Metallography of Aluminum and Aluminum alloys Overview and Preparation

Aerospace

High-performance aluminum alloys require metallography to confirm precipitation hardening, cladding integrity, and uniform grain size. High-strength aluminum alloys (like 2024 or 7075) are used for aircraft structures, wings, and spacecraft components due to their strength-to-weight ratio. Metallography verifies grain refinement and the presence of strengthening precipitates after heat treatments. After forging or rolling an aerospace alloy, metallography ensures proper fiber texture or grain flow in the component. Additionally, when new aluminum-lithium alloys or advanced composites are introduced, metallography and related testing techniques help certify these materials meet stringent aerospace standards.

Esempio: In aircraft-grade aluminum, metallographic examination can confirm that the desired precipitation phase (such as Al2Cu in 2xxx series or MgZn2 in 7xxx series) is properly distributed, indicating the alloy has achieved its required mechanical properties

Automotive

Used in engines, wheels, and structural components. Aluminum alloys are widely employed in engines, cylinder heads, transmission housings, wheels, and structural frames to reduce weight. Special high-temperature alloys and cast aluminum (like Al-Si casting alloys) must be free of excessive porosity and have refined microstructures for durability.

The microstructure of alloy wheels, engine blocks, pistons, and body sheet panels are important. A metallographic cross-section of a die-cast aluminum engine block can reveal porosity or shrinkage cavities that could lead to part failure if too large. For electric vehicles and rail transport (e.g. high-speed trains), where aluminum is used for lightweight structures, metallography confirms that extrusions and castings have consistent quality.

Esempio: Checking the silicon particles in an Al-Si alloy are fine and evenly modified (often achieved by adding modifiers like strontium during casting). Additionally, aluminum testing in this field may include hardness testing of engine components (to verify proper heat treat) alongside microstructural examination.

Additive Manufacturing

3D-printed aluminum shows unique microstructures like cellular grains and melt pool boundaries. A rapidly growing special application is the use of aluminum alloys in additive manufacturing. 3D-printed aluminum components (often using powders of alloys like AlSi10Mg) can exhibit unique microstructures – such as very fine cellular grains and distinct melt pool boundaries – due to rapid solidification. Metallography plays a key role in this emerging field by examining layer bonding, porosity, and microstructural characteristics of printed aluminum parts.

Electronics and Defense

In Al-Li alloys and aluminum heat sinks, metallography ensures homogeneity and bond quality. Some defence applications use aluminum alloys in armored vehicles or high-strength, lightweight enclosures. Special aluminum alloys containing lithium (Al-Li alloys) have been developed to further reduce weight for aerospace and defense; these alloys require careful metallographic examination to ensure the new phases (like Al3Li precipitates) are present and evenly dispersed. Across these industries, aluminum metallography serves as a bridge between material science and practical engineering. It allows engineers to see the internal story of an aluminum part – whether it’s confirming a production process was successful, or diagnosing why a component didn’t perform as expected.

Importantly, metallography is often accompanied by other testing methods (mechanical tests, chemical analysis, etc.), but it provides the visual evidence of material structure that other tests cannot. Many companies have internal laboratories or collaborate with material testing labs to perform metallography on aluminum samples as part of their routine quality assurance.

Aluminum applications can be found in almost all areas of the economy and modern life. According to current statistics, (primary) aluminum production worldwide has almost doubled in the past decade - an unprecedented development for industrial material. Aluminum industry owes this development to its properties like low specific weight, one third of steel which makes it an appealing alternative for energy-saving lightweight construction. The other important properties of Aluminum and its alloys are the wide range of manufacturing options like casting, forming, extrusion, forging etc.

Also, versatile shaping options like machining, deep drawing, stretching, bending, punching etc. make Aluminum a flexible material for many applications. Aluminum and its alloys exhibit excellent corrosion resistance, which can be further enhanced through anodizing and various coating techniques. These materials offer a broad spectrum of mechanical strengths, ranging from 70 to 800 MPa. Aluminum is a non-toxic material, rendering it highly suitable for use as packaging for food products. Additionally, aluminum possesses excellent electrical conductivity and high thermal conductivity.

Cutting

The cutting of pure Aluminum and Aluminum alloys is a challenge because of the softness of the Aluminum. The optimal cut-off disc for these alloys are the ones with SiC abrasive particles. The SiC cut-off discs with harder bonding material can provide the best results. Due to the lower hardness of silicon carbide particles compared to aluminum oxide particles, these cut-off wheels are the optimal choice for cutting soft material like pure Aluminum.

