Faying Surface Research Articles

Challenges in joining of metallic pipes using microwave hybrid heating

Microwave hybrid heating (MHH) is a distinct and emerging alternative joining technique used to join metallic materials compared to available conventional joining processes such as FW, TIG, MIG, etc. MHH shows significant characteristics such as selective and volumetric heating, lower processing time, eco-friendly processing against conventional joining. This paper aims to investigate the joining of 316L stainless steel pipes by incorporating micrometric size (45μm) of 316L stainless steel powder in between the faying surfaces of the candidate material. Charcoal as a susceptor material was placed at the joint zone and exposed to microwave energy at 2.45 GHz inside the domestic microwave applicator at 900 W power levels. The challenges and principles involved in joining SS 316 L pipes using MHH have been discussed and initial experimental observations have been analyzed.

Flash joining of alumina ceramics under a small current density

The ceramic joining using electric field (E-field) has garnered significant research attention due to the decreased joining barrier and enhanced reliability. However, the underlying mechanism of E-field assisted joining remains unclear. Herein, we report the rapid joining of alumina ceramics using a small current. Moreover, the E-field is applied in both perpendicular and parallel directions to the faying surfaces, demonstrating a significant difference in terms of joint strength. These results indicate that the E-field generates defects and promotes the joining process by facilitating ionic diffusion.

Simulation of rotary friction welding using a viscoelastic Maxwell model

A 3-D finite element model of rotary friction welding was developed using a viscoelastic Maxwell model. The thermal softening of the material was accounted for in the viscous part of the material model as well as in an additional friction model. The thermal and mechanical material data were adapted from existing data of tempering steel AISI 4140. The final model was used to predict rotary friction welding of shafts using three different parameter settings. The simulation results showed that the melt temperature was never exceeded at the faying surfaces, which is seen as a main characteristic of friction welding. Subsequent validation also demonstrated that the shortening of the specimen as well as the resulting flash geometry could be predicted successfully.

Technical Note: Notes on Determining Required Connector Strength in Built-up Compression Members

Connections between individual components of a built-up compression member carry no force when the member is perfectly straight in the unbuckled configuration under load. Yet AISC Specification Section E6.1 (AISC, 2016) requires that the end connection of these built-up members be welded or connected by means of pretensioned bolts with Class A or B faying surfaces. Nothing is provided in the AISC Specification or Commentary to help the designer determine the required strength of these connectors. This paper suggests one way of determining these required strengths and provides two LRFD examples. Other assumptions may be used to derive different, but equally acceptable, required strengths.

Ultrasonic investigation of the pressure profile on the faying surface of fastened aluminum plates

Machines, buildings, and vehicles often use fasteners to connect structural elements. Screws and bolts introduce a pressure profile on joint interfaces that varies under different material parameters, preloads, and assembly conditions. Characterization of interfacial pressure at the faying surface of a joint is helpful for understanding the dynamics of the entire built-up system. Examples of interfacial measurements in the literature utilize ultrasonic methods. Some of these previous studies have used separate calibration specimens to convert qualitative ultrasonic measurements to quantitative pressure at an interface. An experiment presented here measured the quantitative distribution of pressure between fastened aluminum plates using an ultrasonic probe and force-sensing washer, without the need of a calibration specimen and test. Beyond traditional measurement methods of this type, boundary effects near the fastener were also accounted for. Scans using this method match finite element results, lending to the utility of the new procedure. The new method has shown to be accurate and more advantageous when compared to previous methods.

