【作者】 田宝艳；

【导师】 毕见强； 张景德；

【作者基本信息】 山东大学 ， 材料学， 2008， 硕士

【摘要】 Fe-Al金属间化合物是介于高温合金与陶瓷之间的一种新型高温材料,该合金中最受关注的主要是Fe₃Al,这种合金具有优良的抗氧化和抗硫化性能、多种介质中的抗腐蚀性和较高的高温强度,密度低,不含贵重合金元素,有望成为新一代廉价耐腐蚀材料。但Fe₃Al室温韧性不足、加工成型困难的问题至今尚未获得圆满解决,从而限制了Fe₃Al作为结构材料在工业中的应用。而利用激光熔覆技术,在钢铁材料表面制备Fe₃Al涂层,可避开其上述缺点并可显著改善钢铁材料的表面性能,从而提高材料的服役寿命。本文以Q235钢为基体材料,利用激光熔覆技术,制备了Fe₃Al涂层。利用SEM,XRD等手段分析熔覆合金组织及相结构,探讨激光熔覆条件下Fe₃Al金属间化合物合金层的组织结构特点。以次亚磷酸钠为还原剂,用化学镀法制备Fe包覆Al复合粉末,通过研究络合剂、镀液pH值、镀液温度以及施镀时间对所得Fe/Al复合粉末镀覆效果的影响,得出优化的工艺参数为:硫酸亚铁8g/L;酒石酸钾钠16 g/L;柠檬酸6 g/L;次亚磷酸钠30～40g/L;pH值9.5;温度75～80℃;镀覆时间:40min。在上述工艺条件下在微米级铝粉表面包覆了一层均匀的团粒状的铁层,得到了以铝为核,铁为壳的核-壳包覆式复合粉末。在Q235钢表面激光熔覆通过化学镀法制得的Fe包覆Al复合粉末,可得到Fe-Al金属间化合物涂层,熔覆合金层主要由Fe₃Al和FeAl₂构成,覆层中分布着树枝晶、柱状晶和颗粒相。分析了激光熔覆中的凝固行为及凝固速度R和温度梯度G对凝固后晶体组织形貌的影响,温度梯度G/凝固速度R（G/R）是凝固组织生长形态选择的控制参量,随着距熔池底部距离的增加,G/R迅速下降,形成了树枝状或柱状晶的涂层区组织特征。由于在激光熔覆过程中熔化的基体材料通过对流传质作用扩散到涂层中,造成了涂层的稀释,从而使涂层成分有较大的改变。而在激光熔覆快速冷却过程中由于涂层中很高的温度梯度以及涂层与基体间的热膨胀系数差造成的热应力,容易导致涂层产生裂纹。为了与激光熔覆Fe包覆Al复合粉末生成铁铝金属间化合物涂层的组织形貌作比较,在Q235钢表面激光熔覆通过机械合金化法制得的Fe₃Al粉末,得到了成分为Fe₃Al的涂层。激光熔覆Fe包覆Al复合粉末同样可以得到Fe₃Al涂层,但与激光熔覆Fe₃Al粉末得到的涂层组织结构不同。前者熔覆层中分布着树枝晶、柱状晶和颗粒相,后者覆层组织为粗大等轴状晶团,等轴状晶团内部由大量极细小的条状Fe₃Al晶粒构成。更多还原

【Abstract】 Fe-Al intermetallic is a relatively new class of high-temperature material, which has melting point between high-temperature alloys and ceramics. Fe₃Al is a potential low-cost corrosion resistant material due to its outstanding properties such as good anti-oxidization and anti-sulphidation property, excellent corrosion resistance, high strength at a high temperature. However, Fe₃Al has poor ductility at room temperature, which in turn limits its applications as structural material. Fabricating Fe₃Al coating on steel substrate by laser cladding technonlogy is expected to combine the advantages of both Fe₃Al and steel—with high strength as steel and good surface property as Fe₃Al.In this paper, we will use laser surface modification technology to fabricate Fe₃Al coating on the surface of Q235 steel. The features of phase structure of the cladding alloy were studied with SEM, XRD et al.Composite powder for electroless iron plating on aluminium-matrix was prepared by chemical plating using sodium hypophosphite as reductant. By analyzing the influences of complexing agent, pH value, plating temperature and time on the coating, we get the optimal technological parameters: the concentrations are 8,16,6,30～40g/L for ferrous sulphate, potassium sodium tartrate, citrate acid, sodium hypophosphite and the pH value is 9.5, plating temperature is 75～80℃, plating time is 40 min. At technological conditions above mentioned, iron plating on aluminium-matrix of micron dimension has a granular surface morphology and composite powder that aluminium as nucleate and iron as shell was obtained.Using Q235 steel as substrate, Fe/Al composite powder produced by chemical plating as laser cladding material, we produce Fe-Al intermetallic coatings by means of laser surface modification technology. The cladding laser is composed of Fe₃Al and FeAl₂ phase, and the morphology contains dendrite crystals, columnar crystals and particles. We analyze the solidification behavior of the coating and the influence of solidification velocity R and temperature gradient G on the crystal structure and morphology after the solidification. The temperature gradient G and solidification rate R（G/R） is the dominant facts of the morphology of solidification structure. With distance from the bottom of the pool, G/R drop rapidly and form the coating characteristics which is constituted of dendrite and columnar crystals. Laser cladding, melted substrate diffused into the coating at the effect of contra-flow and mass transfer. This can make the coating diluted and component of the coating will have a great change. In the process of rapid cooling of laser cladding, thermal stress are produced due to the high temperature gradient in coating and the dilatometric coefficient contrast between substrate and coating, which may lead to cracking.In order to compare the microstructure with the coating produced by Fe/Al composite powder, using Q235 steel as substrate, the Fe₃Al prepared by mechanical alloying, we produce Fe₃Al graded coatings by means of laser surface modification technology. Using Fe/Al composite powder as laser cladding material, we can also obtain Fe₃Al coating, but the microstructure is different from the coating produced by laser cladding Fe₃Al powder. The former cladding layer contains dendrite crystals, columnar crystals and particles, and the latter consists of coarse equiaxed grain-groups, which consist of large amount of banding fine Fe₃Al grains.更多还原