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Progress in Closed Loop Control for Additive Manufacturing

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Closed loop control is commonly used today in many manufacturing processes where high precision is needed such as CNC machining of critical aerospace parts. Real processes continuously deviate very slightly from the programmed intent, these deviations are accurately measured in real time and these deviations are then fed back into the machine control loop to ultimately reduce process deviations and produce a higher quality part. A form of closed loop control called Adaptive Machining is used to machine turbine engine blades and blisks and is accepted today by regulatory authorities for use on these safety-of-flight critical parts. However, that is not the case today for metal additive manufacturing such as LPBF. One reason is that closed loop control for LPBF has yet to demonstrate its quality benefits and its cost effectiveness. We report the results of our program to implement LPBF closed loop control to reduce the harmful effects associated with Spatter particles generated during melt. Spatters are detected and measured for each layer after melt, and then removed using a second ablative machining laser for every layer. The number and size of Spatter particles are surprisingly large and their effect on internal flaws in the finished part from CT scans are reported.

Results of the effectiveness of closed loop Spatter control are also described.

Learning Objectives:

  • Understand that Spatter during LPBF routinely occurs randomly and frequently and causes Part defects related to both powder spreading and powder melting.
  • Realize that insitu inspection methods can now reliably and quickly detect and measure large Spatters welded to the melt layer.
  • Understand that that a method has been developed to mitigate the harmful effects of large Spatter particles by removing welded Spatter from the melt before the next layer is formed.