Joining Mechanism Research Articles

Microstructure and Mechanical Properties of IN690 Ni-Based Alloy/316LN Stainless-Steel Dissimilar Ring Joint Welded by Inertia Friction Welding.

Inertia friction welding (IFW) was used to join large-diameter hollow bars made of Inconel 690 and 316LN successfully. The interfacial characteristics, microstructure, mechanical properties and fracture mechanism of welded joints under different process parameters were investigated. The results indicated that a joining mechanism with mechanical interlocking and metallurgical bonding was found in IFW joints. There was a significant mechanical mixing zone at the welding interface. The elemental diffusion layer was found in the "wrinkles" of the mechanical mixing zone. A tiny quantity of C elements accumulated on the friction and secondary friction surfaces. The tensile strength and impact toughness of the joints increased with the total welding energy input. Increasing the friction pressure could make the grain in all parts of the joint uniformly refined, thus enhancing the mechanical properties of welded joints. The maximum tensile strength and impact toughness of the welded joint were 639 MPa and 146 J/cm2, reaching 94% and 68% of that for Inconel 690, respectively, when the flywheel was initially set at 760 rpm, 200 MPa for friction pressure, and 388 kg/m2 for rotary inertia. Due to the Kirkendall effect in the welded joint, superior metallurgical bonding was at the welding interface close to the Inconel 690 side compared to the 316LN side.

Probing joining mechanism of Ti6Al4V - SS316L steel rods in pressure-controlled joule-heat forge welding

Ti6Al4V-SS316L steel joints are vital in automotive and nuclear industries due to lightweight, creep, and corrosive resistivity; however, their fusion-based weldments readily form brittle phases under uncontrolled melt-solidification dynamics. Plasticity-induced solid-state welds are recourses, but uneven softening and metal upset remain intricate and cause failure. Pressure-controlled joule-heat forge welding offers plastic deformation with the maneuvered pressure, current and forging strokes. Experimental studies reveal local homogeneous plastic flow of dissimilar metal interfaces for a high-strength metallurgical bond at low temperatures using this process. However, the underlying weld mechanics remains to be uncovered. Thus, to address this shortcoming, finite-element-based calculations of concurrent electric resistive heating and pressure-displacement-induced plastic flow of dissimilar interfaces are modeled by a coupled electric, thermal and dynamic stress analysis. The estimated transient thermal histories and different weld geometries provide insight into the joining procedure and are later validated to the measured results. The mechanics of locally controlled plastic deformation are illustrated with evolved stress-strain transients that reasonably indicate differential responses of dissimilar cross-sections to the applied forging, except at the interface that forms metallic joints. Further, the computed results show a homogeneous temperature increment rate at the center and periphery of the deformed cross-sections. The microscopically observed weld structures indicate dynamic recrystallization and smooth hardness transit across bimodal α-β phases with elongated titanium α-grains and refined annealed twin austenite in steel.

Brazing of Cr2AlC and 316H stainless steel using BNi-2 filler: Microstructure, properties and joining mechanism

Cr2AlC, a ternary carbide, has been successfully brazed with 316H stainless steel using the BNi-2 filler, thereby expanding their potential applications. The diffusion of Ni from the filler into both Cr2AlC and 316H dominates the formation of the joint microstructure. High temperature leads to the detachment of Al from the Cr2AlC matrix, retaining the porous Cr7C3 along the diffusion interface. Along with the inward diffusion of Ni from the filler, the NiAl phase will be formed within the porous Cr7C3, collectively constituting the reaction layer. However, these void defects cannot be completely filled, leaving micro-voids along the Cr2AlC interface. It weakens the performance of the brazed joints. Elevated temperature and longer holding time will lead to a thicker reaction layer along the matrix interface, consequently decreasing the joint strength. A maximum joint shear strength of 248 MPa is achieved at 1000 °C/1 min. Inhibiting the excessive detachment of Al can be an efficient way to reduce the thickness of the reaction layer and increase joint performance.

