Strengthening behavior of Al–Cu–Mg alloy sheet in hot forming–quenching integrated process with cold–hot dies

• Cold–hot composite dies were proposed to use in hot forming–quenching integrated process.
• The effects of hot die temperature, quenching condition and precipitate distribution on strengthening were evaluated.
• Lower die heating avoids rapid temperature decrease of heated sheet and results in good strength of parts.
• Upper cold die ensures effective quenching and reduces thermal distortion of final part.

The novel technology combining hot forming and quenching together has been developed to improve formability and avoid thermal distortion for heat-treatable aluminum alloy forming. In this paper, cold–hot composite dies are proposed to use in hot forming–quenching integrated process. Heated lower dies are used to avoid rapid temperature decrease of heated sheet. Water-cooled upper dies are then used to accomplish quenching and reduce thermal distortion. The effects of temperature of lower dies, quenching condition and precipitate distribution on strengthening behavior in this process were investigated systematically. The strengths were measured by Vickers hardness and uniaxial tensile tests. It was found that the upper cold dies could ensure effective quenching. Lower dies could be heated to avoid rapid temperature decrease of heated sheet resulting in good strength of parts. The corresponding hardness, yield and tensile strengths were 140.7 HV, 295.7 and 469.2 MPa, respectively. For comparison, the process with both hot dies at temperatures ranging from 100 to 350 °C was investigated. The temperature of both hot dies could only be improved to 250 °C, otherwise the strength decreased. The strengthening phase was dispersed lath-shaped S phase with an average cross-section of approximately 50 × 100 nm, which was observed with TEM and SEM methods.

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