Additive manufacturing (AM) or 3D Printing (3DP) is a comparatively new fabrication technology that has a number of benefits, including reduced wastages, easier customization, and shorter lead times [1]. This approach, which is frequently referred to as “Art to Part” technology, is regarded as a platform that quickly transforms digital models into physical parts [2]. The additive manufacturing (AM) market is projected to grow at a compound annual growth rate (CAGR) of 23.6%, increasing from USD 27.52 billion in 2024 to an estimated USD 150.20 billion by 2032 [3]. As the interest in AM grows, several sectors, including aerospace, automotive, medical, tooling, energy, natural resources, consumer and defence are incorporating it into their products and services. AM can be categorized based on its core technologies, which include MEX, Binder Jetting, Material Jetting, Vat Photopolymerization, Directed Energy Deposition, Sheet Lamination, and Powder Bed Fusion [4]. MEX, one of the seven major types of AM previously described, commands the largest market share because of its simplicity in manufacturing and usage, broad material acceptance as well as cheap machine cost [5].

Despite widespread enthusiasm, several obstacles still prevent this technology from reaching its full potential. Inconsistent standards, proprietary patents, and ongoing mechanical issues continue to limit the practical uses of MEX printing [6]. While issues like low mechanical strength of the parts, long printing time, anisotropy, low build volume, inferior surface quality and dimensional inaccuracies can be a source of frustration for the “makers”, investors and entrepreneurs, these obstacles also represent a wide opportunity for researchers and traditional manufacturing industries [7–9].

A detailed examination of the MEX-3D printing market reveals that the most popular machines among enthusiasts have a bed volume of 230 mm³ [10–12]. If anything needs to be printed larger than the given bed size, a solution is to purchase an industrial printer with a 500 mm³ capacity, though this will result in the cost and power consumption increasing fourfold [13]. Furthermore, the challenges endured when printing larger sections are typically greater than those faced when using a smaller MEX printer.

An alternative approach to address this problem could involve using a post-processing joining technique, which would allow the parts to be assembled into a larger, more complex final 3D-printed component [14,15]. The joining methods used for MEX 3D printed parts fall into four categories: Mechanical interlocking, Adhesive bonding, Fastening and Welding which includes FSW, FSSW, SFW, UW, MW, Laser welding, etc. [16].

As the use of MEX 3D-printed components is rapidly expanding for both load bearing and primary structural applications, the need for an effective joining method is also increasing [17]. UW, belonging to the cluster of friction-welding techniques, is a crucial and widely used technique for bonding plastic and metal components, with growing applications across diverse industries [18]. This method generates heat by converting high-frequency mechanical vibrations into thermal energy through a combination of contact surface friction and intermolecular friction [19]. UW is highly favoured in industries because of its rapid cycle time (~1 s), higher energy efficiency, ease of automation as well as the ability to produce high-strength joints without the need for additional adhesives or mechanical fasteners [20]. A broad spectrum of industrial and commercial uses, such as aerosol containers, butane fuel canisters, vacuum and pressure valves, pump impellers, electrical switches, floats, and conveyer components, are commonly assembled using UW [21–24].

UW of thermoplastics is a high-frequency welding process that utilizes ultrasonic acoustic vibrations to create solid-state welds between thermoplastic materials [25]. The process involves the conversion of high-frequency electrical energy into mechanical vibrations by a piezoelectric transducer. These vibrations, typically ranging from 20 to 40 kHz, are transmitted through a booster and a sonotrode (horn) to the positioned and aligned thermoplastic workpieces clamped under pressure [18,26,27]. The mechanical vibrations generate frictional heat at the interface of the workpieces, causing the thermoplastic material to soften and melt [21,28]. Upon the termination of the ultrasonic energy, the molten material solidifies under continued pressure, resulting in a strong, homogenous weld [29]. The efficiency of the process is shaped by several parameters like the amplitude of vibrations, welding time, pressure applied, shape of energy directors and the specific material properties of the thermoplastics being welded [30,31].

