The influence of thermal hardening on mechanical properties and microstructural evolutions of as-printed carbon steel

增材制造（AM）是最近的制造概念，用于生产不同金属材料的零件，如钢，钛合金，高温合金，生物相容性金属作为原材料，半成品或最终产品，其中材料逐层添加，而不是通过传统的成型工艺形成。在一些行业，如生物材料、航空航天、海洋；技术准备水平（TRL）已经获得了足够的优势，可以生产与传统版本竞争的认证部件，因为该过程允许根据特定需求调整每个部件。然而，由于增材制造过程中形成的复杂微观结构，其力学性能可能达不到预期。在项目的第一步，采用两种不同的工艺参数制作了两种不同类型的钢。主要目的是研究在打印过程中改变工艺参数可能产生的微观结构改变。利用电子背散射相（EBSP）和局部定向偏差图，我们的第一个成果（前2个月至2019年1月底）是通过解释从EBSD分析中获得的数据来识别打印过程中形成的应变不均匀性。其中一个样品是不锈钢的，另一个是碳钢的。为了使目前所开展的研究工作继续下去，有必要涉及到力学性质。在这方面，最初的硬度测量应用于碳钢热硬化处理后。这种合金是根据我们合作伙伴的要求优先生产的。发现印刷过程中工艺参数的改变会影响碳钢样品的硬度值，导致印刷后热硬化过程中出现严重的宏观开裂。这证实了文献中关于热硬化碳钢在印刷[7]后过早断裂的说法。因此，对于我们合作的下一步，有必要继续研究工作，以一种方式修改打印后所需的热处理，以便根据打印时获得的微观结构确定最佳的热硬化工艺参数。

Additive Manufacturing (AM) is recent concept in fabrication to produce parts with different metallic materialssuch as steels, titanium alloys, superalloys, biocompatible metals as raw, semi-final, or final products for whichmaterial is added layer by layer instead of being formed by conventional shaping processes. In some industriessuch as biomaterials, aerospace, marine; technology readiness level (TRL) has gained enough to producecertified parts that could be competitive with their conventional versions as the process allows adapting eachpart to the specific need. However, the mechanical properties may not meet the expectations in service due tocomplex microstructure formed during AM processes.In the first step of the project, two different types of steels fabricated by two different process parameters werereceived. The main objective was to investigate the possible microstructural modification obtained fromchanging process parameters during printing. Using electron back scattered phase (EBSP) and localmisorientation maps, our first deliverable (2 first months - by end of January 2019) was to identify the strainheterogeneity formed during printing by interpreting data acquired from EBSD analysis. One of the sampleswas a stainless steel and the other one was a carbon steel.For the continuation of research work developed so far, it is necessary to get involved with mechanicalproperties. In this regard, initial hardness measurements were applied on carbon steels after thermal hardeningtreatment. This alloy is prioritized on request of our partner. It was found that changing process parametersduring printing affects the hardness value in carbon steel samples causing severe macro-cracking duringthermal hardening after printing. This confirms the literature talking about premature fracture for thermallyhardened carbon steels after printing [7]. Therefore, for the next step of our collaboration, it is necessary tocontinue the research work in a way to modify the thermal treatment required after printing in order to identifyoptimum thermal hardening process parameters according to the microstructure obtained during printing.