Copper Side Research Articles

Effect of Ultrasound on Microstructure and Properties of Aluminum–Copper Friction Stir Lap Welding

In this paper, the influence mechanism of ultrasound on plastic flow and microstructure features of the aluminum–copper friction stir lap welding (Al/Cu-FSLW) process is systematically investigated by adjusting the welding speed and improving the shear rheology in the plastic stirring zone. Through adjusting the ultrasonic vibration and welding speed, the directional control of mechanical properties is realized. It is found that increasing the welding speed properly is beneficial to enhance the mechanical shear between the tool and the workpiece, thus forming more staggered layered structures at the copper side and improving the tensile strength of the weld. The acoustic softening enhances the viscoplastic fluid mixing and strengthens the mechanical interlock of the Al/Cu lap interface. As the welding speeds increase or ultrasonic vibration is applied, the thickness of Al/Cu intermetallic compound (IMC) decreases, and the tensile strength and elongation of the Al/Cu joints are enhanced. Compared with adjusting the welding speed, the ultrasonic vibration can further refine the copper particles which are stirred into the plastic zone, and the thinning effect of ultrasound on IMC layers is better than that of increasing welding speed. At the welding speed of 60 mm/min, the IMC layer thickness is reduced by 42% under ultrasonic effect. In three welding speed conditions, the UV reduced the absolute value of the effective heat of formation (EHF) for Al2Cu and Al4Cu9 and suppressed the formation of AlCu phase. Meanwhile, only when the welding speed is increased from 60 mm/min to 100 mm/min can the formation of AlCu be suppressed. Under the ultrasonic optimization, the stable improvement of welding efficiency is ensured.

КОМПЬЮТЕРНОЕ МОДЕЛИРОВАНИЕ ТЕМПЕРАТУРНЫХ ПОЛЕЙ В СВАРНОМ ШВЕ ПРИ ИЗМЕНЕНИИ ТЕХНОЛОГИЧЕСКИХ ПАРАМЕТРОВ СВАРКИ ТРЕНИЕМ С ПЕРЕМЕШИВАНИЕМ АЛЮМИНИЯ И МЕДИ

This paper discusses numerical modeling of temperature fields in friction stir welding (FSW) of aluminum (AD1) and copper (M1). A three-dimensional thermomechanical finite element model based on the coupled Euler-Lagrange (CEL) approach is used. The paper models and experimentally confirms the influence of FSW parameters on the distribution and peak temperature values in weld zones. The maximum temperature values are observed on the copper side. This is due to the high thermophysical properties of copper compared to aluminum. Changing the rotation speed of the welding tool from 800 rpm to 1000 rpm leads to an increase in the peak temperature from 492 °C (800 rpm) to 692 °C on the side of the copper plate (1000 rpm). The maximum discrepancy between the calculated and experimental temperature curves does not exceed 9%, which is quite acceptable for assessing the temperature field.

Employing the electrode of different diameters to join dissimilar Al-Cu thin sheets using resistance spot welding

This study aims to clarify the intricate connection between using different electrode tip diameters and the quality of spot joints. By investigating basic principles and process parameters, the research highlights how various combinations of electrode sizes affect weld quality. Specifically, to join aluminum (Al) and copper (Cu), two electrode sizes were employed: 4 mm and 8 mm tip diameter. Given that copper has higher conductivity (398 W/mK) and a higher melting temperature (1085 °C) compared to aluminum (237 W/mK and 660 °C respectively), efforts were made to enhance current density towards the copper side by using the smaller electrode tip diameter (4 mm) on that side. Experiments were conducted using two different combinations of sheet thicknesses (0.5 mm and 1 mm), revealing the need for optimized electrode tip diameter combinations for varying sheet thicknesses and materials with different thermos-physical properties. Overall, this study seeks to deepen our understanding of resistance spot welding, specifically focusing on the importance, challenges, and future prospects associated with varying electrode tip diameters in joining dissimilar metals.

