Engineering
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Production of Single Tracks of Ti-6Al-4V by Directed Energy Deposition to Determine the Layer Thickness for Multilayer Deposition
Chapters
Summary March 13th, 2018
In this research, a rapid method based on melt pool characterization is developed to estimate the layer thickness of Ti-6Al-4V components produced by directed energy deposition.
Transcript
The overall goal of this procedure is to determine the effects of different directed energy deposition process parameters on the layer thickness and other melt pool characteristics to identify the optimal parameters for fabrication of larger metallic parts. Directed energy deposition is an additive manufacturing technology for creating metallic parts with medium-large dimensions from metallic powders. Each powder requires process parameters to produce high-density, defect-free parts.
Conventional process parameters optimization often involves waste from over/under-deposition caused by assumption for layer height. The main advantage of this technique is that it is time and cost saving. This optimization method is based on production and characterization of single tracks from metallic powders.
When handling the titanium six-four powder, wear a respiratory mask, powder-free disposable nitrile gloves, and protective plastic glasses. To begin characterization of the fresh, irregular titanium six-four powder, fix double-sided adhesive carbon tape to an aluminum pin stub for a field emission scanning electron microscope. Apply three grams of the titanium six-four powder to the sticky carbon tape.
Place the pin in the specimen chamber of the FESEM, and evaluate the morphology of the powder. Then, fill a 30 cubic centimeter pre-weighed container with the titanium six-four powder. Weigh the filled container, and calculate the apparent density of the powder.
Perform chemical analysis of the starting powder using combustion analysis and ICP-MS. To begin titanium six-four powder loading, open the powder-feeding system hopper. Remove residual powders with an extractor fan.
Tightly close the top cap of the hopper to prevent gas leakage. Then, clean the titanium six-four sheet with ethanol-soaked paper towels. Measure the weight of the sheet with a centesimal balance.
Place the sheet on the marked location of the working area. Mount a powder-feeding nozzle on the laser head so that the angle between the nozzle and laser is 35 degrees. Ensure that the nozzle is clean.
Then, move the robot to the starting point in the working area. Manually adjust the nozzle position until the vertical distance between the metal alloy sheet and the nozzle tip is five millimeters. Then, turn on the guide laser.
Insert a thin rod through the nozzle to touch the working area. Check if the guide laser spot and the tip of the rod are coincident. Manually adjust the nozzle as needed to align the guide laser and metal rod, retaining the 35-degree angle between the nozzle and laser and a five millimeter vertical distance between the nozzle tip and the alloy sheet.
This manual adjustment of the nozzle should be done very carefully because the deposition of the nozzle can significantly affect the geometry of the melt pool. Connect the nozzle to the powder-feeding system, being careful not to disturb the nozzle alignment. Next, open the robot calibration data in the control software.
Load the code onto the robot, and address any detected code errors. Use the laser control software to enable the laser source module. Manually enable the robot motors from the robot control cabinet.
Verify that the safety LED is illuminated. Then, select the deposition process file from the program list, and load the working path into the main robot routine. Set the laser power and robot speed to the desired values, and apply the parameters.
Once the code has been successfully loaded onto the robot controller, launch the robot routine from the robot control software to deposit the single tracks in an automated process. When the routine has finished, use forceps to pick up the sample from the working area. Remove residual powder from the sample surface with an ethanol-soaked paper towel.
Perform elemental analysis on used titanium four powder and on a deposited alloy block. Examine deposited single tracks under a stereo microscope at 5X magnification. Then, use a precision cutting machine to cut cross-sections of single tracks from the middle of the deposited tracks.
Place the clean, dry cross-section specimens in mounting cups. Mix together 10 grams of resin and six grams of liquid hardener to each sample. Pour each resin mixture over a specimen.
Allow the resin to cure for 30 minutes at room temperature. Then, fix a specimen on a polishing machine. First, grind the specimen with 500, 800, and 1, 200 grit silicon carbide paper.
Then, polish the specimen with three micron and one micron diamond paste in sequence. The grinding and polishing step should be done carefully in order to have scratch-free surfaces for optical microscopy and melt pool size measurements. Grind and polish every specimen in this way.
Then, evaluate the shape and porosity of each polished surface under an optical microscope. Acquire images of the melt pool features at 10X magnification, and analyze the images with image processing software. The chemical compositions of titanium six-four powders in both components were evaluated before and after deposition.
The oxygen and nitrogen contents were greater in the irregular titanium six-four powder than in the standard powder used in additive manufacturing. The oxygen and nitrogen contents of the bulk components increased after deposition, but no significant composition change was observed in the used powder. Increasing the laser power significantly increased the height of single tracks up to a point, after which the power delivered to the melt pool was too great to positively affect deposition height.
Increasing the laser scanning speed decreased the energy input to the melting pool and the powder delivery rate, resulting in decreasing deposition height. Correspondingly, deposition height was found to increase linearly both with specific energy density and with power feed density. These relationships were used to determine the process parameters required to produce layers of a specific height.
Specimens were produced at different powers with an assumed layer height of 0.6 millimeters using the conventional method, resulting in over-and under-deposition. When parameters optimized for a consistent layer thickness were used, greater dimensional accuracy was achieved in a single attempt. In direct energy deposition, powders are directly deposited on a substrate, are melted under a laser and quickly solidified.
The solid is created layer by layer, limiting waste by using only as much powder as needed. This method can help answer key questions in the additive manufacturing field about optimization of the process parameters, such as laser power and laser speed. Characterization of single tracks allowed working conditions to be defined for the shown deposition height.
This kind of process parameter optimization is a fundamental step in the development of a new directed energy deposition instrument. After watching this video, you should have a good understanding of how to determine desired step parameter for multilayer deposition by this directed energy deposition technique without wasting time and material.
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