A literature review aimed at assessing the research on UW of metals and polymers demonstrated that while this joining technique has been comprehensively deliberated for metals, there is a noticeable gap in investigations concerning the welding of polymers and AM components. Parmar and Pandya [32] conducted one of the earliest studies on UW of ABS plastics, focusing on optimizing parameters such as welding time, pressure, and amplitude to achieve strong and dependable joints. The study found that welding strength primarily depends on the amplitude, subsequently by the amount of weld pressure and time. Fernandez Villegas et al. [33] highlighted how flat energy directors can improve weld strength and efficiency compared to traditional designs. The study offers important identifications into optimizing ultrasonic welding parameters for carbon fibre-reinforced Polyphenylene Sulfide composites, suggesting that the choice of energy director significantly impacts joint performance. Villegas and Bersee [19] conducted a detailed study on UW, examining the size, distribution, and alignment of energy directors (EDs) in thermoplastic composites. The research found that proper ED alignment with the load direction increases the weld strength, while optimal ED size and distribution enhances the weld quality by ensuring uniform energy concentration and heat generation. Villegas [26] provided a thorough study of ultrasonic welding for fiber-reinforced thermoplastic composites, focusing on energy directors, process monitoring, and industrial scalability. The paper advances understanding of process parameters and welding dissimilar materials, offering meaningful perspectives for both industry and research. Raza et al. [34] investigated the ultrasonic welding of polymers like polycarbonate (PC), high-density polyethylene (HDPE), and ABS. The study focused on optimizing parameters such as energy input, pressure, and time to enhance weld quality, making ultrasonic welding suitable for various industrial applications requiring strong polymer joints. Li et al. [29] examined the ultrasonic strengthening’s effect on the tensile performance of ABS 3D printed parts. Results displayed an 11.3% upsurge in tensile strength and a 16.7% rise in young’s modulus, along with reduced surface roughness. These findings indicate that ultrasonic treatment can effectively enhance the durability and performance of MEX components, broadening their potential applications. Khatri et al. [28] explored UW for PEEK/carbon-fiber-reinforced PEEK materials. The research demonstrated that incorporating energy directors significantly improved the joint strength and weld quality, leading to enhanced mechanical properties. The results indicate that UW could be well-suited for high-performance thermoplastics in demanding applications. Quader et al. [25] examined the impact of ultrasonic vibration on the tensile, physical and morphological properties of PLA based MEX samples. The study found that ultrasonic treatment enhances tensile strength and improves surface quality by altering the material’s microstructure. Palaniyappan et al. [35] explored different joining techniques for 3D-printed PLA and recycled wood-reinforced PLA composites, focusing on ultrasonic joining. The results demonstrate that samples welded using ultrasonic methods reach a lap shear strength of 16.4 MPa and a Shore D hardness of 80, exceeding the performance of other joining techniques, including adhesives and direct 3D printing. Singh Rana et al. [36] studied UW of PLA specimens from #3D printing and injection moulding with different parameters. They found that cross energy directors outperformed triangular and semi-circular types. Additionally, 100% infill specimens showed 15% higher tensile strength than injection-moulded specimens. The study concludes that ultrasonic welding effectively addresses print bed size limitations for large, complex products.

A comprehensive review of UW disclosed that while the process has been widely studied for metals, research on polymers and additively manufactured (AM) parts is still limited. The current investigation addresses that gap by applying UW to join AM thermoplastics for larger volume components. Critical factors such as material combination, welding time, amplitude, infill percentages, and the shape of energy directors were statistically analysed. Optimal combinations for maximum elongation and lap shear strength were identified, established, and successfully used to weld components, including a car’s rear wing and an agricultural drone, overcoming MEX-3D printer bed size limitations and also further enhancing UW’s potential.

The findings encompassed in the following segments are established on UW of different combinations of ABS and PLA polymers, manufactured using MEX printing. These polymers were chosen for this study due to their compatibility and frequent use in extrusion-based 3D printing. The polymer’s characteristics used during the investigation are presented in Table 1, whereas the printing parameters are depicted in Table 2 below.

Ultrasonic welding of polymers operates by converting vibrational energy into heat through friction, melting the polymer surfaces, and facilitating molecular diffusion and entanglement to create a strong bond as the material cools [30]. The strength of UW joints is heavily influenced by the temperature spread in the weld zone, that further depends on two key factors: the heat generated and the force applied during the welding process [31]. The heat produced is controlled by parameters such as amplitude, polymer type and weld time while the applied force is largely determined by the joint design and the shape of energy directors. Considering these as the critical factors, parameters for the current study were carefully chosen to optimize weld strength of UW 3D printed parts.