Effect of core bar inserted into weld faying part to obtain an ideal pipe joint with non-generating inner flash via friction welding

In this study, the effect of core bar inserted into weld faying part to obtain an ideal pipe joint with non-generating inner flash via friction welding is described. A steel pipe with inner and outer diameters corresponding to 8.0 mm and 13.5 mm was used, and the weld faying surface was machined to a groove shape of a flat (butt) type. The core bar of various materials was inserted in the weld faying part of the pipes, and those pipes were welded with a friction speed of 27.5 s−1 and friction pressure of 30 MPa. The core bars did not decrease inner flash when joints were fabricated with a core bar of some metallic materials with melting points below that of steel; thus, they were melted during the welding process. The joint with an alumina core bar did not decrease inner flash and was crushed by generating an inner flash. However, a commercially pure tungsten (CP-W) core bar was successfully achieved for decreasing the inner flash. Additionally, all joints with a CP-W core bar did not exhibit the tensile strength of the base metal and a fracture in the base metal, when they were fabricated during the same time, the friction torque reached the initial peak. The joint exhibited a fracture in the base metal when it was fabricated with a CP-W core bar and a taper groove shape that was proposed in the previous study. Furthermore, the core bars were easily removed from the joints; thus the joint with almost no inner flash was successfully obtained. To reduce the inner flash of pipe joints, they should be fabricated with a CP-W core bar inserted into the weld faying part with a taper groove shape.

A new approach for dissimilar aluminum-steel impact spot welding using vaporizing foil actuators

Abstract Dissimilar joining of aluminum to steel is an important industrial and academic problem. The present study reports the application of a novel approach in impact welding where a pocket machined on the flyer sheet generates the requisite standoff, enabling joining of commonly used automotive aluminum alloys to high-strength low-alloy (HSLA) steels. The new approach enables impact spot welding of heat treatable AA6111-T4 aluminum (Al) alloy to bare, hot-dip galvanized and e-coated HSLA 340 steel (with local removal of the coating on the faying surface) as well as of high-pressure vacuum die cast Aural 2 Al alloy to hot-dip galvanized HSLA 340 steel. Characterization of the welded samples via lap-shear and coach peel testing demonstrated high joint strengths with button pullout failure modes in all weld pairs. Optical microscopy of the weld cross-sections revealed defect-free joints displaying typical impact welded interface morphology. A significant outcome of the novel impact spot welding approach was the ability to produce joints exhibiting flat external target surfaces with unperturbed coating, a potential enabler for mixed metal joining applications involving pre-treated, e-coated, or painted materials.

PVDF Sensor Foils Employed to Measure Shear Stress and Temperature of Friction Welding

Friction welding is a popular process for joining metal and polymer work pieces by rubbing them against each other. This way, friction heat is generated in a zone of the faying surfaces, thinner than 1 mm. After cooling down, the heated surfaces establish a tight and strong bond. To improve this joining process, a method is desirable allowing measuring generated temperature and shear stress in the zone between the joining work pieces. Unfortunately, this is a very difficult task because the welding zone cannot be accessed with a sensor without significantly altering the process and thereby the desired measurement results. This paper describes how shear stress and temperature change generated by rubbing polymer pieces in a friction welding machine have been measured between the faying surfaces by employing sensor foils from the piezoelectric and pyroelectric polymer polyvinylidene fluoride (PVDF). This way, heating and cooling rates, pressure rise of the pneumatic system, frequency rise of the starting machine, the duration of starting and stopping, the damping of the vibrations after the drive was stopped, and the stress generated by the pullback of the machine head have been measured. A careful characterization of the sensor was necessary to enable distinguishing the measured voltage due to straining, shearing and temperature change.

Impact Behavior for Successful Particle–Particle Bonding in Vacuum Kinetic Spraying

This study investigated the impact behavior of ceramic particles in vacuum kinetic spray (VKS), also known as aerosol deposition method, for successful coating build-up using both experimental methods and numerical simulations. A coating trial and fluid dynamics simulation results showed that there existed a critical impact velocity at which successive coating build-up was possible. This indicated that particle–particle bonding could be achieved through sufficient kinetic energy of impacting particles. According to the results in this study, it is suggested that the critical velocity values for deposition of Al2O3 and SiO2 in VKS were 400 and 300 m/s, respectively. The AUTODYN simulation results revealed that the initial kinetic energy of particles at high impact velocity should be consumed by fracture and deformation of both impacting particles and the pre-deposited layer. Above critical impact velocity, it is expected that high von Mises stress, pressure, and temperature values contribute to dynamic fragmentation of impacting particles, additional size reduction of crystallites, and consolidation for uncontaminated intimate inter-crystallite faying surface bonding. It is expected that these material responses contributed to the dense and strong VKS coating fabrication.

