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. 2018 Apr 3;11(4):547.
doi: 10.3390/ma11040547.

The Influence of Milling and Spark Plasma Sintering on the Microstructure and Properties of the Al7075 Alloy

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Free PMC article

The Influence of Milling and Spark Plasma Sintering on the Microstructure and Properties of the Al7075 Alloy

Orsolya Molnárová et al. Materials (Basel). .
Free PMC article

Abstract

The compact samples of an Al7075 alloy were prepared by a combination of gas atomization, high energy milling, and spark plasma sintering. The predominantly cellular morphology observed in gas atomized powder particles was completely changed by mechanical milling. The continuous-like intermetallic phases present along intercellular boundaries were destroyed; nevertheless, a small amount of Mg(Zn,Cu,Al)₂ phase was observed also in the milled powder. Milling resulted in a severe plastic deformation of the material and led to a reduction of grain size from several µm into the nanocrystalline region. The combination of these microstructural characteristics resulted in abnormally high microhardness values exceeding 300 HV. Consolidation through spark plasma sintering (SPS) resulted in bulk samples with negligible porosity. The heat exposition during SPS led to precipitation of intermetallic phases from the non-equilibrium microstructure of both gas atomized and milled powders. SPS of the milled powder resulted in a recrystallization of the severely deformed structure. An ultra-fine grained structure (grain size close to 500 nm) with grains divided primarily by high-angle boundaries was formed. A simultaneous release of stored deformation energy and an increase in the grain size caused a drop of microhardness to values close to 150 HV. This value was retained even after annealing at 425 °C.

Keywords: gas atomized Al7075 alloy; mechanical milling; microhardness; microstructure; spark plasma sintering.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
The microstructure of the Al7075_A powder: (a) morphology (light microscopy (LM)); (b) internal microstructure in backscattered electron (BSE) contrast; (c) electron back-scatter diffraction (EBSD) micrograph.
Figure 2
Figure 2
The microstructure of the Al7075_M powder particles: (a) morphology (LM); (b) internal microstructure in BSE contrast, scanning electron microscopy (SEM).
Figure 3
Figure 3
The X-ray diffraction (XRD) pattern of the Al7075_A and Al7075_M powders.
Figure 4
Figure 4
The microstructure of the milled powder, Al7075_M: (a) elongated grains, transmission electron microscopy (TEM) and (b) nanometer-sized second phase particles, STEM-HAADF.
Figure 5
Figure 5
The microstructure of the Al7075_AC compact: (a) BSE contrast image; (b) detail of the BSE contrast image, SEM.
Figure 6
Figure 6
The EBSD micrograph of the Al7075_AC compact, (a) inverse pole figures; (b) distribution of grain boundaries, boundaries with misorientation angles above 15° in blue, boundaries between 5° and 15° in red.
Figure 7
Figure 7
The microstructure of the Al7075_MC compact. (a) BSE contrast image; (b) detail of the BSE contrast image, SEM.
Figure 8
Figure 8
The EBSD micrograph of the Al7075_MC compact, (a) inverse pole figures; (b) distribution of grain boundaries, boundaries with misorientation angles above 15° in blue, boundaries between 5° and 15° in red.
Figure 9
Figure 9
The XRD patterns of the Al7075_AC and Al7075_MC compacts.
Figure 10
Figure 10
The microstructure of the Al7075_AC compact (a) and its detail (b), TEM.
Figure 11
Figure 11
The microstructure of the Al7075_MC compact: (a) grain structure, STEM-BF; (b) particles; (c) detail of particles, STEM-HAADF.
Figure 12
Figure 12
The microstructure of the Al7075_AC_HT compact (annealed 425 °C/1 h): (a) BSE contrast image; (b) EBSD inverse pole figure; (c) distribution of grain boundaries, boundaries with misorientation angles above 15° in blue, boundaries between 5° and 15° in red.
Figure 12
Figure 12
The microstructure of the Al7075_AC_HT compact (annealed 425 °C/1 h): (a) BSE contrast image; (b) EBSD inverse pole figure; (c) distribution of grain boundaries, boundaries with misorientation angles above 15° in blue, boundaries between 5° and 15° in red.
Figure 13
Figure 13
The microstructure of the Al7075_MC_HT compact (annealed 425 °C/1 h): (a) BSE contrast image; (b) EBSD inverse pole figure; (c) distribution of grain boundaries, boundaries with misorientation angles above 15° in blue, boundaries between 5° and 15° in red.
Figure 14
Figure 14
The XRD patterns of the annealed Al7075_AC_HT and Al7075_MC_HT compacts.
Figure 15
Figure 15
The microstructure of the compacts annealed 425 °C/1 h: (a) Al7075_AC_HT, TEM; (b) Al7075_MC_HT, STEM.
Figure 16
Figure 16
The microstructure of SPS compacts in BSE contrast: (a) Al7075_AC; (b) Al7075_AC_HT; (c) Al7075_MC; (d) Al7075_MC_HT, SEM.

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