DRSA Inreach

Internal Stresses in Aluminum Engines

Solidification

Outline

• Introduction to solidification of casting alloys.

• Phase diagram for Al-Si.• Phases during cooling.• Different cooling rates throughout

cast engine head.

Phases

Melting and Boiling Temperatures

Simple systems have a melting temperature and a boiling temperature

Engines made from alloy of aluminum and silicon

More complicated in alloys

Phases

What is an alloy Not a compound◦H2O◦CH4

Mixture of two or more metals in any proportion

Aluminum too soft, weak Add silicon to harden and strengthen

Alloys

Aluminum and silicon come out of the melt and form separate components at the microscopic level

Rather than melting and boiling temperatures, look at Phase Diagrams

Alloys

Al-Si Phase Diagram

Al-Si Phase Diagram

Solidus

LiquidusEutectic

Microstructure Al-19%Si

Engine Head

◦Outside cools firstSolid outer shellMolten inner core, cools later, puts outer shell in tension

◦Different cooling rates lead to different microscopic structures◦Mismatch in microscopic structures create internal stresses

Internal Stresses

…Stresses and Strains

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