cut, can be cut quickly, easily obtain a good finish, and do not wear the tooling much; such materials are said to be free machining...
- Cosmetic defect in apparent surfaces
Inclusion types
Oxide films
In contact with ambient air, liquid aluminium reacts with the oxygenOxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
and form an oxide film
Aluminium oxide
Aluminium oxide is an amphoteric oxide with the chemical formula 23. It is commonly referred to as alumina, or corundum in its crystalline form, as well as many other names, reflecting its widespread occurrence in nature and industry...
layer (gama-Al2O3). This layer becomes thicker with time. When aluminium is moved, this oxide film gets mixed inside the melt.
Aluminium carbides
In primary aluminium production, aluminium carbideAluminium carbide
Aluminium carbide, chemical formula Al4C3, is a carbide of aluminium. It has the appearance of pale yellow to brown crystals. It is stable up to 1400 °C. It decomposes in water with the production of methane.-Structure:...
s (Al4C3) originates from the reduction of alumina where carbon anodes and cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...
s are in contact with the mix. Later in the process, any carbon tools in contact with the liquid aluminium can react and create carbides.
Magnesium oxides
In aluminium alloys containing magnesiumMagnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
, magnesium oxide
Magnesium oxide
Magnesium oxide , or magnesia, is a white hygroscopic solid mineral that occurs naturally as periclase and is a source of magnesium . It has an empirical formula of and consists of a lattice of Mg2+ ions and O2– ions held together by ionic bonds...
s (MgO), cuboids (MgAl2O4-cuboid) and metallurgical spinel
Spinel
Spinel is the magnesium aluminium member of the larger spinel group of minerals. It has the formula MgAl2O4. Balas ruby is an old name for a rose-tinted variety.-Spinel group:...
(MgAl2O4-spinel) can form. They result from the reaction between magnesium and oxygen in the melt. More of them will form with time and temperature.
Spinel can be highly detrimental because of its big size and high hardness.
Refractory materials
Particles of refractoryRefractory
A refractory material is one that retains its strength at high temperatures. ASTM C71 defines refractories as "non-metallic materials having those chemical and physical properties that make them applicable for structures, or as components of systems, that are exposed to environments above...
material in contact with aluminium can detach and become inclusions. We can find graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...
inclusions (C), alumina inclusions (alpha-Al2O3), CaO, SiO2, …
After some time, graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...
refractory in contact with aluminium will react to create aluminum carbides (harder and more detrimental inclusions).
In aluminium alloy containing magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
, the magnesium reacts with some refractories to create rather big and hard inclusions similar to spinels.
Unreacted refractory particles coming from the degradation of refractory materials which comes in contact with the melt.
Chlorides
ChlorideChloride
The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...
inclusions (MgCl2, NaCl, CaCl2, …) are a special type of inclusion as they are liquid in liquid metal. When aluminium solidifies, they form spherical voids similar to hydrogen gas porosity
Hydrogen gas porosity
Hydrogen gas porosity is an aluminium casting defect under the form of a porosity or void in an aluminium casting caused by a high level of hydrogen gas dissolved in the aluminium at liquid phase...
but the void contains a chloride crystal formed when aluminium became colder.
Fluxing salt
Fluxing salt, like chlorides are also liquid inclusions. They come from flux treatments added to the melt for cleaning.Intentionally added inclusions
Titanium borideTitanium boride
Titanium diboride is an extremely hard ceramic compound composed of titanium and boron which has excellent resistance to mechanical erosion...
(TiB2) is intentionally added to the melt for grain refinement to improve mechanical properties.
Phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
is added to the melt hypereutectic alloys for modification of the silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
phase for better mechanical properties. This creates a AlP inclusions.
Boron
Boron
Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...
treatment inclusions ( (Ti, V)B2 ) form when boron is added to the melt to increase conductivity
Conductivity
Conductivity may refer to:*Electrical conductivity, a measure of a material's ability to conduct an electric current*Conductivity , also the specific conductance, is a measurement of the electrical conductance per unit distance in an electrolytic or aqueous solution*Ionic conductivity, a measure of...
by precipitating vanadium
Vanadium
Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery gray, ductile and malleable transition metal. The formation of an oxide layer stabilizes the metal against oxidation. The element is found only in chemically combined form in nature...
and titanium
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
.
Less frequently found inclusions
The following inclusion types can also be found in aluminium alloys: alumina needles (Al2O3), nitrides (AlN), iron oxideIron oxide
Iron oxides are chemical compounds composed of iron and oxygen. All together, there are sixteen known iron oxides and oxyhydroxides.Iron oxides and oxide-hydroxides are widespread in nature, play an important role in many geological and biological processes, and are widely utilized by humans, e.g.,...
s (FeO), manganese oxide
Manganese oxide
Manganese oxide is a generic term used to describe a variety of manganese oxides and hydroxides. It may refer to:* Manganese oxide, MnO* Manganese oxide, Mn3O4* Manganese oxide, Mn2O3* Manganese dioxide, , MnO2...
s (MnO), fluoride
Fluoride
Fluoride is the anion F−, the reduced form of fluorine when as an ion and when bonded to another element. Both organofluorine compounds and inorganic fluorine containing compounds are called fluorides. Fluoride, like other halides, is a monovalent ion . Its compounds often have properties that are...
s (Na3AlF6, NaF, CaF2, …), aluminium boride
Aluminium boride
Aluminium boride may refer to*Aluminium diboride, AlB2*Aluminium dodecaboride, AlB12...
s (AlB2, AlB12), borocarbides (Al4C4B).
