Development of metal matrix composite powder cored tubular wire for electron beam freeform fabrication

Abstract

Electron Beam Freeform Fabrication (EBF3) is a technique, developed at NASA Langley Research Center (NASA-LaRC), which uses a computerized numerical controlled electron beam welder and a wire feed system to fabricate large scale aerospace parts. Advantages of using EBF3 as opposed to conventional manufacturing methods include decreased deign-to-product time, decreased wasted material, and the ability to adapt controls to produce geometrically complex parts. The EBF3 process is compatible with a range of aerospace alloys and material properties of parts produced by this process have been shown to be comparable to wrought products. However, to fully exploit the potential of the EBF3 process, development of materials tailored for the process is required. This research used alloy theory, as well as powder cored tubular wire technology, to develop aluminum based metal matrix composite feedstock for the EBF3 system. Five iterations of powder cored tubular wire were made using silicon carbide particulates with different surface conditions. Uncoated particles, as well as particles with high and low amounts of copper and nickel coatings, were incorporated into the powder core. Single and multiple layer deposits were made using each wire. Beam conditions were varied for each wire to determine the optimal combination of feedstock material and electron beam parameters required to create a uniformly distributed metal matrix composite deposit using EBF3. Completely uniform dispersion of the reinforcement particles was not achieved in the matrix, however, it was determined that nickel additions enhanced particle dispersion as well as mitigated solidification cracking and secondary carbide formation. It was also determined that using a lower energy beam also promotes dispersion and mitigates secondary carbide formation. Computational models were created to predict phase transformations and particle dispersion in the matrix for various conditions. The thermodynamic and fluid dynamic models were able to describe trends observed through characterization of EBF3 depositions. Results collected from characterization of the deposits as well as trends observed in the models can be used to plan future iterations of powder cored tubular wire feedstock, as well as deposition parameters, to create homogeneous metal matrix composite structures by Electron Beam Freeform Fabrication.