Strong CAM solutions and streamlined processes that include post-processing and simulation are important factors to enable the commercialization and further development of directed energy deposition (DED) solutions.
How is directed energy deposition different from the more familiar powder bed fusion (PBF) that we think of when we think of 3D printing? Most industrial applications are better served by one process or the other, so they are not necessarily competing processes. PBF is an extension of stereolithography and 3D printing techniques that were initially developed for plastics and polymers, and recently have been applied for various metals. DED processes can be compared to the automation of conventional welding techniques.
DED applications often involve more complex geometries that necessitate multi-axis build inclinations. Keeping the deposition normal to the part surface is typically paramount for success. In addition to building new components, DED is uniquely suited to repairing and modifying existing parts and tooling. PBF techniques fall short in this because the application cannot be used with a complex substrate or starting block. And DED benefits from a machine tool foundation that provides precision and control not possible with manual welding techniques.
Another challenge is procuring the skilled labor needed for increasingly challenging welding operations. Craftsmen of this nature are becoming few and far between. The deposition rates, positional accuracy, orientation of the deposition and control of focal distance that creates ideal melt pools can be controlled when using computer-based machinery. The results are components with negligible voids and near-native material characteristics.
Though toolpath generation is critical in DED, the postprocessing and simulation tasks should not be overlooked and are essential to provide a safe and reliable process.
Multi-axis post-processors are often a challenge in traditional subtractive manufacturing, and the requirements for additive manufacturing are more intensive. The geometric requirements to control the machine axes are similar to the requirements for subtractive processes, but there are many more process parameters to monitor and control. Furthermore, the additive process is less forgiving and requires a greater degree of application control compared to subtractive methods. A milling process can produce a suitable part, even with some parameters such as feed rate, spindle speed or even cutting angle not set at ideal conditions. But additive processes can fail if the parameters such as focal length, deposition rate, gas pressures and others are not set correctly. The latter are often stored in a database within the CAM software and synchronized to the machine tool and deposition materials.
Post-processors are often matched with simulation utilities that can identify additional concerns within additive manufacturing. This is important, as it is difficult to visually observe real-world build processes while the lasers are activated. The required short focal lengths and relatively large size of deposition nozzles can create interference zones that impact process planning, depending on the workpiece geometry. The deposition heads may pose much more of an interference than analogous milling spindle holders that are available in many lengths and diameters, in part to avoid these concerns.
Another point for consideration is the retrofit hardware used to create a hybrid machine tool. Often the deposition hardware is added to an existing platform and can cause constraints regarding axis limits and rotary angle ranges.
OPEN MIND’s hyperMILL VIRTUAL Machining system addresses many of these points with a combined post-processor and simulator, together with an NC Optimizer module that can improve NC code by completely reviewing the machine capability. The system provides accurate collision checks of the entire manufacturing process by comparing G code instructions to the entire work environment.
In one example, an additive delivery system is mounted four inches to the right of the milling spindle. The remaining axis strokes are imbalanced, having a much more negative X range than positive X range. This apparent limitation can be overcome through the hyperMILL VIRTUAL Machining system, which can review machining code and make selections to favor one side or the other and adapt rotary axes to achieve the result, while checking all limits. Throughout the course of a long NC program, the system can continually review instructions between laser activations and determine the preferred solution. This process would be tedious with manual user selections, and exacerbated if the process is repeated following program changes, which is typical in NC programming.
On some five-axis machine tools, the rotary motion is in the head rather than the part. In many of these cases, the rotary motion is limited to one or two revolutions to avoid complications with wiring harnesses inside the machine head. If this type of machine is retrofitted with an additive device, then the toolpath motion must be controlled within the rotary axis limits. The hyperMILL VIRTUAL Machining module can modify the generic toolpath to include these machine limits. Again, the NC Optimizer can look ahead to determine preferred locations in the toolpath to invoke a rewind and deactivate the laser so the DED process can be consistent during the build.
Another popular implementation of DED is to add the deposition nozzle at the end of a multi-axis computer-controlled robot. The robot is lightweight and generally a lower-cost alternative to a machine tool solution. The robot can also be enclosed within a sealed environment to support additive processing if needed. For example, certain materials such as titanium often require a controlled environment for safety and processing reasons. Five controlled axes are sufficient to define a point and vector. Robots often have more axes—up to eight is common. Some of these axes are required to support a system with limited range, joints and singularities. For robot solutions, a five-axis neutral file is output from hyperMILL. An interface module is then used to transfer this manufacturing data, part model, parameters and machine components to a robot solution. In these cases, hyperMILL is still providing toolpath information, nominal collision check, and additive parameters.
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