Pure Aluminum

Pure aluminum is a lightweight metal with a density of approximately 2.7 g/cm³ and a melting point of 660°C. If the purity of Aluminum is more than 99%, it counts as pure Aluminum. The crystallographic structure of pure Aluminum is Face-centered cubic. It is characterized by a bright, silver appearance and excellent corrosion resistance due to the rapid formation of a stable, protective oxide film on its surface. Aluminum is highly ductile and malleable, enabling it to be drawn into wires or rolled into thin sheets. It has relatively low tensile strength in its pure form (about 90 MPa when annealed), but it can be significantly strengthened through alloying or cold working. Pure aluminum is also an outstanding conductor of heat and electricity, non-magnetic, and non-toxic. In the wrought groups 1xxx group represents pure Aluminum like EN AW 1050A. Pure aluminum is typically utilized in non-load-bearing applications, including the manufacture of cables, cans, electronic components, foils, wires, household goods, and packaging materials.

Grinding / Polishing

Among various consumables, silicon carbide (SiC) paper is the most suitable choice for the preparation of pure aluminum samples. Due to the inherent softness of pure aluminum, the material is highly susceptible to deformation during sample preparation. The use of aggressive consumables, such as diamond grinding discs, can introduce preparation artifacts and compromise the microstructural integrity of the sample. Provided that sectioning has been conducted appropriately, initial grinding can commence with SiC paper of P600 grit. This is followed by a second grinding step using SiC paper of P1200 grit; after approximately two minutes of grinding, the sample is ready for the polishing stage.

For the polishing of pure aluminum, soft polishing cloths are recommended to minimize surface deformation. Sigma cloth, a medium-hard silk cloth, is suitable for the initial polishing steps. For the 1 µm polishing stage, Zeta cloth—a short flocked, soft synthetic cloth—should be employed. For final polishing, OMEGA cloth, a chemically resistant soft synthetic cloth, is recommended. The complete sequence of grinding and polishing steps is summarized in the table.

Etching

The objective of etching pure aluminum samples is to selectively reveal the microstructural features, such as grain boundaries or grain size as well as impurities that are otherwise indistinguishable in the polished, unetched state. Etching enhances the contrast between different microstructural components by preferentially attacking specific areas of the sample, allowing for detailed examination of grain size, shape, and distribution under optical or electron microscopy. This process is essential for accurate metallographic analysis and characterization of pure aluminum. There are two main processes to etch the pure Aluminum samples. First is the normal immersing process in etchant. Here the Sodium hydroxide from QATM can be used. The other technique is electrolytic etching. Electrolytic etching of pure aluminum is a metallographic technique used to reveal its microstructure by applying an electrical current in an appropriate electrolyte solution. During the process, the aluminum sample acts as the anode, and selective dissolution occurs at grain boundaries and other microstructural features. This method offers controlled and uniform etching, minimizing mechanical damage and enhancing the visibility of fine details such as grain boundaries and inclusions. Electrolytic etching is particularly effective for pure aluminum due to its softness and tendency to deform under mechanical polishing, resulting in clearer and more reproducible microstructural observations.

QETCH 1000 is a fully automatic electrolytic polisher and etcher with intuitive touch-screen operation. Polishing and etching unit and control unit are separated, also the Qetch 1000 can be used in the lab fume cabinet. A scan function displays the current voltage curve of a material and provides polishing results quickly without structural changes. Handling of the polishing and etching unit was facilitated significantly by the interchangeable 1 ltr. electrolytic tanks. Different electrolytes can be changed and easily stored with lid. The unit is cleaned with water by a washing program.

Casting Aluminum Alloy

DIN EN 1780 governs the classification of castings and casting alloys:2002 (the Aluminum association AA has another classification method)., which utilizes a five-digit designation system as described below

The first digit shows the main alloying element

The second digit shows the type of the alloy.

• The first digit shows the main alloying element
• The second digit shows the type of the alloy.
• The 3rd digit is arbitrary and indicates the special alloy composition.
• The 4th digit is generally 0.
• The 5th digit is always 0 for CEN alloys but never 0 for aerospace AECMA alloys.
• These digits follow by a letter which shows the casting process like S for sand casting, K for gravity casting, D for die casting and L for investment casting.

Subsequently, the following letters and numbers specify the condition of the alloy, as detailed in the table below:

The physical properties of pure Aluminum at 20°C can be seen in the table below:

Grinding / Polishing

Among various consumables, silicon carbide (SiC) paper is the most suitable choice for the preparation of pure aluminum samples. Due to the inherent softness of pure aluminum, the material is highly susceptible to deformation during sample preparation. The use of aggressive consumables, such as diamond grinding discs, can introduce preparation artifacts and compromise the microstructural integrity of the sample. Provided that sectioning has been conducted appropriately, initial grinding can commence with SiC paper of P600 grit. This is followed by a second grinding step using SiC paper of P1200 grit; after approximately two minutes of grinding, the sample is ready for the polishing stage. For the polishing of pure aluminum, soft polishing cloths are recommended to minimize surface deformation. Sigma cloth, a medium-hard silk cloth, is suitable for the initial polishing steps. For the 1 µm polishing stage, Zeta cloth—a short flocked, soft synthetic cloth—should be employed. For final polishing, OMEGA cloth, a chemically resistant soft synthetic cloth, is recommended. The complete sequence of grinding and polishing steps is summarized in the table.