今日更新:International Journal of Plasticity 2 篇
Laser powder bed fused 304L steel shot-peened with various ceramic shot sizes: surface/subsurface characteristics, tensile behavior, and fatigue behavior
Zhang Hongzhuang, Cao Shujie, Li Changyou, Li Bing, Qian Guian
doi:10.1016/j.ijplas.2023.103784
激光粉末床熔融 304L 钢(使用不同尺寸的陶瓷丸进行喷丸处理):表面/次表面特性、拉伸行为和疲劳行为
Shot peening is an efficient post-processing technique for eliminating surface manufacturing defects, inducing compressive residual stress, and improving the mechanical properties of laser powder bed fused (L-PBFed) metallic materials. With a series of microstructural characterizations, this work comprehensively evaluated the influence of ceramic shot size on the surface characteristics, subsurface characteristics, and mechanical properties, especially fatigue performance, of L-PBF manufactured 304L austenitic stainless steel (SS). The mechanical testing process was in situ observed using an infrared thermal camera, and the tensile-fractured and fatigue-fractured microstructures were systematically examined, aiming to explore the life improvement mechanisms caused by different ceramic shot sizes. The results revealed that when compared to small-sized AZB 100 shots, the large-sized AZB 300 shots could result in higher surface roughness, larger thickness reduction, more apparent surface gradient structure, higher strength, and longer fatigue lifetimes. The strength improvement could be associated with the size reduction of L-PBF induced subsurface defects, the grain refinement at the surface layer, and the α'-martensitic transformation during tensile deformation. The life improvement could be primarily attributed to the subsurface defect strengthening, which resulted in the fatigue crack initiation transition from subsurface defects to surface defects. During the fatigue process, the energy dissipation originating from the core materials dominated the thermal equilibrium temperature, whereas the density of micro-defects (crystalline defects) at the surface layer dominated the crack tip temperature. These findings provide a basic understanding of how to advance the shot peening process for L-PBFed materials.
喷丸强化是一种高效的后处理技术,可用于消除表面制造缺陷、诱导压缩残余应力并改善激光粉末床熔化(L-PBFed)金属材料的机械性能。本研究通过一系列微观结构表征,全面评估了陶瓷丸尺寸对 L-PBF 制成的 304L 奥氏体不锈钢(SS)的表面特征、次表面特征和机械性能(尤其是疲劳性能)的影响。使用红外热像仪对机械测试过程进行了现场观察,并对拉伸断裂和疲劳断裂的微观结构进行了系统研究,旨在探索不同陶瓷丸尺寸对寿命改善的作用机制。结果表明,与小尺寸的 AZB 100 射丸相比,大尺寸的 AZB 300 射丸能使表面粗糙度更高、厚度减薄幅度更大、表面梯度结构更明显、强度更高、疲劳寿命更长。强度的提高可能与 L-PBF 诱导的次表面缺陷尺寸减小、表层晶粒细化以及拉伸变形过程中的α'-马氏体转变有关。寿命的提高主要归因于次表层缺陷的强化,这导致疲劳裂纹的起始点从次表层缺陷过渡到了表层缺陷。在疲劳过程中,源自核心材料的能量耗散主导着热平衡温度,而表层的微缺陷(晶体缺陷)密度则主导着裂纹尖端温度。这些发现为如何推进 L-PBFed 材料的喷丸强化工艺提供了基本认识。
Unraveling the transformation of ductile damage mechanisms of void evolution and strain localization based on deformation heterogeneity
Shang Xiaoqing, Fu M.W., Zhang Haiming, Liu Juan, Zhou Xiao, Ying Tao, Zeng Xiaoqin
doi:10.1016/j.ijplas.2023.103785
基于变形异质性揭示空洞演化和应变局部化的韧性损伤机制转变
This study delves into the ductile damage mechanism via exploring the intrinsic nature of the transformation of void and strain localization induced damages. In tandem with this, tensile experiments were conducted by using ductile metals of pure titanium and austenite steel with different grain sizes (d) and sample thicknesses (t). Various damage behaviors were generated: the void damage (Dv) in which the damage is induced by voids and their evolution, the strain localization induced damage (Dl) in which damage is controlled by localized deformation, and their mixed mode. Examinations of damage characteristics show a transformation from Dl to Dv with the increase of t/d or hardening. To identify the decisive factor behind the mechanism, crystal plasticity finite element simulations were performed, and the grain-level non-homogeneous deformation was carefully examined. The materials with severe inhomogeneous deformation were found to have fewer voids, while the ones with more uniform deformation possessed evident void growth. Deformation heterogeneity was thus identified as a pivotal factor for the Dl-Dv transformation. The change of micro-defect configuration of void and grain boundary (GB) with deformation heterogeneity was discovered to be the underlying cause. In the Dv case, the large number of strain localization zones penetrating within the bulk promotes void nucleation and growth from vacancies and dislocations. The Dl case, on the other hand, gets more new GBs but smaller number of voids during deformation. The larger continuous strain localization zone facilitates the dislocation-consuming process of GB formation, resulting in the difficulty of void formation. Additionally, a characteristic parameter representing the deformation heterogeneity degree was defined. A Dl-Dv paradigm, which involves the change of damage mode and micro-defect configuration with deformation heterogeneity, and the characteristic parameter was established. The paradigm was validated to have a wide applicability with its efficiency in interpreting extensive damage phenomena. These explorations are expected to add new insights into the understanding of damage mechanism and support the development of a unified damage prediction platform.
本研究通过探索空隙和应变局部诱导损伤转化的内在本质来深入研究韧性损伤机制。与此同时,研究人员使用不同晶粒大小(d)和试样厚度(t)的纯钛和奥氏体钢等韧性金属进行了拉伸实验。实验产生了各种损伤行为:空隙损伤(Dv),即损伤由空隙及其演变诱发;应变局部诱发损伤(Dl),即损伤由局部变形控制;以及它们的混合模式。对破坏特征的研究表明,随着 t/d 或硬化程度的增加,破坏会从 Dl 转变为 Dv。为了确定该机制背后的决定性因素,我们进行了晶体塑性有限元模拟,并仔细研究了晶粒级非均质变形。结果发现,变形严重不均匀的材料空隙较少,而变形较均匀的材料空隙增长明显。因此,变形异质性被认为是 Dl-Dv 转变的关键因素。随着变形异质性的变化,空隙和晶界(GB)的微缺陷配置也发生了变化,这就是其根本原因。在 Dv 情况下,大量应变局部区渗透到块体内部,促进了空位和位错的空洞成核和生长。另一方面,在 Dl 情况下,变形过程中会产生更多的新 GB,但空隙数量较少。较大的连续应变定位区有利于位错消耗 GB 的形成过程,导致空洞难以形成。此外,还定义了代表变形异质性程度的特征参数。建立了一个 Dl-Dv 范式,其中涉及随变形异质性而变化的损伤模式和微缺陷构型,以及特征参数。经过验证,该范式具有广泛的适用性,能有效解释广泛的损伤现象。这些探索有望为理解损伤机理增添新的见解,并为开发统一的损伤预测平台提供支持。