Additive manufacturing has been emerging as a highly efficient method that minimizes the consumption of virgin materials and energy to rapidly repair the damaged parts and potentially recover the original performance. Here, the microstructures and mechanical properties of polycrystalline titanium (Ti) with surface damage after powder bed fusion (PBF) repair and post-polishing treatment are investigated by molecular dynamics simulations. It is found that during PBF repair of the surface damage by melting Ti nanoparticles, grain boundaries (GBs) in polycrystalline Ti are more susceptible to damage from temperature elevation than the grain interiors. During the solidification process, grains grow in the form of columnar crystals. After PBF repair and post-polishing treatment, the yield strength of damaged samples is recovered to 96.8% of that of perfect ones. This restoration can be attributed to the increased capacity of carrying dislocations due to that the GB directional growth generates dislocation walls after PBF repair. More importantly, the flow stress exceeds that of the perfect sample due to the increase in stacking faults and the appearance of twins after PBF repair, which enhance the resistance of dislocation motion. The fatigue performance of the damaged samples processed by combinatorial PBF repair and post-polishing treatment is found to surpass that of perfect samples. This is attributed to that the elimination of rough surface and the reduction of intragranular dislocations avoid dislocation entanglement that otherwise results in strain concentration, and also prevent the premature occurrence of twins and the rapid increase in stacking faults during the cyclic loading, thus retarding the nucleation of intergranular cracks. Our molecular dynamics simulation results could help shed light on the atomic mechanism of microstructure evolution during PBF repair and mechanical performance after PBF repair.

中文翻译：

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钛表面损伤的粉末床熔融修复：微观结构和力学性能的分子动力学研究

增材制造已经成为一种高效的方法，可以最大限度地减少原始材料和能源的消耗，从而快速修复受损部件并有可能恢复原始性能。在这里，通过分子动力学模拟研究了经过粉末床熔融（PBF）修复和后抛光处理后表面损伤的多晶钛（Ti）的微观结构和力学性能。研究发现，在通过熔化钛纳米颗粒进行 PBF 修复表面损伤的过程中，多晶钛中的晶界 (GB) 比晶粒内部更容易受到温度升高造成的损伤。在凝固过程中，晶粒以柱状晶体的形式生长。经过PBF修复和后抛光处理后，损伤样品的屈服强度恢复到完好样品的96.8%。这种修复可归因于PBF修复后晶界定向生长产生位错壁，从而增加了携带位错的能力。更重要的是，由于PBF修复后堆垛层错的增加和孪晶的出现，流变应力超过了完美样品，增强了位错运动的阻力。通过组合PBF修复和后抛光处理处理的损伤样品的疲劳性能超过了完美样品。这是由于粗糙表面的消除和晶内位错的减少避免了位错缠结而导致应变集中，也防止了孪晶的过早出现和循环加载过程中堆垛层错的快速增加，从而延缓了成核。的晶间裂纹。我们的分子动力学模拟结果有助于揭示PBF修复过程中微观结构演化的原子机制以及PBF修复后的机械性能。