Al Joints Research Articles

Effect of laser spot overlap ratio on surface characteristics and adhesive bonding strength of an Al alloy processed by nanosecond pulsed laser

• Nanosecond laser ablation treatment enhances adhesive bonding strength for Al alloy. • Laser-induced surface physiochemical modifications increase with overlap ratio. • Excessive overlap ratio of laser spots reduces the strength of laser-treated joints. • Micropores in adhesive layer caused by insufficient wetting reduce joint strength. • Corrosion resistance of laser-treated substrate drops with increasing overlap ratio. The laser ablation treatment has been validated to be an effective surface treatment method in improving adhesive bonding performance for metal or composite materials. This study was further conducted to investigate the effect of laser spot overlap ratio on surface characteristics and resultant adhesive bonding strength for AA6022-T4 Al alloy. The laser spot overlap ratio was changed by varying the laser processing speed and line spacing between laser scanning lines. Surface characteristics analyses show that the laser ablation treatment at higher overlap ratio produces rougher surface morphology and thicker aluminum oxide layer on substrate surface, which are considered to be more favorable for adhesive bonding strength. Lap-shear strength results show that the laser ablation treatment at overlap ratio of -49 % (corresponding with separated laser spots) improves the joint strength by 6.8 % and 17.1 % before and after water soak exposure compared to as-received condition. Meanwhile, the desired cohesive fracture mode is also achieved which indicates that the joint strength reaches to the saturated value depending on mechanical performance of adhesive. This explains that the continuous increase of laser spot overlap ratio from -49 % to 63 % exhibits negligible improvement on joint strength. Nevertheless, although inducing a higher degree of surface physicochemical modifications on Al substrates, the laser ablation treatment with an overlap ratio beyond 63 % decreases the joint strength. Detailed studies of SEM micrographs verified that the decreased joint strength is attributed to insufficient wetting of adhesive on the rough surfaces of laser-treated substrates resulting in entrapment of air between substrate and adhesive layer, which forms micropores defects in the adhesive layer and consequently weakens the adhesive-bonding joints. In addition, the improved corrosion resistance of adhesive-bonded Al joints by laser ablation treatment is attributed to the barrier effect of laser-produced rough surface morphology to water diffusion along bonding interface.

Bonding and strengthening mechanism of ultrasonically soldered 7075 Al joint using Ni-foam/Sn composite solder foil

Ni-foam reinforced Sn-based composite solder was employed to join ultrafine grain 7075 Al alloy with the assistance of ultrasound. The solder seam was mainly composed of Ni skeletons, Sn-based solder, α-Al phase and very fine (hundreds of nanometers to several microns diameter) Ni3Sn4 particles. Discontinuous networks surrounded by the fine Ni3Sn4 and α-Al particles were in-situ formed in the solder seam, the diameter of which was decreased with increasing soldering time. Smooth transition of the lattice from Al substrate to Sn-based solder was achieved by the formation of an amorphous Al2O3 interlayer at the interface. The microstructure evolution mechanism and the amorphous Al2O3 interlayer formation mechanism are discussed in detail. The shear strength of Al/Ni-Sn/Al joints increased with prolonging soldering time. The Al/Ni-Sn/Al joint ultrasonically soldered for 60 s exhibits a shear strength of 71.4 MPa, which was approximately 37% higher than that soldered with pure Sn solder.

Investigation of Interfacial Layer for Friction Stir Welded AA7075-T6 Aluminum to DP1180 Steel Joints

Abstract Interfacial layers greatly influence the performance of steel–aluminum friction stir welding (FSW) joints, and understanding the formation and evolution of intermetallic compounds (IMC) can help improve the mechanical properties of the welds. In this study, FSW was used to join DP 1180 high-strength steel to 7075 Al at different welding speeds. The effect of the galvanized layer on the IMC formation and evolution, and the mechanical performance of the steel–Al FSW joints were investigated. It was found that the galvanized steel–Al joints were formed only by metallurgical bonding, a continuous IMC layer composed of FeAl, Fe3Al, and Al–Zn eutectic developed at the joint interfaces. Joints were mechanically and metallurgically bonded in the non-galvanized steel, and a 3 µm thick IMC layer consisting of FeAl existed only in the stir zone (SZ). IMC layer formation was predicted according to thermodynamic principles, which is consistent with the interfacial microstructure evolution identified by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Shear tensile test results showed that the galvanized layer can effectively improve the metallurgical bonding strength of the steel–Al joints, and the optimum tensile properties were found in galvanized steel–Al joints.

