I try to prepare the Ni-Ti alloy by vacuum melting method and then perform press forging and rotary swaging in sequence. However, I observed a external crack with ~5 mm in length on the surface of Ni-Ti bar in the swaging step. It is sometime occurrence. Can anyone help me to explain the phenomenon? Thank you for your help in advance.
In rotary swaging the strain is accumulated by small portions and axially symmetric; that is why the technique works even with relatively brittle matters. However, your intermetallic is particularly brittle and hence previously hardened surficial layers fracture before transmitting the strain inside the bar in next strain steps. Specifically, the surficial concentric (thin) layer would reduce diameter after a swaging pass, if was free. Hence constraint by undeformed core produce residual stress making longitudinal cracks on the surface. Conclusion: elevate temperature of swagging.
Thanks Zisman. If the crack of the sample was transverse,the crack still produced by residual stress? On the other hand, my intermetallic is particularly brittle. Is it connected with precipitate formation of Ni-Ti ingot and inhomogeneous composition?
The Ni-Ti is brittle by nature, so one should avoid tensile stresses in technology. As to transverse cracks, LONG metals under the conditions should undergo parallel cracking. Just as a sausage when boiled !
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