Metallography of aluminum insight - Struers.com

Aluminum alloys

Adding very small amounts of alloying elements to aluminum can increase tensile strength, yield strength and hardness compared to pure aluminum. The most important alloying elements are Si, Mg, Cu, Zn and Mn. These mostly eutectic compounds must be finely dispersed through a hot working process before the alloy can be cold worked.

Ageing of aluminum alloys
Many aluminum alloys are age hardened to improve the mechanical properties. This can be done either naturally or artificially.

• Natural age hardening (example AlCuMg). After solution annealing, the workpiece is quenched and consequently the precipitation of the Al2Cu in the solid solution is sup- pressed. The workpiece is then left to age in ambient temperature. During this process the aluminum lattice precipitates the copper from the supersaturated solution. The resultant strain produced in the aluminum lattice leads to an increase in strength and hardness. The process takes 5-8 days.
• In artificial age hardening, ageing takes place at an elevated temperature, which reduces process time. With an AlMgSi alloy, for example, ageing occurs in 4-48 hours at 120-175 °C after solution annealing and quenching. The precipitation of the Mg2Si phase produces internal strain in the aluminum lattice, which results in an increase in strength and hardness.

Wrought aluminum alloys
The main alloying elements for wrought aluminum alloys are copper, magnesium, zinc and manganese. Silicon and iron affect the mechanical properties and corrosion resistance and can either be impurities or alloying elements, depending on the requested purity and application.

Common uses of wrought aluminum alloys:

• Plates in mechanical engineering and mold construction for rolled products, such as sheets and strips, as well as plated products like radiators and heat exchangers
• Plated sheets for specific semi-finished products for aircraft construction or for decorative applications such as trim and reflectors
• Mechanical engineering, conveying and electro technical applications, as well as high-strength sports and leisure products, such as snowboard bindings and mountain bike gears
• Fiber-reinforced aluminum in the aircraft and aerospace industries

Fig. 1: Aluminum alloy , cast, showing eutectic precipitation on grain boundaries, unetched, 200x


Fig. 2: As Fig. 1, homogenized, unetched, 200x


Fig. 3: As Fig. 2, hot rolled, unetched, 200x

Cast aluminum alloys
Aluminum casts are mainly alloyed to improve the metal’s mechanical properties and are differentiated according to their main alloying elements – silicon, magnesium and copper. Alloy contents that exceed the saturation of the solid solution are precipitated as pure metal, such as silicon, or as eutectics and inter-metallic phases.

Silicon increases the castability of aluminum. In eutectic alloys, such as AlSi12, small amounts of sodium are added before casting to refine the eutectic. In this refining process, instead of precipitating as coarse needles or plates (Fig. 4), the silicon forms a very fine eutectic with the α-solid solution (Fig. 5). The effect of hardening in these alloys is very low and therefore magnesium is added so that they can be age hardened.

Cast alloys with specific properties are used in various product groups, including the fabrication of pistons, slide bearings, parts for mechanical engineering, cylinder heads and brake shoes.

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 Some of the more important cast alloys and their properties  AlSi10Mg  Age hardened. Vibration and corrosion resistant  AlSi5Cu1  Age hardened. Good castability for welding and thin sections   AlMg3  Resistant to seawater AlSi25+ CuNi  Age hardened. Special alloy for pistons; wear resistant due to its high Si content   AlMgSiPb  Suitable for machining   AlSi9Cu3  Castable universal alloy and the most important alloy for pressure die casting 

Fig. 4: Aluminum-silicon cast, unrefined, 500x

Fig. 5: Aluminum-silicon cast, refined, 500x

The metallography of aluminum and its alloys

Metallography of aluminum is used in quality control for grain size determination and to determine microstructure defects on polished and etched specimens. In addition, specimens are often checked for impurities, such as oxides or zirconium aluminides.

Cast alloy aluminum is evaluated for shape, distribution of phases and possible porosity. In wrought material, defects from the rolling and extrusion process are investigated and plating thicknesses measured.

Fig. 6: Oxide in the surface of an aluminum pressure die casting, 50x

Challenges in the metallography of aluminum and its alloys

The metallographic challenges associated with aluminum and aluminum alloys change with the metal’s purity.

• As purity increases, aluminum becomes softer and more susceptible to mechanical deformation and scratches. In high purity aluminum, grinding can cause deep deformation, while grinding and polishing abrasives, such as silicon carbide and diamond particles, can be pressed into the surface.
• As alloying content increases, aluminum becomes harder. Cast alloys are relatively easy to prepare. However, the aluminum matrix must be well polished to avoid errors in structure interpretation.

