Residual stress is an inevitable product of rapid heating and rapid cooling, which is an inherent characteristic of the laser powder bed melting process. Renishaw, a British metal 3D printing manufacturer, has summarized ways to "fight against" residual stress in metal 3D printing.
Each new processing layer is formed by moving the focused laser on the powder bed to melt the top powder and fuse with the processing layer below. The heat in the hot pool is transferred to the solid metal below, so that the molten metal cools and solidifies. This process is very rapid, only about a few microseconds.
When the new metal layer solidifies and cools on the upper surface of the lower metal, shrinkage will occur. However, due to the limitation of the solid structure below, this shrinkage can cause shear forces to form between the layers.
The laser melts the powder on top of the solid substrate to form a new weld bead
The laser moves along the scanning vector and melts the powder. After the heat is transferred to the solid metal below, the molten powder cools down. The metal shrinks during cooling and solidification, thus forming a shear force with the next layer
Residual stress is destructive. When we add a new processing layer above a processing layer, stress will form and accumulate with it, which may cause the part to deform, causing its edges to curl up, or even break away from the support.
In extreme cases, the stress may exceed the strength of the part, causing destructive cracking of the component or deformation of the processing tray.
These effects are most pronounced in parts with larger cross-sections, so the weld bead of such parts tends to be longer, and the shear force acts longer.
Minimize residual stress
One way to solve this problem is to change the scanning strategy and choose the method that is most suitable for the geometry of the part being machined.
When we fill the center of a part with a laser track, we usually move the laser back and forth. This process is called "scanning".
The mode we choose affects the length of the scan vector, and therefore the level of stress that may accumulate on the part. The shorter the scan vector, the smaller the residual stress.
Detour scan mode
• Rotate 67° after each layer scan
• High processing efficiency
• Residual stress gradually increases
• Suitable for smaller and thinner features
Stripe scan mode
• Uniform distribution of residual stress
• Suitable for large parts
• Processing efficiency is higher than checkerboard scanning mode
Checkerboard scan mode
• Each layer is divided into several 5 x 5 mm island areas
• Rotate the overall mode and each island-like area by 67° after each layer scan
• Uniform distribution of residual stress
• Suitable for large parts
We can also rotate the direction of the scan vector when moving from one processing layer to the next, so that the stress will not all be concentrated on the same plane.
Each layer is usually rotated by 67 degrees to ensure that the scanning direction is completely repeated after processing many layers.
Heating the processing tray is also a way to reduce residual stress. In addition, post-sequence heat treatment can also reduce accumulated stress.
Residual stress design recommendations
Design to minimize residual stress
• Avoid large areas of uninterrupted melting
• Pay attention to changes in cross section
• Mixed processing to integrate thicker base plates into additive manufacturing parts
• Use thicker processing pallets where stress may be higher
• Choose the right scanning strategy