Metallurgical and Mechanical Properties of Al–Cu Joint by Friction Stir Spot Welding and Modified Friction Stir Clinching

Dissimilar joints of AA5083 and pure Cu joint are successfully produced and compared by friction stir spot welding and modified friction stir clinching with intermediate layer of Zn interlayer for the first time. Self-reacting behavior of Zn is observed to obtain sound welds resulted from intermixing in stir zone (in FSSW), refilled zone (in MFSC) and brazed zone (in both FSSW and MFSC). MFSC is used to fill the cavity of keyhole that in turn increased 40% strength of dissimilar Cu–Al joints. Presence of lamellar eutectics in brazed zone and intermetallic compounds such as Al2Cu, Al4Cu9, CuZn5 and Cu4Zn in weld zone are confirmed in Cu–Al MFSC joints.

Refill friction stir spot and resistance spot welding of aluminium joints with large total sheet thicknesses (III-1965-19)

Refill friction stir spot welding (RFSSW) is a highly flexible and promising solid-state joining method for aluminium alloys. Alternatively, resistance spot welding (RSW) can be stated as an appropriate joining method which can be automated and used within a high-volume production due to short process times. Both processes do not need any additional elements and a flat surface on both sides of the joints can be realised. In order to meet the modern requirements for crash safety and structural stiffness, thermal and mechanical joining methods are mainly combined by using single-component epoxy resin adhesives. Due to an insufficient knowledge about the application of both thermal joining methods for the abovementioned material combinations combined with additional adhesives, deeper investigations were done regarding the interactions of the polymers and the joining processes. Starting with a brief presentation of the boundary conditions of the investigations and the refill friction stir spot welding and resistance spot welding of high-strength aluminium alloys with sheet thicknesses bigger than 5.8 mm, the paper introduces the process-related joint properties of friction-based and resistance-based welded joints. Afterwards, the paper discusses the influences of the process parameter on the metallographic joint formation and load-bearing capacities for a selected two-sheet and four-sheet material combination. When combining the spot welding technologies with adhesives, the process parameters of the RFSSW process have to be adapted for the two-sheet combination by adding a squeeze-out step, while for RSW, just the preholding time has to be increased. Different challenges for both joining methods are shown. For RFSSW, the gap formation has to be considered when welding big total sheet thicknesses, while for RSW, the shape of the weld nugget is more important for an appropriate joint performance. Additionally, process optimisations for less adhesive incineration will be discussed for both joining processes, and the influences of the adhesive on the joint formation will be addressed with the help of load-bearing capacity evaluations. The paper closes with specific recommendations for the realisation of refill friction stir and resistance spot-welded joints with and without adhesive in the field of Al joints with big total sheet thicknesses which meet the quality demands and an outlook for further research steps will be given.

Stress corrosion behavior of narrow-gap rotating laser welding of thick Al-Mg alloy joint

The stress corrosion resistance of 30 mm-thick narrow-gap rotating laser welding (NG-RLW) 5A06 aluminum (Al) joint was investigated by slow strain rate testing (SSRT). The results show that the average ultimate tensile strength (UTS) of the joints in 3.5% NaCl solution is about 11% lower than that in air, and its elongation is only half of that in air. The stress corrosion index [Formula: see text] of the joint is [Formula: see text], which is at the middle level of the range of [Formula: see text]. Moreover, the failure positions of the SSRT specimens in air are weld metal (WM), while the failure positions in the corrosive medium are base metal (BM). This indicates that the stress corrosion resistance of BM is slightly lower than that of WM due to the different microstructures. Many dimples are formed on the fracture surface of the specimen in air, which is a typical ductile fracture. But the fracture surface shows the characteristics of ductile and brittle mixture with obvious corrosion products. This result indicates that the NG-RLW 5A06 Al joint has a tendency to brittle fracture in 3.5% NaCl solution.

