Spark Plasma Sintering of a Gas Atomized Al7075 Alloy: Microstructure and Properties

Abstract

The powder of an Al7075 alloy was prepared by gas atomization. A combination of cellular, columnar, and equiaxed dendritic-like morphology was observed in individual powder particles with continuous layers of intermetallic phases along boundaries. The cells are separated predominantly by high-angle boundaries, the areas with dendritic-like morphology usually have a similar crystallographic orientation. Spark plasma sintering resulted in a fully dense material with a microstructure similar to that of the powder material. The continuous layers of intermetallic phases are replaced by individual particles located along internal boundaries, coarse particles are formed at the surface of original powder particles. Microhardness measurements revealed both artificial and natural ageing behavior similar to that observed in ingot metallurgy material. The minimum microhardness of 81 HV, observed in the sample annealed at 300 °C, reflects the presence of coarse particles. The peak microhardness of 160 HV was observed in the sample annealed at 500 °C and then aged at room temperature. Compression tests confirmed high strength combined with sufficient plasticity. Annealing even at 500 °C does not significantly influence the distribution of grain sizes—about 45% of the area is occupied by grains with the size below 10 µm.

Nanocrystalline Al7075 + 1 wt % Zr Alloy Prepared Using Mechanical Milling and Spark Plasma Sintering

“…It is believed that, beside the applied pressure, local electric and magnetic fields can activate the powder’s surface, help to break up and remove oxide layers present on the surface of powder particles handled without an inert atmosphere, and promote full densification. Our results confirmed (in accordance with previous experiments on other Al-based alloys [ 20 , 21 , 38 , 39 ]) that SPS is very efficient in the preparation of compacts with nearly full density. The slightly lower density of the Al7075Zr_AC compact (as compared to the theoretical value) may be caused by the presence of some retained oxide particles rather than by porosity.…”

“…Both phases probably also contain Cu; however, this hypothesis cannot be proved by XRD because of the very low weight fraction of both these phases (bellow 1%). Similar results have been found in the gas-atomized Al7075 alloy with a coarser droplet size [ 21 , 22 ]. No peaks of the Al 3 Zr phase were observed despite a nearly negligible equilibrium solubility limit of Zr in Al [ 45 ].…”

“…As already mentioned in the introduction, the temperature gradient along the powder particle radius causes necks to be formed between adjacent powder particles leading to material densification, whereas the powder particle interiors are not significantly influenced and retain their original microstructure. This has been observed in many materials, including a gas-atomized Al7075 alloy [ 21 , 34 ]. The present experiments revealed that no massive grain growth occurred during SPS of the gas-atomized powder ( Figure 12 a).…”

“…A gradual decrease in the temperature gradient and in the rate of the solidification front results in a breakdown of planar solidification front and a formation of cellular, columnar, or dendritic microstructures [ 31 , 32 ]. A mixture of dendritic and cellular microstructure has been observed in the gas-atomized Al7075 alloy, with a mean droplet size of about 50 µm [ 21 , 22 , 33 , 34 ]. Due to the smaller mean droplet size of our material (about 20 µm), the cellular microstructure is the prevailing microstructure type.…”

“…A combination of gas atomization and SPS has been used for the preparation of the Al7075 alloy with the relative density exceeding 99%. The grain size of this material was in the range of between 1 and 10 µm, and remained stable even after annealing at 500 °C [ 21 , 22 ]. Even finer Al7075 material with a grain size at the nano-meter scale has been prepared by a combination of milling of elemental powders followed by hot pressing [ 18 ].…”