Aluminium tooling: Applying the right alloy

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A 7xxx series alloy may have yield strength of 400-490 MPa at ambient temperature (24°C), but at 150°C this strength is reduced by approximately 63%. At 200°C it is reduced by about 82%. Using the same examination for a 6061 alloy, yield strength is 275 MPa at ambient temperature. At 150°C this strength is reduced by approximately 22%, and at 400°C it is reduced by 62%.

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Compare 2xxx series cast mould plate at these temperatures. Yield strength may only be 138 MPa at ambient temperature, but at 150°C this strength is reduced by 22%. At 200°C it is reduced by 45%. Additionally, the cast 2xxx series will be more resistant to thermal fatigue because it is not heat-treated like 6061 and 7xxx series mould alloys. The properties of cast mould plate materials are derived from the alloy composition, not from a heat-treating process. These same alloy-driven properties also apply to 5xxx series cast plate material, which is often used for prototype moulds.

When the 2xxx and 5xxx series cast mould plate returns to ambient temperature, it will have maintained its original properties. After exposure to higher temperatures, the heat-treated alloys will not return to their original strength, however. For example, the properties of the 7xxx series alloys at ambient temperature may drop down by as much as 50% after exposure to a 200°C temperature.

Welding effects on aluminium tools

On welded aluminium moulds, the area around the weld displays three surface appearances: the parent material, filler wire and the heat-affected area due to welding. New techniques and filler wires help to minimise the heat effect on the material from welding, but applying that welding heat to a heat-treated material will cause a loss of metal strength in the welded area. Several aluminium mills have developed welding filler wire that works better with specialised 7xxx series mould alloys, providing improved color match and reduced porosity in the welded area. There also have been advances in welding 7xxx series aluminium, such as a DC pulse welding technique that minimises heat in the welded area, improving part production after the repair.

The art of stable, consistent temperature control

Cast mould plate alloys in the 2xxx and 5xxx series also have specialised welding practices that contribute to successful aluminium mould repair. These include welding techniques that incorporate specialised filler wires, shielding gas and DC pulse procedures. As an additional benefit, these alloy families do not have the same issues with heat-affected zones as heat-treated 7xxx alloys. Since their properties come from the base alloy composition, the parent block and filler wire are the only two areas in the block affected by welding. The area around the weld is not affected, as it would be with 6061 and 7xxx series mould alloys. This ability to maintain strength after welding shows how 2xxx and 5xxx cast mould plate alloys offer increased resistance to thermal fatigue compared with heat-treated alloys.

Aluminium injection moulds are capable of producing more than 1 million parts, making aluminium a viable mould material option for some applications. The design, production and processing of aluminium moulds varies from that of steel, so it is critical to examine all of the material characteristics and application factors when considering an aluminium tool build.

*Reprinted with permission from MoldMaking Technology, Gardner Business Media, moldmakingtechnology.com

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Aluminium tooling: Applying the right alloy

Welding effects on aluminium tools