The applicability of high-frequency ultrasonic testing for the inspection of the bonded interface between a divertor's mono-block and screw pipe

This study investigated the applicability of high-frequency ultrasonic testing to inspect the tungsten-copper bonded interface between a divertor's mono-block and a cooling pipe called the “screw pipe.” Mock-up samples simulating the interface were fabricated by diffusion bonding of a 2.5 mm thick oxygen-free copper plate to a 12 mm thick tungsten block that satisfied the ITER material specifications. After making the bond, grooves were machined on the oxygen-free copper side of the samples. High-frequency ultrasonic tests were performed before and after machining the grooves using an ultrasonic scanner with a 30 MHz probe in the immersion pulse-echo setting. The testing results showed that the original screw pipe has a low capability for inspection with high-frequency ultrasonic testing. The visibility of a defect at the interface decreased because of the reflection of ultrasonic waves from the slope of the grooves and interference by the echoes from the interface and the grooves’ surfaces. These issues were addressed by proposing geometrical modifications for the grooves and testing with high-frequency ultrasonic testing. The proposed modifications considerably increased the visibility of a defect at the interface and improved detectability by high-frequency ultrasonic tests.

A first-principles study of the formation and regulation of the electric double layers at Cu (0 0 1)/mineral oil interfaces

Copper-mineral oil interfaces are key components of oil-impregnated power transformers and are commonly believed to be one of their weak points. The formation of an electric double layer (EDL) at this interface as a result of charge accumulation and transfer is crucial to its insulating properties, but a molecular-level understanding of this phenomenon remains unclear. To understand this fundamental aspect, we have investigated the effect of different EDLs on the electric potential and interfacial potential barrier between copper and mineral oil by using first principle calculations. Based on the calculations, the EDL is shown to reduce the interfacial potential barrier and enhance the diffusion of oil molecules at the interface when the copper side is negatively charged and the mineral oil side is positively charged. In contrast, when the copper side is positively charged and the mineral oil side is negatively charged, the corresponding EDL can increase the interfacial potential barrier and reduce the diffusion of oil molecules at this interface. Our findings shed light on the relationship between the structure of EDLs and their electrical properties in oil-impregnated power transformers.

Establishing theoretical foundations for predicting the structural and morphological characteristics of diffusion-welded joints of the beryllium–copper composite

The paper presents the results of theoretical and experimental studies regarding the quality of diffusion welding of the beryllium–copper composite. Numerical investigations of the parameters of heterodiffusion of diffusants and the thickness of the Be–Cu pair welded joint under varying temperature-time conditions were conducted. The analytical examinations revealed that the thickness of the diffusion weld at the Be–Cu joint varies between 26 and 345 µm, with the temperature increasing from 800 to 1000 °C and the holding time ranging from 20 to 120 min. The calculated layer thickness during the diffusion welding of a Be–Cu pair at 800 °C for 2 h is 65 µm, with 15 µm on the beryllium side and 50 µm on the copper side. Notably, a CuBe3 intermetallic compound zone can form in the diffusion weld, which should be considered an adverse factor that reduces the mechanical properties. To theoretically substantiate the modification of the structure and properties of the diffusion zone, a numerical study of welding was carried out using a 10 μm thick nickel foil spacer, which is readily soluble in beryllium. It was demonstrated that after exposure to temperature-time conditions at 900 °C for 20 min, a 50 µm wide diffusion-bonded joint is formed. Its structure includes two single-phase zones of solid solutions based on copper and beryllium, as well as two two-phase regions consisting of solid solutions hardened with intermetallic compounds. Since the weld lacks structural zones consisting solely of intermetallic compounds (unlike when welding the Be–Cu diffusion pair), there are grounds to anticipate a reduction in the embrittling effect on the weld. The results obtained from the analytical studies can serve as the foundation for a theoretical prediction method for assessing the quality of diffusion welding of the beryllium–copper composite.

Data-driven analysis of surface roughness influence on weld quality and defect formation in laser welding of Cu–Al

