Bu çalışmada 0.85 mm kalınlığındaki 7075 kalite aluminyum alaşımı sacın, sertlik, mukavemet ve darbe direnci gibi mekanik özelliklerine, retrogresyon ve yeniden yaşlandırma ısıl işleminin etkisi incelenmiştir. Başlangıçta T6 temper durumunda olan alaşıma, 220°C’de 15 saniye – 60 dakika arasında retrogresyon uygulanmış, yeniden yaşlandırma 120°C’de 24 saat süreyle yapılmıştır. Retrogresyon ve yeniden yaşlandırma işlemi sonunda, sertlik önce artarak en yüksek değerine ulaşmış, daha uzun retrogresyon sürelerinde ise azalmıştır. Akma ve çekme mukavemeti ise retrogresyon süresine bağlı olarak azalırken, kopma uzaması ve darbe direncinde, retrogresyon süresine bağlı olarak sürekli bir artış görülmüştür. Sonuç olarak, retrogresyon ve yeniden yaşlandırma ısıl işlemiyle, belirli retrogresyon sürelerinde, T6 temper durumu seviyesine eşit ya da daha yüksek sertlik, mukavemet, süneklik ve tokluk elde edilmiştir.Anahtar Kelimeler: Aluminyum, retrogresyon ve yeniden yaşlandırma, çökelme sertleşmesi.In this study, the effect of retrogression and reageing heat treatment on mechanical properties such as hardness, strength and impact resistance of 0.85 mm thick 7075 quality aluminium alloy sheet has been investigated. The alloy, which was initially in T6 temper condition, was retrogressed at 220°C for various times between 15 seconds and 60 minutes, prior to reageing at 120°C for 24 hours. Hardness and electrical conductivity were monitored as an indicator of the microstructural changes during retrogression or reageing stages. Hardness was initially decreased giving a minimum as the retrogression time increases. Further retrogression times beyond increment in hardness resulted in a decrement of hardness again. Electrical conductivity was progressively increased as the retrogression time increased. Hardness of retrogressed and reaged samples initially increased as the retrogression time increased, exhibiting their maximum values, then finally decreased. Yield and tensile strength progressively decreased while elongation at fracture and impact resistance increased with increasing retrogression time. It is concluded that, equal or higher values of hardness, strength, ductility and toughness than those of T6 temper can be obtained with retrogression and reageing heat treatment. Improvement in hardness can be attributed to the increased number of precipitates in retrogressed and reaged microstructure. On the other hand, higher values of impact resistance are accompanied with overageing, which commences in longer retrogression time.Keywords: Aluminium, retrogression and reageing, precipitation hardening
