Pressure dependence of age-hardenability of aluminum cast alloys and coarsening of precipitates during hot isostatic pressing

doi:10.1016/j.msea.2019.04.077

Materials Science and Engineering: A

Volume 757, 29 May 2019, Pages 62-69

https://doi.org/10.1016/j.msea.2019.04.077 Get rights and content

Abstract

The application of hot isostatic pressing is often mandatory to improve the fatigue resistance of aluminum cast alloys in order to meet the high quality requirements defined by automotive and aircraft industries. A pressure difference of 75 MPa between heat treatment and hot isostatic pressing affects the diffusivity and the precipitation kinetics. It is shown that the coarsening of precipitates during hot isostatic pressing is slower compared to the coarsening of precipitates during heat treatment. It is supposed that the high pressure within this densification process reduces the density of vacancies and therefore decreases the diffusivity of silicon and magnesium atoms, resulting in a lower critical cooling rate at which an oversaturated condition of dissolved silicon and magnesium atoms within the aluminum matrix can be achieved. Thus, quenching following hot isostatic pressing can be performed in slightly modified standard hot isostatic presses at lower cooling rates than quenching during standard heat treatment and even then does result in high age-hardenability of the alloy. It becomes possible to combine hot isostatic pressing and solution annealing within a single process step.

Introduction

The high strength of age-hardenable Al–Si–Mg cast alloys is caused by a fine distribution of different intermetallic phases, in particular the $β^{″}$ -phase that precipitates from an oversaturated solid solution during artificial aging [1,2].

The process of pre-precipitate clustering and the formation of precipitates in Al–Si–Mg alloys is reported to be [[3], [4], [5], [6], [7]]:

$α_{oversat} \to$ clusters of Si atoms $\to$ spherical GP-I-zones $\to$ needle-like GP-II-zones $/ β^{″} \to$ rod-like $β^{'} ({Mg}_{2} Si - hcp)$ + lath-like precipitates $\to β ({Mg}_{2} Si - fcc)$ + Si with various morphologies.

The precipitation of these phases is driven by the diffusion of the substitutionally dissolved alloying elements silicon and magnesium. The diffusivity of alloying elements is exponentially increasing with temperature. Precipitates of the type $β ({Mg}_{2} Si)$ as well as silicon precipitates are formed at high temperatures. Their size is predominantly large and they are known to be less effective for an increase of the material's strength. Solution annealing heat treatment is used to form an oversaturated condition of magnesium and silicon atoms dissolved in the aluminum matrix [3,8]. High cooling rates are essential after solution annealing in order to avoid precipitation at high temperatures. If cooling is performed faster than a critical cooling rate, which is documented to be around 4 K/s for the alloy AlSi7Mg0.3 at atmospheric pressure, sufficient amounts of the alloying elements silicon and magnesium are in oversaturated solid solution after solution annealing. The strength relevant $β^{″}$ -phase can then precipitate from solid solution during artificial aging.

Pores and the interface regions between large precipitates and the aluminum matrix are preferred locations for crack initiation [[9], [10], [11], [12], [13], [14]]. It is known that the fatigue resistance of aluminum alloys processed via sand casting is depending on the loading as well as on the shape, the size and the orientation of pores within the relevant cross-section [15,16].

The inner porosity of the material can be decreased by hot isostatic pressing due to thermally activated dislocation creep and diffusional creep thereby increasing the material's fatigue resistance by several orders of magnitude [17,18].

Due to their limited cooling capability, standard hot isostatic presses cannot provide sufficiently high cooling rates to obtain an oversaturated solution of silicon and magnesium atoms in the aluminum matrix during cooling after hot isostatic pressing [19]. Therefore, solution annealing, quenching and aging are to be performed following hot isostatic pressing [20,21]. However, enhanced cooling capabilities offered by modern hot isostatic presses could enable combined densification and solution annealing [22,23].

Hot isostatic pressing of aluminum cast alloys is typically performed at a temperature of 540 °C and a pressure of 75 MPa in Argon atmosphere [18,19]. To the knowledge of the authors, it has not been clarified up to now in which way the high pressure during hot isostatic pressing affects the age-hardenability of aluminum cast alloys.

