Optimasi penambahan persen berat grain refiner titanium boron terhadap sifat mekanis komposit Adc 12 berpenguat partikel SiC melalui proses pengecoran aduk sebagai material untuk brake shoe kereta api = Optimization of titanium boron weight percent addition on mechanical properties of the Adc 12 with SiC reinforced composite through the process of stir casting for the application of train brake shoe