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Hot workability of aluminum particulate composites

Last modified: 2013-06-27

#### Abstract

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].

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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