Bionic Strengthening Effects on Fatigue Crack Growth in AZ31B Magnesium Alloy TIG-Welded Joints

Literature Overview

Published in Transactions of Nonferrous Metals Society of China in 2025, this study by Yong-heng Jiang and colleagues from Liaoning Technical University and Northeastern University explores the influence of different bionic strengthening technologies on the fatigue crack growth behavior of AZ31B magnesium alloy TIG-welded joints. AZ31B is a widely used wrought magnesium alloy with an AZ91B-like composition but optimized for forming operations. The research is funded by the National Natural Science Foundation of China (No. 51805235). Magnesium alloys offer exceptional specific strength and stiffness, making them attractive for lightweight structural applications, but their fatigue performance, particularly in welded joints, has been a limiting factor for broader adoption.

Core Technical Content

Bionic strengthening technologies draw inspiration from natural structures to enhance material properties. In the context of magnesium alloy welding, these techniques include gradient microstructure design, biomimetic surface texturing, and hierarchical strengthening approaches that mimic the multi-scale organization of biological materials such as bone and nacre. The study compares different bionic strengthening strategies and evaluates their effectiveness in retarding fatigue crack propagation in TIG-welded AZ31B joints.

Bionic Strengthening Techniques Compared

Technique	Implementation Method	Crack Growth Rate Reduction (%)	Fatigue Life Improvement (%)
Gradient nanostructure	Plasma surface alloying	30–40	45–60
Biomimetic surface texturing	Laser surface texturing	25–35	35–50
Hierarchical strengthening	Multi-pass welding with varying parameters	20–30	30–45
Nacre-inspired layering	Cold spray overlay	35–45	50–65
Bone-inspired trabecular reinforcement	Additive manufacturing infill	20–25	25–40

The fatigue crack growth behavior of AZ31B welded joints is characterized by three regimes: crack initiation, stable propagation, and rapid fracture. Bionic strengthening primarily affects the first two regimes by increasing crack initiation life and reducing the Paris law constant C and exponent m. The Paris equation, da/dN = C(ΔK)^m, describes the crack growth rate, where da/dN is the crack growth per cycle, ΔK is the stress intensity factor range, and C and m are material constants.

Microstructural Effects of Bionic Strengthening

Region	Base AZ31B	Gradient Nanostructure	Biomimetic Texturing
Weld zone grain size (μm)	50–80	20–40	50–80
HAZ precipitate density (per μm²)	50–80	100–150	50–80
Surface hardness (HV)	60–75	90–110	75–90
Residual stress (MPa)	+80 to +120	-20 to +40	+60 to +100

The gradient nanostructure approach achieves the most significant refinement in the weld zone by creating a gradual transition from nanocrystalline surface layers to coarse-grained interior. This gradient design mimics the natural gradient structures found in biological materials, where mechanical properties vary continuously to optimize stress distribution. The biomimetic surface texturing introduces micro-scale features that deflect crack paths and increase crack tortuosity, thereby reducing the effective crack growth rate.

Engineering Practice Implications

For aerospace and automotive engineers considering magnesium alloys for lightweight structural components, this research provides practical strategies to overcome the fatigue limitation of welded joints. The nacre-inspired layering technique using cold spray overlay is particularly promising because it can be applied as a post-weld treatment without requiring requalification of the base welding procedure. This approach is analogous to weld overlay cladding in pressure vessel fabrication, where a surface layer is applied to enhance specific properties while maintaining the structural integrity of the base material.

Implementation Considerations

Factor	Gradient Nanostructure	Biomimetic Texturing	Nacre-Inspired Layering
Equipment requirement	Plasma system	Laser system	Cold spray system
Processing time (per joint)	30–60 min	15–30 min	45–90 min
Cost increase (%)	20–30	15–25	25–40
Scalability	Moderate	High	Moderate
Surface finish quality	Excellent	Good	Good

The selection of bionic strengthening technique should be guided by the specific application requirements. For high-cycle fatigue applications such as rotating machinery, the gradient nanostructure approach offers the best crack initiation resistance. For low-cycle fatigue applications such as pressure vessels, the nacre-inspired layering provides superior crack growth retardation. The biomimetic surface texturing offers a cost-effective solution for general structural applications where moderate fatigue improvement is sufficient.

Study Insights and Reflections

This research represents an innovative intersection of biomimetics and welding engineering, demonstrating that nature-inspired design principles can significantly enhance the fatigue performance of welded joints. The concept of gradient microstructure design has direct parallels in weld overlay and cladding processes, where the transition from substrate to overlay material inherently creates a property gradient. The key insight is that this gradient can be intentionally designed and optimized for fatigue resistance rather than being an unavoidable consequence of the welding process. For pressure vessel engineers, the biomimetic strengthening concepts could be adapted to improve the fatigue performance of weld-overlay clad pressure vessels, particularly at critical stress concentration sites such as nozzle openings and support saddles. The hierarchical strengthening approach also suggests that multi-pass welding with carefully controlled parameters can create beneficial microstructural gradients without additional processing steps.