Microstructure And Properties Of The Laser Cladding Of The Cobaltbased Alloy With TiC+B₄C Composite

In this thesis,the cladding layer of（TiC+B₄C）/Co-based alloy is prepared on the surface of nodular cast iron using the fiber laser.The effects of the laser parameters and the amount of（TiC+B₄C）addition on the microstructure and properties of cladding layer are investigated.The microstructure and composition distribution near the cladding layer surface are analyzed and the hardness of the laser cladding coating tested.In addition,the effect of the covered powder of B₄C on the microstructure and hardness of the cladding layer of（TiC+B₄C）/Co-based alloy is investigated by surfacing the B₄C.The results show that:（1）The microstructures of laser cladding coating of（TiC+B₄C）/Co-based alloys on nodular cast iron surface mainly are composed of cellular crystal and dendritic crystal,in which There are TiC with unmelted block and TiC with petaloid shape which is precipitated after melting and also B₄C present in the form of unmelted cluster particles.With the content of（TiC+B₄C）increasing,the microstructures of the cladding layer is refined firstly and then coarsened.（2）With the increase of laser power,the dendritic arms spacing in the microstructure of the coating increases and then decreases when the laser scanning speed was fixed.The size of the phase of TiC and clustered particles of B₄C both gradually decreases,which the amount of petaloid TiC increases.With the increas of the laser scanning speed,the number and size of cellular crystals in the cladding layer microstructure decrease gradually,but the dendrite crystals increase with the primary dendrite arm spacing and the secondary dendrite arm spacing decreasing when the laser power is constant.The agglomerating degree of B₄C increase to some extent and the average size of TiC increase with the laser scanning speed.（3）Under the same process parameters,when the addition of（TiC+B₄C）to the laser cladding layer is 30%,its hardness is the highest(1180.45 HV_0.2)which is 4.9 times of the matrix of nodular cast iron.There is the higher average hardness of the cladding layer with1379.28 HV_0.2 which is 5.9 times of the matrix when the mixed powder with the（TiC+B₄C）content of 50%is laser cladded,which the laser power P=1.2 k W and the scanning speed v=4mm/s are adopted.（4）The sharpness and surface energy of B₄C particles covered by Ni are reduced and the dispersibility of the powder effectively improved.There is more disperse distribution of B₄C in the cladding layer with 30 wt.%（B₄C covered with Ni+TiC）than that without Ni cover.After the particles of B₄C are covered with Ni,the primary dendrite arm in the cladding layer is reduced from 4.37μm to 1.91μm and its average hardness is 1467.25 HV_0.2.