PSI - Issue 50

S.V. Panin et al. / Procedia Structural Integrity 50 (2023) 220–227

221

2

S.V. Panin et al. / Structural Integrity Procedia 00 (2019) 000 – 000

1. Introduction Ultrasonic welding (USW) is one of the common technologies for joining thermoplastic parts. This technology is well developed and widely deployed in some industries. The USW process is carried out in three key stages. Firstly, a compression force is gradually risen for squeezing parts to be welded until a threshold level is reached (ultrasonic (US) oscillations are not applied at this stage). Then, at the constant compression force level, US oscillations are turn on (with preset their both frequency and amplitude). During a certain period of time, the USW tool vibrations are transformed into heat via friction of the welded parts, mutually moving relative to each other. As a rule, the compression force is maintained constant at this stage. Finally, the molten material is solidified under the compression force at another level for a certain period. US oscillations are not applied at this stage. From the standpoint of optimizing the USW parameters (the compression force, as well as both period durations of US oscillations and the material solidification), the molecular structure (molecular weight and polarity of units) of joined polymers are important in addition to their glass transition temperatures and the melting points. The compression force determines the diffusion rheological interaction of molecules and the formation of an interlayer at the interface of the welded parts. Such interconnection is effectively implemented when the polymers are in a viscous state, and their molecules have both maximum mobility and minimum packing density [1 – 7]. Previously, USW procedures have not been implemented in the industry for manufacturing laminate reinforced products (prepregs) from thermoplastic composite binders. However, a significant number of papers have been published on the use of USW for the formation of laminate composites based on high-strength high-temperature thermoplastic binders reinforced with high-modulus fibers (fabrics) [8 – 11]. Nevertheless, there are still gaps in knowledge about the optimal USW parameters for joining such composites. In this study, ranking of the key USW parameters for joining laminate composites was carried out using the Taguchi method. To understand the nature of the revealed effects, an analysis of the structure at the fusion zone interface was performed. 2. Materials and Research Methods For the fabrication of samples of laminate composites, the following components were used. Plates were formed by ram extrusion from the ‘770PF’ PEEK powder (Zeepeek, China) [12, 13], a PEEK -based prepreg and a tape (Toray Cetex TC1200, 140 µm thick) made of unidirectional carbon fibers. A PEEK film (Victrex, Aptiv 2000) with a thickness of 250 µm was used as an adhesive film (Energy Director – ED). The film thickness was chosen by analyzing the data, published in [14 – 19]. For the USW procedure, a ‘UZPS - 7’ setup was used (‘SpetsmashSonic’ LLC, V oronezh, Russia). The welded plates were placed in a fixing clamp, which suppressed their mutual movement. The size of a tool for the application of US oscillations to the overlapped plates was 20×20 mm. The Taguchi method [20] was applied to optimize the USW modes for obtaining the ‘PEEK -ED-prepreg-ED PEEK’ joints. In the experiment planning process, the following levels of all factors (the USW parameters) and a range of their possible changes were preset. The USW durations (  USW ) were from 800 up to 1.200 ms, because it was not possible to weld the plates at lower values, while higher levels could result in melting the ED and the prepreg failure. This would inevitably lead to the formation of discontinuities. Both ranges of the compression force ( P compress ) levels and the compression (holding) durations (  compress ) after the USW process were determined based on the technical characteristics of the USW setup, as well as the results of visual inspection of the welded joints. Shear tests of the lap welded join ts were performed with an ‘Instron 5582’ electro -mechanical tensile testing machine according to ASTM D5868. The structural study of the welded joints was performed using a ‘Neophot 2’ optical microscope (Carl Zeiss, Germany). 3. Results and Discussion The obtained mechanical characteristics were ranked according to the Taguchi method. Table 1 presents the summarized factors and their levels. Each level and factor were applied three times. Thus, nine types of the samples