Digital Repository, CONVEGNO IGF XXII ROMA 2013

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Hot workability of aluminum particulate composites
Paola Leo, Emanuela Cerri, Hugh McQueen

Last modified: 2013-06-27


This paper analyzes hot torsion flow curves [1-5], microstructures [1,2,6-9], constitutive equations
and extrusion finite element modeling (FEM) [10-13] of aluminum composites. Those results come mainly from
previous studies of prof. H.McQueen et alii. Metal-matrix composites (MMC) of 6061, 7075, 2618 and A356
alloys with Al2O3 or SiC particles ( 15-30 μm) were produced by liquid metal mixing. Aluminum alloy matrices
reinforced with particles of Al2O3 or SiC possess higher strength and stiffness as well as greater wear resistance
and improved high temperature properties [14-17]. MMC produced by liquid metal mixing are secondarily
fabricated by traditional mechanical forming (extrusion, forging or rolling) [18-21]. Materials were deformed
over the temperature range 300 to 500°C and strain rates 0.1 to 4/5 s-1. At 400°C (and lower T) the strength of
composites is higher than that of the alloys. With exception of 6061 and 2618, there is almost no difference in
strength at 450°C while at 500°C composites appear to be softer than the alloys. 2618 MMC exhibit lower
ductility then A356 and 6061 MMC that exhibit similar ductility. 7075 alloy and MMC decline from good
ductility at 400° to very low at higher T because of GB precipitation [1-5]. The softening of the alloys with
increasing T (and with decreasing strain rate) is due to improved DRV. The softening of composites depends
on more complicated changes in microstructure: DRX occurs to a limited extent along with dynamic recovery.
Furthermore, the composites retain heterogeneous substructures in both quenched and air cooled torsion
specimens since no static recrystallization occurred after torsion [1,2,6-9]. Constitutive analysis was developed
according to Garofalo hyperbolic stress equation and showed that the MMC increase in strength and in
activation energy QHW as alloying element and particles contents rise. The extrusion was modeled using the
finite element software DEFORMtm. This program uses a flow formulation approach and an updated
Lagrangian procedure; it possesses an automatic remeshing scheme to allow the modeling of large or localized
deformations. Extrusion was modeled for a billet with diameter 178 mm and height 305 mm, an extrusion ratio
R = 31 and ram speed VR = 2.6 or 5 mm/s in similarity to previous modeling [10-13]. The constitutive laws
determined by the torsion tests have been used in the model to calculate flow stresses. Models were developed
for the initial billet temperatures TB from 300 to 500°C. From modeling temperature T, strain ε, strain rate έ
and stress σ distribution together with TMax and PMax were determined. The results were validated by comparing
to actual extrusions. The grid distortions and distributions of ε and έ are independent of material properties. As
TB increases (from 300°C to 500°C) the composite extrusion pressure decreases towards that of the bulk
alloys. This applies: from 300 °C to 400°C for A356 and 7075 above which the composite pressure is equal or
lower than that of alloys and from 300°C to 500°C for 6061 and 2618. The maximum load increases in order of
matrix alloy 6061, A356, 7075 and 2618. The temperature increases in the same order. Because of incipient
melting in 2618 is near to 500°C, TB must be limited for this alloy. Since constitutive analysis for a new alloy,
on its adoption for a previous extrusion production, is often available, correlation of maximum extrusion
pressure PMax with activation energies QHW was made [22].

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