Precipitation kinetics during non-linear heat treatment in Laser Additive Manufacturing

在接下来的几年里，激光增材制造（LAM）有可能在复杂和定制金属部件的制造方面引发一场革命。然而，针对LAM优化或定制的合金研究仍然很少。在LAM过程中，合金经历的时间-温度分布与常规加工过程有很大不同。逐层沉积过程中的尖峰式再加热，即“本征热处理”，对LAM中使用的可析出硬化合金的相变有显著影响。该项目的目标是在这种强烈的非线性时间-温度曲线中获得沉淀动力学的基本理解，最终目标是利用它们来开发不需要任何制造后热处理的沉淀强化材料。本项目研究的材料将是一种含Ti和al的马氏体时效钢，预计在固有热处理过程中会出现快速析出。由于本征热处理期间的沉淀动力学无法通过实验获得，因此研究计划将从建立线性热处理期间的沉淀动力学模型开始。它将被调整以匹配动力学，由原位（量热法）和非原位（原子探针断层扫描）实验测量确定。在实验与模型匹配后，将使用该模型模拟LAM非线性热处理过程中的析出，从而了解需要调整的工艺参数以优化本构热处理。模拟结果将与原子探针层析成像结果进行比较。此外，还将研究LAM过程中发生的化学和热不均匀性对沉淀动力学的影响。

Over the next years, Laser Additive Manufacturing (LAM) has the potential to spark a revolution in how complex and customised metallic parts are fabricated. However, research into alloys optimised or tailor-made for LAM remains scarce. The time-temperature profile experienced by alloys during LAM is very different from the one during conventional processing. Spike-like re-heating during the layer-by-layer deposition, i.e. the "intrinsic heat treatment", has a pronounced influence on the phase transformations occurring in precipitation-hardenable alloys employed in LAM. The objective of this project is to gain a fundamental understanding of the kinetics of precipitation during such strongly non-linear time-temperature profiles, with the ultimate goal of exploiting them for materials that are precipitation strengthened without the need for any post-manufacturing heat treatment. The material under investigation in this project will be a Ti- and Al-containing maraging steel that is expected to show rapid precipitation during the intrinsic heat treatment. Because the kinetics of precipitation during the intrinsic heat treatment is not accessible experimentally, the research programme will start by establishing a kinetic model for precipitation during linear heat treatment. It will be tuned to match the kinetics as determined from in-situ (calorimetry) and ex-situ (atom-probe tomography) experimental measurements. After a match between experiment and model has been reached, it will be employed to simulate precipitation during the non-linear heat treatment of LAM, thereby enabling an understanding of the process parameters that need to be adjusted to optimise the intrinsic heat treatment. The results of the simulations will be compared with atom probe tomography results. Additionally, the influence of chemical and thermal inhomogeneity occurring during LAM on the precipitation kinetics will be studied.