BMW X3 Service & Repair Manual: Materials science

1. Aluminum

1.1 Chassis/suspension components READ and observe the Notes on chassis/suspension components before handling aluminum.

1.2 Material influences

Causes Effects/remedies
A galvanic element is created under the effects of moisture by contact with materials such as copper, tin, nickel, iron and zinc.

Tools also used for work on steel components can implant steel particles in the softer surface of an aluminum component.

Surfaces are attacked when fluxing agents are used.

Aluminum/steel grinding dust from adjoining working areas.

This plating process causes aluminum to be removed from the connection point.

This results in surface corrosion or pitting.

New parts and accessory parts which have been approved by the BMW Group for aluminum (screws, washers, nuts etc.) have undergone special surface treatment.

Such parts must not be replaced by conventional parts.

NOTE: Damaged parts lose this protection and must be specially coated or replaced.

Damage caused by contact corrosion is excluded from the warranty.

Surface corrosion or pitting occurs.

A separate tool set is available for processing/machining aluminum.

Soldering is not permitted for joining aluminum components.

Risk of corrosion from chemical factors.

This results in surface corrosion.

Erection of protective barriers.

1.3 Machining properties

Properties of aluminum compared with steel Effects
Aluminum parts are magnetically neutral.

Elasticity is only 1/3 as high.

Elongation failure is approx. 50 % slighter.

Electrical conductivity is almost 4 times higher.

Material expansion during heating is twice as high.

Thermal conductivity is 3 times higher.

Structural transformation between 200 ºC and 250 ºC

Aluminum shows no annealing colors.

Attachment with magnetic tools/working aids is not possible.

Convertibility is limited in comparison with steel.

Overstretching the material results in strain-hardening and an increased tendency of cracking.

Electric welding methods require different equipment (MIG welding).

The material expands more markedly. Shorter heat treatment is necessary for removing dents.

Heat is drained more quickly. Adjoining working areas are affected more heavily e.g. during welding.

Elongation characteristics and thus plasticity are improved.

Strength is reduced.

IMPORTANT: No heat treatment during repair work on the vehicle structure! Workshop operation doesn't make it possible to control the temperature to sufficiently exact levels.

The melting point is 650 ºC. Once the melting temperature has been reached, the material begins to flow without any further indications.

The temperature can only be estimated by means of the paint coloring and the surface warpage.

IMPORTANT: Do not use thermo crayons. Not suitable for workshop operation because the paint runs too quickly.

1.4 Storage

Characteristic features of aluminum Special measures
Corrosion (ageing) in damp environments.

Contact corrosion in event of contact e.g.

with steel components.

Susceptibility to paint infiltration.

Store aluminum components in a dry place.

Always store aluminum components separately or in isolation from steel components.

Do not damage factory protective layer of the surface since this would cause oxidation.

Failure to comply with this requirement would result in more painting work.

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