The present work studies the age-hardenability of the aluminum cast alloy A356 after hot isostatic pressing at a pressure of 75 MPa and after heat treatment in vacuum ( $p_{SA}$  = 5⋅10⁻² Pa). An inductive furnace was constructed to convert the temperature-over-time profile measured during hot isostatic pressing into a thermal treatment under vacuum condition.

The pressure dependence of the coarsening behavior of precipitates is studied after both heat treatment and hot isostatic pressing. The coarsening behavior is used as indicator for the pressure affected diffusivity of the alloying elements silicon and magnesium.

Section snippets

Casting

A schematic illustration of the aluminum castings which were produced via sand casting (Georg Fischer AG, Germany) is shown in Fig. 1. Prior to casting the melt was refined by the addition of strontium. A spark emission spectrometer of the typ SPECTROMAXx LMM16 (Spectro, Germany) was used to analyze the chemical composition of the alloy, see Table 1. The gassing agent Hydral 40 (Foseco, UK) was added to the melt to achieve a casting porosity of $V_{P}$  = 0.17% for our study. Cylinders (diameter

Mechanical properties

Fig. 5 shows the tensile test results for the as-cast condition, the hot isostatically pressed series HIP 540°C 2h -1K/s and HIP 540°C 2h -1K/s -A and the heat treated sample series HT 540°C 2h -1K/s and HT 540°C 2h -1K/s -A. Fig. 6 shows the hardness and the fatigue resistance of the same conditions.

Comparing the yield strength, the ultimate tensile strength and the hardness of the hot isostatically pressed sample series HIP 540°C 2h -1K/s and HIP 540°C 2h -1K/s -A and the heat treated sample

Conclusion

Not only fatigue resistance but also yield strength, ultimate tensile strength and hardness of the age-hardenable aluminum cast alloy A356 were higher after hot isostatic pressing than after standard heat treatment alone. Other than the difference in the fatigue resistance the higher quasi-static mechanical properties cannot be explained by the elimination of the pores that induce cracks at varying loads during fatigue testing. The minor increase in the relevant cross section cannot be made

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Pressure dependence of age-hardenability of aluminum cast alloys and coarsening of precipitates during hot isostatic pressing

Abstract

Introduction

Section snippets

Casting

Mechanical properties

Conclusion

Scripta Mater.

Acta Mater.

Acta Mater.

Acta Mater.

Acta Mater.

Mater. Sci. Eng.

J. Light Met.

Mater. Sci. Eng.

Mater. Sci. Eng.

J. Mater. Process. Technol.

J. Mater. Process. Technol.

Mater. Sci. Eng.

J. Eur. Ceram. Soc.

Acta Metall.

Calphad

Acta Mater.

Acta Metall.

The structure of the metastable precipitates formed during ageing of an Al–Mg–Si alloy

Phil. Mag.

A calorimetric study of precipitation in commercial aluminium alloy 6061

J. Mater. Sci. Lett.

The ageing characteristics of aluminium alloys - electron-transmission studies of Al–Mg–Si alloys

J. Inst. Met.

Influence of casting defect and SDAS on the multiaxial fatigue behaviour of A356-T6 alloy including mean stress effect

Int. J. Fatigue

Solidification and precipitation behaviour of Al-Si-Mg casting alloys

J. Mater. Sci.

Porosity control and fatigue behavior in A356-T61 aluminium alloy

AFS Transactions

Effects of solidification structure on short fatigue crack growth in Al–7%Si–0.4%Mg alloy castings

Mater. Sci. Eng.

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Wärmebehandelndes Heißisostatisches Pressen von Aluminiumgusslegierungen, Dissertation

Heißisostatisches Pressen von Aluminium- und Magnesiumguß, Dissertation

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Heat treatment of aluminum castings combined with hot isostatic pressing

Combined hot Isostatic pressing and heat treatment of aluminum A356 cast alloys

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Kombiniertes Heißisostatisches Pressen (HIP) und Wärmebehandlung von einer A356 Aluminiumgusslegierung

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Aluminium und Aluminiumlegierungen – Gussstücke – Chemische Zusammensetzung und mechanische Eigenschaften