High-strength Joints Research Articles

Experimental investigation of the friction stir welding process of AA5059 and AA7079 and prediction using fuzzy approach

Aluminum alloys AA 5059 and AA7079 have high strength, excellent weldability, and corrosion resistance, widely used in train and truck bodies, armored vehicles, and defense applications. Recent materials engineering tasks call for minimizing material weight and to achieve this objective, the solid-state joining approach of Friction stir welding (FSW) is utilized for material joining. FSW has produced sound welds with high joint strength and good ductility. This research investigates the weldability of AA5059 and AA7079 dissimilar aluminum alloys. In this FSW process, a threaded tapered cylindrical tool pin profile was selected, and welding trials were conducted at three different speeds of 500, 750, and 1000 rpm, with the 75 mm/min constant feed rate. The Taguchi L9 orthogonal array was selected to design the process parameters, that is, tool traversing speed, rotational speed, axial force, feed rate, and effect on dissimilar aluminum weld joints. The Mamdani-type fuzzy logic model predicted welded specimens’ ultimate tensile strength and yield strength. Microstructural analysis revealed that the grains in the stirring zone had undergone a full transformation into homogenous, evenly distributed, refined grains, which had a positive impact on the remarkable mechanical properties.

Influence of the material properties on the clinching process and the resulting load-bearing capacity of the joint

This study focuses on the phenomenological change in material strength caused by a specific heat treatment and the subsequent analysis of the influence on the clinching process and the resulting joint properties. For this purpose, three series of tests were performed. In the first series of tests, the influence of heat treatment up to 340 °C on the mechanical properties of an age-hardenable AlMgSi alloy was investigated. Holding time and temperature were varied and the material strength was evaluated by tensile and hardness tests. Two strength-increasing and two strength-reducing heat treatment parameters were identified. In the second series of tests, selected heat treatment parameters were applied to a larger number of specimens and the joint strength was investigated by shear and head tensile tests. In the shear tensile test, mainly the properties of the punch-side material have an influence on the resulting joint strength. A change in strength of the die-side material can be neglected. In contrast, the properties of both sheets are important in the head tensile test. The strength of the joint will only increase if the strength of both sheets is increased. In general, a strength increasing heat treatment resulted in higher joint strength. In the third series of tests, the factor of punch displacement was considered, which was demonstrated to directly influence the formation of the clinched joint geometry.

Strengthening flat-die friction self-pierce riveting joints via manipulating stir zone geometry by tailored rivet structures

Achieving high-strength joints with flat surfaces is of significant importance for reducing wind resistance and enhancing aesthetic appeal. In this study, a novel flat-die friction self-piercing riveting (flat-die F-SPR) process is proposed. The rivet flaring without die guidance was achieved through the sophisticated design of rivet structures. Three types of rivets with different internal structures were designed to manipulate the material flow and microstructure evolution during the joining process. Based on the method of emergency stop, the load-stroke curves, evolutions of macroscopic morphology, and microstructure of the joints made with different rivets were investigated. A novel mechanism for solid-state bonding of joints was proposed to elucidate the generation and evolution of fine grain regions. The results indicate when downward pressure is applied to the material inside the rivet cavity, a central stirring zone appears. By using a rivet with an annular boss structure, the base material flows continuously into the stirring zone and piled up in the rivet cavity, forming a unique conical-shaped fine grain zone. Finally, a comprehensive assessment of the strength of different types of joints and the transition of the fracture modes were conducted based on different lower sheet thicknesses. The joints of Rivet_B and Rivet_C demonstrate 11.1 % and 6.9 % strength enhancement compared with the joint of Rivet_A, respectively. Two strategies for enhancing the strength of solid-state bonding are proposed, which offers insights for the optimizations of rivet structures.

