Non-isothermal tensile tests during solidification of Al–Mg–Si–Cu alloys: Mechanical properties in relation to the phenomenon of hot tearing

doi:10.1016/j.actamat.2006.06.027

Acta Materialia

Volume 54, Issue 19, November 2006, Pages 5209-5220

https://doi.org/10.1016/j.actamat.2006.06.027 Get rights and content

Abstract

An original set-up has been used to study the mechanical properties of aluminium alloys in tension during solidification with a high cooling rate (70 K s⁻¹). The mechanical behaviour of 6056 aluminium alloy with and without grain refiner has been investigated as well as that of mixtures between AA6056 and AA4047. The results show that the alloys exhibit a viscoplastic behaviour in the mushy state. A transition is observed between fracture in the mushy state and fracture in the solid state as a function of the displacement rate. This displacement rate at the transition depends on the cooling rate and on the composition of the alloy. The displacement before fracture is observed to be independent of displacement rate but to depend on the composition and on the solidification rate. Based on the observations a criterion for fracture in the mushy state is proposed. A simple rheological law describing the mechanical behaviour of the alloys is coupled to a finite element calculation giving the thermal field during the tensile test. This simulation is able to reproduce the mechanical response of the solidifying alloy during a non-isothermal test.

Introduction

The mechanical response of partially solidified metallic alloys subjected to straining is an investigation field of wide interest [1], [2], [3], [4], [5]. From the practical viewpoint, this response needs to be understood to control material processing where substantial deformation occurs in the mushy state, either on purpose as in thixo-forming [6], or inevitably as in the casting of ingots [7], [8], [9], [10], or during welding [11], [12]. In the latter processes, thermal contraction, shrinkage associated with solidification, and the boundary conditions of the process exert tensile stresses on the material in the mushy state. It is well known that these tensile stresses, when acting at low liquid fractions, can lead to macroscopic failure (hot tearing) [13], [14], [15]. It is therefore crucial to understand quantitatively the mechanical behaviour of these materials, both in terms of stress–strain response and in terms of fracture mechanisms.

Characterising the mechanical behaviour in the mushy state is not an easy task. As far as the conditions where hot tearing occurs are concerned, the interesting fractions of solid comprise between 0.9 and 1 [16], [17]. The temperature interval corresponding to this range of solid fractions may be very small, depending on the alloy system. Moreover, it has been repeatedly shown in the literature that the mechanical properties in the mushy state are highly dependent on the thermal path: in fact, the mechanical properties obtained at identical fractions of solid are very different if the material is solidified in situ before the mechanical test, or if it is re-melted from a fully solid sample [18], [19]. In addition, in many practical cases, such as during the solidification of a weld pool, deformation of the solidifying material occurs non-isothermally. The mechanical behaviour in such non-isothermal conditions has not received much attention until now [20].

This paper presents a detailed study of the mechanical behaviour of several aluminium alloys, based on AA6056 + AA4047 mixtures, during non-isothermal straining in the mushy state. Mixing these two alloys is representative of the composition of the nugget of laser welds of AA6056 plates with AA4047 filler wire. Indeed, increasing the Si content in the weld pool is known to decrease sensitivity to hot tearing [8], [15]. Constant crosshead velocity tests were carried out at relatively high cooling rates (25 and 70 K s⁻¹), the material being solidified in situ in the tensile machine before straining started at a given solid fraction. The studied parameters include the cooling rate, the strain rate, and the chemical composition of the alloy. The variation of stress during these experiments is discussed, and modelled using a constitutive law based on isothermal experiments, published in a previous paper [3]. The dependence of fracture mode on the different testing parameters is investigated, and a failure criterion is proposed.

Section snippets

Synthesis of the alloys

The AA6056 alloy was supplied by ALCAN as plates, 6 mm in thickness. It was received in the T4 condition (solution heat treated followed by natural ageing). The composition of this alloy (all concentrations in the following are in wt.%) and of the filler wire (AA4047) are given in Table 1. The filler wire was 1 mm in diameter. Different alloys were investigated: the original 6056 alloy and two alloys obtained by mixing 6056 and 4047, with a resulting Si concentration of, respectively, 2 wt.%

Microstructure and fracture surfaces

The experimental set-up used in this study was aimed at gaining a better understanding of the occurrence of hot tearing during solidification processes which involve fast cooling rates associated with tensile stresses, and particularly during laser welding. Thus, one important point is to reproduce the same fracture mode as encountered in hot tearing of laser welds of the same materials, and to check that the solidification microstructure is qualitatively the same as that in the nugget of such

Mechanical properties of the mushy state before fracture

One of the important results reported above is that the mechanical behaviour in the mushy state can be classified in two characteristic categories when a constant displacement rate is applied during solidification. For high displacement rates, the curves exhibit only a fast stress increase and fracture occurs rapidly. For slow displacement rate, the curves exhibit a more complex shape. When the tensile strain rate is applied, there is an initial increase of the stress to a value of 0.5–1 MPa.

Conclusions

An original experimental set-up was developed to study the fracture behaviour of Al–Mg–Si–Cu alloys during non-isothermal tensile tests carried out in the mushy state during solidification. The mush exhibits a viscoplastic behaviour and shows a transition between fracture in the solid state and fracture in the semi-solid state when the imposed displacement speed is decreased. This critical speed for the occurrence of rupture in the mushy state depends on the composition of the alloy and on the

Acknowledgements

The authors are grateful to the French Ministry of Industry for funding in the framework of the ASA (Allègement des Structures dans l’Aéronautique) RNMP project, carried out in collaboration with EADS and ALCAN, which are also thanked for providing the materials.

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Non-isothermal tensile tests during solidification of Al–Mg–Si–Cu alloys: Mechanical properties in relation to the phenomenon of hot tearing

Abstract

Introduction

Section snippets

Synthesis of the alloys

Microstructure and fracture surfaces

Mechanical properties of the mushy state before fracture

Conclusions

Acknowledgements

Acta Mater

Mater Sci Eng A

Int J Plast

Mater Sci Eng A

Mater Sci Eng A

Mater Sci Eng A

Metall Mater Trans A

Mater Sci Eng A

Proceedings of TMS light metals

Solidif Casting Met

Metall Mater Trans B

Metall Mater Trans A

Welding Res

Scand J Metall

J Mater Process Technol