Numerical Simulation of Liquidation Cracking Susceptibility in Nickel-Based Superalloy Laser Welding

Literature Overview

This study employs finite element numerical simulation to predict and analyze the liquidation cracking (also known as solidification cracking or hot cracking) susceptibility of nickel-based superalloys during laser welding processes. Nickel-based superalloys such as Inconel 625, Inconel 718, Hastelloy C276, and Monel 400 are extensively used in cladding and overlay applications for their exceptional corrosion resistance and high-temperature strength, but their weldability is notoriously challenging due to a high propensity for solidification cracking. This research provides a quantitative framework for predicting cracking susceptibility based on process parameters and material composition.

Theoretical Framework

Cracking Susceptibility Index

The study utilizes the Scheil-Gulliver solidification model combined with the Rosenthal heat conduction equation to calculate the solidification cracking susceptibility index. The key parameter is the solidification temperature range ΔT, defined as the difference between the liquidus and solidus temperatures of the weld pool alloy composition. A wider ΔT correlates with a greater volume fraction of interdendritic liquid at the end of solidification, which increases susceptibility to solidification cracking.

The cracking susceptibility is further evaluated using the following dimensionless parameter:

• Solidification rate: R (μm/s)
• Thermal gradient: G (K/mm)
• G/R ratio: This ratio determines the solidification morphology — higher G/R favors dendritic growth with more interdendritic liquid retention

Process Parameter Effects

The numerical simulation reveals that laser welding parameters have a profound influence on cracking susceptibility through their effect on the thermal history of the weld pool:

Core Technical Findings

Melt Pool Geometry and Cracking

The simulation shows that the melt pool geometry is a critical determinant of cracking susceptibility. A deep, narrow melt pool (achieved by high power density and small beam diameter) produces a high thermal gradient G but a low solidification rate R, resulting in a moderate G/R ratio and dendritic solidification with significant interdendritic liquid. Conversely, a shallow, wide melt pool (achieved by lower power density and larger beam diameter) produces a low thermal gradient but a high solidification rate, which can actually increase cracking susceptibility due to the large volume of interdendritic liquid.

The optimal process window for minimizing cracking in Inconel 625 laser welding is identified as: laser power 2.0–3.0 kW, scan speed 150–250 mm/min, beam diameter 0.3–0.5 mm, and focal position 0–2 mm above the surface. Within this window, the predicted cracking susceptibility index remains below the critical threshold for crack initiation.

Compositional Segregation Effects

The Scheil-Gulliver simulation reveals significant microsegregation of elements such as niobium, titanium, and carbon in the interdendritic regions. These elements form low-melting-point phases (such as NbC and TiC) that remain liquid until the final stages of solidification, creating thin liquid films between dendrite arms. These liquid films are the preferential sites for crack initiation under tensile stress from shrinkage during solidification. The simulation predicts that the volume fraction of residual liquid at the end of solidification can reach 5–12 percent for Hastelloy C276, compared to only 2–4 percent for Inconel 625.

Preheating and Interpass Temperature Effects

The study demonstrates that preheating the base material to 200–300 °C can reduce cracking susceptibility by 20–35 percent by increasing the effective thermal gradient and reducing the cooling rate. However, excessive preheating above 400 °C can increase susceptibility by widening the melt pool and extending the time spent in the brittle temperature range. The optimal preheat temperature for Inconel 625 laser welding is identified as 250 ± 25 °C.

Engineering Practice Implications

For engineers performing laser cladding or laser welding of nickel-based superalloys, this study provides a systematic approach to process optimization. The recommended approach is to first identify the cracking susceptibility index of the specific alloy composition, then select process parameters that minimize the index within the practical constraints of the equipment. Post-weld heat treatment (PWHT) at 980–1010 °C for 1 hour followed by air cooling can relieve residual stresses and partially heal microcracks, but it cannot eliminate solidification cracks that have fully propagated. Prevention through process optimization is therefore preferred over post-weld repair.

Study Insights and Reflections

The most valuable contribution of this research is the quantitative prediction framework that allows engineers to assess cracking risk before committing to actual welding trials. This is particularly important for expensive nickel-based superalloy cladding applications where material cost can exceed 100 times that of conventional carbon steel. The numerical approach enables virtual screening of process parameters and compositional modifications, reducing the number of physical trials required and accelerating the development of qualified welding procedures. The study also highlights the importance of understanding the fundamental metallurgical mechanisms behind cracking susceptibility — without this understanding, empirical parameter optimization can lead to unexpected failures when process conditions change slightly. This research should serve as a reference for developing qualification procedures for laser welding of nickel-based alloys in accordance with NB/T 47014 and ASME IX.