Wire arc additive manufacturing (WAAM) allows for the comparatively cheap and flexible manufacturing of large components due to its high deposition rate and direct deposition of material [1]. However, It can result in high residual stress levels with strongly graded distributions across the component, which can worsen the fatigue behaviour and lead to distortion. Using EDXRD with synchrotron radiation, the lattice strains resulting from these stresses can be determined in-situ in the bulk material of WAAM-processed components, providing insights into their evolution throughout the manufacturing process [2]. In this work, the transformable high strength NiMoCr-steel 10MnNiMoCrSi7-9-6 was used to form a wall structure on a S690 substrate with two different inter-pass temperatures and two different in-situ measuring strategies. The inter-pass temperatures were set below and above the expected martensite start temperature (250°C and 400°C), to explore the effect of transformation kinetics. The two measuring strategies determine the lattice strains in the 3rd and 14th layers of 25- and 28-layer structures respectively. Each layer corresponds to a build-up height of approximately 1.75 mm. The measurements were taken in the core of the material at the midpoint of the deposition path to avoid edge effects and both along the deposition path (LD) and in build direction (BD). Changes in lattice strain were determined in relation to a calculated reference value.

Once deposited, the observed layers experience significant compressive lattice strains in LD. As soon as the number of layers deposited over the observed layer reaches a critical amount, the lattice strain in LD relaxes as illustrated in Figure 1 for the ferrite {211} diffraction line, representative of all diffraction lines of the present phases. The full width at half maximum (FWHM) values of the diffraction lines show a sudden drop during this deposition pass and the martensite stops fully transforming into austenite. The effect was attributed to an annealing effect once the critical number of layers is achieved and the experienced temperature drops below the austenite start temperature. Both the lattice strain evolution and the critical number of layers differs with the inter-pass temperature, being 5 for 250°C and 6 for 400°C, but do not change significantly with the measuring strategy, i.e. the measurement position. After the critical deposition pass, the compressive lattice strain in LD increases with each pass, which is attributed to the successive accumulation of lattice strain. These first results demonstrate both the relevance of inter-pass temperature control for residual strain development, as well as the viability of using in-situ EDXRD to observe these developments during deposition.

Figure 1. Lattice strain determined for the ferrite {211} diffraction line once inter-pass temperatures of 250°C (blue) and 400°C (red) are reached respectively. Values are corrected for thermal expansion.
 
References
[1] D. Herzog, V.Seyda, E. Wycisk, C. Emmelmann Acta Materialia, 2016, 117, 371-392.
[2] H.J. Stone, H.K.D.H. Bhadeshia, P.J. Withers Materials Science Forum, 2008, 571-572, 393-398.