Liquation cracking and its effects in aluminum alloy armatures

Experiments were performed using four different aluminum alloy armatures to study liquation cracking and its effects on armature performance. The four materials tested were Al 1100, Al 6082, Al 7075, and Al 7475. Of these, only the Al 1100 (commercially pure aluminum) armatures were free of gross cracking after exit from the launcher. The Al 6082, Al 7075, and Al 7475 armatures experienced fracture and structural deterioration as observed in high speed photographs of the in-flight bodies and the condition of the recovered armatures. Metallography of the recovered armatures revealed that liquation of the grain boundary phases in Al 7075 and Al 7475 had contributed to the structural failures in these armatures. The results suggest that a composite armature should be designed using different aluminum alloys to separately perform the sliding contact and structural functions.