Sains Malaysiana 45(6)(2016): 977–987

Evolusi Mikrostruktur Aloi A333 melalui Proses Logam Separa Pepejal

(Microstructural Evolution of A333 Alloy through Semisolid Metal Process)

A.M. AZIZ¹*, M.Z. OMAR¹ & M.S. SALLEH^1,2

¹Jabatan Kejuruteraan Mekanik dan Bahan, Fakulti Kejuruteraan dan Alam Bina,

Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor Darul Ehsan, Malaysia

²Jabatan Proses Pembuatan, Fakulti Kejuruteraan Pembuatan, Universiti Teknikal Malaysia Melaka, 76100 Durian Tunggal, Melaka, Malaysia

Diserahkan: 9 Januari 2015/Diterima: 21 Disember 2015

ABSTRAK

Pemprosesan logam separa pepejal yang juga dikenali sebagai pembentukan-tikso merupakan suatu kaedah pemprosesan yang secara relatifnya adalah baharu jika dibandingkan dengan kaedah pemprosesan biasa seperti penuangan dan penempaan. Sebelum pembentukan-tikso dapat dilakukan, aloi berkenaan perlu melalui langkah awal persediaan iaitu dengan mengubah mikrostruktur asal berbentuk dendritik kepada bentuk hampir sfera. Ini diikuti dengan pemanasan semula ke julat suhu separa pejal (iaitu sekitar 30-50% cecair) dan seterusnya proses ubah bentuk ke dalam acuan menggunakan mesin penekanan hidraulik. Antara kaedah utama yang biasa digunakan bagi penyediaan bahan aloi ini adalah tuangan cerun penyejukan, pengadukan mekanik dan kaedah terma langsung. Kajian ini memberi tumpuan kepada evolusi mikrostruktur aloi aluminium A333 (Al-7.5Si-3.1Cu) daripada bentuk dendritik kepada hampir sfera sebagai persediaan untuk diproses melalui pembentukan-tikso kelak. Kaedah tuangan cerun penyejukan digunakan dalam kajian ini kerana kaedah ini diketahui berkesan tapi mudah digunakan. Nilai saiz ira dan faktor bentuk dianalisis berdasarkan kepada suhu tuangan dan panjang cerun penyejukan berbeza yang digunakan. Di samping itu, kaedah kalometri pengimbas perbezaan juga diguna pakai dalam mengenal pasti suhu sempadan pepejal dan sempadan cecair aloi ini. Saiz ira fasa pepejal α-Al paling kecil dan nilai faktor bentuk terbaik yang diperoleh dalam kajian ini berlaku pada suhu tuangan 620°C dan panjang cerun 300 mm, iaitu masing-masing pada 15.3 μm dan 0.54.

Kata kunci: Aloi aluminium; rawatan haba sesuhu; tuangan cerun penyejukan

ABSTRACT

Semi solid metal processing also known as thixoforming is a relatively new processing method compared to the conventional casting and forging processes. Prior to the thixoforming process, the candidate alloy must go through a preparation procedure to transform its normally dendritic structure to a near spheroidal form. This is followed by a re-heating process at a semi solid temperature range (30-50% liquid range) before forming in a mould using a hydraulic pressing machine. Among the mostly used alloy preparation methods were cooling slope casting, mechanical stirring and direct thermal methods. This study focused on the microstructural evolution of A333 (Al-7.5Si-3.1Cu) aluminium alloy from dendritic to a near spheroidal microstructure as a preparation for the later thixoforming process. The cooling slope casting method was selected in this study because the method was known to be effective yet simple to use. The resulting grain size and shape factor of the solid phase were analysed in response to the pouring temperatures and the lengths of the cooling slope plate selected. In addition, the identification of the solidus and liquidus temperatures was carried out using a difference scanning calometry method. The finest grain size and the best shape factor for the α-Al solid phase were obtained at 620°C pouring temperature and 300 mm cooling slope plate length, i.e. at 15.3 μm and 0.54, respectively.

Keywords: Aluminum alloy; cooling slope process; isothermal heat treatment

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*Pengarang untuk surat-menyurat; email: ahmadaziz@siswa.ukm.edu.my