| Abstract Scope |
Introduction: Aerospace flight panels must provide high strength with low mass. For aluminum panels, it is common practice to begin with a wrought plate and remove the majority of the material by machining to reveal the desired thin rib, light weight structure. As an alternative, this study implements hybrid manufacturing, where aluminum was first deposited on a thin baseplate only at the rib locations using additive friction stir deposition (AFSD). Structured light scanning was then used to measure the printed geometry. The printed geometry was finally used as the stock model for computer numerically controlled (CNC) machining. This paper details the hybrid manufacturing process that consists of AFSD to print the preform, structured light scanning to generate the stock model and tool path, three-axis CNC machining, and post-process measurements.
Experimental procedures: To represent an aerospace flight panel, a ribbed structure with a center hole and boss was designed. The preform was fabricated using a MELD Manufacturing L3 additive friction stir deposition machine to deposit 6061-T6 aluminum onto a 6061-T6 aluminum baseplate. The total thickness of the multi-layer deposition was approximately 12 mm.
The printed preform was measured using a GOM ATOS Q structured light scanner. The scan model was imported into computer-aided manufacturing (CAM) software, where it was used as the stock model for toolpath generation. The scan model was aligned with the computer-aided design (CAD) model of the ribbed aerospace panel and the coordinate system was assigned using the corner of the baseplate.
To select optimal machining parameters, the frequency response functions (FRFs) of each cutting tool were measured using impact testing. Here, a modal hammer (PCB model 086C04) was used to excite the tool tip and the response was measured by a low-mass accelerometer (PCB model 352C23). The FRFs were used to generate stability maps, which enabled the selection of optimal, stable machining parameters.
Once the machining parameters and toolpaths were selected, the preform was clamped to the table of a Haas VF-4 three-axis CNC milling machine. The part was then probed with the machine’s touch trigger probe to locate the part and align the machine coordinate system with the CAM coordinate system using a coordinate rotation. Facing, contour milling, and boring operations were all implemented to create the ribbed structure with a hole and boss in the center.
Results and discussion: Each step in the hybrid manufacturing process was completed with the expected results. The AFSD toolpaths deposited material in the desired locations on the baseplate. The structured light scan provided a stock model for the CAM software that was used to set a coordinate system and define the toolpaths required to remove the excess material. Stable machining conditions were observed using the optimized parameters obtained from tap testing. In summary, the structured light scanning strategy to provide a CAM stock model and local coordinate system was successfully implemented.
To confirm the machined part geometry, measurements were completed using the structured light scanner. The scan results were compared to the CAD model by creating a best fit alignment and observing the differences. The alignment showed a maximum deviation of 0.25 mm. One potential cause of these deviations is from the release of internal stresses during the machining process. In follow-on testing, measurements will be performed before deposition, after deposition, and after machining to record any part distortion.
Conclusions: This paper describes the combination of AFSD, structured light scanning, and CNC machining in a hybrid manufacturing scenario. The demonstration part is an aluminum aerospace flight panel, although the material and application are not limited to this domain. The AFSD material demonstrated machinability similar to wrought aluminum. The structured light scanning procedure provided an accurate stock model for the CAM software, while also ensuring the desired part geometry was contained within the printed preform. Tool tip FRF measurements enabled the selection of optimal machining parameters. Post-process measurements were used to compare the final part with the intended CAD design. Ultimately, this work demonstrates a hybrid manufacturing approach that leverages AFSD, metrology, and machining to provide a new option for the production of aerospace flight panels, as well as other metallic components traditionally obtained from wrought plate, castings, or forgings. |