IN MAKING ALUMINUM INGOTS

TECHNOLOGICAL FIELD

[0001 ] The technological field of the disclosure relates to making aluminum ingots. More particularly, the technological field relates to improving grain structure in the finished aluminum ingots.

BACKGROUND OF THE DISCLOSURE

[0002] Ideally, when producing aluminum ingots, there are no bubbles and few if any flaws in the grain structure of the solid metal.

[0003] Grain structure is determined by constituents in the molten metal. Not all the constituents are in liquid state. Some insoluble materials, such as entrained oxides, remain in the form of particles that range from relatively larger particles to very fine particles, called "fines" or "fine inclusions." Particles are typically at least 5 microns across. Fines are typically less than 5 microns across.

[0004] Fines are essential in making high quality aluminum metal because they serve as nucleation sites around which the atoms of the cooling metal will coalesce. However, the concentration of fines in molten metal must be controlled because too large a concentration adversely affects the quality of a casting and too small a concentration may result in coarser grain

development. Accordingly, the concentration of fine inclusions must be managed and controlled to have sufficient nucleation sites, yet avoid

"overdosing" from too many fines, which can lead to pinholes in light gauge foil, flange cracks in beverage containers, surface streaks in bright automotive trim and lithographic material, breakage in wire drawing operation, increased tool wear and tear, increased porosity, loss of pressure tightness of engine blocks, poor machinability, and cosmetic defect in apparent surfaces.

[0005] However, control of fine inclusions in the prior art process is difficult because accurate measurement of the density of fines in molten metal is nearly impossible. Measurement systems, such as that used by JW

Aluminum under the trademark MetalVision MV20/20 system, are useful in measuring the concentrations of particulate in molten metal. However, they cannot analyze the concentration of fine inclusions because particle size is simply too small.

[0006] Bubbles may also occur in cast aluminum, for example, when hydrogen gas is trapped in the metal as it cools. Hydrogen gas has a high solubility in molten aluminum and is readily available from various sources, including combustion products from burners, moisture in raw materials, and humidity in the surrounding atmosphere. The principle reaction for hydrogen uptake is:

3H20 + 2AI 6 H + AI203.

[0007] Because the solubility of hydrogen in aluminum drops significantly

during solidification, hydrogen gas can become trapped in the metal matrix, resulting in pores in the cast product. Therefore, reducing dissolved hydrogen content while the aluminum is still molten is a priority in quality casting of aluminum. Traditionally hydrogen has been removed by injecting an inert gas, such as argon, into the molten aluminum. The difference in hydrogen potential between the injected gas and aluminum drives the hydrogen from the aluminum into the argon. [0008] A newer method for removing hydrogen gas from molten aluminum has been developed that relies on ultrasound-induced cavitation. The tip of a probe is inserted into the molten metal and vibrated at ultrasonic frequencies. The vibrating tip causes cavitation in the molten aluminum. Hydrogen is drawn into these cavities because of their lower density. Ultrasonic degassing successfully removes hydrogen and has several advantages over the use of argon described above. See US patents US8652397 and US8574336, and US Publication US2015/0135901 , which are incorporated herein in their entirety by reference, for a description of ultrasonic degassing of hydrogen.

[0009] However, despite the advantages of hydrogen degassing, it results in an increase in fine inclusion concentration. Filters more easily become clogged by the fines and restrict flow to the caster as a result, and have to be replaced after a few hours rather than the normal service life of several days.

[0010] A method for casting aluminum that results in fine grain structure with few or no bubbles would be advantageous.

SUMMARY

[001 1 ] An improvement in production of aluminum is disclosed. In particular, the process of making aluminum metal is improved by filtration of only larger particulate upstream of a hydrogen removal rather than downstream as taught by the prior art. Furthermore, surprisingly, with upstream filtration of larger particulate, the use of ultrasonic degassing does not result in excessive fine inclusions. The removal of larger particulate upstream limits the concentration of fine inclusions downstream because it removes them as a source of downstream fines. It has been found that the hydrogen degasser pulverizes larger particles and turns them into fines. Failing to remove larger particulate before it is pulverized may result in an overdose of fine particulate in the molten aluminum, which in turn results in poorer quality castings. Removal of larger particulate prior to degassing, however, results in much more appropriate level of fines downstream of the hydrogen degasser and superior grain structure in the finished product.

[0012] An aspect of the disclosure is a method for making aluminum ingots comprising the steps of melting aluminum to form molten aluminum, filtering particulate from said molten aluminum, then ultrasonically degassing said molten aluminum after filtration; and finally casting said degassed molten aluminum.

[0013] Another aspect of the disclosure is the step of measuring particulate in the melted aluminum before the ultrasonic degassing step and recirculating the melted aluminum through the filtration step if the particulate density is greater than 1 .25 mm ² /kg. By filtering particulate including the larger particulate prior to ultrasonic degassing of the melt, the quantity of fine inclusions in the casting can be controlled.

[0014] Another aspect of the disclosure is particle filtering using ceramic foam filters, filter beds, or tube filters. If ceramic filters are used, they may be staged in at least two stages with the melt flowing, for example, through a first filter and then a second filter.

[0015] Another aspect of the disclosure is filtering of particles through a first filter having at most 1 1 .8 pores per cm and at most 15.8 pores per cm, and, in particular, a first filter having a pore size of not less than 1 1 .8 pores per centimeter followed by a second filter having a pore size of not less than 15.8 pores per centimeter.

