We limit the hot cracking discussion to the following nine points:
1. Dendrite coherency and back-filling.Theback-filling may be beneficial, as in the non-coherent region of Fig 1.37, or detrimental as with backfill cracking, (Ch. 1.1.16). The name with the dash seems to be used for the beneficial one in several references. Dendrite coherency hampers the back-fill healing of cracks. Maximum cracking is expected where Solidification Cracking Temperature Range (SCTR,Lippold, Lin 1996 [55]) coincides with a region of
maximal coherency, which most likely is at the maximum solubility, pointBin the upper, and region II in the lower part of Fig. 1.38. The shaded region is the coherent zone.
2. Eutectic reaction: Increasing eutecticum is expected to lessen the susceptibility to hot cracking (Fig. 1.38 at concentration Ce).
Fig. 1.39.Two types of hot cracks and their combination. A:
Hot crack along center line. A1 from Top; A2 from side.
B: Hot cracks along grain and cell boundaries. B1 from top, B2…4 from side.
CL= cracks in centerline and GB in grain boundary direction. C: Combination of the two from top and side.
Cracks in B3…B5 form a mesh similar to found in our experiments on Cu. With inclusions (arrows O and P) they form what we call “spongy cracking”).B1: Lundin et al. [101], Brooks, Robino 2003[102]; C1:Feng et al. [103]; C2:Cam et al, [104]
Fig. 1.40. Transverse solid. Zacharia 1994 [58]
Welding direction is from right to left.
The solidification range and the hot cracking susceptibility are inter-connected in eutectic systems, Fig.1.38. The solidification temperature range, divided by G, gives the width of PMZ, or the dendrite length from tip to the last non-solidified liquid in the coherency region in Fig.1.37. The maximum SCTR occurs in the alloy with solute content of maximum solubility (Fig. 1.38). This is the alloy with maximum hot cracking propensity..
3. Liquid rupture. Crack initiates only after the liquid films, fenced-in between the dendrites, rupture..
4. Centerline and grain/cell boundary hot cracks. As seen in Fig.1.39 (and our experiments), two types of fusion zone hot cracks are prominent in practice, namely: (A) hot cracks along the weld centerline (Fig.1.39A)and (B) hot cracks along grain and cell boundaries (Fig.
1.39B). Combinations of the two - (C) - are common. Our results endorse the theory that a planar solidification front bulldozes the concentration gradient to the centerline [4 p.446][3 p.291], while solidification fronts consisting of needle-tipped cells and dendrites fence-in a part of the solute gradient at the cell/dendrite lines, spreading the solute more evenly across the fusion zone.
5. Meshes of cracks. Fig. 1.39B5 shows what appear to be meshes of cell boundary cracks intersected by perpendicularcross-jump cracks. These are similar to our findings in welds in Cu with v<100cm/min, (see Ch.6.3.2). The cracks in Fig. 1.39 B1 curve - along the cell- and grain boundaries - asymptotically towards the weld C/L.
High welding speed may make the grains straight, non-curving, resulting in a head-on collision with the C/L, as with the two protuberances of the centerline crack CL in the laser weld in C2(see tear-drop weld pool form in Ch.
1.119).
Fig.1.41. Crack sensitivity in Al alloyed with (a) Si, (b) Cu, (c)Mg and (d) Mg2Si. Dudas and Collins 1966 [59]
Fig. 1.42. Hot cracking sensitivity vs.
equilibrium diagram of the Fe-C-0,5% Si system.Tamaki, Kawakami, Suzuki 2003 [60]
Fig. 1.43. Three areas in weld metal classified by the type of the dendritic structure.Tamaki et al. 2003 [60]
6. Equiaxed dendrites. The least susceptible growth morphology to hot cracking appears to be the equiaxed dendritic. It distributes the solute most evenly in the weld, albeit in the centerline region [3 p.188][4 p.445].
7. Dynamic stresses in weld metal hot cracking.Zacharia proposes the stress distribution in Fig. 1.40
Zacharia 1994 [58], showing a tensile (+) stress behind line B in the just solidified weld metal. Visual inspection in our experiments suggests that cracks open a few millimeters behind the trailing pool end. It is not impossible that between the lines A and B the crack is back-filled from the pool, and after line B the crack is in the dendrite coherency
region, with the remaining liquid – not yet rupturing - giving the structure resilience. With this assumption, Fig. 1.40 would be well in accordance with Fig. 1.37
.
8.Hot cracking in aluminum.
Nil-segregation causes negligible hot cracking susceptibility in pure aluminum (Fig 1.41 a, b, c and d, area I). Increasing alloy composition increases this susceptibility to maximum at area II, where the amount of segregated low-melting alloy is sufficient to form a continuous network surrounding the grains but insufficient to back-fill the forming cracks. The decreasing hot cracking susceptibility in the region II-III-IV is due to an increasing amount of low-melting solute-rich melt - in these Al-alloys - not increasing cracks, but healing them by back-filling.
9. Hot cracking in carbon steel.
In carbon steel, the cracking susceptibility is minimal at and below region I of Fig. 1.42 because high amount of δ-ferrite is able to dissolve harmful P [60].
The susceptibility may be minimal also at region II – following the dotted line in Fig 1.42 (a) - if no peritectic reaction occurs. Generally hot cracking susceptibility increases outside range of the peritectic reaction in region III with increasing γ+L range.
The possible peritectic reaction may cause deleterious hot cracking (Fig. 1.42 (a) solid line in the region II). This is a complex process including weld metal shrinkage, changes of P solubility, of δ−γ -transformation kinetics among others [60 p.31]. The peritectic reaction is problematic and the hot cracking occurs in it with varying kinetics, but the process has common features with the afore-described Al alloy cracking.
Fig. 1.43 is included as an example of an approach dividing the dendritic sub structures into cellular dendritic and columnar dendritic at least seemingly similarly as Savage (Fig’s 2.19B, 2.22, 2.23 and Appendix 37).
Fig 1.45. Solidification growth modes vs. the relative degree of constitutional supercooling. The present mainstream classification.
Kou 2003 [3],Messler 1999 [4 p.428]