Microstructure and corrosion evolution of additively manufactured aluminium alloy AA7075 as a function of ageing

Oumaïma Gharbi,Nick Birbilis,Paula Roberta De Lima,Shravan Kumar Kairy,Derui Jiang,Juan Nicklaus

doi:10.1038/s41529-019-0101-6

Abstract

Additively manufactured high strength aluminium (Al) alloy AA7075 was prepared using selective laser melting (SLM). High strength Al-alloys prepared by SLM have not been widely studied to date. The evolution of microstructure and hardness, with the attendant corrosion, were investigated. Additively manufactured AA7075 was investigated both in the “as-produced” condition and as a function of artificial ageing. The microstructure of specimens prepared was studied using electron microscopy. Production of AA7075 by SLM generated a unique microstructure, which was altered by solutionising and further altered by artificial ageing—resulting in microstructures distinctive to that of wrought AA7075-T6. The electrochemical response of additively manufactured AA7075 was dependent on processing history, and unique to wrought AA7075-T6, whereby dissolution rates were generally lower for additively manufactured AA7075. Furthermore, immersion exposure testing followed by microscopy, indicated different corrosion morphology for additively manufactured AA7075, whereby resultant pit size was notably smaller, in contrast to wrought AA7075-T6.

Highlights

A recent study discovered the predominant presence of a nanometre scale icosahedral quasicrystalline particle, termed ν-phase.[13]. This (ν) phase was comprised of Zn–Mg–Cu(–Al), a structurally and chemically unique in the Al-matrix of as-SLMed AA7075.13 As a consequence of this discovery, a more detailed study upon as-SLMed AA7075, including as a function of artificial ageing, is essential in the context of understanding microstructural evolution—and the corrosion response in contrast to wrought AA7075-T6
Ageing resulted in pits to discuss the role of pitting developed upon SLMed AA7075 in with size greater along grain boundaries than within grains
The total fatigue life decreased with the severity of corrosion damage,[54,55,62] the number of cycles to initiate a fatigue crack did not depend on the depth of pits.[63,64]

Summary

INTRODUCTION

Metal additive manufacturing (AM) is a promising technology for complex and net-shape prototyping and production, with minimal waste generation.[1,2,3,4,5] With the prospect of net shape production of high strength aluminium (Al) alloys,[6,7,8,9,10,11,12] the interest in AM technology to produce components from high strength 7xxx series (Al–Zn–Mg(–Cu)) Al-alloys is increasing.[6,13,14,15,16,17,18] To date, the microstructural evolution and corrosion behaviour of AM prepared 7xxx series Al-alloys have not been studied and not understood in contrast to wrought 7xxx series Al-alloys presently used in aerospace applications. Several second phase particles (including dispersoid phases or non-strengthening constituent particles), such as Al7Cu2Fe, Al3Fe, Mg2Si, Al2Cu, Al2CuMg, MgZn2, Al6Mn, Al3Ti and Al3Zr, with varying size, shape and distribution have been previously determined to populate wrought AA7075.19–28 Second phase particles influence corrosion behaviour of wrought AA7075.26,29–35. A recent study discovered the predominant presence of a nanometre scale icosahedral quasicrystalline particle, termed ν-phase.[13] This (ν) phase was comprised of Zn–Mg–Cu(–Al), a structurally and chemically unique in the Al-matrix of as-SLMed AA7075.13 As a consequence of this discovery, a more detailed study upon as-SLMed AA7075, including as a function of artificial ageing, is essential in the context of understanding microstructural evolution—and the corrosion response in contrast to wrought AA7075-T6. The second phase particles in as-SLMed AA7075 were mostly spherical; some elongated and thread-like

RESULTS

DISCUSSION

METHODS

10 DATA AVAILABILITY