ABSTRAK
Paduan aluminium banyak digunakan di industri manufaktur karenasifatnya yang ringan, mudah di cor, dan tahan korosi. Oleh karena itu, penggunaanpaduan aluminium sebagai material turbin Organic Rankine Cycle (ORC)diharapkan mampu meningkatkan efisiensi kerjanya. Untuk meningkatkankekuatan mekanis aluminium perlu dilakukan penambahan elemen paduan sepertiSi, Mg, dan Cu. Selain itu paduan Al-Si-Mg-Cu dapat ditingkatkan sifatmekanisnya dengan melakukan pengerasan penuaan atau pengerasan presipitasi.Studi ini dilakukan untuk mengetahui pengaruh elemen paduan Cu sebesar0,38, 3,82, dan 6 wt.% pada paduan Al-7Si-4Mg dan mengamati respon paduanterhadap pengerasan penuaan dengan melakukan solution treatment padatemperatur 495 oC selama 2 jam, pendinginan cepat, dan penuaan (ageing) padatemperatur 130 oC. Karakterisasi yang dilakukan yakni pengujian kekerasan,pengamatan struktur mikro dengan mikroskop optik dan Scanning ElectronMicroscope (SEM) dilengkapi Energy Dispersive X-Rays Spectroscopy (EDX), danpengujian Simultaneous Thermal Analysis (STA).Hasil penelitian menunjukkan peningkatan kekerasan as-cast seiringdengan penambahan Cu. Proses pengerasan penuaan juga meningkatkan kekerasanpaduan hingga kekerasan puncak sebesar 64,47, 65,8, dan 70,1 HRB padapenambahan Cu berturut- turut 0,38, 3,82, dan 6 wt.%. Penambahan Cu mampumembentuk fasa kedua Al2Cu dan Al5Cu2Mg8Si6 yang akan meningkatkankekerasan. Pembentukan presipitat GP-zone, θ'', dan θ' terjadi pada temperatur 48,240, dan 296 oC dan tidak ada pengaruh penambahan Cu terhadap temperaturtersebut.

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ABSTRACT
Aluminum alloys have been used in manufacturing industries because of their light-weight, high castability, and high corrosion resistant. Therefore, the use of aluminum alloys for turbine impeller of Organic Rankine Cycle (ORC) is expected to increase the efficiency. The addition of alloying elements such as Si, Mg, and Cu are necessary to improve mechanical properties. Further improvement of the properties can be achieved through age hardening or precipitation strengthening. This study was aimed to determine the effect of Cu addition of 0.38, 3.82 and 6 wt.% in Al-7Si-4Mg alloy on hardness and age hardening response. The alloys were solution treated at 495 °C for 2 hours, quenched and aged at 130 oC. Characterization included hardness test, microstructural observation by an optical microscope and Scanning Electron Microscope (SEM) combined with Energy Dispersive X-Rays Spectroscopy (EDX), as well as Simultaneous Thermal Analysis (STA) testing. The results showed an increase in as-cast hardness along with the addition of Cu. Peak hardness increased to 64.47, 65.8 and 70.1 HRB by addition of 0.38, 3.82 and 6 wt.% Cu, respectively. The addition of Cu promoted the formation of Al2Cu and Al5Cu2Mg8Si6 which contibuted to higher as-cast hardness. Formation of GP-zone, θ'' dan θ' was observed at 48, 240 and 296 oC, respectively and no effects of Cu on these temperatures.