Microstructural evolution and bonding characteristic in multi-layer laser cladding of NiCoCr alloy on compacted graphite cast iron

Laser cladding of NiCoCr alloy powder on cast iron was performed with a 1 kW Nd:YAG continuous laser. Despite numerous advantages, one of the most critical issues is the formation of martensite and a brittle layer in the vicinity of the bonding interface due to the inherent rapid cooling, which might result in cracking nearby and a decrease in bonding strength. The objective of this research is to produce well bonded NiCrCo alloy coating that is free of pores and cracks on cast iron without preheating the whole substrate. Considering the poor weldability of cast iron, the strategy of reciprocated deposition, namely multi-layer laser cladding, was applied. The microstructural evolution of the multi-layer coating on the cast iron was investigated with the emphasis on the variation of the bonding zone. The results showed that NiCoCr coatings with different layers consisted of fine solid solution dendrites surrounded by an inter-dendritic network of precipitates. The multi-layer clad coating had a gradual distribution of elements, which was different from that in the single layer coating. The martensite near the bonding interface was transformed into tempered sorbite under the thermal influence of multi-layer deposition, leading to a decrease in the high micro-hardness zone width. A bending test confirmed that the bonding brittleness was reduced and the mechanical properties were improved through the use of the multi-layer laser cladding approach.