The Interaction between Grains during Columnar-to-Equiaxed Transition in Laser Welding: A Phase-Field Study

Lingda Xiong,Ping Jiang,Chunming Wang,Zhimin Wang

doi:10.3390/met10121647

Lingda Xiong, Ping Jiang + Show 2 more

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https://doi.org/10.3390/met10121647

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Journal: Metals	Publication Date: Dec 7, 2020
Citations: 8	License type: CC BY 4.0

Affiliation: Huazhong University of Science and Technology

Abstract

A phase-field model was applied to study CET (columnar-to-equiaxed transition) during laser welding of an Al-Cu model alloy. A parametric study was performed to investigate the effects of nucleation undercooling for the equiaxed grains, nucleation density and location of the first nucleation seed ahead of the columnar front on the microstructure of the fusion zone. The numerical results indicated that nucleation undercooling significantly influenced the occurrence and the time of CET. Nucleation density affected the occurrence of CET and the size of equiaxed grains. The dendrite growth behavior was analyzed to reveal the mechanism of the CET. The interactions between different grains were studied. Once the seeds ahead of the columnar dendrites nucleated and grew, the columnar dendrite tip velocity began to fluctuate around a value. It did not decrease until the columnar dendrite got rather close to the equiaxed grains. The undercooling and solute segregation profile evolutions of the columnar dendrite tip with the CET and without the CET had no significant difference before the CET occurred. Mechanical blocking was the major blocking mechanism for the CET. The equiaxed grains formed first were larger than the equiaxed grains formed later due to the decreasing of undercooling. The size of equiaxed grain decreased from fusion line to center line. The numerical results were basically consistent with the experimental results obtained by laser welding of a 2A12 Al-alloy.

Highlights

CET occurs when the growth path of columnar grains is blocked by the equiaxed grains form ahead of the columnar front
The results indicated that the columnar zone length and the equiaxed grain size increased with the decrease of cooling rate [11]
We found that the maximum of undercooling ahead of S/L interface during linear growth stage was so large (>17 K) that if the seeds for nucleation were set at the beginning of the simulation, CET would occur before S/L underwent a Mullins–Sekerka instability and columnar dendrites grew when nucleation undercooling was smaller than 17 K

Summary

Introduction

CET (columnar-to-equiaxed transition) occurs when the growth path of columnar grains is blocked by the equiaxed grains form ahead of the columnar front. CET significantly changes the morphology and size of microstructure [1]. It is observed that CET often happens during the solidification process in the molten pool after welding. The microstructure in the fusion zone determines the mechanical properties of welding joints [2]. A better understanding of the CET is of great importance to obtain weld joints with high quality. Numerous experiments have been made to reveal the nucleation mechanism and the factors that affect the CET during solidification process. Gandin et al conducted a directional solidification experiment with Al–Si alloy. The experimental results showed that nuclei of equiaxed grains may come from heterogeneous nucleation or dendrite arm detachment/fragmentation [3].

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