Numerical study of the ultrasonic impact on additive manufactured parts
Graphical abstract
Introduction
Metal-based additive manufacturing (AM) has attracted considerable attention across multiple industries due to its digitized method by building up metal components layer-by-layer that increases the design freedom and manufacturing flexibility of complex components [1]. Benefits include the absence of mould, a significant reduction in material wastage and in time to market [2]. In the highly localized melting and rapid solidification process of AM, a fine-grained nonequilibrium microstructure has been formed [3]. However, there are also some inherent characteristics that are slowing its wider implementation of the as-deposited metal parts. Directional columnar or dendritic grains caused by strong temperature gradient lead to the mechanical anisotropy [4]. Besides, high residual tensile stress produced during the highly localized rapid solidification process is also undesirable [5]. Micro-voids are sometimes also formed due to improper processing parameters [6]. Until now, some methods have been developed and applied to improve those immanent issues, such as optimization of processing parameters [7], post-heat treatment [8] and hot isostatic pressing (HIP) [9,10]. Additionally, some mechanical treatments such as rolling [11,12], shot peening (SP) [13], [14], [15], laser shock peening [16], [17], [18], ultrasonic surface mechanical attrition treatment (SMAT) [19,20] and ultrasonic impact treatment (UIT) [21] have been employed to improve the performance.
UIT is one of the most promising post-processing methods applied in many industries, particularly used in welded joints to induce beneficial compressive residual stress for enhancing fatigue performance [22]. During the UIT process, the cylindrical pin is continuously excited and accelerated by the low-amplitude high-frequency ultrasonic vibrations of the concentrator and repeatedly impacts the sample surface. The surface layer receives severe plastic deformation and redistribution of residual stress. Some new investigations of UIT-assisted wire and arc additive manufacturing (WAAM) of Ti-6Al-4V alloy shows that residual stress could be significantly reduced and a novel refined bamboo-like structure was formed in the hybrid manufacturing process [23]. Compared with the high-pressure rolling, the UIT process is operated under low or zero additional pressure and high energy density input can be achieved during the repeated impacting, which confirms that the UIT can be used for complex thin-walled components without the deformation and destroy of previously-deposited part. Therefore, the UIT technique used in AM process is beneficial to improve the mechanical performance of additive manufactured metal parts.
Considering the dynamic characteristics of high transient impacting in the UIT process, conventional quasi-static experiments are difficult to detect and analyze the pin movement and the dynamic stress field, and therefore numerical methods have also been employed by some authors. For example, a single-impact finite element model and a two-impact finite element model were utilized to simulate the UIT process [24]. The energy transformation of kinetic energy, strain energy and plastic dissipation energy in the contact and rebound processes was used to analyze the equivalent plastic strain. For example, Yuan and Sumi conducted a three-dimensional finite element model including thermo-mechanical welding and dynamic elastic-plastic numerical analyses for welded joints in UIT process [25]. The predicted residual stress and the evaluation of fatigue strength were employed to simulate UIT process in engineering structure. Furthermore, a peen-rebound-peen finite element analysis approach was proposed to investigate the residual stress field and the effect of gap width between the pin and the sample [26]. The Johnson-Cook plasticity model and fracture model were applied and the results revealed that as the peening duration reaches a critical value, the compressive residual stress remains unchanged but the risk of fracture increases.
As one of the AM methods, laser metal deposition (LMD) builds parts with high efficiency by applying the feedstock powders into the melt pool created by the scanning laser beam [27,28], which shows that UIT technique can be easily combined into the LMD process. In our current work, a hybrid fabrication method combining LMD and UIT was established to investigate the stress and plastic deformation behavior of the as-deposited parts in the process of UIT. The 304 stainless steel (SS) sample was firstly prepared by LMD technique. The properties of the as-deposited sample, particularly the dynamic hardening properties, were measured. Then the as-deposited sample was post-treated by UIT. Microhardness and electron backscatter diffraction (EBSD) tests were conducted for investigating the plastic deformation zone of the treated sample. Finite element method was applied to analyze the impact-rebound-impact process of UIT, including the pin velocity, stress filed and plastic strain field.
Section snippets
Material preparation
Fig. 1 shows the schematics of the combined LMD and UIT process. The 304 SS powders were utilized to prepare samples. The spherical powders with an average particle size between 80 μm and 140 μm are shown in Fig. 2(a). The processing parameters were given as: the laser power was 1800 W, the deposition velocity was 8 mm/s, the hatch spacing was 1.2 mm and the height increment of each deposited layer was 0.7 mm. The as-deposited bulk sample was prepared as shown in Fig. 2(b). The as-deposited 304
Dynamic hardening behavior of the as-deposited 304 SS sample
Fig. 7(a) shows the typical waveforms measured by strain gauges in the SHPB test with a gas pressure of 0.50 MPa. Three typical waves, incident, reflected and transmitted waves, were obtained, and they were smoothly processed using a hundred point smoothing algorithm [38] as shown in Fig. 7(b). The loading process can be divided by three stages: Ⅰ. rapid loading in which the incident wave increases rapidly; Ⅱ. platform loading in which the incident wave is nearly constant; Ⅲ. unloading of
Conclusions
Experimental methods and numerical analyses were conducted to study the UIT effects on additive manufactured metal parts. The following conclusions were drawn from the current investigation:
(1) The strain rate controlling factors and stress-strain relationship of the as-deposited 304 SS sample in the SHPB tests were theoretically analyzed. The factors that affect the strain rate of the sample in SHPB tests can be divided into experimental parameter (the striker impact velocity v0), material
CRediT authorship contribution statement
Changping Zhou: Conceptualization, Methodology, Writing - original draft. Jiandong Wang: Data curation. Chunhuan Guo: Investigation. Chengzhi Zhao: Visualization. Guorui Jiang: Validation. Tao Dong: Supervision. Fengchun Jiang: Project administration, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Key R&D Program of China (No. 2017YFB1103701); the National Natural Science Foundation of China (No. 51671065); Equipment Development Department for Commission of Science and Technology (No. 41423030504); Applied Technology Research and Development Plan of Heilongjiang (No. GA18A403).
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