Study on Fatigue Strength and Fracture Behavior of Magnesium Powder Metallurgical Structural Components for Aerospace Applications

The application of magnesium powder metallurgical structural components in the aerospace field (such as unmanned aerial vehicle frames and satellite supports) is becoming increasingly widespread due to their lightweight advantages, but their fatigue strength and fracture behavior directly affect the safety of component service. The research focuses on microstructural characteristics, load environment effects, and failure mechanism analysis, aiming to provide theoretical support for the design and structural optimization of high-performance magnesium alloys.

1、 The regulatory mechanism of microstructure on fatigue performance

The fatigue strength of magnesium powder metallurgical components is closely related to powder particle size, densification degree, and second phase distribution:

Powder particle size effect

Components prepared with fine powder (<50 μ m) are prone to forming "microcrack initiation zones" due to the presence of more sintering interfaces, but the strengthening effect of fine grains can increase the fatigue limit by 10% -15% (such as AZ91 alloy from 60 MPa to 70 MPa).

After sintering, the porosity of coarse powder (>100 μ m) is relatively high (>5%), and the pores act as stress concentration sources, leading to premature initiation of fatigue cracks.

Second phase distribution

The introduction of rare earth elements (such as Y and Nd) forms thermally stable phases such as Mg ₂₄ Y ₅ and Mg ₁₂ Nd, which can pin dislocations and hinder crack propagation. For example, the nanoscale precipitates in Mg-Gd-Y alloy reduce the fatigue crack propagation rate by 30%.

The undissolved coarse Mg ₁₇ Al ₁₂ phase (such as AZ91 alloy) is prone to debonding from the matrix under cyclic loading, forming the starting point of microcracks.

2、 Multiscale characteristics of fatigue fracture behavior

Crack initiation stage (<1 mm)

Mainly caused by surface processing defects (such as grinding scratches), internal pores, or detachment of the second phase interface. In high cycle fatigue (>10 ⁶ cycles), the resident slip band (PSB) caused by grain boundary slip is a key factor in crack initiation.

Crack propagation stage

Low speed expansion zone (da/dN<10 ⁻⁶ mm/cycle): fatigue striations can be seen on the fracture surface, and cracks propagate along the transgranular path, significantly affected by grain orientation.

High speed expansion zone (da/dN>10 ⁻⁴ mm/cycle): Cracks turn and propagate along the grain, accompanied by the generation of secondary cracks, and the fracture surface presents a sugar like morphology.

Instantaneous fracture stage

When the crack size exceeds the critical value (K ₁ C), the component undergoes brittle fracture, and the fracture surface shows a mixture of ductile dimples and tearing edges.

3、 The influence of load environment and surface state

Characteristics of alternating load

When the stress ratio (R=σ -min/σ _max) decreases, the crack closure effect weakens and the propagation rate significantly increases. For example, the fatigue life of AZ31 alloy is reduced by 50% when R=-1 compared to when R=0.1.

Random spectral loading in vibration loads (such as aircraft turbulence loads) is more likely to trigger multi-source crack initiation than sine wave loading.

Surface strengthening treatment

Shot peening strengthening: Introducing a 100-300 MPa compressive stress layer on the surface can increase fatigue life by 2-3 times. For example, the fatigue limit of Mg-3Al-1Zn alloy increased from 55 MPa to 75 MPa after shot peening.

Laser shock peening: The shock wave induces surface grain refinement (at the nanometer level) and an increase in dislocation density, suppressing crack initiation.

4、 Failure prediction and structural optimization

Fatigue life prediction model

The Paris formula based on fracture mechanics (da/dN=C (Δ K) ⁿ) can describe the crack propagation process, and combined with CTOD (crack tip opening displacement) testing, the remaining life can be estimated.

Machine learning models (such as BP neural networks) improve prediction accuracy by 20% compared to traditional empirical formulas by training microstructure parameters (porosity, grain size) and fatigue data.

Structural topology optimization

By using finite element simulation (such as ANSYS) to identify high stress concentration areas (such as bolt holes and chamfers), variable cross-section design or circular arc transition is adopted to reduce stress gradients, resulting in a 15% -20% increase in fatigue strength.

5、 Challenge and Development Direction

The current research difficulty lies in the fatigue failure mechanism under multi field coupling (force heat corrosion) and the fatigue reliability assessment of complex geometric components. In the future, it is necessary to combine in-situ CT scanning technology to track crack evolution in real time, develop magnesium based composite materials (such as Mg GNP) to improve fatigue fracture resistance, and establish a full life cycle monitoring system based on digital twins, providing key technical support for the lightweight and high reliability design of aerospace components.