Full answers to UT/RT flaw detection and mechanical properties testing of titanium castings

A complete analysis of the quality inspection standards for titanium castings: multi-dimensional control from non-destructive testing to mechanical properties
I. The core system of titanium casting quality inspection: international standards and industry specifications
The inspection standards for titanium castings are guided by material properties and usage scenarios, and mainly include:
International general standards:
ASTM B368 (American Society for Testing and Materials): specifies the size, chemical composition, and mechanical properties of titanium and titanium alloy castings;
ISO 5832-2 (International Organization for Standardization): Biocompatibility and purity standards for titanium alloy castings for surgical implants;
Industry-specific standards:
AMS 4930 (American Aerospace Material Specification): Non-destructive testing and mechanical property thresholds for titanium castings for aviation;
GB/T 6614 (Chinese National Standard): General technical conditions for titanium and titanium alloy castings.

II. Nondestructive testing (NDT): All-round scanning from internal defects to surface flaws
1. Ultrasonic flaw detection (UT): “Perspective eyes” for internal defects
Detection principle: Through high-frequency sound waves (2-10MHz) penetrating castings, reflected echoes are generated when encountering defects (pores, shrinkage, cracks), and the size and position of defects are judged based on the echo amplitude and position.
Specific parameters for titanium castings:
Coupling agent: Use glycerin or engine oil (to avoid chlorine-containing components from corroding the titanium surface);
Probe type: Focused probe (diameter 6-12mm), used for thin-walled castings (thickness <10mm); straight probe (diameter 14-20mm), used for thick-walled parts (thickness ≥10mm);
Acceptance criteria (taking ASTM B368 as an example):
Aviation-grade titanium castings: Single defect echo amplitude ≤ 20% of the bottom surface echo, and defect size <1.5mm;
Industrial grade: Defect echo ≤ 40% of the bottom surface echo, and defect spacing > 5 times the defect diameter.
2. Radiographic testing (RT): Visualized internal structure “CT scan”
Detection principle: X-rays or γ-rays are used to penetrate the casting, and the defective area forms a grayscale contrast image on the film/detector due to density differences.
Key points for titanium casting applications:
Ray energy: 200-400kV X-rays (applicable to thickness ≤50mm), cobalt-60 γ-rays (applicable to 50-150mm);
Sensitivity requirements: Defects of ≥1% of the casting thickness must be identified (such as 10mm thick castings must identify cracks of ≥0.1mm);
Typical defect image features:
Porosity: round or elliptical low-density shadows;
Shrinkage: cloud-like or mesh-like low-density areas;
Oxidation inclusions: irregular high-density bright spots (because titanium easily reacts with oxygen to form TiO₂).
3. Penetrant testing (PT): “Developer” for surface defects
Testing process: Spray penetrant (fluorescent/coloring) → Penetrate for 5-10 minutes → Remove surface residue with cleaning agent → Adsorb penetrant in defects with developer → Observe under UV light (fluorescent) or white light (coloring).
Special requirements for titanium castings:
The penetrant must be a chlorine, sulfur, and fluorine-free formula (to avoid residual corrosion on the titanium surface);
Applicable to detecting open defects (such as surface cracks and cold shuts), with a minimum identifiable width of 0.02mm

III. Mechanical properties test: “Serviceability” verification from strength to toughness
1. Tensile test: “Basic indicator” for measuring tensile strength and plasticity
Test standard: ASTM E8, sampling direction must be consistent with the main force direction of the casting
2. Impact test: “Toughness scale” for resistance to brittle fracture
Test method: Charpy V-notch impact (ASTM E23), temperature 23℃±2℃.
Titanium casting characteristics:
The impact absorption energy of Ti-6Al-4V castings is ≥20J (better than 15J of stainless steel 316L);
The impact energy decreases by ≤15% in low temperature environment (-20℃) (reflecting good low temperature toughness).
3. Fatigue test: “Life prediction” under simulated cyclic load
Test conditions: sine wave loading, stress ratio R=-1, frequency 10-50Hz, target number of cycles 10⁷ times.
Key data of titanium castings for aviation:
The fatigue strength limit (10⁷ times) of Ti-6Al-4V castings is ≥400MPa (twice that of aluminum alloy castings);
When the surface roughness Ra≤1.6μm, the fatigue life can be increased by 30% (for every increase of 1 level of roughness, the fatigue strength decreases by 5%-8%).
4. Hardness test: “Quick evaluation” of surface wear resistance and machinability
Common methods: Rockwell hardness (HRB/HRF), suitable for annealed titanium castings; Vickers hardness (HV), suitable for thin sections or surface hardened layers.
Typical hardness values:
Ti-6Al-4V annealed castings: HRB 85-95 (about HV 170-200);
Solution + aging treatment castings: HRB 100-110 (HV 200-230).