 Overview of metallographic challenges and solutions  Challenge: Solution:  Pure aluminum is very soft and prone to mechanical deformation and scratching Plane grinding with the finest possible SiC Foil or Papers Silicon carbide and diamond particles can be pressed into the specimen surface Diamond polishing and/or final polishing need to be long enough to remove all embedded particles Severely worked and deformed wrought alloys are difficult to contrast, making structure interpretation difficult - Final polishing with colloidal silica suspension
- Anodization with Barker’s reagent

Read further for a detailed method description of how to prepare aluminum and its alloys for metallographic analysis quickly and accurately.

Fig.7: Embedded diamond particles in pure aluminum after polishing with 3 μm, 200x

Preparation of aluminum and its alloys: Mechanical grinding & diamond polishing

When working with aluminum and its alloys, we recommend mechanical grinding, followed by diamond polishing. For many pure aluminum and wrought alloy specimens, electrolytical polishing is also recommended.

Mechanical grinding

Plane grinding should be carried out with the finest possible grit to avoid excessive mechanical deformation.

• The hardness, size and number of specimens should be considered. However, even with large specimens of pure aluminum, plane grinding with 500# SiC Foil or Paper is usually sufficient.
• Large cast parts of aluminum alloys can be ground with 220# SiC or 320# SiC Foil. It is important that the grinding force is low to avoid deep deformation and to reduce friction between the grinding SiC Foil or Paper and specimen’s surface.

Diamond polishing

Diamond polishing should be carried out until all deep scratches from grinding have been removed. If water soluble constituents must be identified, we recommend polishing with water-free diamond suspension and lubricant.

Final polish for pure aluminum and aluminum alloys: The polish/check sequence

• Begin polishing. After 1 minute of polishing with OP-U suspension, check the specimen under the microscope.
• If necessary, continue polishing for another minute and check the specimen again.
• Continue this polish/check sequence until the required quality has been achieved.
• If diamond particles have been pressed into the surface during polishing, they can lead to erroneous interpretations of the structure. Therefore, the polish/check sequence may need to be relatively long. Continue the sequence until you can no longer see bright and dull areas on the surface of the specimen with the naked eye.
• Approximately 30 seconds before the end of polishing, pour water onto the polishing cloth to rinse the specimen and cloth.
• Finally, wash the specimen again with clean water and then dry it.

Note: Polishing for too long with silicon dioxide suspension OP-S NonDry can cause a pronounced relief, see Fig.11.

*Alternatively, MD-Dac

* To avoid coarse scratches, the SiC Foil or Paper can be rubbed with wax before grinding.
** Alternatively, MD-Dac

* To avoid coarse scratches, the SiC Foil or Paper can be rubbed with wax before grinding.'

Fig. 9: Aluminum-silicon cast, after polishing with 3 μm diamond small scratches are still visible, 200x

Fig.10: Structure as in Fig. 9, but fine polished with OP-U suspension. The matrix is well polished, and the eutectic has more contrast, 200x

Fig.11: Aluminum-silicon cast polished for too long with OP-S suspension, silicon precipitates stand in relief, 100x

Electrolytic polishing

Electrolytic polishing of aluminum leaves a scratch-free surface and is often used in quality control as it delivers fast and reproducible results. However, it is not recommended for many cast alloy specimens, due to the many different phases in cast alloys.

Pure aluminum and wrought alloys
Electrolytic polishing is especially suitable for pure aluminum and wrought alloys.

• For routine grain size determination at 100x, pre-grinding with # SiC Foil is sufficient.
• Rolled or drawn surfaces do not need any grinding or polishing.
• For pure aluminum and precise examinations of grain shapes, the specimen should be fine ground to # and sometimes even to # before electrolytic polishing.

If you anodize the specimen with Barker’s reagent after the polishing, it will result in a color contrast that is particularly suited for grain size evaluation. To obtain the color effect, view the specimen under polarized light with a λ1⁄4 sensitive tint plate.

Fig.12: Plated sheet, anodized, grain areas are clearly visible and suitable for automatic image analysis, polarized light with λ1⁄4-plate, 100x

Discover the parameters for electrolytic polishing of aluminum in our application note here.

• Get more knowledge, expertise and insight on our grinding and polishing section
• See our range of grinding and polihsing equipment and products
• Get consumables and accessories for metallographic grinding and polishing

Fig.13: Aluminum pressed part, macro etching, primary and heterogeneous precipitates are revealed

Below are frequent questions and corresponding answers about EPA's Facts and Figures About Materials, Waste and Recycling.