The laser welding of Cu–Al alloys for battery applications in the automotive industry presents significant challenges due to the high reflectivity of copper. Inadequate bonding and low mechanical strength may occur when the laser radiation is directed toward the copper side in an overlap configuration welding. To tackle these challenges, a laser surface treatment technique is implemented to enhance the absorption characteristics and overcome the reflective nature of the copper material. However, elevating the surface roughness and heat-energy input over threshold values leads to heightened temperature and extreme weld. This phenomenon escalates the formation of detrimental intermetallic compounds (IMC), creating defects like cracks and porosity. Metallurgical analysis, which is time-consuming and expensive, is usually used in studies to detect these phases and defects. However, to comprehensively evaluate the weld quality and discern the impact of surface structure, adopting a more innovative approach that replaces conventional cross-sectional metallography is essential. This article proposes a model based on the image feature extraction of the welds to study the effect of the laser-based structure and the other laser parameters. It can detect defects and identify the weld quality by weld classification. However, due to the complexity of the photo features, the system requires image processing and a convolutional neural network (CNN). Results show that the predictive model based on trained data can detect different weld categories and recognize unstable welds. The project aims to use a monitoring model to guarantee optimized and high-quality weld series production. To achieve this, a deeper study of the parameters and the microstructure of the weld is utilized, and the CNN model analyzes the features of 1310 pieces of weld photos with different weld parameters.

Adhesion Characterization and Enhancement between Polyimide-Silica Composite and Nodulated Copper for Applications in Next-Generation Microelectronics.

As the need for high-speed electronics continues to rise rapidly, printed wiring board (PWB) requirements become ever-more demanding. A typical PWB is fabricated by bonding dielectric films such as polyimide to electrically conductive copper foil such as rolled annealed (RA) copper and is expected to become thinner, flexible, durable, and compatible with high-frequency 5G performance. Polyimide films inherently feature a higher coefficient of thermal expansion (CTE) than copper foils; this mismatch causes residual thermal stresses. To attenuate the mismatch, silica nanoparticles may be used to reduce the CTE of PI. A nodulated copper surface can be used to enhance the Cu/PI adhesion by additional bonding mechanisms that could include a type of mechanical bonding, which is a focus of this study. In this investigation, a 90° peel test was used to measure the peel strength in copper/polyimide/copper laminates containing nodulated copper and polyimide reinforced with 0, 20, and 40 wt % silica nanoparticles. The influence of silica nanoparticles on the peel strength was quantitatively evaluated. Laminates incorporating polyimide films lacking silica nanoparticles had a ∼3.75× higher peel strength compared with laminates reinforced with 40% silica. Their failure surfaces were analyzed by using scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDX), and X-ray photoelectron spectroscopy to identify the mode of failure and to understand bonding mechanisms. The key bonding mechanism, mechanical interlocking, was achieved when the polyimide surrounded or engulfed the copper nodules when the laminate was created. Post-testing failure surface analysis revealed the presence of copper on the polyimide side and polyimide on the copper side, indicating mixed mode failure. An analytical model was developed to determine the impact of applied pressure, temperature, and time on the polyimide penetration and mechanical interlocking around the copper nodules. The model was validated by measuring the peel strength on another set of specimens fabricated using increased temperature and pressure that showed a 3× increase in peel strength compared to lower temperature/pressure processing conditions. This enhanced adhesion resulted from the lower polymer material viscosity at higher temperatures, which fosters deeper and more complete penetration around the copper nodules during processing at higher pressures for longer durations. The methodology of combining peel testing, viscosity and CTE measurement, SEM/EDX, surface chemical analysis, and penetration depth calculation developed herein enables the calculation of the desired processing parameters to enhance functionality and improve adhesion.

Control of pore morphology and mechanical strength of alumina ceramics produced by freeze-casting

In this work, unidirectional and bidirectional freeze-casting techniques were employed to fabricate alumina monoliths using camphene as solvent. Monolith samples with aligned porosity were produced in unprecedented molds designed using materials with different thermal conductivities (technyl and copper) and varying the alumina monoliths concentrations (15, 20 and 25 % of solid load). The total porosity of the produced monoliths ranged within 50–70 % and it was observed that the different molds did not influence the total porosity of the material. The SEM images showed that the samples produced in the technyl/copper mold presented a change on the perpendicular alignment of the pores in the transition region from copper to technyl. The copper side exhibited more aligned structures forming continuous channels, that become less noticeable when closer to the transition region, reaching the technyl region where the dendrites were not well aligned. The monoliths prepared in the technyl mold showed a better mechanical resistance with values of 19.6; 23.34 and 34.2 MPa for 15, 20 and 25 % of the solid load, respectively.