Diffusion bonding behaviour of Nb–Si and TiAl alloys by spark plasma sintering

Nb–Si alloy and high niobium β-γ TiAl alloy(HNBG) was joined by diffusion bonding ranging from 1000 to 1100 °C at 10 MPa for 20 min by spark plasma sintering. The phase sequence from HNBG to Nb–Si side was HNBG||β/B2+α2-Ti3Al||δ-(Nb, Ti)3Al+σ-(Nb, Ti)2Al + Ti rich γ-(Nb, Ti)5Si3||Nb–Si in the joint. The sound joints without any pores were obtained with the bonding temperature above 1050 °C. The nano-indentation test results showed that Ti rich γ-(Nb, Ti)5Si3 and σ-(Nb, Ti)2Al phases had relatively high hardness and elastic modulus in the diffusion zones II. The highest shear strength of 269 MPa was achieved at 1050 °C. Fracture mainly occurred along the interface of the diffusion zones due to low interfacial adhesion induced by lower bonding temperature. As the bonding temperature was higher, the cracks propagated mainly from in the interface to through the phases due to the increase of interfacial adhesion. The joining mechanism of HNBG/Nb–Si joint was analysed through diagram.

Joining of Polymer to Aluminum Alloy AA1050 By Friction Spot Welding

The friction spot welding process is used to join the hybrid joint of aluminium to polymer. AA 1050 Aluminum alloy and polyethylene polymer (type of plastic) were used for joining. Aluminium surface treatment was done to increase the wettability of it. The surface treatment was done by electrochemical treatment. Two parameters of the joining process were used: (1, 1.5 and 2) min. (920, 1340 and 1500) rpm for time and rotational speed, respectively. Many tests for the joint are done: shear test and Scanning electron microscopy (SEM). Using the Minitab program, the effect of the parameters on the joint was analysed. The maximum value of the shear force was (1587 N) at 1.5 min. time and 1340 rpm rotational speed. The minimum shear force was (589 N) at 1 min. time and 920 rpm rotational speed. The joints which were tested failed by an interfacial shear with a ductile fracture of the polymer side. The process of joining mechanism occurred through a mechanical interlocking between the molten polymer and the surface of the AA 1050 aluminium specimen, cohesive and adhesive. The average thickness for the joint of the tested specimens was 4µm.

Experimental evaluation of T-peel strength on functionally graded Al5083 and HDPE tri-laminated composites fabricated by colding-assisted friction stir additive manufacturing

Functionally graded composites offer impressive promise in overcoming the inherent weaknesses of Metal-polymer hybrid structures. In this study, functionally graded 5083 aluminum (Al5083) and high-density polyethylene (HDPE) tri-laminated composites were fabricated by colding-assisted friction stir additive manufacturing (CA-FSAM) and friction stir additive manufacturing (FSAM). Functionally graded laminate composites have been used to overcome these drawbacks by varying the thickness of the raw laminate. The thickness changes in the user sheets were functionally 0.75 mm compared to the previous laminate. Finally, the initial sheets were transformed to three thicknesses: 3, 2.25, and 1.5 mm. The bond strength between the sheets was measured using the T-peel test. In the T-peel test, the initial crack length was 25 mm and the length of the weld zone was 111 mm. The results showed that the bond strength between the laminates improved with cooling after the CA-FSAM process. The bond strength is essentially determined by the amount of covalent bonding, which, in turn, is a function of the density on the treated surface. Dislocation forest at the surface of the tri-laminate composite can be considered a consequence of T-peel test. The joining mechanism could be ascribed to mechanical interlocking, adhesion bonding at the interface of the Al5083 alloy and HDPE polymer, and metal-chip reinforcement in HDPE of the stir zone. The maximum force (Fmax) obtained in the FSAM and CA-FSAM specimens were 1057.58 and 1254.20 N, respectively. Average force of T-peel test (Favg peel) obtained in the FSAM and CA-FSAM specimens was 984.36 and 1024.32 N, respectively.