On this page:

• What are the most recent Facts and Figures data? From what year?
• What's the difference between a material and a product?
• What's the difference between a nondurable good, a durable good, and containers and packaging?
• How many cell phones and computers are generated and recycled each year?
• How much packaging is generated, recycled, combusted with energy recovery, and landfilled each year in the United States? What trends exist?
• How much clothing is generated, recycled and disposed of each year?
• What percentage of plastic or plastic bottles are recycled? What is the percentage of paper cartons recycled? And the percentage of aluminum cans recycled?
• What are the overall findings of the Facts and Figures Report?
• Approximately how many shopping bags do Americans consume every year? How many of those bags are recycled?
• How much food waste is generated in America?
• Where can I download the summary data tables and view the research memos?
• Is the municipal solid waste data in the Facts and Figures analysis pre-consumer or post-consumer?
• How is textiles generation calculated?
• Is the data in the Facts and Figures analysis provided in U.S. (short) tons or metric tons?

What are the most recent Facts and Figures data? From what year?

The most recent data for all materials and products are from .

What's the difference between a material and a product?

EPA uses two categories to characterize the 292.4 million tons of municipal solid waste (MSW) generated in . The first is by material. This category includes paper and paperboard, glass, metals, plastics, food, yard trimmings, rubber and leather, textiles, wood, and other. The second is by these major product categories, which include durable goods, nondurable goods and containers and packaging.

The distinction between products and materials is that products are manufactured out of materials. Also, products are what people buy and handle, such as newspapers, bottles and cans. A material is a raw item before it is shaped into something else, such as a piece of leather before it is made into a glove. EPA tracks products to learn how people are consuming, using and discarding materials. This information allows the Agency to target activities that will ultimately maximize source reduction, recycling and composting of materials.

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What's the difference between a nondurable good, a durable good, and containers and packaging?

• Durable goods last three years or more;
• Nondurable goods generally last less than three years; and
• Containers and packaging are assumed to be discarded the same year the products they contain are purchased.

How many cell phones and computers are generated and recycled each year?

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The Advancing Sustainable Materials Management (SMM): Facts and Figures  does not currently break down electronic waste by category. The category “Selected Consumer Electronics,” which includes information on the recycling rate for consumer electronics such as TVs, VCRs, DVD players, cell phones, video cameras, fax machines, telephones and computer equipment, has consistently tracked this same subset of electronics throughout the report’s history starting in .

According to this report, an estimated 2.7 million tons of consumer electronics goods were generated in . Of this, 1.04 million tons of selected consumer electronics were collected for recycling for a 38.5 percent recovery rate.

The Electronics section of the Durable Goods page has information on generation, recycling, combustion with energy recovery and landfilling for selected consumer electronics.

The purpose of this report is to track generation and recycling rates. The report does not provide insight into policies and behaviors that influence the trends.

How much packaging is generated, recycled, combusted with energy recovery and landfilled each year in the United States? What trends exist?

For general information on containers and packaging (on what is generated, recycled, combusted with energy recovery and landfilled), refer to the Containers and Packaging page of this website. Figures 17 and 18 in the Advancing Sustainable Materials Management: Facts and Figures  Data Tables also provide useful information. Figure 17 summarizes containers and packaging materials (glass, metals, plastic, etc.) and Figure 18 summarizes containers and packaging items (corrugated cardboard, steel packaging, aluminum packaging, PET bottles and jars, etc.).

Some examples of recent trends in containers and packaging recycling are found on Table 25 of the  Data Tables. Corrugated boxes were recycled at 67.3 percent in . This rose to 96.5 percent in . Polyethylene Terephthalate (PET) bottles were recycled at a rate of 22.1 percent in and 29.1 percent in .

Information on recycling of Styrofoam packaging can be found in Table 8 of the  Data Tables. Styrofoam containers are known as polystyrene (PS) containers. In , 80 thousand tons were generated, and a negligible amount (less than 5,000 tons) was recycled in the United States. Additionally, 140 thousand tons of polystyrene bags, sacks, and wraps and 330 thousand tons of other packaging were generated in with 20 thousand tons recycled (3.6 percent of PS in containers and packaging).

How much clothing is generated, recycled and disposed of each year?

The Clothing and Footwear section on the Nondurable Goods page of this website shows that 13 million tons of clothing and footwear were generated in . "Generated" means having reached end-of-life—ready for recycling, combustion with energy recovery or landfilling. Reuse occurs before generation, and EPA does not have estimates for the reuse of textiles, such as from donations or to groups such as Goodwill.

Table 19 in the Advancing Sustainable Materials Management: Facts and Figures  Data Tables shows that 13 percent of clothing and footwear was recycled in . These numbers do not include the amount of textiles donated or resold in the United States for reuse. Table 20 of the  Data Tables shows that 2.2 million tons of clothing and footwear were combusted with energy recovery in . Table 21 of the  Data Tables shows that 9.1 million tons of clothing and footwear were landfilled in .

