Interfacial Intermetallic Compounds Research Articles

Corrosion Behavior and Mechanical Property of 5182 Aluminum/DP780 Steel Resistance Spot Welding Joints

Corrosion behavior is critical to the application of lightweight aluminum/steel joints using new resistance spot welding (RSW) technology. The study investigated the corrosion mechanism and the shear strength of RSW joints comprising 1.2 mm 5182 aluminum and 1.5 mm DP780 galvanized steel. Electrochemical corrosion tests were conducted on the base materials and various positions of the welds in a 3.5% NaCl solution. This result revealed that the corrosion susceptibility of the interfacial intermetallic compound (IMC) layer was not accelerated by the aluminum nugget because of the noble corrosion potential. Subsequently, the spray acceleration test was employed to investigate the corrosion mechanism. It is noteworthy that microcracks, as well as regions enriched with silicon and oxygen at the interface front, are preferential to corrosion during salt spray exposure, instead of the IMC layer. Moreover, the shear strength of the joints decreases with the reduction in the effective joint area after the salt spray exposure of the weld joints. This research systematically explored the corrosion behavior and its relationship with the mechanical properties of Al alloy/steel RSW joints.

An Innovative High-Entropy Alloy Solder for High-reliability Low-Temperature Bonding in 3D Electronic Packaging——Based on nano InSnBiZnAg particles

To address the demand for multilevel package, a low-melting-point high entropy alloy (HEA) solder was developed. It consists of 43In28Sn14Bi9Zn6Ag micro- and nanoparticles with a melting point of about 62.80 °C. We obtained 43In28Sn14Bi9Zn6Ag nanoparticles by combining ultrasonic heating interaction with a "hot shock" process. Upon reflowing at 115 °C for 5 min, a Cu/Zn/Ag ternary intermetallic compound (IMC) formed at the interface between the solder and the copper substrate, leading to a shear strength of 40 MPa. The microstructure of this joint exhibits a complex composition with at least three phases, including Bi3In5 phase, Ag5Zn8 phase, and In0.2Sn0.8 solid solution. The stable Ag5Zn8 phase not only enhances the overall strength of the alloy matrix but also impedes the diffusion of In and Bi atoms, thereby preventing the segregation of the Bi3In5 phase. The high mixing entropy and pronounced lattice distortion in this high entropy alloy hinder the diffusion of elements within the solder matrix, mitigating the issue of excessive interfacial IMC growth. Consequently, the solder joints exhibit little degradation even after a high-temperature aging test. This high-entropy alloy solder has promising potential for multi-level chip stacking in 3D packaging.

Investigating the impact of cobalt incorporation on the transformation of Cu6Sn5 layer to (Cu, Co)6Sn5: Microstructural and mechanical insights

In this study, cobalt (Co) particles were incorporated as reinforcing agents into the Sn58Bi solder. The solderability of the solder and the strength of the solder joints were investigated. The results indicated that adding Co particles did not alter the low melting point characteristics of the Sn58Bi solder, yet its undercooling was reduced, and the wettability was enhanced. In the Sn58Bi/Cu solder joint, adding Co led to forming free (Cu, Co)6Sn5 intermetallic compound (IMC) and generating the CoSn2 phase through microalloying. The interfacial IMC was gradually transformed from a fan-shaped Cu6Sn5 to a flatter layer-like (Cu, Co)6Sn5 IMC, eventually exhibiting a coral-like structure with increased Co content. Adding Co particles increased the thickness of the IMC layer, but at low levels, the Co could refine the IMC grains. Simultaneously, implementing grain refinement and Orowan strengthening resulted in heightened microhardness and shear strength. In the shear test, the fracture of solder joints predominantly transpired within the solder matrix. Excessive Co particles were prone to aggregating within the solder, forming irregular block-shaped (Cu, Co)6Sn5. Concurrently, the coral-like IMC at the interface led to a decline in the mechanical performance of the solder joints, with some interfacial IMC being exposed in the fracture region.