Improved mechanical properties of SiC/SiC brazed joints via added Al in Si–Mg composite fillers

SiC components were joined under vacuum at 1100 °C for 10 min via Si–Mg composite fillers with added Al (XAl) ranging over 0–10 at.%. The joining mechanism involves Mg addition to Si, which lowers the melting point of Si. The transformation of the molten liquid phase into a solid Si layer is induced by Mg evaporation. In SiC/SiC joining, however, Mg evaporation was significantly suppressed when XAl = 0 at.%, with a large amount of Mg remaining in the joint, forming a brittle bonding layer of Si, Mg2Si, and MgO. Notably, Mg evaporation was enhanced by the addition of Al to the filler, resulting in the formation of a dense Si layer. The joint tensile strength increased with increasing XAl in the range of 0–6 at.% and then decreased with further increases in XAl. The changes in strength were correlated with microstructural changes in the bonding layer. From the microstructure and the phase diagrams, it was concluded that the added Al increased the thermodynamic degrees of freedom, which hindered the rate of Mg-evaporation-induced Si solidification from the liquid during joining. Because the amount of liquid surrounded and trapped by the solid Si decreased, more Mg in the liquid could diffuse to the surface.

Characterization of friction stir-based linear continuous joining of aluminium alloy to structural polymer

Dissimilar continuous joining of lightweight metal alloys to structural polymer is a major manufacturing challenge, demanding feasible and reliable solutions. Through-slot extrusion joining (TSEJ) is a friction stir-based processing technique investigated in the manufacturing of continuous linear joints between aluminium alloy AA5754 overlapping structural polymer polyether ether ketone (PEEK). An intermediate rigid and thin titanium extrusion die protects the polymer locally from thermal degradation and promotes the formation of a continuous double hook-like feature of extruded aluminium into the polymer component along the joint path. The structure of the joint provides macro-mechanical interlocking between the joined components. A set of four tools, and other key process parameters, were investigated for process stability, tensile-shear strength, and microstructure. The best TSEJ condition is chosen for microstructural analysis via optical microscopy and scanning electron microscopy. Three distinct failure modes were identified. The best condition provided an average tensile-shear load of 110 kN/m. The tool position with bias toward the flow side of the extrusion slot shows to improve the strength of joints. The microstructural analysis along the interface of AA5754 to PEEK exhibits micro-mechanical interlocking, intercalated layers of these materials and adhesion as joining mechanisms.

Comprehensive influence of silane layer and interfacial thermo-mechanical effect on joining mechanism of silane pretreated Al/CFRP FSLW joint: FEA and experiment

Al/CFRP composite structure has the advantage of lightweight and high specific strength, making it a promising material for aviation, railway, and automotive applications. Silane coupling pretreatment is a chemical surface pretreatment method that can improve the mechanical properties of the Al/CFRP friction stir lap welding (FSLW) joint. To further investigate the strengthening mechanism of silane-treated Al/CFRP FSLW joint, a 6061-T6 Al/CF-PA66 FSLW experiment was carried out and the finite element analysis (FEA) was used. The thermophysical properties of semi-crystalline resin during the welding process were also considered. The joining mechanism of silane surface pretreated Al/CFRP FSLW joints was further revealed by comparing the changes in interface morphology, temperature, stress, and mechanical properties. The results indicated that the silane coupling agent forms a silane film by an Al-O-Si chemical bond at the interface. The silane film can enhance the Al/CFRP interface's tightness and reduce interfacial defects. The FSLW process affected the joint morphology and mechanical properties through interfacial temperature and stress, which were decided by welding parameters and thermophysical properties of the resin. By adjusting the thermal-mechanical effect of the Al/CFRP interface, defects can be reduced and the joining force can be improved. According to the analysis of interfacial chemical bonds and mechanical properties, the introduction of silane film enhances the hydrogen bond between Al and resin and thus enhances the mechanical properties of the interface. This study provides a new method for revealing the strengthening mechanism of silane-pretreated FSLW Al/CFRP joint.