Quasi-static and dynamic performance of novel interlocked hybrid metal-composite joints

This study presents a novel hybrid technique for joining composites to metal, employing an array of macro-scale interlocking features on the faying surfaces of adhesively bonded adherends. Single-lap, interlocking adhesive joints (IAJs) and baseline adhesive joints (BAJs), are tested at quasi-static and transient dynamic (0.5 m/s and 3 m/s) loading rates. The joint deformation mechanisms are examined and fractography analysis is performed at the macro and micro scales. Results indicate a 10% increase in lap-shear strength, and 75–120% increase in work to failure for the IAJs compared to the BAJs, at all loading rates. In addition, IAJs exhibit improved damage tolerance compared to adhesive joints, due to reduced joint rotation, more stable adhesive fracture growth, and the ability to sustain load even after cracks have propagated through the adhesive at the ends of the overlap region. The high energy absorption capacity (23–38 J) of IAJs indicates they could be used to significantly improve the crashworthiness performance of multi-material transportation structures.

The joining of gamma titanium aluminides via the powder interlayer bonding method

Powder interlayer bonding (PIB) is a joining technique originally developed to enable high-integrity repairs of aerospace components. The technique has previously been employed for the joining of titanium and nickel alloys utilised in the aerospace industry. This study expands on the application of the novel joining technique known as powder interlayer bonding (PIB), to the bonding of γ titanium aluminide (TiAl) material. PIB has been used to facilitate high-integrity joints in gamma titanium aluminides (TiAl), where full densification of the joint was achieved. The PIB technique described here used a metallic powder interlayer between the two faying surfaces of γ TiAl specimens. Bonds were formed in an inert atmosphere under induction heating. The PIB technique proved capable of producing high-integrity bonds in terms of microstructural evaluation, with very limited porosity retained after the bonding cycle. A brittle Ti2Al phase can be produced with heavily oxidised powder which is susceptible to cracking and will negatively affect mechanical properties.

Diffusion Bonding of Al–Fe Enhanced by Gallium

In this research, diffusion bonding was carried out to produce transition joints between mild steel A36 (Fe A36) and aluminium Al 5083 (AA5083) with the presence of gallium (Ga) as an interlayer between the two faying surfaces. The microstructural development and interfacial growth of intermetallic compounds at the interface layer between Fe A36 and AA5083 after the diffusion bonding process were investigated. The joining was performed by clamping the two materials with a Ga interlayer and then heated in a furnace. The interlayer developed from this diffusion heating in air condition provides an average thickness of 30 μm. Characterization of intermetallic compounds was conducted using SEM-EDX and XRD. The results showed that SEM-EDX confirmed the occurrence of interdiffusion of elements from Fe A36 and AA5083 present at interlayer. XRD analysis reveals the formation of Fe3Al at the diffusion layer.

Strengthening of steel decks for cable-stayed bridge using ultra-high performance concrete: A case study

Fatigue cracking induced by vehicle load is a prevalent problem in orthotropic steel decks. In addition, pavement debonding in steel bridge decks is another familiar issue resulting from low slip resistance in the faying surface between the steel and asphalt concrete. The present study proposed a strengthening method that uses ultra-high performance concrete to stiffen a repeatedly maintained cable-stayed bridge in order to help address these two problems. The existing issues of the real bridge and the corresponding causes were investigated. Following this, an ultra-high performance concrete paving system was designed to improve the stiffness of the orthotropic steel decks. For this paving system, a 45-mm ultra-high performance concrete layer was connected to the deck by welded shear studs. The local stresses at the typical vulnerable fatigue cracking points were determined by means of a finite element model and of a field loading test to evaluate the strengthening effect. The results showed that this strengthening method can prevent the propagation of fatigue cracks. The local stresses of the U-ribs and diaphragms were reduced by 45.4% and 40.0%, respectively. The repaired bridge has sufficient resistance against fatigue cracking based on the in situ observations.

