Advanced Applications of Industrial CT in the Inspection of Semi-Solid Forming of Magnesium Alloys

In today's era, characterized by the pursuit of automotive lightweighting and the high integration of electronic products, semi-solid forming technology for magnesium alloys is gradually displacing traditional full-liquid die casting. This shift is driven by the technology's inherent advantages, such as lower forming temperatures, minimal shrinkage rates, and a dense microstructure. However, the semi-solid process involves a complex interplay of "solid-liquid" two-phase flow, rendering traditional defect detection methods increasingly inadequate for meeting the rigorous demands of quality control. Industrial Computed Tomography (CT)—widely regarded as the pinnacle of non-destructive testing—is therefore emerging as a critical technological pillar in this field.

I. The "Specific Pathologies" of the Semi-Solid Process

Compared to standard die-cast parts, defects in semi-solid magnesium alloy components are often more insidious and complex:

Entrapped Air and Porosity: Although the mold-filling process in semi-solid forming is relatively smooth, minute air bubbles can still become entrapped during the high-velocity injection phase.

Shrinkage Cavities and Microporosity: Due to the wide solidification range of magnesium alloys, fine-scale microporosity is highly prone to occurring in thicker sections of the component.

Non-Uniform Distribution of Primary Phases: The accumulation or segregation of solid-phase particles (specifically α-Mg particles) within the flow channels directly compromises the consistency of the component's mechanical properties.

Cold Laps and Flow Marks: The semi-solid slurry possesses high viscosity; consequently, if mold temperature control is inadequate, minute, unfused interfaces can easily form within the interior of the part.

II. Core Applications of Industrial CT in Semi-Solid Inspection

1. "Qualitative, Quantitative, and Positional" Analysis of Defects

Through high-resolution tomographic scanning, industrial CT is capable of detecting minute voids and defects at the micron level.

Spatial Localization: It precisely determines the three-dimensional coordinates of shrinkage defects within the component's interior, enabling an assessment of whether these defects are situated in stress concentration zones or on surfaces designated for subsequent machining.

Volume Measurement: It automatically calculates the volume of individual defects as well as the total porosity level—metrics that are crucial for evaluating the leak-tightness and fatigue life of semi-solid components.

2. Investigation of Solid-Phase Particle Segregation

This capability represents a unique value proposition that distinguishes industrial CT from traditional X-ray inspection methods. The fundamental essence of the semi-solid process lies in the content and distribution of the solid-phase particles.

Application: By leveraging its high contrast resolution, CT can effectively differentiate between the varying density levels of the distinct phases present within the magnesium alloy matrix. By analyzing CT values within specific regions, engineers can observe the distribution of primary particles across the part's wall thickness; this, in turn, enables them to optimize the injection speed and pressure parameters of the die-casting machine.

3. Wall Thickness Analysis and Full-Dimension Comparison

Semi-solid magnesium alloys are frequently used to manufacture complex, thin-walled components (such as laptop casings and magnesium alloy central control frames).

Wall Thickness Heat Maps: Industrial CT systems can instantly generate wall thickness distribution maps for an entire part, identifying areas of thinning or deformation caused by incomplete mold filling.

CAD Comparison: By aligning the scanned point cloud model with the original design model, deviation analysis can be performed with an accuracy of $10–50 \mu m$.

III. Key Considerations for Selecting CT Equipment for Magnesium Alloys

Due to the low atomic number of magnesium alloys, their absorption of X-rays is relatively weak. When selecting industrial CT equipment, the following parameters require particular attention:

Micro-focus Source: To clearly visualize the fine microstructure of semi-solid materials, a micro-focus X-ray source—typically operating at 225 kV or even lower energy levels but featuring a smaller focal spot size—is required to achieve high spatial resolution.

High Dynamic Range Detector: Magnesium alloy parts often feature complex geometries with significant variations in wall thickness. A flat-panel detector with a high dynamic range effectively prevents "burn-through" artifacts in thin-walled sections while minimizing "noise" interference in thicker, denser areas.

Algorithmic Compensation: To address the issue of X-ray scattering—a common occurrence with magnesium alloys—advanced Scatter Correction algorithms are required to enhance image clarity and sharpness.

IV. Industry Value: From "Sampling Inspection" to "Process Optimization"

In the field of semi-solid magnesium alloys, the value of industrial CT applications extends far beyond simple "pass/fail" determinations:

Shortened Development Cycles: During the mold trial phase, CT scanning allows for the early detection of defects and the rapid adjustment of runner designs, thereby eliminating the need for costly and time-consuming iterative mold modifications.

Failure Analysis: By scanning the internal fracture surfaces of failed components, engineers can trace the root cause of the fracture—determining whether it originated from a "flow mark" layer or a segregation zone characteristic of the semi-solid processing method.

Data Closed Loop: CT scan data can be imported into CAE simulation software for comparative validation, thereby improving the accuracy and predictive power of simulation models. Industrial CT serves not only as a "truth-revealing mirror" for assessing the quality of semi-solid magnesium alloy components but also as a "microscope" for process R&D. As equipment costs gradually decrease and inspection efficiency improves, industrial CT is transitioning from high-end research laboratories to production floors, thereby facilitating the widespread application of magnesium alloys in sectors such as aerospace and new energy vehicles. Shenhai Image (Shenzhen) Co., Ltd. is a manufacturer and solution provider of X-ray inspection equipment, dedicated to offering users diverse industrial X-ray and CT inspection system solutions characterized by low maintenance costs. Our core product portfolio includes industrial CT imaging systems and industrial X-ray digital imaging systems. Contact: Mr. Lu at 139-2212-7456.