Effect of Pressure on the Linear Friction Welding of a Tool Steel and a Low‐Alloy Carbon Steel

This study investigates the effect of pressure (burn‐off and forging) on the mechanical properties of the joint between a wear‐resistant tool steel and a low‐alloy steel using linear friction welding. The authors have previously demonstrated the feasibility of joining these dissimilar materials, but the impact of pressure on the mechanical properties of the bimaterial joint remains unclear. To address this, weld samples are prepared using different pressures and are characterized through microstructural analysis, microhardness, tensile testing, and fractography. The results show that the strength of the joint between the wear‐resistant tool steel and the low‐alloy carbon steel increases as the pressure increases up to a certain point, after which a decrease is observed. The highest joint strength is achieved at a pressure of 360 MPa. The microhardness profile measurement reveals a distinct transition zone at the interface between the two materials, with varying hardness values. The hardness of the low‐alloy carbon steel increases near the interface, while that of the wear‐resistant tool steel decreases. This transition zone is found to be narrower at higher pressures. Microstructural characterization shows that the grain structure near the interface differs from that of the starting base materials.

Dissimilar Resistance Welding of NiTi Microwires for High-Performance SMA Bundle Actuators

Shape memory alloys (SMAs) are becoming a more important factor in actuation technology. Due to their unique features, they have the potential to save weight and installation space as well as reduce energy consumption. The system integration of the generally small-diameter NiTi wires is an important cornerstone for the emerging technology. Crimping, a common method for the mechanical and electrical connection of SMA wires, has several drawbacks when it comes to miniaturization and high-force outputs. For high-force applications, for example, multiple SMA wires in parallel are needed to keep actuation frequencies high while scaling up the actuation force. To meet these challenges, the proposed study deals with the development of a resistance-welding process for manufacturing NiTi wire bundles. The wires are welded to a sheet metal substrate, resulting in promising functional properties and high joint strengths. The welding process benefits from low costs, easy-to-control parameters and good automation potential. A method for evaluating the resistance-welding process parameters is presented. With these parameters in place, a manufacturing process for bundled wire actuators is discussed and implemented. The welded joints are examined by peel tests, microscopy and fatigue experiments. The performance of the manufactured bundle actuators is demonstrated by comparison to a single wire with the same accumulated cross-sectional area.

Experimental investigation into friction stir welding of AA6061-T6 and magnesium AZ31B alloy using copper backing plate

Aluminium-magnesium alloys are lightweight and possess superior properties, making them ideal for applications in aerospace, railway, automotive, and marine structures. However, these alloys are challenging to fabricate using traditional fusion welding due to various metallurgical issues. This study investigates the feasibility of joining 3 mm thick dissimilar alloys of aluminium AA6061-T6 and magnesium AZ31B using the friction stir welding (FSW) process with copper as a backing plate. The effects of FSW process parameters, including tool rotational speed (380, 545, 765 rpm), weld speed (20, 31.5, 50 mm min−1), and tilt angle (1°, 2°, 3°), on tensile properties were examined using Taguchi-based grey relational analysis. Temperatures were recorded during the FSW process under different conditions on both sides of the joint. The highest tensile strength of 130.72 MPa, with a joint efficiency of 67.85%, was achieved at a tool rotational speed of 765 rpm, a welding speed of 50 mm min−1, and a tilt angle of 2°. Analysis of variance (ANOVA) for the grey relational grade indicated that the tilt angle had the highest percentage contribution of 43.60%. This study demonstrates the significant influence of FSW parameters on the mechanical properties of aluminium-magnesium joints and highlights the optimal conditions for achieving high joint strength and efficiency.

