Numerical study of the ultrasonic impact on additive manufactured parts

Highlights

• A hybrid fabrication method combining additive manufacturing (AM) and ultrasonic impact treatment (UIT) techniques was further investigated.

• The strain rate controlling factors and stress-strain relationship of the additive manufactured sample in SHPB tests were theoretically analyzed.

• A finite element model was established to analyze the impact-rebound-impact process of UIT.

• The stress and plastic strain fields were analyzed numerically and experimentally.

Abstract

A hybrid fabrication method combining additive manufacturing (AM) and ultrasonic impact treatment (UIT) techniques was developed to improve the microstructure and mechanical properties of additive manufactured metal parts. Experimental and numerical methods were conducted to analyze the stress and strain fields of ultrasonic impact on additive manufactured parts. Laser metal deposition (LMD) technique was applied to prepare the 304 stainless steel (SS) samples and then the samples were post-treated by UIT. Considering high strain rate effect of metallic materials in the UIT process, the dynamic hardening properties of the as-deposited 304 SS sample were experimentally measured using a split Hopkinson pressure bar (SHPB) technique. The strain rate controlling factors and stress-strain relationship of the as-deposited sample in the SHPB tests were theoretically analyzed. The dynamic and high transient impact-rebound-impact process of UIT including the pin velocity, stress field and plastic strain field were investigated numerically via a three-dimensional finite element model. The impact stress field parameters such as the magnitudes and directions of principal stress and principal shear stress were investigated to further analyze the plastic deformation behavior of the deposited sample. The experimental results of plastic deformation zone obtained from optical microscopy, electron backscatter diffraction (EBSD) and microhardness testing are in good agreement with the numerical results. Both the experimental and numerical results confirm that UIT can effectively improve the performance of additive manufactured metal parts.

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.