The AlSi7Mg sample was additive manufacturing by the PBF method and heat treated 480℃-6h and 550℃-6h.
When the residual stress of the top view and the side view(Figure) was measured by XRD, the following results were obtained.
As-built
Top : 47.4 MPa
Side : 134.9 MPa
480℃-6h
Top : -33.7 MPa
Side : 2.1 MPa
550℃-6h.
Top : -38.8 MPa
Side : -36.1 MPa
So, I have some questions about the results.
1. In case as-built sample, why is there such a big difference between Top and Side?
2. In case 480℃-6h heat treatment, after heat treatment, the Top changed greatly from tension to compression stress, but why didn't the Side change like this?
3. After heat treatment, Why tensile stress change to compressive stress?
The powder bed fusion manufacturing process, such as selective laser melting (SLM) or electron beam melting (EBM), involves the use of a high energy density laser or electron beam to melt and fuse together successive layers of powdered material. The high energy input and rapid cooling during the solidification process can lead to the development of high levels of residual stress within the resulting parts. In the case of the AlSi7Mg sample, it is possible that the residual stress observed in the top view was influenced by the manufacturing process, while the residual stress observed in the side view may have been less affected.
The heat treatment applied to the sample may also have contributed to the differences in residual stress between the top and side views. Heat treatment is a process that involves heating a material to a specific temperature for a specified time and then cooling it at a controlled rate. Different parts of the sample may have been subjected to different cooling rates during the heat treatment process, leading to variations in the residual stress.
It is also possible that the X-ray diffraction analysis itself may have influenced the measured residual stress. X-ray diffraction is a non-destructive technique that measures the diffraction of X-rays by a sample to determine its crystal structure. However, the results can be affected by the surface conditions of the sample, such as the surface roughness and morphology. If the top and side views of the sample had significantly different surface conditions, this could have contributed to the observed differences in the measured residual stress.
Additionally, the microstructure of the sample may also play a role in the difference in residual stress between the top and side views. In AlSi7Mg, the dendrite microstructure that is formed during solidification, can cause the residual stress to be more in the horizontal direction, that is why in side view the residual stress is high but in the top view it could be less.
2. In the case of the heat treatment at 480℃, the residual stress in the top view of the sample changed greatly from tensile stress to compressive stress. This change in residual stress may be due to the heat treatment process, which can alter the crystal structure and microstructure of the material and cause the residual stress to change. The heat treatment process can cause the material to expand and release the tensile stress, leading to a decrease in residual stress.
However, the side view of the sample may not have changed as much because the microstructure and crystal structure may not have been affected as much. In addition, the side view of the sample have been affected by the heat treatment in a similar way as the top view, and it could be that the residual stress in the side view was already close to a compressive state. The residual stress changes that occurred after heat treatment might not have been enough to change the state of the residual stress. Hope this answers your question.
Could you share the scanning strategy and rotation used in the build?
Is your hatching direction kept perpendicular to the inert gas flow for all layers irrespective of rotation? If so, which side of the cube is referred as "side" with respect to the argon flow?
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