Microstructural evaluation of interfacial intermetallic compounds between Sn58Bi and ENEPIG

BackgroundLow carbonization and Restriction of Hazardous Substances (RoHS) regulations have promoted the development of low-temperature lead-free solder. The solder and substrate are critical entities of joint connections. MethodsSn58Bi has been considered a promising low-temperature solder. Electroless nickel/electroless palladium/immersion gold (ENEPIG) is a good surface finish that prevents solder joints from oxidizing and extends the shelf life of the substrates. In this study, the interfacial reactions between the Sn58Bi solder and ENEPIG surface finish were systematically analyzed. Findings(Au,Pd,Ni)(Sn,Bi)4 was observed after reflow at 160 °C. Au and Pd dissolved into solder rapidly in the beginning. NiSn4 and Ni3Sn4 with small amounts of Au and Pd dissolution formed at the interface after aging. The morphology of NiSn4 evolved from block-like to needle-shaped and finally to a layered configuration as the aging temperature and time increase. Ni3Sn4 was observed between NiSn4 and substrate above 100 °C. This study established the growth mechanism of intermetallic compounds (IMCs) in the reaction between Sn58Bi and ENEPIG for low-carbon emitting processes.

Microstructural response characteristics of SAC305/Cu column joint under friction embedded welding conditions

The friction welding method for macro-scale weldments was introduced into the manufacturing of electronic products. Without mold assistance, the positioning and connection of micro copper columns in column grid array packaging (CGA) devices were achieved. The average connection strength of 40.6 N and the intermetallic compound interface layer meant that effective connections were achieved. In the poorly identified stirring-mixing zone (SMZ) in scanning electron microscopy (SEM) images, electron backscatter diffraction (EBSD) results showed that this region was composed of recrystallized grains, twin grains, and Ag3Sn and Cu6Sn5 particles distributed inside or at the grain boundaries. The grain size was about 2 μm, and the Ag3Sn and Cu6Sn5 particle sizes were broken to 250 nm and 120 nm due to stirring. The SMZ at the bottom of the copper column only underwent deformation-induced dynamic recrystallization, while the SMZ on the side was discharged from the bottom of the copper column, so it underwent dynamic recrystallization through the process of cutting crushing-extruding and flow-bonding-deformation. Therefore, the hardness of SMZ was significantly reduced after welding. Unlike traditional friction welding, in the thermo-mechanical affected zone (TMAZ), the solder not only underwent deformation but also formed 47.8% recrystallization microstructure and 14.6% recovery microstructure. Therefore, the effect of deformation strengthening was weakened and recrystallization led to a decrease in hardness. Recovery was the main response behavior in the micro-deformation zone (MDZ). Combined with the numerical simulation results, the temperature and stress driving conditions of the microstructure of each micro-region in instantaneous friction embedded welding (FEW) were also discussed.

Influence behavior and mechanism of crystal orientation of Cu6Sn5 coating on interfacial intermetallic compounds growth in Sn-0.7Cu/Cu solder joint

Herein, the Cu6Sn5 coatings with principal crystal orientations were prepared on the Cu substrate using magnetron sputtering to inhibit the growth of interfacial intermetallic compounds (IMCs) in Sn-0.7Cu/Cu solder joint. The effect of sputtering parameter on the principal crystal orientation of Cu6Sn5 coating was researched firstly, followed by the investigation on the influence of Cu6Sn5 coating principal orientation on the interfacial IMCs growth kinetics. The influence mechanisms were studied using molecular dynamics simulations and first-principles calculations. The results revealed that the increasing sputtering power from 40 W to 60 W and 80 W can not only improve the bonding property between the prepared Cu6Sn5 coating and Cu substrate, but also promote the transformation of the coating principal orientation from (132) to (132)/(22-1) and (132)/(22-1)/(42-2). Moreover, the three prepared Cu6Sn5 coatings can inhibit IMC growth at SC07/Cu solder joint, and the Cu6Sn5 coatings with (132)/(22-1) and (132)/(22-1)/(42-2) principal orientations exhibit more significant inhibitory effects on IMC growth than that with (132) principal orientation. It can be proved in this work that the growth of interfacial IMCs in Sn-0.7Cu/Cu solder joint at aging stage is mainly caused by the diffusion of Cu from Cu substrate toward Cu6Sn5 phase. The difference in the growth behavior of IMCs can be attributed to the fact that when the Cu atoms diffuse across Cu6Sn5 (22-1) and Cu6Sn5 (42-2), the bonding strength between the Cu atom at the saddle point and its surrounding atoms is significantly larger than when diffusing across Cu6Sn5 (132), leading to the smaller diffusion coefficients and a slower interfacial IMCs growth rate.

