Effect of quenching rate on age hardening in an Al–Zn–Mg alloy sheet

• Published in: Journal of Japan Institute of Light Metals Vol. 67; no. 2; pp. 41 - 48
• Main Authors: Yoshida, Hideo, Watanabe, Takero, Hatta, Hidenori
• Format: Journal Article
• Language: Japanese; English
• Published: Tokyo The Japan Institute of Light Metals 2017; Japan Science and Technology Agency
• Subjects: Al–Zn–Mg alloy; furnace cooling; GP zone; pre-aging at room temperature; quench sensitivity
• ISSN: 0451-5994; 1880-8018
• DOI: 10.2464/jilm.67.41

The effect of quenching rate on artificial age hardening in an Al–6.0% Zn–0.75% Mg alloy sheet was investigated by changing the quenching rate like water quenching (WQ), air cooling (AC) and furnace cooling (FC) followed by pre-aging at 20°C and then artificial aging at 120, 160 and 200°C. In pre-aging at 20°C for 10080 min, the quenching rate little influence the peak hardness at 120 and 160°C aging. However the peak hardness at 200°C aging of a sheet quenched by WQ and AC is lower than those quenched by FC regardless of the holding time at 20°C. In a long pre-aging at 20°C, many GP(I) zones are formed during pre-aging at 20°C even if the cooling rate is as slow as FC. These GP(I) zones transform into GP(II) ones which quickly transform into η′ phase during heating or aging. Therefore the strength of a sheet quenched by FC followed by 120 and 160°C aging is as high as those by WQ and AC. However these GP(I) and GP(II) zones dissolve by reversion during heating to 200°C. Therefore the strength of the sheet quenched by WQ and AC followed by 200°C aging decreases. On the other hand, it is considered that stable clusters or unknown GP zones form during FC, which did not dissolve up to 200°C and transform into η′. Therefore higher strength was obtained in FC at 200°C compared with WQ and AC.