|Alternative Title||Investigation of weld pool flow, heat and mass transfer in laser welding of 304SS and Ni|
|Place of Conferral||北京|
|Keyword||异种激光焊接 传导焊接 深熔焊接 数值模拟 热质输运|
With the advantages of concentrated energy density of the laser heat source, high processing efficiency and easy automation, laser welding technology for dissimilar metals stands out from various dissimilar metal fusion technologies and is used in large numbers in industries such as chemical, aerospace and vehicle engineering. The differences between the thermal properties of dissimilar base metals can lead to more challenging weldability than laser welding with the same metal, and the mixing and redistribution of alloy components during the welding process makes the mechanisms affecting the quality of the weld more complex. For example, differences in melting points lead to different melting sequences between the two metals, which can cause damage to the mechanical properties of the weld, and the formation of intermetallic compounds (IMCs), which can occur when the alloying elements reach a certain concentration during the mixing of the metal components. However, the process of laser welding of dissimilar metals involves many physical effects, and the parameters and process conditions that influence the final weld are so complex that it is extremely challenging to regulate the defects and composition of the weld and to achieve a target depth of melt with low thermal stress, and strain and a dense and fine microstructure. These objectives are closely related to the dynamic changes in heat and alloying element concentration in the melt pool, and it is one of the main trends in research to link the mesoscopic transport behaviour of the melt pool with the macroscopic mechanical properties and microstructure of the weld. Therefore, this paper combines the practical application of engineering for the important needs of laser welding of dissimilar metals, based on the status of relevant research, for dissimilar metal laser welding multi-physical field phenomenon, refining the scientific aspects of the impact of thermal mass transport in the melt pool during the welding process. This paper investigates the behavioural characteristics of heat and mass transport in the melt pool of dissimilar metals welding in conduction mode and keyhole mode, respectively, using a solver (based on the finite volume method and SIMPLE algorithm) for three-dimensional heat and mass transfer problems in laser welding as an example. The main contents and findings of this paper are as follows.
1. Based on the Navier-Stokes equation, coupled temperature field, fluid flow, concentration field to establish a three-dimensional numerical model, The model takes into account the complex physical mechanisms of phase change heat transfer in the base metal, the dissipation of momentum in the mushy zone, and the coupling effect of Marangoni convection on heat transport and mass transport.
2. For numerical simulation of laser conduction welding of 304SS and Ni, the calculated melt pool size and alloy element distribution values and experimental results were compared to verify the validity of the model. Firstly, the dominant mechanism of heat and mass transfer in the evolution of the melt pool of conduction mode laser welding was analyzed by means of dimensional analysis. The numerical simulations were then combined with orthogonal parameter design and extreme difference analysis to systematically investigate the effects of laser power, spot offset, and scan speed on the redistribution of alloying elements in the joint of nickel and 304SS. The average elemental content of Fe flowing into the Ni side characteristics the elemental distribution in the melt pool and the relative importance of the process parameters is investigated by means of a polar difference analysis: 9.45% for the scanning speed levels, 9.17% for the spot offset and 1.11% for the power. The mean elemental concentration of Fe flowing into the Ni side was negatively correlated with the scanning speed and positively correlated with the offset. The physical mechanism behind the influence of process parameters on elemental distribution was further explored. Appropriately lowering the scanning speed and shifting the spot towards the 304SS side facilitated a more adequate dilution and uniform distribution of Fe elements.
3. Keeping the amount of defocusing and scanning speed constant, deep penetration welding experiments were carried out by varying the laser power between 304SS and industrial pure nickel thin plate. The impact of power density on the geometry of the weld seam was investigated, found that with the increase in power, the depth to width ratio of the weld seam gradually increased. For the characteristics of deep fusion welding of thin plates, a three-dimensional numerical model was established for laser deep fusion welding of 304SS with industrial pure nickel. After calculation, the results of the simulated weld profile under different parameters are consistent with the experiment. Through dimensional analysis, conduction was found to be the main mechanism in the melt pool of 304SS and industrial pure nickel sheet deep fusion welding, while the mass transfer mechanism was dominated by convection in the melt pool. The quasi-steady state of the melt pool was then studied in relation to the heat transport characteristics of different locations, the flow of the melt pool and the solute transport behaviour.
|李梓洵. 304SS和Ni激光焊接的熔池流动及传热传质研究[D]. 北京. 中国科学院大学,2021.|
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