GTA Weld Brazing a Joint of Aluminum to Stainless Steel

The relationship between heat input, microstructures, and mechanical properties was studied in the gas tungsten arc (GTA) weld brazing process of aluminum alloy and stain-less steel dissimilar metals with a pure aluminum core wire (ER1100). The mechanisms involved were further revealed by hot-dip aluminizing experiments. The intermetallic compounds (IMCs) of the brazing interface consisted of a thin layer of -Fe2Al5 next to the steel, and a thick layer of -Fe4Al13 adjacent to the Al joint, surrounded by eutectic identified to be Al-FeAl6 and Al-Fe2Al9 distributed uniformly in the weld metal. With increasing heat input, the total thickness of the IMCs decreased first, reaching a minimum value of 3.5 m, and then increased. The decrease in IMC thickness with heat input was observed for the first time and had never been reported before in welding or brazing processes. The joint strength increased to a maximum value of 238 MPa and then declined, suggesting an inverse relationship between IMC thickness and joint strength. The results of the hot-dip aluminizing, regarding the relationship between IMC thickness and temperature, were consistent with the change of IMC thickness in the welds. A possible mechanism in action is that interfacial kinetics and thermodynamics play a role in the dissolution and decomposition of the thick layer of -Fe4Al13 into the FeAl6 and Fe2Al9 phase. Promoting the IMC dissolution or decomposition by adjusting welding procedures is a promising new way to control the IMC growth and improve the joint strength.

Exploring the mechanical and sensing capabilities of multi-material bonded joints with carbon nanotube-doped adhesive films

Single lap shear tests are carried out at the joints of similar and dissimilar materials, and their electrical resistance through thickness is monitored to evaluate their sensing capabilities. Al-Al and carbon fibre reinforced polymer (CFRP)-Al joints are manufactured with two types of surface treatments of the Al substrates: brushing (B) and grit blasting (GB). The lap shear strength (LSS) is higher with grit blasting due to the optimum surface roughness and the creation of an active surface. Carbon nanotube (CNT)-doped samples are detrimental to the LSS in Al-Al joints compared to the neat adhesive, while a slight increase in LSS for CFRP-Al and CFRP-CFRP is observed due to a better CNT-CFRP interaction and good load transfer between the substrates. Electrical monitoring tests are strongly influenced by the surface treatment with better adhesive-substrate interfaces leading to a more stable response and higher sensitivity, proving the high applicability of the proposed technique.

Grain refinement caused by intensified cavitation within narrow channel and its improvement to ultrasonically soldered Al joint property.

In this work, grain refinement of the aluminum soldered joint was obtained by applying cavitation within narrow channels and the possible grain refinement mechanism was proposed. Aluminum sheets with different channel widths were ultrasonically soldered by pure Sn in air. An ultrasonic system with a TC4 sonotrode, was operated at a frequency of 20 kHz and power of 1000 W during soldering. The effect of channel width on grain size, element distribution and strength of the soldered joint was studied. Results showed that the grain size decreased from 2.62 to 1.04 µm and the element ratio of Al in solder increased from 0.93 to 4.86% when the channel width decreased from 0.8 to 0.2 mm. Instant solidification of Sn grains was readily observed in the joint before cooling due to the large undercooling induced by the intensified cavitation inside the narrow channels. The random cavitation induced nucleation of Sn was believed to be mainly responsible for the grain refinement of the soldered joint. The shear strength of the joint increased from 29.5 to 48.8 MPa and the hardness increased from 16.5 to 25.2 HV due to the grain refinement of Sn and the presence of Al transferred from the substrates.

