Materials Science Forum Vol. 1185

Abstract: Friction stir welding (FSW) is widely used to bond metals and polymers to create large structures from standardized geometric components. However, FSW has found more limited use in traditional continuous fiber reinforced composites due to the risk of damage to the reinforcing fibers, including fiber misorientation and fragmentation. The process is more amenable to discontinuous fiber composites, which often take the form of injection molding compounds reinforced with short, milled fibers, for which the mechanical performance is significantly degraded. An emerging class of materials, recycled carbon fiber nonwovens, contain long discontinuous fibers that imbue their composites with mechanical performance that bridges the gap between injection molding compounds and continuous fiber laminates, while also decreasing the energy footprint of the materials. This work evaluates this class of materials, alongside injection molding compounds, to develop new insights into the bonding of composite structures using friction stir welding. The bridging effect of discontinuous fibers across the bondline is evaluated using optical microscopy to link processing conditions and fiber length to the resulting performance. Fiber migration was observed in the weld area, though mechanical interlocking was the primary mechanism of bonding in the weld zone. While samples failed at a fraction of the neat materials nominal strength, increased fiber length was found to have a beneficial effect on the apparent tensile and lap shear strength on welds in this study. The new insights gained represent an important step towards the adoption of the FSW process as a means of rapidly manufacturing large composite structures made of carbon fiber/PPS or other fiber-reinforced polymers to enable deployable structures from standardized geometric components.

Abstract: Rotary friction welding (RFW) is a solid-state process used to join similar and dissimilar materials, such as steel and aluminium. In RFW, interface temperature development and its distribution are essential factors influencing material bonding. It governs bond strength, the formation of intermetallic phases (IMPs) and the evolution of the heat-affected zone (HAZ). Thus, precise prediction of temperature distribution is vital for the reliable design and optimization of the RFW process, as well as for the prediction and control of IMP formation. This work presents an experimental investigation of the thermo-mechanical behaviour of EN AW-6082 and 20MnCr5 during RFW in addition to a corresponding novel numerical modelling framework. A systematic parameter study was conducted to evaluate the influence of the friction pressure, friction time, forging pressure, forging time and rotational speed on the peak temperature, sledge path and flash formation. In-situ temperature measurements were performed using thermocouples (TC) embedded in the steel component, while axial force, displacement and rotational speed were recorded. The results demonstrate that, within the investigated parameter ranges, the rotational speed is the dominant factor governing frictional heat generation and the peak temperature, while the friction pressure primarily influences the sledge path. In parallel, a 2D axisymmetric finite-element model with a user-defined subroutine was developed to compute the heat flux based on process parameters and contact conditions, providing a transparent and extensible numerical framework for RFW. The experimental findings establish a robust basis for the calibration and future validation of the numerical model.

Abstract: With the increasing demand for lightweight materials, the combination of aluminum and magnesium sheets enables the development of advanced laminates with a balanced combination of strength and ductility, making them suitable for forming applications. This work investigates the effect of rolling temperature on the mechanical behavior and formability of AA1050/AZ31/AA1050 sheets produced by roll bonding in the temperature range of 250–450°C. Tensile tests showed that the yield stress is weakly affected by rolling temperature, whereas the ultimate tensile strength increases up to 350°C and then stabilizes. The elongation at fracture increases monotonically with temperature, indicating improved ductility at higher rolling temperatures. Microhardness measurements revealed softening of the aluminum sheets with increasing temperature, while limited variations were observed in the AZ31 sheet. Formability was evaluated by Erichsen Cupping test. The maximum load and extension at break remained nearly constant over the investigated temperature range; however, higher rolling temperatures led to reduced delamination and improved interfacial bonding integrity during deformation. The results indicate that roll bonding at elevated temperatures promotes better strain distribution and enhanced bonding quality. Overall, roll bonding at 450°C provides the most favorable combination of mechanical performance, formability, and interfacial stability, making the produced sheets suitable for lightweight forming applications.

