Microstructure and Mechanical Properties of the Aluminum Alloy Billet Recycled by Using Direct Chill Casting Technique

KARDO RAJAGUKGUK, Prof. Dr. Ir. Harwin Saptoadi, M.SE., IPM.; Dr. Indraswari Kusumaningtyas, S.T., M.Sc.; Prof. Dr. Suyitno., M.Sc. IPM.

2024 | Disertasi | S3 Teknik Mesin

Daur ulang limbah logam semakin menarik perhatian industri mengikuti kebijakan global untuk mengurangi konsumsi energi serta dampak lingkungan. Di antara logam yang didaur ulang, aluminium (Al) saat ini menjadi salah satu bahan populer karena aplikasinya yang sangat luas. Beberapa teknologi daur ulang telah dikembangkan, termasuk yang berbasis pada proses peleburan ulang dan pengecoran. Salah satu tantangan utama dalam proses daur ulang aluminium adalah variasi signifikan dalam komposisi limbah dan keberadaan elemen pencemar atau pengotor. Dalam penelitian ini, teknologi pengecoran direct chill (DC) digunakan sebagai alat untuk mendaur ulang limbah aluminium menjadi paduan billet aluminium. Metode pengecoran direct chill merupakan metode pengecoran yang menjanjikan untuk menghasilkan billet aluminium cor karena efisiensinya yang tinggi dan cocok untuk produksi berskala besar. Penelitian ini bertujuan untuk menyelidiki cacat cor dan menentukan parameter cor yang optimum selama proses pengecoran. Struktur mikro dan sifat mekanis pada empat lokasi sepanjang penampang radius billet yang dicor menggunakan metode pengecoran DC casting dievaluasi. Distribusi suhu dari awal hingga akhir proses pengecoran juga disajikan dalam penelitian ini. Distribusi suhu yang tidak merata dalam pengecoran DC menghasilkan variasi struktur mikro di sepanjang penampang melintang billet aluminium cor, yaitu struktur butir yang kasar (coarse grains) di pinggir, butiran berserabut (feathery grains) di tengah, dan butiran equiaxed kasar di tengah. Struktur mikro billet cor menunjukkan struktur seperti aksara Cina (Chinese-script), terdiri dari ?-Al dan fase intermetalik sepanjang batas butiran. Hasil analisis komposisi dan struktur mikro menunjukkan bahwa komposisi billet dalam penelitian ini terdiri dari ?-Al dan fase intermetalik seperti ?-Al₈Fe₂Si, ?-Al₁₂Fe₃Si₂, Al₃Fe, and Al₄MgSi. Kekuatan tarik (UTS) dan kekerasan di daerah dekat permukaan lebih rendah dibandingkan di bagian tengah billet. Sementara itu, pori berukuran mikro juga ditemukan di daerah dekat permukaan billet aluminium cor. Ukuran butir (grain size) rata-rata billet aluminium cor sedikit lebih besar di bagian tengah, meningkat dari permukaan ke tengah. Daerah dekat permukaan menunjukkan kekuatan tarik ultimate (UTS) dan kekerasan yang lebih rendah dibandingkan dengan bagian tengah, dengan nilai yang meningkat dari tengah ke permukaan. Si menunjukkan segregasi makro negatif ke arah permukaan, positif di tengah, dan negatif di tengah. Kandungan Mg menunjukkan segregasi negatif, sedangkan kandungan Fe meningkat dari permukaan ke tengah. Mendaur ulang limbah aluminium menjadi billet menggunakan metode pengecoran direct chill casting merupakan metode yang menjanjikan untuk masa depan. Namun, berdasarkan temuan dalam penelitian ini, keberadaan fase intermetalik ?-Al₅FeSi dan elemen segregasi makro dalam billet aluminium cor adalah masalah yang perlu diatasi karena berdampak buruk pada sifat mekanis billet cor pengecoran langsung. Upaya lebih lanjut harus dilakukan untuk meningkatkan kualitas billet aluminium cor dengan menggunakan limbah aluminium. Beberapa upaya dapat dilakukan seperti mengontrol bahan baku, standarisasi komposisi paduan, penghalusan struktur butir, dan homogenisasi billet setelah proses pengecoran selesai dilakukan.

Recycling of metallic scrap has increasingly attracted the attention of industries following the global policy to reduce energy consumption as well as environmental impacts. Among the recycled metals, aluminum (Al) has nowadays become one of the popular materials owing to its wide applications. Several recycling technologies have been developed, including those that are based on the remelting and casting process. One major challenge in the aluminum recycling process is the significant variation in scrap compositions and the presence of impurity elements. In this research, direct chill (DC) casting technology was introduced as a tool for recycling aluminum scrap into an as-cast billet alloy. The direct chill method is a promising approach for producing wrought aluminum billets, known for its high efficiency and suitability for large-scale production. This study aims to investigate the casting defects and determine the optimum casting parameters during casting processes. The microstructure and mechanical properties of four locations along a cross-section of the as-cast billet radius were evaluated. The temperature distribution from start-up until the final casting process was also presented in this work. Non-uniform temperature distribution in DC casting resulted in varied microstructures across the as-cast aluminum billet's cross-section i.e., coarse columnar grains at the periphery, feathery grains in the middle, and coarse equiaxed grains at the center. Microstructures of as-cast billet showed a Chinese-script-like structure, comprising ?-Al and intermetallic phases along grain boundaries. The results of the composition and microstructural analysis showed that the as-cast billet in this research was composed primarily of ?-Al and the intermetallic phases such as ?-Al₈Fe₂Si, ?-Al₁₂Fe₃Si₂, Al₃Fe, and Al₄MgSi. The ultimate tensile strength (UTS) and the hardness at the region near the surface were lower than those at the bulk of the billet. Meanwhile, micro-sized pores were also reported in the region near the surface of the as-cast Al billet. The average grain size of the as-cast Al billet was slightly larger in the middle, increasing from surface to center. Near-surface region exhibited lower ultimate tensile strength (UTS) and hardness than the bulk, with values increasing from the center to the surface. The macrosegregation of silicon (Si) shows a negative trend towards the surface region of the billet, a positive trend in the middle, and a negative trend at the center of the billet. The macrosegregation of magnesium (Mg) elements shows a negative trend along the cross-section of the billet. The segregation of iron (Fe) elements tends to increase from the surface to the center of the billet. Recycling aluminum scrap into billet using direct chill casting is a promising method for the future. However, based on the findings in this research, the presence of the ?-Al₅FeSi intermetallic phase and the macrosegregation elements in as-cast billet are a problem that needs to be handled since it has a detrimental effect on the mechanical properties of the DC casting as-cast billet. Further efforts should be made to improve the quality of as-cast aluminum billet by utilizing aluminum scrap. Several works can be done such as controlling the raw material scrap, standardizing the alloy composition, refining the grain structure, and homogenizing the recycled as-cast billet after the direct chill casting process.

Kata Kunci : direct chill casting, aluminum scrap, recycling, microstructure, mechanical properties