I want to know the exact reasons for residual stress formation and the standard adopted practices to avoid them during 3D printing.
The answer is yes. The main reason is that normally in EBM machines the base plate for powder bed is heated up to at least 700 cent degrees. Additionally, as you have mentioned, the process is held at higher temperatures than at SLM. As a result, the as-printed product is usually unstressed.
I believe you are right. The reason for residual stresses in AM parts is that there is a temperature gradient generated during the process. EBM reduces the gradient mainly because the electron beam is used for preheating the powder bed before the process starts.
The answer is yes. The main reason is that normally in EBM machines the base plate for powder bed is heated up to at least 700 cent degrees. Additionally, as you have mentioned, the process is held at higher temperatures than at SLM. As a result, the as-printed product is usually unstressed.
Yes. Temperature gradient is lower in EBM and in all AM processes, it is higher at the base material e.g. substrate and reduces as the layers move upward.
Ojo Philip Bodunde : Why is the residual stress reducing with higher layers of the build. It was mentioned that with increasing build height, the stress increases.
Actually, residual stresses are higher on the build platform e.g. substrate material and reduce with higher build. It is simply as a result of higher temperature gradient. Just like temperature gradient reduces up, residual stress also reduces up the build. They are dependent on each other.
It is true that the parts produced by EBM are less residual stresses than the parts produced by SLM. In fact, electron beam–processed parts can be used directly without any stress-relieving operation. The main reason is because the EBM process is a hot process, while the SLM is a cold process. That means that the EBM process occurs under high temperatures because the vacuum environment during the EBM process ensures a good thermal isolation and higher temperature can be reached without oxygen uptake. Furthermore, the EBM process, unlike the laser-based AM process, involves the preheating of the powders before the melting phase, which reduces the temperature gradient, keeps the chamber warm and helps avoid the formation of heat cracks. Even the final cooling phase, at the end of the building, is slow and takes few hours before it is possible to remove the part from the machine. Parts manufactured by laser-based system require post heat treatments to release internal stresses which are mainly due to the rapid component cooling and solidification. In fact, high thermal gradients between the melt pool and the adjacent powder (that is almost the environment temperature) can occur and can be increased by the gas flow during the SLM process. At the end of the process, the part can be removed almost immediately from the machine. The residual stress formation cannot be avoided during SLM process, but it can be managed by designing proper support structures and suitable post heat treatments (that have to be carry out before to remove the part from the build table and of course before to remove the supports).
If you are interested you can find more detail about the EBM process in this paper: " A literature review of powder based electron beam melting focusing on numerical simulations".
Best regards
Manuela
Manuela Galati :
1.How is the gas-flow in SLM chamber increases the thermal gradients during the process?
2. Normally, annealing and other heat treatments are done after the part is removed from the chamber. what are the in-situ heat treatments available for the SLM and the EBM process?
Thanks in advance!
1. Because the gas flow accelerates the cooling of the layer
2. What you mean for " in-situ heat treatments"? In the AM machine? In my experience, the AM commercial systems do not have this feature, also because you need to remove the powder before to treat the part termically, otherwise the powder will be sintered during the treatment . The sintered powder cannot (or it is very difficult) be removed, especially for complex geometries. I should specify that during a PBF process, the material is subject to a numerous thermal cycles that, from the point of view of the material (microstructure and chemestry), represent local thermal treatments.
Thank you Manuela Galati.
The residual stress is an area of rapid interest in SLM, as it is a challenge to avoid its formation during the process. As was mentioned by you that in EBM, one preheats the powder to the state where it does not sinter, but could reduce the thermal gradients and avoid cracking.
Why is it not implemented in the SLM, eventhough one could do a base-plate cooling till 200 C or even higher in some machines.
Is there any other practical methods to avoid the residual stress. Could you suggest some papers?
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