Crack Paths 2012

Hot tensile behaviour and cavitation analysis in as-cast and

solutionized Al-5.5Mg-Zn alloys

P.Leo1, E.Cerri1 and S. Spigarelli2

1Dip.di Ingegneria dell’Innovazione,Università del Salento, via per Arnesano 73100

Lecce (Italy), paola.leo@unisalento.it

2 D i p . di Ingegneria Industriale e Scienze Matematiche, Università Politecnica delle

Marche, v. Brecce Bianche 60131- Ancona (Italy), s.spigarelli@univpm.it

ABSTRACTH.ot tensile test have been performed on Al-Zn-Mg alloy at 250-400°C and

10-3-10-5s-1 . At the lowest test temperature the stress-strain curves shows a gradual

softening after peak while at the other temperatures a plateau is obtained. The

microstructural analysis of the polished longitudinal surface of all deformed samples

shows that the alloy exhibits cavitation. The cavities have been detected mainly at grain

boundaries and particles inclusions. The phenomena limits the hot workability of

materials and can cause premature fracture.

This paper studies both the relationship between flow curve behaviour and

microstructural

evolutions and between cavitation, microstructure and process

parameters. The main conclusions are the following:

1) Precipitated Zn-Mg particles not are active in crack nucleation, in fact at 250°C

cavitation remains almost constant as strain rate decreases. The role of nucleation

point of the cavity seems to be related to the constituent particles and segregation as

shown by strong reduction of cavitation in the sample solutionized at 490°C-2h before

tensile deformation at 250 and 400°C

2) In the investigated range of experimental conditions, the cavity growth is mainly

driven by plastic straining and the fracture type is transgranular ductile.

I N T R O D U C T I O N

Al-Zn-Mg alloys (without Cu addition) are employed for land transport and

architectural applications. They have high strength (good response to age hardening),

good weldability, low quench sensitivity, good corrosion resistance (due to the absence

of Cu addition) and good extrudability (higher than 6061 alloy) [1-5].

During hot working of aluminium alloy, generally dynamic recovery (DRV) is the sole

restoration mechanism. As a consequence, the flow curves (stress Vs strain;

Vs )

exhibit strain hardening (SH) to a steady state regime even if a peak of stress followed

by rapid softening can be observed due respectively to dynamic precipitation (DPN)

and particles coarsening [6-10]. In this study the flow curve behaviour has been justified

and related to microstructural evolution induced by both temperature (by static heat

treatment) and deformation.

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