Welding is the most effective way to join metals permanently; over 50% of global domestic and engineering products are estimated to contain welded joints. In welding, work-pieces are mixed with filler materials and melted, to form a pool of metal that upon solidification becomes a strong, permanent joint. Cracking may occur during solidification of the melt pool, and solidification cracking is an important issue in welding, casting and solidification-related additive manufacturing processes. A sustained deficiency in understanding the fundamental mechanism for damage has driven experimental efforts towards the observation of the phenomenon in situ. At beamline ID19, the team installed a novel strain-based deformation stage. The beamline has a small source size while the divergence and the length of 145 m allows macroscopically-large beam diameters at the position of the experimental hutch. Furthermore, the length of ID19 reduces the effective source size contribution to the images and therefore allows the coherence properties of the beam to be exploited by means of inline X-ray phase contrast.

Researchers observed the solidification cracking during welding of steel in situ using high-speed, high-energy radiography. Synchrotron X-ray micro-tomography was used to visualise and analyse the 3D crack network (Figure 144). Isolated micro-cavities are situated away from the bulk crack, in the region where damage initiation is observed during in situ radiography. Quantitative analysis of these isolated cavities reveals a size range between 10-27 µm with highly spherical morphologies. In some cases, the micro-cavities coalesce to form isolated micro-cracks, indicating that coalescence is the dominant mechanism for growth in the early stages of damage development.

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Fig. 144: Transverse cross-sectional tomography images showing the solidification crack growth path: a,b) early stage growth; c,d) mid stage growth.

Damage initiation is dependent upon strain rate. Higher strain rates induce cracking at a relatively higher volume fraction of liquid. The true strain required to initiate the damage in this thermodynamic state is higher than for low strain rate due to the increased presence of liquid in the semi-solid skeleton. The extra liquid maintains permeability within the semi-solid skeleton, allowing liquid metal to remain mobile and heal any deformation-induced cavity openings. As a result, the strain required for damage initiation increases.

The scientists also found that cracks grow by linking micro-porosities in the meshing zone in the solidifying weld pool.

 

Principal publication and authors

Initiation and growth kinetics of solidification cracking during welding of steel, L. Aucott (a), D. Huang (b), H.B. Dong (a), S.W. Wen (a,c), J.A. Marsden (c), A. Rack (d) and A.C.F. Cocks (b), Sci. Rep. 7, 40255 (2017); doi: 10.1038/srep40255.
(a) University of Leicester (UK)
(b) Oxford University (UK)
(c) Tata Steel, Research & Development, Rotherham (UK)
(d) ESRF