Achieving outstanding mechanical property after long-cycle thermal shock of reflow/ultrasonic-assisted Kovar/SnSb10 interfaces

The present study analyzed the microstructure evolution corresponding to the mechanical properties of reflow/ultrasonic-assisted Kovar/SnSb10/Fe joints after long-cycle thermal shock (TS). The results indicate that the thickness of interfacial intermetallic compounds (IMCs) at Kovar/SnSb interface with ultrasonication was less than 1 μm even after 2000 cycles, and almost no defects were detected, exhibiting superior microstructure stability. The ultimate shear strength with and without ultrasonication reached 58 MPa and 45 MPa, respectively. From TEM analysis, the nanoprecipitates inside Sn or IMC region primarily consisted of Ni3Sn4 and Ni3Sn phases. The ultimate outstanding mechanical property after long-cycle TS was attributed to dispersed strengthening and dislocation strengthening. This work provides valuable insights into the stability and reliability of the Kovar/SnSb10 system.

Improved shear property of Sn-3.0Ag-0.5Cu/Ni micro solder joints under thermal shock between 77 K and 423 K by adding TiO2 nanoparticles

The rapid development of space exploration technology was intensifying the desire for lead-free solders with high performance and reliability under extreme temperature environments. In this study, the influence of TiO2 nanoparticle addition on the microstructure, growth behavior of interfacial intermetallic compounds (IMCs) and shear property of Sn-3.0Ag-0.5Cu (SAC305)/Ni micro solder joints under thermal shock from 77 K to 423 K was systematically investigated. The results indicated that the grain size of β-Sn, IMC (Ag3Sn and (Cu,Ni)6Sn5) particles inside the solder was refined by adding 0.05 and 0.1 wt% TiO2 nanoparticles. Due to the spalling of interfacial IMCs into the solder occurring in all micro joints during thermal shock, the thickness of interfacial IMCs did not show a monotony increasing trend with increasing thermal cycles. However, the micro composite solder joint added with 0.05 and 0.1 wt% TiO2 nanoparticles always exhibited thinner interfacial IMC thickness and higher shear force compared with the plain micro joints during thermal shock. The inhibiting effect of TiO2 nanoparticles on the interfacial IMC growth could be explained by the adsorption theory and grain boundary pinning theory. The improvement in shear property of micro composite solder joints was primarily induced by the refinement of IMC particles in the solder and the reduced growth rate of interfacial IMCs by adding TiO2 nanoparticles.

Insight into the influence of Cu6Sn5/Cu micro-interface configuration on growth behavior of Cu-Sn interfacial intermetallic compounds in Sn/Cu solder joint

