激光熔覆钛基复合涂层中原位自生TiC的形态特征与生长机制

Morphological characteristics and growth mechanism of in-situ TiC in titanium-base composite coating formed by laser cladding

为研究激光熔覆过程中原位自生TiC的生长机制,使用扫描电子显微镜对TC4+10%Cr3C2混合粉末CO2激光熔覆涂层中原位自生TiC的分布与形态特征进行了观察。结果表明,涂层中TiC主要以枝晶、颗粒丛和平均尺寸在0.5μm左右的密集小颗粒3种形态出现,并表现出了"枝晶(表面附近)→颗粒丛(涂层中部)→密集小颗粒(结合界面附近)"的分布特点。涂层中的枝晶具有块状颗粒线性排列的特征,多呈十字状,二次晶臂间表现出较一致的取向;颗粒丛形貌中不同TiC颗粒间取向杂乱,表现为随机粘结,无明显取向;结合界面附近的密集TiC小颗粒多呈随机分布,表面光滑近球状,无明显的棱角。分析表明,涂层中不同形态的TiC分布主要取决于对应位置的过冷度和冷却速度。大的过冷度能够显著增加形核密度,而过高的冷却速度将导致TiC晶体长大不充分。涂层中原位自生TiC所表现出的上述分布与形态特征为TiC晶体自身的晶体学特点和液固前沿的温度、浓度梯度等因素共同作用的结果。

In order to study the growth mechanism of in-situ TiC ,the scanning electric microscope（SEM） was used to observe the distribution and morphological characteristics of in-situ TiC in a Ti-base composite coating formed by CO2 laser cladding with preplaced TCA ＋ 10% Cr3C2 powder. The results show that the TiC are with the dendrite, cluster and crowded together grains（ with a smaller average dimension of about 0.5 μm）morphologies in the coating and shows a distribution characteristic of “ dendrite （near the surface ）→ cluster（ in the middle part）→crowded together grains （near the bonding zone）”. The dendrites are usually in cross-like shape with the characteristics of being composed of grains in a linear arrangement, and the dendrite arms＇ orichalc are consonant;the orichalc of TiC grains for cluster morphologies are usually randnm; for the single grain, the distribution are random and the surface are even with ball-like shape. The distribution characteristic of TiC is due to the degree of supercooling and cooling rate. A high supercooling can increase the density of crystallization nucleus,and high cooling rate can cause the growth of TiC dendrite insufficient. The characteristics of TiC in the composite coating are mainly attributed to the effects of their crystallography characteristics, temperature and concentration gradient at the liquid-solid boundary.