Ultrasonic and Laser Welding Technologies on Al/Cu Dissimilar Materials for the Lithium-Ion Battery Cell or Module Manufacturing

The global technical trend of the automobile industry has been moving to environmental friendly. Typical gasoline cars will be replaced by green cars such as hydrogen, electric and hybrid cars. Therefore, the demand for battery cells is expected to increase remarkably. Welding and joining technologies are key technology for assembling tab to tab or tab to bus bar, and it is important to secure stable and robust joints. This paper provides a comprehensive review of joining technologies for the ultrasonic and laser welding, respectively. It discusses the advantages and disadvantages of the joining technologies for the Al and Cu dissimilar joints as related to battery manufacturing, and discusses corresponding issues. Key words: Technical trend, Aluminum, Copper, Dissimilar joint, Battery cell & pack, Ultrasonic welding, Laser welding

Effect of CNT dispersion methods on the strength and fracture mechanism of interface in epoxy adhesive/Al joints

The tensile and shear strengths of adhesively bonded aluminum (Al) joints were inspected in the presence of amino functionalized multi-wall carbon nanotubes (MWCNTs). Tensile and shear tests were carried out using butt and lap-shear joints. The main goal was to compare the effects of dispersion methods of functionalized-CNT into epoxy on the mechanical performances and failure mechanisms of Al joints. Two different types of dispersion procedures, distributing CNT in the hardener (HH method) and distributing CNT in the resin (RR method), were applied. To identify the failure mechanisms, the morphology of fracture surfaces were analyzed using scanning electron microscopy (SEM). Comparing two dispersion methods against one another ascertained that following the RR method for dispersing CNTs in the adhesive displayed larger shear strength, while applying HH method offered fairly greater tensile strength. Moreover, dispersing CNTs in the resin induced more uniform dispersion of CNTs as compared to distributing nanofillers in the hardener. Following RR method, CNTs good dispersion as well as the presence of effective crack growth dissipating mechanisms, increased the shear strength of CNT reinforced adhesive joint. Incorporating CNTs using HH approach encouraged the plastic void formation of epoxy around the agglomerated CNTs, and as a consequence, promoted the plastic deformation under tension.

Application of pulsed Yb: Fiber laser to surface treatment of Al alloys for improved adhesive bonded performance

Surface treatment of the Al surface by pulsed Yb: fiber laser has proven to be effective in improving the adhesive bonded performance of sheet Al substrates. This study was conducted to expand the initial investigation to include sheet, extrusion, and cast Al alloys and measure the effect of laser ablation treatment on the bond strength both in ambient and after water soak. It is found that laser ablation treatment improved the joint strength for all three Al alloys through modification of the surface characteristics and exceeded the target bond strength. Moreover, the results after water soak support the conclusion that the laser ablation treatment exhibited a significant improvement in corrosion resistance of the adhesive-bonded Al joints compared to as-received condition and resulted in the required cohesive fracture mode even after water soak. The reason for this is attributed to the retarding effect of rough laser-ablated surfaces on water diffusion along the bonding interface of laser-treated joints. Besides, the improvement of laser ablation treatment on joint strength before and after exposure increases with the laser fluence. The optimized laser fluence for all three Al alloys is 26.3 J/cm2, which is attributed to a higher degree of laser-induced surface modifications.

Tungsten inert gas welding of dissimilar metals aluminum to titanium with aluminum based wire

The T40 and 5A06 alloy were welded by tungsten inert gas (TIG) welding-brazing technology. The effects of welding parameters on cross-section macrostructure, microstructure, mechanical properties and fracture features were investigated. During the TIG welding-brazing of Ti–Al, the low melting point Al alloy and filler metal melted and resulted in the formation of fusion welded joint, whereas the molten filler metal spread on the solid Ti plate and resulted in the brazed joint. TiAl3 IMC layer formed at the brazing area and the thickness of IMC increased with the increase of welding current. Besides, the thickness of IMC layer of lower zone is less than that of upper zone. The optimal performance of Ti–Al joint is gained under the current of 80 A and the highest joining strength reaches 235 MPa. However, as the current exceeded 80 A, tensile strength of joint decreased owing to the metallurgical defects at the fusion welding area and brazing area.

