A numerical model for simulating the effect of strain rate on eutectic band thickness
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RESEARCH PAPER
A numerical model for simulating the effect of strain rate on eutectic band thickness J. Draxler1
˚ ¨ 2 · J. Edberg2 · L.-E. Lindgren2 · S. Singh3 · T. Raza1 · J. Andersson1 · P. Akerstr om
Received: 14 November 2019 / Accepted: 4 May 2020 © The Author(s) 2020
Abstract Large tensile strains acting on the solidifying weld metal can cause the formation of eutectic bands along grain boundaries. These eutectic bands can lead to severe liquation in the partially melted zone of a subsequent overlapping weld. This can increase the risk of heat-affected zone liquation cracking. In this paper, we present a solidification model for modeling eutectic bands. The model is based on solute convection in grain boundary liquid films induced by tensile strains. The proposed model was used to study the influence of strain rate on the thickness of eutectic bands in Alloy 718. It was found that when the magnitude of the strain rate is 10 times larger than that of the solidification rate, the calculated eutectic band thickness is about 200 to 500% larger (depending on the solidification rate) as compared to when the strain rate is zero. In the paper, we also discuss how eutectic bands may form from hot cracks. Keywords Macrosegregation · Solidification · Hot cracking · Alloy 718
1 Introduction A pure metal has the same solidus and liquidus temperatures. An alloy, on the other hand, has a solidus temperature that is always lower than the liquidus temperature. This results in the formation of a partially melted zone (PMZ) when the alloy is being welded. The PMZ is in the heataffected zone (HAZ), immediately adjacent to the fusion zone (FZ). In this zone, the base material has only been partially melted. This contrasts with the FZ where all material has been fully melted. The PMZ can be susceptible to HAZ liquation cracking, which is a type of hot crack. HAZ liquation cracking is normally intergranular and is formed by rupture of a grain boundary liquid film (GBLF) [1–3]. The rupture occurs at the terminal stage of the solidification and is caused by tensile stresses that are acting on the GBLF.
Recommended for publication by Study Group 212 - The Physics of Welding J. Draxler
[email protected] 1
University West, 46132 Trollh¨attan, Sweden
2
Lule˚a University of Technology, 97187 Lule˚a, Sweden
3
Chalmers University of Technology, 41296 G¨oteborg, Sweden
The susceptibility to HAZ liquation cracking depends on the size of the PMZ and the thickness of the GBLFs in the PMZ. For a polycrystalline alloy, without any particles or eutectic, the outer boundary of the PMZ is traced out by an isotherm that is a few degrees below the solidus temperature of the alloy. This temperature difference is because of the grain boundaries, which are high-energy sites, that slightly lower the melting temperature [1]. However, if the alloy contains particles, melting can start to occur by a eutectic reaction between a particle and the matrix at the eutectic temperature [3]. As the eutectic temperature can be significantly lower than
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