Development of In-Process Monitoring Closed-Loop Feedback for Use in Aluminum Alloy Additive Manufacturing (AM) Applications



OBJECTIVE: Develop and demonstrate in-process monitoring and closed-loop feedback methods that can be utilized in metallic additive manufacturing processes to improve repeatability for geometric dimensions, material properties, and quality. 

DESCRIPTION: Army rotorcraft components require structural integrity to be flight safe. Traditional manufacturing methods have been refined over time to achieve high reliability such as casting processes used for gearbox housings or machining used for mounts, fittings, and pitch-link horns. Recent progress with use of Additive Manufacturing (AM), especially powder bed fusion processes, has demonstrated manufacture of complex components as a single part, which may save manufacturing labor, cost, and reduce production time. The application of optimized topology in design of parts can have the added benefit of weight savings unfeasible using traditional manufacturing processes. In order for additive manufacturing to transition to widespread use in aerospace, the AM processes must be repeatable and reliable to meet aerospace qualification standards. There are several challenges with AM processes that limit the use for manufacturing. Some of the common challenges/limitations for metal are: 1. Residual stresses can be high in AM parts, which limit the loading of parts. Optimization strategies must be developed as part of the effort. 2. Density of the material throughout the part can be inconsistent. Density can be influenced by un-melted entrapped powders. Overcoming this challenge needs to be addressed as part of the effort. 3. The rapid cooling rates associated with AM processes can affect the microstructure of the base material resulting in variations in desired strength, ductility, toughness, and modulus. The new AM process control system must mitigate the effects to material properties. 4. Geometry and surface finish of parts can be inconsistent from part to part. The relationship between AM process parameters and part quality have been studied and reported [1]. Porosity/density is affected by laser power, laser speed, and layer thickness. Temperature can affect residual stresses, material microstructure, and geometry. Many of the process parameters such as temperature and laser speed, can be controlled. In-situ sensors can provide information such as melt pool temperatures, layer thickness, laser power, and laser track. Methods are needed for in-process monitoring and closed-loop feedback for AM processes to improve repeatability for geometric dimensions, material properties, and quality. The methods need to monitor and control the AM process parameters, identify flaw areas, and provide feedback to AM equipment during the build of each layer. It is also desired that any flaws, such as un-melted powder or voids, be corrected by the AM equipment prior to building the subsequent layers. The closed-loop feedback methods must integrate with AM equipment computer controls. Technologies should enable determination of the boundaries of the molten pool within 0.001 (in order to define the size and shape), measurement of temperature over the range from 700 °F to 3000 °F (representative of the molten pool and surrounding regions) to within 25 °F, measurement of geometric features to within +0.005, detect flaws in the range of 0.010 - 0.001, and determine chemical composition within 1 weight percent. For Phase I and Phase II, the technology shall concentrate on aluminum alloy applications to achieve equivalent or superior mechanical properties of Aluminum A357 (AMS 4219). The demonstration of the technology should be the manufacturing of an Army helicopter gearbox (e.g., intermediate or tail rotor gearbox). Offerors are encouraged to team with a helicopter Original Equipment Manufacturer (OEM). 

PHASE I: Demonstrate the feasibility of sensors for use as an in-process monitoring and feedback system for additive manufacturing. Efforts should show that the sensors can meet the demands of the AM process environment and provide feedback to the computer control system. 

PHASE II: Create a closed loop feedback system to optimize the AM processes for flaw density control, thermal stresses, surface finish and material properties. Demonstrate the improved AM processes by manufacturing several sets of coupons and testing them for yield strength, ultimate strength, fatigue strength, hardness testing, etc. Test the system on AM metallic powders. Compare coupon performance to baseline properties using other AM and traditional processes. Manufacture at least two full-size gearboxes for testing to demonstrate the technology in a relevant part. 

PHASE III: Transition the new or optimized AM process closed loop feedback system via aerospace Original Equipment Manufacturers (OEM) and/or qualified suppliers for Army rotorcraft. Demonstrate the AM process for actual aircraft components. Potential commercial / dual-use applications include aviation, medical, automotive, marine and industrial applications. 


1: Mani, Mahesh, et al. Measurement Science Needs for Real-time Control of Additive Manufacturing Powder Bed Fusion Processes, NISTIR 8036. National Institute of Standards and Technology.

2: Manfredi, D. et al. Additive Manufacturing of Al Alloys and Aluminum Matrix Composites (AMCs), Contract FP7-2012-NMP-ICT-FoF-313781. European Space Agency.

3: Measurement Science Roadmap for Metal-Based Additive Manufacturing, NIST, May 2013

4: McLellan, D., "Tensile Properties of A357-T6 Aluminum Castings," Journal of Testing and Evaluation, Vol. 8, No. 4, 1980, pp. 170-176,

5: AMS 4219. Aluminum Alloy Castings 7.0Si - 0.55Mg - 0.12Ti - 0.06Be (A357.0 T6) Solution and Precipitation Heat Treated, SAE International. October 2015.

6: Hu, Dongming, Radovan Kovacevic. Sensing, modeling and control for laser-based additive manufacturing, International Journal of Machine Tools and Manufacture. Volume 43, Issue 1, January 2003, Pages 51-60.


8: ASTM E1479 “ 99 (2011), Standard Practice for Describing and Specifying Inductively-Coupled Plasma Atomic Emission Spectrometers, ASTM International.

9: ASTM B822-10, Standard Test Method for Particle Size Distribution of Metal Powders and Related Compounds by Light Scattering, ASTM International.

10: ASTM B311-13, Standard Test Method for Density of Powder Metallurgy (PM) Materials Containing Less Than Two Percent Porosity, ASTM International.


KEYWORDS: In-process, Closed-loop, Monitoring, Additive Manufacturing, Rotorcraft 

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