Pengaruh Variasi Fraksi Volume Penguat SiC terhadap Karakteristik Komposit Alumunium ADC12 Melalui Proses Pengecoran Aduk Untuk Bahan Kampas Rem Kereta Api = The Effect Of Volume Fraction of SiC Reinforce on The Properties of Aluminium ADC12 Composite Through Stir Casting Process for Brake Shoe Materials of Train