Herein, the behavior and mechanism of Cu atom diffusing across Cu6Sn5/Cu interface was researched for analyzing the influence of Cu6Sn5/Cu micro-interface configuration on growth behavior of Cu-Sn interfacial intermetallic compounds (IMCs) in Sn/Cu solder joint. The results indicate that, during Cu atoms diffusing across Cu6Sn5 (001)/Cu (110), Cu6Sn5 (110)/Cu (001), Cu6Sn5 (110)/Cu (111) interfaces, VA2→VA3 stage, VA2→VA3 stage and IA→VA1 stage are the decisive stages, respectively. At those diffusion decisive stages, the difference of average Mulliken population between Cu atom at saddle point and initial state with the surrounding atoms for Cu6Sn5 (110)/Cu (111) interface is 0.62, much greater than that for Cu6Sn5 (001)/Cu (110) and Cu6Sn5 (110)/Cu (001) interfaces with average Mulliken population difference of 0.29 and 0.20. Therefore, when Cu atom diffusing across Cu6Sn5 (110)/Cu (111) interface, the magnitude of increase in bonding strength between Cu atoms and the surrounding atoms is the largest, leading that the diffusion rate of Cu atom across Cu6Sn5 (110)/Cu (111) interface is several orders of magnitude smaller than the Cu6Sn5 (001)/Cu (110) and Cu6Sn5 (110)/Cu (001) interfaces. As a result, Cu6Sn5 (110)/Cu (111) interface shows the strongest ability for inhibiting Cu atom diffusion, and has the smallest IMC growth rate.

Interfacial intermetallic compounds growth kinetics and mechanical characteristics of Ga-Cu interconnects prepared via transient liquid phase bonding

Driven by continuous miniaturization, higher performance, and functional robustness in three-dimensional (3D) embodiments of electronic devices, Ga which possesses a low melting point (29.8°C) has shown its potential uses in electronics packaging and integration. The use of Ga metal as a solder material to achieve low-temperature transient liquid phase bonding (TLPB) has emerged as a viable solution for reliable interconnections of future-generation products. This study investigates the interfacial intermetallic compounds (IMCs) structure, growth kinetics, and mechanical characteristics of Ga-Cu TLPB joints in the temperature range of 150–220°C. The Cu9Ga4 phase were observed and the growth rate of Ga-Cu IMCs has been investigated. The research findings indicate that the growth mechanism of interface IMCs is primarily diffusion-controlled, with CuGa2 grains exhibiting anisotropic growth. Their shear strength has reached 23.8 MPa, with a porosity of 1.82±0.07%. Furthermore, the fracture interface is presented and correlated with the experimental observations.

Influence of doping Si3N4 nanoparticles on the properties and microstructure of Sn58Bi solder for connecting Cu substrate

Purpose This study aims to explore the feasibility of adding Si3N4 nanoparticles to Sn58Bi and provides a theoretical basis for designing and applying new lead-free solder materials for the electronic packaging industry. Design/methodology/approach In this paper, Sn58Bi-xSi3N4 (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0 Wt.%) was prepared for bonding Cu substrate, and the changes in thermal properties, wettability, microstructure, interfacial intermetallic compound and mechanical properties of the composite solder were systematically studied. Findings The experiment results demonstrate that including Si3N4 nanoparticles does not significantly impact the melting point of Sn58Bi solder, and the undercooling degree of solder only fluctuates slightly. The molten solder spreading area reached a maximum of 96.17 mm2, raised by 19.41% relative to those without Si3N4, and the wetting angle was the smallest at 0.6 Wt.% of Si3N4, with a minimum value of 8.35°. When the Si3N4 nanoparticles reach 0.6 Wt.%, the solder joint microstructure is significantly refined. Appropriately adding Si3N4 nanoparticles will slightly increase the solder alloy hardness. When the concentration of Si3N4 reaches 0.6 Wt.%, the joints shear strength reached 45.30 MPa, representing a 49.85% increase compared to those without additives. A thorough examination indicates that legitimately incorporating Si3N4 nanoparticles into Sn58Bi solder can enhance its synthetical performance, and 0.6 Wt.% is the best addition amount in our test setting. Originality/value In this paper, Si3N4 nanoparticles were incorporated into Sn58Bi solder, and the effects of different contents of Si3N4 nanoparticles on Sn58Bi solder were investigated from various aspects.

