Objective  DD5 nickel-based single-crystal (SX) alloy has been widely applied to manufacture the aeroengine turbine blades due to its excellent high-temperature strength and creep resistance. However, many types of damage to SX turbine blades, e.g., blade tip erosion, crack, are unavoidable in the harsh working environment, which shortens the service life of SX turbine blades. Therefore, it is urgent to study the repair of damaged SX turbine blades. The laser deposition technology, which can provide high temperature gradients and allows the addition of controlled amounts of material to required locations, is beneficial to repair the damaged SX alloy parts. According to investigation on laser deposition repair of SX alloy by scholars, the damaged SX alloy can be successfully repaired by properly controlling laser process and repairing SX alloy with different materials has bright prospects. At present, minimal reports have discussed the laser deposition repair of DD5 SX alloy using different materials. Therefore, the DD5 SX alloys are repaired by laser deposition technology using the DZ125 superalloy powder. The influence of laser power, scanning velocity and powder-feeding on dendrite epitaxial growth is systematically investigated by the orthogonal experiment method. The microstructure and microhardness of the single multi-layer as-deposited sample are analyzed. This study is aimed at providing a guide for the repair of damaged DD5 SX alloy.

  Methods  The gas-atomized DZ125 superalloy powders were used as the depositing materials in this experiment and the cast DD5 SX alloys with the crystallographic orientation ((001)/100) normal to the depositing surface were applied as the substrate. Firstly, the DD5 SX alloys were repaired by laser deposition technology. The influence of laser power, scanning velocity and powder-feeding on dendrite epitaxial growth is systematically investigated by the orthogonal experiment method. Then, the laser deposition experiment of single multi-layer was carried out. The microstructure of the single multi-layer as-deposited sample was characterized by optical microscope, scanning electron microscope and the chemical composition was determined by EDS analysis. Finally, the microhardness of substrate and deposition zone was tested by Vickers hardness tester to explore the variation trends of microhardness and the relationship between the variation trends of microhardness and the microstructure.

  Results and Discussion   Under the conditions of different heat input and powder-feeding rate, the dendrite epitaxial growth in the deposition zone is different (Fig.4). It is obvious that the influence of laser powder and powder-feeding rate on the dendrite epitaxial growth is remarkable, and the effect of scanning speed on the dendrite epitaxial growth is relatively weak (Fig.6). An increase in laser powder can heighten epitaxial growth height, it also significantly decreases the ratio of epitaxial growth. Similarly, the influence of powder-feeding rate on the variation trends of the height and ratio of dendrite epitaxial growth is similar to the laser powder. Therefore, the ratio of dendrite epitaxial growth can be prominently improved with the lower heat input and powder-feeding. When the laser power is 420 W, the scanning speed is 6 mm·s⁻¹ and the powder-feeding rate is 1.5 g·min⁻¹, the ratio of dendrite epitaxial growth is about 100% (Fig.7). According to the microstructure of the single multi-layer as-deposited sample, it is known that the dendrites are planar crystals and columnar crystals along the deposited direction at the bottom and middle of the deposited zone. There are equiaxed crystals at the top (Fig.8). Moreover, the γ′ particles in dendrite epitaxial region of deposition zone unevenly distribute in the γ matrix and the size of γ′ particles in the inter-dendrite is much bigger than that in the core-dendrite (Fig.9(c)). In addition, short rod-like MC carbides with high Ta content are distributed in the inter-dendritic region at the bottom of the deposition zone (Fig.12(c)). Small blocks and octahedral MC carbides are randomly distributed at the top (Fig.12(e)). This is because the heat accumulation at the bottom of the deposition zone is serious and the top of deposition zone is relatively weak. Through the analysis of the microhardness of deposition zone, it is concluded that the average microhardness of the deposition zone is 449 HV_0.5, which is slightly higher than that of the substrate 425 HV_0.5 (Fig.13). Moreover, the microhardness of the different deposition zone is slightly different. The microhardness at the bottom of the deposition zone is higher than that at the middle and top due to the higher content of Ta in MC carbides (Tab.3).

  Conclusions  The DD5 SX alloys are repaired by laser deposition technology using the DZ125 superalloy powder. The specific conclusions are as follows: 　　(1) The influence of laser powder and powder-feeding rate on the ratio of dendrite epitaxial growth is remarkable and an decrease in the heat input and powder-feeding rate can effectively increase the ratio of dendrite epitaxial growth. When the laser power is 420 W, the scanning speed is 6 mm·s⁻¹ and the powder-feeding rate is 1.5 g·min⁻¹, the ratio of dendrite epitaxial growth is about 100%.　　(2) The dendrites are planar crystals and columnar crystals along the deposited direction at the bottom and middle of the single multi-layer deposition zone. There are equiaxed crystals at the top. In addition, the size of γ′ particles in the inter-dendrite is much bigger than that in the core-dendrite due to the higher content of elements of Al and Ta in the inter-dendritic.　　(3) Affected by the high temperature of the molten pool, the carbides in the heat affected zone can dissolve in γ matrix, which reduces the carbides size. The carbides are distributed in the inter-dendritic region at the bottom and middle of deposition zone, while the carbides are randomly distributed at the top. Due to the heat accumulation, the shape of the carbides at the bottom and middle of deposition zone are mostly short rod-like. Compared with the bottom and middle of the deposition zone, the heat accumulation at the top is weak, which induces the formation of small blocks and octahedral carbides.　　(4) The average microhardness of the deposition zone is slightly higher than that of the substrate. Compared with the middle of the deposition zone, the microhardness at the bottom and top is slightly higher, and the microhardness at the bottom is the highest.