Studi pengaruh kombinasi komposisi 0,02 wt% Sr dan 0,055, 0,078 dan 0,087 wt% Ti terhadap ketahanan aus paduan aluminium AC4B = Effect of composition combination of 0.02 wt.% Sr and 0.055, 0.078, and 0.087 wt.% Ti to wear resistance of aluminum alloy AC4B