The cracks in AM depend upon a number of factors. To zero down on the exact mechanism you need to analyze if the cracks are formed during the printing process itself or during heat treatment. Depending on the printing parameters and residual stresses some defects/ cracks can be avoided

What kind of substrate are you using? If the thermal expansion of the additively manufactured material and the substrate are different, then the chance to get such cracks during the heating and cooling periods are quite expected ... Good luck, Dirk

And in addition to my first answer - slow cooling and heating may help.

A second comment: If you have pores (which you will typically have after additive manufacturing) and some remaining material (or a contamination) is included in the pores, then an evaporation of volatile species during the heat treatment may also cause cracks ... Keep us updated, Dirk

It depends on the composition of the respective Inconel 625. Because each alloy grade behaves slightly different. Nevertheless, I think that the rate of lowering for air cooling (max. to RT) is the reason for the occurrence of the cracks. A more precise assessment could be achieved using a structural damage analysis.

The cracking mechanism?

Mechanism of hot crack welding made of Inconel 625 nickel alloy

Inconel 625 nickel superalloy generally has good weldability. However, its tendency to hot cracks when welding should be taken into account. The gamma matrix is reinforced by the alloying elements chromium, titanium and niobium. These elements promote the formation of carbides and intermetallic compounds, which can cause embrittlement and hot cracks in a welded joint. Based on research, the Gleeble 3800 system and the Transvarestraint test established a criterion for susceptibility to hot cracks and a high embrittlement temperature range (HTBR). Analysis of the results allowed evaluation of the hot crack structure mechanism of welded joints on Inconel 625.