Researchers from Oak Ridge National Laboratory (ORNL), Oak Ridge, Tennessee, USA; University of California, Berkeley; General Electric Global Research Center, Niskayuna, New York; and Japan Atomic Energy Agency, Ibaraki, Japan, have released a paper studying neutron scattering to measure internal strain in additively manufactured samples before, during and after annealing. Controlling residual strain in metal parts manufactured via Laser Beam Powder Bed Fusion (PBF-LB) helps prevent cracks and failures. The research was published as ‘Monitoring residual strain relaxation and preferred grain orientation of additively manufactured Inconel 625 by in-situ neutron imaging’ in the journal Additive Manufacturing.
Researchers at General Electric (GE) needed to better understand where residual strain forms and at what temperatures annealing should be conducted to relieve the strain to optimise Additive Manufacturing component design and annealing time & temperature. The scientist then performed neutron experiments and computational modelling to understand the AM and annealing process.
Post-processing heat treatments reduces strain in parts after they are additively manufactured, but too much heat can cause unwanted structural changes. Using neutron diffraction, the ORNL researchers measured the strain inside samples of additively manufactured Inconel 625. The researchers performed the initial neutron calibration experiments at the NOBORU beamline at the Japan Proton Accelerator Research Complex (J-PARC). They then used neutron imaging, which enabled them to observe the samples inside a high-temperature furnace, in real time, during annealing. The neutrons easily penetrated the furnace walls and allowed mapping the strain relaxation throughout the entire part during annealing.
Data from neutron scattering validate computer models that simulate the amount and distribution of residual strain formed during the Additive Manufacturing process. The new model is said to be able to more accurately predict whether slightly changing the design of a part will make it stronger by minimising residual strain formation during production. The new model can also indicate if changing the diameter of the Additive Manufacturing laser beam or the speed at which it travels will improve production quality.
The researchers compared the measured stress to computer simulations and conducted simulations of the AM process to predict the residual stress distributions within the samples as a function of the process parameters. Comparisons of the simulation results to the room temperature experimental measurements showed good agreements when the simulation data are averaged over the volume of the part, confirming the usefulness of the experiments for validating simulation results. The results are helping GE validate its computer models and adjust component designs to reduce residual strain formation during Additive Manufacturing. This data will also enable GE to anneal its products and optimise the strain relaxation without causing undesirable structural problems.
‘Monitoring residual strain relaxation and preferred grain orientation of additively manufactured Inconel 625 by in-situ neutron imaging’ was written by Tremsin, AS; Gao, Y; Makinde, A; Bilheux, HZ; Bilheux, JC; An, K; Shinohara, T; and Oikawa, K. The paper is available here in full.