Non-metallic inclusions in aluminum extrusion ingots
What are non-metallic inclusions
Any aluminum bar-pillars for pressing always contain a certain amount non-metallic inclusions. Several types of non-metallic inclusions in aluminum poles are known., such as aluminum oxides, magnesium oxides, furnace lining particles, spinels and carbides. However, quantitative data on the permissible content of non-metallic inclusions in aluminum melt, very few bars and aluminum profiles.
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This work provides practical quantitative data on the problem of non-metallic inclusions in aluminum ingots of alloy columns 6060 and 6063.
Production of round aluminum ingots
Interesting, that compares the quality of two swedish manufacturers – with knowingly high quality products - pillars from primary aluminum and pillars of secondary aluminum, i.e. from molten aluminum scrap. Both manufacturers used very modern technology., including:
- Obtaining the original molten aluminum.
- slag removal.
- Loading alloy elements into the melt – magnesium and silicon, mixing and again removing slag.
- Tilting the furnace and feeding the melt into the chute for transfer to the casting machine.
- Degassing of the melt in the SNIF rotary unit by purging with argon at a flow rate of 15 m3/h - inclusions are removed, impurities of alkaline elements, and most importantly - hydrogen.
- Melt filtration on a ceramic foam filter.
- Feeding Al-Ti-B to the melt for grinding grain in an amount of about 1 kg per 1 ton of melt. This bar, by the way, is a source of non-metallic inclusions TiB2.
- Pillar casting on a vertical foundry machine.
- Homogenization of ingots at temperature 550 ºС for several hours.
Primary aluminum bullion manufacturer used only one furnace mixer, into which liquid aluminum was loaded. The second ingot manufacturer used two furnaces: a smelter – for melting scrap and a mixer - for processing the melt and feeding it to a casting machine.
The inclusions in aluminum melt
On the classification of inclusions in molten aluminum, their sources and methods of cleaning see. Inclusion of aluminum ingots for pressing-pillars.
Measurement of inclusions in aluminum melt
Having information on the number and size of inclusions in the aluminum melt is very important for foundry. There are several instrumental methods for assessing the content of non-metallic inclusions in aluminum melt - very complex and expensive.. Each of them has its own advantages and disadvantages.. Here are some of them - LiMCA method, PoDFA method, метод Prefil-Footprinter, ultrasonic inclusion detection method. See. Methods for measuring inclusions in aluminum melt.
The metallographic method is the most practical
The most accessible method for assessing the content of non-metallic inclusions in an aluminum ingot is the classical metallography. It includes clipping, grinding, polishing, sometimes chemical treatment of samples, and then inspecting them with the naked eye or under a microscope.
Mechanical sample preparation
- Thick aluminum cross-cut templates 15-20 mm at a given distance from the beginning or end of the ingot.
- One of the surfaces of the template is treated on a grinding wheel. The treatment was carried out until all traces of the saw cut and scratches were completely removed., as well as to obtain a flat surface. Grinding wheel rotation speed - from 2000 to 1400 revolutions per minute depending on the roughness of the sample.
- Then the surface is treated first on a coarse sandpaper (number 20 GOST 3647), and then - on a thinner one (number 4). Thus achieve the smoothest surface, since inclusions will hide behind irregularities on a rough surface and cannot be seen after deep etching.
Deep alkaline etching of samples
The so-called deep etching method is used to estimate the number and distribution of non-metallic inclusions over the cross section of samples. One of the advantages of this method for analyzing inclusions in aluminum columns is that, that it allows us to estimate the distribution of inclusions along the column, which is impossible for other methods.
With deep alkaline etching apply 15 %-solution of sodium hydroxide (NaOH) at a temperature 65 oC. To prepare it, first, the required amount of water is heated in a suitable vessel, and then sodium hydroxide is added and mixed gradually. It is important to control the temperature with a thermometer. Every three experiments, the solution is changed.
When the temperature of the solution is 65 ºС immersed in it. First, the temperature of the solution decreases, and then increases. Sample etched over 15 minutes, then thoroughly washed with water and clean off all traces of plaque on its surface. The etched surface is about 0,3 mm.
In deep alkaline etching, non-metallic inclusions are etched. Pits form in their place., which are visible to the naked eye. These pits show the location of inclusions, but their sizes are approximately 10 times more, than the sizes of real inclusions (figure 1).
Figure 1 - Etched etching pits
Etching pit counting
To quantify the distribution of inclusions over the cross section of the column, its cross section was divided into three regions of equal area, as shown in Figure 1 - central C, medium M and surface S.
Figure 2 - Column cross-sectional separation scheme
All the fossa, which are visible to the naked eye, marked with a black marker. They made a paper copy of the surface with marked dimples on the copier..
Depending on the size of the pits, they were sorted into two groups: large and small. Pits with a diameter greater than 0,5 mm was considered big, less 0,5 mm - small. It means, that inclusions are larger 50 um are considered large.
Distribution of non-metallic inclusions in ingots
The measurement results showed the following:
- Cross-sectional distribution of inclusions - mainly in the central zone.
- The number and distribution of inclusions along the length of the columns depends on the duration of exposure of the melt in the furnace before casting. Melt exposure for 10 minutes are not enough. Longer exposure times more 30 minutes does not give a positive result.
- With a decrease in the volume of “swamp” in the furnace at the end of the pillar casting, it leads to an increase in the number of inclusions at the end of the columns of the last casting.
Inclusions in a primary aluminum ingot
Typical distribution of inclusions in an ingot with a diameter 152 mm of primary metal is shown in the figures 1 and 2, post diameter 228 mm from recycled metal - in the figure 3.
a) the beginning of the pillar
b) the end of the post
Figure 3 – Distribution of inclusions in an ingot with a diameter 152 mm from primary aluminum: a) the beginning of the ingot, b) the end of the ingot
Figure 4 – Distribution of inclusions along the length of a primary aluminum ingot with a diameter 152 mm
Inclusions in secondary aluminum ingots
Figure 5 - Distribution of inclusions along the length of a column of secondary aluminum with a diameter 228 mm - first casting
Figure 6 – View of etch pits in an ingot sample from a distance 440 cm from the end of the ingot – last cast
Figure 7 - Micrograph of oxide film from the “swamp” of the furnace
Figure 8 - The difference in the distribution of inclusions
first and last casting of ingots
Ghadir Razaz |, Casting practice influencing inclusion distribution in Al-billets, Karlstad University, 2012.