[0016] Another aspect of the disclosure is filtering particles through a first filter that removes up to 100% of particulate above 30 microns followed by a second, finer filter which leaves up to 50% of particulate of less than 30 microns. Alternatively, the melt can be directed through a first filter that removes up to 100% of particulate above 30 microns followed by a second, finer filter that removes up to 50% of particulate above 5 microns.

[0017] Those skilled in the art of aluminum casting will appreciate other

aspects of the disclosure from a careful reading of the Detailed Description accompanied by the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In the drawings,

[0019] FIG. 1 is a schematic view of the prior art process for casting an

aluminum ingot;

[0020] FIG. 2 is a schematic view of aspects of the present disclosure of a process for casting an aluminum ingot with two alternative additional features shown within dashed lines; and

[0021 ] FIG. 3 is a graph of particulate removal efficiency using staged 30 and 40 micron ceramic foam filters.

DETAILED DESCRIPTION OF ASPECTS OF THE DISCLOSURE

[0022] According to FIG. 1 , which shows the prior art process of making

aluminum ingot, the first step in making aluminum ingots is to melt aluminum stock in a melter. The molten aluminum is then subject to hydrogen degassing, as described above, either with the argon method or hydrogen degassing, to remove hydrogen gas from the melt. The degassed, molten aluminum is then filtered to remove particulate. The reason for filtration is to capture and segregate potential non-aluminum particles from the melt that may have been in the aluminum prior to melting or picked up subsequently from the melter, the degasser, or the vessels and channels before it is cast. The molten, degassed, filtered aluminum is then cast into ingots.

[0023] Downstream of the hydrogen degasser, the concentration of fines is impossible to measure and to control. Fines, unlike larger particles that are effectively removed by ceramic foam filters, pass through such filters freely but may be in such quantity that the fines and particulate blind the filters quickly, increasing manufacturing expense or in being too restrictive so that too few fines pass for a fine grain in the solidified ingot.

[0024] However, it has been learned that the cause of the presence of large concentrations of fines downstream of the hydrogen degasser is from the vibration energy of the ultrasonic degasser itself, which pulverizes larger particles into smaller particles. Smaller particles entering the degasser are largely unaffected. Thus, pulverization of larger particles is a side-effect of the use of an ultrasonic degasser.

[0025] According to the present disclosure, and as illustrated schematically in FIG. 2, in order to reduce the level of fines in the molten metal at the time of casting to a level that is acceptable for good nucleation without adverse effects in casting, the filter is moved upstream of the ultrasonic degasser so as to intercept larger particles and remove them prior to the melt reaching the hydrogen degasser. The larger particles thus no longer serve as a source of additional fines in the molten metal from their pulverization by the degasser. When the larger particulate is removed upstream of the degasser, the concentration of fines is reduced and is thus controlled.

[0026] According to FIG. 2, the molten aluminum is passed through at least one filter upstream of the degasser. A first filter 50 removes coarser particulate but not finer particulate. A second filter 60 removes smaller particulate. The filters may be ceramic filters, bed filters, tube filters and, as an example, particle density after filtration may be less than 1 .25 mm ² /kg.

[0027] As an example, first filter 50 may be coarser and have 30 pores per inch (1 1 .8 pores per centimeter) and may be followed by second filter 60 that is finer, and may have 40 pores per inch (15.7 pores per centimeter).

Inclusions larger than 30 microns should be removed and approximately half of those smaller than 30 microns but at least 5 microns in size should be removed. Approximately half of the smaller (i.e., 5 micron to 30 microns) particles remain in the melt to serve as grain refiners. The filtered molten aluminum proceeds to hydrogen degassing and then to casting.

[0028] FIG. 2 illustrates two alternative, additional steps, shown in dashed lines. In the first alternative, additional step, shown to the left of the main process, samples of molten metal are taken for testing to determine compliance following filtration. The particulate density is measured and stored.

[0029] In the second alternative, additional step, the molten aluminum is measured and recirculated through the filtration step if the quantity of particulate is excessive. [0030] FIG. 3 illustrates a graph of the particulate removal efficiency of the foregoing two-stage filtration example for ceramic foam filters using flow rates of about 0.7 - 2.1 kilograms/minute across 1000 mm ² area (about 1 -3 pounds minute across 1 in ² ).

[0031 ] The molten metal, with reduced larger particulate concentration, is then subjected to ultrasonic degassing to remove hydrogen. On exit from the degasser, the concentration of fine inclusions may be within an acceptable range based on removal of larger particulate and sufficient removal of smaller particulate, as described above.

[0032] Fine inclusions in the molten metal are needed to achieve the benefits of grain refining yet excessive fine inclusions must be avoided because of the disadvantages of plugging the filters after the ultrasonic degasser, of resulting in dirt in the lines, and of adversely affecting the casting grain structure.

[0033] The absence of inclusion-related defects is indicated visually by the absence of surface defects on the finished casting such as marks or shades and by looking at images of the microscopic grain structure using an electron microscope. Mechanical testing - tensile strength, yield strength, formability, and toughness - indirectly and macroscopically indicates of grain size but is objective as a way to confirm the quality of the ingot.

[0034] Those skilled in the art of casting aluminum will understand that many modifications and substitutions may be made in the foregoing descriptions of aspects of the disclosure without departing from the spirit and scope of the invention which is defined by the appended claims.