Joint formation mechanism and performance of resistance rivet welding (RRW) for aluminum alloy and press hardened steel

The options for joining aluminum (Al) alloy to ultra-high strength steels are extremely limited due to the great differences in their thermophysical properties. In this work, a novel process, e.g., resistance rivet welding (RRW), was proposed to join Al alloys to press hardened steels (PHS) through combining traditional self-piercing riveting and resistance spot welding. The materials plastic flow, element diffusion and nugget growth characteristics during RRW were studied in depth to reveal the nugget formation mechanism. It was found that in RRW process, the intercepted Al alloy, internal wall of rivet shank and PHS near the faying surface were melted together and produced a unique Al/steel mixed nugget. The microstructure of the mixed nugget was equiaxed ferrite with a large number of B2-structured intermetallic compounds (IMCs) FeAl dispersed within it. The competition between the nugget and Al sheet determined the joint mechanical performance. The nugget dominated joint strength when the heat input is comparatively low while the Al sheet played a significant role with the increase of heat input.

Effects of tensile strength on friction welding condition and weld faying surface properties of friction welded joints between pure copper and austenitic stainless steel

This paper reported the effects of tensile strength on friction welding condition and weld faying surface properties of friction welded joints between pure copper (OFC) and austenitic stainless steel (AISI 304). The joining phenomena and the joint tensile strength at various friction welding conditions were investigated. The maximum temperature of the joint at a friction pressure of 90 MPa was lower than that of 30 MPa. Also, the central portion of the weld interface of the joint with high friction pressure was not joined completely. Hence, it was showed that the joint should be made with a low friction pressure. In addition, the friction torque curve, joint appearance, flash quantity, and axial shortening of joints differed by virtue of the polishing timing at the weld faying surface, and they were influenced by the surface condition of the OFC side before welding. As a conclusion, the good joint with the fracture in the OFC side should be made with a low friction pressure such as 30 MPa, a friction time after the friction torque reached the initial peak such as 3.6 s, and a high forge pressure such as 270 MPa. Furthermore, the weld faying surface of the OFC side should be polished just before welding, and it was suggested.

Design variables influencing the fatigue of Al 2024-T3 in riveted aircraft lap joints: Squeeze force and initial fit tolerance

This study experimentally analyzed the influence of squeeze force and initial fit tolerance on the fatigue property of a riveted aircraft lap joint. All specimens were prepared under load control and tested under six levels of fatigue loads. The results showed that an initially oversized rivet might be detrimental to fatigue-life improvement. Increasing the squeeze force within an allowable range could extend the fatigue life generally. A relative decrease was also observed at a higher range of squeeze force. A higher fatigue load was likely to weaken the effect of the interference fit. The reduced fatigue life at higher squeeze forces could be clearly explained in terms of the fretting characteristics on the faying surface as a result of different squeeze forces and load levels. This study suggests that, to fully extend the fatigue life of riveted lap joints, the influence of working fatigue loads must be considered in the structure design.

Experimental Study on the Performance of GFRP–GFRP Slip-Critical Connections with and without Stainless-Steel Cover Plates

Composite structures have become increasingly popular in civil engineering due to many advantages, such as light weight, excellent corrosion resistance and high productivity. However, they still lack the strength, stiffness, and convenience of constructions of fastener connections in steel structures. The most popular fastener connections in steel structures are slip-critical connections, and the major factors that influence their strength are the slip factors between faying surfaces and the clamping force due to the prevailing torque. This paper therefore examined the effect that changing the following parameters had on the slip factor: (1) replacing glass fiber reinforced plastic (GFRP) cover plates with stainless-steel cover plates; (2) adopting different surface treatments for GFRP-connecting plates and stainless-steel cover plates, respectively; and (3) applying different prevailing torques to the high-strength bolts. The impact on the long-term effects of the creep property in composite elements under the pressure of high-strength bolts was also studied with pre-tension force relaxation tests. It is shown that a high-efficiency fastener connection can be obtained by using stainless-steel cover plates with a grit-blasting surface treatment, with the maximum slip factor reaching 0.45, while the effects of the creep property are negligible.