Bonding mechanism of laser joining of Poly-Ether-Ether-Ketone film which used for 5G antenna substrate to copper foil

The bonding of copper foil to Poly Ether Ether Ketone (PEEK) film has become a hotspot in research of the 5G RF (Radio Frequency) antenna substrate. In order to make the flexible copper clad laminates(FCCL) in the RF substrate thinner and more resistant to bending, there is an urgent need to replace the use of adhesive to join the copper foil to the PEEK film with a direct joining method. In this paper, a nanosecond laser was used to join the copper foil to the PEEK film effectively and the comprehensive property of joint was enhanced by the laser-textured grid on the surface of copper foil. The influence of textured grid on the adhesion work between copper foil and PEEK film was investigated. The interfacial morphology was observed and the result showed that molten PEEK completely filled the textured grid and formed a mechanical interlocking with copper. The complex reaction between Cu and PEEK was indicated by detecting the red-shift for the wave number of functional groups and the weak chemical bond between Cu and PEEK. Besides, the bonding mechanism was proved by density functional theory (DFT) calculation. The tensile shear strength of Cu/PEEK joint was significantly enhanced by laser texturing treatment, and the maximum force of joint with 0.4 mm-textured grid was increased by approximately 1.29 times compared to the untreated condition. This study proposes the use of nanosecond laser joining to replace the traditional use of adhesives to join copper foils to PEEK films.

Process parameter assessment on the dissimilar deposition of AA2024-T351 on AA7475-T761 by friction surfacing

Friction surfacing as a solid-state deposition process allows the joining of materials with different chemical and physical properties at temperatures below their respective melting points. This experimental work focuses on generating sound, defect-free metallurgical joints between single friction surfacing deposits and substrate surfaces from dissimilar Al alloys of the 2xxx and 7xxx series. In this context, the influence of axial force and deposition speed on surface morphology and deposit geometry of the two heat-treatable Al alloys AA7475 as substrate and AA2024 as deposit are investigated. Process parameter variation shows that an increase in axial force from 8 to 12 kN, in conjunction with a deposition speed of 8 mm/s, leads to smooth surface morphology and consistent deposit width along its length. The AA2024 deposits consist of fine-grained microstructure with higher hardness at the top and lower hardness at the deposit-substrate interface. The joining mechanism is by interdiffusion, with a 7.5 μm thick diffusion zone across the dissimilar interface. Three-point bending tests reveal excellent bonding in the lateral surface of the advancing side due to the absence of delamination for all conditions tested. Minor delamination appears predominantly on the retreating side region for process parameter sets with low axial force. Tensile test results reveal that the AA2024 deposit on the AA7475 substrate presents an ultimate tensile strength equivalent to the AA7475-T651 base material and an increase of 37 % in ductility.

Joining of 304 stainless steel to PET by semiconductor laser conduction welding

304 stainless steel (304SS) and polyethylene terephthalate (PET) were lapped joined by laser conduction welding using a semiconductor laser with a flat-top thermal distribution. Experimental investigation and numerical simulation were conducted to analyze the weld morphology, pores distribution, interfacial microstructure, temperature field, joint strength and fracture behavior of the laser dissimilar joints. Because of the uniform thermal distribution of the laser, the decomposition of the PET base material was well controlled with pore-free joints obtaining at the welding speed over 25 mm/s. Besides, a compound layer was generated at the interface between 304SS and PET with its thickness decreased when the welding speed increased. Chemical reactions and mechanical anchoring were both observed at the interface suggesting a dual joining mechanism. The joint fracture load first increased then decreased with the increased welding speed. Besides, a ductile to brittle failure transition was clearly seen. The underlying mechanism was also discussed.

Achieving high strength friction lap spot joints of carbon fiber reinforced thermosetting composite to aluminum alloy with additional thermoplastic interlayer

