Our take on metals additive manufacturing (AM) is that it has made it past the “valley of death” in the so-called hype curve. It is being used widely and sometimes for unexpected applications. For example, I was impressed to hear a presentation from the Sonova Group about printing custom hearing-aid earpieces in titanium, for which the unexpected benefit was much better robustness against being dropped on the floor and crushed underfoot.
Despite the dominance of laser powder-bed fusion (LPBF) – because of its maturity and ability to make parts in a number of different materials – other technologies are advancing and becoming more competitive, such as powder feed and binder jetting. For larger parts, wire feed in what are essentially robotic welding systems offer considerable potential, notwithstanding the challenges of adjusting for thermal distortion during build and subsequent heat treatment. The range of alloys available for printing continues to increase, including adaptation of existing alloys to accommodate the constraints of AM.
That being said, qualification and certification of parts for demanding applications remains a challenge. Where fatigue is the dominant loading, hot isostatic pressing will generally result in acceptable performance, but this is an expensive processing step. There is limited evidence in the technical literature that paying careful attention to print parameters can result in fatigue lives comparable to conventionally processed material provided that part surfaces are machined to a good surface condition. It is accordingly not surprising that there is strong interest in developing procedures, protocols and standards that will enable users of 3D printers to qualify their process and certify parts.
Anyone can easily find public discussions of how companies and agencies are developing such protocols. The National Institute for Standards and Technology (NIST) is taking a leading role in this as are the professional societies (e.g., ASTM, SAE, TMS, EWF, ASME, TWI, etc.). An essential contribution to this effort is the generation of data that supports standards development. As others have pointed out, purely statistical testing of metals AM processes is an expensive and, by most measures, impractical approach because of the large number of process parameters that can affect the outcome.
We are fortunate at Carnegie Mellon University to have NASA support through its University Leadership Initiative for developing a qualification ecosystem for LPBF. We have developed spreadsheets for recording the relevant data for each build that have upward of 50 columns of data, which illustrates the complexity.
The counterblast to these issues is to focus on the physics of the printing process and the importance of melt-pool size, shape and overlap. This leads naturally to process maps in which process windows can be defined that are predictable, to a large extent, as a function of the parameters. Operating within the “process box” then allows the user to be more confident that their qualification of a given machine is valid and likely to remain so.
The main process variables to pay attention to are laser power, scan speed, hatch spacing, spot size, layer thickness and preheat. If you are concerned about LPBF qualification, please do not hesitate to contact us and get involved.