BDCS Notes - Aluminum Testing and Properties

Aluminum Testing and Properties

Tests for aluminum are similar to those for steel. They typically include stress-strain tensile tests to determine elastic modulus, yield strength, ultimate strength, and percent elongation.

In contrast to steel, aluminum alloys do not display an upper and lower yield point. Instead, the stress-strain curve is linear up to the proportional limit and then is a smooth curve up to the ultimate strength. Yield strength is still based on the .2% offset method.

The modulus of elasticity for aluminum is 69 GPa (10,000 ksi).

The coefficient of thermal expansion for aluminum is .000023/ degree C. This is twice that of steel and concrete. Therefore, joints between aluminum and steel or concrete must be designed to accommodate differential movement.

Obviously, strengths of aluminum are considerably affected by temperature. At temperatures above 150 C, tensile strengths are reduced considerably.

Welding and Fastening

Aluminum pieces are joined either by welding or fastening. Welding requires that tough oxide coating on aluminum be broken and kept from reforming during welding, so arc welding helps shield oxygen from reentering the atmosphere.

Two kinds of arc welding are:
Gas Metal Arc Welding (GMAW): Filler wire serves as the electrode.
Gas Tungsten Arc Welding (GTAW): Tungsten electrode and a separate filler wire.

Fastening: Either aluminum or steel can be used. When steel bolts are used, they must be galvanized, aluminized, cadmium plated, or made of stainless steel to prevent galvanic corrosion.

Corrosion: Aluminum develops a thin oxidation layer immediately upon exposure to the atmosphere. A tough oxide film protects the surface from further oxidation. Alloying elements alter the corrosion resistance of the aluminum.

Airplane Metal: Give extra protection by painting or cladding with a thin coat of a corrosion-resistant alloy.

Galvanic Corrosion: Occurs when aluminum is in contact with any of several metals in the presence of an electrical conductor, such as water. Best protection method is to break the path of the galvanic cell by painting, using an insulator, or keeping dissimilar metals dry.

BDCS Notes - Aluminum Production

Aluminum Production

Historical Production of Aluminum: Uses processes that were developed in the 1880s. Bayer developed the sodium aluminate leaching process to produce pure alumina (Al2O3). Hall and Heroult developed an electrolytic process for reducing the alumina to pure aluminum.

Process: The production of aluminum follows the following process:

1. The mining of the aluminum ore, bauxite.
2. Crushing of the bauxite, which is then washed, to remove clay and silica materials, and kiln dried afterwards.
3. Mixing the crushed bauxite with soda ash and lime and passed through a digester, pressure reducer, and settling tank to produce a concentrated solution of sodium aluminate. (This removes silica, iron oxide, and other impurities).
4. Seeding of the solution with hydrated alumina crystals in precipitator towers. The seeds attract other alumina crystals and form groups that settle out of solution.
5. The resulting alumina is reduced with the Hall-Heroult process and melted in a cryolite bath (a molten salt of sodium-aluminum-fluoride).
6. A current is passed between anodes and cathodes of carbon, which separates the aluminum from the oxygen and the molten aluminum is collected at the cathode at the bottom of the bath.

The final molten aluminum is either shipped to a foundry for casting into final products or is cast into ingots. The ingots are formed by a direct chill process that produces sheets for rolling mills, or square billets for production of wire, rod and bar stock.

Final products are made by either casting or deforming solid aluminum stock. Casting is the oldest method, and can be done in three ways:

1. Die Casting – Forcing molten aluminum into mold under high pressure.
2. Permanent Mold Casting – Pouring aluminum into reusable metal mold.
3. Sand Casting – Sand with a binder packed around a pattern, aluminum poured into pattern, reproducing the shape.

The deformation processes are:

1. Forging
2. Impact Extrusion
3. Stamping
4. Drawing
5. Drawing Plus Ironing

Recycling Aluminum: Scrap stock is melted in a furnace. The molten aluminum is purified and alloys are added.

BDCS Notes - Aluminum Metallurgy

Aluminum Metallurgy

Aluminum has a FCC lattice structure. It is very malleable. Its tensile strength is around 28 MPa and the modulus of elasticity for aluminum is approximately 69 GPa.

Aluminum Alloys: Can be as much as 15 times stronger than pure aluminum, through the addition of small amounts of alloying element, strain hardening by cold working, and heat treatment.

Common Alloys: Copper, manganese, zinc, silicon, magnesium.

Cold Working: Increases strength by causing a disruption of the slip planes in the material that resulted from production process.

Alloy Designation System: Classification starts by separating the product according to its production method (either casting or wrought methods).

The designation system for wrought alloys consists of a four-digit code. The first digit indicates the alloy series. The second digit, if different from 0, indicates a modification in the basic alloy. The third and fourth digits identify the specific alloy in the series; these digits are arbitrarily assigned, except for the 1xxx series, in which the final two digits indicate the minimum aluminum content.

For the 1xxx series, the aluminum content is 99% plus the last two digits of the code, expressed as a decimal fraction. An example would be 1060, which contains a minimum aluminum content of 99.60%.

Cast alloys are assigned a three-digit number followed by one digit after a decimal point. The first digit represents the alloy series. The second and third digits are arbitrarily assigned to identify specific alloys.

Temper Treatments: Specifications of an aluminum material must include the manner in which the product was tempered (either strong (heat-treatable) or common (non-heat-treatable)).

Strong Alloys: Contain elements, constituents that have a considerable solid solubility at elevated temperatures and limited solubility at lower temperatures.

Common Alloys: Contain elements that remain substantially in solid solution or that form insoluble constituents.

BDCS Notes - Aluminum

Aluminum

Aluminum: The most plentiful metal on Earth. Represents 8% of the Earth’s crust.

Aluminum exists primarily as oxides, and the process of extracting aluminum from oxides is very intensive. Approximately 2%-3% of the electricity used in the U.S. is used for this process.

Properties of Aluminum: Not intended for structural applications. Alloying elements are almost always added to aluminum when it is required for industrial applications.

Production of Aluminum: In terms of metal produced, aluminum is only second to steel. About 25% of aluminum is used for containers, 20% used for architectural applications (doors, windows, and siding) and 10% for electrical conductors.

Cost of Aluminum: Has limited usage because of high initial cost when compared with steel and the lack of performance for structural applications.

Favorable Characteristics of Aluminum:

1. 1/3 the density of steel
2. Good thermal and electrical conductivity
3. High strength-to-weight ratio
4. Can be given a hard surface by anodizing and hard coating
5. Has alloys that are weldable
6. Will not rust
7. Has high reflectivity
8. Can be die cast
9. Is easily machined
10. Has good formability
11. Is nonmagnetic
12. Is nontoxic

It’s high strength-to-weight ratio and ability to resist corrosion are the primary factors that make aluminum an attractive structural engineering material.