The dissimilar materials of the 5182-aluminum alloy and continuous carbon fiber reinforced bismaleimide (CF-BMI) were joined by friction lap spot joining (FLSJ) directly and via the addition of the polyamide 6 (PA6) interlayer, and the effect of interlayer on the joining mechanism and the joint mechanical property was investigated. It was found that no effective joints were achieved by FLSJ directly, which was mainly attributed to the poor re-forming ability and liquidity of the thermosetting BMI, and its difficulty in reacting to metals. After the addition of the PA6 interlayer, the effective dissimilar joints were successfully realized by FLSJ. The maximum average tensile shear force of the joint reached 3.68 kN, with the average normal tensile shear strength of 20.9 MPa at the optimum parameter with the rotation rate of 2000 rpm, dwell time of 7 s, plunge depth of 0.7 mm and the PA6 interlayer of 0.3 mm. A new COAl chemical bond was found at the interface of the PA6 interlayer and aluminum alloy, which was the result of the chemical reaction between the amide polar function of PA6 and surface oxide of aluminum alloy. The great increase in the joint strength by the addition of the PA6 interlayer was mainly attributed to the great reduction of the interface defects and the formation of the chemical bonding, as the result of the great improvement of the interface fluidity and chemical reaction ability of PA6. This study provides an effective way to achieve high strength metal/continuous carbon fiber reinforced thermosetting composite joints.

Contact reactive brazing of TC4/304 stainless steel dissimilar metals by interrupted arc pulsed welding

The most challenging aspect of producing durable joints between titanium alloy and stainless steel (Ti/SS) is preventing the formation of brittle Ti–Fe intermetallic compounds, particularly the TiFe2 phase. In this paper, the interrupted pulsed arc welding (IPAW) was proposed to join TC4 and 304SS dissimilar metals. The joining mechanisms, microstructures and shear strength of the joints were investigated. The precise regulation of arc pulsed time in the millisecond range enabled the creation of a joint without the formation of the TiFe2 phase through contact reactive brazing. A Ti-rich reaction layer consisting of β-Ti and TiFe phases was formed in the brazing joint, and the shear strength of the joint reached 106.3 MPa. By increasing the arc pulsed current or time, the brazing joint was transformed into the fusion joint. The formation of two reaction layers was observed: Ti-rich phases (TiFe and β-Ti) near the TC4 and Fe-rich phases (TiFe2 and TiCr2) near the 304SS, which exhibited high brittleness. The presence of a high brittleness fusion zone resulted in immediate cracking post-welding.

Mechanical properties and joining mechanisms of magnetic pulse welding joints of additively manufactured 316L and conventional AA5052 aluminum alloy

Magnetic Pulse Welding (MPW) is a new solid-phase welding (no heat input and no auxiliaries) technique. This results in joints with almost no brittle intermetallic compounds (IMC) and hence provides excellent joint quality. In this study, 316L fabricated by additively manufactured (AMed 316L) and conventional rolled AA5052 aluminum alloy were welded by MPW. Moreover, it was compared with conventional rolled 316L (C316L). Shear tests were used to characterize the mechanical properties of the joints. Laser confocal microscopy, Scanning Electron Microscope (SEM), and Energy Dispersive Spectroscopy (EDS) were used to characterize the microstructure and elemental diffusion of the joints. It was found that the strength of both joints was higher than that of the base material. In addition, sufficient elemental diffusion was achieved at the weld interface. No significant weld defects and IMC generation were observed at the interface of the AA5052-AMed 316L joint. Finally, Smoothed Particle Hydrodynamic (SPH) was used to analyze the formation mechanism of the transition zone morphology. This study demonstrates for the feasibility of MPW technology for welding additive manufacturing materials.

Laser shock dynamic compaction of aluminum–copper (Al–Cu) composite metal powder

Laser impact dynamic compaction experiments and 2D multi-particle finite element method (MPFEM) simulations were carried out on aluminum–copper (Al–Cu) binary-based composite powders. Al–Cu compacts with densities of about 95% were successfully prepared at the maximum laser energy of 1.8 J. Research trends indicate that higher power lasers could nearly achieve the theoretical densities. Analysis of the influence laws of laser energy and copper powder mass fraction on the densities of Al–Cu compacts was carried out, and the effects of microstructure and microhardness of Al–Cu compacts on their mechanical properties were investigated, as well as the stresses and velocities in the dynamic compaction process of Al–Cu composite powders were analyzed. The results showed that the relative densities of composite powder compacts of Al-5 wt%Cu gradually exceeded those of Al-10 wt%Cu and Al-15 wt%Cu with the increase of laser energy. In laser impact dynamic compaction the mechanism of inter-particle joining was solid-state press-welding, and the increase in the hardness of the Al–Cu compacts could be attributed to the inherent hardness of the hard particles. The upper particles have a localized maximum velocity of more than 300 m/s, which met the explosion welding velocity, thus localized cold welding occurred and contributed to the inter-particle connection.

