激光熔覆316L粉末多层堆积过程中熔覆层Cr元素分布机制研究

Distribution Mechanism of Cr Element in Laser Cladding Layer During 316L Powder Multilayer Stacking

为了研究激光熔覆316L粉末多层堆积过程中熔覆层的Cr元素分布机制,建立了一种基于体积平均法的三相熔化凝固模型,模拟多层激光熔覆过程中的熔池流动、传热和传质现象。对模拟结果与实验结果进行比较分析,验证了模型的可靠性。通过分析激光熔覆多层316L粉末过程中的温度场、流场以及元素分布的瞬态变化,以Cr元素作为粉末的示踪元素获得了熔覆层的元素分布机制。结果表明:由于Marangoni效应,熔池内形成了位于熔池前端的顺时针方向涡流和后端的逆时针涡流,熔池最大流速出现在熔池后端表面;在前三层熔覆过程中,随着层数增加,熔池表面温度梯度下降导致熔池最大流速减小,而在热积累作用下熔池体积明显增大,但熔池内部流态不发生改变。同时,第二、三层熔覆时熔覆层发生部分重熔,重熔区内的基体元素在熔池内被粉末元素再次稀释,导致粉末元素的含量在熔覆层内逐层上升,最终获得从基体元素向粉末元素过渡的熔覆层。

Objective In the laser cladding process,a multi-cladding layer with a large thickness is required to satisfy the requirements of industrial production.To improve the performance of the cladding layer,the powder metal is different from the matrix,and therefore,the elements in the cladding layer need to change from matrix to powder elemental composition.The properties of the cladding layer are affected by the distributions of elements.The faster the cladding elements change from matrix elements to powder elements,the more metal powder elements are contained in the cladding layer,which has better abrasion resistance.Therefore,it is of great significance to analyze the transient changes in the temperature field,flow field,and element distribution by numerical simulation of the laser cladding 316L powder multilayer stacking process as well as study the distribution mechanism of Cr elements in the cladding layer,providing a theoretical basis for the cladding layer to contain a higher proportion of powder elements and fewer matrix elements.Methods The multilayer laser cladding process of 316L powder on a 45-steel matrix is studied using a three-phase melting and solidification model based on the volume averaging method.The distribution mechanism of elements in the process of cladding layer stacking is clarified by comparing and analyzing the changes of temperature field,flow field,and solute field in the first three layers.The simulation results are verified from four aspects:the geometric morphologies and Cr concentrations of the first and second cladding layers.Results and Discussions The geometric morphologies of the molten pool and element distributions of the cladding layers are verified by comparing the experimental and simulation results of the first and second cladding layers in the stacking process(Figs.4,5,and 6).The Cr element is used as a tracer element to analyze the distribution mechanism of the cladding layer element(Fig.10).The simulation results show that the molten pool morphologies and Cr element distributions of the first three layers are highly similar to the experimental results.During laser cladding,the matrix and powder continuously absorb energy,leading to a rapid increase in temperature and the formation of a small molten pool.As the laser beam moves,the molten pool continues to increase and becomes stable after a certain period(Fig.7).Under the influence of heat accumulation,a W-shaped temperature field distribution is formed during the cladding of the second and third layers,forming longer and deeper molten pools(Fig.8).The maximum flow velocity in the molten pool appears on the upper surface of the molten pool and decreases in the cladding process of the second and third layers.When cladding the second and third layers,the original cladding layer is partially remelted(Fig.9),and the matrix elements in the remelted area enter the molten pool under the force of Marangoni and mix with powder elements.As the molten pool moves,the powder is continuously sent into the molten pool,leaving a stable area with a higher Cr concentration at the back end of the molten pool.Conclusions To study the element distribution mechanism in the cladding process of the first three layers,combined with experimental verification,we simulate the stacking process of multilayer laser cladding,and achieve an accurate prediction of element distribution after cladding layer stacking.The technological parameters to form the elements of the cladding layer similar to the metal powder elements with a minimum layer number can be subsequently studied.This provides a theoretical basis for the repair of highend parts.The main conclusions are as follows.The reliability of the model is verified by comparing the molten pool morphologies and Cr element distribution results of the first and second layers obtained by simulation and experiment.The results indicate that the melting height error of the first layer is 5.81%,the melting depth error of the first layer is 3.23%,the melting height error of the second layer is 2.33%,and the melting depth error of the second layer is 3.23%.The slight errors in the molten pool morphology and the Cr distributions in the cladding layer obtained by the experiment and simulation are consistent,which proves that the current numerical model is reliable.In the first three layers during multilayer laser cladding,clockwise vortices exist in the front of the pool and counterclockwise vortices exist in the back of the pool,caused by the Marangoni effect in each layer.The length and depth of a molten pool increase because of heat accumulation.For the first three layers,the temperature gradients in the molten pool on the upper surface are G_(1)>G_(2)>G_(3).The decrease of the temperature change rate leads to the decrease of the maximum velocity.The original cladding layer partially remelts for the second and third layers,and the matrix elements in the remelting area enter the molten pool and are diluted by powder elements.Therefore,the cladding layer elements further transition from matrix elements to powder elements.After selecting Cr element as the tracer element of powder element,we find that the mass fraction of Cr element progressively increases with the height in the cladding layer,approximately 0.004 for each layer.Cr is easily enriched near the interface between the remelting and nonremelting areas,and the mass fraction of Cr increases by approximately 0.002 in the enrichment area.