Common evaluation methods for friction stir welding seam quality of aluminum alloy die castings
Friction stir welding can currently weld a wide range of materials, such as aluminum alloys, magnesium alloys, copper, titanium, stainless steel and even composite materials. Currently, this process has been widely used in electronic controls, charging housings or domain controller housings in new energy vehicles. Also because the friction stir welding area involves water channels, quality problems such as weld defects lead to leakage and frequent market problems. Therefore, effective detection and evaluation methods for friction stir welding (FSW) weld quality of aluminum alloy die castings are particularly important. Choosing effective detection and evaluation methods also requires comprehensive consideration of material characteristics, defect types, process requirements and application scenarios.
Currently, the commonly used detection and evaluation methods for friction stir welding seam quality of three-voltage casting shells are as follows:
🔍Common defect types
- 1 Internal defects: holes, incomplete welding, tunnel defects, oxide inclusions, etc.
- 2 Surface defects: grooves, flash, surface cracks.
- 3 Microscopic defects: grain coarsening and heat-affected zone softening.
⚙️Detection methods and principles
At present, electronic control, charging or domain control die casting shells are commonly used to detect the quality of friction stir welding seams, such as visual inspection, non-destructive testing (X-ray, phased array), destructive testing, air tightness requirements of welding, and layer milling testing.
Each detection method is described in detail below:
1️⃣ Visual inspection
1) Visual inspection and requirements should be carried out in accordance with ISO 17637 or in accordance with the customer's appearance standards for the weld bead. The appearance, size, position and existing condition of the weld must comply with the requirements specified in the drawings and relevant standards.
2) Surface grooves and back-side penetrating defects are not allowed on the weld surface.
3) Serious welding collapse defects are not allowed on the weld surface. If there is machining after welding, and the depth of the collapse defect on the weld surface doesn't exceed the machining allowance, the defect is acceptable. It is recommended to control the processing allowance of the weld bead. If no processing is possible, try not to process it to avoid damaging the surface dense layer and causing the risk of leakage.
4) Serious flash defects are not allowed on the weld surface. For mild flash that doesn't damage the thickness of the part, it can be removed by CNC machining or grinding.
5) Height difference of welded products: ≤0.2mm; gap between two welded products before welding: within 0.05mm on one side; deformation after welding: ≤0.4mm; weld offset: ≤0.4mm;
2️⃣ Non-destructive testing
Verification method: Welds may be non-destructively tested in accordance with ISO 17636 using appropriate non-destructive, volumetric testing methods (penetrant testing, radiography or ultrasonic testing).
Judgment criteria: There are no obvious tunnels or holes that affect the welding performance;
2.1 Penetrant inspection
Principle: Capillary action is used to allow the developer to penetrate into the surface opening defects and display through the developer.
Applicability: Detecting surface microcracks (>1μm), opening defects only.
Application timing: Generally, the surface has been clearly confirmed. It is used when confirming whether it is penetrated. It is mainly used for problem analysis; it is not commonly used in the actual factory mass production process of aluminum alloy die-casting parts.
2.2 Ultrasonic detection
Principle: High-frequency sound waves propagate in materials, and the location and size of defects are determined through reflected signals.
Improved technology: The core advantages of phased array ultrasonic testing are high precision, multi-angle scanning capabilities and sensitivity to complex defects, with a resolution of up to 0.1mm.
Applicability: Internal defect detection (such as holes, incomplete welding) requires coupling agent and the surface needs to be smooth.
At present, some OEMs or first-level parts factories have implemented 100% testing on electronic control housings or charging housings.
2.3 X-ray detection
Principle: X-rays are used to penetrate materials and reveal internal defects through film or digital imaging.
Applicability: Three-dimensional defect imaging (such as holes, inclusions), but the resolution of tiny cracks is limited.
Inspection frequency: 3 pieces/first piece&inspection/tool replacement&annual testing.
Note: At present, some OEMs or first-level parts factories have implemented 100% testing on electronic control housings or charging housings.
2.4 Industrial CT scanning
Principle: Multi-angle X-ray projection reconstructs the three-dimensional structure and accurately analyzes defect distribution.
Applicability: High-precision internal defect analysis, high cost, suitable for laboratories.
Frequency of inspection: process verification, problem analysis or annual testing.
Purpose: It is mainly used to analyze the location, size and porosity evaluation of specific pores, and is also used to compare and confirm the effectiveness and accuracy of X-ray flaw detection.
3️⃣ Welding air tightness detection
Common air tightness testing is mainly gas testing or helium testing. However, due to the accuracy of gas parts detection, leakage problems in the market can’t be effectively identified, so most of them currently use helium detection.
Specific helium inspection standards are formulated based on actual application scenarios. Common ones include: helium concentration ≥80%, the remainder is nitrogen, 3.2bar absolute pressure; cabinet vacuum: 20pa; ambient temperature between 15°C and 45°C; inflation time: 20s, test time: 10s.
Judgment standard: leakage volume ≤5*10-6mbar L/s
Detection frequency: 100%.
Note: The density ratio of helium and nitrogen or argon is related to the judgment standard.
4️⃣ Destructive testing
4.1 Layer milling detection
Verification method: Place the welded product on the CNC fixture, perform CNC layered processing along the welding track, 0.5mm/0.2mm 1 time/day (tool change) 0.2mm, layer milling 15 times.
Judgment criteria: No obvious tunnels or holes that affect sealing performance
Frequency of inspection: 1 piece/first piece/tool change & annual test
Purpose: Mainly to confirm the specific depth and size of the stomata. Each layer of milling must be visually confirmed and photos taken for record keeping.
4.2 Metallographic analysis
Principle: intercept the weld section, observe the microstructure (grain size, defect distribution) after polishing and etching.
Purpose: To evaluate the degree of softening, dynamic recrystallization and microscopic defects in the heat-affected zone. Metallographic methods are also usually used to confirm the effective weld penetration and whether the weld bead is offset.
Evaluation criteria: The grain size of the heat-affected zone shouldn't be significantly coarsened to avoid performance degradation caused by softening. The weld penetration meets the design standards and the offset meets the requirements.
Frequency of inspection: 1 piece/per shift/tool change, early process verification, problem analysis or annual type test.
Note: The deepest point of penetration is not necessarily the center point of the weld.
4.3 Mechanical property testing
Tensile test: measure the weld strength and elongation to determine whether the base material properties are achieved.
Bend test: evaluates weld ductility and bond quality.
Hardness test: Analyze the hardness changes in the heat-affected zone to reflect the degree of softening.
Judgment criteria: Meet customer technical standards or reach more than 80% of the performance of the base material (generally set according to application scenarios); testing can be conducted according to ISO 4136.
Frequency of inspection: 1 piece/tool change, initial process verification, annual type test.
📌Suggestions on method selection
Online rapid inspection: visual + eddy current/ultrasonic + X flaw detection
Comprehensive laboratory analysis: industrial CT + metallography + mechanical testing.
High-demand scenarios such as aerospace and aerospace: PAUT+industrial CT+fatigue testing.
⚠️Precautions
Characteristics of aluminum alloy die-casting parts: The original pores in the die-casting part body may affect the quality of FSW and need to be combined with X-ray pre-inspection.
Surface treatment: Clean the surface oxide layer before testing to avoid interfering with eddy current/penetration results.
Standard adaptation: Adjust detection parameters with reference to industry standards (such as automotive ISO 4063, aviation NAS 410).