Failure mechanism of parallel gap resistance welding joint between Ag foil and GaAs solar cell by temperature cycling

Reliability of joints in solar arrays significantly influences the service life of satellites. Interface between solar cell and interconnector experiences serious temperature cycling during space service which would further lead to failure. To further improve the interface joining thermal reliability, elucidation of the interface formation and corresponding microstructure evolution during thermal fatigue is necessary. In this study, parallel gap resistance welded (PGRW) multi-layered joint between GaAs solar cell and Ag foil are subjected to different temperature cycling tests (−160–120 °C, −165–160 °C) with various cycles. Obtained results confirm the joining mechanism of the joint as solid-solution interdiffusion between Ag foil and Au surface of solar cell electrode. Also, conducted temperature cycling essentially lead to thermal fatigue process at Ag/Au interface, therefore more serious interface strength degradation is generated by larger temperature cycling range. Joint failure is initiated by thermal fatigue induced dislocation and residual strain concentrations around dissimilar interface. And the large mismatch in coefficients of thermal expansion (CTE) of the multilayer structure amplifies the thermal fatigue effect.

Large sandwich sheets enabled by joining rolled sheets and additively manufactured core structures for forming

To overcome the size limitations of conventional laser-powderbed fusion (L-PBF) machines, a new methodology to manufacture large sandwich sheets by joining rolled sheets with additively manufactured core structures is developed, which potentially can be used as parts with a high amount of functional integration for heat exchanger systems or battery housings. Different joining mechanisms are directly applied to the core structure and tested for their strength in shear- and bending-experiments. Design guidelines for L-PBF are especially important to guarantee good geometrical accuracy of the joining mechanisms. The joint strength exceeds the face sheet material strength with certain geometric parameters. A process window is developed showing that bending of the joined sandwich sheets is possible up to a bending radius R to sandwich sheet height H ratio of R/H = 3.33. The global axial compliance of the buckling-joining mechanism leads to an increased overlap of the joining mechanism and the face sheet, thus a high bonding strength is achieved.

Insights into microstructure evolution and interface joining mechanism of ultrasonic spot welded Cu/Cu joint

To understand the underlying microstructure evolution and joining mechanism of ultrasonic spot welded Cu/Cu joint, the microstructure and crystallography analysis were investigated. The grain size, grain boundary, misorientation angle, Kernel Average Misorientation, Schmid factor and crystal orientation along the weld thickness were heterogeneous and unsymmetrical. The changes of corresponding microstructural and crystallographic features were more obvious for the top sheet comparing with the bottom sheet. The {111} <110> texture, nano-sized equiaxed induced by discontinuous dynamic recrystallization and refined elongated grains because of continuous dynamic recrystallization were formed at the welding interface. The nucleated fine grains between the elongated grains attributing to continuous dynamic recrystallization were beneficial to the grain boundary migration and consequent the achievement of the interfacial metallurgical bonding.

Joining mechanism of connection between Ag-plated Kovar interconnector and stranded Ag-plated Cu wire produced via parallel gap resistance welding

A space solar array includes GaAs solar cell, interconnector, and stranded Cu wire. Due to the preferential high working efficiency, the assembly of solar array is mostly performed using parallel gap resistance welding (PGRW). Since strengthening of the PGRW joints is the key to further extend service life of solar array, it is necessary to elucidate joining mechanism of each connection. In the present research, to clarify the controversy over PGRW joining mechanism between Ag-plated Kovar interconnector and stranded Ag-plated Cu wire, various advanced material characterization methods are conducted to joining interfaces. Experimental results confirm that, in addition to the known Ag/Cu eutectic interface, solid evidence of metallurgy bonding is also found in Cu/Cu interface. Such finding has significant implication for further enlarge the PGRW process window and produce stronger joints.