Study of Structure Formation in Multilayer Composite Material AA1070-AlMg6-AA1070-Titanium (VT1-0)-08Cr18Ni10Ti Steel after Explosive Welding and Heat Treatment

Multilayer composite materials, consisting of layers of aluminum alloy and steel, are used in the manufacturing of large engineering structures, including in the shipbuilding and railcar industries. Due to the different properties of aluminum alloys and steels, it is difficult to achieve high-strength joints by conventional welding. Therefore, these joints are produced by explosive welding. In the present work, the structure of a multilayer material, AA1070-AlMg6-AA1070 (aluminum alloys)-VT1-0-08Cr18Ni10Ti (steel), was investigated after explosive welding and heat treatments were performed under different conditions. The microstructure of the AlMg6 layer at the AlMg6-AA1070 interface consists of shaped anisotropic grains extending along the weld interface. The AA1070 layer is enriched with magnesium due to its diffusive influx from AlMg6. In the AlMg6 and VT1-0 layers, adiabatic shear bands are found that start at the weld interface and propagate deep into the material. The optimal temperature for the heat treatment is 450–500 °C, as internal stresses are reduced at this temperature and the grain structure of the AlMg6 layer is not coarse. Tear strength testing revealed that the tear strength of the composite material after explosive welding was 130 ± 10 MPa, which exceeded the strength of the AA1070 alloy.

A new strategy for preparing high strength and high precision diffusion bonding GH536 joints via pulsed current and subsequent heat treatment

GH536 as a typical solid solution strengthened Ni-based superalloy, has been widely used in the preparation of laminated cooling structure. Diffusion bonding, which is highly promising as a joining process in laminated cooling structure, generally requires high strength and high precision. However, the high strength joint fabricated by the conventional hot pressure diffusion bonding (HPDB) often leads to severe deformation of base materials. Here, we developed a new diffusion bonding strategy to overcome this issue in GH536 via pulsed current diffusion bonding (PCDB) and subsequent heat treatment. At low temperature and short time (900 °C/30 min), a joint with a high shear strength of 535 MPa and a low deformation rate of 0.42 % was obtained using the PCDB, and these were 1.60 times and 0.28 times that of the conventional HPDB joint, respectively. Meanwhile, the low bonding temperature and short bonding time hindered the formation and coarsening of carbides, allowing them to be eliminated in a short time heat treatment. The unbonded zones healing, carbide dissolution and recrystallization during heat treatment significantly improved the joint shear strength, and the joint shear strength after heat treatment reached 561 MPa.

DOCOL 1100M WELDING IN THE CONSTRUCTION OF ELECTRIC MEANS OF TRANSPORT

Advanced high-strength steels are important for the automotive sector. Metal active gas (MAG) is the most popular method for joining grades of steel. The goal of the paper is to analyze the mechanical properties of the MAG welding joint made of high-strength DOCOL 1100M intended for the construction of electric vehicles. The manuscript shows a basic understanding of the properties of DOCOL joints. This type of material is characterized by a martensitic microstructure, which makes it difficult to make a proper joint. The tensile strength, metallographic structure, and type of non-metallic inclusions were analyzed as a function of the oxygen amount in the protective gas mixture. Investigations of oxide non-metallic inclusions were carried out using scanning electron microscopy. This article attempts to obtain high joint strength of the electric vehicle structure by controlling the average size of non-metallic inclusions in the weld, which is influenced by shielding gas in the MAG welding process. The solution has application potential for the automotive industry, especially for electric vehicles.

Interface strengthening mechanism of TZM/Q235 joint by ultrasonic-assisted electron beam welding with high entropy alloy interlayer

The high-strength joints of TZM/CoCrFeNiMo0.2/Q235 were realized by ultrasonic-assisted electron beam welding. The effects of ultrasonic vibration on the microstructure evolution and mechanical properties of the reaction layers were analyzed. A reaction layer with a thickness of >10 μm was observed at the TZM side region. The reaction layer was characterized by columnar Fe2Mo phases growing perpendicular to the fusion line and FCC solid solutions with different grain orientations. Under the action of ultrasonic vibration, the eutectic structure of Fe2Mo phases and FCC solid solutions was evenly distributed in the reaction layer, and the grain orientation of the FCC solid solutions in the reaction layer changed to be the same as that in the fusion zone. Compared with the joint without ultrasonic vibration, the difference of nanoindentation hardness between the Fe2Mo phases and FCC solid solutions and the residual stress inside the reaction layer of the welding joint with an ultrasonic power of 80 W were significantly reduced. The maximum tensile strength of the joints was 318.4 ± 26 MPa, which was 18.9% higher than that of the joint without ultrasonic vibration.

