Solidification Cracking in Welded Alloys

Solidification cracking is a defect that arises during the last stage of weld metal solidification, when tensile stresses act upon a semi-solid mixture of solid grains and residual liquid. As an alloy cools through its mushy zone – the temperature range between liquidus and solidus – dendritic grains form and impinge, leaving interconnected liquid channels that must feed shrinkage and accommodate strain. If the liquid fraction falls below a critical level or cannot flow rapidly enough, micro-cavities develop at grain boundaries or interdendritic regions. Under imposed strain from thermal contraction or external restraint, these cavities coalesce and propagate as cracks. Susceptibility is governed by alloy composition, solidification microstructure (cellular versus dendritic), solid fraction evolution, grain boundary character and welding parameters (heat input, cooling rate, temperature gradient and travel speed). Solidification cracking compromises structural integrity in applications ranging from aerospace aluminium assemblies to high-strength steels in automotive and power-generation equipment, and poses a significant challenge in fusion and additive manufacturing processes. Advances in alloy design, process control and predictive modelling aim to mitigate cracking by promoting fine equiaxed grains, optimising feeding paths and tailoring solidification pathways.

Research from Nature Portfolio

Recent studies have demonstrated that control of nucleation and grain growth can eliminate solidification cracking in high-strength aluminium alloys. A novel composite wire, comprising a zirconium core and aluminium shell, coupled with an oscillating laser-arc hybrid welding technique, promotes equiaxed, fine-grained solidification in 2024 aluminium alloy. The lattice-matched Zr core seeds uniform nucleation, while tailored oscillations refine the mushy zone, yielding crack-free joints with tensile strength approaching that of solid-state processes.

Foundational in situ synchrotron X-ray radiography investigations have elucidated the initiation and growth kinetics of solidification cracks in steel welds. Micro-cavities nucleate at low true strains within interdendritic liquid pockets and grow by coalescence under increasing strain. Crack propagation follows grain boundary networks from weld core to surface at millimetre-per-second speeds, revealing the interplay between local strain, triaxial stress and interdendritic permeability.

Solidification Cracking in Welded Alloys publication trend

The graph below shows the total number of articles in solidification cracking in welded alloys across all publications each year (not limited to Nature Index journals).

Technical terms

Mushy zone: Temperature interval between liquidus and solidus where solid and liquid coexist.

Primary dendrite arm spacing (PDAS): Distance between main arms of a dendritic crystal, influencing liquid permeability.

Phase-field modelling: Computational method to simulate microstructure evolution during solidification.

CALPHAD: Technique for calculating phase equilibria and solid fraction as a function of temperature and composition.

Grain coalescence: Process by which neighbouring solid grains bridge, reducing liquid continuity and affecting feeding.

References

1. High-strength and crack-free welding of 2024 aluminium alloy via Zr-core-Al-shell wire. Nature Communications (2024).
2. Initiation and growth kinetics of solidification cracking during welding of steel. Scientific Reports (2017).
3. Influence of the temperature gradient and the pulling velocity on solidification cracking susceptibility during welding: A phase field study. Materials & Design (2023).
4. Multiphase-field simulation of grain coalescence behavior and its effects on solidification cracking susceptibility during welding of Al-Cu alloys. Materials & Design (2021).
5. Predicting Susceptibility to Solidification Cracking and Liquation Cracking by CALPHAD. Metals (2021).