Bone ash
Bone ash
Bone ash is the white, powdery ash left from the burning of bones. It is primarily composed of calcium phosphate. It is commonly used in fertilizers, polishing compounds, and in making ceramics...
(Ca3(PO4)2) sometimes added to patch cracks in the through can be found as inclusions in the melt.
Inclusion measurement
Several methods exist to measure the inclusion content in liquid aluminium. The most common methods are PoDFA, Prefil, K-Mold and LiMCA. Measuring the inclusions is of great help to understand the impact of furnace preparation, alloying practice, feedstock mix, settling timeSettling time
The settling time of an amplifier or other output device is the time elapsed from the application of an ideal instantaneous step input to the time at which the amplifier output has entered and remained within a specified error band, usually symmetrical about the final value.Settling time includes a...
, and similar parameters on melt cleanliness.
PoDFA
The PoDFA method provides information on the composition and concentration of the inclusions in molten aluminum. PoDFA is widely used for process characterization and optimization, as well as product improvement. It allows to quickly and accurately assess the effects of various operating practices on metal cleanliness or identify filtrationFiltration
Filtration is commonly the mechanical or physical operation which is used for the separation of solids from fluids by interposing a medium through which only the fluid can pass...
efficiency.
The PoDFA method was developed by Rio Tinto Alcan in the 70s. The metallographic analysis method has been optimized for over the years on a wide variety of alloys.
The measurement principle is the following: A predetermined quantity of liquid aluminum is filtered under controlled conditions using a very fine porosity filter. Inclusions in the melt are concentrated at the filter surface by a factor of about 10,000. The filter, along with the residual metal, is then cut, mounted and polished before being analyzed under an optical microscope
Optical microscope
The optical microscope, often referred to as the "light microscope", is a type of microscope which uses visible light and a system of lenses to magnify images of small samples. Optical microscopes are the oldest design of microscope and were possibly designed in their present compound form in the...
by a trained PoDFA metallographer.
Prefil
The Prefil method is similar to PoDFA but, in addition to the metallographic analysis, Prefil provides also a immediate feedback on metal cleanliness from the metal flowrate through the filter. Because everything about the filtration is well controlled (pressurePressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
, metal temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
, ...), the only parameter affecting the filtration speed is the inclusion content. One can determine the cleanliness level from the filtration curve (weight of metal filtered as a function of time).
K-Mold
K-Mold is a fractureFracture
A fracture is the separation of an object or material into two, or more, pieces under the action of stress.The word fracture is often applied to bones of living creatures , or to crystals or crystalline materials, such as gemstones or metal...
test method. Liquid metal is cast into a mold containing notches. Once solidified, the resulting bar is bent to expose a fracture surface. The visual observation of inclusions on the fracture is used to determine a K-value for the melt and compared to a preset standard. This method is rather imprecise and therefore only suitable when metal contents large inclusions and inclusion clusters.
LiMCA
The LiMCA method measures the total concentration and size distribution of inclusions present in aluminum alloys. Its measuring principle is based on an objective and user-independent method. The LiMCA CM system can characterize the cleanlinessCleanliness
Cleanliness is both the abstract state of being clean and free from dirt, and the process of achieving and maintaining that state.Cleanliness may be endowed with a moral quality, as indicated by the aphorism "cleanliness is next to godliness," and may be regarded as contributing to other ideals...
of a melt at time intervals in the order of one minute. It can therefore monitor, in real-time, the evolution of cleanliness along a cast as a function of process parameters and melt-handling practices.
The heart of the LiMCA measuring system consists of a closed glass tube (electrically insulating material) bearing a small orifice at its bottom. The tube is positioned in liquid metal
Liquid metal
Liquid Metal may refer to:* Metals in a liquid state** Metals which have a relatively low melting point, such as tin or lead.** A fusible alloy** A poor metal.* Liquid Metal , a radio channel* XM Liquid Metal, a radio channel...
. By creating a vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
inside the tube, the metal with the suspended inclusions to be detected is forced through the small orifice. Two electrodes are necessary: one inside the tube and the other outside. Both electrodes are immersed in the liquid metal. A constant electrical current is applied between the electrodes. The current flows through the
liquid metal by the small orifice in the tube. When an inclusion enters the orifice, it displaces its volume of conducting fluid, temporarily rising electrical resistance. The increase of resistance generates a voltage pulse. The magnitude of the voltage pulse is a function of the volume of the particle. The duration of the pulse is related to the
transit time of the inclusion. The voltage pulses are amplified and their amplitude
Amplitude
Amplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...
measured digitally. The size distribution and total concentration are displayed in real-time on a computer screen.
Inclusion removal
In order to get a good quality product, removing the inclusion becomes necessary. Liquid metal filtration through a ceramicCeramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
media is an efficient way to clean the metal. Different types of ceramic media are used in-line in foundries, such as, ceramic foam filters, porous tube filters, bonded ceramic filters, and deep bed filters.