Metal Additive Manufacturing has transformed the way parts are designed and manufactured within aerospace, medical, automotive, and energy sectors. Complex structures can be manufactured using lightweight lattices, consolidated parts, and designs impossible to achieve via machining or casting. However, it is a very fussy process. Due to the interaction of laser, powder, gas, and heat, there are many potential issues that could occur. Knowing what the most common issues are and why they happen is the first step to manufacturing repeatable and certifiable parts.
1. Porosity
This is the most commonly talked about issue in powder bed fusion. The main types of porosity are:
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Gas porosity – Small circular voids due to gas within the melt pool. Gas can be present due to moisture within the powder, encapsulated argon, and hollow powder particulates.
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Keyhole porosity – Too much laser energy results in metal vapourising, creating a deep, unstable ‘keyhole’ which collapses.
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Lack of fusion porosity – Irregular angular voids due to low energy or large hatching resulting in poor fusion of adjacent layers/tracks.
Porosity reduces fatigue life.
2. Cracking
Thermal stresses can cause cracks in the metal. Temperature gradients become large due to fast heating and cooling. Some alloys are very susceptible to cracking.
Solidification (hot) cracking – This is caused when the final intergranular liquid solidifies. Tensile stresses cause the metal to crack. Certain nickel superalloys and high-strength aluminium alloys are susceptible.
Cold cracking – Happens post-solidification. Often due to residual stress and brittle microstructures.
This will be increased by poor parameters, wrong alloy chemistry, and insufficient pre-heating.
3. Warping/Distortion
As new layers are laid down, they cool down, causing contraction. This creates tension in the previous layers. Over many hundreds or thousands of layers, this causes residual stress within the part. This causes the part to distort, edge lifting, and, in extreme cases, recoater crashes. Thin walls, large flat areas, and asymmetric geometries are more prone to this issue. Causes are commonly:
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Insufficient or incorrectly placed supports.
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Poor part orientation.
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No build plate heating.
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High-intensity scan strategies.
It is standard practice to perform stress relief heat treatments before parts are removed from the build plate.
4. Balling/Poor Surface Finish
Balling is when molten metal does not lay down as a continuous track but creates small balls. This can be caused by high scan speeds, low laser power, and oxidation in the melt pool area. This causes a poor surface finish with uneven, rough layers. This can cause issues with the powder flow and then cause other issues.
Rough surfaces can be created due to step-laddering on angled surfaces, partially fused powder clinging to edges, and unsupported downward surfaces. Most metal parts will require post-processing such as machining, polishing, and abrasive processes to meet tolerance.
5. Contamination/Inclusions
Contamination can occur with oxygen pick-up, alloy contamination, and foreign particulates in the powder. This can be caused by reusing powder without correct sieving/testing. It can also be caused by not tightly controlling the inert gas atmosphere. Spatter can be caused by the melt pool. This can land on the powder bed and get stuck in future layers.
6. Delamination/Support Failure
Parts can delaminate or collapse if the layers do not fuse and/or the supports fail. This is usually due to residual stress, incorrect energy input, and thin supports.
How To Minimise Defects
Most defects can be traced back to a few controllable factors. Controlling your parameters, ensuring good quality powder, having good part orientation, good support strategy, and ensuring the machine is in good condition will help reduce defects. Parameters should be qualified for each alloy and machine. Ensure powder is tested and stored. Simulate prior to printing to predict distortion. Monitor the melt pool. Perform non-destructive testing such as CT scanning. Use HIP to seal any internal porosity.
Why Choose ADDMAN Engineering For Your Metal Parts?
Not only is it important to know what the defects are, but having in-depth knowledge, controlled processes, and equipment will ensure they are avoided. ADDMAN Engineering has experts in design for additive, with stringent quality processes and finishing processes to create production-ready metal components. We can help with design reviews, simulation of the build, and post-processing. This will reduce waste, reduce lead times, and give you confidence in your parts. By working with a team that understands these failure modes, you can save a lot of time when going from prototyping to production.
Conclusion
The most common issues in metal 3D printing are porosity, cracking, distortion, balling, contamination and delamination. Each issue has a cause that can be identified. With controlled parameters, good quality powder, good design, and thorough inspection, the issues can be minimised or avoided completely. As the technology develops, it is less about whether it can be done and more about whether it is done with discipline and expertise.
Frequently Asked Questions
1. What is the most common defect in metal 3D printing?
Porosity is the most common. It includes gas pores, keyhole voids, and lack-of-fusion defects, and it can significantly reduce a part’s fatigue strength.
2. Can defects in metal 3D printed parts be fixed after printing?
Some can. Hot isostatic pressing (HIP) can close internal pores, and heat treatment relieves residual stress. Surface flaws can be machined or polished away. Large cracks or severe distortion usually require reprinting.
3. How are internal defects detected?
Industrial CT scanning is the most thorough method. Ultrasonic testing, dye penetrant inspection for surface cracks, density measurements, and in-process melt pool monitoring are also commonly used.
