Mechanism and elimination of hot cracks in laser additive manufacturing of NiFe based superalloy

To solve the problem of hot cracking in laser additive manufacturing process of NiFe based superalloy, the formation mechanism of hot crack was investigated, and the method of interlayer temperature control and powder nitriding to reduce the sensitivity of hot crack was proposed in the laser additive process of NiFe based superalloy. The results show that the occurrence of hot crack is mainly caused by element segregation and thermal stress. Most of the hot cracks are located at the high angle grain boundary, which own higher grain boundary energy to extend existing time of liquid film in grain boundaries during the cooling process, so the obvious segregation phenomenon occurs. When the interlayer temperature is lower, the cooling rate is faster, which can reduce the growth of harmful carbides in the superalloy. The proportion of high angle grain boundary is reduced, and the probability of hot crack is reduced further. Another method is to form stable nitrides by pre-nitriding Ti, Nb elements in the powder, which inhibits element segregation, increases the nucleation points and promotes grain refinement, therefore, the hot crack sensitivity of the superalloy is decreased.