Research paper
Dynamic stress-strain response of high-energy ball milled aluminium powder compacts

https://doi.org/10.1016/j.mechmat.2020.103337Get rights and content

Highlights

  • Dynamic compaction of green pellets of high-energy ball milled al powders was performed.

  • Milled and as-received powders were found to be largely insensitive to the effect of strain-rate.

  • A second order P-α model can represent the irreversible compaction of the porous pellets.

  • The strengths are primarily a function of porosity and pre-treatment, twice as high with milling.

  • The milling did not affect strain hardening coefficient or strength index.

Abstract

Ball milling is a bulk powder manufacturing process used in the creation of dispersion strengthened and nanostructured materials. Fundamentally, these powders have not been dynamically characterized in a green state prior to hot consolidation. The understanding of high strain-rate compaction on void collapse and particle interaction for such systems can help the development of predictive models for impact events of porous metallic structures that may be employed as energy absorbers, reactive structures, and intermetallic materials. This study investigates high strain-rate impact of porous green compacts of as-received and high-energy ball milled (HEBM) aluminium powders characterized under dynamic compression using a split-Hopkinson pressure bar (SHPB) in a passive confinement configuration. The plastic deformation of the powder compacts and crush up were shown to be strain-rate insensitive within the strain rate range of 1000–2100 s−1 and as a result, were modelled adequately with a second order P-α model. The as-received aluminium and HEBM aluminium powders appear to have the same strain-hardening coefficient and strength index as solid aluminium after yielding. The respective stress-strain responses of green compacts follow the same trend but differ only in strength as result of porosity and pre-strain experienced prior to dynamic compression. The HEBM powder was found to be twice as strong as the untreated as-received aluminium powder.

Introduction

High-energy ball milling is a manufacturing technique used to reduce, mix, and alloy particles in the ceramic and powder processing industries (Koch, 1993), originally created as a bulk powder process to create dispersion strengthened superalloys by the hot consolidation of ball milled powders (Benjamin, 1970). As a result, there has been extensive work done to characterize and model the quasi-static and dynamic mechanical behaviours of these dispersion strengthened alloys and nanocrystalline composites (Khan et al., 2006; Farrokh et al., 2009). Since then, it has also been used to create nano-lamellar intermetallic composites resulting in intermetallics with increased reactivity (Schwarz et al., 1992; Gunduz et al., 2014; Reeves R et al., 2010), and to entrain propellant additives in aluminium (Al) based fuel systems (Sippel et al., 2013; Terry et al., 2016; Rubio et al., 2017). However, these ball milled powder systems have not been studied under high strain-rate compression without prior sintering.

To date, it is not well understood how porosity affects the dynamic mechanical response of metallic powder systems. Dynamic characterization of high-energy ball milled (HEBM) metallic powders has broad implications for the understanding and creation of energetic structural members and energy absorbers under impact. Experimental parameters gathered from this work can aid in the modelling and testing of such impact events on Al powder-based structures.

This work aims to characterize the dynamic response of porous green compacts of HEBM and as-received Al powders after sieving the powders to an identical particle size distribution. The compaction was done using a split-Hopkinson pressure bar (SHPB) with the specimen under passive confinement, which was adapted from the study of porous geomaterials like concrete and sand (Gong and Malvern, 1990; Bragov et al., 2008). This allows for porous materials to be tested in the SHPB by applying weak radial stress. As a result, HEBM Al compacts of 70–90% theoretical maximum density (%TMD) were characterized with the SHPB in a bi-axial stress condition.

Section snippets

Material preparation

The as-received Al powder (Alfa Aesar, 99.98% purity) had a particle size range of 44–420 μm and the HEBM Al originated from a particle size of 7–15 μm. The initial 7–15 μm Al was ball milled with a planetary ball mill (Retsch GmbH, model PM100, Germany) in a 250-ml stainless steel jar with 175 g of 9.5 mm stainless-steel media. A wet milling condition was used to achieve the desired particle size with 20 ml of hexane added to the 10:1 mass ratio of media to the Al powder mixture. The jar was

Stress-strain response of as-received and HEBM Al powders

Fig. 6 provides the generated stress-strain responses for both the as-received and HEBM Al. Fig. 6a and Fig. 6c give the stress-strain curves for the two materials at the various initial densities and strain-rates. Each curve is an average of five SHPB tests at a given initial density and strain-rate taken at room temperature (20°C). The end of the stress-strain curves are defined by the end of the incident pulse. It is clearly seen that the HEBM strengthens the powder significantly; the HEBM

Conclusions

The dynamic compaction of as-received and high-energy ball milled non-sintered green compacts of Al powders have been characterized in a passive confinement configuration using the SHPB. A second order P-α model was used to model the irreversible compaction of the porous Al compacts. The dynamic responses of the compacts were found to be largely insensitive to the effect of strain-rate within the limited tested range of 1000–2100 s−1 and their strengths appear to be a function of porosity and

CRediT authorship contribution statement

A.W. Justice: Investigation, Formal analysis, Writing - original draft, Visualization. M.T. Beason: Investigation, Formal analysis, Writing - review & editing, Visualization. I.E. Gunduz: Methodology, Investigation, Writing - review & editing, Visualization, Supervision. W. Chen: Methodology, Resources, Writing - review & editing, Supervision, Funding acquisition. S.F. Son: Methodology, Resources, Writing - review & editing, Supervision, Funding acquisition.

Declaration of Competing Interest

All authors have participated in (a) conception and design, or analysis and interpretation of the data; (b) drafting the article or revising it critically for important intellectual content; and (c) approval of the final version.

This manuscript has not been submitted to, nor is under review at, another journal or other publishing venue.

The authors have no affiliation with any organization with a direct or indirect financial interest in the subject matter discussed in the manuscript.

Acknowledgements

We would like to thank H. Liao, B.H. Lim, and C.D. Kirk for their experimental support with the split-Hopkinson pressure bar. This work was supported by the Department of Energy, National Nuclear Security Administration, under the award number DE-NA0002377 as part of the Predictive Science Academic Alliance Program II. M.T. Beason was supported by the Department of Defense (DoD) through the National Defense Science & Engineering Graduate Fellowship (NDSEG) Program during this work.

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