摘要
金属表面镀层处理是材料表面改性的重要方法,随着环保、节能和生产成本等的新要求,逐步形成了“先对基材表面镀层处理,后对带金属表面镀层材料精密塑性成形”的加工方式。目前,有限元法已成为研究板料成形的一种重要方法,这种方法不仅可以预测板料成形过程中的应力、应变分布、成形缺陷以及分析缺陷产生原因,而且可以比较准确的分析工艺参数对成形过程的影响。
本文首先对板料成形的研究现状进行综合评述,然后讨论镀层薄板有限元成形模拟中的关键问题,最后采用ANYSY/LS-DYNA软件模拟镍镀层薄板成形过程,研究冲压工艺参数与模具参数对镍镀层薄板成形的影响,并对部分模拟结果进行实验验证。本文的主要研究内容及结果如下:
(1)通过对镍镀层薄板冲压成圆筒件的过程进行模拟,得到了镍镀层和基体的应力应变场、厚度变化及冲压力行程曲线。计算结果表明镀层和基体在凹模圆角处的应力应变场最大,镀层的应力分布较之基体复杂,表现为圆筒件底部分布不均匀。同时发现在凸模圆角处板料最易拉裂,基体与镀层的减薄率分别达到了4.8%与6.7%,冲压力先增后降,当冲压深度为12.62571mm时,冲压力达到最大值15.41846KN。
(2)为了对模拟结果进行验证,对镍镀层薄板冲压圆筒件进行了实验研究,结果表明模拟得到的圆筒件形状与实验很好的吻合,模拟圆筒件深度为15.6mm,实际的深度为16mm。最后,通过扫描电镜观察了界面的结合情况,同时采用扫描电镜附带的软件测量了镀层厚度变化,发现界面未见开裂现象,镀层厚度变化趋势与模拟结果一致。
(3)研究了压边力对镍镀层薄板成形质量的影响。发现此板料在没有加压边力的情况下会出现起皱,而施加较小的压边力(400N、800N),冲压的后期阶段会出现起皱,当压边力为1KN时,冲压件质量较好。最后为了改善板料的成形性能,设计了一条变压边力曲线,结果表明此压边力不仅能够防止起皱,而且有效的防止拉裂。
(4)采用有限元法研究了摩擦系数、凹模半径、冲压速度等工艺参数对镍镀层薄板成形的影响。发现摩擦系数增加,冲压力随之增加,基体与镀层在最薄处的厚度均变薄,镀层变薄较之基体更严重。同时发现凹模半径减少,冲压速度增加均会增加板料的拉裂机会。
通过本文的研究,得到了镍镀层薄板的成形特征,了解了工艺参数对镍镀层薄板成形的影响。研究结果对镍镀层薄板的工业冲压具有一定的指导意义。
Coating treatment of metal surface has been an important way of material surface modification, along with new requirement of energy-saving, environment protect and production cost, the processing method that firstly nickel was electrodeposited on the substrate and then metal plating material are plastic formed precisely has gradually formed. Recently, finite element method (FEM) has been an important method to study sheet metal forming, this method can not only forecast the distributions of stress-strain fields, discover the forming drawbacks and analyze the reason of forming drawbacks, but also it can analyze precisely the process parameter’s impact to the forming process.
In this thesis, the synthetical review on the present situation of sheet metal forming was firstly presented. Then some attention problems during simulation of coating material were investigated. Finally, the forming process of electrodeposited nickel coating was simulatly studied by ANSYS/LS-DYNA. The effects of process and module parameters were studied during the forming process. Some simulation results were validated by experiments. The main contents and results contain the following:
(1) Through simulating the cup-stamping of electrodeposited nickel coating, the stress-strain fields, thickness variations of nickel coating and steel substrate were obtained. The force-displacement curves were also obtained. The results of the simulation illustrated that the stress-strain fields of the coating and substrate were maximum at die fillet. It was found that the stress field of the coating was more complicated than that of the steel sheet, which showed that the stress-strain distributed non-homogeneously at the bottom of cylindrical cup. Meanwhile, it was found the punch-nose radius was the most unsubstantial part for the intensity of the entire deep-drawing part and the thinnest part. It was the danger zone for the break and at this zone. The thickness thinning-rate of the steel sheet and the nickel coating were up to 4.8% and 6.7% respectively. The punch force increase first and decrease afterwards, when the depth of stamping was 12.62571mm and the punch force reached maximum 15.41846KN.
(2) In order to validate the results of simulation, the cup-stamping of electrodeposited nickel coating was done by experiment. The results showed that the shapes of cylindrical cup were obtained by FEM and experiment results were agreed. The depth of cylindrical cup by FEM was 15.6mm and the depth of cylindrical cup by experiment was 16mm. At last, the interface combination status of the coating was observed by scanning electron microscopy (SEM). The thickness of the coating was measured by the measurement software of SEM, which showed the interfacial delamination would not happen and the thickness variation tendency of coating was similar to the simulation results.
(3) The influences of blankholder force (BHF) on the forming quality of the electrodeposited nickel coating were studied. It was found that the sheets would wrinkle without BHF. When BHF (400N, 800N) was too small, the sheets would wrinkle during later stage. When BHF was 1KN, the quality of stamping parts was better. In order to improve the formability of the sheet, a curve of variable BHF (VBHF) was designed by us. The results illustrated that the VBHF could not only prevent wrinkle, but also it could prevent crack.
(4) The influences of process parameters such as friction coefficient, the radius of die and the velocity of punch were studied during stamping process of the electrodeposited nickel coating by FEM. The results showed that higher friction coefficient would increase the punch force, the thickness of the substrate and coating at thinnest parts would thinner and the thinning-rate of the coating was higher than that of the substrate. Meanwhile, it was found that decrease of the radius of die and the raise of punch velocity would increase the tendency of crack.
Through the research work, the forming characteristics of electrodeposited nickel coating and the influences of process parameters on the forming process were obtained. The results may serve as a reference for the stamping process of electrodeposited nickel coating in industrial fields.
引文
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