激光熔覆NJ-4镍基合金涂层显微硬度的探究

目的探究晶体尺寸、组织结构和过冷度对NJ-4镍基合金涂层显微硬度的影响规律,找出一定组织结构和晶体尺寸下的最佳显微硬度。方法采用正交实验对基体进行激光熔覆,然后分析组织结构、晶体尺寸和过冷度对合金涂层硬度的影响。结果不同组织结构的NJ-4镍基合金涂层显微硬度有很大差异。从熔覆层的上表面到下表面依次为树枝晶、等轴晶、胞状枝晶、等轴晶、树枝晶、板条状马氏体。晶体结构依次变化时,显微硬度先增大、后减小、再增大,在熔覆层上部的等轴晶处的显微硬度最大。此外显微硬度还受到晶体尺寸和过冷度的影响。激光为熔池凝固提供特殊的冷却环境,抑制了凝固过程中杂质的析出,降低了缺陷的产生概率,提高了熔覆层硬度。激光熔覆层产生的板条状马氏体镶嵌在基体和熔覆层之间,提高了冶金结合强度,测量发现熔覆层的显微硬度是基体的2.5倍以上。结论熔覆层的显微硬度最终由组织结构、晶体尺寸和过冷度决定。

Study on Microhardness of Laser Cladding NJ-4 Powder

The work aims to study the influence of crystal size, structure and super-cooling on the microhardness of NJ-4 alloy coating and find out the optimum microhardness of the structure and crystal size. Orthogonal experiment was used to carry out laser cladding to the substrate. Then, the influence of structure crystal size and super-cooling degree on hardness of alloy coating was analyzed. From the results, the microhardness of different tissue structures was very different. Dendrite crystal, equiaxed crystal, cellular crystal, equiaxed crystal, dendrite crystal and banded martensite were respectively distributed from the upper surface to the lower surface of cladding layer. When the crystal structure changed in turn, the microhardness firstly increased, then decreased and finally increased again. The microhardness was greatest in the equiaxial crystals at the upper part of the cladding layer. The microhardness was also affected by crystal size and overcooling. The special cooling environment provided by the laser for solidification of molten pool inhibited the precipitation of impurities in solidification process and reduced the production probability of defects to improve the hardness of the cladding layer. The lamellar martensite produced by the laser cladding layer was inlaid between the substrate and the cladding layer and improved the metallurgical bonding strength. Through the measurement, the microhardness of the cladding layer was more than 2.5 times of that in the substrate. The micro- hardness is determined by the structure, crystal size and the degree of supercooling.