Spring-back and metal flow in forming shallow dishes by explosives

Disks of the aluminium alloys DTD.687 and HS.10, in the fully heat-treated condition, have been formed into a shallow spheroidal-section cavity in a steel die using water to transmit the shock pulse.

The main conclusion is that when large charges are employed, a movement of metal takes place towards the centre (the pole position), giving a progressive increase in thickness, or buckling if the metal is of thin gauge. This effect is attributed to the high pressure produced by the impact of the specimen on the die, the line of contact with the die being initially an annular ring which then sweeps towards the centre of the specimen.

With the minimum charge necessary to form the dish, the metal becomes progressively thinner on approaching the centre, as would be expected from published experimental and theoretical work on free-forming (without a die).

Graphs showing spring-back from the die plotted against distance from the pole position exhibit several features which can be qualitatively explained by the changes in metal flow with increasing charge weight. With the minimum charge to ensure complete contact with the die, the spring-back of the DTD.687 alloy at the pole position was≈0·1 in., compared with a calculated value of 0·048 in. for slow-speed expansion by hydrostatic pressure. Moderate increases in the charge weight reduced the spring-back.