Interfacial wetting and reacting between pyrolytic graphite and Sn–Ag–Cu–Al alloy under ultrasonication at a low temperature

Poor wetting and bonding serve as significant challenges in the joining of carbon materials at low temperatures. In this work, the ultrasonically assisted hot dipping of pyrolytic graphite was investigated in a SnAgCu–2Al liquid metal at a low temperature of 250 °C in air. During the ultrasonic-assisted hot-dipping process, the filler metal filled into the surface grooves of pyrolytic graphite. A closely contacting interface formed between the pyrolytic graphite and the filler metal. Interlayer penetration induced by the cavitation effect, and the amount and depth of penetration increased with ultrasonication time. A transition layer of Al2O3 was formed at the interface between pyrolytic graphite and the filler metal. Large amounts of O atoms were induced at the interface during the ultrasonically assisted hot-dipping process, and the reaction of Al and O preferentially occurred between the liquid metal and solid C. This was consistent with the thermodynamic calculation results. The joining of metallizing-pyrolytic graphite was achieved by soldering at same temperature of 250 °C in air, where the highest joint strength could reach 17.52 MPa, achieving a joining module with a high thermal conductivity of 392 W/(m·K) and reaching 90 % of the base material.

Construction of microstructures on the Cu substrate using ultrafast laser processing to enhance the bonding strength of sintered Ag nanoparticles

Silver nanoparticle (Ag NP) pastes become a potential die-attachment material with the increased electronic power density. However, the weakness of bonding interface between sintered Ag NPs and bare Cu substrate limits the applications of the Ag NPs paste, thereby reducing the shear strength of the sintered joint. In this work, ultrafast laser processing is utilized to enhance the bonding strength of the sintered Ag joint by fabricating a microstructure interface. The microstructure dimensions are tunable by controlling laser parameters, and then high-strength joints could be obtained. Different substrate microstructures were constructed, and the enhanced bonding mechanism was analyzed by characterizing the cross section and fracture surface morphologies of joints. The ultrafast laser processing could increase the surface energy of Cu substrates to form a more reliable connection with Ag NPs and more energy required for crack extension with the increasing connection area, thereby resulting in a significant improvement in the shear strength of the Ag NP joints. The patterned microstructures on the Cu substrate using this technique showed improved surface energy and increased number of connection areas on the substrate, showing potential for the use in third-generation semiconductors for highly reliable packaging.

Demonstration and Analysis of Conditions to Obtain a High Strength Inconel 625 to Stainless Steel 304L Interface by Directed Energy Deposition

Functional grading (FG) is often used to bond dissimilar metals. However, that approach is complicated from a manufacturing perspective, and the associated challenges can outweigh the benefits of FG. Here, we investigate a directly bonded interface by transitioning from stainless steel 304L (SS304L) to Inconel 625 (IN625) using powder-feed directed energy deposition with a laser beam energy source (DED-LB). Both cracking and the presence of carbide phases have been reported in this multi-materials system. Conditions that unambiguously achieve crack-free joints have not yet been established. With DED-LB, we consistently observe solidification cracking in melt pools containing > 50 wt pct SS304L, while no cracking is observed in melt pools with < 40 wt pct SS304L. Variations on the most up-to-date solidification cracking model are applied to gain insight into the cracking dependencies. Parameters that give rise to defect-free single layers also enable defect-free multilayer prints despite the additional thermal cycling. Upon printing and testing full-sized ASTM E8 tensile specimens, the interface is sufficiently strong that failure occurs solely within the SS304L region, indicating a joint strength of > 650 MPa. Thus, a simple method to attain high strength joints for these dissimilar metal alloys is demonstrated.

