Pengaruh %Vf SiC terhadap sifat mekanik komposit aluminium 6061 / SiC dengan penambahan modifier Sr dan grain refiner TiB melalui metode stir casting = Effect of the addition of %Vf SiC to mechanical properties composite aluminium 6061 / SiC with the addition of modifier AlSr and grain refiner TiB through stir casting method