Effect of laser remelting on the microstructure and mechanical properties of AerMet100 steel fabricated by laser cladding

AerMet100 ultrahigh-strength steel is used in safety-critical applications with limited repair methods. A repair technology using laser cladding (LC) has recently been developed, but it also has limitations due to mechanical anisotropy of the columnar crystal in the cladding layer. Herein, laser remelting (LR) is a process in which an LR layer with lower power density but without powder delivery has been applied after each LC layer is deposited to inhibit the epitaxial growth of columnar crystals and relieve the mechanical anisotropy. It was found that the LR process melted columnar crystal tips and rod-shaped carbides of the LC layer of AerMet100 steel into fragments and smaller sized granular carbides, respectively. In the LR melt pool, the fragments blocked the columnar solidification front to hinder its growth, or nucleated at the bottom of the melt pool to form short-thick columnar and equiaxed crystals to inhibit the epitaxial nucleation of the columnar crystals. The undissolved granular carbides clustered at the tips of the columnar dendrites and stopped the growth of columnar crystals. Moreover, the undissolved carbides also served as heterogeneous nucleation sites in the LR melt pool, extending the equiaxed crystal solidification zone to higher thermal gradient and lower solidification rate, which resulted in the formation of the LR layer whose microstructure morphology was mainly equiaxed grains. The AerMet100 forged steel repaired by the LC plus LR process minimised the impact of the crystal texture of the LC layers on the mechanical anisotropy due to the slowdown of crack propagation in the equiaxed crystal zone. It exhibited superior mechanical properties to those of AerMet100 steel repaired by LC alone.