The effect of Sn interlayer on weld structure, intermetallic compounds, and joint properties of Al/Mg friction stir welding with ultrasonic assistance

ABSTRACT: The formation of intermetallic compounds (IMCs) is a primary factor that affects the welding joint between dissimilar materials. In this study, experiments were performed using Al/Mg friction stir welding (FSW) and ultrasonic vibration enhanced FSW (UVeFSW) with varying thicknesses of Sn interlayer. Detailed characterization and tests were conducted to explore the effect of Sn interlayer with or without ultrasonic vibration on weld surface morphology, macro-micro structure, generation of IMCs, and joint tensile strength. The study discusses the effect of ultrasonic vibration coupled with Sn interlayer and provides ideas for IMCs regulation when joining dissimilar materials. The study showed that adding a Sn interlayer cannot eliminate the generation of Al-Mg IMCs, but it can significantly reduce their thickness. Applying ultrasonic vibration with Sn interlayer can further reduce the thickness of Al-Mg IMCs and improve the interface microstructure from ‘IMC + matrix +IMC + matrix’ multilayer structures to simple IMCs structures. Furthermore, the highest joint strength was achieved when the Sn interlayer thickness was 0.1 mm during FSW. UVeFSW’s joint strength can be further improved with 0.05 mm and 0.1 mm Sn interlayer.

The preparation and wettability of the Sn-9Zn-2.5Bi-1.5In solder paste for SMT process and high shear ball performance

Purpose This study aims to focus on the surface mount technology (SMT) mass production process of Sn-9Zn-2.5Bi-1.5In solder. It explores it with some components that will provide theoretical support for the industrial SMT application of Sn-Zn solder. Design/methodology/approach This study evaluates the properties of solder pastes and selects a more appropriate reflow parameter by comparing the microstructure of solder joints with different reflow soldering profile parameters. The aim is to provide an economical and reliable process for SMT production in the industry. Findings Solder paste wettability and solder ball testing in a nitrogen environment with an oxygen content of 3,000 ppm meet the requirements of industrial production. The printing performance of the solder paste is good and can achieve a printing rate of 100–160 mm/s. When soldering with a traditional stepped reflow soldering profile, air bubbles are generated on the surface of the solder joint, and there are many voids and defects in the solder joint. A linear reflow soldering profile reduces the residence time below the melting point of the solder paste (approximately 110 s). This reduces the time the zinc is oxidized, reducing solder joint defects. The joint strength of tin-zinc joints soldered with the optimized reflow parameters is close to that of Sn-58Bi and SAC305, with high joint strength. Originality/value This study attempts to industrialize the application of Sn-Zn solder and solves the problem that Sn-Zn solder paste is prone to be oxidized in the application and obtains the SMT process parameters suitable for Sn-9Zn-2.5Bi-1.5In solder.

Influence of the depth of friction-welded dowels on the strength of rotary welded joints

One of the important factors in rotary welding is the depth of welding of the dowel as well as the direction of welding of the dowel. In the studied interval (welding depth of 15 to 30 mm), the pull-out force increased when welding the dowel parallel to the wood grain and perpendicular to the wood grain. The strength of the welded joint increased from 15 to 20 mm, and then it continuously decreased towards a welding depth of 30 mm. The reason for this is that the tip of the dowel is intensively worn, and with a welding depth of 20 mm, it is approximately equal to the diameter of the hole. Therefore, by increasing the welding depth, the pull-out force increases slightly (due to the friction between the dowel and the hole wall), and the strength of the joint decreases. The highest joint strength was achieved at a welding depth of 20 mm for specimens welded parallel to the grain (PV) and specimens welded perpendicular to the grain (RTV). In welded joints where the dowels are loaded only by tensile force, it is recommended to use a welding depth of 30 mm.

