How to conduct quality inspection after metal 3D printing?

一, Non-destructive testing technology: looking at things from the outside to find internal flaws
The main way to check the quality of metal 3D printing is by non-destructive testing (NDT), which can find internal flaws without affecting the items' structure. Based on distinct detection principles, the most common technologies can be put into four groups:
1. Micro CT, or industrial computed tomography
Micro CT employs X-rays to go through parts and get data from multiple angles. After being reconstructed by a computer, it creates three-dimensional tomographic images that can find defects with a resolution of micrometers. A Micro CT system with a 450kV X-ray source can find pores with a diameter of 0.02mm inside an aluminum alloy cylinder head and measure things like porosity and crack length. Its main benefits are:
Full dimensional inspection: can find both interior flaws (like cracks and pores) and outward geometric aberrations (like wall thickness and deformation) in parts at the same time.
Quantification with high accuracy: 3D reconstruction technology can correctly estimate the size, location, and distribution density of flaws.
Non-contact operation: Don't harm precision parts again.
2. Radiographic testing (RT)
According to the GB/T 35351 standard for "Non Destructive Testing of Metallic Materials - Radiographic Testing," radiographic testing finds internal flaws by looking at the changes in how X-rays or gamma rays pass through parts. For instance, while checking titanium alloy aviation blades, radiographic testing can find interlayer non-fusion problems and measure detection sensitivity using image quality indicators (IQI). It has some problems, such as:
Limitation of penetration capability: High-density materials, like tungsten alloys, need high-energy radiation sources;
Limitations of two-dimensional imaging: Overlapping projections can hide problems in complicated structural parts.
3. Testing using sound waves (UT)
Ultrasonic testing uses the way high-frequency sound waves bounce off and travel through parts to find near-surface flaws like cracks and inclusions. For instance, phased array ultrasonic technology (PAUT) may swiftly find and photograph flaws in 316L stainless steel molds using multi-element probes. Some of its traits are:
Very sensitive: can find cracks as small as a few microns;
Directional dependence: The angle of the probe needs to be set just right for the geometry of the part.
4. Testing with Laser Ultrasonic (LUT)
LUT employs laser pulses to make stress waves move on the surface of parts and finds flaws by looking at how sound waves move through them. The Nanyang Technological University team built a laser ultrasonic system that can find cracks in titanium alloy parts in 15 minutes with a resolution of 0.1mm. This method is good for finding difficult curved parts online.
二, Checking the quality of the surface, from the microstructure to the macroscopic shape
The surface quality of metal 3D printed products has a direct impact on how long they last and how well they resist corrosion. The following dimensions should be checked during surface inspection:
1. Measuring the roughness of the surface
To find the arithmetic mean deviation (Ra) of the surface profile of the part, use a surface roughness meter like the MarSurf series. For instance, the surface Ra value of Ti6Al4V titanium alloy parts made by the SLM method is normally between 6 and 10 μm. To fulfill aviation standards, this value needs to be lowered to less than 0.8 μm using electrolytic polishing.
2. Analysis of the microstructure
Use scanning electron microscopy (SEM) to look at the parts' grain structure, phase composition, and defect morphology. Hot isostatic pressing (HIP) can change the shape of aluminum alloy objects, and SEM photos can demonstrate this.
3. Testing the chemical makeup
To find out what chemicals are in the pieces, use an X-ray fluorescence spectrometer (XRF) or an inductively coupled plasma mass spectrometer (ICP-MS). For instance, checking the content deviation of Cr, Co, W, and other elements in nickel-based high-temperature alloys that have been 3D printed to make that they meet the ASTM F3001 standard.
三, Testing mechanical performance: checking how much weight parts can hold
It is important to verify the mechanical qualities of metal 3D printed objects to make sure they are up to par:
1. Test for tensile strength
The GB/T 228.1 standard says to use a universal testing machine to check the parts' tensile strength (Rm), yield strength (Rp0.2), and elongation (A). For instance, the Rm of 17-4PH stainless steel parts made with the SLM method must be 1000MPa or higher.
2. Test for fatigue
Use a rotary bending fatigue testing machine, like an R-R testing machine, to see how long parts last when they are under cyclic strain. For instance, aviation fasteners need to go through 10 cycles of load testing, and the crack propagation rate needs to be less than 1 × 10⁻⁶ mm/cycle.
3. Testing for hardness
You can use a Vickers hardness tester (HV) or a Rockwell hardness tester (HRC) to find out how hard the surface of items is. For instance, turbine blades need pieces made of Inconel 718 that have an HV value of 450–500 when printed with DMLS technology.
四, Industry Practice: Trends in Standardization and Intelligence
1. Building a national standard system
The three national standards for 3D printing (GB/T 35351-2025, GB/T 45675-2025, and GB/T 45667-2025) that went into effect in September 2025 give the industry a single way to judge quality. For instance, GB/T 45675 says how to assess the surface roughness of SLM parts and requires that the Ra value detection repeatability error be ≤ 5%.
2. Use of smart detection technologies
The use of machine learning and artificial intelligence is making detection more efficient. For instance, Nanyang Technological University created an optical imaging-based crystal orientation analysis system that can finish the microstructure evaluation of titanium alloy parts in just 15 minutes and costs only 1/10 of the SEM method.
3. Quality control for the whole process
Leading companies have set up a closed-loop system for "design printing testing feedback." For instance, GE Aviation has added an in-situ monitoring system to its SLM equipment. This lets them change the laser intensity and scanning speed in real time, which has lowered the failure rate of components from 8% to less than 0.5%.