Effects of heat treatment on the ballistic properties of AA6070 aluminium alloy

Abstract

The ballistic properties of the aluminium alloy AA6070 in different tempers are studied, using target plates of 20 mm thickness in tempers O (annealed), T4 (naturally aged), T6 (peak strength) and T7 (overaged). The stressstrain behaviour of the different tempers was characterised by quasi-static tension tests and was found to vary considerably with temper in regards to strength, strain hardening and ductility. Ballistic impact tests using 7.62 mm APM2 bullets were then carried out, and the ballistic limit velocities were obtained for all tempers. In the material tests it was shown that the O-temper was most ductile and almost no fragmentation took place during the ballistic impact tests. The T6-temper proved to be least ductile, and fragmentation was commonly seen. The experiments show that despite fragmentation, strength is a more important feature than ductility in ballistic impact for this alloy, at least for the given projectile and within the velocity range investigated. A thermoelasticthermoviscoplastic constitutive relation and a ductile fracture criterion were determined for each temper, and finite element analyses were performed using the IMPETUS Afea Solver with fully integrated 3 rd -order 64-node hexahedrons. The numerical simulations predicted the same variation in ballistic limit velocity with respect to temper condition as found in the experiments, but the results were consistently to the conservative side. In addition, analytical calculations using the cylindrical cavity expansion theory (CCET) were carried out. The ballistic limit velocities resulting from these calculations were found to be in good agreement with the experimental data.

A nano-scale material model applied in finite element analysis of aluminium plates under impact loading

“…The results obtained by this approach are validated against an experimental study reported in Holmen et al [17] on the perforation of 20 mm AA6070 plates by 7.62 mm AP…”

“…The plates were heat treated to tempers O (annealed), T4 (naturally aged), T6 (peak strength) and T7 (overaged) according to Table 2. The material was characterised by quasi-static tensile tests in different in-plane directions at ambient temperature to determine the yield strength, work hardening, tensile strain to failure and plastic anisotropy, see [17] for details. It was found that the anisotropy in strength and work hardening was negligible, while the tensile strain to failure varied considerably with direction.…”

“…Flow-stress curves in terms of the equivalent von Mises stress versus the equivalent plastic strain for the 0° orientation are shown in Figure 1b). These curves were determined from the experimentally-obtained true stress-strain curves in Figure 1a) using finite element simulations in combination with an optimisation tool [17]. It is evident from these figures that the different heat treatments lead to substantial differences in the stressstrain behaviour of the materials, and this will influence the impact resistance of the plates.…”

“…The AA6070 plates in different tempers were struck by APM2 bullets fired from a smooth-bore rifle at various impact velocities [17]. The bullet consisted of a brass jacket, lead filler and an ogive-nose, hardened steel core.…”

High-velocity impact deformation and perforation of fibre metal laminates

“…Further, again in contrast to the results shown in Fig. 11 and Table 4, it is of interest to note that significantly changing the mechanical properties of an aluminium alloy, via a heat treatment, is known to affect the ballistic performance of both thin [36] and thick [37,38] monolithic aluminium alloy sheets. However, from the results shown in Fig.…”