Diamond Annual Review 2019/20

54 55 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 1 9 / 2 0 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 1 9 / 2 0 Internal flow inmelt pools during arc welding and additivemanufacturing ofmetals Related publication: Aucott L., Dong H., MirihanageW., Atwood R., Kidess A., Gao S.,Wen S., Marsden J., Feng S.,TongM., ConnolleyT., Drakopoulos M., Kleijn C. R., Richardson I. M., Browne D. J., Mathiesen R. H. & Atkinson H.V. Revealing internal flowbehaviour in arc welding and additivemanufacturing of metals. Nat. Commun. 9 , 5414 (2018). DOI: 10.1038/s41467-018-07900-9 Publication keywords: Synchrotron X-ray; Fluid flow;Welding; Additivemanufacturing D uring advanced manufacturing processes, such as fusion welding and additive manufacturing, metallic alloys undergo rapid transitions in state between liquid and solid. Fluid flow inside the melt pool significantly influences the properties of the manufactured component. The dynamic nature of the flow and the high temperature and opaque nature of moltenmetal make the direct experimental observation of the flow evolution inside the melt pool challenging. Researchers were able to achieve direct time-resolved imaging of melt pool flow dynamics using synchrotron radiation on beamline I12, which has a high enough energy to penetrate throughmetal. Theywere able, for the first time, to track internal flow streams during arc welding of steel. Theirmeasurements showed instantaneous flow velocities ranging from 0.1 m/s to 0.5 m/s. The results showed that when the temperature-dependent surface tension coefficient is negative, bulk turbulence is the main flow mechanism and the critical velocity for surface turbulence is below the limits identified in previous theoretical studies. When the alloy exhibits a positive temperature-dependent surface tension coefficient, surface turbulence occurs, and damaging oxides can be trapped within the subsequent solid as a result of higher flow velocities. Thesefindings provide insights into internalmelt pool flowduringarcmelting.Theydemonstrate that controlling internalmelt flowthrough adjusting surface-active elements can optimise both thewidely used arc welding and the emergingwire arc additivemanufacturing routes. In welding and additive manufacturing, metal is melted to form a molten pool that upon solidification can form high integrity products for applications such as automobiles, ships and other large metallic structures. Correct control of the melt pool is essential in manufacturing processes to avoid catastrophic failures 1 . Several fundamental physical phenomena govern the flow in themelt pool, including convection, plasma drag, electromagnetic forces and surface tension effects 2,3 . For steels, surface tension driven Marangoni forces dominate the melt pool flow, and are key to determining the shape and properties of the solidified structures 4,5 . Direct experimental observation of the flow evolution inside melt pools has been hindered by the dynamic nature of the flow as well as the high temperature and opaque nature of molten metal. In this study, a high-energy synchrotron micro-radiography technique at I12 has been employed to observe melt pool formation and flow dynamics in situ during advanced manufacturing of metallic alloys in a time-resolved way. These novel experimental observations have allowed the development of understanding of how the melt pool forms and evolves under realistic fusion welding conditions or wire arc additive manufacturing of metal. The experimental setup, illustrated in Fig. 1, was positioned to transmit the incoming high-flux synchrotron white beam through the molten metal pool. Melt pools were created in solid steel bars using an electric arc. X-ray radiographs of the molten region, illustrated in Fig. 1b, were captured by employing a scintillator coupled fast CMOS camera, at 2 kHz frame rate, covering the whole molten region. Tungsten (W) particles, approximately 50 µm in size, were used as tracers to visualise the flow in the pool. In comparison to the iron and other constituting elements of the sample material,W particles exhibit significantly higher X-ray attenuation. W particles in the melt pool appear darker than the surrounding liquid metal in the projected images. Therefore, the trajectories of the W particles can be tracked to reveal the internal flow in the melt pool. The morphological evolution of melt pools in low sulphur (S) (0.0005 wt%) and high S (0.3 wt%) steels are presented in Fig. 2. Both melt pools were createdusingexactlythesameprocessparametersandsampledimensions.The images represent three instances to assess the overall geometric evolution in