What percentage of plastic or plastic bottles are recycled? What is the percentage of paper cartons recycled? And the percentage of aluminum cans recycled?

Nationwide, in , our most current data shows the actual recycling rate for some key beverage containers was:

• Glass beer and soft drink bottles—39.6 percent
• Glass wine and liquor bottles—39.8 percent
• Aluminum beer and soft drink cans—50.4 percent
• Polyethylene terephthalate (PET) plastic bottles and jars—29.1 percent
• High-density polyethylene (HDPE) plastic natural bottles—29.3 percent

The figures above come from Table 25 of the Advancing Sustainable Materials Management (SMM): Facts and Figures  Data Tables.

EPA keeps its data on the generation, recycling, composting, energy recovery and landfilling of materials and products, such as beverage containers, in weight, rather than in volume. The amount generated is the amount available for recycling, composting, energy recovery and landfilling.

What are the overall findings of the Facts and Figures Report?

In the United States in , 292.4 million tons (U.S. short tons unless specified) of Municipal Solid Waste (MSW) (trash) were generated. About 94 million tons of MSW were recycled and composted, resulting in a 32.1 percent recycling rate. An additional 17.7 million tons was managed through other food pathways (refer to the Food: Material-Specific Data web page.)

Approximately how many shopping bags do Americans consume every year? How many of those bags are recycled?

EPA collects information on solid waste generation, recycling, composting, combustion with energy recovery and landfilling for the Advancing Sustainable Materials Management: Facts and Figures report. The Data Tables include the following information:

We have statistics for plastic bags, sacks and wraps (which includes shrink wrap), as a category. In the  Data Tables, Table 22 shows that about 4,200,000 tons (4.20 million tons) of plastic bags, sacks and wraps were generated in , while Table 24 shows that 420,000 tons (0.42 million tons) were recycled. Table 26 shows that 740,000 tons (0.74 million tons) of plastic bags, sacks and wraps were combusted with energy recovery. Table 28 shows that 3,040,000 (3.04 million tons) of plastic bags, sacks and wraps were landfilled. The recycling rate for plastic bags, sacks and wraps was 10 percent in (Table 25).

How much food waste is generated in America?

EPA and USDA have been working on the issue of sustainable management of food since the s.  In , more than 63 million tons of food waste was generated from residential, commercial, and institutional sectors, with 4.1 percent diverted from landfills and combustion facilities for composting and another 28.1 percent diverted through other food management pathways (animal feed, codigestion/anaerobic digestion, bio-based materials/biochemical processing, donation, land application and sewer/wastewater treatment). While wasted food generation estimates increased in due to changes in methodology, the percentage of food landfilled is lower than previous estimates: 55.9% in , compared to 75.31% in . Still, EPA estimates that more food reaches landfills and combustion facilities than any other single material in our everyday trash, constituting 24 percent of the amount of municipal solid waste (MSW) landfilled and 22 percent of the amount of MSW combusted with energy recovery.. Since food is a major contributor to the amount of methane generated by and released from landfills, and since landfills are one of the top sources of methane emissions, taking action on wasted food will help reduce the amount of methane that is released. The most recent data EPA has on the generation and management of wasted food in America is from . Refer to the Material-Specific page on Food for more information. The most recent data also is captured in the “Advancing Sustainable Materials Management: Facts and Figures Report.”

Where can I download the summary data tables and view the research memos?

Click this link to find out how to download the summary data tables and view the research memos.

Is the municipal solid waste data in the Facts and Figures analysis pre-consumer or post-consumer?

All of the municipal solid waste data in the Facts and Figures analysis is post-consumer waste. Sources of MSW include residential waste, including waste from single and multi-family housing, as well as waste from commercial and institutional locations, such as restaurants, grocery stores, other businesses, schools, hospitals, and industrial facilities. Industrial facility waste includes waste from sources such as offices, cafeterias and packaging, but not process waste.

How is textiles generation calculated?

The information on textiles includes generation, which is the weight of materials and products as they enter the waste management system. After calculating total generation, EPA estimates several pathways, such as recycling, composting, combustion with energy recovery and landfilling, to understand how waste is managed. Reuse, such as the reuse of clothing through donation to secondhand stores, occurs before generation, so it is not included in the generation or recycling estimates. EPA currently does not have an estimate for the amount of textiles reused.

The textiles material-specific web page includes textiles such as those in furniture. On another page in this website, there is a section on clothing and footwear and a section on towels, sheets and pillowcases.

Is the data in the Facts and Figures analysis provided in U.S. (short) tons or metric tons?

The data in the Facts and Figures analysis is provided in U.S. (short) tons unless otherwise noted. One ton equals 2,000 pounds.

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