Numerical Simulation of Stress Evolution of Thin-Wall Titanium Parts Fabricated by Selective Laser Melting
KE Linda1,YIN Jie2(),ZHU Haihong2,PENG Gangyong2,SUN Jingli1,CHEN Changpeng2,WANG Guoqing3,LI Zhongquan1,ZENG Xiaoyan2
1. Shanghai Engineering Technology Research Center of Near-Net-Shape Forming for Metallic Materials, Shanghai Spaceflight Precision Machinery Institute, Shanghai 201600, China 2. Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China 3. China Academy of Launch Vehicle Technology, Beijing 100076, China
Cite this article:
KE Linda,YIN Jie,ZHU Haihong,PENG Gangyong,SUN Jingli,CHEN Changpeng,WANG Guoqing,LI Zhongquan,ZENG Xiaoyan. Numerical Simulation of Stress Evolution of Thin-Wall Titanium Parts Fabricated by Selective Laser Melting. Acta Metall Sin, 2020, 56(3): 374-384.
Selective laser melting (SLM) is a very promising additive manufacturing (AM) technology for fabrication of thin-walled parts due to its high forming accuracy with complex shape. The higher temperature gradient in rapid heating and cooling process is prone to produce larger thermal stress, which will induce warpage deformation of SLMed parts. However, most of the current SLM stress studies focus on the residual stress, and only a few reports on the transient stress in the thermal cycle during SLM. In this work, a thermal-mechanical coupled transient dynamic finite element model was established to study the effects of laser scan rate and layer thickness on stress evolution during SLM processing. The results show that under the action of thermal cycle, the internal stress evolution in SLM of titanium alloy thin-walled parts presents a thermal stress cycle. Under the relief annealing of the thermal stress cycle, the peak thermal stress increases first and then decreases in the heating stage, and stabilizes and approaches the value of residual stress in the cooling stage. The residual stress of SLMed thin-walled parts is less than the transient peak stress during heating. After several thermal cycles with stress relief annealing effect, the peak thermal stress of SLM thin-walled parts can be reduced by more than 30%.
Fund: National Natural Science Foundation of China(61805095);National Natural Science Foundation of China(51701116);Shanghai Science and Technology Innovation Action(17JC1402600);Shanghai Aerospace Science and Technology Innovation Fund(SAST2017-58);Shanghai Sailing Program(16YF1405000)
Fig.1 Temperature dependency of thermal conductivity (a) and volumetric enthalpy (b) of Ti-6Al-4V alloy(Tliquid—liquidus temperature, Tα→β—temperature of transition from α phase to β phase of Ti-6Al-4V alloy)
Fig.2 Temperature dependency of elastic modulus (a) and yield strength (b) of Ti-6Al-4V alloy
Fig.3 Temperature distributions and molten pool evolutions in the longitudinal mid-section during selective laser melting(SLM) of Ti-6Al-4V thin-walled parts under different laser scan rates of 1000 mm/s (a), 700 mm/s (b) and 500 mm/s (c) (The colored arrow vectors represent the maximum heat flow directions at the tail of the molten pool boundary)Color online
Fig.4 Comparisons of simulation and experimental results of the molten pool width and depth at the cross-section under different laser scan rates (Insets show the OM images of molten pools under scanning velocities of 500, 700 and 1000 mm/s, respectively)
Fig.5 Comparisons of simulation and experimental results of grain growth orientation during SLM of Ti-6Al-4V thin-walled parts (The tilt angles between the maximum heat ?ow directions and the building direction can be calculated from the temperature gradients at the tail of the molten pool boundary (θsim), and also measured experimentally by analyzing optical micrographs (θexp))Color online
Fig.6 Comparisons of simulation and experimental results of residual stress in SLM of thin-wall parts(a) left nodes (b) right nodes
Fig.7 Effects of laser scan rate on the stress distribution in SLM of thin-wall parts (MN indicates the minimum residual stress, and MX indicates the maximum residual stress)Color online(a) 500 mm/s (b) 700 mm/s (c) 1000 mm/s
Fig.8 Schematic of D1 node at the upper surface of 1st layer of the thin-wall parts (a), and effects of layer thickness on the thermal stress cycle (b) and thermal cycle (c) (σn_max and σn_min represent the maximum stress and the minimum stress in each thermal stress cycle during SLM, respectively; σresidual is the residual stress in the final cooling stage; Tn_max represents the maximum temperature during each thermal cycle; Tannealing is the annealing temperature of Ti-6Al-4V alloy)
Fig.9 Decreases in amount (a) and percentage decreases (b) of the thermal stress of D1 node in cooling stage as a function of subsequent thermal cycle under different layer thicknesses
Fig.10 Decreases in amount (a) and percentage decreases (b) of the thermal stress of D1 node in heating stage as a function of subsequent thermal cycle under different layer thicknesses
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