A CAD model with dimensions of 96 × 25 × 4 mm³ (length × breadth × thickness) following the ASTM 3163-01 (with several adjustments) was designed using Solid Works [39]. A length of 96 mm was selected to ensure that the total length after welding would be 170 mm, allowing it to fit comfortably within the 3D printer’s build area. An overlapping region of approximately 21.25 mm (25% of the length of the specimen) was designated for ultrasonic welding. Mechanical stops and alignment tabs were implemented to maintain a consistent overlapping area and to ensure precise positioning of the specimens during testing, as illustrated in Fig. 1. Further, three types of Energy directors, namely Rectangular, Semi-circular as well as Triangular were also 3D printed on the test specimens, shown in Fig. 2 while the dimensions of the energy directors, depicted in Table 3 below. The energy director dimensions and their shapes were chosen based on the near-field welding principle and standard practice in the literature. Printing below 1 mm is challenging with FDM, and features larger than 2 mm could negatively affect the melt flow volume below the overlap region during welding. Therefore, 1 mm was selected as the practical lower limit, while 2 mm was chosen to avoid disrupting the melt flow [36]. Further, the ultrasonic welding nature of specimens having rectangular energy director (RED) profile has not been explored till date.

CAD files with an STL extension were imported into Cura slicing software, which converted them into GCode. This GCode then guides the printer’s tool head, determining when and where to deposit the thermoplastics on the print bed. The MEX 3D printer used in this study was the Creality Ender-3 V3 SE, which has a bed measuring 190 mm. A matrix of experiments matrix was created by Minitab^® 16 with a Taguchi design consisting of 5 factors at 3 levels, as detailed in Table 4 below. Further, Statistical evaluation and optimization were conducted through ANOVA.

The experimental levels were chosen based on the pilot study, which demonstrated that for the amplitude 80% to 100%, welding time 1.5 to 2 s, infill greater than 20% the welds were defect-less. In total, 162 specimens were printed for the optimization experiments, which consisted of 27 experiments with 2 pairs and 3 trials each. The three repetitions for each trial were averaged using the arithmetic mean, providing a single representative value per trial. After printing, two 3D printed polymeric samples would be sandwiched and clamped tightly using a die and placed below the horn for the welding. Fig. 3a–d shows the process schematically. The overview of the research work conducted is as depicted in the Fig. 4 below.

A high-performance ultrasonic welding machine, provided by Johnson Plastosonic Pvt. Ltd., Pune, operating at a frequency of 20 kHz and a power capacity of 1500 W, was utilized to weld the 3D-printed samples. All trials were conducted according to the DOE (Table 5) outlined below. Following the welding process, the samples were kept in the fixture for approximately 2 min to allow cooling, preventing warpage and ensuring optimal weld strength. Welding was performed on only one side, following which, the welded parts were subjected to additional testing and property evaluation.

The welded samples were examined under standard conditions (34°C and 84% relative humidity) using a universal testing machine (UTM) as presented in Fig. 3d. The tests were performed with a Zwick Roell-Z020 (Germany) UTM, stocked with a 20 kN load frame and a crosshead speed of 3 mm/min, following the ASTM D-638 Type-V standard [40–42]. For each trial, three tests were carried out to evaluate laps shear strength and elongation. Additionally, failure modes and defects in the welded samples were analysed using a stereomicroscope fitted with a 5 MP camera. The key findings from these tests are discussed in the following section.

Two primary goals guided the conduct of this investigation: First, to successfully demonstrate the achievability of UW on 3D-printed polymers with the aim of overcoming the part size restrictions of MEX-3D printers. Second, to experimentally and statistically evaluate the effects of different materials, welding parameters, and energy director geometries on the UW of 3D-printed parts. The experimental observation and analysis yielded the following significant findings: •

When compared to the majority of commercially available joining techniques, such as FSW, FSSW, SFW, AB, MW, UW welding produced the best joint efficiency (174.52%).

From a broader perspective, UW stands out for its quickness and economy, making it an excellent choice for welding AM parts to produce complex, high-strength components. However, the proper selection of welding parameters, particularly the material and the geometry of the energy directors, is essential to the weld’s success.