Heat Assisted Local Forming and Joining Processes: Simulation and Experimentation

汽车和航空航天工业对制造零件的更高性能的需求日益增长，这与成形过程中可能涉及的金属板厚度较低以及最终这些零件的重量较轻密切相关。主要问题是高等级材料的成形性差，特别是在室温下。出于这个原因，许多努力实际上是朝着温暖的高温金属板材制造工艺，但仍然有一个重要的差距，因为在高温下成形过程中的微观结构的复杂演变的数值模型的预测和实际的过程。热辅助成形工艺之后的一个方面是相变材料中可能发生的微结构中的相变诱导塑性效应（TRIP）。在这种转变之后表现出更高的硬化能力，并且人们可以利用所制造的部件的增强的性能来进一步减小它们的厚度以减轻重量，然后提高运输中的燃料效率。涉及难以成形的高强度材料的动态热辅助成形工艺的典型示例是用于航空航天应用的金属板旋压。该过程逐渐将扁平金属坯料转变成中空形状，主要具有轴对称轮廓。该方法使用工具，即辊，其迫使金属坯料到旋转心轴上。辊压靠在坯料上，并轴向移动到复杂的轨迹中，以最终获得中空形状的产品。由于过程中涉及大量参数（速度、温度相关特性、应变率、工具轨迹、诱导各向异性），综合实验设计在成本和专用时间方面要求太高。有限元模型对于再现复杂过程有很大帮助，并且可以提供有关零件正在进行的成形过程和最终性能的非常深入的细节。关键是在数值模型中输入准确的信息，以及精确再现传热、应变硬化、各向异性和最终TRIP现象的能力。在这个项目中，重点将放在模拟模型及其在材料中随时间演变的这些现象的能力。晶体塑性模型将在有限元分析中实施，以跟踪这些复杂的演变。最后，相变可以通过在成形工具接触区的适当量的局部热输入来监测，以提高金属板的可成形性。其目标是通过在难以成形的高强度材料的成形步骤中添加高科技加热源（如激光加热），将传统成形工艺提升到新的技术水平，从而提高材料性能并以尽可能低的成本保持热辅助操作。

The increasing demand in the automotive and aerospace industries for higher properties in their manufactured parts is closely connected to the lower thickness of the sheet metal that could be involved in the forming processes and consequently the lighter weights of these parts at the end. The main concern is the poor formability of high grade materials, especially at room temperature. For this reason, many efforts are actually made towards warm to high temperature sheet metal manufacturing processes but there is still an important gap between the prediction of the numerical models and the actual processes because of the complex evolution of the microstructure during forming at elevated temperature. One aspect following a heat assisted forming process is the transformation induced plasticity effect (TRIP) in the microstructure that can occur in phase changing materials. Higher hardening capability are exhibited following this transformation and one can take an advantage of the enhanced properties of the manufactured parts to decrease furthermore their thicknesses for weight reduction and then improved fuel efficiency in transportation. A typical example of a dynamic heat assisted forming process involving hard-to-shape high strength materials is the sheet-metal spinning for aerospace applications. The process progressively transform flat metal blank into hollow shapes, mainly with axisymmetric profiles. The method uses a tool i.e. a roller which forces a metal blank onto a revolving mandrel. The roller is pressed against the blank and is moved axially into complex trajectories to achieve finally a hollow-shaped product. With the great number of parameters (speed, temperature dependent properties, strain rate, tool trajectories, induced anisotropy) involved in the procedure a comprehensive design of experiments would be too demanding in terms of costs and dedicated time. Finite element models are of great help to reproduce complex processes and can offer very in deep details on the ongoing forming process and the final properties of the parts. The key is the accurate information input in the numerical model and its ability to reproduce precisely the heat transfer, the strain hardening, the anisotropy and eventually the TRIP phenomena. In this project, the focus will be on the simulation model and its ability to represent these phenomena evolving with time in the material. A crystal plasticity model will be implemented in the finite element analysis to follow these complex evolutions. Eventually, phase transformation could be monitored with the appropriate amount of locally heat input at the forming tool contact zone to enhance the sheet metal formability. The goal is to bring conventional forming processes to new technological levels by adding high tech heating sources (like laser heating) in the forming steps of hard to shape high strength materials, enhancing material properties and maintaining heat assisted operation at lowest cost as possible.