Effects of Bionic Strengthening Technologies on Fatigue Crack Growth of AZ31B Magnesium Alloy TIG Welded Joints

Literature Overview

This study, published in Transactions of Nonferrous Metals Society of China (2025, Vol. 35, No. 10), investigates the effects of two bionic strengthening technologies on the fatigue crack growth (FCG) behavior of AZ31B magnesium alloy TIG-welded joints. The research is conducted by the College of Materials Science and Engineering at Liaoning Technical University and the Key Laboratory of Electromagnetic Processing of Materials at Northeastern University, supported by the National Natural Science Foundation of China. The work addresses a critical challenge in magnesium alloy structural applications: the inherently poor fatigue performance of welded joints under cyclic loading.

Core Technical Approach

The study compares two bionic treatment methods applied to TIG-welded AZ31B joints:

1. Laser bionic treatment, which uses a focused laser beam to create a textured surface pattern mimicking biological structures.
2. Ultrasonic impact bionic treatment, which uses ultrasonic vibration to create a patterned surface with compressive residual stresses.

Both treatments aim to refine grains at the joint surface, improve microhardness, and reduce fatigue crack growth rates. The bionic approach draws inspiration from the microstructural features observed in biological materials, which often exhibit hierarchical structures optimized for fatigue resistance.

Key Findings

The results demonstrate that both bionic treatment methods are effective in improving FCG behavior:

• Both treatments refine grains on the joint surface.
• Both treatments improve microhardness at the treated region.
• In the stable crack growth stage, both bionic samples exhibit a lower FCG rate compared to untreated samples.
• Both treatments reduce the probability of crack initiation.
• Both treatments partially promote crack deflection, which increases the crack path tortuosity and consumes more energy during crack propagation.

Technical Parameter Summary

Mechanism of Fatigue Crack Growth Improvement

The improvement in FCG behavior through bionic treatment operates through multiple synergistic mechanisms:

1. Grain refinement at the surface increases the number of grain boundaries, which act as barriers to crack propagation. When a crack encounters a grain boundary, it must change direction or nucleate a new crack in the adjacent grain, consuming additional energy.
2. Increased microhardness at the treated region raises the local resistance to crack initiation and propagation.
3. Compressive residual stresses introduced by the treatment counteract the tensile component of the applied cyclic stress, effectively reducing the stress intensity factor range at the crack tip.
4. Crack deflection caused by the textured surface pattern increases the crack path length and reduces the effective driving force for crack growth.

Comparison of Bionic Treatment Methods

While both methods show positive results, they differ in their mechanisms and practical applicability:

• Laser bionic treatment offers precise control of the surface texture geometry and can be programmed to create specific patterns. However, it requires expensive laser equipment and may introduce thermal effects that could affect the HAZ if not carefully controlled.
• Ultrasonic impact bionic treatment is simpler to implement and does not involve thermal effects. It introduces compressive residual stresses through mechanical peening. However, the texture pattern is less precisely controllable compared to laser treatment.

For engineering applications, the choice between the two methods should be guided by the specific application requirements, equipment availability, and cost considerations.

Integration with Engineering Practice

AZ31B magnesium alloy is increasingly used in lightweight structural applications in automotive, aerospace, and biomedical fields. The poor fatigue performance of welded joints has been a major barrier to broader adoption. The bionic treatment approach offers a post-weld enhancement strategy that can be applied to existing welded components without requiring changes to the welding process itself.

Practical implementation considerations include:

1. Apply bionic treatment to the weld surface and HAZ region where fatigue cracks typically initiate.
2. Verify the effectiveness of treatment through microhardness mapping and surface roughness profiling.
3. Conduct fatigue crack growth testing on treated joints to confirm the expected improvement in FCG resistance.
4. Establish acceptance criteria for the treated surface based on grain size, microhardness, and residual stress measurements.
5. Consider combining bionic treatment with other surface engineering techniques such as shot peening for synergistic effects.

Study Insights and Reflections

This study is significant because it introduces the concept of bionic strengthening to the field of magnesium alloy welding. The inspiration from biological materials, which have evolved over millions of years to optimize mechanical performance under complex loading conditions, provides a novel perspective on surface engineering for welded joints.

The finding that both methods reduce crack initiation probability and promote crack deflection is particularly encouraging. Crack deflection is a well-known mechanism for improving fracture toughness, as it increases the surface area of the crack and reduces the effective stress intensity factor. The bionic approach effectively engineers the surface to promote this beneficial mechanism.

However, the study also implies that bionic treatment alone may not be sufficient to fully address the fatigue challenges of magnesium alloy welded joints. The inherently low ductility of AZ31B and the presence of weld defects such as porosity and microcracks in the HAZ remain significant concerns. A comprehensive approach combining bionic surface treatment with optimized welding parameters, post-weld heat treatment, and rigorous non-destructive testing would likely yield the best results.

Reference Value and Outlook

This paper provides a valuable proof of concept for bionic strengthening of magnesium alloy welded joints. The demonstration that both laser and ultrasonic impact bionic treatments improve FCG behavior opens new avenues for enhancing the fatigue life of magnesium alloy structures. Future research should explore the long-term durability of bionic-treated surfaces, the effects of environmental factors such as corrosion on the treated region, and the scalability of bionic treatment to large production volumes.