respect to time, until the melt pools grow to their maximum size. Quantitative Imaging andMicroscopy Group Beamline I12 measurements of melt pool width evolution show that in the initial 500 ms the width evolution is nearly identical. After 500 ms, the low S sample begins to grow wider than the high S sample. The difference in the shape of the weld pool is radical and the high S sample immediately grows at a higher rate than the low S sample. The melt pool of the low S sample reached only 1.34 mm depth after 2000 ms. The high S steel melt pool appears to favour downward growth to penetrate a depth of 3.59 mm. Melt pool shape evolution is mainly determined by the internal flow. As illustrated in the measured instantaneous flow (Fig. 3), for low S steel, the tracer particles follow anti-clockwise paths in the left-half of the melt pool and therefore there is an outward flow in the upper part of the pool. Consequently, the highest temperature liquid metal under the heat source is being convectively transported horizontally away from the melt pool centre towards its lateral extremities. This leads to the development of the shallow, wide melt pool. The flow in the high S steel melt pool is in the opposite direction. The flow tracked by tracer particles reveals inward flow in the upper part of the melt pool. Therefore, the highest temperature liquid metal in the upper centre region of the melt pool is transported vertically downwards to the centre bottom.The bottomof themelt pool receives more heat load, which stimulates further melting of the solid substrate at the bottom of the pool. As a result, the melt pool in the high S steel is deeper than that in low S steel. Instantaneous particle (flow) velocities for low and high S steel melt pools are presented in Fig 3. The flow velocities can exceed 0.5m s -1 in high S steel, whereas, in low S steel, flow velocities do not exceed 0.3m s -1 . Observations allow us to comprehend how melt pool formation and evolution progress under realistic manufacturing conditions. The widely used arc welding and the emerging wire arc additive manufacturing routes can be optimised by controlling internal melt flow through adjusting surface-active elements such as sulphur. References 1. Bonnín Roca J. et al. Policy needed for additive manufacturing. Nature Mat. 15 , 815 (2016). DOI: 10.1038/nmat4658 2. David S. A. et al. Current issues and problems in welding science. Science 257 , 497 (1992). DOI: 10.1126/science.257.5069.497 3. Tong M. et al. Multiphysics numerical modeling of fusion welding with experimental characterization and validation. JOM 65 , 99 (2013). DOI: 10.1007/s11837-012-0499-6 4. Wang Y. et al. Effects of surface active elements on weld pool fluid flow and weld penetration in gas metal arc welding. Metallurgical and Materials Transactions B 32 , 501 (2001). DOI: 10.1007/s11663-001-0035-5 5. Aucott L. et al. Initiation and growth kinetics of solidification cracking during welding of steel. Sci. Rep. 7 , 40255 (2017). DOI: 10.1038/srep40255 Funding acknowledgement: This research work was supported by the European Commission as part of the FP7 program, as the project, Modelling of Interface Evolution in AdvancedWelding, Contract No. NMP3-SL-2009-229108. Corresponding authors: Prof Helen Atkinson CBE FREng, Cranfield University, helen.atkinson@ cranfield.ac.uk , Prof Hongbiao Dong, University of Leicester, h.dong@ leicester.ac.uk Figure 1: (a) Schematic diagram of the experimental setup and (b) an example radiograph annotated to show the key elements under observation during the experiment. A white beam of ~ 50–150 keV was used and the beam size was 12x50 mm 2 (H x W) and was transmitted through the entire melt pool. The detector was a Vision Research Phantom v7.3 CMOS camera, lens-coupled to cadmium tungstate or caesium iodide scintillators. With an optical magnification of 1.8x, the linear resolution was 13 µm/pixel. Imaging was acquired at frame rates up to 2 kHz at 800x600 pixels per frame. Figure 2: Synchrotron X-ray radiographs of the evolving melt pool for low S and high S steels at three time instances. The corresponding measured geometries are below the respective radiographs. The melt pools are created using the same melting parameters and sample dimensions. (a–f) are from a low S steel melt pool, while (g-l) are from a high S steel, showing that the high S melt pool favours downward growth to penetrate deeper than the low S melt pool. All scale bars=1 mm. Figure 3: Measured flow direction and instantaneous velocity in (a) low S steel, and (b) high S steel in 2seconds (s) after the inception of melting. r and z denote the distance in the radial and vertical axis, respectively.