The effects of Al surface treatment, adhesive thickness and microcapsule inclusion on the shear strength of bonded joints

The influences of various Al surface treatments, adhesive thicknesses as well as the incorporation of synthesized microcapsules into epoxy adhesive on the shear strength of adhesive/ Al joints have been investigated using lap-shear tensile tests. First, the influence of adhesive thickness on the shear strength of joints has been presented. Then, the effects of various Al surface treatments on the surface roughness of Al and shear strength of joint have been researched. Atomic force microscopy was used to study the Al surface morphologies and textures. Finally the few micron-sized polymeric microcapsules were synthesized and the shear performances of microcapsule filled epoxy adhesives were inspected. It was observed that the HCl acid based etching increased both micro-roughness and nano-texture of the Al surface and led to the peak shear strength. Moreover, HCl-nitric acid treatment offered the maximum value for the cohesive failure. Capsule inclusions into the adhesive displayed different influences on the joint shear performances depending on the capsule morphology and the surface treatment of Al.

The effect of changing chemical composition on dissimilar Mg/Al friction stir welded butt joints using zinc interlayer

In this study, solid state joining technology of friction stir welding (FSW) was carried out for Al and Mg butt joints. To improve the microstructural characteristics of the banded structure zone, before to the FSW process, a Zn interlayer was also inserted in the butt joints. Optimal conditions for friction stir welding were obtained using a combination of two travel speeds (25 and 35 mm/min) and three rotation speeds (550, 600 and 650 rpm). Mg-Zn and Mg-Al-Zn IMCs, Al solid solution and residual Zn were the most common phases in the stirred zone, eliminating Al-Mg intermetallic formation. The joint fabricated at 35 mm/min and 600 rpm, in which tensile strength of the weld improved from 141 to 175 MPa and an elongation from 0.7 to 1.9 was achieved, gave the maximum mechanical properties compared with the Zn free sample FSWed under identical conditions. The fracture micrographs were compatible with the corresponding ductility results and the fracture surfaces of Zn added samples possessed a fine texture combining brittle and ductile fracture features. Meanwhile, the hardness level was lower than that of the Zn free sample because of the change in the IMCs composition of the Zn added samples.

A study on intermetallic compound formation in Ag–Al system and evaluation of its mechanical properties by micro-indentation

In electronic packaging in recent years, silver (Ag) alloy wires have been widely adopted in wire-bond processes. The bond pads on the device chips are mostly aluminum (Al). Thus, the bonding interface is mainly Ag–Al. In recent Ag alloy wire-bond publications, it is unclear what intermetallic phases form at the interface. In this research, experiments were designed to understand the Ag–Al intermetallic compound (IMC) formation in the Ag–Al system and evaluate its mechanical properties. First, the Ag–Al alloys with compositions from 19 to 43 at.% Al were evaluated to identify the phase equilibrium and crystal structure of the Ag–Al intermetallic phases. Microstructures and phase compositions of the designed Ag–Al alloys are presented. To further study the intermetallic compound formation at the Ag/Al interface, the interfacial reaction of the Ag/Al joints at 200 °C was investigated. The µ-Ag3Al and δ-Ag2Al IMC were identified to form at the Ag/Al interface and stabilize after long-time annealing at 200 °C. At last, deformation and fracture behaviors of the bulk µ-Ag3Al and δ-Ag2Al were analyzed by the micro-indentation. The measured results reveal that µ-Ag3Al exhibits significantly higher hardness and lower fracture toughness as compared to δ-Ag2Al. Indentation crack propagation in µ-Ag3Al demonstrates the fracture characteristics of brittle materials. In the case of δ-Ag2Al, the presence of slip bands exhibits the ductility of δ-Ag2Al to endure plastic deformation prior to fracture. The effect of the mechanical properties of the µ-Ag3Al and δ-Ag2Al IMC on the Ag–Al joint reliability is discussed. New information obtained in this research is important for future study of the joint reliability and failure mechanism of Ag–Al wire bonds.