Abstract: Optimizing the performance and reliability of welding techniques for dissimilar aluminum (Al) to titanium (Ti) is a promising way to establish new applications in aerospace industry. Due to structural weight reduction, lightweight materials can help to minimize fuel consumption and save emissions. Solid-state welding technologies allow short joining cycles and metallurgical changes, residual stresses and severe intermetallic compound formation can be reduced by limited thermal exposure. Besides temperature and plastic deformation, intimate contact plays an important role for diffusion. In this work, AlMgSi alloys with systematic variations of Mg and Si alloying elements, were welded to Ti6Al4V (Ti64) by refill Friction Stir Spot Welding. The focus lays on the effect of Ti64 sheet surface roughness, varied by different surface preparations. Additionally, the influence of the plunge depth, the distance between the tool and the Ti64 sheet surface is analyzed. It was found that a reduced tool to interface spacing has a beneficial influence on joint integrity. Grinding trenches allowed better bonding compared to the pit-like surface structure generated by sandblasting, which led to an increase in mechanical lap-shear properties. Knurling the grinded surfaces resulted in high standard deviation, as most likely not the whole interface area was bonded. However, the partially outstanding properties showed that a beneficial effect can be expected due to mechanical interlocking mechanisms, when sufficient diffusion is ensured.

Abstract: Self-piercing riveting (SPR) is a well-established joining technique in lightweight construction, as it enables the joining of different materials without requiring pre-drilling. However, the necessary adaptation of the rivet-die combination to the respective material and thickness combinations requires a large number of specific tool sets, which significantly limits the process's flexibility. To overcome these limitations, the versatile self-piercing riveting (V-SPR) was developed, which features enhanced punch actuation in combination with a multi-range-capable rivet . In this context, the concept of a movable die was introduced, which enables an extended process window and adaptable joint formation. Kappe et al. presented initial studies demonstrating the potential of this approach . However, a detailed numerical understanding of the underlying mechanisms remains lacking. This paper presents a numerical analysis of V-SPR with a movable die using a finite element (FE) model. The model includes deformable rivets, sheet metal materials and a kinematically controlled die with adjustable movement. A parameter study was conducted to analyse the influence of die movement on the material flow of the rivet and sheets, as well as joint formation. The simulations were validated using selected experimental data. The goal is to compare the joint geometries achieved with fixed and moving dies and expand the process windows of VSPR. The results demonstrate that the movable-die concept significantly enhances the material flow of both the sheets and the rivet, resulting in a noticeably larger and more reliable interlock than what is achievable with V-SPR using a fixed die. The numerical analyses support the observations reported by Kappe et al. and extend them by providing a quantitative description of how die displacement influences the resulting interlock size. Moreover, the ability to precisely control the die movement makes it possible to join challenging sheet-metal combinations that are difficult to process with conventional setups, particularly in cases involving thicker sheet materials.

Abstract: Clinched joints with non-rotationally symmetric geometries exhibit orientation-dependent mechanical behavior that is commonly neglected in structural-scale simulations. Reuleaux triangle shaped clinched joints, in particular, show pronounced in-plane anisotropy depending on their orientation. While such effects have been studied at joint and specimen scale, their relevance at the structural level remains largely unexplored. In this work, the influence of joint orientation on the bending response of a joined structure is investigated using numerical simulations. A simplified joint replacement model based on the *CONSTRAINED_SPR2 point-connector formulation in LS-DYNA is employed, with parameters calibrated from previously obtained experimental force displacement data. A hat shaped profile structure subjected to three-point bending is analyzed in a parametric study considering variations in joint orientation, joint spacing, and profile geometry. The results show that joint orientation has little influence during the initial deformation phase but becomes increasingly significant at larger displacements, where joint behavior governs load transfer. Orientation dependent effects are found to influence the global force displacement response and local load redistribution among joints, with magnitudes comparable to those induced by changes in joint spacing and structural geometry. The findings confirm that joint orientation effects remain relevant at the structural level and should be considered in the design of structures assembled using non-rotationally symmetric clinched joints.

Abstract: This paper proposes, for the first time, that the gas pressure loading path significantly influences the weld interface ratio of SPF/DB four-layer sandwich structures. Based on a gas pressure loading curve incorporating back pressure, experimental verification and analysis were conducted. Ultrasonic C-scanning, metallographic examination, and scanning electron microscopy (SEM) were employed to observe and analyze unwelded defects, elucidating their causes and formation mechanisms. Key process parameters—including back pressure time, back pressure value, and gas inlet delay time—were extracted and defined. The influence of these parameters on the weld interface ratio of four-layer sandwich structures was systematically investigated. Finally, with the objectives of eliminating surface grooves and achieving a high weld interface ratio, reference ranges for these process parameters are provided.