Microstructure and Properties of Spark Plasma Sintered Al-Zn-Mg-Cu Alloy

Abstract

The microstructure of an aluminum alloy containing 53 wt% Zn, 2.1 wt% Mg and 1.3 wt% Cu as main alloying elements has been studied with the focus on the precipitation behavior during the spark plasma sintering process. The starting material was an atomized AlZnMgCu powder with the particle size below 50 µm. The particles showed a solidication microstructure from cellular to columnar or equiaxed dendritic morphology with a large fraction of the alloying elements segregated in form of intermetallic phases, mainly (Zn,Al,Cu)49Mg32 and Mg2(Zn,Al,Cu)11, at the cell and dendrite boundaries. The microstructure of the sintered specimens followed the microstructure of the initial powder. However, Mg(Zn,Al,Cu)2 precipitates evolve at the expense of the initial precipitate phases. The precipitates which were initially continuously distributed along the intercellular and interdendritic boundaries form discrete chain-like structures in the sintered samples. Additionally, ne precipitates created during the sintering process evolve at the new low-angle boundaries. The large fraction of precipitates at the grain boundaries and especially at the former particle boundaries could not be solved into the matrix applying a usual solid solution heat treatment. A bending test reveals low ductility and strength. The mechanical properties suer from the precipitates at former particle boundaries leading to fracture after an outer ber tensile strain of 3.8%.

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

“…Figure 7 documents the phase composition estimated using XRD. The atomized powder contains predominantly the Mg 32 (Zn,Al,Cu) 49 phase [ 20 ] with the weight fraction below 1 wt %. The fraction of this phase decreased at the expense of the Mg(Zn,Al,Cu) 2 phase during sintering.…”

“…No systematic study of oxide layers was performed in our material. Their presence can be nevertheless indirectly deduced from results of mechanical tests where fracturing was observed at surfaces of original powder particles during bending tests [ 20 ] or high temperature tensile tests [ 32 ].…”

“…New precipitates of larger size were formed at surfaces of original powder particles. The detailed EDS investigation of the microstructure [ 20 ] also revealed the presence of particles containing impurities like Fe and Si.…”

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

“…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 ].…”