On understanding hot cracking in Nickel based alloys processed by SLM

Nickel-based superalloys have great applications in the fabrication of turbine blades, jet engines, and other high-value metal components found in marine applications, industrial or nuclear reactors. Using additive manufacturing technology to build these components can offer significant benefits. However, the laser processability of these alloys shows they are prone to cracking.

Superalloys

Nickel-based alloys exhibit great high temperature and oxidation resistance. This characteristics makes them excellent candidates for hot gas path in aero and stationary gas turbines. The high temperature strength of these alloys is due mainly to [1a]:

• Solid solution strengthening of the FCC (face centered cubic) continuous (gamma)-Ni matrix;
• Precipitation hardening by ordered, coherent intermetallics such as gamma’-Ni3(Al, Ti) or gamma’’-Ni3Nb. Gamma’ is a coherently precipitating phase: the crystal planes of the precipitates are in sync with the gamma matrix with an ordered FCC crystal structure. These phases are extremely stable and show only low driving force for coarsening due to their low lattice mismatch.

Processing of superalloys by SLM

Selective laser melting allows the production of complex parts out of metal powders. Successive layers are selectively melted using a laser beam scanned across according to a digital CAD file. The final component is fused as a stack of melted and resolidified metal as a fully dense and functional part. In terms of chemical composition, the final component shows homogeneous composition. Given the small melt pools typical size and the fast cooling rates, only little – short distance - segregations mechanisms occur during machining.

Processing of these Ni based superalloys by SLM has mainly focused on solid solution hardened superalloys and the gamma´´-strengthened alloy IN718. These alloys are all considered to be “easy to weld” [1] due to their low content of the gamma’ forming elements Al+Ti. For high load applications, superalloys with higher gamma’ volume fraction are required. This class of alloys is inherently more difficult to process and tends to form severe hot cracks during SLM processing at room temperature [2] for the alloy CM247LC.

Cracking

In literature related to welding, many mechanisms are proposed to explain the cracking that occurs during resolidification [2b,15]. The best known are:

• the shrinkage brittleness theory [3-5],
• the strain theory [6-8],
• the generalized theory [9-12],
• and the technological strength theory [13-15].
  

These theories differ in their approach to actual cracking mechanistics. Yet the conscensus is that cracking occurs in a discrete temperature range: the brittle temperature range.

Metallurgically, this temperature range varies between:

• Tmax=the temperature where liquid is confined within the solidification structure
• Tmin=the temperature where the boundary liquid is partially or completely solidified and the material recovers its ductility.

 
Mechanically, the ductility of the material is essentially zero over this temperature range and susceptible to cracking.

References

[1a] Engeli, Roman, Etter, Thomas, Hddotovel, Simone, Wegener, Konrad, Processability of different IN738LC powder batches by selective laser melting.Journal of Materials Processing Technology http://dx.doi.org/10.1016/j.jmatprotec.2015.09.046
[1] Prager, M., Shira, C.S., 1968. Welding of precipitation-hardening nickel-base alloys. Welding Research Council Bulletin 1.

[2] Carter, L.N., Attallah, M.M., Reed, R.C., 2012. Laser Powder Bed Fabrication of Nickel-Base Superalloys: Influence of Parameters; Characterisation, Quantification and Mitigation of Cracking, in: Proceedings of the Twelfth International Symposium on Superalloys. Presented at the Superalloys 2012: Twelfth International Symposium on Superalloys, TMS Publication, pp. 577–586.

[2b] LIN, J. C. LIPPOLD AND W. A. BAESLACK, An Evaluation of Heat-Affected Zone Liquation Cracking Susceptibility, Part I: Development of a Method for Quantification, WELDING RESEARCH SUPPLEMENT I 135-s, 1993
[3] Medovar, B. I. 1954. On the nature of weld hot cracking. Avtom. Svarka 7(4):12-28. Brutcher Translations No. 3400.
[4] Torpov, V. A. 1957. On the mechanism of hot cracking. Metallovedenie Obrabotka Metallov (6):54-58. Brutcher Translations No. 3982.
[5] Pumphrey, W.)., and Jennings, P. H. 1948. A consideration of the nature of brittleness above the solidus in castings and welds on aluminum alloys. Journal of the Institute of Metals 75:235-256.
[6] Pellini, W. S. 1 952. Strain theory of hot tearing. The Foundry 80(11 ):1 25-133.
[7] Apblett, W. R„ and Pellini, W. S. 1954. Factors which influence weld hot cracking. Welding Journal 33(2):83-s to 90-s.
[8] Bishop, H. F., Ackerlind, C. E., and Pellini, W. S. 1952. Metallurgy and mechanics of hot tearing. Trans. American Foundrymans Society 60:818-833.
[9] Borland, J. C , and Younger, R. N. 1960. Some aspects of cracking in welded Cr-Ni austenitic steels. British Welding Journal 6(1):9^16.
[10] Borland, j . C. 1961. Generalized theory of super-solidus cracking in weld and casting. British Welding Journal 7(81:508-512.
[11] Borland, J. C. 1979. Fundamentals of solidification cracking in welds. Welding and Metal Fabrication Parti, (1/2):19-29. Part II, (3):99-107.
[12] Matsuda, F., Nakagawa, H., and Sorada, K. 1982. Dynamic observation of solidification and solidification cracking during welding with optical microscope. Trans. 7W/?l11(2):67-77.
[13] Prokhorov, N. N. 1962. The technological strength of metals while crystallizing during welding. Welding Production 9(4):1-8.
[14] Prokhorov, N. N., and Prokhorov, N. Nikol. 1 971. Fundamentals of the theory for technological strength of metals while crystallizing during welding. Trans. JWS 2(2):109-117
[15] Transgranular liquation cracking of grains in the semi-solid state S. Karagadde, P.D. Lee, B. Cai, J.L. Fife, M.A. Azeem, K.M. Kareh, C. Puncreobutr, D. Tsivoulas, T. Connolley & R.C. Atwood, 2015, NATURE COMMUNICATIONS | DOI: 10.1038/ncomms9300