Aluminum Workshop: Is aluminum ductile or brittle?

A: Charpy V-notch samples are used to determine a material’s toughness, that is, whether the material is ductile or brittle in the presence of a notch. This is done by taking a standard sample with a notch machined into it, impacting the sample so the tip of the notch is in tension, and measuring the energy absorbed to fracture the sample. Samples that are ductile absorb a lot of energy. Samples that are brittle absorb little energy. Depending on the filler alloy used, the details of the welding procedure, and the testing temperature, steels can exhibit either ductile or brittle behavior. Needless to say, brittle behavior is undesirable, and the CVN test tries to screen out a WPS that would lead to brittle behavior.

All steels are ferritic, body-centered, cubic materials. While they will fracture in a ductile manner at room temperature, as the temperature is reduced you will eventually reach a temperature (actually a narrow range of temperature) at which fracture behavior becomes brittle. This is called the ductile-brittle transition temperature (DBTT). The DBTT is different for each type of steel. In general, the DBTT becomes lower (this is good) as alloys such as Ni, Cr, and V are added to the steel and as the microstructure is improved.

Charpy V-notch Testing and Aluminum

Aluminum is different. As a face-centered cubic material, it has no DBTT. As the temperature is lowered, aluminum becomes stronger but remains ductile. In fact, aluminum alloys are ductile even at liquid helium temperatures (-425 degrees F). That’s why aluminum alloys are used so often in fabricating cryogenic equipment. The work establishing the toughness of aluminum alloys, and aluminum welds, was performed in the 1950s and 1960s. Ever since then no fabrication code requires Charpy samples to be performed on aluminum alloys or aluminum welds.

In fact, Charpy testing is not only irrelevant, it also can be misleading for aluminum alloys and welds. Because of the details of the Charpy test, tests on aluminum usually show very low values. This does not mean the aluminum is brittle, only that the test doesn’t perform well on aluminum. There is a standard toughness test for aluminum alloys and welds. It’s called the Kahn tear test. Its use and interpretation are documented in the ASTM B871 specification.