Strong composite SnAgCu solder joints with internal aligned carbon fibers by magnetic field

Lightweight carbon fibers with high mechanical performance are referred to as an ideal reinforcement for an extensive variety of composite materials. However, the addition of carbon fibers into electronic interconnection solders is extremely limited so that the expected reinforcement in the mechanical properties of the composite solder joints has not been completely realized. Here, we used magnetic fields to align Ni-coated carbon fibers in the Sn-3.0Ag-0.5Cu solder vertically with the Cu substrate and prepared the oriented composite solder joints. We achieved a further improvement in the shear performance of composite solder joints (∼58 MPa) with a small addition of Ni-coated carbon fibers (0.5 wt%), which is 25% higher than that of pristine joints. It is found that these vertically oriented carbon fibers changed the propagation path of shear cracks and thus increased the shear resistance. Ni-coated carbon fibers also promoted nucleation of interfacial Cu6Sn5 in the solder joints during reflow, but prevented the formation of brittle Cu3Sn phase during isothermal aging, thus ensuring the high shear strength of composite solder joints after isothermal aging. This success provides a new pathway to fabricate a strong composite solder joint with a limited addition of reinforcements by aligning their orientation.

The effects of laser line energy on welding strength of PASF/PC using magnesium zinc alloy powders as absorbents in laser transmission welding

The metal powder absorbents have the advantage of fabricating clean and high-strength joints in the process of laser transmission welding dissimilar transparent thermoplastics. However, the joining behavior between metal powders and thermoplastics is vague. Herein, polyarylsulfone (PASF) and polycarbonate (PC) are selected as model materials to investigate the joining behavior using magnesium zinc alloy (MZA) powders at different laser line energies. The shear force test was performed to reveal the effect of laser line energy on the welding. The µ-CT images reveal the evaluation of welding strength is attributed to the specific joint morphology. The multi-scale morphology characterization shows that the fractured surface height increases with increasing the laser line energy. The voids between MZA powders and resins increase the risks of interface failure, and the bubbles in the resins increase the risks of cohesion failure. The effects of laser line energy on the joint characteristics are elaborated based on the thermo-physical property test and transient temperature field analysis. The above results indicate the optimized welding temperature locates the melting temperature of PASF, because of the good fusion of PASF and less decomposition of PC. The study can provide a guideline for choosing proper process parameters when choosing metal powders as laser absorbents in welding dissimilar transparent thermoplastics.

Surface microstructuring by Ultrasonic Vibration Assisted Deformational Machining (UVADM) – Simulation and experimental results

Polymer-metal hybrids are a lightweight solution for many industrial applications, especially in the automotive or aircraft sector. However, due to their poor chemical bonding the manufacturing of such compounds requires the use of adhesive layers or the conditioning of the metallic surface to achieve form closure and high joint strength. Ultrasonic Vibration Assisted Deformational Machining (UVADM) is a novel process that generates functional microstructures with high production rates, aiming to form defined burr structures with undercuts, that have a significant impact on the interlaminar strength of polymer-metal hybrids (PMH). A 3D FEM simulation using Abaqus/Explicit software was applied to design and analyze process conditions including tool geometry. Material deformation was described by the Johnson-Cook model. Simulation results were validated experimentally using two different MCD tools. For the UVADM experiments, specimens of the aluminum alloy EN AW-6082 were used. In the tests, a constant ultrasonic frequency (20.42 kHz), machining speed (100 m/min and 200 m/min), and peak-to-peak amplitude (7 µm) were used. Geometrical surface properties were analyzed by optical methods and SEM. The study demonstrates a conformity between simulated and experimental results, highlighting the potential of UVADM for an increased productivity in the surface conditioning for polymer-metal hybrids.

Influence of the temperature changes on stresses in carbon composite fastening bolts

High joint strength of composite structures can be achieved by using mechanical fasteners. The materials used for rivets and bolts have lower coefficients of thermal expansion than composites in the direction perpendicular to the reinforcement fibers. It is suspected, therefore, that temperature changes can cause stress changes in mechanical fasteners. The purpose of this study was to experimentally determine and analytically substantiate the values of stress changes resulting from a change in temperature in the range from -20 to 60°C in M8 steel bolts fastening carbon composite. The value of stress changes was estimated to be around 100 MPa, which can consequently lead to joint unsealing or plastic deformation of the mechanical fastener.