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首页> 外文会议>International Conference on Solidification and Gravity; 20040906-09; Miskolc-Lillafured(HU) >MARANGONI CONVECTION AND FRAGMENTATION IN LASER TREATMENT
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MARANGONI CONVECTION AND FRAGMENTATION IN LASER TREATMENT

机译：激光治疗中的Marangoni对流和碎片化

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Epitaxial Laser Metal Forming (E-LMF) consists in impinging a jet of metallic powder onto a molten pool formed by controlled laser heating and thereby, generating epitaxially a single crystal deposit onto a single crystal substrate. It is a near net-shape process for rapid prototyping or repair engineering of single crystal high pressure/high temperature gas turbines blades. Single crystal repair using E-LMF requires controlled solidification conditions in order to prevent the nucleation and growth of crystals ahead of the columnar dendritic front, i.e., to ensure epitaxial growth and to avoid the columnar to equiaxed transition. A major limitation to the process lies in the formation of stray grains which can originate either from heterogeneous nucleation ahead of the solidification front or from remelting of dendrite arms due to local solute enriched liquid flow, .i.e fragmentation. To study this last aspect, heat and fluid flow modelling is required to establish the relationship between process parameters such as laser power, beam diameter and scanning speed, and the local solidification conditions plus the fluid flow in the vicinity of the mushy zone. Surface tension driven convection known as the Marangoni effect needs to be included in the model owing to its large influence on the development of eddies and on the shape of the liquid pool. The 3D model implemented in the FE software calcosoft® is used to compute the fluid convection within the liquid pool and to assess the risk of fragmentation using a criterion based on the local velocity field and thermal gradient. The computed results are compared with EBSD maps of laser traces carried out at EPF-Lausanne in re-melting experiments.
机译：外延激光金属成形（E-LMF）包括将金属粉末射流撞击到通过受控激光加热形成的熔池上，从而在单晶衬底上外延生成单晶沉积物。这是用于单晶高压/高温燃气轮机叶片的快速原型制造或维修工程的近净成形工艺。使用E-LMF的单晶修复需要受控的固化条件，以防止晶体在柱状树枝状前沿之前成核和生长，即，确保外延生长并避免柱状向等轴转变。该方法的主要限制在于形成杂散晶粒，该杂散晶粒既可以源自凝固前沿之前的异质形核，也可以源自因局部溶质富集的液体流动而导致的枝晶臂的重熔，即碎裂。为了研究这最后一个方面，需要进行热和流体流动建模，以建立工艺参数（例如激光功率，光束直径和扫描速度）与局部凝固条件以及糊状区域附近的流体流动之间的关系。由于表面张力驱动的对流对涡流的发展和液池的形状有很大的影响，因此需要在模型中包括称为Marangoni效应的表面张力驱动对流。 FE软件calcosoft®中实现的3D模型用于计算液池内的流体对流，并使用基于局部速度场和热梯度的标准评估碎裂的风险。将计算结果与EPF-Lausanne在重熔实验中执行的激光迹线的EBSD图进行比较。

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