Bonding formation in vacuum kinetic-sprayed Y2O3 particles induced by high-velocity impact

This study proposed a satisfactory bonding formation mechanism between inter-crystallites fabricated by a vacuum kinetic spray (VKS). This paper confirmed the necessity of high-velocity impact for particle deposition. Based on both experimental (coating trials) and numerical approaches (ANSYS AUTODYN), the impact behavior of Y2O3 particles at different impact velocities was carefully analyzed. There existed a critical impact velocity above which a successive coating buildup with nanocrystal size was possible according to coating thickness measurement. Microstructural observations showed that dynamic fragmentation (creation of non-contaminated faying surface) of the impacting particle and consolidation of the pre-deposited layer occurred favorably at a high impact velocity. The localized deformation at the crystallite surface was attributed to inter-crystallite bonding (frictional heat generation during crystallite rotation). A numerical approach also supported the generation of sufficient energy for strong bonding by high von Mises stress, pressure, and temperature in the contact region at high velocity. Proper kinetic energy of the impact particle is key to successful Y2O3 coating deposition.

Application of ultrasonic welding to repair adhesively bonded short carbon fiber reinforced Nylon 6 composites

A method of applying ultrasonic welding to repair adhesive-bonded 2.3 mm thick injection molded short carbon fiber reinforced Nylon 6 (Cf/PA6) composite with 30 wt% fiber was developed. The method included positioning the ultrasonic horn and anvil proximate the region on the workpieces having the partially adhesive-bonds and applying an ultrasonic oscillation at the workpieces where insufficient adhesive was dispensed at the faying surfaces. The effects of horn force, oscillation time, and inadequate adhesive on the quasi-static strengths of the nominal, partially adhesive-bonded and repaired joints were investigated. It was found out that the strengths of the partially adhesive-bonded joints with 0%, 25%,50% and 75% adhesive coverages were restored and achieved over 90% of the nominal joint strength (i.e., full adhesive coverage). Microstructure analyses of the repaired joints demonstrated that the second ultrasonic pulse produced viscoelastic heating and frictional heating, and consequently fused carbon fiber composites at the faying surfaces of the workpieces. The strengths of the repaired (i.e., weld-bonded) lap-shear joints were primarily from the combined contribution of adhesive-bond and ultrasonic weld at the faying surfaces, which were virtually unchanged regardless the amount of adhesive missing. Therefore, for the substrate and joint configuration and dimensions studied here, application of ultrasonic welding virtually restored the quasi-static strengths of partially adhesive-bonded lap-shear Cf/PA6 composites with 30 wt% fiber. Consequently, by the application of this repair method the scrap of the component assembly would be reduced.

The Effect of Processing Variables on Powder Interlayer Bonding in Nickel-Based Superalloys.

Powder Interlayer Bonding (PIB) has been considered as a lower-energy joining technology for nickel-based superalloys compared to conventional methods; such as friction welding. Typically; nickel-based superalloys exhibit high energy requirements for joining due to their high operating temperatures. However; PIB utilizes a localized temperature gradient created by an induction current; reducing the energy requirements for the process. PIB is a solid-state joining method that compresses and heats a powder interlayer between two faying surfaces to produce one joined workpiece. It has been successfully used to bond titanium alloys; and the objectives of this work were to explore its application as a joining method for nickel-based superalloys. Initial results showed that joining nickel-based superalloys via PIB is possible; and bondlines with very little porosity were observed. Further analysis showed that these bonded areas had lower porosity than the base material; suggesting PIB could be a successful joining method for difficult-to-join nickel-based superalloys.