材料期刊网

DD8单晶镍基高温合金经激光扫描快速熔凝后, 熔池表面宽度随扫描速率增加变窄; 一次枝晶间距随冷却速率增加而减小. 分析表明: 熔凝后元素的枝晶偏析不明显, 合金成分趋于均匀. 熔凝后的凝固形态由熔池底部到表面依次为准平面凝固、胞状凝固、胞枝状凝固和细密枝晶凝固. 准平面和胞状凝固区由γ固溶体及弥散γ'相 (20 nm) 组成. 在胞枝晶和枝晶凝固区, 枝晶间出现一种高Ti含量条状相. 在过渡区, γ固溶体及γ'相皆部分熔凝, 未全溶的γ'相成为熔凝再析出弥散γ'相的核心, 过渡区γ'相呈堆状分布.

The laser glazing rapid solidification can make the dendrite refined and reduce the segregation of alloying elements, so it is very favorable to
mechanical properties of alloys, e.g., the fatigue life of Ni--based superalloy can be prolonged by laser surface melting process due to improved
resistance to stress corrosion cleavage. The laser surface melting process can be used for modification of surface or repair of casting defects (such
as surface porosity, surface stray grains) in single crystal components. Recently, this technique has become an attractive research subject with latent application. A successful laser glazing rapid solidification to single--crystal should ensure the preservation of the single--crystal nature, i.e., the
re--solidified surface layer needs to be epitactic with the substrate. The microstructure after laser re--solidification is closely related to the processing parameters, however, the relationship between microstructure and re--solidification conditions or processing parameters is not well understood
and needs further study. In the present work, the microstructure of laser glazing solidified Ni--based single crystal DD8 was investigated by OM, SEM as well as TEM. The results show that the surface width of the melted pool becomes narrow with an increase of scanning rate, so the melted pool has a relatively high interface/volume ratio which leads to a higher cooling rate. The primary dendrite arm spacing decreases with an increase of cooling rate, and gradually reaches a minimum about only 3 μm, which is two orders of magnitude smaller than that of the untreated part of alloy. EDXA shows that the dendritic segregation is not obvious and the chemical compositions tend to be homogeneous after re--solidification, which is caused by the higher solidification rate and related partition coefficients of alloying elements. The solidified structures are composed of the quasi--plane front, cellular and fine dendrites from substrate to surface of melted pool. The structures in the regions of the quasi--plane front and cellular consist of γ--solid solution and dispersive γ' precipitates with about 20 nm in size. In the regions of the cellular--dendrite and the dendrite, however, there is a eutectic structure with a higher Ti content in the interdendrites. These eutectic structures are small in size and look like leaves. In the transition region, both γ and γ'--phases are not completely melted, and there are some dislocations distributed on the interfaces of the γ'--phase particles induced by thermal stress between γ and γ' phases at higher cooling rate. The unmelted γ' particles can act as nuclei of  the second precipitated γ' phases which have a size of about 10 nm and a pile--up--like morphology.