Thermal simulation & warping prevention
Predict residual stresses and prevent pressure failures through virtual simulation.
1. Introduction: The invisible enemy in the powder bed
In the Laser Powder Bed Fusion (PBF-LB) process, the laser fuses the metal powder at temperatures above 1,500 °C. The surrounding powder and the underlying layers are often only 200 °C hot. This huge temperature gradient in milliseconds leads to extreme thermal expansion and rapid shrinkage as it cools.
The result is enormous residual stresses. If these stresses are greater than the yield point of the material, the component bends upwards (warping), tears away from the support structures or, in the worst case, causes the recoater to block (crash) - which immediately destroys a construction job that lasted for days.
Why not just screw everything down with support?
Of course you could print massive support structures to forcefully hold the component on the build plate. But these supports waste valuable metal powder, cost a lot of machine time and have to be painstakingly sawn or milled away (often by hand). The aim is therefore to design as support-free as possible.
2. Thermal simulation as a savior
Instead of printing according to the trial-and-error principle and wasting days of machine time after a crash, the industry today uses thermomechanical simulation software (e.g. Simufact Additive, Amphyon or Ansys). This software virtually calculates the entire printing process in advance.
- Voxel-based calculation: The CAD model is broken down into small cubes (voxels). The software calculates the heat input, expansion and shrinkage behavior for each layer.
- Stress visualization: The designer can see on the screen (in red) where the internal stresses are greatest and where the component is at risk of cracking.
3. Preventive measures against warping
If the simulation shows that the component will warp, the engineer has several tools at his disposal:
- Pre-Deformation: This is the most elegant trick. When the simulation calculates that the corners of the part are bending up, the software intentionally distorts the CAD model in the opposite direction (the corners are bending down). When the component then pulls upwards due to the heat under real pressure, it lands exactly in the desired, dimensionally stable target geometry.
- Optimization of the alignment (orientation): It is often enough to rotate the component by a few degrees in the installation space. Solid cross-sections should not be parallel to the squeegee line, as the laser introduces too much heat into a small space.
- Exposure strategies: The laser does not have to scan stubbornly from left to right. "Checkerboard" or "Island" strategies distribute the heat input evenly across the component, which reduces stress peaks.
4. The hurdle: computing time
A physically absolutely correct calculation of each of the tens of thousands of layers at the micro level would take weeks on supercomputers. The industry therefore often uses macro approaches or replacement models (inherent strain method), which greatly simplify the process in order to deliver a result in just a few hours. This is a balancing act between computational accuracy and practical speed.
5. Conclusion: First-Time-Right
Thermal simulation is indispensable in additive series production. The goal is “first time right” – the first printing attempt must produce a usable component. Without the use of predictive simulation software, the economic risk for complex components made of titanium or nickel-based alloys is simply too high.