Formation mechanism for the interface between Cu and Sn formed by magnetic pulse welding

The formation of interfacial intermetallic compounds (IMCs) such as Cu6Sn5, Cu3Sn or platelet-shaped Ag3Sn which are formed in reflow soldering process will directly destroy the soldering reliability. Magnetic pulse welding (MPW), as a solid state welding technique can effectively restrain or even impede the formation of interfacial IMCs. In this paper, Cu and Sn dissimilar metals were welded by MPW under the discharge energy from 17 to 31 kJ. The typical wavy interface and metal jet flow were found in all samples. When the discharge energy reached 31 kJ, a typical gradient nanograined (GNG) interface has been observed to undergo an evolution from a sub-micron secondary solid solution of Cu6.26Sn5 IMCs to a nanoscale secondary solid solution of Cu81Sn22 IMCs, followed by the formation of nanoscale equiaxed α-Cu grains, and ultimately leading to the deformed elongated nanoscale α-Cu grains. This discontinuous distribution of the aforementioned phases occurs specifically at the interface of the 23 kJ_1.5 mm sample. In contrast, when subjected to a discharge energy of 17 kJ, there were no indications of a secondary solid solution of Cu6.26Sn5 IMCs or elongated nanoscale α-Cu grains. However, a discontinuous secondary solid solution of Cu81Sn22 IMCs was observed to form at the Cu/Sn interface under these conditions. Based on these findings, the utilization of microprojection welding (MPW) techniques involving Cu and Sn-based solder in the microscale is anticipated to serve as a highly efficient and automated technology within the field of electronic micropackaging.

Formation and effect of intermetallic compounds in the interface of copper/aluminum composites under rolling conditions

Rolling and annealing are common composite processes of Copper/Aluminum composites (Cu/Al), but the formation of intermetallic compounds (IMCs) at Cu/Al interfaces and their relationship with processing conditions still lack a nanoscale understanding. Here, we use molecular dynamics (MD) method to simulate the formation of IMCs at Cu/Al interface under different rolling and annealing process parameters with their formation mechanisms quantitatively analyzed. XRD, RDF and potential energy analysis show that rolling promotes the formation of interfacial IMCs, because copper and aluminum atoms tend to diffuse into each other at the interface and combine to form IMCs during rolling. The effects of different size ratios of copper to aluminum and annealing on IMCs are also evaluated. By studying the equivalent phonon thermal conductivity of the interfacial layer and shear strength, it is shown that IMCs enhances interfacial phonon heat transfer, but reduce the interfacial shear strength due to their brittle and hard properties. In addition, the rolling and annealing processes are able to change the dislocation density, thereby affecting the phonon scattering and shear strength of Cu/Al. These results are expected to guide improved directions for the process optimization of high-performance Cu/Al.

Molecular dynamics and first-principles calculations on the wetting behavior and mechanism of molten Al on nickel-infiltrated steel: A cross-scale simulation

Wetting behavior and mechanism of melting Al on Ni-infiltrated steel were investigated using the cross-scale simulations. The molecular dynamics calculations were employed firstly to intuitively exhibit the influence of amount of nickel infiltration (on the surface of steel substrate) on wettability of Al droplet. The micro-mechanisms affecting wettability, such as the bonding characteristic and charge distribution on wetting interface, were researched using the first-principles calculations. The results indicate that, the chemical reactions occur on Al droplets/Ni-infiltrated steels wetting interfaces. With the increasing amount of Ni infiltration, the main interfacial intermetallic compound (IMC) transforms from Fe2Al5 to NiAl and finally to Ni3Al. Although the atomic bonds on Al/Fe2Al5, Al/NiAl, and Al/Ni3Al interfaces all show both ionic and covalent characteristic, the work of adhesion for the three Al/IMC interfaces follows the order of Al/Fe2Al5 > Al/NiAl > Al/Ni3Al. This phenomenon indicates that the interfacial bonding strength between Al droplet and the generated IMC decreases gradually with the increasing amount of Ni infiltration, and leading to the improving wettability because the wettability is negatively correlated with the Al/IMC bonding strength on wetting interface.

