铝合金熔滴复合电弧增材组织演化及外延生长特性

Microstructure Evolution and Epitaxial Growth Characteristics of Droplet and Arc Deposition Additive Manufacturing for Aluminum Alloy

金属增材制造可实现铝合金构件高性能一体化成形,其中熔滴复合电弧增材是一种高效、低成本的增材工艺。为了揭示其沉积过程中微观组织生长演变规律,从而优化工艺提升构件力学性能,本工作研究了2319铝合金沉积过程中的温度场分布、微观组织演化以及外延生长特性。首先,通过有限元结合生死单元法计算沉积过程中温度场并提取熔池不同位置的凝固参数,然后代入相场模型实现跨尺度耦合获得熔池不同位置微观组织生长演变过程,发现在熔池底部和中部区域形成柱状晶结构,在中上部出现从柱状晶向等轴晶转变的现象。同时在相场模型中引入取向偏差角,研究凝固过程外延生长特性,结果表明,取向偏差角越大,对枝晶形貌影响越明显,在竞争生长中越容易淘汰。金相分析显示从沉积层底部到上部,微观组织经历了从柱状晶到等轴晶的转变过程,并且柱状晶存在外延生长现象,与模拟结果吻合较好。

Aluminum alloys are widely used in the automobile,rail transportation,and aerospace industries owing to their excellent properties such as low density,high thermal conductivity,and high specific strength.Metal additive manufacturing(MAM)enables the high-quality integrated forming of aluminum alloy components.Among the MAM techniques,droplet and arc additive manufacturing(DAAM)is a newly proposed method that offers advantages,such as high efficiency and low cost.In DAAM process,a droplet generation system is designed above the substrate fixed on a three-dimensional motion platform.Below the droplet generation system,an arc heat source with variable polarity is tilted.During the DAAM process,the metal droplets drop vertically and sequentially into the molten pool generated by the arc heat source to realize metallurgical bonding.Layer-by-layer deposition of aluminum alloy components is achieved by moving the substrate.This study focuses on the DAAM process for 2319 aluminum alloy.The temperature field distribution,microstructure evolution,and epitaxial growth characteristics were investigated.First,the temperature field distribution during the deposition process was calculated using the finite element method combined with element birth and death techniques.Based on the temperature field analysis,the solidification parameters at different positions of the molten pool were calculated.These parameters were then substituted into a phase field(PF)model to determine the growth and evolution of the microstructure at different positions in the molten pool.Columnar crystal structures were formed in the bottom and middle regions of the molten pool.From the bottom to the upper part of the molten pool,the temperature gradient decreased and the solidification speed increased.Therefore,columnar crystals to equiaxed transition occurred in the middle and upper regions.Additionally,misorientation angles were introduced in the PF model to investigate the epitaxial growth characteristics of the solidification process.Larger misorientation angles had a more obvious influence on dendrite morphology and were more likely to be eliminated during competitive growth.Finally,the metallographic analysis showed that from the bottom to the upper part of the deposition layer,the microstructure changed from columnar to equiaxed crystals,and the presence of columnar crystal epitaxial growth agreed well with the simulation results.