Microstructure and mechanical properties of T2 copper /316L stainless steel explosive welding composite with small size wavy interface

Copper/stainless steel composite is often used for preparation of superconducting busbar “double box” lap joints. However, it is a challenge for the large thickness T2 copper/316L stainless steel composite is fabricated by explosive welding, because of the bonding interface is always present many problems such as large ripple size, many microscopic defects, low bonding strength and poor sealing etc. In this paper, the big-thickness T2 copper/316L stainless steel composite with small size wavy interface was successfully fabricated by explosive welding. Microstructure of bonding interface indicated that the welding process could be divided into three stages: detonation growth stage, detonation stabilization stage, detonation descent stage. The morphology of bonding interface presented a regular wavy interface and no defects such as cracks, voids and intermetallic compound were formed in the stable stage, which was consistent with the results of ultrasonic and permeability quality inspection. The hardness of copper side and stainless steel side close to the bonding interface reached 131HV and 432HV, respectively. Dynamic recrystallization and fine grain generated in the bonding interface. The mechanical properties of tensile strength, pull-off strength and shear strength in the stable detonation stage is greater than 110 MPa, which is meeting the requirements of preparation of superconducting busbar “double box” lap joints. The deformation failure mainly occurred in the copper side near the bonding interface, which presented a higher bonding strength than copper. Some dimples and tearing edges with different sizes on the fracture surfaces, indicating that the fracture was a typical ductile fracture.

Microstructure and mechanical properties of ultrasonic welded copper to aluminum cables joints

The experiments of ultrasonic welding were carried out for BVR2.0 copper (Cu) cables and BLV6.0 aluminum (Al) cables with different welding parameters. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), temperature measurement, microhardness evaluation and tensile tests were conducted to investigate the microstructure and mechanical properties of Cu−Al cables joints. The results show that the tensile load of Cu−Al joints reaches 355.6 N, while the welding parameters are set as follows: welding pressure of 0.21 MPa, amplitude of 31%, and time of 860 ms. The diffusion thickness of well-welded Cu−Al joint is only about 3 μm and the peak temperature is 212 °C, which avoids the formation of intermetallic (IMC) effectively. The joint fractures on the copper side rather than the interface of the Cu−Al with a ductile−brittle hybrid fracture mode, indicating that the joint has excellent properties.

Fundamental study on high-quality welding of copper and aluminum by angled and superposed irradiation of blue and near-infrared lasers

In order to achieve sustainable development goals, decarbonization and low-carbonization are required. Electric and hybrid vehicles are indispensable for the conservation of natural environment, and the lightweight construction and the effective transfer of electricity become important. Thus, copper and aluminum have been increasingly recognized as important materials because of their excellent materials’ properties. However, in the welding of these materials, it is difficult to obtain strong joints because there are problems in the brittle intermetallic compounds and the welding defects due to different melting points between copper and aluminum. Especially in the joining of copper and aluminum by copper side irradiation, aluminum-rich intermetallic compounds (IMCs) of brittle state result in the decrease of mechanical strength. Therefore, mild heat input from copper to aluminum would be necessary to reduce the brittle IMC. Angled irradiation might result in the mild energy input to aluminum because it can be expected that aluminum would be heated by the reflected light inside the keyhole generated in copper according to its high light reflection. In addition, stable welding can be expected by the superposed irradiation of blue and near-infrared lasers because of high light absorption rate of blue laser to copper. The angled and the superposed irradiation could achieve a stable welding state, and the generation of aluminum-rich IMC becomes smaller. Angled irradiation of a near-infrared laser showed equivalent joining strength to the superposed irradiation of two wavelengths, and the combination of angled and superposed irradiation achieved a remarkable increase of joining strength in a cross tensile test by 80%.

Microstructure and mechanical properties of vacuum electron beam welded joints of Ti/Cu dissimilar metals

Sheets of TC4 titanium alloy and T2 pure copper were joined by vacuum electron beam welding with an offset on the copper side. Optical microscopy (OM) and scanning electron microscopy (SEM) were used to study the microstructure of the welded joint, and the composition of the intermetallic compounds (IMCs) at the reaction layer was studied by energy dispersive spectrometry (EDS) and transmission electron microscopy (TEM), followed by tensile testing to investigate the mechanical properties of the welded joints. The results show that electron beam welding can achieve an effective connection of Ti/Cu dissimilar metals. A multilayer structure composed of an IMCs layer, a final solidification zone, and a weld zone is formed at the Ti/Cu interface. The main composition of the IMCs layer is the faceted phase of TiCu and a small amount of the faceted phase of Ti2Cu, while the FS zone is composed of tiny equiaxed grains of copper. The tensile test results show that the ultimate tensile strength of the joints reaches 190 MPa, and the elongation rate reaches 4.27%. The joint presents brittle fracture characteristics with fracture occurring in the IMCs layer.

