The strain rate and temperature dependence of the dynamic impact properties of 7075 aluminum alloy

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Abstract

The dynamic impact properties of 7075 aluminum alloy are studied experimentally using a split Hopkinson bar. Cylindrical specimens of 10 mm height and 10 mm diameter are compressed dynamically at temperatures ranging from 25 to 300°C and at constant strain rates of from 103 to 5×103 s−1. The influence of strain rate and temperature on the mircrostructural evolution, the fracture mechanisms and the occurrence of shear localization is investigated. It is found that the compressive stress–strain response depends sensitively on the applied strain rate and test temperature. Considering the effects of strain rate, temperature, strain hardening, rate sensitivity and thermal softening of the material, a constitutive equation is used successfully to describe the dynamic impact deformation behavior of 7075 Al alloy. Microstructural observations reveal that the size of the initial coarse equi-axial grains is reduced as the strain rate and temperature increase due to dynamic recrystallization. In contrast, the second phase increases in size in response to increasing strain rate and temperature. SEM observation of the fracture surfaces makes evident an adiabatic shearing mechanism along the fracture planes accompanying crack formation.

Introduction

7075 aluminum alloy is one of the most important engineering alloys and has been utilized extensively in aircraft structures because of its high strength-to-density ratio. A considerable amount of work has been carried out on the plastic flow of this material under low strain rates and various temperatures [1], [2]. However, up to now, there has been little work concerning the systematic effects of strain rate and temperature on the plastic flow response, as well as the evolution of the microstructure, during dynamic impact deformation. From the deformability viewpoint and for structural design purposes, it is necessary to characterize the mechanical properties of 7075 Al alloy over a wide range of temperatures and strain rates up to impact loading.

The compression split Hopkinson bar (SHPB) is used widely to determine the mechanical properties of structural materials under high loading rates [3]. Using this technique, the impact response of metals, alloys and, more recently, many non-metallic and composite materials, has been studied, and several reviews of this field have been published [4], [5], [6], [7]. It is clear that, to a greater or lesser extent, most materials show a significant change in mechanical response under increased strain rates or temperatures. Several mechanisms such as dislocation damping have been proposed to understand high velocity deformation. It is generally recognized that thermally-activated strain-rate analysis can provide fundamental insight into temperature and strain rate effects on stress [8], [9].

It is also essential to clarify the structural evolution during deformation over a wide range of temperatures and strain rates. Several authors have found that the significant strain-rate and temperature dependence of the flow stress of materials such as copper and titanium alloy in the high strain-rate ranges can be understood in connection with the evolution of the structure during deformation [10], [11]. Actually, the mechanical behavior of a material depends not only on the strain rate and temperature but also on its current microstructure, and changes in microstructure result in changes of the plastic flow behavior. Hence, the establishment of more physically-based constitutive models to describe the complex loading processes of 7075 alloy requires a knowledge of the coincident influences of temperature, strain rate and microstructure on the high-strain-rate mechanical responses of the alloy.

The objective of this study is to characterize the behavior of 7075 aluminum alloy during dynamic compression using a split Hopkinson bar over a temperature range of from 25 to 300°C, at constant strain rates of from 103 to 5×103 s−1. The stress–strain relation, the evolution of the microstructure and the fracture characteristics are discussed in terms of the test conditions. A constitutive equation expressing the plastic flow behavior is developed by analysis and regression of the test results.

Section snippets

Experimental procedure

The material tested was a commercially produced 7075 aluminum alloy with the following chemical composition: Zn 5.65, Mg 2.7, Cu 1.58, Cr 0.2, Fe 0.07, Ti 0.02, Mn 0.05, balance Al (all in mass pct). This material was supplied as hot-rolled plates of 12.7 mm thickness, which were formed in the annealed (O temper) condition and subsequently heat treated to the T6 temper (120°C for 24 h). Cylindrical compression specimens of height 10 mm and diameter 10 mm were prepared from the plates with the

Stress–strain behavior

The true compressive stress–strain curves of 7075 Al alloy deformed at four temperatures under strain rates of 1300, 2400 and 3100 s−1 are shown in Fig. 2, Fig. 3, Fig. 4, respectively. The flow stress as well as the shape of the flow curves are sensitively dependent on temperature and strain rate. For all of the specimens, after initial yielding, the flow stress increases monotonically with different strain-hardening rates. Comparing these curves with one another, it is found that, for a

Conclusions

Dynamic impact experiments on 7075 Al alloy have been conducted to investigate the influence of the loading rate and the temperature on the mechanical properties and microstructure variations. The compressive stress–strain response of this material is found to depend on both the strain rate and the temperature. By means of the experimentally-determined material parameters, a proposed deformation constitutive equation is used successfully to describe the behavior of the material under dynamic

Acknowledgements

The authors would like to acknowledge both their department and the National Science Council of the Republic of China for their financial support. The grant from the NSC is numbered 86-2212-E006-015.

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