Objective Renovating surface defects of Ni-based single crystal superalloy blades is the key to prolong their manufacturing life, and repair mechanism is also a hot research topic in material physics and chemistry. In this study, laser melting method combined with theoretical model calculation is used, and the self-developed DD series Ni-based superalloys in China is taken as the research object. On the basis of previous studies, the substrate orientation has rotated 45°around the direction of [010] during laser melting of single crystal alloys, then rotate different angles ξ along the direction of [001], the distribution and variation of different crystal orientation regions with the increase of ξ are studied. It is found that with the increase of ξ, the distribution of single crystal domain increases on the side of molten pool. Substrate orientation 112/[201ˉ] with single crystal region on one side of molten pool is obtained. Repairing operation on its substrate, because of the disappearance of [010] domain in the molten pool of epitaxial growth repair zone, the grain boundary [001]/[010] also disappears, the ability of stray grain formation is greatly reduced. The substrate orientation suitable for laser cladding repair is obtained, and the mechanism of the effect of changing substrate orientation on the formation ability of stray grain in the epitaxial growth structure is demonstrated.
Methods In our previous research, the mechanism of substrate orientation for the formation of equiaxed crystals has been elaborated in detail. It is defined that when the laser scans along the [100] direction on the crystal plane, that is, the substrate orientation relationship is the initial substrate orientation. When the initial matrix is rotated 45° around the crystal direction (i.e., the y-axis), the growth of heterocrysts can be effectively suppressed. On this basis, this study continues to take the crystal direction as the rotation axis. Through the second rotation of different ξ and laser melting technology, the mechanism of inhibiting the growth of heterocrystals is revealed, and the optimal substrate orientation is obtained to effectively control the single crystal nature of the repaired area.
Results and Discussions According to the experimental results, for the initial substrate crystal orientation (001)/[100], the value of the single-layer repair height is limited by the position of the [001]/[100] crystal zone boundary in the laser cladding epitaxial growth structure, and the position of the [001]/[100] crystal zone boundary in the weld pool is related to the initial substrate crystal orientation. For the matrix (101)/[101]ˉ crystal orientation rotated 45° around the y-axis, as shown in Fig. 2 (a) and Fig. 2 (b). In this case, because there is no crystal zone boundary in the epitaxial growth structure by laser cladding, the height can be effectively raised in the single-layer repair. However, the monocrystal property of the repaired structure is affected by the existence of the sum zone on the left and right sides. However, when the substrate with ξ=45° is used (as shown in Fig. 10), due to the disappearance of the [010] crystal zone in the weld pool of the epitaxial growth repair structure, the intersection line of the [001]/[010] crystal zone on one side of the corresponding molten pool is generated, and all the crystal regions are [001] crystal regions on one side of the molten pool, so the substrate orientation with only a single crystal zone can be obtained. Based on the above matrix orientation combined with multi-channel and multi-layer laser cladding experiment, the single crystal structure of laser cladding repair can be obtained completely, which plays a great role in improving the single crystal property of laser cladding epitaxial growth structure.
Conclusions In this paper, we propose a laser cladding repair method for the secondary rotating matrix, and obtain the substrate orientation suitable for the growth of single dendrite direction. The specific conclusions are as follows:
1. In the experiment of laser cladding epitaxial growth and repair, the choice of the initial substrate orientation can affect the ability of heterocrystal formation in cladding epitaxial growth structure under the condition of other repair parameters such as laser parameters, scanning speed and so on.
2. When the initial crystal direction rotates 45° around the y-axis, the crystal direction is rotated by the rotation axis (secondary rotation). When ξ=45°, the laser cladding repair process is carried out along the crystal direction on the surface. The single-layer repair height increases and the single-layer dendrite growth direction is ensured on one side of the weld pool, so as to ensure the monocrystalline of the laser cladding epitaxial growth structure.
3. The internal mechanism of this situation is that after changing the initial crystal substrate orientation, the crystal area in the laser melting pool expands and occupies the area where the original crystal zone is located, and the boundary of the crystal zone disappears in the molten pool. Therefore, a large single crystal area can be adopted, which ensures the repair efficiency and single crystal property of the laser cladding epitaxial growth structure.
