Eliminating warping caused during metal 3D printing [Source: Science Direct]
Researchers have found a way to possibly reduce thermal stress during metal 3D printing.
Thermal stress is perhaps the most unfortunate aspect of metal 3D printing. Caused by the extreme temperatures created by energy sources (usually lasers) to rapidly melt fine metal powder, these temperatures cause all kinds of issues during and after printing.
The problem results from the fact that metals tend to expand and contract while being heated and cooled. Don’t think of this as a “hot or cold” situation. It’s more of the difference in temperature between segments of the print.
The hottest point is always the melt pool, where the laser is currently traversing. That puts the metal at the highest temperature, and it gradually cools down after the laser passes by.
This cooling is quite complex; radiative cooling is affected by surface area. A thin segment might cool more rapidly than a thicker segment. Lower layers are more cooled than higher layers that have been more recently fused. This creates highly complex thermal flows in the print structure.
If not managed, a print could suddenly deform as support structures are sliced off, thus allowing built-up stress caused by thermal effects to release. That’s definitely not good if you’re hoping to print a dimensionally accurate metal part.
Metal 3D Print Thermal Management
To account for this, most metal 3D printing operations feature a squad of engineers to carefully model the thermal flows and then set up the print job to minimize the effect, or at least focus the effects on portions of the print that matter the least.
Now, researchers from Lawrence Livermore National Laboratory (LLNL) have developed a technique that may assist in solving this ongoing issue, in a paper entitled “Reducing residual stress by selective large-area diode surface heating during laser powder bed fusion additive manufacturing”. They explain:
”By illuminating the surface of a build with homogeneously intense, shaped light from a set of laser diodes, the thermal history was controlled thereby reducing the residual stress in as-built parts. 316L stainless steel bridge-shaped parts were built to characterize the effect of in situ annealing on the residual stress. A reduction in the overall residual stress value of up to 90% was realized without altering the as-built grain structure (no grain growth).”
In-Situ Annealing of Metal 3D Prints
Their approach was to simply illuminate the current print layer with laser diodes to rapidly increase the temperature. These operations would typically hit as high as 1000C, after which the temperature would be slowly cooled.
They found that the thermal effects were dramatically reduced by performing this operation during each layer of a metal 3D print job.
This heating process is effectively a type of annealing, a post-processing step often used to increase metal strength on 3D prints. However, the difference here is that it’s done on each layer. The stronger layers thus resist warping much more than normal.
This could be a very promising approach that has so far not been integrated into any known commercial metal 3D printer offering. However, we may soon see this done.
Metal 3D Printing Financial Challenges
I do have one observation, however. An annealing step at each layer, particularly if it includes a lengthy period to cool, could dramatically increase the elapsed time of each print job.
Thus it remains to be seen if this is a financially feasible approach. Large metal 3D printers are quite expensive, and they are made profitable by running them as close to 24/7 as possible, making objects for clients. If the prints take much longer, then that’s a direct hit on revenue from operations.
Via Science Direct