增材制造技术是在计算机辅助制造程序的控制下，通过逐层增加材料制造构件的技术，该技术能够制作出传统制造方法难以制造的形状复杂的构件，在航空航天、医药等领域得到了广泛应用。在增材制造领域，残余应力引起的构件损伤、变形和破坏问题至关重要，因此，本文围绕增材制造样品的残余应力和变形展开研究。通过有限元模拟方法分析了激光熔化沉积316L不锈钢成形件的残余应力的发展过程。此外，本文通过实验研究了不同扫描策略对激光沉积样品变形的影响，通过优化扫描策略减小了样品的变形。本文的主要研究内容如下：
首先研究了激光沉积过程中残余应力的发展。本文依次建立了单道10层薄壁件、单道50层薄壁件和单层15道沉积层的激光沉积的热力耦合有限元模型，模拟了激光沉积316L不锈钢薄壁件和沉积层的温度场和应力场的发展过程。模拟结果表明，激光沉积过程中，温度和残余应力随时间往复变化，随着沉积层数的增加，温度和残余应力的变化幅度变小。随着高度的增加，薄壁件底部的纵向应力逐渐减小，薄壁件顶部的纵向应力略有减小，薄壁件边界的竖直方向应力逐渐增大。在样品刚沉积结束的道次附近，拉应力最大。
然后研究了减小激光沉积过程中残余应力导致的变形的策略。本文通过实验研究了分形扫描策略和基于分形曲线的分区扫描策略对激光沉积样品变形的影响。结果表明，三种分形扫描策略对应的基板最大变形量分别为：Peano曲线3.3 mm，Sierpinski曲线2.5 mm，Lebesgue曲线3.8 mm，明显小于传统扫描策略对应的基板最大变形量7.5 mm，因此，分形扫描策略可以显著减小样品变形。三种基于分形曲线的分区扫描策略对应的基板最大变形量分别为：Hilbert曲线顺序3.5 mm，Sierpinski曲线顺序3.2 mm，Lebesgue曲线顺序5.4 mm，明显小于传统的分区扫描策略，因此，基于分形曲线的分区扫描策略可以显著减小样品变形。此外，在基于分形曲线的分区扫描策略下，扫描线段的方向和数量可以灵活的调节。在综合考虑扫描线设计的灵活性和变形量的情况下，基于Sierpinski曲线的分区扫描策略为最优策略。
综上所述，本文的研究有助于深入的了解激光沉积过程中的残余应力的分布规律和发展过程，此外，对于减小激光沉积过程中的残余应力和变形具有较好的指导意义。

Additive manufacturing is a technique for fabricating components layer by layer under the control of computer-aided manufacturing programs. This technology can manufacture components with complex shapes that are difficult to manufacture with traditional methods and it has been applied in aerospace, medicine and other fields. Component damage, deformation and failure caused by residual stress is one of the most important problems in additive manufacturing. Therefore, this paper focuses on residual stress and deformation in additive manufacturing. Finite element models are built to simulate residual stress evolution of laser deposited 316L stainless steel parts. In addition, the effects of different scanning strategies on the deformation of the laser-deposited samples are investigated experimentally, and the deformation of the samples is reduced by optimizing the scanning strategy. The content of this paper is as follows.
This paper studies residual stress evolution in laser deposition first. Coupled thermomechanical finite element models about a 10 layer thin-walled part, a 50 layer thin-walled part and a 15 track deposited layer are built. The temperature field and stress field are simulated by the models. The simulation results show that the temperature and residual stress change reciprocally during the laser deposition process. With the increase of deposited layers, the changes in temperature and residual stress become smaller. As the height increases, the longitudinal stress at the bottom of the thin-walled part decreases gradually, the longitudinal stress at the top of the thin-walled part decreases slightly, and the vertical stress at the boundary of the thin-walled part gradually increases. In the vicinity of the just deposited pass, tensile stress is the largest.
Besides, this paper studies strategies to reduce the deformation caused by residual stress during laser deposition. The effects of fractal scanning strategies and subarea scanning strategies based on fractal curves on the deformation of laser deposition samples are investigated experimentally. The results show that the maximum substrate deformation of three fractal scanning strategies is 3.3 mm for the Peano curve, 2.5 mm for the Sierpinski curve, and 3.8 mm for the Lebesgue curve, which is significantly smaller than the maximum substrate deformation of traditional scanning strategy. The strategies can significantly reduce sample deformation. The maximum substrate deformation of three subarea scanning strategies based on fractal curves is: Hilbert curve order 3.5 mm, Sierpinski curve order 3.2 mm, Lebesgue curve order 5.4 mm, which is significantly smaller than traditional subarea scanning strategy. The subarea scanning strategies based on fractal curves can significantly reduce sample deformation. In addition, under the subarea scanning strategy based on fractal curve, the direction and number of line segments can be flexibly adjusted. Considering the flexibility of segments design and deformation, the subarea scanning strategy based on Sierpinski curve is the optimal.
To sum up, the simulation on the temperature field and residual stress field in this paper is helpful for a deep understanding of the distribution and evolution of residual stress during the laser deposition process. In addition, this paper has a guiding significance for reducing the residual stress and deformation in laser deposition.