Vacuum Brazing and Performance Evaluation of T2 Copper Block and 316L Stainless Steel Tube

The International Thermonuclear Experiment Reactor (ITER) Thermal shield (TS) serves as a cryogenic heat exchanger to maintain the thermal stability of the ITER superconducting magnet coil, which is critical to the control of the plasma during the operation of the ITER device. The TS is composed of long-length 316L stainless steel (SS) and copper as brazed joints. In this case, a feasible fabrication design for the CCS TS is presented, accomplished by three kinds of joining processes (vacuum brazing, friction stir weld, and TIG weld). In the reliable fabrication design, the brazing quality of the as-brazed long-distance 316L SS and copper joints plays a critical role in the thermal conductivity performance of the ITER thermal shield. Therefore, a high-quality vacuum brazing process of long-length SS/Cu joints applied in a low-temperature superconductor magnet system was first studied. The macro metallography analysis demonstrates the braze ratio of the samples is 100%, and no crack or defect is observed in the samples. The microstructural characterization reveals the brazing seams are composed of silver-based Ag-rich eutectic. The micro-shear test indicates that the shear strength of the 316L tube and copper joint is 205 MPa, with the fracture position located on the copper side; this zone will be the most vulnerable zone of the joints. In addition, the SEM results illustrated that the shear fracture morphology displayed a ductile fracture feature. The test results demonstrated that the highly precise depth drilling employed in this paper ensured a good control of the brazing clearance, resulting in a 100% braze ratio for the long-length SS/Cu joints. Therefore, it can be concluded that the brazing process can be applied in the ITER TS for the good thermal conductivity performance of long-length SS/Cu-brazing joints.

Failure investigation of brazed copper winding bar strands of 250 MW stator

The present failure analysis investigates the failure of copper connectors and bus-bar windings of a 250 MW generator unit, of a steam powered turbine, which failed during service. An exhaustive microstructural and nanomechanical characterization was conducted in-order to determine the failure of copper bus-bar. Microstructure analysis revealed porosities in the brazed region with formation of numerous microconstituent phases. Furthermore, microstructure analysis revealed a network of numerous intergranular porosities on the failed copper side. SEM-EDS analysis of different microconstituents revealed a presence of Ag-rich dendritic phase, Cu-rich phase and lamellar eutectic microconstituents, highlighting a diverse microstructure. Nanoindentation measurements of different microconstituents and phases revealed a stark difference in the indentation hardness, thereby highlighting a strong mismatch between the observed microconstituents and phases.

Selective laser melting of 316L–CuCrZr bimetallic structure via IN718 transition layer

Steel/copper bimetallic structural materials combine the good strength and corrosion resistance of steel with the high thermal and wear resistance of copper and have been widely used in fusion reactors, conformal cooling channels, and heat transfer devices. Due to the large difference in thermal and physical properties of steel and copper, and the immiscibility of steel and copper, the bonding interface usually produces a large residual stress during the forming process, which leads to the formation of interface cracks. In order to combine CuCrZr with 316L stainless steel by using selective laser melting technology, IN718 was deposited on 316L stainless steel as a transition layer. The effects of the IN718 alloy transition layer on the microstructure, phase composition, and mechanical properties of the interface of 316L–CuCrZr were studied. IN718–316L had no defects such as pores and cracks at the interface and achieved good metallurgical bonding. The IN718–CuCrZr interface area and copper side had round key holes and relatively small pores. In the solidification process, IN718 preferentially solidified in the shape of fine particles, providing a nucleation point for the copper grain, thus forming equiaxed grains because of the high cooling rate of the interface. No macroscopic defects were observed at the bonding position of the 316L–IN718–CuCrZr samples, and the elements at the interface were fully diffused by each other. The tensile strength of the 316L–IN718–CuCrZr samples reached 403 MPa, and the interface strength was significantly improved upon the addition of the transition layer.