Ultrafine grained structure and improved mechanical properties of low temperature friction stir spot welded 6061-T6 Al alloys

In this study, 1mm thick 6061-T6 aluminum alloy plates were subjected to friction stir spot welding (FSSW) at a very low rotation speed of 30, 40 and 50rpm, aiming to prevent the hardness reduction in the heat affected zone (HAZ) of this precipitate hardening Al joints. Two kinds of rotating tools, namely, one with a 1mm long probe and the other one with a flat shoulder without a probe, were used and the corresponding force applied to the rotating tools was ranged from 4 to 8tons. Under this special welding condition, the highest welding temperature was measured lower than 200°C. As a result, no hardness reduction of the HAZ region could be detected and an ultrafine grain structure with a high density of dislocations was formed in the stir zone. The average grain size obtained in the stir zone was less than 500nm for all the welds, which was much smaller than that in conventional FSSW processed 6061-T6 Al joints. In addition, no hook-like bonding ligaments were formed in the joints due to the very different material flows and the shear tensile tests showed that the spot welds prepared by the low speed FSSW can reach a maximum shear load of about 5400N, much higher than those prepared by the conventional FSSW technique.

High mechanical performance of similar Al joints produced by a novel spot friction welding technique

In this study, the protrusion friction stir spot welding (PFSSW) was introduced in this study while a specially designed back plate (anvil) with a protrusion on its surface and a pinless tool were used. The effects of the tool dwell time (6, 12, 18 s) were investigated on the microstructure and mechanical properties of samples. Surface appearance of the welding zone showed that the keyhole was not formed and the appearance of weld was relatively smooth. Microstructure and mechanical results indicated that the welding zone with uniform and refine structure due to dynamic recrystallization presents higher hardness and strength than base metal while can be affected by dwell time. The tool penetration depth, the stir zone (SZ) depth and the bonding area at the interface of two sheets increased with increasing dwell time. Fracture surfaces of the failed specimens present the shear fracture at dwell times of 6 and 12 s and the interfacial fracture at 18 s. The proper dwell time of tools in this process was obtained 12 s. Considerable hardness (220 HV) and maximum failure load (6000 N) were obtained in present work compared to other welding methods.

Ultrasonic-assisted soldering of fine-grained 7034 aluminum alloy using Sn-Zn solders below 300 °C

The fine-grained Al alloys prefer to be soldered at as low as temperature to keep their mechanical properties. Solders of Sn-4Zn, Sn-9Zn, and Sn-20Zn alloys were used to solder fine-grained 7034 Al alloy pieces by ultrasonic-assisted soldering below 300°C in air. The joint using Sn-4Zn solder had the highest tensile strength of 201MPa and the fractures occurred in both β-Sn and Sn-Zn eutectic phases. Such joint was much stronger than the 1060 Al joint using Sn-4Zn solder, and its strength had approached the strength of 7034 Al joint using Zn-5Al solder. The strength of the joints using Sn-9Zn and Sn-20Zn solders dropped to∼160MPa due to the appearance of weak interfaces between η-Zn and eutectic phases in the bond layers. All the joints using Sn-Zn solders had very strong interfacial bonding, and alumina interlayers were identified at all the interfaces. Al dissolved in the bond layer reacted with the O rapidly to form alumina interlayers at the interfaces under the ultrasonic action. Zn segregated at the interface and formed strong bonds with both the Al terminated surface of alumina and the bond layer, resulting in strong interfacial bonding between Sn-Zn solders and Al alloys.

Influence of Zn coating on friction stir spot welded magnesium-aluminium joint

ABSTRACTFor friction stir spot welded (FSSW) magnesium–aluminium joints, the formation of Mg–Al intermetallics in the hook region and cracking at the interface damaged seriously the strength of Mg–Al joints, resulting in lower joint load of only 0.8 ± 0.2 kN. When adding the hot-dipped Zn coating on the Al substrate surface prior to FSSW, a brazed layer, composed of Mg–Zn and Al–Zn diffusion zones at the edge of the shoulder, and a transition layer, composed of MgZn2, Zn-rich zone and residual Zn in the hook region, were formed in the FSSW Mg–Al joint, eliminating the cracking and Mg–Al intermetallics in the FSSW joint without the Zn coating. The load of the joint with the Zn coating increased to 3.7 ± 0.3 kN.