Revealing evolutions of intermetallics in a solder joint under electromigration: A quasi-in situ study combining 3D microstructural characterization and numerical simulation

The evolution of primary and interfacial intermetallic compounds in solder joints during electromigration (EM) significantly influences the mechanical properties and reliability of solder joints. In this work, by combining high-resolution x-ray computed tomography and finite element modeling, the 3D evolution of Cu6Sn5 in a solder joint containing a single β-Sn grain with representative orientation was revealed. It is found that the growth of primary Cu6Sn5 rods and the formations of Cu6Sn5 rods/particles within the matrix and on the anode/cathode interfaces are all heavily influenced by local Cu concentrations and Cu diffusion fluxes in β-Sn, which are controlled by the β-Sn orientation and geometry of the solder joint. The unobvious growth of some Cu6Sn5 is also attributed to the high angles between [0001]Cu6Sn5 and [001]β-Sn (>45°). The orientation relationship between β-Sn and Cu6Sn5 also contributes to the growth direction of the newly formed Cu6Sn5 rod. This study provides insight into the mechanisms of EM-induced microstructure evolutions in ball-grid-array solder joints.

Enhancement of SAC105 solder for vacuum soldering with Cu substrates through incorporation of activated Ti nanoparticles

This study incorporated activated Ti nanoparticles (Ti NPs) into Sn1·0Ag0·5Cu (SAC105) solder for vacuum soldering with Cu substrates. The wettability, microstructure, interfacial, and mechanical properties of SAC105-xTi (x = 0.1, 0.2, 0.3, 0.4, 0.5 wt%) composite solder were systematically investigated. The results showed that adding Ti NPs improved the wettability of SAC105 solder, in which the wetting angle of SAC105-0.3Ti solder was 19.51°. Additionally, Ti NPs helped to refine the microstructure and inhibited the growth of interfacial intermetallic compounds (IMC). The shear strength of SAC105/Cu joints increased and then decreased with increasing Ti NPs content, and the optimum shear strength was obtained for SAC105-0.3Ti/Cu joints at 27.98 MPa. The strength of the solder matrix and the IMC layer was altered by adding Ti NPs, resulting in a modification of the fracture path of the joint, bringing it closer to the IMC layer. Excessive addition of Ti NPs could lead to their agglomeration within the joints and result in damage to the mechanical properties of the joints. Furthermore, adding Ti NPs increased the microhardness of SAC105 solder.

Reliability improvement of SnAgCu interconnections under extreme temperature condition by TiO2 nanoparticles doping: Experiments and first principles calculations

Electronic components of the spacecraft during their service may endure intricate and extreme temperature environments. In this study, the interfacial microstructure evolution and reliability of Cu/Sn-3.0Ag-0.5Cu (SAC305)/Ni and Cu/Sn-3.0Ag-0.5Cu-0.05TiO2 (SAC305–0.05TiO2)/Ni interconnections under thermal shock from −196 °C to 150 °C are investigated. Short-column-type (Cu,Ni)6Sn5 intermetallic compounds (IMCs) are developed at the solder/Ni interface for both interconnections after the reflow process, and their morphology changes to long-rod type after 200 cycles. Extremely thin and discontinuous (Ni,Cu)3Sn4 phases are generated between (Cu,Ni)6Sn5 and Ni only in SAC305 interconnections after 200 cycles. The rate of interfacial IMCs growth in SAC305–0.05TiO2 interconnections is slower than that in the SAC305 interconnections. Cracks are formed in the IMC layer of the SAC305 interconnection after 200 cycles, which propagate through the whole IMC layer after 250 cycles. While for the SAC305–0.05TiO2 interconnection, no crack is observed until after 250 cycles. TiO2 nanoparticles doping could suppress the interfacial IMC growth, leading to an enhancement in the interconnection reliability under thermal shock. First-principles calculations are performed to further understand the mechanism for phase transformation of Ni-Cu-Sn IMCs and the reliability improvement of interconnections.