Material flow behavior and mechanical characterization of dissimilar friction stir welded Cu-Mg alloy joints

Magnesium alloys are considered to be the lightest material, whereas pure copper is known for better electrical and thermal conductivity finds their applicability especially in automotive and electronic industries. Joining of pure copper and Mg alloy has been studied by many researchers. However, very limited work are available for joining of Cu alloy with Mg Alloy. In the present study, friction stir welding was performed to join Cu-8Zn alloy and Mg AZ31 alloy plates of 3 mm thickness. The process parameters were selected as 1200 rpm rotational speed, 20 mm/min traverse speed, 3° tilt angle, and 0.5 mm tool offset towards copper side. The weld samples were studied for microstructural and mechanical properties. The obtained results by SEM and XRD revealed that there is a complex intercalated microstructure and Mg2Cu intermetallic compounds (IMCs) are present in the stir zone (SZ). Variation in hardness has been observed in the microstructure and maximum hardness of 162.54 HV was found in the SZ due to the presence of Mg2Cu and dynamic recrystallization of grains. The tensile strength of 72 MPa has been attained.

Nb Phase Position Marking for Clarifying the Formation Process of Cu-Al Composite Interfacial Phases in Continuous Composite Casting

Cu-Al composites are widely applied materials exhibiting advanced properties of both matrix metals. Controlling the brittle interfacial phases is a key factor in improving the interfacial strength of Cu-Al composites. This paper studied the interfacial formation process of Cu-Al composites fabricated by continuous composite casting. The phase formation sequence, growth direction and formation mechanism were clarified via element marking and thermodynamic calculations. The spatial distribution of the interfacial phases from the aluminum side to the copper side is as follows: the α + θ layer (α-Al + CuAl2), the θ layer (CuAl2) and the γ layer (Cu9Al4). Moreover, insular η phases (CuAl) and δ phases (Cu3Al2) exist in the γ phase sublayer. The formation sequence of interfacial phases is as follows: the θ phase, the η phase, the δ phase and the γ phase. The θ layer and α + θ layer are transformed from a liquid diffusion layer formed by scouring the surface of copper with liquid aluminum, the η and δ phases grow towards the θ layer and the γ phase simultaneously grows towards both the copper matrix and the θ layer.

Unveiling slip/nanotwin/nano shear band deformation mechanisms at the copper side in a roll-welded aluminum/copper couple

In this study, the co-deformation behavior of Cu in an Al-Cu couple was evaluated. It is observed that slip, twinning, and partial shear banding were responsible for deformation in Cu during cold welding. For characterization purposes, transmission electron microscopy (TEM) was employed to detect deformation mechanisms after the rolling. It was demonstrated that the formation of nanotwins could have evidently influenced the microstructure of the Cu side. The junction of twin/twin, twin/slip, and twin/shear bands was another major observation and confirmed complex microstructure during rolling. Also, the resulting co-deformation has a noticeable effect on microhardness as compared to the aluminum side. Moreover, the textural investigations were conducted by the X-ray diffraction (XRD) and electron back-scattered diffraction (EBSD) methods after rolling. Textural investigations exhibited a transition from Cube to rolling texture (mainly Brass) and shear (Rotated Cube) textures. For validation of shear deformation texture in Al-Cu, a finite element method (FEM) was employed, and approved experimental textural studies regarding shear texture formation.

Numerical simulation of the temperature field of T2 copper/304 steel dissimilar metal electron beam welding

ANSYS workbench finite element analysis software was adopted to simulate and study the thermal process of electron beam welding of T2 copper and 304 steel heterogeneous materials in this paper. A solid model was established according to the size of the specimens, and a double ellipsoidal moving heat source model of the temperature field of the specimens of the three-dimensional dynamic simulation was imposed. Copper and steel welding transient temperature field distribution pattern and molten pool morphology were studied, and the temperature field welding thermal cycle curve was analyzed. The results show that the peak temperature of the steel side is higher than the copper side, the morphology of the welding pool is basically biased toward the steel side, and the highest temperature during the welding process appears in the center of the weld on the steel side about 0.5 mm.