What are the quality inspection methods for metal printing components?

一, Microstructure characterisation: uncovering the 'genetic code' of material attributes.
The macroscopic mechanical properties of metal printing components are directly influenced by their microstructure, including grain size, orientation, and phase composition. Scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) are traditional methods for detection, however they have problems including high costs and long processing times. The combination of optical imaging and machine learning technology in recent years has opened up a new way to quickly analyse microstructures.
1. Machine learning and optical imaging
A group of people from Nanyang Technological University in Singapore has devised a cheap detecting system that uses optical cameras, lamps, and laptops. Chemical etching reveals the microstructure of the metal surface, and machine learning algorithms look at the patterns made by the reflected light on distinct crystal surfaces to generate a "crystal orientation map." This method can do the inspection in 15 minutes and costs only 1/40 of typical EBSD. It is good for quickly checking important parts like turbine blades and aviation supports.
2. Analysis of a metallographic microscope
After making metallographic samples of the printed parts, use a metallographic microscope to look at their microstructure. For instance, printed items made of titanium alloy should have a consistent α+β dual phase structure. If you see coarse martensite or a layered structure, it means that there was a problem with the heat treatment. For a certain aircraft engine blade project, metallographic research showed that some places had coarse grains that were not usual. By changing the laser power and scanning speed, the grain size was kept to within 50 μm.
3. X-ray diffraction (XRD)
XRD can measure the phase composition and residual stress of printed parts in a precise way. For instance, the volume percentage of the γ 'phase in printed parts made of nickel-based high-temperature alloy directly influences their strength at high temperatures. XRD can reliably measure its presence. Also, detecting residual stress helps stop parts from bending or cracking when stress is released during use.
二, Internal flaw detection: a way to see inside the "black box"
The main reasons why metal printing parts fail are because they have intrinsic flaws such pores, fissures, and a lack of fusion. Ultrasonic testing and radiographic testing are two traditional non-destructive testing methods that have trouble finding complex geometric parts. Micro CT and the water immersion ultrasonic scanning microscope (SAM) have become very important tools for solving this challenge.
1. Micro CT (Micro CT)
Micro CT employs X-rays to go through parts and collect projection data to make a three-dimensional model. This model can find cracks that are at least 10 μm long and pores that are at least 5 μm wide. A business that makes medical implants employed Micro CT to find titanium alloy hip joint cups and found that the interior flow route was blocked by leftover powder. By fine-tuning the printing settings, the blockage rate dropped from 12% to 0.5%. Micro CT can also assess geometric factors like wall thickness and how much parts change shape with an accuracy of 5 μm.
2. Scanning microscopy using immersion ultrasonic technology (SAM)
SAM scans all the parts in all directions using a high-frequency ultrasonic transducer (5MHz-70MHz) and makes high-resolution images by looking at the reflected signals of ultrasonic waves at the fault interface. During the inspection of a 3D printed radar part, SAM found an interior crack that was 12mm deep and 0.3mm wide. Traditional ultrasonic testing can only find flaws that are close to the surface.
3. Laser Ultrasonic Testing (LUT)
LUT employs laser pulses to make ultrasonic waves move, and it finds faults by picking up signals that bounce off of sensors. This approach doesn't need coupling agents and works well for finding curved pieces. A project for an aviation engine fuel nozzle used LUT detection to find a 0.5mm micro crack in the interior cooling channel. The detection accuracy was 0.1mm.
三, Checking geometric accuracy: making sure that digital models and physical items are the same
The geometric precision of metal printing parts has a direct effect on how well they fit together and how well they work. Standard coordinate measuring machines (CMM) aren't very good at finding complicated surfaces, but 3D scanning technologies can find them quickly and accurately without touching them.
1. 3D scanning with structured light
Using high-precision structured light scanners (like the XTOM-5M) to get point cloud data from different angles, compare it to the original CAD model to make chromatograms, and show size differences visually. A 3D scan of a certain car engine mounting showed that the local wall thickness was 0.2mm too thick during the examination. The dimensional qualifying rate went up from 85% to 98% by changing the thickness of the printing layer and the support structure.
2. Measurement with a laser tracker
The laser tracker can find the dynamic accuracy of huge parts, like aeroplane frames, by measuring the spatial coordinates of the target ball that is reflected. The laser tracker found a 0.3mm distortion in a structural part of a spacecraft as it was being printed. The distortion was kept to within 0.1mm by adjusting the temperature of the substrate and the printing path.
四, Mechanical Performance Evaluation: Checking performance from the lab to the service environment
Standardised testing must be used to check the mechanical qualities of metal printing parts, such as tensile strength, fatigue life, and impact toughness. Tensile testing and impact testing are still common ways to evaluate things, but in-situ testing technology gives us a new way to look at performance.
1. Test for tensile strength
Make standard samples that meet the ASTM E8 standard and test their yield strength, tensile strength, and elongation. The tensile test of a certain aluminium alloy printed part indicated that it could handle 320MPa of tensile strength and 18% of elongation, which is what aviation structural parts need.
2. Test for fatigue
Use a rotational bending fatigue testing machine to see how long parts last when they are under different loads. The stress test of a titanium alloy printed blade indicated that its fatigue limit is 450MPa, which is 10% lower than the limit for typical forged blades. To make the material better at handling fatigue, it is required to get rid of internal flaws by hot isostatic pressing (HIP) treatment.
3. Technology for testing in situ
The in-situ tensile SEM combination approach can see how cracks start and spread in real time as the material is being stretched. In situ testing of a nickel-based alloy printed part showed that cracks tend to start in the area that isn't fused. By fine-tuning the printing settings, the number of unfused faults was cut by 80%, and the fatigue life was tripled.
五, Using detection technologies in a fusion application to construct a quality closed-loop control system
The quality check for metal printing parts needs to happen at every step of the process, from design to printing to post-processing. For instance, the testing plan for the combustion chamber of a given aeroplane engine is as follows:
Before printing, use a powder particle size analyser (such the Malvern Mastersizer 3000) to find out the powder particle size distribution and make sure that D50 is between 45 and 60 μ m.
Printing: Using LUT to keep an eye on the state of the molten pool in real time and changing the laser power to avoid making porosity;
Micro CT finds internal flaws, 3D scanning checks geometric accuracy, and tensile testing checks mechanical characteristics after printing.
Before service: Do a corrosion test and a high-temperature endurance test to make sure that the parts will work well in extreme conditions.