Insight into the improvement of service performance of Sn/Cu solder joint by Pt doping in Cu6Sn5 interfacial intermetallic compound

The effects of Pt doping on the mechanical properties of (Cu,Pt)6Sn5 interfacial intermetallic compound (IMC), bonding properties of the (Cu,Pt)6Sn5/Cu interface, and growth behavior of (Cu,Pt)6Sn5 IMC during the aging stage were investigated using first-principles calculations. The results indicate that Pt doping improves both the deformation resistance and ductility of the (Cu,Pt)6Sn5 phase, and that Cu2Pt4Sn5 demonstrates the highest ductility, as indicated by the maximum B/G value (3.14) and Poisson’s ratio (0.356). This phenomenon can be attributed to the smallest standard deviation of the charge density of 0.097 e/Å3. Comparisons of the interfacial work of adhesion and tensile strength of the Cu2Pt4Sn5/Cu and Cu6Sn5/Cu interfaces show that Pt doping enhances interfacial atomic interaction. Based on semi-classical Boltzmann theory, the electrical conductivity of the Cu2Pt4Sn5/Cu interface is smaller than that of the Cu6Sn5/Cu interface, with an amplitude of 9.26%, owing to the enhanced electron localization after Pt doping and resulting in a slight increase in the service temperature. Nevertheless, Pt doping still significantly suppresses the diffusion of Cu atoms, as evidenced by the millions of times reduction in the diffusion coefficient of Cu atoms across the Cu2Pt4Sn5/Cu interface compared to that across the Cu6Sn5/Cu interface because of the latter interface’s larger diffusion activation energy (3.52 eV > 2.08 eV), which was investigated by climbing image nudged elastic band methods. Therefore, Pt doping not only strengthens the mechanical properties of the (Cu,Pt)6Sn5 phase and improves the bonding strength of the (Cu,Pt)6Sn5/Cu interface, but also suppresses the growth of IMC during the aging stage, playing a positive role in obtaining a highly reliable Sn/Cu solder joint.

Effect of pin offset on interface evolution and fracture behavior of aluminum/copper dissimilar friction stir welded

In this study, triangular prismatic threaded structural pins at different offsets were used to study the forming and performance of Al/Cu dissimilar joints. In the experiments, the Al alloy was placed on the advancing side, and the stirring pin was offset 0.5 mm, 0.8 mm, 1.0 mm, and 1.2 mm towards the Al side, respectively. The results show that sound joints can be obtained under each offset. The number and size of Cu particles in the stirred zone of the joint with a pin offset of 0.5 mm was large. The Cu particles in the stirred zone gradually refine as the stirring pin is offset towards the Al side. According to the EDS analysis, the stirring zone and interfacial intermetallic compounds (IMCs) mainly consist of Al2Cu and Al4Cu9, and a lamellar structure with IMC layers was observed on the Cu side near the Al/Cu interface. The size and number of Cu particles as well as the thickness of the IMCs layer had a significant effect on the tensile strength of the joints, with a maximum tensile strength of 203.4 MPa (equivalent to 71.3% of the Cu base metal) obtained at a pin offset of 1 mm, which was fractured in the heat-affected zone on the Al side, and the fracture surfaces demonstrated ductile fracture.

Improvement of weldability and fracture behavior of mild steel and A7075 aluminum alloy dissimilar friction stir welded joints by increasing welding speed

In this study, dissimilar friction stir welding of mild steel and A7075 aluminum alloy was conducted at a constant tool rotational speed (900 rpm) and different welding speeds of 100–400 mm/min with an interval of 100 mm/min. A thermocouple was employed to record the thermal cycle and the maximum reached temperatures of the A7075/steel interface during the joining process. The higher the welding speed, the lower the maximum temperature recorded during the joining process. Conducting the joining process at a high welding speed also enhanced the cooling rate and, thus, effectively contributed towards eliminating the thermal residual stresses and suppressing the interfacial intermetallic compounds growth. As a result, 100% joint efficiency with a steel base metal fracture was obtained for the joints fabricated at the welding speeds of 300 and 400 mm/min.