Cracking formation mechanism of mild steel parts fabricated by surfacing rapid forming

The micro morphology of cracks for the parts fabricated by surfacing rapid forming was observed with SEM.The results showed that the cracks initiated from the first layer of the part,extended along the deposition direction,and finally terminated under the surface of the part.The segregation and inclusion in the formed part were studied by OM and EDS,moreover,the content of residual austenite was measured by XRD.The results indicated that the cracks were liquid hot cracks.In course of forming with multi-layers and multi-paths,the segregation comprised of Mn and Si oxidate,Mo carbide and Ni intermetallic compound were the internal cause for the cracks formation;while the tension stress was the external cause.By properly matching the contents of Mn,Ni and Si elements,purifying the weld metal with rear earth elements cerium,adopting the orthogonal deposition paths layer by layer and prolonging the interformational intervals,the cracks could be avoided.