TABLE OF CONTENTS For the armature to expand to its largest diameter without breaking up, a compromise between material softness and strength needed to be found. Full annealing of wrought aluminum alloys results in the greatest amount of ductility in the material and is given the temper designation "O". Full annealing of 6061 aluminum alloys can be achieved by heating the material to a temperature of 415 C (775 F), and maintaining the annealing temperature for 2 3 hours. The cooling should be conducted at a rate of 30 C/hr. (50 F/hr.) from the annealing temperature to 260 C (500 F). Rate of cooling below 260 C is unimportant.
In general, larger expansion ratios lead to better FCG performance since the initial inductance is larger for a given stator diameter. We found that partial annealing of 6061 aluminum and OFHC copper armatures worked best in producing intermediate mechanical properties needed for expansion ratios of up to ~ 3 before breakup. Partial annealing of aluminum armatures (temper designation H-2) is achieved at temperatures below those needed to produce extensive recrystallization, thereby resulting in incomplete softening due to sub-structural changes in dislocation density. For partial annealing of aluminum, we heated the armatures to 345 C (650 F), followed by subsequent cooling to room temperature. No holding time at annealing temperature, or controlled cooling rate is required for this purpose. In the case of OFHC copper armatures, partial annealing was achieved by heating the armatures to 240 C (465 F) for 20 minutes under...
