目的 针对基板表面在激光定向能量沉积转角结构时易出现热积累与残余应力集中问题,研究转角角度与沉积速度对温度场和应力场演化的影响规律。方法 以316L不锈钢为研究对象,建立激光定向能量沉积的热-力耦合有限元模型,并进行实验验证模型的准确性。结果 转角角度减小会显著增强热积累,并增大内外角的温度梯度差。转弯后转角区域边界的残余应力相比直线熔道明显升高,沉积速度对残余应力的影响呈现非线性特征,在低速区（10~15 mm/s）和高速区（20~25 mm/s）影响较大。在10 mm/s低速沉积小角度（15°）转角区域时发生局部重熔效应,可释放热应力,最终使表面残余应力相较离开转角区域时的边界下降27.4%。最大残余应力位置随转角角度增大由集中在转角区域边界变为均匀分布。在沉积层和基板结合的边界处,残余应力主要集中在外角、内角、沉积起始和终止位置。外角处残余应力大于内角,终止位置残余应力大于起始位置。结论 转角角度和沉积速度是热应力变化的主要影响因素,激光的重熔效应可以调控残余应力。本研究结果可为表面具有复杂路径特征的激光定向能量沉积工艺优化与残余应力调控提供理论依据。
Abstract
Laser directed energy deposition (LDED) offers high flexibility and efficiency, enabling rapid fabrication of large components in small batches as well as surface enhancement and repair and has become an advanced technology for surface structure design and remanufacturing. Complex geometries mainly include cross-shaped, ring/columnar, S-shaped, and polygonal structures. Corner structures are the fundamental elements of these complex geometries. Overfilling is a key challenge in manufacturing of corner structures. In addition to causing geometric deviations, excessive heat accumulation in corner zones increases the risk of stress concentration and distortion.
To investigate the relationships between corner angle, deposition velocity, and the temperature and stress fields, numerical simulations were combined with experiments. Under constant laser power, powder feed rate and stand-off distance, corner angles of 15°, 30°, 45°, 60°, 90°, 120°, 150°, and 180° and deposition velocities of 10, 15, 20, and 25 mm/s, the geometric structures were examined. Since deposition velocity changed the molten pool size and thus the geometry of the deposited layer, separate finite element mesh models were established for the four deposition velocities, with a uniform element size of 0.3 mm in the deposited layer. Both powder and substrate materials were 316L stainless steel. Straight-line deposition at 180° was used for parameter calibration and model validation.
By analyzing peak temperature, cooling-stage temperature, cooling-stage temperature gradient, temperature change rate, and equivalent residual stress at different locations of corner structures, the effects of secondary laser scanning and corner geometry on the temperature and stress fields were clarified. The results showed that corner angle significantly affected heat accumulation and residual stress. Smaller angles (e.g., 15°) intensified heat accumulation and increased temperature gradients and cooling rates, which markedly raised residual stress. When the corner angle exceeded 90°, the temperature and stress distributions approached those of straight-line deposition, and the angle effect weakened. Deposition velocity affected the temperature and stress fields in a nonlinear manner. In the low-velocity (10-15 mm/s) and medium-velocity (15-20 mm/s) ranges, temperature and residual stress varied strongly with velocity. Changes were smaller at velocity speed (20-25 mm/s), although excessively high velocities might increase residual stress. The relationship between residual stress and deposition velocity was non-monotonic. In this work, a medium deposition velocity helped reduce stress concentration.
During low-velocity deposition at 10 mm/s for a small-angle (15°) corner structure, local remelting occurs within the corner zone, which releases thermal stress. As a result, the residual stress in the remelted region (241 MPa) is reduced by 27.4% compared with that at the boundary upon leaving the corner zone (322 MPa). As the corner angle increases, the location of maximum residual stress shifts from concentration at the corner boundary to a more uniform distribution. At the interface between the deposited layer and the substrate, residual stress mainly concentrates at the outer corner, inner corner, and the start and end of deposition. Residual stress at the outer corner is higher than that at the inner corner, and the end position shows higher residual stress than the start position.
Corner angle and deposition velocity are the primary factors governing thermal stress evolution, and laser remelting provides an effective means for regulating residual stress. These findings provide a theoretical basis for process optimization and residual stress control in laser directed energy deposition of complex path features.
关键词
激光定向能量沉积 /
转角角度 /
沉积速度 /
温度场 /
应力场 /
数值模拟
Key words
laser directed energy deposition /
corner angle /
deposition velocity /
temperature field /
stress field /
numerical simulation
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基金
中国科学院基础与交叉前沿科研先导专项（XDB0910000）;国家自然科学基金（52205370,52206252）
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